JP7756572B2 - Organic wastewater treatment method, organic wastewater treatment device, and polymer flocculant - Google Patents

Description

The present invention relates to a method for treating organic wastewater, an organic wastewater treatment device, and a polymer flocculant, and in particular to a method for treating organic wastewater, an organic wastewater treatment device, and a polymer flocculant that are suitable for treating difficult-to-dewater organic wastewater that contains at least human waste or septic tank sludge.

In recent years, there has been an increase in treatment methods that directly dehydrate organic wastewater generated from human waste or septic tanks. Because organic wastewater from human waste or septic tanks contains large amounts of impurities, the impurities are removed as needed, and then the wastewater is directly dehydrated, with the separated liquid then subjected to biological treatment, dilution treatment, etc.

Since this type of dewatering process produces a large amount of dewatered cake of organic sludge, various measures to reduce the volume of the dewatered cake are being considered. Furthermore, in order to reuse the resulting dewatered cake as a combustion improver or compost, it is necessary to reduce its moisture content. Therefore, a method is being used in which a flocculant is added to organic wastewater from human waste or septic tanks to aggregate it into flocs, which are then dewatered.

Sewage and septic tank sludge are generally collected separately and transported to sewage treatment plants, but the amount of sludge brought in and the properties of the sludge at the time of delivery are not constant, and the mixing ratio fluctuates, so the properties of the sludge used for dehydration treatment vary greatly. Because sewage contains a large amount of chlorides in particular, a high mixing ratio of sewage can result in poor dehydration. Therefore, it is necessary to frequently adjust the amount of polymer flocculant added depending on the properties of the sludge, or to change the type of polymer flocculant used.

One way to solve this problem is to condition the sludge by adding an inorganic coagulant such as polyferric sulfate or aluminum sulfate prior to coagulation with a polymer coagulant. However, this method is not a sufficient solution. In particular, if the pH of the sludge to be dewatered is low, adding an inorganic coagulant will further lower the pH, causing the metal salts of the inorganic coagulant to dissolve, making it impossible to add the inorganic coagulant.

JP 2014-159000 A (Patent Document 1) describes how a method of alternating between a process of adding a flocculant to a mixed sludge of human waste, organic sludge, and excess sludge to dehydrate it, and a process of dehydrating the mixed sludge without adding a flocculant and subjecting the separated liquid to biological treatment, can reduce the moisture content and volume of the resulting dehydrated cake.

JP 2020-100917 A (Patent Document 2) describes an example of a method for using a polymer to remove micro-pitch particles with a particle size of 150 μm or less from separated white water during the paper manufacturing process.

JP 2014-159000 A Japanese Patent Application Laid-Open No. 2020-100917

Patent Document 1 describes the use of ordinary inorganic flocculants or polymer flocculants for treatment. However, the amount and properties of organic sludge such as human waste or septic tank sludge vary greatly depending on the time of day and season when it is delivered. When treating organic sludge with inconsistent amounts and properties at the time of delivery, the sludge subjected to dewatering treatment becomes difficult to dewater. Conventional polymer flocculants may not be suitable for such difficult-to-dewater organic sludge. Furthermore, Patent Document 2 is an invention related to a method and treatment device for treating pitch-containing water, but does not mention its application to organic wastewater, nor is its effectiveness clear.

In light of the above-mentioned problems, the present invention provides a method for treating organic wastewater, an organic wastewater treatment device, and a polymer flocculant that can stably dehydrate organic wastewater, particularly organic wastewater that is difficult to dehydrate.

The inventors conducted extensive research to solve the above problems and discovered that it is effective to perform a flocculation treatment using a specific polymer flocculant on difficult-to-dewater organic wastewater, followed by a dewatering treatment.

In one aspect, one embodiment of the present invention, which was completed based on the above findings, is a method for treating organic wastewater that involves adding a polymer flocculant to difficult-to-dewater organic wastewater containing at least one of human waste and septic tank sludge, satisfying the following conditions: (1) a 0.5% salt viscosity of 10 to 90 mPa·s, and (2) a cationic monomer content of 25 to 90 mol% among the molar amounts of cationic monomer, anionic monomer, and nonionic monomer contained in the total monomers constituting the copolymer contained in the polymer flocculant, and the total molar amount of anionic monomer and nonionic monomer is 10 to 75 mol%, followed by a flocculation treatment, followed by a concentration treatment, and then dewatering the concentrated sludge.

In another embodiment of the organic wastewater treatment method according to the present invention, the polymer flocculant has a cation equivalent value of 1.0 to 5.0 meq/g at pH 4 and a cation equivalent value of 4.5 meq/g or less at pH 10.

In yet another embodiment of the organic wastewater treatment method according to the present invention, one or more copolymers selected from the group consisting of quaternary methyl chloride salt of dimethylaminoethyl acrylate, quaternary methyl chloride salt of dimethylaminoethyl methacrylate, acrylamide, and acrylic acid are used as the polymer flocculant.

In yet another embodiment of the organic wastewater treatment method according to the present invention, the conductivity of the difficult-to-dewater organic wastewater is 50 to 1500 mS/m, the difference between the evaporation residue and the suspended solids is 1500 mg/L or more, the cation demand is -0.01 to -2.00 meq/L, and the capillary suction time (CST) is 300 seconds or more.

In yet another embodiment of the organic wastewater treatment method according to the present invention, the method further includes biologically treating the concentrated separated liquid and dehydrated separated liquid obtained from the concentration treatment and dehydration treatment, and adding the excess sludge obtained from the biological treatment to the difficult-to-dewater organic wastewater.

In another aspect, the present invention provides an organic wastewater treatment apparatus comprising: a reaction tank into which difficult-to-dewater organic wastewater containing at least one of human waste and septic tank sludge is introduced; a polymer flocculant having a 0.5% salt viscosity of 10 to 90 mPa·s, in which the molar ratio of cationic monomers to anionic monomers to the total monomers constituting the copolymer is 25 to 90 mol%, and the total molar ratio of anionic monomers and nonionic monomers is 10 to 75 mol%; a flocculant addition means for adding the polymer flocculant to the reaction tank; a concentration means for concentrating the organic wastewater to which the polymer flocculant has been added; and a dehydration means for dehydrating the concentrated sludge obtained by the concentration means.

