JPH07214100A - Method for dehydrating sewage digested sludge - Google Patents

Abstract

PURPOSE:To lower the water content of a sludge dehydrated cake without increasing the cost of chemicals and to save the fuel for incineration by adding an inorg. flocculant to a sewage digested sludge and using an acrylate polymer flocculant to dehydrate the sludge. CONSTITUTION:The sewage digested sludge generated from a sewage disposal plant is treated as follows. Namely, an inorg. flocculant is firstly added to the sewage digested sludge, and then an acrylate polymer flocculant is used to dehydrate the sludge. At this time, the cation equivalent of the acrylate polymer flocculant at pH 3 is controlled to >=2meq/g and the limiting viscosity (eta) to >=5dl/g. Consequently, the chemical cost is not increased when the sludge is dehydrated, the water content of the dehydrated cake is lowered, and the fuel for incineration is saved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sludge dewatering method for efficiently treating digested sludge generated from a sewage treatment plant.

[0002]

2. Description of the Related Art Organic sludge produced from sewage, night soil treatment plants, organic industrial wastewater, etc. is dehydrated with a screw decanter, a belt press or the like by adding a polymer flocculant. The sludge that has been dehydrated is sometimes used for landfill, but it is mainly incinerated. Most of the fuel used for incineration of dehydrated sludge is used for evaporation of water in dehydrated sludge (hereinafter referred to as dehydrated cake). Generally, the water content in the dehydrated cake is 1%
If it decreases, it is possible to save about 10% of the fuel used for incineration.

However, in recent years, sludge has become difficult to be dehydrated due to the sophistication of water treatment, changes in the properties of wastewater to be treated, diversification, etc., and the fuel cost for incineration disposal has increased. A lower water content is demanded.

In particular, digested sludge generated from a sewage treatment plant that is performing digestion treatment has the following characteristics and is very difficult to dehydrate compared to other sludges. I. The particle size of suspended solids (SS) is small and the strength is weak,
Further low specific gravity. B. Extremely low sand and fiber content that form the core of floc. C. It contains a large amount of perishable organic substances shown in the sludge colloid value, and has strong adhesiveness.

As a result, in the dewatering treatment of sewage digestion sludge, the method using a belt press results in poor peeling of the dewatered cake, side leak, and high water content cake, and cannot be satisfactorily treated. In addition, the method using a screw decanter has a high water content, and the dehydrated cake obtained has a highly sticky mass, which makes transportation and incineration very difficult. Incidentally, the water content of the dehydrated cake is often in the range of 84 to 88%, although it depends on the properties of the sludge. In addition, recently in sewage treatment facilities, the organic matter in sludge tends to increase due to the increase in the diversion type inflow method, and the water content tends to deteriorate.

Regarding sludge dewatering, as a method for reducing the water content of the dehydrated cake, a method of using both a cationic and anionic flocculant in combination (JP-A-57-32797), a mineral acid is added to the sludge to adjust the pH. And then dehydration (JP-A-57-204299) and a method of adding an inorganic coagulant and then dehydrating with an amphoteric coagulant (JP-A-63).
No. 158200), and a method of dehydrating with anionic and cationic flocculants after adding an inorganic flocculant (JP-A-61-61).
-200897) has been proposed.

However, the combined use of the cationic and anionic coagulants requires a larger amount of the coagulant as compared with the case where the cationic coagulant is used alone, and the chemical cost is more than doubled, and the inorganic coagulant is also used. The method of dehydrating with an amphoteric flocculant or anionic and cationic flocculant after addition is an effective means, but since the alkalinity of digested sludge is very high, a large amount of inorganic flocculant is required and This is not practical because the chemical cost is more than doubled as compared with the case where the coagulant is used alone. Further, in the method of adding an inorganic flocculant to digested sludge and then treating it with a general cationic flocculant, the flocs formed are small and weak, and good dehydration treatment is difficult.

[0008]

As described above, in the prior art, the cost of chemicals is reduced in spite of the desire to reduce the water content of the dehydrated cake when dehydrating sewage digested sludge and to perform good dehydration treatment. There was no way to reduce the water content of the dehydrated cake without increasing the water content. An object of the present invention is to provide a method for solving these problems.

[0009]

Means for Solving the Problems The gist of the present invention is a method for dehydrating sludge, which comprises adding an inorganic flocculant to sewage digestion sludge and then performing a dehydration treatment using an acrylate polymer flocculant. is there.

