Hagino, Takao (Tokyo, JP)
Ishikawa, Hideyuki (Tokyo, JP)
Fuchu, Yuichi (Tokyo, JP)
Plaque It!
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| 7153436 | Method for enhancing cyclonic vessel efficiency with polymeric additives | December, 2006 | Bair et al. | 210/709 |
1. A crystal separation apparatus for separating crystals from sludge or from separated water generated when sludge is subjected to a concentration process or a dewatering process, comprising: a crystallization reactor for precipitating the crystals by adding a chemical to the sludge or separated water; a hydrocyclone separating the crystals from the sludge or separated water introduced into the hydrocyclone after being treated by said crystallization reactor; a first return pipe for returning said sludge or separated water which flows out from an overflow riser of said hydrocyclone to said crystallization reactor, and a second return pipe for returning the sludge or separated water which flows out from said overflow riser of said hydrocyclone to a sludge or separated water introduction portion of said hydrocyclone.
2. The crystal separation apparatus according to claim 1, further comprising a third return pipe for returning all or a part of said crystals separated or concentrated by said hydrocyclone to said crystallization reactor.
3. The crystal separation apparatus according to claim 2, wherein a cleaning pipe for cleaning said hydrocyclone is connected to said third return pipe.
4. The crystal separation apparatus according to claim 1, further comprising an extraction pipe for extracting crystals deposited on a base portion of said crystallization reactor to the exterior of the system such that, when an amount of the sludge or raw water to be supplied to said crystallization reactor is Q1, an amount of the chemical to be added to said crystallization reactor is Q2, an amount of the crystals separated or concentrated by said hydrocyclone to be returned to said crystallization reactor is Q3, and an amount of the crystals to be extracted by said extraction pipe is Q4, Q1+Q2+Q3<Q4.
5. The crystal separation apparatus according to claim 1, wherein a recovery pipe for recovering crystals separated or concentrated by said hydrocyclone is connected to a base portion of said hydrocyclone.
6. The crystal separation apparatus according to claim 5, wherein a cleaning pipe for cleaning said hydrocyclone is connected to said recovery pipe.
7. A crystal separation apparatus for separating crystals from sludge, or from separated water generated when sludge is subjected to a concentration process or a dewatering process, comprising: a crystallization reactor for precipitating the crystals by adding a chemical to the sludge or separated water; a plurality of hydrocyclones arranged in parallel for separating the crystals from the sludge or separated water introduced into said hydrocyclones after being treated by said crystallization reactor; and a return pipe for returning all or a part of said crystals separated or concentrated by said hydrocyclones to said crystallization reactor.
8. The crystal separation apparatus according to claim 7, further comprising: a flow meter for measuring a flow rate of the sludge or separated water disposed in a pipe for introducing the sludge or separated water into said crystallization reactor, and means for determining a number of said hydrocyclones to be operated from a relationship between a value measured by said flow meter, a preset flow rate range, and a number of operational hydrocyclones of said plurality of hydrocyclones.
9. The crystal separation apparatus according to claim 7, further comprising: a sludge concentration meter disposed in said crystallization reactor, and means for determining a number of hydrocyclones to be operated in accordance with a detection value of said sludge concentration meter.
10. The crystal separation apparatus according to claim 7, further comprising: a screen residue removal apparatus disposed upstream of said crystallization reactor or between said crystallization reactor and said hydrocyclones, means for monitoring a phosphorus concentration and a pH value of the sludge or separated water treated by said crystallization reactor, said hydrocyclones and said crystallization reactor; means for calculating a supersaturation ratio on the basis of the phosphorus concentration and the pH value measured by said monitoring means; and means for determining an amount of magnesium to be added and a number of hydrocyclones to be operated in accordance with the supersaturation ratio.
11. A digested sludge treatment apparatus for treating digested sludge generated by subjecting organic waste matter to anaerobic digestion, comprising: an apparatus for decarbonating the digested sludge; a removal apparatus for removing screen residue from the digested sludge, wherein said removal apparatus is a wet-type vibrating screen equipped with two or more types of sieve bodies having different hole diameters; and an apparatus for separating or concentrating crystals containing magnesium ammonium phosphate from the digested sludge after the digested sludge passes through said decarbonation apparatus and said removal apparatus.
12. A digested sludge treatment apparatus for treating digested sludge generated by subjecting organic waste matter to anaerobic digestion, comprising: a crystallization reactor for precipitating magnesium ammonium phosphate from the digested sludge; a removal apparatus for removing screen residue from the digested sludge, wherein said removal apparatus is a wet-type vibrating screen equipped with two or more types of sieve bodies having different hole diameters; and an apparatus for separating or concentrating crystals containing magnesium ammonium phosphate from the digested sludge after the digested sludge passes through said crystallization reactor and said screen residue removal apparatus.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus and method for separating and concentrating crystals from anaerobic or aerobic sludge, separated water generated when sludge is subjected to a concentration process or dewatering process, or various kinds of waste water. The present invention also relates to an apparatus and method for treating digested sludge generated when organic waste matter or waste water is subjected to anaerobic digestion.
2. Description of the Related Art
In a treatment facility for treating organic waste water containing phosphorus and nitrogen, such as sewage, waste water, and screen, first the raw sludge (also referred to as preliminary sludge hereafter) is subjected to liquid/solid separation in a preliminary sedimentation tank, whereupon the separated supernatant liquid is subjected to activated sludge treatment to remove organic matter. During the activated sludge treatment, the multiplied activated sludge is discharged as excess sludge. Incidentally, when raw sludge or organic waste matter such as excess sludge, screen, or raw refuse is subjected to anaerobic digestion, the organic matter in the waste matter is broken down by the action of acidogenic bacteria and methanogenic bacteria, as a result of which the amount of sludge is reduced and gas containing methane, carbon dioxide, and the like is generated together with waste water having high nitrogen and phosphorus concentrations. Nowadays, a great deal of research is being conducted into the utilization of generated methane gas as a heat source, as well as the generation of MAP from digested sludge liquor obtained through the dewatering of digested sludge and the effective use of this MAP in fertilizers, chemical raw materials, and so on (Japanese Unexamined Patent Application Publication 2003-117306).
Moreover, improvements are currently being made in the efficiency of anaerobic digestion tanks in the areas of energy recovery and sludge reduction. For example, by subjecting raw sludge, excess sludge, or mixed sludge containing both raw sludge and excess sludge to physical/mechanical treatment, chemical liquefaction treatment, heat treatment, and so on such that the sludge is solubilized, improvements have been achieved in the methane gas recovery rate and sludge reduction rate during a subsequent anaerobic digestion process. The aforementioned physical/mechanical treatment includes ultrasonic treatment and crushing using a mill. Chemical liquefaction treatment includes treatment using ozone, hydrogen peroxide, acid, and alkali. Heat treatment includes treatment using thermophilic bacteria. For example, Japanese Unexamined Patent Application Publication 2002-336898 describes a method of solubilizing sludge by treating the sludge in an ultrasonic treatment process.
As the efficiency of anaerobic digestion increases in the manner described above, waste water having ever higher nitrogen and phosphorus concentrations is produced. Originally, organic waste matter contains elements such as nitrogen, phosphorus, and also magnesium and the like, and when the organic waste matter is solubilized, these elements migrate into the solution. When waste water containing such high concentrations of nitrogen and phosphorus is returned to a water treatment system, the nitrogen and phosphorus loads on the water treatment system increase, causing deterioration in the quality of the treated water.
Hence, in a known technique, MAP is generated by subjecting the digested sludge or digested sludge liquor to aeration treatment, and a part of the sludge that settles in the sedimentation tank is returned to the aeration tank to serve as seed crystals. In so doing, the phosphorus concentration of the return water decreases, and the phosphorus can be recovered easily as MAP. In Japanese Examined Patent Application Publication H7-115979, digested sludge is decarbonated, whereupon a magnesium compound is added to precipitate MAP, thereby reducing the phosphorus concentration of the dewatered separated liquid. In both cases, the phosphorus concentration of the liquid is reduced by precipitating MAP, and as a result, phosphorus can be prevented from circulating endlessly between water treatment process and sludge treatment process.
Nowadays, in order to achieve effective utilization and efficient treatment of digested sludge, in some regions, all wastewater treatment plants and the like are connected by sewers (pipes), and generated digested sludge is transported through the sewers, gathered at a single treatment plant, and treated collectively in the single treatment plant. Similarly, in order to treat the separated liquid that is generated by dewatering digested sludge efficiently, in some regions, all wastewater treatment plants and the like are connected by sewers, and the generated separated liquid is transported through the sewers to be treated collectively in a single treatment plant. The construction cost of the sewers is cheaper than that of a treatment facility, and since the advantage of scale applies to sludge treatment facilities (i.e. the unit cost thereof decreases as the scale increases), sewers are considered to be more economical in locations such as urban areas, where residential buildings are in close proximity.
However, when digested sludge or the separated liquid thereof is transported through pipes, so-called MAP precipitate is generated when the magnesium ions of the sludge chemically combine with the phosphate ions and ammonium ions, leading to possible blockages of the sludge pipe.
To solve this problem, a method of transporting the sludge through a sludge pipe after removing and recovering the MAP by aerating the digested sludge in a reactor in advance to generate MAP particles, and then subjecting the sludge containing the MAP particles to centrifugal separation, is known. Further, a part or all of the MAP particles is returned to the reactor following centrifugal separation to serve as kernels for generating new MAP particles in the reactor. By performing such an operation, problems such as blockages of the sludge pipes caused by MAP particles can be avoided.
As noted above, as the efficiency of anaerobic digestion increases, waste water having ever higher nitrogen and phosphorus concentrations is produced. Originally, organic waste matter contains elements such as nitrogen, phosphorus, and also magnesium and the like, and when the organic waste matter is solubilized, these elements migrate into the solution. Nitrogen, phosphorus, and magnesium are the constituent components of MAP, and at high concentrations in liquid, or when the pH rises, the nitrogen, phosphorus, and magnesium easily equal or exceed the solubility product of the MAP such that the MAP precipitates spontaneously in a digestion tank. When the MAP precipitates into a draft tube in the digestion tank, the flow of the digested sludge deteriorates, and scale trouble such as pipe blockages during pump extraction occurs frequently. Moreover, the MAP is disposed of together with the dewatered sludge rather than being recovered, and hence there is demand for an efficient MAP recovery method.
When MAP is precipitated by decarbonating the digested sludge or adding a magnesium compound thereto, the phosphorus concentration of the dewatered separated liquid decreases, and therefore the phosphorus load on the water treatment system is reduced so that the treated water can be maintained at a favorable quality. However, this method focuses on phosphorus removal rather than the recovery of phosphorus resources, and hence there is demand for a treatment method which satisfies aspects of both phosphorus removal and phosphorus recovery.
Moreover, when MAP is recovered through aeration and centrifugal separation, the recovered matter contains digested sludge and screen residue as well as the MAP, and hence it is not always possible to recover MAP having a high degree of purity. When recycling phosphorus, purity is required, and hence there is demand for a method of recovering MAP having a high degree of purity.