In one embodiment of the organic wastewater treatment device according to the present invention, the flocculant addition means further adds an inorganic flocculant to the organic wastewater.

In another embodiment, the organic wastewater treatment device according to the present invention further includes a control means capable of controlling the addition rate of the polymer flocculant added by the flocculant addition means based on the properties or supply amount of the organic wastewater.

In yet another embodiment, the organic wastewater treatment device according to the present invention further comprises a night soil receiving tank for receiving night soil, a pretreatment device for removing impurities contained in the night soil, a night soil storage tank for storing pretreated liquid treated in the pretreatment device, and a delivery means for delivering the pretreated liquid to a reaction tank or a storage tank for storing organic wastewater to be delivered to the reaction tank, and for adjusting the flow rate of the organic wastewater in the reaction tank or storage tank.

In yet another aspect, the present invention provides a polymer flocculant for difficult-to-dewater organic wastewater containing at least one of human waste and septic tank sludge, which satisfies the following conditions: (1) a 0.5% salt viscosity of 10 to 90 mPa·s, and (2) of the molar amounts of cationic monomer, anionic monomer, and nonionic monomer contained in all monomers constituting the copolymer contained in the polymer flocculant, the cationic monomer constitutes 25 to 90 mol%, and the total molar amount of anionic monomer and nonionic monomer is 10 to 75 mol%.

The present invention provides an organic wastewater treatment method, an organic wastewater treatment device, and a polymer flocculant that can stably dehydrate organic wastewater, particularly organic wastewater that is difficult to dehydrate.

1 is a schematic diagram illustrating an example of an organic wastewater treatment device according to an embodiment of the present invention. FIG. 2 is a schematic diagram illustrating an example of a configuration of an apparatus upstream of the storage tank of FIG. 1. FIG. 10 is a schematic diagram illustrating a modified example of the organic wastewater treatment device according to the embodiment of the present invention.

(organic wastewater)
The organic wastewater to be treated can be organic wastewater containing organic substances such as sewage, human waste, and kitchen waste, and particularly can be difficult-to-dewater organic wastewater containing at least one of human waste and septic tank sludge. The method can particularly stably treat difficult-to-dewater organic wastewater having properties such as a conductivity of 50 to 1500 mS/m, a difference between the total solids (TS) and suspended solids (SS) in the organic wastewater of 1500 mg/L or more, a cation demand of -0.01 to -2.00 meq/L, and a capillary suction time (CST) of 300 seconds or more, preferably 500 seconds or more.

In one embodiment, the electrical conductivity of the organic wastewater to be treated is 50 mS/m or more, preferably 100 mS/m or more, and even more preferably 400 mS/m or more. The upper limit of electrical conductivity is 1500 mS/m, but in the case of organic wastewater containing at least one of human waste and septic tank sludge, in one embodiment it is 1500 mS/m or less, and may be 1200 mS/m or less, or even 1100 mS/m or less. The electrical conductivity is measured according to the electrical conductivity measurement method in accordance with JIS K0102 (2021).

The TS of the organic wastewater is not limited to the following, but in one embodiment is 2000 mg/L or more, further 5000 mg/L or more, and even more 7000 mg/L or more. The upper limit of the TS of the organic wastewater is not particularly limited, but in the case of organic sludge containing at least one of human waste and septic tank sludge, it is 30,000 mg/L or less, and even 25,000 mg/L or less.

The SS of the organic wastewater is not limited to the following, but in one embodiment it is 2000 mg/L or more, further 5000 mg/L or more, and even more 7000 mg/L or more. The upper limit of the SS of the organic wastewater is not particularly limited, but in the case of organic sludge containing at least one of human waste and septic tank sludge, it is 28,500 mg/L or less, and even more preferably 23,500 mg/L or less.

Organic wastewater with a difference between TS and SS (TS - SS) of 1500 mg/L or more is difficult to dewater, and even if a dewatering process is performed after adding a conventional flocculant, the moisture content of the dewatered cake may not be significantly improved. In one embodiment, the difference between TS and SS (TS - SS) of the organic wastewater is 1500 mg/L or more, or even 2000 mg/L or more, or even 3000 mg/L or more. While there is no particular upper limit for the difference between TS and SS (TS - SS), in the case of organic sludge containing at least one of human waste and septic tank sludge, it is 6000 mg/L or less, or even 4000 mg/L or less. Here, TS is measured in accordance with the Sewage Testing Method (2012), measuring the weight of the evaporation residue after heating at 105°C for two hours. SS complies with the Sewage Testing Method (2012) and measures the weight of sediment in a centrifuge at 3000 rpm for 10 minutes.

When the cation demand of organic wastewater is between -0.01 and -2.00 meq/L, more typically between -0.4 and -1.50 meq/L, or even between -0.43 and -1.00 meq/L, it is difficult to dewater, and even if a general coagulant is added and then dewatered, the moisture content of the dewatered cake may not be significantly improved. The cation demand is measured by centrifuging the sludge and then measuring the separated liquid phase in accordance with the Sewage Testing Methods (2012).

In one embodiment, the CST of difficult-to-dewater organic wastewater is 300 seconds or more, and even 500 seconds or more. There are no particular limitations on the upper limit of the CST, but for organic sludge containing at least one of human waste and septic tank sludge, the CST is typically within the above range. The CST is measured in accordance with the Sewage Testing Methods (2012).

(polymer flocculant)
As the polymer flocculant, a cationic polymer flocculant or an amphoteric polymer flocculant is used. As the cationic polymer flocculant, a flocculant obtained by copolymerizing a cationic monomer or a cationic monomer with a nonionic monomer is used. The cationic monomer constituting the copolymer of the cationic polymer flocculant is preferably a cationic vinyl monomer, and examples thereof include neutralized salts or quaternized products of various acrylate or methacrylate monomers such as those listed below. Specific examples of cationic vinyl monomers include dimethylaminoethyl acrylate or methacrylate, diethylaminoethyl acrylate or methacrylate, diethylaminopropyl acrylamide or methacrylamide, dimethylaminopropyl acrylamide or methacrylamide, and their neutralized salts with hydrogen halides, sulfuric acid, nitric acid, acetic acid, etc., and quaternized products with alkyl halides, benzyl halides, dimethyl sulfate, diethyl sulfate, etc.

