JP7628485B2 - Method and device for concentrating liquid waste - Google Patents

Description

The present invention relates to a method and an apparatus for concentrating liquid waste, and in particular to a method and an apparatus for concentrating liquid waste, which form flocs from liquid waste and obtain concentrated sludge and separated liquid from the liquid waste containing the flocs.

Liquid waste is sludge, which is formed when suspended solids in water settle or float to the surface and become muddy, or liquid waste generated during the treatment process of human waste, septic tank sludge, water purification, sewage treatment, and private wastewater. There are two types of liquid waste: organic sludge and inorganic sludge. The former is organic waste collected as sludge or industrial waste generated in facilities that treat organic wastewater from sewage treatment plants, food factories, paper and pulp factories, etc. The main component of organic waste is organic matter including mineral oil and animal and vegetable oil.

The latter includes drinking water sludge, which is a coagulation and settling sludge from civil engineering sites and water purification plants, and sludge generated in facilities that treat wastewater containing large amounts of sand and metal components, such as in metal plating factories.

Digested sludge discharged from digesters and methane fermenters that decompose organic matter such as biomass and sewage sludge under anaerobic conditions is also a liquid waste.

To reduce the volume of sludge, it is thickened or thickened and dewatered.

Thickening is the process of increasing the concentration of sludge to turn it into thickened sludge.

Dewatering is the process of squeezing water out of thickened sludge using mechanical pressure, resulting in a dehydrated cake with a low moisture content.

In sludge dewatering, the higher the sludge concentration, the more efficient the dewatering process, so various methods have been proposed for concentrating sludge prior to dewatering.

Figure 6 is a schematic diagram showing the process flow of a conventional method for concentrating and dehydrating liquid waste.

As mentioned above, thickening is generally used as a pretreatment to increase the concentration of sludge before it is fed to the dehydrator. After the sludge is conditioned in a coagulation tank where a polymer coagulant is added to create coagulated flocs, the conditioned sludge is separated in a thickener into concentrated sludge and separated liquid.

The rate at which polymer flocculants are added to sludge is approximately 0.2 to 5 wt% of the sludge solids.

The concentrated sludge is dehydrated in a dehydrator to further reduce its weight. A polymer flocculant is used in this dehydration process as an aqueous solution. There are no particular restrictions on the concentration of the aqueous solution, but it is usually 0.05 to 0.8 wt%.

The dehydrator produces the dehydrated filtrate and the dehydrated cake, which is either transported to a landfill or incinerated in the facility's own incinerator. In sewage treatment, the separated liquid and the dehydrated filtrate are sent to the sewage treatment process as return water and treated.

Thickening is also used as a pretreatment process to increase the concentration of sludge supplied to anaerobic digesters and methane fermentation tanks in order to improve the anaerobic treatment performance of these tanks.

Figure 7 is a schematic diagram showing the process flow of a conventional method for concentrating liquid waste as a pretreatment for anaerobic digestion.

As shown in the diagram, a polymer coagulant is added to the sludge in a coagulation tank and the sludge is conditioned to create coagulated flocs. The conditioned sludge is then separated into concentrated sludge and separated liquid in a thickener. The organic matter in the concentrated sludge is broken down into methane gas and carbon dioxide gas in an anaerobic digester. The methane gas is used as boiler fuel, etc. The separated liquid is sent to the sewage treatment process for water treatment.

The sludge from the anaerobic digester (digested sludge) is also periodically removed from the anaerobic digester and thickened and dewatered as shown in Figure 6.

Gravity thickeners and granular thickeners have traditionally been used to thicken sludge.

Figure 8 is a schematic diagram showing the conventional liquid waste treatment flow using a gravity concentration device.

As shown in the figure, a polymeric coagulant is added to the sludge in the coagulation tank and the sludge is conditioned to create coagulated flocs. The conditioned sludge is then separated into thickened sludge and separated liquid in a gravity thickening tank. The thickened sludge is sent to a dehydrator or anaerobic digester.

The conditioned sludge flows into the gravity thickener tank where solids and liquids are separated. The separated liquid is discharged from the separated liquid outlet pipe. The sludge that settles in the gravity thickener tank is stirred by a picket fence inside the tank, which drains the water between the sludge particles to obtain thickened sludge. The thickened sludge is discharged from the bottom of the tank.

Figure 9 is a schematic diagram showing the processing flow of liquid waste using a conventional granulation concentrator.

As shown in the figure, the granulation concentrator uses an agitator and draft tube to mix the sludge flowing in from the bottom of the tank with a polymer coagulant, turning the flocs into large granules. The granules and separated liquid are separated by a screen, and the separated liquid is discharged from the separated liquid outlet pipe. The granules become concentrated sludge, and are discharged from the granulation concentrator via the concentrated sludge outlet pipe.

Patent Document 1 also discloses a sludge concentration device that includes a coagulation reaction tank that reacts sludge with a coagulant to produce coagulated sludge, a concentrator that thickens the coagulated sludge extracted from the coagulation reaction tank to form concentrated sludge, a sludge supply path that supplies coagulated sludge from the coagulation reaction tank to the concentrator, a sludge circulation path that circulates a portion of the concentrated sludge formed by the concentrator to the coagulation reaction tank, a discharge path that discharges the remainder of the concentrated sludge formed by the concentrator to the outside, and a first coagulant injection device that injects the coagulant into the coagulation reaction tank.

According to the sludge thickening device of Patent Document 1, a portion of the thickened sludge is returned to the coagulation reaction tank, so that the residence time of the thickened sludge in the coagulation reaction tank can be extended, which increases the density and mechanical strength of the coagulated flocs and further reduces the water content of the dehydrated cake after dehydration.

Patent Document 2 discloses a method for producing pure water, which includes: (1) a flocculation step in which an inorganic flocculant is added to raw water and returned sludge containing a polymer flocculant is added to carry out a flocculation reaction; (2) a solid-liquid separation step in which the flocculated flocs produced in the flocculation step are separated into solid and liquid; (3) a desalination step in which treated water obtained from the solid-liquid separation step is passed through a reverse osmosis membrane device and/or an ion exchange device to desalt the treated water; (4) a sludge return step in which a portion of the flocculated sludge discharged from the solid-liquid separation step is returned to the flocculation step; and (5) a step in which a polymer flocculant is added to the flocculated sludge returned to the flocculation step.