In the present invention, the sewage digested sludge means a sludge obtained by anaerobic digestion of a mixed raw sludge generated from a sewage treatment plant [a mixed sludge of a first settled sludge and an excess (concentrated) sludge]. The present inventors, for such sewage digested sludge, as a result of investigating and examining the effect on the properties and dehydration properties, various polysaccharides, proteins and the like are present in the sewage digestive sludge in large amounts. It was found that the substance not only makes the sludge sticky but also has an important influence on the sludge dewatering performance (particularly, dehydrated cake water content and cake peelability).

The various substances such as polysaccharides and proteins contained in the above-mentioned sewage digested sludge react well with the inorganic flocculant and coagulate to make the sewage digested sludge hydrophobic. However, flocs simply coagulated with an inorganic coagulant are so fine that they cannot be dehydrated with a screw decanter, which is a commonly used dewatering machine, or a belt press machine, and a flocculant is used in combination. It is necessary to coarsen the floc.

On the other hand, since a large amount of alkali metal salts is present in the sewage digested sludge, the sludge pH is high even if various polysaccharides, proteins and the like react with the inorganic flocculant, and the colloid value of the sludge is negative. is there.

In the conventional dehydration treatment method using a combination of an inorganic flocculant and an amphoteric polymer flocculant or an anionic polymer flocculant, it is essential to positively charge the colloidal value of sludge, It was necessary to add a large amount of an inorganic flocculant until the metal salt or the like was neutralized, the pH was lowered, and it became positively charged. Therefore, an attempt was made to coarsen the flocs with a general cationic polymer flocculant, but no sufficient effect was obtained.

As a result of intensive studies by the present inventors, the use of an acrylate-based cationic polymer flocculant as a polymer flocculant suppresses the addition amount of the inorganic flocculant even in the dehydration treatment of sewage digestion sludge, We were able to dramatically increase the cost of dehydration treatment within the range comparable to the conventional method.

As the inorganic flocculant used in the present invention, commercially available products such as PAC (polyaluminum chloride), sulfuric acid band, ferric chloride, ferric sulfate, and polyferric sulfate can be used. An inorganic flocculant is preferably used.

The time required for the reaction when the inorganic coagulant is added does not need to be particularly considered and may be mixed with the sludge before the addition of the polymer coagulant.

The acrylate polymer flocculant used in the present invention is a homopolymer of quaternary dimethylaminoethyl acrylate or a quaternary dimethylaminoethyl acrylate and other cationic vinyl monomer or nonionic copolymerizable therewith. A copolymer with a monomer selected from the group of vinyl monomers. Examples of the cationic vinyl monomer include dimethylaminoethyl methacrylate tertiary salt or quaternary salt, and dimethylaminopropylacrylamide represented by the following chemical formula. In addition, examples of the nonionic vinyl monomer include acrylamide and methacrylamide.

[0018]

[Chemical 1]

In these acrylate-based polymer flocculants, the cation equivalent value C _V is preferably 2 meq / g or more, and values less than this value tend to lower the dehydration performance, which is not preferable. Further, the intrinsic viscosity [η] indicating the molecular weight of the polymer coagulant is suitably 5 dl / g or more, and the higher the intrinsic viscosity [η] is, the larger and stronger the flocs become, and the stable dehydration treatment becomes possible. If the intrinsic viscosity [η] is less than 5 dl / g, flocs are small and weak, and the dehydration treatment efficiency tends to decrease, which is not preferable.

In the present invention, the cation equivalent value C _V of the organic polymer coagulant can be determined by the colloid titration method shown below. [Measurement of Cation Equivalent Value C _V ] I. Take 90 ml of deionized water in a conical beaker, add 10 ml of 500 ppm sample, and add pH with hydrochloric acid solution.
To 3.0 and stir for about 1 minute. Then add 2-3 drops of toluidine blue indicator and titrate with N / 400-polyvinyl potassium sulfate reagent (N / 400-PVSK).
The titration rate is 2 ml / min, and the end point is when the test water changes from blue to reddish purple for 10 seconds or more. II. Preparation of 500 ppm aqueous solution of sample 0.2 g of sample (not converted into dry product) is precisely weighed and dissolved with 100 ml of deionized water using a triangular Korben with a ground-in stopper. This 25 ml is made up with deionized water in a 100 ml volumetric flask. III. Calculation method