Cationic polymer flocculants also include those in which the above-mentioned cationic monomers are copolymerized with nonionic monomers. Nonionic monomers are preferably nonionic vinyl monomers, such as acrylamide, methacrylamide, acrylonitrile, methacrylonitrile, (meth)acrylic acid esters such as dimethylaminoethyl acrylate and dimethylaminoethyl methacrylate, and vinyl acetate, with acrylamide being particularly preferred.

The amphoteric polymer flocculant is a polymer copolymerized with a cationic monomer, an anionic monomer, and, if necessary, a nonionic monomer. Typically, the amphoteric polymer flocculant preferably contains a polymer copolymerized with a vinyl cationic monomer unit, a vinyl anionic monomer unit, and a vinyl nonionic monomer unit within the molecule. Examples of vinyl cationic monomers copolymerized with anionic or nonionic monomers include neutralized salts or quaternized products of cationic vinyl monomers. These vinyl cationic monomers may be used alone or in combination of two or more.

The anionic monomer used in the amphoteric polymer flocculant is preferably an anionic vinyl monomer. Specific examples include acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, vinyl sulfonic acid, allyl sulfonic acid, methallyl sulfonic acid, styrene sulfonic acid, 2-acrylamidoethanesulfonic acid, 2-acrylamido-2-methylpropanesulfonic acid, 2-methacrylamidoethanesulfonic acid, 2-methacrylamido-2-methylpropanesulfonic acid, 2-acryloyloxyethanesulfonic acid, 3-acryloyloxypropanesulfonic acid, 4-acryloyloxybutanesulfonic acid, 2-methacryloyloxyethanesulfonic acid, 3-methacryloyloxypropanesulfonic acid, 4-methacryloyloxybutanesulfonic acid, and metal salts such as alkali metals or ammonium salts thereof. These anionic monomers may be used alone or in combination of two or more.

The nonionic monomer used in the amphoteric polymer flocculant is preferably a nonionic vinyl monomer. Specific examples include acrylamide, methacrylamide, acrylonitrile, methacrylonitrile, (meth)acrylic acid esters such as dimethylaminoethyl acrylate and dimethylaminoethyl methacrylate, and vinyl acetate, with acrylamide being particularly desirable.

In this embodiment, a polymer flocculant with a cross-linked structure is preferred. Generally, when a polymer flocculant is added to sludge containing a large amount of soluble inorganic salts, it is thought to cause thread clumping, and sufficient flocculation effects cannot be achieved. However, a polymer flocculant with a cross-linked structure can prevent thread clumping and form flocs sufficient for dewatering.

Polymer flocculants with a crosslinked structure can be produced by using a crosslinking agent during polymerization. Examples of crosslinking agents include divinyl compounds such as N,N'-methylenebis(meth)acrylamide, triallylamine, ethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, 1,3-butylene glycol di(meth)acrylate, and divinylbenzene, vinyl-based methylol compounds such as methylol methacrylamide, vinyl-based aldehyde compounds such as acrolein, and vinyl-based compounds such as methyl acrylamidoglycolate methyl ether. The crosslinking agent is preferably blended at a ratio of 0.0005 to 0.003 wt% based on the total amount of monomers.

A cationic polymer flocculant with an amidine structure can also be used as the polymer flocculant. By using a cationic polymer flocculant with an amidine structure, the viscosity of the flocculated sludge does not decrease even when heated, and the filtrate becomes easier to separate, making it possible to reduce the moisture content of the dehydrated cake without causing problems such as leakage from the screen holes.

Specific examples of cationic polymer flocculants having an amidine structure include:
(a) Cationic polymers containing repeating units of the azimine structure represented by the following (A) and/or (B) can be used.
(wherein R ¹ and R ² represent a hydrogen atom or a methyl group, and X ⁻ represents an anion)

Such cationic polymer flocculants having an amidine structure can be mixed with known cationic polymer flocculants and/or amphoteric polymer flocculants to the extent that the objectives of the present invention can be achieved.

The mixing ratio of polyamidine to a polymer primarily composed of aminoalkyl (meth)acrylate is preferably 10% by weight or more, more preferably 50% by weight or more, and even more preferably 70% by weight or more of polyamidine.

If the polyamidine content is less than 10% by weight, the cake moisture content will not be sufficiently reduced, and strong flocs that can withstand compression in a dehydrator will not form. The blending ratio of polyamidine to polymers primarily composed of aminoalkyl (meth)acrylate can be adjusted as desired depending on the properties of the organic wastewater and the dehydrator used.

As the polymer flocculant, a polymer flocculant that satisfies the following conditions (1) and (2) is used.
(1) 0.5% salt viscosity is 10 to 90 mPa·s;
(2) When the sum of the number of moles (C) of cationic monomers, the number of moles (A) of anionic monomers, and the number of moles (N) of nonionic monomers contained in all monomers constituting the copolymer contained in the polymer flocculant is taken as 100 mol%, the constituent ratio of the cationic monomers {C/(C+A+N)} is 25 mol% to 90 mol%, and the constituent ratio of the total of the anionic monomers and nonionic monomers {(A+N)/(C+A+N)} is 10 mol% to 75 mol%.

The 0.5% salt viscosity of the polymer flocculant is more typically 17 to 80 mPa·s, and even more typically 20 to 70 mPa·s. Note that 0.5% salt viscosity refers to the viscosity at 25°C of a sample in which 0.5% of the polymer flocculant is dissolved in a 4% NaCl solution.

If the cationic monomer composition ratio {C/(C+A+N)} is less than 25 mol% or more than 90 mol%, the floc diameter of the flocs will not increase significantly during the flocculation treatment, and the moisture content of the dehydrated cake in the subsequent dehydration treatment may not increase significantly. The cationic monomer composition ratio {C/(C+A+N)} is more preferably 40 mol% to 90 mol%, and even more preferably 45 mol% to 80 mol%.