According to the pure water production method of Patent Document 2, the SS concentration of the sedimentation or flotation treated water is reduced, improving the quality of the treated water.

JP 2020-001016 A Japanese Patent Application Publication No. 11-104696

According to Patent Document 1, compared to conventional concentration methods in which a portion of the concentrated sludge is not returned to the coagulation reaction tank, the density and mechanical strength of the coagulated flocs are increased, and the water content of the dehydrated cake after dewatering can be further reduced. However, the addition of the polymer coagulant to the coagulation reaction tank increases the viscosity within the tank, which reduces the dispersibility of the concentrated sludge, sludge, and polymer coagulant in the coagulation reaction tank, and there is a risk of insufficient mixing.

Therefore, the coagulation reaction is greatly influenced by the operating conditions of the coagulation reaction tank, such as the stirring conditions of the coagulation reaction tank and the amount of thickened sludge returned, which affects the concentration.

In Patent Document 2, a polymer flocculant is added to a portion of the flocculated sludge discharged in the solid-liquid classification process, and the flocculated sludge is returned to the flocculation process. However, since the object to be treated is not liquid waste (sludge) to be disposed of, but raw water that is the raw material for pure water, the viscosity of the flocculation process is also low, and the problem of non-uniform dispersion does not occur. In addition, the object to be treated is raw water, and the technical field is different.

The objective of the present invention, which was made in consideration of the above problems, is to provide a method and device for concentrating sludge that improves dispersibility during the coagulation reaction and enables stable concentration even when the operating conditions differ slightly.

The inventors have conducted extensive research to achieve the above-mentioned object, and have discovered that in a sludge thickening method involving flocculation in a flocculation tank, thickening of the sludge in a thickener, and returning a portion of the thickened sludge to the flocculation step, adding a polymer flocculant to the thickened sludge being returned before it is mixed with the sludge to be treated makes it possible to make the flocculated flocs in the flocculation tank larger and denser, and that not adding a polymer flocculant to the flocculation tank prevents an increase in viscosity within the flocculation tank. As a result, they discovered that the concentration efficiency in the thickener is improved stably even with slight differences in operating conditions, and have completed the present invention.

That is, the above object can be achieved by a method for concentrating liquid waste, comprising a flocculation step for forming flocs from liquid waste, a concentration step for concentrating the liquid waste containing the flocs to obtain concentrated sludge and separated liquid, a removal step for removing at least a portion of the concentrated sludge obtained in the concentration step, and a return step for adding a polymer flocculant to the removed concentrated sludge and returning the obtained flocculated concentrated sludge to the flocculation step, characterized in that the flocculation flocs are formed in the flocculation step by mixing the returned flocculated concentrated sludge with the liquid waste.

A preferred embodiment of the method for concentrating liquid waste according to the present invention is as follows.
(1) After the removing step, the method further includes a dewatering step of dewatering the remaining part of the removed concentrated sludge.

According to this, by dewatering the concentrated sludge having a high concentration, it is possible to improve the dewatering performance more than before.
(2) The polymer flocculant is a polymer flocculant for water supply. By using a polymer flocculant for water supply as the polymer flocculant, large and strong flocs can be produced from the water supply sludge, and therefore the thickening property of the obtained thickened sludge is improved.
(3) The polymer flocculant is a polymer flocculant for water supply that does not contain acrylamide monomers. This improves the concentration of concentrated sludge obtained from water supply sludge, and prevents carcinogenic acrylamide monomers from being returned to the upstream process of water purification treatment even when separated water produced during concentration is returned to the upstream process of water purification treatment.

The above object can also be achieved by a liquid waste concentrating device comprising: a flocculation means for forming flocs from liquid waste; a concentration means for concentrating the liquid waste containing the flocs to obtain concentrated sludge and a separated liquid; a piping means for removing at least a portion of the concentrated sludge obtained by the concentration means; a mixing means for adding and mixing a polymer flocculant with the concentrated sludge removed by the piping means to obtain a flocculated concentrated sludge; and a return means for returning the flocculated concentrated sludge obtained by the mixing means to the flocculation means.

A preferred embodiment of the liquid waste concentrating apparatus according to the present invention is as follows.
(1) The concentrating means is a gravity concentrating device, or the flocculating means and the concentrating means are a granulating concentrating device.

According to the present invention, instead of adding a polymer flocculant directly to liquid waste to form flocs, a polymer flocculant is added to and mixed with at least a portion of concentrated sludge to obtain flocculated concentrated sludge, and this flocculated concentrated sludge is mixed with liquid waste to form flocs. This suppresses the increase in viscosity during flocculation and improves dispersibility during the flocculation reaction compared to when a polymer flocculant is added directly to liquid waste. Furthermore, because flocs are formed using the flocculated concentrated sludge as a nucleus, the flocs are larger and stronger than before.

Therefore, stable concentration of liquid waste is possible even if the conditions of the coagulation reaction are slightly different.

1 is a flow chart for explaining an example of a method for concentrating liquid waste according to the present invention. FIG. 1 is a schematic diagram showing a typical example of a treatment flow according to a method for concentrating liquid waste of the present invention. 1 is a schematic diagram showing a liquid waste treatment device 10 of the present invention. FIG. 2 is a schematic diagram showing a liquid waste treatment device in which the concentrating means is a gravity concentrating device. FIG. 2 is a schematic diagram showing a liquid waste treatment device in which the flocculation means and the concentration means are granulation concentration devices. FIG. 1 is a schematic diagram showing a process flow of a conventional method for concentrating and dehydrating liquid waste. FIG. 1 is a schematic diagram showing a process flow of a conventional method for concentrating liquid waste as a pretreatment for anaerobic digestion. FIG. 1 is a schematic diagram showing a conventional treatment flow of liquid waste using a gravity concentration device. FIG. 1 is a schematic diagram showing a conventional treatment flow of liquid waste using a granulation concentrator.

