JP7130949B2 - Sludge dewatering method and sludge dewatering device - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for dehydrating digested sludge (also called digestive juice) discharged from an anaerobic digestion tank, and a sludge dewatering apparatus.

Expanding use of methane generated from anaerobic digestion as renewable energy, such as biogas power generation by anaerobic fermentation of organic waste such as primary sedimentation sludge and surplus sludge of sewage, or livestock manure and food residue. is doing. The residue after finishing fermentation by anaerobic digestion is discharged as digested sludge. This digested sludge is generally landfilled or incinerated after being dewatered.

As a method for dewatering digested sludge, a method of flocculating with a polymer flocculant and dehydrating with a dehydrator is generally performed. However, since digested sludge is difficult to dewater, it has a high moisture content and produces a large amount of dehydrated cake. Therefore, a technique for reducing the weight of the dehydrated cake by reducing the moisture content is required.

Patent Document 1 describes a method in which digested sludge is centrifugally dewatered, heated and pressurized to 130 to 180° C., and then centrifugally dewatered again.

Patent Document 2 discloses a method of subjecting digested sludge to flocculation treatment, concentrating the flocculated sludge with a thickener, and then dehydrating it with a dehydrator such as a screw press. A method of dewatering sludge with indirect heating is described.

JP-A-55-116500 JP 2016-107265 A

The method according to Patent Document 1 performs heat treatment at a high temperature of 130 to 180° C. under pressure, and the cost of heating is enormous, and the cost of equipment such as the need for a pressure-resistant reactor increases. The method of Patent Document 2 requires equipment for pre-concentration, but anaerobic digestion sludge has poor sedimentation separation properties, and flocculation treatment with a polymer flocculant is essential for concentration. Also, a special dehydrator such as a screw press with an indirect heating mechanism is required.

SUMMARY OF THE INVENTION An object of the present invention is to provide a sludge dewatering method and a sludge dewatering apparatus that are inexpensive in equipment cost and running cost for dehydration.

The sludge dehydration method of the present invention is a sludge dehydration method in which digested sludge is heated to 60° C. or higher for 0.5 hours or longer, then a polymer flocculant is added to aggregate the sludge, and the aggregated sludge is dewatered.

The sludge dewatering apparatus of the present invention comprises a heating tank into which digested sludge is introduced and having a residence time of 0.5 hours or more, and a heating means for heating the digested sludge in the heating tank to 60° C. or more attached to the heating tank. and a flocculating/dehydrating means for flocculating digested sludge discharged from the heating tank with a polymer flocculant to obtain flocculated sludge and dewatering the flocculated sludge.

The sludge dewatering apparatus of the present invention comprises a heating tank into which digested sludge is introduced and having a residence time of 0.5 hours or more, and a heating means for heating the digested sludge in the heating tank to 60° C. or more attached to the heating tank. A flocculating tank for obtaining flocculated sludge by flocculating digested sludge discharged from a heating tank with a polymer flocculant, and dewatering means for dehydrating the flocculated sludge discharged from the flocculating tank.

In one aspect of the present invention, the polymer flocculant is a polymer flocculant having a hydrophobic group.

In one aspect of the present invention, the polymer flocculant having a hydrophobic group is a DAM-based polymer flocculant.

In one aspect of the present invention, the heating temperature is 60-80° C. and the heating time is 0.5-5 hours.

Anaerobic digestive fluid (sludge) contains a large amount of proteins with relatively large molecular weights (tens of thousands or more) derived from metabolites of anaerobic bacteria. Proteins have a three-dimensional structure, and contain water molecules in the three-dimensional structure. Since protein is likely to be incorporated into a cationic polymer flocculant with water molecules still present, the water content of flocculated sludge from digestive fluid tends to be high.

In the present invention, focusing on the fact that digested sludge contains a large amount of protein, the protein is at least partially denatured at 60 ° C. or higher, for example, 60 to 80 ° C., and the digested sludge containing denatured protein has a high Aggregates and dehydrates using a polymer flocculant that exerts a dehydrating effect. As a result, dehydrated sludge having a low moisture content can be obtained.

In the present invention, the heating temperature of the digested sludge is as low as about 60 to 80° C., and pressurization during heating is unnecessary, so that the heating cost and facility cost required for heating can be suppressed. In addition, since it is sufficient to use only a polymer flocculant as a flocculant, the cost of chemicals can be reduced.

In the present invention, since the digested sludge is directly flocculated without being subjected to a thickening treatment, equipment for thickening the digested sludge is not required, and equipment costs can be reduced. Also, the dehydrator does not require an indirect heating mechanism, and a dehydrator generally used for organic sludge dehydration, such as a centrifugal dehydrator, belt press, or screw press, can be used.

It is a flow diagram showing an example of the present invention. 4 is a graph showing the results of Examples and Comparative Examples.

The present invention will be described in more detail below.

In the present invention, as shown in FIG. 1(a), digested sludge is heated to 60.degree. Also, as shown in FIG. 1(b), the digested sludge heated in the heating tank is introduced into the flocculating tank, flocculated, then introduced into the dehydrator and the dewatered cake is taken out. The dehydrated wastewater is discharged out of the system and treated separately.