If the total composition ratio of the anionic monomer and the nonionic monomer {(A+N)/(C+A+N)} is less than 10 mol% or exceeds 75 mol%, the floc diameter of the flocs will not increase significantly during the flocculation treatment, and the moisture content of the dehydrated cake in the subsequent dehydration treatment may not improve significantly. The total composition ratio of the anionic monomer and the nonionic monomer {(A+N)/(C+A+N)} is more preferably 10 mol% to 65 mol%, and even more preferably 15 mol% to 60 mol%.

The polymer flocculant preferably has a cation equivalent value of 1.0 to 5.0 meq/g at pH 4 and 4.5 meq/g or less at pH 10. Using such a polymer flocculant increases the frequency of contact between the polymer flocculant and organic wastewater containing at least one of human waste and septic tank sludge, producing flocs with relatively large diameters, enabling stable treatment of difficult-to-dewater organic wastewater.

The cation equivalent value of the polymer flocculant at pH 4 is more preferably 1.1 to 4.5 meq/g, and even more preferably 2.0 to 3.5 meq/g. The cation equivalent value at pH 10 is more preferably 3.0 meq/g or less, even more preferably 1.0 meq/g or less, and even more preferably 0.65 meq/g or less. There is no particular lower limit for the cation equivalent value at pH 10, but it is typically 0.01 meq/g or more, and more preferably 0.25 meq/g or more.

The cation equivalent value of a polymer flocculant is measured according to the following procedure. First, the polymer flocculant is dissolved in water to prepare a 500 mg/L polymer flocculant sample solution. Next, 90 mL of pure water and 10 mL of the polymer flocculant sample solution are mixed to prepare a polymer flocculant aqueous solution, and the solution is adjusted to pH 4.0 or pH 10.0 with 0.1 N dilute hydrochloric acid or dilute sodium hydroxide, respectively. Toluidine blue is added to this solution, and while stirring with a magnetic stirrer, titration is performed with a polyvinyl potassium sulfate solution (N/400 PVSK solution) for colloid titration. The titration rate is 2 mL/min, and the point at which the color changes from blue to reddish purple and this color persists for 20 seconds or more is set as the endpoint, and the sample PVSK titer is measured. Separately, a blank test is performed using 100 mL of pure water, and the blank PVSK titer is measured. Next, the colloid equivalent is calculated according to the sample PVSK titer and the blank PVSK titer, and this colloid equivalent is used as the cation equivalent value.
Colloid equivalent (meq/g) = {(PVSK titration amount - blank PVSK titration amount) × F}/2
(F is a factor, typically 1.0)

Any polymerization method can be used to produce a polymer flocculant, including precipitation polymerization, bulk polymerization, dispersion polymerization, and aqueous solution polymerization. As an example, but not limited to, a manufacturing method using a quaternary methyl chloride salt of dimethylaminoethyl acrylate and acrylamide will be described. First, a predetermined amount of a quaternary methyl chloride salt of dimethylaminoethyl acrylate, acrylamide, and ion-exchanged water is weighed and adjusted to a predetermined temperature, then placed in an insulated container. Dissolved oxygen is replaced with nitrogen gas, and chemicals such as a polymerization initiator are added. The polymerization initiator is not particularly limited; common azo initiators can be used. As the polymerization progresses, the temperature reaches its peak, and the mixture is allowed to age for a certain period of time. The polymer gel is then removed from the reaction vessel, cut, dried, and pulverized to obtain a powdered polymer flocculant.

The molecular weight of the polymer flocculant is typically 3 to 25 million, more typically 3 to 15 million. The polymer flocculant can be used in either powder or liquid form. When using a polymer flocculant with a crosslinked structure, the crosslinked polymer obtained as an emulsion liquid can be granulated or powdered by spray drying using a spray dryer or similar device to produce dried granules or powder.

The polymer flocculant is preferably added to the organic wastewater as an aqueous solution in water. The concentration of the polymer flocculant in the aqueous solution is not particularly limited, but is typically 0.05 to 0.8% by mass, more typically 0.1 to 0.5% by mass. The addition rate of the polymer flocculant to the organic wastewater varies depending on the properties of the organic wastewater, but can be approximately 0.2 to 5% by mass relative to the solids in the organic wastewater. For the organic wastewater of this embodiment, the addition rate can be 100 to 500 mg/L, or even 150 to 350 mg/L.

(Other flocculants)
In addition to the polymer flocculant, other inorganic flocculants or organic coagulants may be added. Examples of inorganic flocculants include aluminum sulfate, aluminum chloride, polyaluminum chloride (PAC), ferric chloride, and polyferric sulfate. Examples of organic flocculants that can be used include one or more selected from polyamines, dicyandiamides, polydicyandiamides, polydiallyldimethylammonium chlorides (also referred to as "polyDADMAC"), amino condensation systems, and melamine acid colloids.

More specifically, one or more of the following can be used: polyalkylpolyamine, polyethyleneimine, diallyldimethylammonium chloride, ethylenediamine-epichlorohydrin polycondensate, methylolmelamic acid colloid, dicyandiamide-ammonium chloride-formaldehyde polycondensate, polyethylene-polyamine-dimethylamine-epichlorohydrin polycondensate, dialkylamine-epichlorohydrin polycondensate (particularly dimethylamine-epichlorohydrin polycondensate), polyallylamine hydrochloride, polydiallylmethylamine hydrochloride, copolymer of diallyldimethylammonium chloride and sulfur dioxide, copolymer of diallyldimethylammonium chloride and acrylamide, copolymer of diallylamine hydrochloride and sulfur dioxide, etc. The addition rate of other flocculants to organic wastewater varies depending on the properties of the organic wastewater, but is typically around 100 to 5000 mg/L, or even around 1000 to 4000 mg/L.

When flocculation treatment is performed using the above-mentioned polymer flocculants and/or other flocculants, the floc diameter of the flocs in the organic wastewater is typically greater than 2 mm and less than 10 mm. When the floc diameter is approximately 3 to 10 mm, stable dewatering treatment can be performed while suppressing the occurrence of dewatering failures, even for organic wastewater that is difficult to dewater.