<Method for concentrating liquid waste>
FIG. 1 is a flow chart for explaining an example of the method for concentrating liquid waste of the present invention, and FIG. 2 is a schematic diagram showing a representative example of a treatment flow according to the method for concentrating liquid waste of the present invention.

As shown in FIG. 1, the present invention includes an aggregation step (S100), a concentration step (S110), a removal step (S120), and a return step (S130).

(Liquid waste)
Liquid waste includes wastewater with a suspended solids concentration of 1000 mg/L or more, sludge formed when suspended organic or inorganic matter in water, hydrolysis products of inorganic coagulants or powdered activated carbon added as water treatment chemicals, etc., precipitate or float to form a mud-like (slurry-like) state, water supply sludge containing solid particles, algae, fungi, hydrolysis products of inorganic coagulants, etc., sewage and septic tank sludge, and liquid waste generated during water treatment processes such as water purification processes, sewage treatment processes, and public wastewater treatment processes.

The method for concentrating liquid waste of the present invention will be described below with reference to Figures 1 and 2.

[Agglomeration step (S100)]
In this process, flocculated flocs are formed from the liquid waste. The flocculated flocs are formed by mixing the returned flocculated and concentrated sludge (concentrated sludge to which a polymer flocculant has been added) with the liquid waste. The flocculated and concentrated sludge will be described later. This process is generally carried out in a flocculation tank.

The sludge concentration (SS concentration) in the coagulation tank into which the coagulated and concentrated sludge and liquid waste flow is preferably 5.0 to 20 g/L, and the SS concentration is more preferably 10 to 20 g/L. Therefore, by monitoring the sludge concentration in the coagulation tank, the optimal return flow rate of the coagulated and concentrated sludge can be determined in the return process (S130) described below.

A mixing tank may be provided upstream of the coagulation tank to add and mix the inorganic coagulant.

When an inorganic coagulant is added in the mixing tank, commercially available products such as aluminum sulfate, polyaluminum chloride (PAC), polyferric sulfate (polyferric iron), ferric chloride, or a mixture of these can be used. In addition, when these inorganic coagulants are used, the pH of the liquid waste during conditioning drops, so commercially available caustic soda or the like is used as an alkaline agent to adjust the pH to the appropriate coagulation pH.

In addition to the inorganic coagulant added in the mixing tank, or instead of the inorganic coagulant, an organic coagulant may be added. When an organic coagulant is added in the mixing tank, a commercially available product may be used as the organic coagulant, and for example, one or more of polyalkyl polyamine, polyethylene imine, diallyl dimethyl ammonium chloride, ethylene diamine epichlorohydrin polycondensate, dicyandiamide-ammonium chloride-formaldehyde polycondensate, polyethylene polyamine-dimethylamine-epichlorohydrin polycondensate, dialkylamine-epichlorohydrin polycondensate, etc. may be used.

The organic coagulant can be used undiluted or diluted as desired with water. When an organic coagulant is added, the addition rate of the organic coagulant to the liquid waste is generally 0.1% to 10% by mass, preferably 0.5% to 5% by mass, based on the solids TS of the sludge (liquid waste).

The liquid waste in the coagulation tank is mixed with the coagulated and concentrated sludge, and after conditioning to create coagulated flocs, the liquid waste containing the coagulated flocs is transferred to the concentration step (S110) (this concludes the coagulation step (S100)).

[Concentration step (S110)]
In this process, the liquid waste containing the flocs is thickened to obtain thickened sludge and separated liquid. As the thickener used for thickening, for example, a gravity thickener such as a gravity thickener, a centrifugal thickener, a screen thickener, a belt-type filtration thickener, a granulation thickener, a flotation thickener, or other commercially available thickeners can be used.

When the liquid waste is coagulation-sedimentation sludge generated from the coagulation-sedimentation process of a water purification plant, i.e., drinking water sludge, the thickener used in this process is generally a gravity thickener.

The sludge concentration (SS) of liquid waste before thickening is 5g/L to 15g/L, but the sludge concentration (SS) of the thickened sludge after thickening is 20-40g/L, depending on the sludge properties, type of thickener, and operating conditions.

Concentrated sludge with a high sludge concentration of 20 to 40 g/L may be transferred to an anaerobic digestion tank as desired. In this case, the high concentration of concentrated sludge increases the retention time in the anaerobic digestion tank, improving anaerobic digestion performance (concentration process (S110)).

[Removal step (S120)]
In this step, at least a part of the concentrated sludge obtained in the concentration step (S110) is taken out. To take out the concentrated sludge, for example, a return line including a dedicated return pipe connected at one end to the concentrator and at the other end to the coagulation tank or an upstream position of the coagulation tank, and a return pump provided in the return pipe can be used. This allows the coagulated and concentrated sludge obtained by adding a polymer coagulant to be returned to the coagulation tank at a stable sludge flow rate in the subsequent return step (S130).

The amount of concentrated sludge removed is based on the preferred mixing ratio of the coagulated concentrated sludge and liquid waste described below, and the amount removed is the amount required for this mixing ratio (removal process (S120)).

[Returning process (S130)]
In this step, a polymer flocculant is added to the extracted thickened sludge, and the resulting flocculated and thickened sludge is returned to the flocculation step (S100).

The polymer flocculant may be added at the suction or discharge part of the return pump (not shown) for the concentrated sludge, at a line mixer installed in the return piping, or in a mixing tank installed at the mixing site in Figure 2. The mixing tank installed in the return piping may be mechanically mixed or mixed by a water flow caused by the concentrated sludge.

The residence time of the concentrated sludge in the mixing tank installed in the return pipe varies depending on the mixing method, mixing strength, return flow rate and its concentration, but can be, for example, 0.5 to 3 minutes. If the residence time is 0.5 minutes or more, the polymer flocculant and concentrated sludge are mixed sufficiently. On the other hand, if the residence time is 3 minutes or less, the possibility of concentrated sludge flocs accumulating in the mixing tank installed in the return pipe and the possibility of the flocculation properties decreasing can be reduced.