Furthermore, when MAP is recovered by subjecting the MAP to centrifugal separation using a hydrocyclone and the concentration of MAP or other inorganic solids introduced into the hydrocyclone is high, the hydrocyclone itself may become blocked. In certain cases, the MAP concentration in an overflow riser may also increase, leading to deterioration in the recovery rate. Hence, there is demand for a separating method enabling treatment to be performed with stability and a high MAP recovery rate.
Further, when digested sludge containing MAP is transported by pipe to a facility for treating sludge collectively, a large amount of MAP scale is generated in the pipe, leading to deterioration in the efficiency with which the sludge is transported. Following its initial generation, MAP scale continues to grow. If MAP scale is left on the inside of the pipe, the entire sewer eventually becomes covered in MAP scale, making sludge transportation difficult, and hence cleaning and complicated maintenance must be performed periodically.
Nowadays, due to the increase in sewerage, advancements in advanced treatment, and so on, the amount of sewage-treated sludge is increasing. In order to detoxify and solubilize the sludge, sludge smelting treatment is becoming more widely used. However, when sewage sludge is subjected to smelting treatment at a high temperature between 1200 and 1400° C., a part of the phosphorus contained in the sludge is emitted into the slag without solidifying, and as a result, the phosphorus becomes stuck during an exhaust gas treatment process, leading to problems such as machinery corrosion, an increase in the phosphorus load of the return water produced by exhaust gas wet cleaning, and so on. Hence, there is demand for a technique for removing phosphorus from sludge in advance.
As for crystallization reactors, when attempts have been made in the past to reduce the size of the crystallization reactor, the concentration ability of the hydrocyclone has posed a problem since it becomes difficult to maintain the MAP in the crystallization reactor at a high concentration.
SUMMARY OF THE INVENTION
An object of the present invention is to solve the problems illustrated above by providing a treatment method and apparatus which satisfy aspects of both phosphorus removal and phosphorus recovery, enable the recovery of MAP having a high degree of purity, and enable stable treatment in which the MAP recovery rate is high and blockages of a hydrocyclone do not occur.
The present invention may be applied not only to the separation and recovery of MAP crystals from anaerobically digested sludge and the like, as described above, but also to the separation and recovery of various types of crystal from various types of waste water. For example, the present invention may be applied to the recovery of calcium phosphate (Ca 3 (PO 4 ) 2 ) crystals or hydroxyapatite (Ca 10 (PO 4 ) 6 (OH) 2 :HAP) crystals from waste water such as return water obtained from the secondary treated water of sewage or a sludge treatment system; the recovery of calcium fluoride (CaF 2 ) crystals from the waste water of a semiconductor factory or the like; the recovery of calcium carbonate crystals from service water using ground water as a raw water, waste water, and refuse leachate; the recovery of calcium carbonate (CaCO 3 ) crystals from hard water containing a large amount of carbonate ions; the recovery of manganese carbonate (MnCO 3 ) crystals generated from Mn in the form of an impurity contained in running tap water; and so on.
This specification mainly describes an example in which MAP crystals are separated and recovered from anaerobically digested sludge and the like.
As means for achieving the object described above, an aspect of the present invention provides the crystal separation apparatus illustrated below.
According to one aspect of the present invention, there is provided a crystal separation apparatus for separating crystals from sludge, or from separated water generated when sludge is subjected to a concentration process or a dewatering process, comprising a hydrocyclone for separating the crystals from the sludge or separated water introduced into the hydrocyclone and a return pipe A for returning the sludge or separated water which flows out from an overflow riser of the hydrocyclone to a sludge or separated water introduction portion of the hydrocyclone.
According to another aspect of the present invention, there is provided the crystal separation apparatus as defined hereinbefore, wherein a crystallization reactor for precipitating the crystals by adding a chemical to the sludge or separated water is provided before the. The apparatus further comprises a return pipe B for returning all or a part of the crystals separated by the hydrocyclone to the crystallization reactor.
According to another aspect of the present invention, there is provided a crystal separation apparatus for separating crystals from sludge, or from separated water generated when sludge is subjected to a concentration process or a dewatering process, comprising a crystallization reactor for precipitating the crystals by adding a chemical to the sludge or separated water, a hydrocyclone for separating the crystals from the sludge or separated water introduced into the hydrocyclone after being treated by the crystallization reactor and a return pipe C for returning the sludge or separated water which flows out from an overflow riser of the hydrocyclone to the crystallization reactor.
According to another aspect of the present invention, there is provided the crystal separation apparatus as defined hereinbefore, further comprising a return pipe A for returning the sludge or separated water which flows out from the overflow riser of the hydrocyclone to a sludge or separated water introduction portion of the hydrocyclone.
According to another aspect of the present invention, there is provided the crystal separation apparatus as defined hereinbefore, further comprising a return pipe B for returning all or a part of the crystals separated by the hydrocyclone to the crystallization reactor.
According to another aspect of the present invention, there is provided the crystal separation apparatus as defined hereinbefore, further comprising an extraction pipe for extracting the crystals deposited on a base portion of the crystallization reactor to the exterior of the system. When an amount of the sludge or raw water to be supplied to the crystallization reactor is Q1, an amount of the chemical to be added to the crystallization reactor is Q2, an amount of the crystals separated by the hydrocyclone to be returned to the crystallization reactor is Q3, and an amount of the crystals to be extracted via the extraction pipe is Q4, Q1+Q2+Q3<Q4.
According to another aspect of the present invention, there is provided the crystal separation apparatus as defined hereinbefore, wherein a surface area of a plane cross section of the base portion of the crystallization reactor is smaller than a surface area of a plane cross section of an upper end of the crystallization reactor.
According to another aspect of the present invention, there is provided the crystal separation apparatus as defined hereinbefore, wherein a recovery pipe for recovering the crystals separated or concentrated by the hydrocyclone is connected to a base portion of the hydrocyclone.
According to another aspect of the present invention, there is provided the crystal separation apparatus as defined hereinbefore, wherein a cleaning pipe for cleaning the hydrocyclone is connected to the return pipe C or the recovery pipe.
According to another aspect of the present invention, there is provided a crystal separation apparatus for separating crystals from sludge, or from separated water generated when sludge is subjected to a concentration process or a dewatering process, comprising a crystallization reactor for precipitating the crystals by adding a chemical to the sludge or separated water, a plurality of hydrocyclones arranged in series for separating the crystals from the sludge or separated water introduced into the hydrocyclones after being treated by the crystallization reactor and a return pipe B for returning all or a part of the crystals separated or concentrated by the hydrocyclones to the crystallization reactor.
According to another aspect of the present invention, there is provided the crystal separation apparatus as defined hereinbefore, wherein a flow meter for measuring a flow rate of the sludge or separated water is disposed in a pipe for introducing the sludge or separated water into the crystallization reactor. The apparatus further comprises means for determining a number of hydrocyclones to be operated from a relationship between a measured value of the flow meter, a preset flow rate range, and a number of operational hydrocyclones.
According to another aspect of the present invention, there is provided the crystal separation apparatus as defined hereinbefore, wherein a sludge concentration meter is disposed in the crystallization reactor, and the apparatus further comprises means for determining a number of hydrocyclones to be operated in accordance with a detection value of the sludge concentration meter.
According to another aspect of the present invention, there is provided the crystal separation apparatus as defined hereinbefore, further comprising means for monitoring a phosphorus concentration and a pH value of the sludge or separated water treated by the crystallization reactor and the hydrocyclones, means for calculating a supersaturation ratio on the basis of the phosphorus concentration and the pH value measured by the monitoring means and means for determining an amount of magnesium to be added and a number of hydrocyclones to be operated in accordance with the supersaturation ratio.
According to another aspect of the present invention, the crystal separation apparatus as defined hereinbefore, further comprising a screen residue removal apparatus disposed before said crystallization reactor or between the crystallization reactor and the hydrocyclone, means for monitoring a phosphorus concentration and a pH value of said sludge or separated water treated by the crystallization reactor, the hydrocyclones and said crystallization reactor, means for calculating a supersaturation ratio on the basis of the phosphorus concentration and the pH value measured by the monitoring means and means for determining an amount of magnesium to be added and a number of hydrocyclones to be operated in accordance with the supersaturation ratio.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a treatment flow diagram pertaining to an embodiment of the present invention;
FIG. 2 is a treatment flow diagram pertaining to an embodiment of the present invention;
FIG. 3 is a treatment flow diagram pertaining to an embodiment of the present invention;
FIG. 4 is a treatment flow diagram pertaining to an embodiment of the present invention;
FIG. 5 is a treatment flow diagram pertaining to an embodiment of the present invention;
FIG. 6 is a treatment flow diagram pertaining to an embodiment of the present invention;
FIG. 7 is a treatment flow diagram pertaining to an embodiment of the present invention;
FIG. 8 is a treatment flow diagram pertaining to an embodiment of the present invention;
FIG. 9 is a treatment flow diagram pertaining to an embodiment of the present invention;
FIG. 10 is a treatment flow diagram pertaining to an embodiment of the present invention;
FIG. 11 is a graph showing the relationship between a MAP amount in a crystallization reactor and a phosphorus recovery rate;
FIG. 12 is a treatment flow diagram pertaining to an embodiment of the present invention;
FIG. 13 is a treatment flow diagram pertaining to an embodiment of the present invention;
FIG. 14 is a treatment flow diagram pertaining to an embodiment of the present invention;
FIG. 15 is a treatment flow diagram pertaining to an embodiment of the present invention;
FIG. 16 is a treatment flow diagram pertaining to an embodiment of the present invention;
FIG. 17 is a treatment flow diagram pertaining to an embodiment of the present invention;
FIG. 18 is a treatment flow diagram pertaining to an embodiment of the present invention;
FIG. 19 is a graph showing the relationship between a sludge circulation ratio in a hydrocyclone and a MAP concentration following circulation, determined in example 3;
FIG. 20 is a flow diagram of apparatuses used in comparative example 1;
FIG. 21 is a flow diagram of apparatuses used in comparative example 2;
FIG. 22 is a flow diagram of apparatuses used in example 4;
FIG. 23 is a flow diagram of apparatuses used in example 5;
FIG. 24 is a flow diagram of apparatuses used in comparative example 3;
FIG. 25 is a flow diagram of apparatuses used in comparative example 4;
FIG. 26 is a flow diagram of apparatuses used in example 6;
FIG. 27 is a flow diagram of apparatuses used in example 7;
FIG. 28 is a flow diagram of apparatuses used in example 8;
FIG. 29 is a flow diagram of apparatuses used in example 9;
FIG. 30 is a flow diagram of apparatuses used in example 10;
FIG. 31 is a flow diagram of apparatuses used in example 11;
FIG. 32 is a flow diagram of apparatuses used in comparative example 5;
FIG. 33 is a flow diagram of apparatuses used in example 14;
FIG. 34 is a flow diagram of apparatuses used in comparative example 6;
FIG. 35 is a flow diagram of apparatuses used in comparative example 7; and
FIG. 36 is a flow diagram of a comparative flow in example 16.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiments of the present invention will now be described in detail with reference to the drawings. However, it is to be understood that the present invention is not limited to these embodiments. As noted above, the present invention may be applied to the separation and recovery of various types of crystal from various types of waste water, but in the following description, an example is mainly used in which MAP crystals are separated and recovered from anaerobically digested sludge or the like.