The amount of sewage and septic tank sludge received and the sludge properties at the time of delivery fluctuate constantly depending on the receiving conditions and the season, which can cause significant variations in the properties of the sludge used for dehydration. Because sewage contains a large amount of chloride, a high mixing ratio of sewage can lead to poor dehydration. Controlling the amount of flocculant added is one solution, but daily control changes can be practically difficult. The polymer flocculant used in the present invention can coarsen fine flocs during the flocculation process to produce strong flocculants, even in response to these ever-changing sludge properties depending on the receiving conditions and the season. This allows for easy dehydration without poor dehydration. Therefore, stable treatment can be achieved over long periods of time without the need to carefully control the amount of flocculant added depending on the fluctuating sludge properties.

The polymer flocculant according to an embodiment of the present invention can coarsen fine flocs during the flocculation treatment to produce strong flocs, even for organic wastewater or sludge that is difficult to dewater and contains high salt concentrations, such as those containing human waste and/or septic tank sludge, making it easy to dehydrate without causing dewatering problems. Therefore, stable treatment can be carried out over long periods of time, regardless of the amount of human waste or septic tank sludge brought in or fluctuations in its properties.

(Organic wastewater treatment device)
Next, an organic wastewater treatment apparatus according to an embodiment of the present invention will be described. As shown in Figure 1, the organic wastewater treatment apparatus according to the embodiment of the present invention includes a storage tank 30 for storing organic wastewater, a reaction tank 40 connected to the storage tank 30, a flocculant addition means 90 for adding a polymer flocculant to the organic wastewater in the reaction tank 40, a concentration means 50 connected to the reaction tank 40 for concentrating the organic wastewater to which the polymer flocculant has been added, a dewatering means 60 for dewatering the concentrated sludge obtained by the concentration means 50, a storage tank 70, a biological treatment tank 80, and a control means 100 for controlling the amount of polymer flocculant added by the flocculant addition means 90.

The storage tank 30 stores organic sludge, including at least one of human waste and septic tank sludge. Human waste and septic tank sludge may be directly transferred to the storage tank 30 upon delivery from an external facility, or the human waste and septic tank sludge may be received separately and then delivered to the storage tank 30. For example, as shown in FIG. 2, an organic wastewater treatment device may further include a human waste receiving tank 10 for receiving human waste, a pretreatment device 11 for removing impurities contained in the human waste, a human waste storage tank 12 for storing the pretreated human waste liquid obtained in the pretreatment device 11, and a delivery means, such as a pump, for delivering the pretreated liquid to the storage tank 30 or the reaction tank 40 and controlling the flow rate to adjust the SS, TS, conductivity, cation demand, CST, and other properties of the organic wastewater in the storage tank 30 or the reaction tank 40. The organic wastewater treatment device may also include a septic tank sludge receiving tank 20 for receiving septic tank sludge, a pretreatment device 21 for removing impurities and the like contained in the septic tank sludge, and a septic tank storage tank 22 for storing the pretreated septic tank sludge liquid obtained in the pretreatment device 21.

By sending the pretreated liquid, which has undergone a specified pretreatment process in pretreatment equipment 11 and 21 for human waste and septic tank sludge, to storage tank 30, the properties of the mixed liquid sent to storage tank 30 can be adjusted to a state appropriate for subsequent treatment. Of course, pretreatment equipment 11 and 21 and the tanks shown in Figure 2 may be added or omitted depending on the size and receiving capacity of the human waste treatment plant or sewage treatment plant. The organic wastewater stored in storage tank 30 is sent to reaction tank 40.

As shown in FIG. 1, organic wastewater containing at least one of human waste and septic tank sludge from the storage tank 30 is supplied to the reaction tank 40, where a polymer flocculant is added via a flocculant addition means 90 to perform flocculation treatment. The reaction tank 40 may be equipped with an agitator (not shown) to thoroughly stir the polymer flocculant in the organic wastewater. As shown in FIG. 3, a separate reaction tank 41 may be installed upstream of the reaction tank 40, and an inorganic flocculant and/or organic coagulant may be added in the reaction tank 41 to perform conditioning treatment of the organic wastewater. The organic wastewater in the reaction tank 40 to which the polymer flocculant has been added is sent to a concentration means 50.

The concentrating means 50 is a means for concentrating organic wastewater to which a polymer flocculant has been added (hereinafter also referred to as "flocculated sludge") to obtain concentrated sludge and concentrated separated liquid, and various commonly available concentrating devices can be used. From the perspective of treatment efficiency, it is preferable to use a mechanical concentrating device as the concentrating means 50.

For example, an elliptical plate thickener equipped with a gravity thickening section in which a slit-shaped elliptical plate rotates relative to the flocculated sludge delivered from the reaction tank 40 and a water collection section located below the gravity thickening section for capturing the concentrated separated liquid can be used as the thickening means 50 in Figure 1. Because elliptical plate thickeners do not require wash water, the amount of wash water required for typical excess sludge thickening processes can be reduced, enabling more environmentally friendly thickening processes. Furthermore, because elliptical plate thickeners have a relatively compact exterior, the space required for installation can be reduced, allowing for the overall downsizing of the equipment. Furthermore, to ensure more stable concentration in the thickening means 50, inorganic flocculants and/or organic coagulants may be added to the flocculated sludge. The thickened sludge thickened by the thickening means 50 is delivered to the dewatering means 60, and the concentrated separated liquid is delivered to the storage tank 70 via piping or the like.

The dewatering means 60 is a means for dewatering the concentrated sludge concentrated by the concentration means 50 to obtain dewatered sludge and dewatered separated liquid. The dewatering means 60 can be a centrifugal dehydrator, belt press dehydrator, filter press dehydrator, screw press dehydrator, rotary press dehydrator, electroosmosis dehydrator, or the like. Screw press dehydrators are particularly preferred because they can achieve a low moisture content with low power consumption.