Polymer flocculants that can be used are commercially available products, and anionic polymer flocculants, cationic polymer flocculants, or amphoteric polymer flocculants can be used alone or in combination. Polymer flocculants can also be used in powder or liquid form (dispersion or emulsion).

The anionic polymer flocculant may be any one or more selected from the group consisting of polyacrylic acid, sodium polyacrylate, potassium polyacrylate, ammonium polyacrylate, polymethacrylic acid, sodium polymethacrylate, potassium polymethacrylate, and ammonium polymethacrylate.

Polyacrylamide-based polymer flocculants can also be used, which are copolymers of acrylamide monomers and (meth)acrylate salts.

When the liquid waste is coagulation-sedimentation sludge generated from the coagulation-sedimentation process of a water purification plant, i.e., drinking water sludge, a polymer coagulant for water supply is used as the polymer coagulant.

Polymer flocculants for water supply are standardized by the Japan Water Works Association (JWWA K163:2019, Polyacrylamide for water supply). As the polymer flocculant for water supply, polyacrylamide for water supply can be used in this process. On the other hand, since acrylamide is carcinogenic, from the viewpoint of avoiding the separation liquid and dehydration filtrate from the concentration process (S110) in which acrylamide monomers remain being returned as return water to the previous process of water purification treatment, polymer flocculants for water supply that do not contain acrylamide monomers, such as polyacrylic acid and sodium polyacrylate, are preferred.

The cationic polymer flocculant has a cationic monomer as an essential component, and one or more selected from homopolymers or copolymers of cationic monomers, copolymers of cationic monomers and nonionic monomers, etc. Examples of cationic monomers include dimethylaminoethyl acrylate, dimethylaminoethyl methacrylate, diethylaminoethyl acrylate, diethylaminoethyl methacrylate, or alkali metal salts or quaternary ammonium salts of these.

In addition, amphoteric polymer flocculants are copolymerized with cationic monomers, anionic monomers, and nonionic monomers, and have cationic units, anionic units, and nonionic units in the molecule.

When the liquid waste is sludge after biological treatment, the polymer flocculant can be a cationic polymer flocculant or an amphoteric polymer flocculant of any cationic degree or molecular weight.

When the liquid waste is drinking water sludge, it is preferable to add a water supply polymer flocculant of poly(meth)acrylate salt with a 0.1% salt viscosity of 2 to 5 mPa·s and an anion equivalent of -9.0 or less to the concentrated sludge, but the water supply polymer flocculant is not limited to this. Water supply polymer flocculants containing acrylamide monomers and water supply polymer flocculants not containing acrylamide monomers can both be used depending on the purpose.

The salt viscosity is a value measured at 25°C using a Brookfield viscometer on a sample prepared by dissolving poly(meth)acrylate in a 1N aqueous sodium chloride solution to a concentration of 0.1% by mass, and is expressed in mPa·s.

The anion equivalent is a value that can be determined by the following measurement method, and is expressed in units of meq/g. Prepare a 0.1% aqueous solution of poly(meth)acrylate, add 5 ml of methyl glycol chitosan solution (N/200), stir, add 2-3 drops of touidine blue indicator, and titrate with potassium polyvinyl sulfate solution (PVSK, N/400). The end point is the point at which the color changes and is maintained for 10 seconds or more. A blank test is performed using the same procedure as above, but without adding the sample, and the anion equivalent Av is calculated using the following formula.

Anion equivalent (Av) [meq/g]=
(Blank titer ml - Sample titer ml) x 1/2 x PVSK titer

The rate of polymer flocculant added to the concentrated sludge is 0.02 wt/wt% to 2.0 wt/wt% of SS or more. If the rate of polymer flocculant added is 0.02 wt/wt% to SS or more, the polymer flocculant solution mixes well with the concentrated sludge, strong flocs are formed in the coagulation tank, and concentration performance is improved. If the rate of polymer flocculant added is 2.0 wt/wt% to SS or less, strong flocs are formed in the coagulation tank from the coagulated concentrated sludge and liquid waste while reducing the amount of polymer flocculant used, and concentration performance is improved.

By monitoring the predetermined polymer flocculant addition rate to the concentrated sludge (wt/wt% vs. SS or wt/wt% vs. TS), the sludge concentration and sludge flow rate in the coagulation tank into which the coagulated and concentrated sludge and liquid waste flow in, and the concentrated sludge concentration, the optimal return flow rate of the coagulated and concentrated sludge to be returned to the coagulation process (S100) can be determined.

In addition, by installing a sludge flow meter and a sludge concentration meter in the return piping of the return process (S130) to monitor the return flow rate and sludge concentration, the amount of flocculated and concentrated sludge returned to the flocculation process (S100) can be controlled, allowing for stable concentration.

To detect the sludge concentration, commercially available sludge concentration meters such as near-infrared sludge concentration meters, laser light sludge concentration meters, and microwave sludge concentration meters can be used. To detect the sludge flow rate, commercially available electromagnetic flow meters and ultrasonic flow meters can be used. The pump that returns the coagulated and concentrated sludge can be a commercially available product, and the flow rate can be adjusted to a set value by controlling the rotation speed before being returned.

According to the method for concentrating liquid waste of the present invention, the formation of flocs from the liquid waste in the coagulation step (S100) is achieved by mixing the returned coagulated and concentrated sludge with the liquid waste, rather than by adding a polymer coagulant directly to the liquid waste, so that the increase in viscosity during the coagulation reaction is suppressed more than in the past, improving dispersibility during the coagulation reaction. Therefore, compared to the conventional method of setting the injection rate of the polymer coagulant based on the flow rate of the sludge (liquid waste) flowing into the coagulation tank, it is possible to always add the optimal amount of polymer coagulant regardless of the properties of the liquid waste.

This makes it possible to optimize the amount of polymer flocculant used for efficient processing while still achieving stable, continuous high flocculation and concentration.