In the drawings, constitutional elements having identical functions may be described using identical reference numerals. Furthermore, where appropriate, description relating to constitutional elements having identical functions has been omitted. The technical matter relating to each constitutional element described in relation to the flow of each drawing may be applied to identical constitutional elements in the flows of the other drawings.
Examples of the MAP-containing sludge which is treated in the present invention include screen, treatment tank sludge, sewerage sludge, agricultural sludge, livestock waste, raw refuse, and food waste. The organic waste matter is typically in liquid slurry form, or has a high moisture content when in solid form. To enable smooth treatment, it is preferable to introduce waste water or the like into non-slurry form waste matter such that the waste matter is treated in slurry form. The present invention may also be applied to separated water obtained by concentrating or dewatering the sludge. In the following description, an example will be used in which sludge obtained by subjecting excess sewerage sludge to anaerobic digestion is employed as the MAP-containing sludge.
FIG. 1 shows a treatment flow in which anaerobically digested sludge is used as raw sludge 1 , MAP 3 in the sludge 1 is concentrated using a hydrocyclone 2 and discharged via a micro-particle discharge pipe 4 , and the sludge from which the MAP 3 has been removed is caused to overflow through a sludge discharge pipe (also referred to as an overflow riser) 5 .
The lower portion structure of the hydrocyclone 2 shown in FIG. 1 takes an inverted conical form with a hydrocyclone inflow pipe 6 (also referred to as an introduction pipe) provided on the side portion, the micro-particle discharge pipe 4 provided on the lower portion, and the overflow riser 5 provided on the upper portion. In the hydrocyclone 2 , the digested sludge 1 containing the MAP 3 is transmitted through the hydrocyclone inflow pipe 6 by the pressure of a pump, and is then caused to swirl down the inverted cone-shaped wall surface in the interior of the hydrocyclone 2 . Micro-particles containing the MAP 3 , which have a greater specific gravity than the digested sludge, are collected and concentrated on a lower wall surface side by means of centrifugal force. The concentrated micro-particles are discharged either continuously or intermittently from the micro-particle discharge pipe 4 . Further, sludge (treated sludge) 8 from which the micro-particles containing the MAP 3 have been removed is extracted through the overflow riser 5 and discharged.
The pipe diameter of the micro-particle discharge pipe 4 and the pipe diameter of the overflow riser (sludge discharge pipe) 5 may be altered, and by altering both, the flow rate and particle diameter distribution can be varied.
In the present invention, the sludge that flows out of the sludge discharge pipe of the hydrocyclone 2 is returned to the hydrocyclone inflow pipe 6 through a treated sludge return pipe 7 (return pipe A). The return amount may be set to a desired proportion of the raw sludge 1 introduction amount, but the return amount is preferably determined such that the MAP concentration of the sludge introduced into the hydrocyclone remains at or below a predetermined concentration, and the set value thereof is preferably varied according to the size of the hydrocyclone 2 , the diameter of the sludge discharge pipe, and the diameter of the micro-particle discharge pipe. The micro-particle discharge speed in relation to the sectional area of the micro-particle discharge pipe (kg/mm 2 /hr) is a particularly important factor in determining whether or not the hydrocyclone will become blocked since the hydrocyclone becomes blocked when the discharge speed reaches or exceeds a certain speed. For example, a two-inch hydrocyclone becomes blocked at approximately 10 kg/mm 2 /hr or more, although this differs according to factors such as the introduction pressure and the shape of the hydrocyclone, and therefore the concentration of the MAP-containing micro-particles in the hydrocyclone inflow pipe is preferably reduced such that the micro-particle discharge speed is no greater than 5 kg/mm 2 /hr, and more preferably no greater than 2 kg/mm 2 /hr.
As will be described below in Example 3, the treated sludge return amount is preferably no less than 0.1 times the amount of introduced raw sludge 1 , and although there is no specific upper limit, the return amount is preferably set to no more than 50 times the amount of introduced raw sludge 1 in order to achieve an economical circulation ratio.
The sludge that flows out of the sludge discharge pipe 5 may be returned to the cyclone inflow pipe 6 after being stored temporarily in a storage tank (not shown), or may be returned without passing through the storage tank. The remaining sludge is recovered as the treated sludge 8 and applied appropriately to a dewatering process or the like.
By returning the treated sludge that flows out from the overflow riser of the hydrocyclone to the cyclone inflow pipe in this manner, the concentration of the MAP-containing micro-particles in the cyclone inflow pipe decreases, and as a result, blockages of the cyclone can be prevented. Moreover, by reducing the concentration of the MAP-containing micro-particles in the cyclone inflow pipe, the MAP concentration of the sludge that is discharged through the overflow riser also decreases, and thus the MAP recovery rate improves.
FIG. 2 shows the treatment flow of another embodiment of the present invention, constituted by a crystallization reactor 9 and the hydrocyclone 2 .
In the crystallization reactor 9 , MAP is precipitated by adding a magnesium compound 10 serving as a crystallization chemical to the digested sludge and digested sludge liquor 1 extracted from the anaerobic digestion tank, and by causing the magnesium compound 10 to react with PO 4 —P dissolved in the liquid. If aeration, decompression, or a similar process is also performed at this time, the sludge is decarbonated, leading to an increase in the pH, and hence MAP can be precipitated more efficiently. Needless to say, a chemical such as sodium hydroxide, magnesium hydroxide, or magnesium oxide may be added to raise the pH. As the added magnesium compound 10 , magnesium chloride, magnesium hydroxide, magnesium oxide, sea water, and so on may be used. As regards the amount of added magnesium, a molar ratio between 0.1 and 10, preferably between 0.5 and 3.0, and more preferably between 0.8 and 1.2 in relation to the aqueous orthophosphoric acid concentration of the digested sludge is suitable. The pH in the reaction may be between 7.0 and 11.0, and preferably between 7.5 and 8.5.
Seed crystals are preferably added to the crystallization reactor 9 to ensure that MAP is precipitated efficiently. The MAP precipitated by the crystallization reactor 9 or MAP precipitated in a separate reactor may be used as the seed crystals. Alternatively, MAP that precipitates spontaneously in the digestion tank may be used.
Micro-particles or the like containing the MAP 3 , which are recovered by the hydrocyclone 2 , may also be used. In this case, the micro-particles that are separated and recovered by the hydrocyclone 2 may be returned to the crystallization reactor 9 through a return pipe 11 (return pipe B). When separated water from the separation process, effluent water, or the like contains MAP, each of them may also be used.
Alternatively, a powder or granular substance such as rock phosphate, dolomite, bone charcoal, activated carbon, silica sand, or calcium silicate may be used. The particle diameter of the seed crystals is arbitrary, but is preferably set between 0.05 and 3.0 mm, and more preferably between 0.1 and 0.5 mm. By precipitating new MAP on the surface of the seed crystals, subsequent separation of the digested sludge and MAP 3 in the hydrocyclone 2 can be performed favorably. The seed crystal charging amount is extremely important for precipitating MAP on the surface of the seed crystals. The charging amount is determined in consideration of the introduced phosphorus amount and the seed crystal particle diameter such that the phosphorus introduction amount in relation to the seed crystal surface area (the phosphorus surface area load hereafter) is no more than 100 g-P/m 2 /d, preferably no more than 30 g-P/m 2 /d, and more preferably no more than 10 g-P/m 2 /d. In other words, when a high concentration of MAP is maintained in the reactor 9 in relation to a constant particle diameter, the volume of the reactor can be reduced, enabling a reduction in initial costs.
There are no particular limitations on the form of the reactor 9 used in the crystallization process, and a complete mixing type reactor comprising a mechanical stirring apparatus, a jet stream type stirring reactor using a pump, a fluidized bed reactor charged with seed crystals at a high density, an internal circulation type reactor comprising a draft tube, an external circulation type reactor, or another reactor may be used.
The MAP precipitated in the crystallization process is extracted from the base portion of the crystallization reactor 9 , introduced into the aforementioned hydrocyclone 2 through the hydrocyclone inflow pipe 6 , and subjected to separation and recovery in the hydrocyclone 2 . In this case also, the sludge that flows out from the overflow riser 5 of the hydrocyclone 2 is returned to the hydrocyclone inflow pipe 6 through the return pipe 7 (return pipe A). By diluting the MAP concentration, which was maintained at a high level in the crystallization process, with the treated sludge 8 , blockage of the cyclone 2 can be prevented, and a stable treatment performance with a high MAP recovery rate can be obtained. Note that in FIG. 2, 10 denotes a Mg compound. An Mg compound for generating MAP is added to the crystallization reactor, and therefore MAP generation occurs in the crystallization reactor 9 . Sludge may be introduced into the hydrocyclone 2 either continuously or intermittently.
FIG. 3 is a flow diagram showing an example of another embodiment of the present invention, in which a screen residue removal apparatus 11 is provided before the sludge 1 is introduced into the hydrocyclone 2 . Conventionally, no screen residue separation process is provided, and therefore problems such as blockages caused by screen residue and the like occur in the downstream hydrocyclone 2 . As a result, long-term stable treatment is difficult. In the present invention, screen residue is removed, enabling a dramatic improvement in the stability of MAP particle separation using the hydrocyclone. Moreover, by separating screen residue in advance, MAP with a high degree of purity can be obtained. The screen residue removal apparatus 11 may also be attached midway along the hydrocyclone inflow pipe 6 .
The removed screen residue may be discharged outside of the system or introduced into a dewatering process for dewatering the treated sludge 8 . In the latter case, the dewatering performance is preferably enhanced.
The crystallization process and the screen residue separation process may performed in any order. The crystallization process may precede the screen residue separation process, or the screen residue separation process may precede the crystallization process.
A method for separating the screen residue may employ a centrifugal settler, a sedimentation tank employing gravity separation, and so on, and a micro-particle separation method using differences in particle diameter may employ a vibrating screen, a drum screen, a filter layer, a classification layer type separation layer, and so on.
A further embodiment of the present invention is illustrated in FIGS. 4 through 6.
FIG. 4 shows a treatment flow constituted by the crystallization reactor 9 comprising a stirring apparatus, the hydrocyclone 2 , and a circulating water tank 55 .
To ensure that the particles and liquid are mixed together well in the crystallization reactor 9 , stirring is preferably performed to maintain a favorable particle current. The stirring method may employ a mechanical stirring apparatus equipped with a motor, or stirring may be performed with the use of an air supply.