For example, a screw press dehydrator has a cylindrical outer tube with a screw shaft and screw blades concentric with the cylindrical outer tube. It has a thickening section on the mixed sludge supply side and a squeezing section on the dehydrated cake discharge side, where the space between the cylindrical outer tube and the screw shaft gradually narrows in the direction of mixed sludge flow. The cylindrical outer tube is equipped with multiple apertures for discharging separated liquid. A sliding-shaft screw press dehydrator has a mechanism in which the screw shaft moves parallel to the dehydrated sludge outlet direction and forcibly discharges the dehydrated sludge. Using such a screw press dehydrator can significantly reduce the moisture content of the dehydrated cake. In addition to dehydration devices that combine separate screens and dehydrators, dehydration devices that integrate a screen and dehydrator, including a thickening section in the front stage that performs the function of a screen and a squeezing section in the rear stage, are more preferable because they do not require a separate screen and simplify the device configuration. To facilitate the dehydration process in the dehydration means 60, an inorganic flocculant and/or an organic coagulant may be added to the dehydration means 60 via the flocculant addition means 90.

The concentrated separated liquid obtained by solid-liquid separation in the concentration means 50 and the dehydrated separated liquid obtained by solid-liquid separation in the dehydration means 60 are stored in the storage tank 70 and then subjected to biological treatment in the biological treatment tank 80, and are then subjected to dilution treatment, etc., as necessary. Biological treatment is not particularly limited, but examples include aerobic biological treatment using activated sludge methods (standard activated sludge method, membrane separation activated sludge method, batch activated sludge method), biofilm treatment methods (fixed-bed biofilm method, fluidized-bed biofilm method), etc.

The concentrated separated liquid produced by the concentration means 50 and the dehydrated separated liquid produced by the dehydration means 60 can also be used as raw water for phosphorus recovery. Alternatively, excess sludge produced in the biological treatment tank 80 is allowed to flow into the storage tank 30 and mixed with the human waste and septic tank sludge from the human waste receiving tank 10 and the septic tank sludge receiving tank 20 shown in Figure 2. By circulating the excess sludge from the biological treatment tank 80 to the storage tank 30, the amount of sludge generated and discharged outside the treatment system as a whole can be reduced.

The control means 100 is connected to a pump 31 that supplies organic wastewater from the storage tank 30 to the reaction tank 40 (reaction tank 41 in Figure 3), and to a flocculant addition means 90. The control means 100 controls the supply of polymer flocculant and inorganic flocculant or organic coagulant by the flocculant addition means 90. For example, the control means 100 can control the addition rate and type of flocculant to be added depending on the amount of organic wastewater supplied into the reaction tank 40 via the pump 31.

The control means 100 may control the type and amount of flocculant supplied to the reaction tanks 40, 41, the concentration means 50, and the dewatering means 60 depending on the properties of the organic wastewater contained in the storage tank 30. For example, if easily dewaterable organic wastewater, such as that described in Patent Document 1, is supplied from the storage tank 30 to the reaction tanks 40, 41, and the addition of a general polymer flocculant or inorganic flocculant provides sufficient flocculation effects, the control means 100 adds a commonly available, ordinary flocculant to the reaction tank 40. On the other hand, if the organic wastewater supplied from the storage tank 30 to the reaction tank 40 is difficult to dewater due to fluctuations in the properties of the organic wastewater caused by seasonal fluctuations, climate change, fluctuations in the amount and timing of delivery, etc., the control means 100 adds a polymer flocculant according to an embodiment of the present invention to the reaction tank 40.

In this way, the control means 100 can control the type of polymer flocculant based on the properties of the organic wastewater in the storage tank 30, thereby optimizing and streamlining the treatment. When treating organic wastewater that is difficult to dewater, using the polymer flocculant according to this embodiment allows for stable dewatering without causing problems such as poor dewatering, resulting in more efficient and stable treatment than ever before.

The properties of the organic wastewater in the storage tank 30 may be measured manually. Alternatively, a measuring device may be installed inside the storage tank 30 to measure the conductivity, cation demand, and other properties of the organic wastewater inside the storage tank 30. Based on the measurement results from the measuring device, the control means 100 may output a control signal to the flocculant addition means 90 to select the type of flocculant. In addition to the type of flocculant, the control means 100 can also appropriately control the flocculant addition rate, addition time, start and end of addition, etc.

(Method for treating organic wastewater)
An example of an organic wastewater treatment method according to an embodiment of the present invention will be described using the organic wastewater treatment apparatus shown in Figures 1 to 3. The organic wastewater treatment method according to an embodiment of the present invention includes: adding a polymer flocculant satisfying the following conditions (1) and (2) to difficult-to-dewater organic wastewater containing at least one of human waste and septic tank sludge, for example, organic wastewater having a conductivity of 50 to 1500 mS/m, a difference between evaporation residue and suspended solids of 1500 mg/L or more, a cation demand of -0.01 to -2.00 meq/L, and a capillary suction time (CST) of 300 seconds or more, flocculating the organic wastewater, followed by concentrating the organic wastewater; and dehydrating the concentrated sludge obtained by the concentration treatment.
(1) 0.5% salt viscosity is 10 to 90 mPa·s;
(2) When the sum of the number of moles (C) of cationic monomers, the number of moles (A) of anionic monomers, and the number of moles (N) of nonionic monomers contained in all monomers constituting the copolymer contained in the polymer flocculant is taken as 100 mol%, the constituent ratio of the cationic monomers {C/(C+A+N)} is 25 mol% to 90 mol%, and the constituent ratio of the total of the anionic monomers and nonionic monomers {(A+N)/(C+A+N)} is 10 mol% to 75 mol%.

In the coagulation treatment, the above-mentioned polymer coagulant is added to organic wastewater having the above-mentioned properties, and coagulation treatment is performed to generate coagulated flocs with a floc diameter of approximately 2 to 10 mm in the liquid. An inorganic coagulant and/or organic coagulant may be added before or after the condensation treatment.

In the thickening process, inorganic flocculants and/or organic coagulants may be added as needed to the flocculated sludge containing the flocs obtained in the flocculation process. The thickened sludge obtained in the thickening process is then dehydrated using a dehydrator or the like to obtain dehydrated sludge (dehydrated cake). In this thickening process, the flocculated sludge obtained in the flocculation process is preferably treated using an elliptical plate thickener equipped with a gravity thickening section in which a slit-shaped elliptical plate rotates and moves, and a water collection section located below the gravity thickening section for capturing the concentrated separated liquid. This allows for efficient and stable treatment of the flocculated sludge.