If the flocculated and concentrated sludge with the optimal amount of polymer flocculant added is returned to the flocculation step (S100), there is no need to add additional polymer flocculant in the flocculation step (S100). In fact, if additional polymer flocculant is added in the flocculation step (S100), it may take a long time to disperse and homogenize the solution in the flocculation tank due to the viscosity of the polymer flocculant solution, which may actually reduce the flocculation and concentration properties.

Indicators of sludge concentration include SS (suspended solids) and TS (total solids). In sludge with a low dissolved salt concentration (concentration of dissolved salts such as salt), such as sewage, the measured values of SS and TS are the same, but in sludge with a high dissolved salt concentration, such as sewage treatment, when the SS and TS values differ, SS is used as the sludge concentration.

In the present invention, SS and TS are defined as follows.
SS:K 0102:2019 Industrial wastewater test method 14.1 Suspended solids TS:K 0102:2019 Industrial wastewater test method 14.2 Total evaporation residue

An example of the method for concentrating liquid waste according to the present invention has been described above with reference to Figures 1 and 2, but the present invention is not limited to this example.

For example, at least a portion of the concentrated sludge obtained in the concentration step (S110) is removed and returned to the flocculation step (S100) as condensed concentrated sludge, but the remaining portion of the concentrated sludge obtained in the concentration step (S110) may be dewatered.

As shown in FIG. 1, the method for concentrating liquid waste of the present invention includes a dehydration step (S140) as an optional step. The dehydration step (S140) is described below.

[Dehydration step (S140)]
In this step, the remaining portion of the concentrated sludge removed after the removing step (S120) is dewatered.

Dewatering involves applying mechanical pressure to the thickened sludge to squeeze out the water from the thickened sludge, resulting in a dehydrated cake with a low moisture content.

The dehydrator used in this process may be a conventionally known device, such as a belt press dehydrator, a centrifugal dehydrator, a pressurized dehydrator, a vacuum dehydrator, a granulation and conditioning type high-efficiency direct dehydration method, a multiple disk type dehydrator, a screw press type dehydrator, or an electro-osmotic dehydrator.

The resulting dehydrated cake is disposed of by being transported to a landfill or incinerated in an incinerator, as in conventional methods (this concludes the dehydration process (S140)).

According to the liquid waste concentrating method of the present invention, which includes a dewatering step (S140), the concentrated sludge obtained after the concentration step (S110) contains large and strong flocs, so the dewatering performance in the dewatering step (S140) is improved and the moisture content of the obtained dewatered cake is lower than in the past. Therefore, the transportation cost of the dewatered cake and the fuel cost for incineration can be reduced compared to the past.

In addition, in the method for concentrating liquid waste of the present invention, a polymer flocculant is added to the concentrated sludge, and an inorganic flocculant or an organic coagulant is optionally added to the liquid waste, but other additives may also be used. For example, a short fiber agent may be added to the liquid waste to improve the concentrating ability.

Examples of short fiber agents include natural short fibers such as waste paper and cotton, chemically synthesized short fibers, and recycled short fibers. Short fibers made from recycled threads made from plastic waste and short fibers made from viscose rayon are preferred.

Among these, short fiber medicines made of viscose rayon (such as Evergloss U-700 series, manufactured by Suing Co., Ltd.) have excellent concentration effects.

From the viewpoint of excellent dispersibility in liquid waste and affinity with liquid waste (sludge), short fiber agents with fiber lengths of 5 to 10 mm and moisture contents of 30 to 80 wt/wt% are suitable.

The addition rate of the short fiber-like chemical is 0.02 to 1.0 wt/vol% per liquid waste, and preferably 0.1 to 0.5 wt/vol%.

By setting the addition rate of the short fiber chemical to 0.02 wt/vol% or more per liquid waste, the liquid waste and fibers can produce stronger flocs, improving the concentration in the concentration step (S110). This results in a more concentrated concentrated sludge, which reduces the SS concentration and BOD concentration of the separated liquid.

In addition, by setting the addition rate of the short fiber chemical to less than 1.0 wt/vol% per liquid waste, the short fiber chemical can be uniformly mixed and dispersed in the liquid waste, effectively capturing the oil and SS in the liquid waste, and is also economically advantageous.

<Liquid waste concentrating device>
3 is a schematic diagram showing a liquid waste treatment device 10 of the present invention. As shown in the figure, the liquid waste treatment device 10 of the present invention has a flocculation means 20, a concentration means 30, a piping means 40, a mixing means 50, and a return means 60.

The flocculation means 14 is a means for forming flocs from liquid waste, and is generally a flocculation tank. The mixed liquid waste and the returned flocculated and concentrated sludge are stirred in the flocculation tank, causing the flocs to grow and form.

A mixing tank may be provided upstream of the coagulation tank to add and mix an inorganic coagulant. Also, an organic coagulant may be added in addition to or instead of the inorganic coagulant added in the mixing tank.

The sludge concentration (SS concentration) in the coagulation tank, the return flow rate of the coagulated and concentrated sludge, the inorganic coagulant, and the organic coagulant are as described in the method for concentrating liquid waste above, and will not be described here. Downstream of the coagulation means 20, a concentration means 30 is provided.

The concentrating means 30 is a means for concentrating liquid waste containing flocs to obtain concentrated sludge and separated liquid.

As the concentrating means 30, a conventionally known concentrating means can be used, for example, a commercially available concentrating device such as a gravity concentrator such as a gravity concentrator tank, a centrifugal concentrator, a screen concentrator, a belt-type filtration concentrator, a granulation concentrator tank, or a flotation concentrator.

In particular, it is preferred that the concentrating means 30 is a gravity concentrating device, or that the flocculating means 20 and the concentrating means 30 are a granulating concentrating device.