The crystallization reactor 9 is connected to a raw water 1 supply pipe, a chemical 10 supply pipe, a return pipe 51 whose lower portion communicates with the reactor, an extraction pipe 52 for extracting sludge and particles from the interior of the reactor, and a discharge pipe 53 for discharging micro-particles from the hydrocyclone, while a circulating water effluent pipe 54 is connected to the return pipe 51 . The sludge of the raw water contains between 100 and 600 mg/L of phosphate ions, and between 50 and 3000 mg/L of ammonia nitrogen. A magnesium source for generating MAP is supplied as the chemical 10 . Magnesium chloride, magnesium hydroxide, magnesium oxide, and so on may be employed as the magnesium source.
The crystallization reactor 9 is preferably structured such that the surface area of its base portion planar cross section is smaller than the surface area of its top end planar cross section. Furthermore, the planar cross section from the top end to a predetermined lower position preferably has a constant shape and a constant sectional area, and from the predetermined position downward, the cross section preferably decreases steadily.
As shown in FIG. 4, for example, the base portion preferably takes an inverted conical form. The angle of the inverted cone is preferably no less than 45° in relation to the horizontal, and more preferably no less than 60°. By setting the angle in this manner, the MAP particles which settle inside the crystallization reactor 9 become concentrated at a single point in the base portion of the reactor, which is preferable. Furthermore, the particle extraction pipe 52 is preferably connected to the vicinity of the point at which the settled particles concentrate. As a result, blockages caused by sediment can be prevented.
The micro-particle discharge pipe 53 of the hydrocyclone 2 is used to discharge the MAP particles that are concentrated by the hydrocyclone 2 from the base portion of the hydrocyclone 2 and to return these MAP particles to the crystallization reactor 9 . By returning the MAP particles concentrated by the hydrocyclone 2 to the crystallization reactor 9 , the MAP particle concentration in the reactor can be raised, and as a result, the MAP particle surface area can be maintained at a high level. The crystallization reaction is constituted by a particle nucleation phenomenon and a growth phenomenon, but as the particle surface area increases, the growth phenomenon takes precedence over the nucleation phenomenon, thereby facilitating solid/liquid separation. A recovery pipe 56 for recovering the concentrated MAP particles may also be connected to the base portion of the hydrocyclone 2 or the micro-particle discharge pipe 53 .
An important constitutional requirement of the embodiment shown in FIG. 4 is that the return pipe 51 be disposed in the crystallization reactor 9 . The return pipe 51 is connected to the overflow riser 5 of the hydrocyclone 2 and the circulating water inflow pipe 54 , and also communicates with the crystallization reactor 9 at the base portion. Following the removal of MAP particles in the hydrocyclone 2 , the sludge passes through the overflow riser 5 and is supplied in its entirety to the return pipe 51 . The return pipe 51 maintains an identical water level to the water surface of the reactor 9 such that a part of the returned sludge is returned to the reactor 9 , while the remainder is supplied to the circulating water tank 55 through the circulating water inflow pipe 54 .
The inflow rate into the hydrocyclone is determined principally according to the size of the employed hydrocyclone. With a two-inch hydrocyclone, an inflow rate of approximately 4 m 3 /hr is available, and with a four-inch hydrocyclone, an inflow rate of approximately 20 m 3 /hr is available. When the actual inflow rate is larger or smaller than a predetermined flow rate, the treatment performance varies. In a conventional crystallization reactor, return pipe 51 is not provided, and it is therefore difficult to match the sum total of the raw water 1 supply rate, the under flow rate of the hydrocyclone (the flow rate of the micro-particle discharge pipe 53 at the base portion of the reactor), and the chemical 10 supply rate with the extraction rate from the extraction pipe 52 . This leads to increases or decreases in the water surface of the reactor 9 . Hence, conventionally a level meter or the like must be provided to control the water level. In the embodiment shown in FIG. 4, this problem can be solved by providing the return pipe 51 in the crystallization reactor 9 . More specifically, when the return pipe 51 communicates with the reactor 9 such that the sludge discharged from the overflow riser 5 of the hydrocyclone 2 is returned to the reactor 9 via the return pipe 51 , and when an amount of sludge corresponding to the raw water supply amount and chemical supply amount is caused to overflow by the return pipe 51 constantly, the water level of the reactor 9 can be maintained at a constant level. As a result, no level meter need be provided, continuous introduction into the cyclone can be performed, and the water level is stable, enabling stable treatment.
The sludge which overflows from the return pipe 51 is led to the circulating water tank 55 via the circulating water inflow pipe 54 . A part of the sludge overflows and is extracted as the treated sludge 8 , while the remainder is introduced into the hydrocyclone 2 through the hydrocyclone inflow pipe 6 .
The MAP particles in the reactor 9 are extracted via the extraction pipe 52 at a predetermined flow rate, and introduced into the hydrocyclone 2 via the hydrocyclone inflow pipe 6 . An important constitutional requirement here is that the extraction pipe 52 be connected to the hydrocyclone inflow pipe 6 such that the MAP particles are introduced into the cyclone after the particle concentration in the extraction pipe 52 is diluted by circulating water (sludge) from the circulating water tank 55 . The concentration of the MAP particles introduced into the hydrocyclone is an important operational factor affecting the treatment performance of the hydrocyclone, and therefore to prevent reductions in the particle concentration of the treated water and blockages of the cyclone under (cyclone base portion), the concentration is preferably as low as possible.
When the raw water supply amount supplied to the crystallization reactor 9 is defined as Q1, the chemical supply amount supplied to the crystallization reactor 9 is defined as Q2, the return amount of MAP crystals separated in the hydrocyclone 2 and then returned to the crystallization reactor 9 is defined as Q3, and the extraction amount of MAP crystal particles extracted from the extraction pipe 52 of the crystallization reactor 9 is defined as Q4, a relationship of
Q 1 +Q 2 +Q 3 <Q 4
is preferably maintained. By maintaining this relationship, the flow direction of a return water amount Q5 passing through the return pipe 51 can be maintained in a direction heading toward the crystallization reactor 9 , and the MAP particles in the crystallization reactor 9 can be prevented from flowing out through the circulating water effluent pipe.
When the pH, water temperature, or sludge concentration in the reactor varies, a pH meter 57 , water temperature meter, sludge concentration meter, or the like is preferably provided, and in accordance with the measured value, a pH adjusting agent is preferably added, or a heating/cooling operation or sludge concentration adjustment operation is preferably performed.
The lower portion structure of the hydrocyclone 2 preferably takes an inverted conical form. The hydrocyclone inflow pipe 6 , micro-particle discharge pipe 53 , and overflow riser (sludge discharge pipe) 5 are connected to the hydrocyclone 2 . In the hydrocyclone 2 , sludge containing MAP particles is introduced through the hydrocyclone inflow pipe 6 by the pressure of a pump, and is then caused to swirl down the inverted cone-shaped wall surface of the hydrocyclone 2 . The MAP-containing particles, which have a greater specific gravity than the sludge, are collected and concentrated on a lower wall surface side by means of centrifugal force. The concentrated micro-particles are returned to the reactor either continuously or intermittently via the micro-particle discharge pipe 53 . Alternatively, the micro-particles may be recovered via the recovery pipe 56 appropriately.
The concentration of the particles in the hydrocyclone inflow pipe 6 is an important operational factor affecting the treatment performance of the hydrocyclone 2 , and to prevent reductions in the particle concentration of the treated water and blockages of the cyclone under, the concentration is preferably as low as possible. The degree to which the particle concentration should be reduced varies according to the concentration performance of the cyclone, but a preferable concentration is approximately several tens of grams per liter.
As described above, sludge from which MAP particles have been removed flows out from the overflow riser 5 . The overflow riser 5 is connected to the return pipe 51 of the crystallization reactor 9 , and hence all of the sludge from which the MAP micro-particles have been removed is supplied to the return pipe 51 .
In the hydrocyclone 2 , the pipe diameter of the micro-particle discharge pipe and the pipe diameter of the sludge discharge pipe may be altered, and by altering both, the flow rate and particle diameter distribution can be varied.
The circulating water inflow pipe 54 , which is connected to the return pipe 51 of the crystallization reactor 9 , the hydrocyclone inflow pipe 6 , and the treated water (sludge) 8 discharge pipe are connected to the circulating water tank 55 . A part 8 of the sludge that flows into the circulating water tank 55 through the circulating water inflow pipe 54 overflows into the treated water discharge pipe. The circulating water tank 55 stores circulating water (having identical properties to the treated water) temporarily, and its size may be determined arbitrarily. Needless to say, a stirring apparatus may be provided to ensure that the circulating water is mixed evenly.
A screen residue removal apparatus may be provided in the extraction pipe 52 , which is connected to the sludge 1 supply pipe and/or the base portion of the crystallization reactor 9 . By removing screen residue, a dramatic improvement in the stability of MAP particle separation using the hydrocyclone 2 can be achieved. Moreover, by separating screen residue in advance, MAP with a high degree of purity can be obtained.
In the example shown in FIG. 5, the circulating water tank 55 is disposed directly above the return pipe 51 . In so doing, the circulating water inflow pipe 54 of the embodiment in FIG. 4 can be omitted.
As shown in FIG. 6, the crystallization reactor 9 may be provided with a base portion having a columnar or other shape below the inverted conical part, and the extraction pipe 52 may be connected to this columnar part.
As shown in FIGS. 4 to 6, the particle recovery pipe 56 for recovering the MAP particles concentrated by the hydrocyclone 2 may be connected to the base portion of the hydrocyclone 2 or the micro-particle discharge pipe 53 . Typically, the MAP particle concentration in the crystallization reactor 9 increases as the crystallization reaction progresses, and hence by extracting the MAP particles from the hydrocyclone 2 appropriately, the MAP particle concentration in the crystallization reactor 9 can be maintained at a constant concentration. The MAP particles concentrated by the hydrocyclone 2 are returned to the crystallization reactor 9 through the micro-particle discharge pipe 53 , and hence by connecting the recovery pipe 56 to the micro-particle discharge pipe 53 , a part of the concentrated MAP particles can be discharged to the outside through the recovery pipe 56 . The concentrated MAP particles may be discharged by means of valve switching, for example. The valve may be switched in accordance with a fixed timer setting, or the particle concentration in the crystallization reactor 9 may be detected so that the valve is switched in accordance with the detection value.
The recovered MAP particles may be subjected to a process such as cleaning, dewatering, or drying in preparation for reuse.
Further, a cleaning pipe (not shown) for supplying cleaning water to clean the hydrocyclone may be connected to the micro-particle discharge pipe 53 or the recovery pipe 56 . To ensure that the under (base portion) of the hydrocyclone 2 does not become blocked, measures such as removing large particles in advance and reducing the concentration of the particles that are introduced into the hydrocyclone 2 are usually taken. Even so, in certain cases blockages occur. In such cases, cleaning water may be supplied to flow in an upward flow from the cyclone under (base portion) so as to push the particles out to the cyclone main body side. When supplying cleaning water, the micro-particle discharge pipe and recovery pipe may be closed using a valve or the like.