As the digestive sludge, digestive fluid discharged from an anaerobic digestion tank made from food waste or sewage sludge (sludge containing a large amount of protein mainly composed of extracellular macromolecular substances; dissolved by filtering with a 0.45 μm filter 386 mg/L or more as a concentration analyzed by the Lowry method of sexual proteins). The water content of this digested sludge is usually about 80 to 90 wt%.

The heating temperature of the digested sludge is set at 60° C. or higher for protein denaturation. Moreover, in order to suppress the heating cost, the heating temperature is preferably 80° C. or lower. The heating time is preferably 0.5 hours or more, for example 0.5 to 5 hours, particularly 0.5 to 3 hours. Note that the heating time is the time during which the digested sludge is held at 60° C. or higher, and does not include the low temperature time period during the initial heating.

In order to heat the digested sludge, it may be heated with hot water, or it may be heated with a high-temperature gas such as steam. For heating with hot water, for example, a part of the liquid in the tank is taken out from the heating tank and passed through a heat exchanger for heat exchange with hot water.

Examples of heat sources for heating include, but are not limited to, the following.
(b) The boiler is heated using the methane gas generated in the digestion tank.
(b) Boiler heating using heavy oil.
(c) If there is waste heat in the plant system (for example, waste hot water discharged from a generator), the waste heat may be used for heating.
(d) Exhaust heat in the plant system is used for preheating (heating up to 60°C, for example), and the amount of heating that does not reach the set temperature (e.g., 80°C) is, for example, part of the methane gas generated from the digestion tank. You may heat a boiler using a part.
(e) Use waste heat in the plant system for preheating (heating up to 60°C, for example), and then heat the part that does not reach the set temperature (e.g., 80°C) by heating the boiler using, for example, heavy oil. Also good.
(f) For heating with gas, for example, a gas blowing means can be immersed in a heating bath and high temperature gas can be blown into it.

Under these heating conditions, proteins contained in the digested sludge are denatured and lose their three-dimensional structures. When the three-dimensional structure collapses, the water contained in the three-dimensional structure comes out of the three-dimensional structure of the protein. In addition, it is thought that the collapse of the three-dimensional structure causes the hydrophobic groups that were folded inside to come out on the surface of the protein. Since this heating condition is relatively low temperature and the heating time is short, the cost required for the heat treatment can be reduced as compared with the conventional technology.

Polymer flocculants such as aminoalkyl methacrylates, aminoalkyl acrylates, and polyvinylamidines are effective for the sludge heated in this way, but polymer flocculants having hydrophobic groups are preferred. Added. As a result, aggregated flocs containing aggregates in which denatured proteins are bound together are formed, so that a dehydrated cake with a low moisture content can be obtained.

Among water-soluble polymer flocculants, a polymer flocculant having a hydrophobic group is more effective in reducing the moisture content of heat-treated digested sludge than a polymer flocculant having a hydrophilic group. Proteins usually have a three-dimensional structure with water molecules inside. When a protein denatures, its three-dimensional structure collapses and unravels into a string-like structure, so the water molecules contained in the three-dimensional structure are released to the outside, and the inner hydrophobic groups come to the surface, resulting in a denatured protein. It is presumed that the water molecules are difficult to hold. Denatured proteins are more likely to bind to macromolecular flocculants having hydrophobic groups than undenatured proteins. As a result, it is considered that the moisture content of the dehydrated cake can be significantly reduced.

As the polymer flocculant having a hydrophobic group, a DAM (alkylaminomethacrylate quaternary salt polymer) polymer flocculant is suitable.

The amount of the polymer flocculant added to the digested sludge is preferably about 0.5-4.0%/TS, especially about 1.0-3.0%/TS.

The polymer flocculant may be added without cooling the heated digested sludge, or the polymer flocculant may be added after the temperature is lowered (for example, returned to room temperature).

The flocculated sludge produced by the flocculation treatment is dehydrated by a dehydrator. The dehydrator does not require an indirect heating mechanism. As the dehydrator, a centrifugal dehydrator, a screw press, a belt press, an electroosmotic dehydrator, and the like can be used. When using a centrifugal dehydrator, it is preferable to follow the flow of FIG. 1(a), and when using other dehydrator, it is preferable to follow the flow of FIG.

The sludge dehydration method of the present invention has a relatively low heating temperature of about 60 to 80 ° C. and a short heating time of about 30 minutes to 5 hours, so that the cost required for heat treatment can be suppressed. can be done.

In addition, since the digested sludge is heated as it is discharged from the anaerobic digestion tank, that is, without being thickened, a thickener is not required. Therefore, equipment costs can be suppressed and space can be saved. In addition, since only the polymer flocculant is used for flocculation, chemical costs can be suppressed. Furthermore, a general-purpose dehydrator is sufficient, and the equipment cost is low.

In the following examples and comparative examples, food residue anaerobic digested sludge (water content is shown in Tables 1 to 3) was used as the digested sludge.

Moreover, the following was used as a polymer flocculant.