In order to stably dehydrate difficult-to-dehydrate organic wastewater containing at least one of human waste and septic tank sludge, the concentrated sludge obtained in the concentration process is preferably dehydrated using, for example, a screw press dehydrator to obtain dehydrated sludge and dehydrated separated liquid. The concentrated separated liquid and dehydrated separated liquid obtained by the concentration and dehydration processes are stored in storage tank 70 and then introduced into biological treatment tank 80 for biological treatment. By returning the excess sludge generated in the biological treatment to storage tank 30 and mixing it with organic wastewater containing at least one of human waste and septic tank sludge, the amount of excess sludge generated in the system can be reduced, improving treatment efficiency.

The organic wastewater treatment method according to an embodiment of the present invention makes it possible to stably dehydrate sludge, including human waste and/or septic tank sludge, which are prone to fluctuations in properties.

The following examples of the present invention are presented together with comparative examples. These examples are provided to provide a better understanding of the present invention and its advantages, and are not intended to limit the invention.

(Test 1)
Polymer flocculants A to H were prepared, each having the composition and properties shown in Table 1. The abbreviations for the components constituting the polymer flocculants are as follows:
DAA: methyl chloride quaternary salt of dimethylaminoethyl acrylate DAM: methyl chloride quaternary salt of dimethylaminoethyl methacrylate AAm: acrylamide AA: acrylic acid

The 0.5% salt viscosity of the polymer flocculant was measured at 25°C using a Brookfield viscometer on a sample in which 0.5% of the polymer flocculant was dissolved in a 4% NaCl solution.

The cation equivalent value of the polymer flocculant at pH 4 and 10 was measured using the same method as described above. Specifically, the polymer flocculant was dissolved in water to prepare a 500 mg/L polymer flocculant sample solution. The solution was then adjusted to pH 4.0 or 10.0 with 0.1 N dilute hydrochloric acid or dilute sodium hydroxide, respectively, and titrated with a polyvinyl potassium sulfate solution (N/400 PVSK solution) for colloid titration. The colloid equivalent was calculated based on the sample PVSK titer and the blank PVSK titer, and this was used as the cation equivalent.

In Test 1, 200 ml of sewage (pH: 7.5, conductivity: 973 mS/m, TS: 9,100 mg/L, SS: 5,700 mg/L, TS-SS: 3,400 mg/L; cation demand: -0.91 meq/L, CST: 735 seconds) delivered to a certain sewage treatment plant was placed in a 500 ml beaker, to which 310 mg/L of a 0.2% aqueous solution of polymer flocculants A to H listed in Table 1 was added. The mixture was stirred for 10 seconds at 800 rpm using a commercially available hand mixer to obtain flocculated sludge. This flocculated sludge was gravity filtered through a 60-mesh nylon filter cloth, and the volume of filtrate after 30 seconds was measured. The flocculated sludge on the filter cloth was quickly transferred to a beaker, and then polyferric sulfate was added dropwise and stirred 10 times with a spatula to re-flocculate the sludge. The obtained flocculated sludge was then sandwiched between two filter cloths and squeezed for 1 minute at a pressure of 2 kg/ ^cm² using a dehydrator, and the moisture content of the obtained dehydrated cake was measured. The results are shown in Table 2.

In the following, "floc self-sustainability" was evaluated as follows: when flocculated sludge was placed on the filter cloth in the dehydrator according to the above procedure, if the sludge stood on its own, it was evaluated as "Good", if it protruded from the filter cloth, it was evaluated as "Good", and if it did not fit on the filter cloth at all, it was evaluated as "Poor". "Floc diameter" was evaluated by visually evaluating the overall floc diameter of the sludge when the flocculated sludge was placed on the filter cloth in the dehydrator according to the above procedure, and the average value was taken as the floc diameter. "Moisture content of dehydrated cake" was evaluated based on the change in weight of the sludge before and after dehydration treatment.

The human waste used in Test 1 was difficult to dewater, with a high salt concentration, TS-SS of 3,400 mg/L, and an extremely high CST value. However, by using polymer coagulants A to D, the flocculated flocs were able to self-sustain, allowing for easy dewatering. As a result, dewatered cake with a low moisture content of 69.9% to 72.6% was consistently obtained. On the other hand, when polymer coagulants E to H were used, the floc diameter was too small or flocs did not coagulate, preventing stable filtration. No effect was observed from adding inorganic coagulants. It was also difficult to measure the moisture content of the dewatered cake, and the self-sustainability of the flocs was poorly evaluated.

(Test 2)
As the polymer flocculants, polymer flocculants I to M having the compositions and properties shown in Table 3 were prepared.

In Test 2, 100 ml of a mixed sludge of sewage and septic tank sludge delivered to a certain sewage treatment plant (pH: 7.1, conductivity: 1,040 mS/m, TS: 10,400 mg/L, SS: 7,000 mg/L, TS-SS: 3,400 mg/L; cation demand: -0.96 meq/L; CST: 1,625 sec) was placed in a 300 ml beaker, and 170 mg/L of a 0.2% aqueous solution of polymer flocculants I to M having the physical properties shown in Table 3 was added. The mixture was stirred at 800 rpm for 10 seconds using a commercially available hand mixer. The flocculated sludge was gravity filtered through a 60-mesh nylon filter cloth, and the amount of filtrate after 30 seconds was measured. Next, the sludge was sandwiched between two filter cloths and squeezed for 1 minute using a dehydrator at a pressure of 2 kg/ ^cm² . The moisture content of the resulting dehydrated cake was measured. The results are shown in Table 4.