A further embodiment of the present invention provides a treatment system in which a plurality of hydrocyclone are connected in series so that the crystallization reactor can be reduced in size, blockages of the hydrocyclone can be eliminated, and stable treatment maintaining a high MAP particle concentration in the crystallization reactor can be performed. An example of this embodiment is shown in FIGS. 7 to 10.
FIG. 7 shows a treatment flow in which the crystallization reactor 9 is used to crystallize phosphorus dissolved in the raw sludge 1 , and the hydrocyclone 2 is used to concentrate the MAP contained in the sludge and to discharge the concentrated MAP through the micro-particle discharge pipe 53 such that the sludge 8 from which MAP has been removed overflows from the sludge discharge pipe (overflow riser).
Contaminants such as screen residue are preferably removed from the raw sludge 1 in advance by using a screen residue removal apparatus (also referred to as a contaminant removal apparatus). A centrifugal settler, a sedimentation tank employing gravity separation, and so on may be employed as the screen residue removal apparatus, while a vibrating screen, a bar screen, a drum screen, a filter layer, a classification layer-type separation tank, and so on, or an apparatus combining the principles thereof, may be employed as a micro-particle separation method using differences in particle diameter. By disposing the screen residue removal apparatus before the crystallization reactor 9 and/or the hydrocyclone 2 , contaminants can be prevented from becoming caught in the stirring apparatus inside the crystallization reactor 9 , and hence blockages of the hydrocyclone 2 can be prevented. As a result, treatment can be stabilized, and the frequency with which cleaning and maintenance must be performed can be reduced. Moreover, by separating screen residue in advance, MAP having a high degree of purity can be obtained. Note that the screen residue removal apparatus may be disposed in the hydrocyclone inflow pipe 6 , which is connected to the base portion of the crystallization reactor 9 .
The crystallization reactor 9 is connected to the raw sludge 1 supply pipe, the magnesium 10 supply pipe, the micro-particle discharge pipe 53 of the hydrocyclone 2 , and the sludge discharge pipe (overflow riser) 5 . In certain cases, a pH meter and pH adjusting agent supply pipe are also provided in the crystallization reactor 9 . There are no particular limitations on the form of the reactor, and a complete mixing type reactor comprising a mechanical stirring apparatus, a jet stream type stirring reactor using a pump, a fluidized bed reactor charged with seed crystals at a high density, an internal circulation type reactor comprising a draft tube, an external circulation type reactor, or another reactor may be used.
When sludge is the treatment subject, a complete mixing type reactor comprising a mechanical stirring apparatus which is capable of applying a strong shearing force is preferably employed.
In the crystallization reactor 9 , MAP is precipitated by adding the magnesium compound 10 to the sludge or the separated water 1 that is generated as a result of sludge concentration or dewatering treatment. If aeration, decompression, or a similar process is also performed at this time, the sludge is decarbonated, leading to an increase in the pH, and hence MAP can be precipitated more efficiently. Needless to say, a chemical such as sodium hydroxide, magnesium hydroxide, or magnesium oxide may be added to raise the pH. As the added magnesium compound, magnesium chloride, magnesium hydroxide, magnesium oxide, sea water, and so on may be used. As regards the amount of added magnesium, a molar ratio between 0.1 and 10, preferably between 0.5 and 3.0, and more preferably between 0.8 and 1.2 in relation to the aqueous orthophosphoric acid concentration of the digested sludge is suitable. The pH in the reaction should be between 7.0 and 11.0, and preferably between 7.5 and 8.5.
Seed crystals are preferably added to the crystallization reactor 9 to ensure that MAP is generated efficiently. The MAP that precipitates spontaneously in the digestion tank, the MAP that precipitates in the crystallization reactor 9 , the MAP that is concentrated by the hydrocyclone 2 , MAP precipitated in a separate crystallization reactor, and so on may be used as the seed crystals.
Alternatively, a powder or granular substance such as rock phosphate, dolomite, bone charcoal, activated carbon, silica sand, or calcium silicate may be used. The particle diameter of the seed crystals is arbitrary, but is preferably set between 0.05 and 3.0 mm, and more preferably between 0.1 and 0.5 mm. By precipitating new MAP on the surface of the seed crystals, separation of the digested sludge and MAP in a subsequent separation process can be performed favorably. The seed crystal charging amount is extremely important for precipitating MAP on the surface of the seed crystals.
FIG. 11 shows the relationship between the MAP amount in the crystallization reactor and the phosphorus recovery rate. As is evident from FIG. 11, the recovery rate rises as the MAP charging amount increases, and therefore by setting the MAP amount charged into the reactor to at least 10 g/L, at least 30% of the phosphorus can be recovered.
Further, the phosphorus introduction amount in relation to the seed crystal surface area (referred to as the phosphorus surface area load, hereafter) should be set to no more than 100 g-P/m 2 /d, preferably no more than 30 g-P/m 2 /d, and more preferably no more than 10 g-P/m 2 /d.
The MAP concentration in the crystallization reactor is determined according to the raw sludge supply amount and the concentration of the MAP contained in the raw sludge, the amount and concentration of the MAP that is concentrated by the hydrocyclone and returned to the crystallization reactor, and the crystallization amount of the reactor. As noted above, it is important to increase the crystal surface area by raising the MAP concentration in the reactor, but since the MAP concentration of the raw sludge is approximately 1/10 to 1/500 that of the MAP which is concentrated by the hydrocyclone, the MAP concentrated by the hydrocyclone is diluted by the raw sludge. As a result, it becomes more and more difficult to maintain the MAP in the reactor at a high concentration as the raw sludge amount increases.
Hence, in the embodiment shown in FIGS. 7 to 10, a plurality of the hydrocyclones 2 are disposed in series such that each hydrocyclone is capable of independent control. Further, the MAP that is concentrated by each hydrocyclone 2 is returned partially or entirely to the crystallization reactor 9 by the micro-particle discharge pipe 53 . By employing this constitution, the MAP concentration in the crystallization reactor 9 can be maintained at a desired concentration, and since a larger amount of the MAP concentrated by the hydrocyclones 2 is returned, the MAP concentration in the crystallization reactor 9 can be maintained at a high concentration. Consequently, this greatly contribute to reduce the size of the crystallization reactor 9 .
The number of hydrocyclones 2 may be set at a desired number of two or more. The number of hydrocyclones 2 may be determined in consideration of the concentration flow rate of the hydrocyclones, the concentration of the concentrated MAP, and the desired MAP concentration in the crystallization reactor.
Further, in the embodiment shown in FIGS. 7 to 10, the MAP concentration in the reactor can be adjusted by connecting the recovery pipe 56 to the base portion of the hydrocyclones 2 or the micro-particle discharge pipe 53 and extracting a part of the MAP concentrated by the hydrocyclones, as shown above in FIG. 4 and so on. The concentrated MAP particles may be extracted intermittently or continuously from a specific cyclone, each cyclone in sequence, or all of the cyclones simultaneously. The extracted MAP may be put to effective use as a fertilizer, an inorganic chemical, a chemical raw material, and so on.
Note that a part of the sludge 8 that flow out from the overflow riser 5 of the hydrocyclones following recovery of the micro-particles may be returned to the hydrocyclone inflow pipe 6 . In this case, the sludge 8 preferably passes through a relay tank.
When a plurality of hydrocyclones are disposed in series, the number of operational hydrocyclones may be controlled in accordance with the raw water flow rate, for example as shown in FIG. 8. More specifically, a flow meter 71 is disposed on the raw sludge 1 supply pipe to monitor the inflow rate of raw sludge into the crystallization reactor 9 . By determining a number of operational pumps disposed at the inflow portion of each hydrocyclone 2 from the relationship between a preset flow rate range and the number of operational hydrocyclones 2 , the number of operational hydrocyclones 2 can be determined. In so doing, the interior of the crystallization reactor 9 can be maintained at a desired MAP concentration even when the raw sludge inflow rate varies.
The number of operational hydrocyclones 2 can be also controlled in accordance with the MAP concentration in the crystallization reactor, as shown in FIG. 9. The MAP concentration in the crystallization reactor 9 can be measured using a concentration meter 73 employing a transmitted light/scattered light method, a laser light diffusion method, an ultrasonic method, a microwave method, a near infrared method, or another method. The MAP concentration may be measured together with the organic matter concentration, and since there is little variation in the organic matter concentration in the crystallization reactor 9 , the MAP concentration may be calculated by creating a calibration curve.
Further, as shown in FIG. 10, by monitoring the phosphorus concentration and pH of the sludge or separated water treated in the crystallization reactor and hydrocyclones, or in other words the sludge or separated water 8 that flows out from the overflow riser 5 of the hydrocyclones 2 , using a phosphorus concentration meter 74 and a pH meter 75 , the supersaturation ratio can be calculated on the basis of the resultant measurement data, and in accordance with the supersaturation ratio, the magnesium addition amount and the number of operational hydrocyclones can be controlled.
The pH and phosphorus concentration can be measured using an arbitrary analytical instrument, and monitoring may be performed continuously or at fixed time intervals. It is preferable that the total phosphorus, including the phosphorus dissolved in the sludge, and particles as well as the soluble phosphorus, be measured.
The supersaturation ratio is calculated by a calculation apparatus using the pH and phosphorus concentration obtained as a result of the monitoring. A personal computer or the like is a representative example of the calculation apparatus. An input apparatus, display apparatus, or other apparatus may be employed.
The supersaturation ratio will now be described.
The supersaturation ratio is calculated in the following manner.
Supersaturation ratio=[Mg 2+ ][NH 3 + ][PO 4 3− ]/SP (1)
Here, SP denotes the solubility product of MAP, which is 10 −12.6 (General Water Quality Chemistry; Kyoritsu Shuppan Ltd.). [Mg 2+ ] and [NH 3 + ] denote the molarities of respective ions, which vary according to the pH, and hence the number of unknown quantities for calculating the supersaturation ratio is four. The supersaturation may be expressed in other ways such as
supersaturation=([Mg 2+ ][NH 3 +][PO 4 3− ]) (1/3) /SP (1/3) (2)
or
supersaturation=[Mg 2+ ][NH 3 + ][PO 4 3− ]−SP (3).
In all cases, deviation in the solubility product is used as an index. Here, calculation is performed using the supersaturation ratio of Equation (1) as an index.
When the supersaturation ratio is 1 in Equation (1), this indicates that the sludge or liquid is in a state of equilibrium, and hence even if local crystallization and dissolution are occurring repetitively, as a whole crystallization is not underway. When the supersaturation ratio is greater than 1, this indicates a state of supersaturation, i.e. a region in which the concentration which exceeds the ionic product is crystallized such that MAP scale is generated. MAP scale is generated until a state of equilibrium is achieved, and the amount of MAP scale increases as the supersaturation ratio increases. When the supersaturation ratio is no more than 1, this indicates a state of unsaturation, i.e. a region in which MAP scale is not generated.
The ammonium ion concentration in the anaerobically digested sludge of the sewerage sludge also varies according to regionality and treatment method, but is set at a fixed value between approximately 500 and 3000 mg/L.