[0021]

[Equation 1]

The intrinsic viscosity [η] can be determined as follows. I. Viscosity measurement of solvent blank Ubbelohde-type viscometer is set in a constant temperature bath adjusted to 30 ± 0.05 ° C., 1N-sodium nitrate solution is put in the viscometer, and this solution is left for 10 to 30 minutes. The flow-down time required to pass between the upper and lower marked lines displayed on the viscometer when the temperature was raised and then allowed to flow naturally was measured three or more times, and the average value was used to determine the viscosity (t
₀ ). II. Calculation of reduced viscosity The same operation as above was performed for sample concentrations of 0.10 to 0.02%
Using N-sodium nitrate solution,
The average flow-down time required to pass between the upper and lower marked lines displayed on the viscometer is measured, and the average value thereof is taken as the viscosity (t) of each sample solution. Next, the ratio t / t ₀ for each sample is calculated to obtain the relative viscosity η _r , the specific viscosity η _sp = η _r −1 is calculated from these, and these are divided by the respective sample concentrations to obtain the respective samples. Reduced viscosity η _sp /
Calculate C. III. Calculation of intrinsic viscosity Plot the concentration (g / d1) of the sample solution and the reduced viscosity,
Determine the intrinsic viscosity [η] by extrapolation.

In the present invention, an acrylate polymer flocculant is added to form flocs and then dehydrated by a known method. Examples of the dehydrator include a screw decanter type dehydrator and a belt press type dehydrator. Machine etc. can be applied.

[0024]

The present invention will be described in more detail with reference to the following examples. [Examples 1 to 4 and Comparative Examples 1 to 8] pH: 7.0, TS
(Evaporation residue): 2.2%, VTS (loss on ignition): 6
A sewage digested sludge having a property of 7.8% and M-alkalinity of 5,520 mg / l was used to perform a dehydration test by a belt press. The belt press machine used was a test machine manufactured by the applicant company. This belt press machine has a structure in which sludge is filtered and concentrated on a belt, and the concentrated sludge is sandwiched between two belts and pressed and dehydrated by an air cylinder. The belt is T-118 manufactured by Shikishima Canvas.
9 was used.

First, 300 ml of sludge was added to a 500 ml beaker.
ml is collected, and various inorganic flocculants shown in Table 2 are added and mixed as a stock solution. Further, each 0.5% aqueous solution of each polymer flocculant shown in Table 1 was added by a predetermined amount with a syringe, and then mixed with stirring with a spatula to generate various floc flocs. The generated flock is opened on the belt, and after concentration, it is sandwiched between two belts and 0.5 kg / cm ² ,
It was dehydrated for 1 minute under the condition of 1.0 kg / cm ² . The results of these tests are shown in Table 2. As is clear from Table 2, the method according to the present invention can reduce the water content by 5-6% as compared with the conventional method, and further improve the cake peeling property.

[0026]

[Table 1]

[0027]

[Table 2]

[Examples 5 to 12 and Comparative Examples 9 to 12] p
H: 6.9, TS: 2.43%, VTS: 57.6%,
Using a sewage digested sludge having the property of M-alkalinity: 3,775 mg / l, a dehydration test was carried out by a screw decanter (HS-40L manufactured by Ishikawajima Harima Heavy Industries). The test results are shown in Table 3. The general flow is as follows. 1. The sludge is supplied to a dehydrator by a mohno pump, and mixed with various polymer flocculants shown in Table 1 (injected into the dehydrator) in the dehydrator for dehydration treatment. 2. The supply of various inorganic coagulants shown in Table 3 was carried out by attaching a service nozzle to the outlet of the sludge supply pump and injecting predetermined amounts by the diaphragm pump. 3. Various polymer flocculants were dissolved in 0.3% concentration before use.

As is apparent from Table 3, the method according to the present invention can reduce the water content by 4-5% and the shape of the dehydrated cake as compared with the conventional method.

[Table 3]

Claims (2)

[Claims] 1. A method for dehydrating sludge, which comprises adding an inorganic coagulant to sewage digested sludge and then performing a dehydration treatment using an acrylate polymer coagulant. 2. The sludge dewatering method according to claim 1, wherein the acrylate polymer flocculant has a cation equivalent value at pH 3 of 2 meq / g or more and an intrinsic viscosity [η] of 5 dl / g or more.