DAM-based polymer flocculant: Kurita Water Industries Co., Ltd. Clifarm PC895
Amidine-based polymer flocculant: Kurita Water Industries Ltd. Clifix CP111
DAA-based polymer flocculant: Kurita Water Industries Ltd. Clifix PC688

[Examples 1 to 3, Comparative Examples 1 and 2]
The Erlenmeyer flask containing the digested sludge was heated using a water bath at the temperature shown in Tables 1 to 3 for the time.

A polymer flocculant shown in Table 1 was dissolved in pure water to a concentration of 0.3 wt %.

After returning the heated digested sludge to room temperature, 200 mL thereof is put into a high-speed mixer, 60 mL of polymer flocculant solution (900 mg / L as a polymer flocculant) is added, and immediately stirred strongly at 4000 rpm for 5 seconds. The digested sludge and the polymer flocculant were allowed to react with each other.

The flocculated sludge was gravity filtered using a Bronner funnel fitted with a filter cloth. The flocculated sludge after filtration was filled into a cake mold (30 mmφ×17.5 mmH) using a spatula, and pressed using a desktop press under conditions of a surface pressure of 1.0 kg/cm ² and 60 s. Then, using a metal spatula, it was separated from the filter cloth to obtain a dehydrated cake. This cake was dried in a constant temperature machine at 105° C. for 12 hours to determine the moisture content. Each result is shown in Tables 1-3.

As shown in Tables 1 to 3, dewatered sludge having a low moisture content can be obtained by setting the heating time to 0.5 hours or longer, the heating temperature to 60 to 80° C., and using a DAM-based polymer flocculant.

If the temperature is higher than 90°C, a large heating/cooling facility is required and the energy efficiency is poor, so 60 to 80°C is economical.

In the heat treatment at 50 ° C., it was not sufficiently dehydrated (Comparative Example 1-1), and even in the heat treatment at 60 ° C., the water content was lower when the heating time was 10 minutes compared to the heat treatment at 80 ° C. ( Comparative Examples 2-1, 2-3). This is believed to be due to the small proportion of denatured protein.

As shown in Table 2, if the heating time is 30 minutes or longer at 60°C or 80°C, a sufficient dehydration effect can be obtained. Although not shown in Tables 1 to 3, no significant improvement in the dehydration rate was observed even after 5 hours.

As shown in Table 3, DAM-based polymer flocculants having hydrophobic groups are superior in dehydration effect to other polymer flocculants having no hydrophobic groups. It is considered that the denatured protein was preferentially aggregated and the moisture content was effectively lowered.

[Example 4, Comparative Example 4]
The digested sludge was heated for 0, 30, 60 or 180 minutes, and immediately after heating, a DAM system (Kurita Water Industries Co., Ltd. Clifarm PC895) or a DAA system (Kurita Water Industries Ltd. PC688) was added as a polymer flocculant. Others were the same as in Examples 3-1 and 3-3. Figure 2 shows the water content of the dehydrated sludge.

As shown in FIG. 2, even if the digested sludge is not returned to room temperature after being heated and is coagulated immediately after heating, the water content is sufficiently reduced if the heating time is 30 minutes or longer. It was also found that the polymer flocculant having a hydrophobic group (KP201H in the figure) greatly reduced the moisture content of the dehydrated cake.

Claims (7)

After heating the digested sludge discharged from the anaerobic digestion tank at 60 to 80°C for 0.5 hours or more, a DAM-based polymer flocculant is added to the heated digested sludge to flocculate it, and the flocculated sludge is obtained. A method for dewatering sludge to be dewatered. 2. The method of dewatering sludge according to claim 1, wherein the digested sludge discharged from the anaerobic digestion tank is heated as discharged from the anaerobic digestion tank. 3. The method of dewatering sludge according to claim 1 or 2 , wherein the heating time is 0.5 to 5 hours. a heating tank having a residence time of 0.5 hours or more into which digested sludge discharged from the anaerobic digestion tank is introduced;
heating means for heating the digested sludge in the heating tank attached to the heating tank to 60 to 80 ° C.;
A sludge dewatering device comprising a coagulating and dewatering means for obtaining coagulated sludge by coagulating heated digested sludge discharged from a heating tank with a DAM-based polymer coagulant and dewatering the coagulated sludge. a heating tank having a residence time of 0.5 hours or more into which digested sludge discharged from the anaerobic digestion tank is introduced;
heating means for heating the digested sludge in the heating tank attached to the heating tank to 60 to 80 ° C.;
a flocculation tank in which the heated digested sludge discharged from the heating tank is flocculated with a DAM-based polymer flocculant to obtain flocculated sludge;
and dewatering means for dewatering the coagulated sludge discharged from the coagulating tank. 6. The sludge dewatering apparatus according to claim 4 , wherein the digested sludge discharged from the anaerobic digestion tank is introduced into the heating tank as it is discharged from the anaerobic digestion tank. 7. The sludge dehydrating apparatus according to any one of claims 4 to 6 , further comprising means for controlling the heating time to be 0.5 to 5 hours.

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