Figure 4 is a schematic diagram showing a liquid waste treatment device when the concentration means is a gravity concentration device. As shown in the figure, the gravity concentration device introduces liquid waste (conditioned sludge) containing coagulated flocs from the coagulation tank into a center well, and gently stirs the conditioned sludge to separate the water and air bubbles in the conditioned sludge by rotating the picket fence and sludge scraping plate with a drive device. The sludge is separated by gravity into thickened sludge and separated liquid, the separated liquid is drawn out from the separated liquid outflow pipe, and the thickened sludge is drawn out from the thickened sludge withdrawal pipe below the gravity concentration device.

According to the present invention, the flocculation is strong, so even if the sludge is stirred with a picket fence in the tank, the sludge does not break down, and the water between the sludge particles is easily discharged, making it possible to obtain highly concentrated concentrated sludge. Since highly concentrated concentrated sludge is obtained, the flow rate of the concentrated sludge to be returned can be reduced, which reduces the power cost of the return pump (not shown). In addition, if a dehydration facility is provided in the downstream stage, the dehydration performance of the dehydrator is improved and the retention time of the concentrated sludge in anaerobic digestion is extended, improving the organic matter removal effect.

Figure 5 is a schematic diagram showing a liquid waste treatment device in which the flocculation means and the concentration means are granulation concentration devices. As shown in the figure, the granulation concentration device has a granulation concentration tank equipped with an agitator having an agitation blade, and liquid waste is introduced into the tank through a sludge inlet pipe provided at the bottom, and flocculated and concentrated sludge is introduced through a return pipe also provided at the bottom.

The liquid waste and coagulated, concentrated sludge are mixed and stirred in a draft tube and agitator to form coagulated flocs, which then form larger granules. From there, the mixture is separated into granulated concentrated sludge and separated liquid by a screen. The granulation concentration device produces strong, large coagulated flocs, which makes it possible to enlarge the mesh size of the screen, which is prone to clogging, resulting in stable concentration and highly concentrated concentrated sludge.

If the liquid waste is organic sludge, the separated liquid is sent to a sewage treatment process or the like for water treatment. If the liquid waste is inorganic sludge, the separated liquid is returned as return water, for example to a previous process of water purification treatment, or is discharged. The concentrated sludge is removed from the piping means.

The piping means 40 is a means for extracting at least a portion of the concentrated sludge obtained by the concentration means 30, and may be, for example, a return line including a dedicated return pipe connected at one end to the concentration means 30 and the other end to the bottom of the concentration means 30 when the concentration means 20 or the concentration means 30 is a granulation concentration device, and a return pump provided on the return pipe. A mixing means 50 is provided on the return line.

The mixing means 50 is a means for adding and mixing a polymer coagulant to the concentrated sludge extracted by the piping means 40 to obtain a coagulated concentrated sludge. Examples of the mixing means 50 include a line mixer or a mixing tank provided at the suction or discharge part of a return pump (not shown) for the concentrated sludge or in the middle of the return piping. The mixing tank may be mechanical or may be a water current caused by the concentrated sludge. The residence time in the mixing tank and the polymer coagulant to be added are as explained in the method for concentrating liquid waste, and will not be explained here.

The return means 60 is a means for returning the coagulated and concentrated sludge obtained by the mixing means 50 to the coagulation means. The specific configuration of the return means 60 is as described above for the piping means 40 as the return line.

A sludge flow meter and a sludge concentration meter may be installed in the return piping of the return line to monitor the return flow rate and sludge concentration. The detection of sludge concentration is as explained in the method for concentrating liquid waste above, and so a description thereof will be omitted here.

As shown in FIG. 3, in the liquid waste treatment device 10 of the present invention, the return pipe branches into a return means 60 that returns at least a portion of the concentrated sludge from the concentration means 30 to the flocculation means 20 via the mixing means 50, and a pipe that transports the remainder of the concentrated sludge downstream. A dewatering means (not shown) may be provided at the end of the pipe.

The dewatering means applies mechanical pressure to the concentrated sludge to squeeze out water from the concentrated sludge and obtain a dewatered cake with a low moisture content. Any conventionally known means can be used as the dewatering means. The obtained dewatered cake is transported to an external location for disposal in a landfill or incinerated in an incinerator.

The liquid waste treatment device of the present invention, which includes a dewatering means, can obtain strong coagulated flocs, improving the dewatering performance when dewatering concentrated sludge, and reducing the moisture content of the dewatered cake compared to conventional methods. This reduces the transportation costs when removing the waste, and also reduces the fuel costs required for incineration.

The present invention will be explained in more detail below with reference to the following examples.

1. Example 1
A concentration test was conducted on excess sludge (liquid waste) (pH 6.5, SS 10g/L) from a standard activated sludge treatment facility of a separate sewerage treatment plant, using the concentration test device shown in Table 1, with an excess sludge flow rate of 4.5 ^m3 /day and a liquid temperature of 20-25°C, using a wire slit screen with a width of 1 mm. The sludge concentration of the concentrated sludge and the SS concentration of the separated liquid were measured.

The excess sludge (liquid waste) in the coagulation tank in Table 1 was mixed with coagulated concentrated sludge, which was prepared in advance by adding cationic polymer coagulant (EvaGrose C-104G, manufactured by Suing) at 0.02-0.5 wt% per SS weight of the excess sludge (0.03-0.74 wt% per SS weight of the concentrated sludge), and the coagulated flocs were concentrated using a screen.

The thickened sludge that had been prepared in advance was thickened in advance using the thickening test device shown in Table 1 after conditioning the excess sludge with a cationic polymer coagulant at a rate of 0.3 wt/wt% relative to SS (SS 38 g/L, test number No. 11 in Table 2).

Tests were also conducted in which the polymer flocculant was added directly to the coagulation tank without returning the concentrated sludge to the coagulation tank, and in which the polymer flocculant was added directly to the coagulation tank and then concentrated sludge without the addition of polymer flocculant was added.

The results are shown in Table 2.

The polymer flocculant addition rate (wt/wt% vs. SS) in Table 2 is the addition rate of the polymer flocculant relative to the weight of the SS in the flocculation tank.

In Table 2, (i) "Polymer flocculant is added to concentrated sludge" refers to the case where the required amount of polymer flocculant is added to concentrated sludge using the method of the present invention, and concentrated sludge containing the polymer flocculant (flocculated concentrated sludge) is returned to the coagulation tank.