The magnesium concentration and phosphorus concentration are preferably measured in a similar manner, and since magnesium is added as a chemical such that Mg/P=1.0, the concentration of the remaining Mg can be estimated between 10 and 100 mg/L.
MAP scale is less likely to occur as the supersaturation ratio, calculated in the manner described above, decreases, and therefore a supersaturation ratio of no greater than 30, preferably no greater than 10, and more preferably no greater than 5 is desirable.
By means of the supersaturation ratio calculated in this manner, the number of operational hydrocyclones can be controlled. As noted above, when the MAP concentration in the crystallization reactor decreases, the MAP recovery rate falls, and hence the supersaturation ratio of the treated sludge 8 increases. In this case, when the number of operational hydrocyclones is increased such that the MAP concentration in the crystallization reactor rises, the phosphorus concentration of the treated sludge decreases, leading to a reduction in the supersaturation ratio, and hence scale generation can be suppressed to a minimum.
The amount of magnesium to be added to the crystallization reactor can also be controlled by calculating the supersaturation ratio. More specifically, in a case where the supersaturation ratio is calculated by measuring the soluble phosphorus concentration, the phosphorus concentration is high when the supersaturation ratio is high, and hence the phosphorus concentration can be reduced by increasing the magnesium addition amount.
Although not shown in FIGS. 7 to 10, the treated sludge from which MAP particles have been removed, which flows out from the overflow riser 5 of the hydrocyclones 2 , may be returned to the inflow pipe 6 of the hydrocyclones 2 , as described above in the flow of FIGS. 2 to 6. Also, the particle slurry concentrated by the hydrocyclone 2 may be extracted and recovered appropriately.
Providing a plurality of hydrocyclones and controlling operations thereof may be adopted likewise in a flow having the constitution shown in FIGS. 2 to 6, or flows having the constitutions shown in FIGS. 12 to 17 and FIG. 18 to be described below, for example.
According to a still further aspect of the present invention, there is provided a treatment process for treating digested sludge generated by subjecting organic waste matter to anaerobic digestion, comprising steps for: decarbonating the digested sludge or crystallizing MAP from the digested sludge; removing screen residue from the digested sludge; and separating or concentrating crystals containing MAP from the digested sludge after the digested sludge passes through the steps for decarbonating or crystallizing and removing the screen sludge. Also, the present invention includes the following aspects.
According to a further aspect of the present invention, there is provided a digested sludge treatment apparatus for treating digested sludge generated by subjecting organic waste matter to anaerobic digestion, comprising: an apparatus for decarbonating the digested sludge; a removal apparatus for removing screen residue from the digested sludge; and an apparatus for separating or concentrating crystals containing magnesium ammonium phosphate from the digested sludge after the digested sludge passes through the decarbonation apparatus and the removal apparatus.
According to a further aspect of the present invention, there is provided a digested sludge treatment apparatus for treating digested sludge generated by subjecting organic waste matter to anaerobic digestion, comprising: a crystallization reactor for precipitating magnesium ammonium phosphate from the digested sludge; a removal apparatus for removing screen residue from the digested sludge; and an apparatus for separating or concentrating crystals containing magnesium ammonium phosphate from the digested sludge after the digested sludge passes through the crystallization reactor and screen residue removal apparatus.
According to a further aspect of the present invention, there is provided the digested sludge treatment apparatus as defined hereinbefore, wherein said screen residue removal apparatus is a wet-type vibrating screen equipped with two or more types of sieve bodies having different hole diameters.
According to a further aspect of the present invention, there is provided the digested sludge treatment apparatus as defined hereinbefore, further comprising a cleaning apparatus for cleaning said separated crystals containing magnesium ammonium phosphate.
According to a further aspect of the present invention, there is provided a method of preventing scale from forming on the inside of a pipe when digested sludge generated by subjecting organic waste matter to anaerobic digestion is transported via the pipe, wherein the digested sludge is treated in a decarbonation process and a screen residue removal process, micro-particles containing MAP are separated from the digested sludge following decarbonation and screen residue removal, and following removal of these micro-particles, the digested sludge is transported via the pipe.
In this method of the present invention, following micro-particle removal, the digested sludge is preferably subjected to processing to lower the pH and/or processing to reduce at least one of the magnesium ion concentration, phosphate ion concentration, and ammonium ion concentration, and then transported via the pipe. The anaerobic digestion may be performed following a discharge process for discharging phosphate ions from the organic waste matter and a concentration process for concentrating the organic waste matter following the discharge process. Further, the digested sludge that is transported via the pipe may have a magnesium ion concentration of no more than 20 mg/L, and preferably no more than 5 mg/L.
A further aspect of the present invention provides an apparatus for preventing scale from forming on the inside of a pipe when digested sludge generated by treating organic waste matter in an anaerobic digestion tank is transported via the pipe, comprising: an apparatus for decarbonating the digested sludge; a removal apparatus for removing screen residue from the digested sludge; and an apparatus for separating micro-particles containing MAP from the digested sludge after the digested sludge passes through the decarbonation apparatus and screen residue removal apparatus.
This apparatus of the present invention preferably further comprises a chemical adding apparatus for lowering the pH of the digested sludge following micro-particle removal, and/or a chemical adding apparatus for reducing at least one of the magnesium ion concentration, phosphate ion concentration, and ammonium ion concentration thereof. Further, discharging means for discharging phosphate ions from the organic waste matter and a concentration apparatus for concentrating the organic waste matter following the discharge process may be disposed before the anaerobic digestion tank.
A further aspect of the present invention provides a digested sludge treatment method for treating digested sludge generated by subjecting organic waste matter to anaerobic digestion, wherein the digested sludge is treated in a crystallization process for precipitating MAP by adding a magnesium compound and screen residue removal process for removing screen residue such as night soil residue from the digested sludge, micro-particles containing MAP are recovered from the digested sludge in a separation process following the crystallization process and screen residue removal process, and following removal of these micro-particles, the digested sludge is dewatered in a dewatering process. In this method of the present invention, the MAP-containing micro-particles recovered in the separation process, or a portion or all of MAP-containing effluent generated in the separation process, may be returned to the crystallization process, and a portion or all of the screen residue removed in the screen residue removal process may be supplied to the dewatering process. Further, following micro-particle removal, the digested sludge is preferably subjected to processing to lower the pH and/or processing to reduce at least one of the magnesium ion concentration, phosphate ion concentration, and ammonium ion concentration, and then transported to the dewatering process.
A further aspect of the present invention provides a digested sludge treatment apparatus for treating digested sludge generated by subjecting organic waste matter to anaerobic digestion, comprising: a crystallization apparatus having adding means for adding a magnesium compound in order to precipitate MAP during treatment of the digested sludge; a screen residue removal apparatus for removing screen residue from the digested sludge; a separation apparatus for recovering micro-particles containing MAP from the digested sludge after the digested sludge passes through the crystallization apparatus and screen residue removal apparatus; and a dewatering apparatus for dewatering the digested sludge following removal of the micro-particles. In this apparatus of the present invention, a return path may be provided for returning a portion or all of the MAP-containing micro-particles recovered by the separation apparatus to the crystallization apparatus, and supply means may be provided for supplying a portion or all of the screen residue removed by the screen residue removal apparatus to the dewatering apparatus. Further, a chemical adding apparatus for lowering the pH of the digested sludge following micro-particle removal, and/or a chemical adding apparatus for reducing at least one of the magnesium ion concentration, phosphate ion concentration, and ammonium ion concentration thereof, may be provided after the separation apparatus.
According to the present invention, by employing the constitution described above, MAP scale on the inside of a pipe for transporting digested sludge can be reduced greatly, and/or phosphorus can be recovered efficiently from the inside of a digestion tank.
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be illustrated hereinafter with reference to FIGS. 12 to 17. In FIGS. 12 to 17 and FIGS. 33 to 35 that will be mentioned hereinafter, each of reference numerals in the drawings denotes the following meaning.
| 101: | introduced sludge | |
| 102: | anaerobic digestion tank | |
| 103: | digested sludge (extracted sludge) | |
| 104: | decarbonation process | |
| 105: | screen residue removal process | |
| 106: | screen residue | |
| 107: | process for separating micro-particles containing MAP | |
| 108: | recovery of micro-particles containing MAP | |
| 109: | out of the system | |
| 110: | pH adjustor | |
| 111: | concentrated excess sludge | |
| 112: | phosphorus discharge tank | |
| 113: | BOD source | |
| 114: | concentration process | |
| 115: | separated water | |
| 116: | dewatering process | |
| 117: | concentration-adjusted water | |
| 118: | sludge liquor | |
| 201: | introduced sludge | |
| 202: | anaerobic digestion tank | |
| 203: | extracted sludge (digested sludge) | |
| 204: | crystallization process | |
| 205: | Mg compound | |
| 206: | screen residue removal process | |
| 207: | screen residue | |
| 208: | process for separating micro-particles containing MAP | |
| 209: | recovery of micro-particles containing MAP | |
| 210: | dewatering process | |
| 211: | sludge liquor | |
| 212: | out of the system | |
| 213: | micro-particle transportation pipe | |
| 214: | screen residue transportation pipe | |
FIG. 12 is a flow diagram showing an example of a treatment flow according to the present invention, which is constituted by an anaerobic digestion tank 102 , a decarbonation process 104 for decarbonating digested sludge, a screen residue separation process 105 , and a micro-particle separation process 107 for separating and recovering micro-particles containing MAP.
Excess sludge and/or primary sludge are introduced into the anaerobic digestion tank 102 . The interior of the anaerobic digestion tank is heated and maintained at a temperature of approximately 55° C. or approximately 35° C. In the anaerobic digestion tank, the sludge is broken down into a gas such as methane, carbon dioxide, hydrogen sulfide; water-soluble nitrogen and phosphorus, and so on by the action of acid-fermentative bacteria and methanogenic bacteria. The generated methane gas may be recovered and used as energy. The amount of generated methane gas is increased by introducing easily-decomposable raw sludge in addition to excess sludge. As the sludge decomposes, phosphorus, magnesium, and ammonium are dissolved into the liquid side, and thus MAP is generated spontaneously in the anaerobic digestion tank.
As a result of this MAP precipitation, scale trouble occurs on the draft tube, the base portion of the anaerobic digestion tank, the sludge discharge pipe, and so on.
The ratio of the phosphorus, magnesium, and ammonium in the digested sludge is generally phosphorus magnesium:ammonium=100 to 500: several to several tens:1000, and hence the magnesium concentration is overwhelmingly lower than the phosphorus and ammonium concentrations. During MAP generation in the digestion tank, the magnesium concentration clearly serves as a rate control.
The digested sludge contains micro-particles containing MAP, and has a pH in the vicinity of 7, a phosphorus concentration between 100 and 500 mg/L, a magnesium concentration between several and several tens of mg/L, and an ammonium concentration between 500 and 4000 mg/L. Conventionally, a large amount of MAP scale is generated in a sewer when digested sludge having these properties is transported via pipe, leading to problems such as blockages.