The mixed sludge of sewage and septic tank sludge used in Test 2 was difficult to dewater, with a high salt concentration, TS-SS of 3,400 mg/L, and an extremely high CST value. However, by using polymer coagulants I to L, the flocculated flocs became more self-sustaining, allowing for easy dewatering. As a result, dewatered cake with a moisture content of approximately 75.5% to 76.3% was consistently obtained. On the other hand, when polymer coagulant M was used, the floc size did not become coarse, and the amount of filtered water was reduced compared to when polymer coagulants I to L were used. When polymer coagulant M was used, the flocculated sludge did not adhere to the filter cloth when placed on it during dewatering, making it impossible to measure the moisture content of the dewatered cake.

(Test 3)
As the polymer flocculants, polymer flocculants N to V having the compositions and properties shown in Table 5 were prepared.

In Test 3, 200 ml of a mixed sludge of sewage and septic tank sludge delivered to a certain sewage treatment plant (pH: 7.2, conductivity: 429 mS/m, TS: 7,300 mg/L, SS: 5,800 mg/L, TS-SS: 1,500 mg/L; cation demand: -0.44 meq/L, CST: 835 seconds) was placed in a 300 ml beaker, and 150 mg/L of a 0.2% aqueous solution of polymer flocculants N to V having the physical properties shown in Table 5 was added. The mixture was stirred at 800 rpm for 10 seconds using a commercially available hand mixer. The flocculated sludge was gravity filtered through a 60-mesh nylon filter cloth, and the amount of filtrate after 30 seconds was measured. Next, the sludge was pressed between two filter cloths using a dehydrator at a pressure of 2 kg/ ^cm² for 1 minute, and the moisture content of the resulting dehydrated cake was measured. The results are shown in Table 6.

The mixed sludge of sewage and septic tank sludge used in Test 3 was difficult to dewater, with a TS-SS of approximately 1,500 mg/L. However, by using polymer coagulants P to T, the flocs became more self-sustaining, allowing for easy dewatering. As a result, dewatered cakes with moisture contents of approximately 81.2% to 84.3% were consistently obtained. Polymer coagulant N had flocs with too small a diameter, resulting in poor self-sustainability and poor handling, making it more difficult to dewater than polymer coagulants P to T. Polymer coagulant U had a moisture content of 82.5%, enabling it to achieve the same level of dewatering as polymer coagulants S and T. However, the amount of filtered water was less than that of polymer coagulants S and T, and the flocs' small diameter resulted in poor self-sustainability, making stable dewatering difficult. Polymer flocculants O and V also produced flocs with small diameters and poor self-sustaining properties, and although dewatering was possible, the moisture content of the dehydrated cake was higher than when polymer flocculants P to T were added, making it more difficult to carry out stable dewatering compared to polymer flocculants P to T.

(Test 4)
As the polymer flocculants, polymer flocculants W to Z and AA to DD having the compositions and properties shown in Table 7 were prepared.

In Test 4, 200 ml of a mixed sludge of sewage and septic tank sludge delivered to a certain sewage treatment plant (pH: 7.5, conductivity: 486 mS/m, TS: 7,300 mg/L, SS: 5,800 mg/L, TS-SS: 1,500 mg/L; cation demand: -0.66 meq/L, CST: 987 seconds) was placed in a 300 ml beaker, and 150 mg/L of a 0.2% aqueous solution of polymer flocculants W to Z and AA to DD having the physical properties shown in Table 7 was added. The mixture was stirred at 800 rpm for 10 seconds using a commercially available hand mixer. The flocculated sludge was gravity filtered through a 60-mesh nylon filter cloth, and the amount of filtrate after 30 seconds was measured. Next, the sludge was squeezed between two filter cloths using a dehydrator at a pressure of 2 kg/ ^cm² for 1 minute, and the moisture content of the resulting dehydrated cake was measured. The results are shown in Table 8.

The mixed sludge of human waste and septic tank sludge used in Test 4 was difficult to dewater, with a TS-SS of approximately 1,500 mg/L. However, the addition of polymer coagulants Y-Z and AA-BB improved the self-sustaining properties of the flocs, making dewatering easy. As a result, dewatered cakes with moisture contents of approximately 76.5% to 81.1% were consistently obtained. Polymer coagulants W and U produced flocs with floc diameters too small, resulting in poor self-sustaining properties and poor handling, making dewatering more difficult than with polymer coagulants P-T. Polymer coagulants X, CC, and DD also produced flocs with small diameters and poor self-sustaining properties. While dewatering was possible, the moisture content of the dewatered cake was higher than with polymer coagulants Y-Z and AA-BB, making stable dewatering more difficult than with polymer coagulants Y-Z and AA-BB.

10...Sewage receiving tank 11...Pretreatment device 12...Sewage storage tank 20...Septic tank sludge receiving tank 21...Pretreatment device 22...Septic tank storage tank 30...Storage tank 31...Pump 40...Reaction tank 41...Reaction tank 50...Concentration means 60...Dehydration means 70...Storage tank 80...Biological treatment tank 90...Flocculant addition means 100...Control means

Claims (2)

A method for treating difficult-to-dewater organic wastewater containing at least one of human waste and septic tank sludge, having an electrical conductivity of 50 to 1500 mS/m, a difference between evaporation residue and suspended solids of 1500 mg/L or more, an evaporation residue of 2000 to 30000 mg/L or less, a cation demand of -0.01 to -2.00 meq/L, and a capillary suction time (CST) of 300 seconds or more, the method further comprising the step of :
(1) 0.5% salt viscosity is 17 to 80 mPa·s;
(2) Of the molar number of cationic monomers, the molar number of anionic monomers, and the molar number of nonionic monomers contained in the total monomers constituting the copolymer contained in the polymer flocculant, the constituent ratio of the cationic monomers is 25 to 90 mol%, and the total constituent ratio of the anionic monomers and the nonionic monomers is 10 to 75 mol%,
1. A method for treating organic wastewater, comprising the steps of: adding a polymer flocculant satisfying the above requirement to carry out a flocculation treatment; thereafter, concentrating the flocculated sludge; and dehydrating the concentrated sludge. 2. The method for treating organic wastewater according to claim 1 , further comprising subjecting the concentrated separated liquid and the dehydrated separated liquid obtained by the concentration treatment and the dehydration treatment to biological treatment, and adding excess sludge obtained by the biological treatment to the difficult-to-dehydrate organic wastewater.