The polymer flocculant addition rate relative to the SS weight of the concentrated sludge is shown in parentheses (Test No. 1 to 9).

In Table 2, (ii) "Polymer flocculant added to coagulation tank without returning thickened sludge" refers to the case where polymer flocculant was added to the coagulation tank without adding thickened sludge or returning thickened sludge to the coagulation tank (Test Nos. 10 to 13).

In Table 2, (iii) "Concentrated sludge was returned and polymer flocculant was added to the coagulation tank" refers to the case where concentrated sludge without polymer flocculant was returned to the concentrated sludge in the coagulation tank and polymer flocculant was added to the coagulation tank (Test No. 14 to 17).

A polymer flocculant was added to the thickened sludge at a rate of 0.3 wt/wt% relative to the weight of SS in the coagulation tank, and the resulting coagulated thickened sludge was returned to the coagulation tank so that the SS in the coagulation tank was 20 g/L. The coagulated thickened sludge and excess sludge were coagulated in the coagulation tank and screen thickened. The SS of the thickened sludge was 65 g/L, and the SS of the separated liquid was 110 mg/L (Test No. 5).

Neither the coagulated and concentrated sludge nor the concentrated sludge was returned to the coagulation tank, and instead a polymer coagulant was added to the coagulation tank at a rate of 0.3 wt/wt% of SS based on the weight of SS in the coagulation tank, and the sludge was coagulated and screen-concentrated. The SS of the concentrated sludge was 38 g/L, and the SS of the separated liquid was 220 mg/L (Test No. 11).

The concentrated sludge without the addition of polymer coagulant was returned to the coagulation tank, the SS in the coagulation tank was adjusted to 20 g/L, and polymer coagulant was added to the coagulation tank at a polymer coagulant addition rate of 0.4 wt/wt% to SS based on the weight of SS in the coagulation tank. After coagulation and screen thickening, the concentrated sludge had a SS of 44 g/L and the separated liquid had a SS of 300 mg/L (Test No. 15).

As in the present invention, by adding the required amount of polymer flocculant to the concentrated sludge, returning the concentrated sludge containing the polymer flocculant (flocculated concentrated sludge) to the flocculation tank, and conditioning the excess sludge (liquid waste) in the flocculation tank and concentrating it using a screen, the SS concentration of the concentrated sludge obtained was increased even with a reduced amount of polymer flocculant added, and the SS concentration of the separated liquid was also reduced.

2. Example 2
0.5 L of the concentrated sludge obtained in Example 1 was taken and conditioned with the polymer flocculant used in Example 1. The addition rate of the polymer flocculant to the concentrated sludge was 0.5 to 2.0 wt/wt% relative to SS. After conditioning, the conditioned concentrated sludge was placed on a filter cloth with a filter cloth area of 100 ^cm2 , and uniaxially pressed at a pressure of 50 kPa for 10 minutes, and a pressure dehydration test was performed using a pressure dehydration tester. After the pressure dehydration test, the dehydrated cake was taken out and the moisture content was measured. In addition, the SS of the dehydrated filtrate discharged from the filter cloth was measured.

The results are shown in Table 3.

The three types of concentrated sludge used in the dewatering tests are shown below. The concentrated sludge preparation methods (i) to (iii) shown in the column for polymer flocculant addition rate for concentrated sludge in Table 3 and the concentrated sludge concentrations are as follows:

(i) This is concentrated sludge obtained by adding a polymer flocculant to concentrated sludge, and corresponds to test number No. 5 in Table 2 (Example 1), and the concentrated sludge concentration is SS 65 g/L.

(ii) This is concentrated sludge obtained by adding a polymer coagulant to a coagulation tank without returning the concentrated sludge, and corresponds to test number 11 in Table 2 (Example 1), and the concentrated sludge concentration is SS 38 g/L.

(iii) This is the concentrated sludge obtained by returning the concentrated sludge and adding a polymer coagulant to the coagulation tank, and corresponds to test number 15 in Table 2 (Example 1), and the concentrated sludge concentration is SS 44 g/L.

When polymer flocculant was added to the concentrated sludge of test number 5 in Table 2 (Example 1) at 1.0 wt/wt% relative to SS and pressure dewatered after conditioning, the moisture content of the dewatered cake was 80%, and the SS of the dewatered filtrate was 310 mg/L (test number 2).

When polymer flocculant was added to the concentrated sludge of test number 11 in Table 2 (Example 1) at 1.5 wt/wt% relative to SS and pressure dewatered after conditioning, the moisture content of the dewatered cake was 82%, and the SS of the dewatered filtrate was 380 mg/L (test number 6).

When polymer flocculant was added to the concentrated sludge of test number 15 in Table 2 (Example 1) at 1.5 wt/wt% relative to SS and pressure dewatered after conditioning, the moisture content of the dewatered cake was 82%, and the SS of the dewatered filtrate was 350 mg/L (test number 10).

As in the present invention, by adding the required amount of polymer flocculant to the thickened sludge, returning the thickened sludge containing the polymer flocculant (flocculated thickened sludge) to the coagulation tank, and conditioning the excess sludge (liquid waste) in the coagulation tank and dewatering the thickened sludge obtained by screen thickening, a dehydrated cake with a low moisture content can be obtained even if the amount of polymer flocculant added is reduced.

The concentrated sludge obtained by this invention contributes to improving the dewatering performance in the subsequent stages.

3. Example 3
In the rapid filtration equipment of a water purification facility, river water with a turbidity of 3 to 5 degrees was used as raw water for drinking water, and a PAC injection rate of 20 mg/L was used to coagulate and sediment the water. The coagulated and sedimented sludge discharged from the sedimentation tank (pH 6.5, SS 5.1 g/L or less, drinking water sludge (liquid waste)) was subjected to a concentration test using the concentration test equipment shown in Table 1.