As a result of committed research into the phenomenon of scale generation, performed by the present inventors and others, it was ascertained that pH variation and gas phase intermixing inside the pipe leads to decarbonation, causing the generation of MAP and hence scaling. In other words, the digested sludge continues to possess a latent ability to generate MAP (referred to as MAP generation ability, hereinafter). To make matters worse, the MAP micro-particles generated spontaneously in the digested sludge act as seed crystals, promoting scale generation.
The present inventors focused on the remnant MAP generation ability described above, and thus discovered a need to reduce the remnant MAP generation ability before the digested sludge is introduced into a sewer. The present inventors discovered that the MAP generation ability of digested sludge is greatly reduced by decarbonating the digested sludge to raise the pH such that MAP is generated in advance, and then separating the MAP contained in the digested sludge and the MAP generated in the decarbonation process from the digested sludge, as in the present invention.
Aeration treatment or decompression treatment may be employed in the decarbonation process 104 . Aeration treatment involves aerating the digested sludge such that the carbon gas in the sludge is dispersed into the gas phase, thereby raising the pH such that MAP is generated in a quantity corresponding to the amount of remaining magnesium. In decompression treatment, a degassing apparatus such as that disclosed in Japanese Unexamined Patent Application Publication H7-136406 (referred to as a thin film vacuum degassing apparatus hereafter) is preferably used. Specifically, a subject liquid is put in acceleration by the centrifugal force of a sifting body which has a base and is rotated in a vacuum container, whereby the subject liquid collides with the inner wall surface of the vacuum container such that the gas in the subject liquid is removed. As a result of the decompression treatment, decarbonation occurs, leading to an increase in the pH and the generation of MAP.
In the treatment described above, if the magnesium ion concentration of the liquid decreases, the MAP generation ability disappears, and hence scale generation is suppressed. For example, if the pH is raised from 7 to 8, the magnesium ion concentration of the digested sludge generally falls to 1/10 to ½. In the present invention, the magnesium ion concentration of the digested sludge is set at no more than 20 mg/L, and preferably no more than 5 mg/L. When the magnesium ion concentration is no more than 20 mg/L, almost no MAP supersaturation occurs even when the pH in the sewer varies, or when the gas phase is intermixed such that the pH rises, for example, and hence MAP precipitation can be prevented.
In addition to a decarbonation method by aeration and decompression, a decarbonation by chemical addition may be employed. Needless to say, these operations may be combined and performed in any order. The decarbonated digested sludge is then introduced into the screen residue removal process 105 . Conventionally, no screen residue separation process is performed, and therefore when a hydrocyclone is used in the subsequent micro-particle separation process 107 , blockages caused by screen residue and the like occur. For this reason, there is demand for a treatment method exhibiting long-term stability. In the present invention, screen residue are removed, enabling a vast improvement in the stability of the subsequent micro-particle separation process 107 , particularly when this process employs a hydrocyclone.
The removed screen residue 106 may be discharged outside of the system or mixed into the digested sludge following the micro-particle separation process 107 . When a dewatering process is provided, the screen residue 106 may be introduced in the dewatering process. In this case, the dewatering performance is enhanced, and it is therefore preferable to introduce the screen residue 106 into the dewatering process if such a process is provided.
Note that the decarbonation process 104 and screen residue removal process 105 may be performed in any order. The decarbonation process 104 may precede the screen residue removal process 105 , or the screen residue removal process 105 may precede the decarbonation process 104 , as shown in FIG. 12 and so on.
Following the decarbonation process 104 and screen residue removal process 105 , the micro-particles containing MAP precipitated in the digestion tank and during the decarbonation process are separated from the digested sludge. A hydrocyclone, a centrifugal settler, a sedimentation tank employing gravity separation, and so on may be employed as a method of separating the micro-particles from the digested sludge using the specific gravity difference between the two, while a vibrating screen, a drum screen, a filter layer, a classification layer-type separation tank, and so on may be employed as a micro-particle separation method using differences in particle diameter.
A hydrocyclone has a reverse conical-form lower portion structure, and is constituted by a hydrocyclone inflow pipe, a micro-particle discharge pipe, and a sludge discharge pipe. In the hydrocyclone, the digested sludge containing MAP is caused to swirl down the wall surface of the reverse conical form by the pressure of an extractor pump. The MAP-containing micro-particles, which have a greater specific gravity than the digested sludge, are collected and concentrated on a lower wall surface side by means of centrifugal force. The concentrated micro-particles are extracted either continuously or intermittently.
In the present invention, screen residue are separated before the MAP-containing micro-particle separation process 107 , and therefore problems such as blockages of the hydrocyclone caused by screen residue and the like are solved. Having passed through the decarbonation process 104 , screen residue separation process 105 , and micro-particle separation process 107 , the digested sludge is transported through a sewer to a collective sludge treatment facility, other type of sludge treatment facility, or a sludge treatment facility within the same premises. As a result of the processes described above, the magnesium ion concentration of the digested sludge is greatly reduced and MAP micro-particles are removed from the digested sludge. Hence, according to the present invention, the MAP generation ability is reduced even upon pH variation or gas phase intermixing, and therefore MAP scale generation is reduced greatly.
In the example shown in FIG. 13, processing to lower the pH and/or processing to reduce at least one of the magnesium ion concentration, phosphate ion concentration, and ammonium ion concentration are performed following the micro-particle separation process 107 . A pH adjustor may be added as a method of reducing the pH. A chemical for lowering the pH of digested sludge, such as hydrochloric acid, sulfuric acid, aluminum salt, or iron salt, is used as the pH adjustor. With aluminum salt and iron salt, the soluble phosphorus contained in the digested sludge is fixed, causing a reduction in the solubility concentration. Ammonia stripping, fixation by adhesive, and so on may be used as a method of reducing ammonium ions. MAP is precipitated when any of the phosphorus concentration, magnesium concentration, ammonium concentration, and pH rises. Conversely, as described above, when the pH of the digested sludge falls or at least one of the magnesium ion concentration, phosphate ion concentration, and ammonium ion concentration is reduced, the MAP generation ability deteriorates. According to the present invention, the MAP generation ability deteriorates, reducing the likelihood of MAP scale generation. Note that with the addition of iron salt, the generation of hydrogen sulfide and the like from the digested sludge can be suppressed.
In the example shown in FIG. 14, excess sludge or concentrated excess sludge 111 is introduced into a phosphorus discharge tank 112 . Phosphorus is discharged from the sludge by adding a BOD under anaerobic conditions. As a BOD source 113 , organic waste matter containing raw sludge, a portion of the solubilized sludge that is generated when sludge solubilization treatment is performed, or a portion of the organic waste water from which the excess sludge is generated is used. Also, a chemical such as methanol may be added as a BOD source. In the phosphorus discharge tank 112 , phosphorus is discharged from the excess sludge and concentrated excess sludge, and a part of the magnesium contained in the sludge is also eluted to the liquid side. Particularly when a biological dephosphorylation method such as an anaerobic/aerobic process is performed in a water treatment system, the phosphorus concentration and magnesium concentration of the liquid increase dramatically. When phosphorus is discharged from concentrated excess sludge, the phosphorus concentration and magnesium concentration of the liquid fall to approximately 50 to 400 mg/L and 50 to 200 mg/L, respectively. Meanwhile, ammonium elution is small, leading to an ammonium concentration of approximately 50 to 150 mg/L. Having undergone this phosphorus discharge treatment, the discharged sludge is separated into concentrated sludge 101 and concentrated sludge liquor 115 in a sludge concentration apparatus 114 , or separated into sludge cake and sludge liquor in a dewatering apparatus. The sludge concentration apparatus 114 employs a method such as flotation separation, gravity separation, or mechanical separation. The dewatering apparatus employs a dewatering method such as centrifugal dewatering, belt pressing, or screw pressing.
The separated liquid and sludge liquor 115 contain concentrated phosphorus, and it is of course desirable that the phosphorus be removed and recovered by precipitating a phosphorus compound from this waste water. In the anaerobic digestion tank 102 , the sludge is broken down such that the phosphorus, magnesium, and ammonium migrate to the liquid side, but by discharging the phosphorus and magnesium from the sludge in advance, before introduction into the anaerobic digestion tank, such that the phosphorus and magnesium concentrations decrease, as in the present invention, the amount of MAP that is generated spontaneously in the anaerobic digestion tank can be reduced. As a result, scale trouble caused by MAP and the like can be suppressed. Moreover, the amount of MAP that is discharged together with the digested sludge can be reduced. In FIG. 14, the digested sludge is decarbonated, screen residue are removed, and then the MAP-containing micro-particles are separated. The effects of these processes are as described above.
Another example of the treatment flow according to the present invention is illustrated in FIG. 15. The flow in FIG. 15 is constituted by an anaerobic digestion tank 202 , a crystallization process 204 , a screen residue removal process 206 , a micro-particle separation process 208 , and a dewatering process 210 . Note that in the flow illustrated in FIG. 15 and the following FIGS. 16 and 17, the dewatering process 210 is not essential, and may be omitted.
Excess sludge and/or primary sludge 201 are introduced into the anaerobic digestion tank 202 . The interior of the anaerobic digestion tank is heated and maintained at a temperature of approximately 55° C. or approximately 35° C. In the anaerobic digestion tank, the sludge is broken down into methane, carbon dioxide, a gas such as hydrogen sulfide, water-soluble nitrogen, phosphorus, and so on by the action of acid-fermentative bacteria and methanogenic bacteria. The generated methane gas may be recovered and used as energy. The amount of generated methane is increased by introducing easily-decomposable raw sludge in addition to the excess sludge. As the sludge decomposes, phosphorus, magnesium, and ammonium migrate to the liquid side. When the respective ion concentrations thereof reach or exceed the MAP solubility product, MAP is generated spontaneously in the anaerobic digestion tank. As a result of this MAP precipitation, scale trouble occurs on the draft tube, the base portion of the anaerobic digestion tank, the sludge discharge pipe, and so on.
Normally, the ratio of the phosphorus, magnesium, and ammonium in the digested sludge is generally phosphorus:magnesium:ammonium=100 to 500: several to several tens:1000. The magnesium concentration is overwhelmingly lower than the phosphorus and ammonium concentrations, and therefore during MAP generation in the digestion tank, the magnesium concentration clearly serves as a rate control.
In the following crystallization process 204 , MAP is precipitated by adding a magnesium compound to the digested sludge and digested sludge liquor extracted from the anaerobic digestion tank 202 . If aeration, decompression, or the like is also performed at this time, the sludge is decarbonated, leading to an increase in the pH, and hence MAP can be precipitated more efficiently. Needless to say, a chemical such as sodium hydroxide, magnesium hydroxide, or magnesium oxide may be added to raise the pH. As the added magnesium compound, magnesium chloride, magnesium hydroxide, magnesium oxide, sea water, and so on may be used. As regards the amount of added magnesium, a molar ratio between 0.1 and 10, preferably between 0.5 and 3.0, and more preferably between 0.8 and 1.2 in relation to the aqueous orthophosphoric acid concentration of the digested sludge is suitable. The pH in the reaction should be between 7.0 and 11.0, and preferably between 7.5 and 8.5.
Seed crystals are preferably added to the crystallization process 204 to ensure that MAP is generated efficiently. The MAP that precipitates spontaneously in the digestion tank, the MAP that precipitates in the crystallization process 204 , or MAP precipitated in a separate reactor may be used as the seed crystals.
As shown in another flow diagram of the present invention in FIG. 15, MAP-containing micro-particles and so on recovered in the separation process 208 may be supplied to the crystallization process 204 through a pipe 213 and be used as seed crystals. Separated water generated midway through the separation process or MAP contained in effluent or the like may also be used.
Alternatively, a powder or granular substance such as rock phosphate, dolomite, bone charcoal, activated carbon, silica sand, or calcium silicate may be used as seed crystals. The particle diameter of the seed crystals is arbitrary, but is preferably set between 0.05 and 3.0 mm, and more preferably between 0.1 and 0.5 mm. By precipitating new MAP on the surface of the seed crystals, separation of the digested sludge and MAP in the subsequent separation process can be performed favorably. The seed crystal charging amount is extremely important for precipitating MAP on the surface of the seed crystals. The charging amount is determined in consideration of the introduced phosphorus amount and the seed crystal particle diameter such that the phosphorus introduction amount in relation to the seed crystal surface area (the phosphorus surface area load hereafter) is no more than 100 g-P/m 2 /d, preferably no more than 30 g-P/m 2 /d, and more preferably no more than 10 g-P/m 2 /d.
Next, the digested sludge is introduced into the screen residue removal process 206 . Conventionally, no screen residue separation process is performed, and therefore when a hydrocyclone is used in the subsequent micro-particle separation process, blockages caused by screen residue and so on occur. For this reason, there is demand for a treatment method exhibiting long-term stability. In the present invention, screen residue is removed, enabling a vast improvement in the stability of the subsequent micro-particle separation process 208 , particularly when this process employs a hydrocyclone. Furthermore, when a sedimentation tank is used, the screen residue, digested sludge, and MAP are mixed together, and hence in the past it has been impossible to obtain MAP having a high degree of purity. In the present invention, MAP with a high degree of purity can be obtained by separating the screen residue in advance. The separated screen residue may be discharged outside of the system or may be introduced into the dewatering process 210 through a pipe 214 , as shown in another flow diagram of the present invention in FIG. 17. In this case, the dewatering performance is enhanced, and hence the screen residue 106 are preferably introduced into the dewatering process.
The crystallization process 204 and screen residue separation process 206 may be performed in any order. The crystallization process 204 may precede the screen residue separation process 206 , as shown in FIG. 15, or the screen residue separation process 206 may precede the crystallization process 204 , as shown in FIG. 16.
Following the crystallization process 204 and screen residue removal process 206 , the micro-particles containing MAP precipitated in the digestion tank and during the crystallization process are separated from the digested sludge. A hydrocyclone, a centrifugal settler, a sedimentation tank employing gravity separation, and so on may be employed as a method of separating the micro-particles from the digested sludge using the specific gravity difference between the two, while a vibrating screen, a drum screen, a filter layer, a classification layer-type separation layer, and so on may be employed as a micro-particle separation method using differences in particle diameter. The digested sludge is viscous, making it difficult to separate the MAP from the digested sludge through natural sedimentation, and therefore a mechanical separation method using a hydrocyclone or the like is preferable. A hydrocyclone has a reverse conical-form lower portion structure, and it is constituted by a hydrocyclone inflow pipe, a micro-particle discharge pipe, and a sludge discharge pipe. In the hydrocyclone, the digested sludge containing MAP is caused to swirl down the wall surface of the reverse conical form by the pressure of an extractor pump. The MAP-containing micro-particles, which have a greater specific gravity than the digested sludge, are collected and concentrated on a lower wall surface side by means of centrifugal force. The concentrated micro-particles are extracted either continuously or intermittently.
In the present invention, screen residue such as night soil residue are separated before the MAP-containing micro-particle separation process 208 , and therefore problems such as blockages of the hydrocyclone caused by screen residue and the like are solved.
In the dewatering process 210 , the digested sludge separated from the MAP-containing micro-particles is dewatered. Belt pressing, screw pressing, filter pressing, centrifugal dewatering, and so on may be used as a dewatering method. As noted above, by introducing the screen residue separated in the screen residue separation process 206 into the dewatering process 210 , the dewatering effect can be improved, and hence this is preferable.
To prevent pipe blockages caused by MAP scale following this process, processing to lower the pH and/or processing to reduce at least one of the magnesium ion concentration, phosphate ion concentration, and ammonium ion concentration are preferably performed following the micro-particle separation process 208 . A pH adjustor may be added as a method of reducing the pH. A chemical for lowering the pH of digested sludge, such as hydrochloric acid, sulfuric acid, aluminum salt, or iron salt, is used as the pH adjustor. With aluminum salt and iron salt, the soluble phosphorus contained in the digested sludge is fixed, causing a reduction in the solubility concentration.
Ammonia stripping, fixation by adhesive, and so on may be used as a method of reducing ammonium ions. MAP is precipitated when any of the phosphorus concentration, magnesium concentration, ammonium concentration, and pH rises. Conversely, as described above, when the pH of the digested sludge falls or at least one of the magnesium ion concentration, phosphate ion concentration, and ammonium ion concentration is reduced, the MAP generation ability deteriorates. According to the present invention, the MAP generation ability deteriorates, reducing the likelihood of MAP scale generation. Note that with the addition of iron salt, the generation of hydrogen sulfide and the like from the digested sludge can be suppressed. Needless to say, each of the above processes may be performed in the same treatment facility, or the sludge may be piped to a different treatment facility for each process.
By means of the above processes, phosphorus can be recovered efficiently from the digested sludge. In the past, it has been particularly difficult to recover MAP generated spontaneously in a digestion tank, but with the present system, MAP can be recovered easily.
The crystallization reactor and hydrocyclone of the various embodiments illustrated above in FIGS. 1 to 10 may be employed as the apparatuses used in a crystallization process and a micro-particle separation process of the flows shown in FIGS. 12 to 17.
Furthermore, in the present invention a wet-type vibrating screen comprising two or more sieve bodies having different hole diameters may be used as a screen residue removal apparatus. FIG. 18 shows an example of a treatment flow using a two-stage, wet-type vibrating screen comprising two or more sieve bodies having different hole diameters.
FIG. 18 illustrates a treatment system in which anaerobically digested sludge generated in a sewage treatment plant serves as raw sludge 1 . In this treatment system, pre-existing MAP is recovered from the sludge, further MAP is generated using phosphate ions and ammonia ions, which are soluble components of the sludge, as substrates, and the generated MAP is recovered efficiently.
According to an investigation performed by the inventors, when an iron or aluminum additive is not used in the water treatment system, the anaerobically digested sludge of sewage may contain MAP particles having a particle diameter within a range of 0.1 to 2.0 mm at approximately 0.8 g/L on average. In addition to these MAP particles, the sludge is also intermixed with a large amount of contaminant particles having a comparatively large particle diameter of at least 1.0 mm, such as screen residue, plant seed particles, hair, straw, and wood waste. The flow shown in FIG. 18 is constituted by a two-stage vibrating screen 81 , the crystallization reactor 9 , the hydrocyclone 2 , a MAP cleaning apparatus 82 , and a cleaned MAP slurry dewatering apparatus 83 . First, the sludge 1 is introduced into the two-stage vibrating screen 81 comprising two sieves having different hole diameters. A first stage sieve 81 a having coarse holes and a second stage sieve 81 b having narrow holes are provided in the two-stage vibrating screen 81 . The first stage sieve 81 a is used to separate contaminants such as screen residue, while the second stage sieve 81 b is used to separate MAP particles having a large particle diameter for recovery. The matter introduced into the two-stage vibrating screen 81 is divided into three types: contaminant particles 85 recovered by the first stage sieve 81 a , such as screen residue, which have a comparatively large particle diameter of approximately 0.8 to 3.0 mm or greater and are not to be recovered; particles 86 constituted mainly by MAP that is to be recovered, which pass through the first stage sieve 81 a and are recovered by the second stage sieve 81 b , and have a particle diameter of approximately 1.0 mm; and sludge 87 containing micro-particles having a particle diameter of approximately 0.3 to less than 1.2 mm, which passes through the second sieve. Following separation from the sludge, the contaminant particles 85 such as screen residue which are separated by the upper stage coarse sieve 81 a may eventually be added to the micro-particles of separated sludge following recovery of these micro-particles by the hydrocyclone 2 , and be discharged. By separating contaminants such as screen residue and the like, pipe blockages in the hydrocyclone 2 and the like can be prevented, an improvement in the purity of the recovered MAP can be achieved, and by adding the contaminant particles 85 to the micro-particles of separated sludge, deterioration of the treatment performance during dewatering of the discharged sludge can be prevented. The particles 86 mainly constituted by MAP, which are separated by the lower stage narrow sieve 81 b , may be recovered as MAP particles without passing through the downstream crystallization reactor 9 . Alternatively, when the amount of seed crystals in the crystallization reactor 9 is insufficient during system start-up or the like, the particles 86 mainly constituted by MAP that are separated by the lower stage narrow sieve may be introduced into the crystallization reactor 9 . The sludge 87 containing micro-particles which passes through the lower stage narrow sieve is introduced entirely into the downstream crystallization reactor 9 . By separating the particles 86 mainly constituted by MAP from the sludge, the particle diameter of the MAP particles in the downstream crystallization reactor 9 can be controlled, and MAP accumulation or pipe blockages in the reactor 9 can be suppressed.
A magnesium source and a pH adjusting agent are added to the crystallization reactor 9 as needed, and MAP crystallization is performed using the phosphate ions and ammonia ions dissolved in the sludge as substrates. When the sludge flowing into the reactor 9 contains a large amount of comparatively large MAP particles having a particle diameter of at least 1.0 mm, the crystallization reaction in the reactor 9 causes these MAP particles to grow even larger such that the particle diameter of a part of the MAP particles may increase to several millimeters or more. When the MAP particle diameter reaches several millimeters or more, the MAP particles may accumulate in the reactor 9 or on the bottom of the pipes, causing blockages. Furthermore, a catalytic function for promoting the MAP crystallization reaction exists on the MAP particle surface, and therefore MAP generation progresses more quickly as the total surface area of the MAP particles in the sludge inside the reactor 9 increases. However, as the proportion of MAP particles having a large particle diameter in the sludge inside the reactor 9 increases, the MAP surface area decreases in relation to the MAP mass, and hence the reaction promotion effect may decrease in relation to the MAP mass. On the other hand, when the proportion of MAP particles having a small particle diameter increases, the MAP particle recovery rate in the subsequent MAP particle separation operation performed by the hydrocyclone 2 and so on may de