The water supply sludge (liquid waste) and pre-prepared concentrated sludge with an acrylamide-free anionic water supply polymer flocculant (EvaGrose WA-200, manufactured by Suing Co., Ltd.) added at 0.2-0.4 wt% per SS weight of the water supply sludge (0.35-0.81 wt% per SS weight of the concentrated sludge) were added to the coagulation tank shown in Table 1, mixed, and coagulated, and the coagulated flocs were concentrated using a screen.

The thickened sludge that had been prepared in advance was made by conditioning the drinking water sludge with the same non-acrylamide anionic water supply polymer flocculant at a rate of 0.4 wt/wt% vs. SS, and then thickening it in the thickening test device shown in Table 1 (SS 36 g/L, test number No. 7 in Table 4).

As in Example 1, tests were also conducted in which the polymer flocculant was added directly to the coagulation tank without returning the concentrated sludge to the coagulation tank, and in which the polymer flocculant was added directly to the coagulation tank and then concentrated sludge without the addition of polymer flocculant was added.

The results are shown in Table 4.

In Table 4, (i) "Polymer flocculant added to concentrated sludge" refers to the case where the required amount of polymer flocculant is added to concentrated sludge that has been prepared in advance using the method of the present invention, and the concentrated sludge containing the polymer flocculant (flocculated concentrated sludge) is returned to the coagulation tank.

The polymer flocculant addition rate relative to the SS weight of the concentrated sludge is also shown in parentheses in Table 4 (Test No. 1 to 5).

In Table 4, (ii) "Polymer flocculant added to coagulation tank without returning thickened sludge" refers to the case where polymer flocculant was added to the coagulation tank without adding thickened sludge or returning thickened sludge to the coagulation tank (Test Nos. 6 to 8).

In Table 4, (iii) "Concentrated sludge was returned and polymer flocculant was added to the coagulation tank" refers to the case where concentrated sludge without polymer flocculant was returned to the coagulation tank and polymer flocculant was added to the coagulation tank (Test Nos. 9 to 11).

A polymer flocculant was added to the thickened sludge at a rate of 0.3 wt/wt% relative to the weight of SS in the coagulation tank, and the resulting coagulated thickened sludge was returned to the coagulation tank so that the SS in the coagulation tank was 10 g/L. The coagulated thickened sludge and excess sludge (liquid waste) were coagulated in the coagulation tank and screen thickened. The SS of the thickened sludge was 44 g/L, and the SS of the separated liquid was 100 mg/L (Test No. 2).

Neither the coagulated and concentrated sludge nor the concentrated sludge was returned to the coagulation tank, but instead a polymer coagulant was added to the coagulation tank at a rate of 0.4 wt/wt% of SS based on the weight of SS in the coagulation tank, and the sludge was coagulated and screen-concentrated. The SS of the concentrated sludge was 36 g/L, and the SS of the separated liquid was 260 mg/L (Test No. 7).

The concentrated sludge without the addition of polymer coagulant was returned to the coagulation tank, the SS in the coagulation tank was adjusted to 10 g/L, and polymer coagulant was added to the coagulation tank at a polymer coagulant addition rate of 0.4 wt/wt% to SS based on the weight of SS in the coagulation tank, and the sludge was coagulated and screen-concentrated. The SS of the concentrated sludge was 39 g/L, and the SS of the separated liquid was 330 mg/L (Test No. 10).

As in the present invention, by adding the required amount of polymer flocculant to the concentrated sludge, returning the concentrated sludge containing the polymer flocculant (flocculated concentrated sludge) to the flocculation tank, and conditioning the water supply sludge (liquid waste) in the flocculation tank and screen concentrating it, the SS concentration of the concentrated sludge obtained was increased even with a reduced amount of polymer flocculant added, and the SS concentration of the separated liquid was also reduced.

Claims (8)

a flocculation step for forming flocs from the liquid waste;
a concentration step of concentrating the liquid waste containing the flocs to obtain concentrated sludge and a separated liquid;
A removal step of removing at least a portion of the concentrated sludge obtained in the concentration step;
a suspended solids concentration measuring step of measuring a suspended solids concentration (SS concentration) of the extracted concentrated sludge;
A returning step of adding a polymer flocculant to the extracted concentrated sludge in an amount corresponding to the measurement result of the SS concentration measuring step , and returning the obtained flocculated concentrated sludge to the flocculation step,
2. A method for concentrating liquid waste, comprising the steps of: forming the flocs in the flocculation step by mixing the returned flocculated and concentrated sludge with the liquid waste. The method for concentrating liquid waste according to claim 1, further comprising a dehydration step for dehydrating the remaining part of the concentrated sludge that has been removed after the removal step. The method for concentrating liquid waste according to claim 1 or 2, characterized in that the polymer flocculant is a polymer flocculant for water supply. The method for concentrating liquid waste according to claim 3, characterized in that the polymer flocculant is a polymer flocculant for water supply that does not contain acrylamide monomers. 3. The method for concentrating liquid waste according to claim 1, wherein the liquid waste has a suspended solids concentration (SS concentration) of 1000 mg/L or more. 3. The method for concentrating liquid waste according to claim 1, wherein in the coagulation step, no coagulant is added except for a polymer coagulant contained in the coagulated and concentrated sludge. a flocculation means for forming flocs from the liquid waste;
a concentrating means for concentrating the liquid waste containing the flocs to obtain concentrated sludge and a separated liquid;
A piping means for extracting at least a portion of the concentrated sludge obtained by the concentration means;
a sludge concentration meter provided in the middle of the piping means and capable of measuring a suspended solids concentration (SS concentration) of the concentrated sludge in the piping means;
a mixing means for adding and mixing the concentrated sludge taken out by the piping means with an amount of polymer flocculant corresponding to the measurement result by the sludge concentration meter to obtain a flocculated concentrated sludge;
A returning means for returning the flocculated and concentrated sludge obtained by the mixing means to the flocculation means;
1. A liquid waste concentrating apparatus comprising: 8. The liquid waste concentrating apparatus according to claim 7, wherein the flocculating means and the concentrating means are granulating concentrating apparatus.

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