JPH021560B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method of sewage treatment. More particularly, the present invention relates to a method for treating sewage sludge. The present invention is most useful for domestic and municipal sewage sludge treatment. Generally, sludge treatment consists of the following steps: concentration;
It consists of stabilization, conditioning, dewatering and either its reuse or disposal of the treated sludge. Generally, the step of sludge thickening is an essential part of the sludge treatment process. Sludge thickening by gravity is the most economical. Sludge stabilization is done to prevent sludge rot. Anaerobic digestion in methane tanks is the most widely used stabilization method. In order to provide better dewatering of the sludge, conditioning or conditioning thereof is carried out beforehand with the aim of improving the dewatering properties of the sludge by changing the structure of the sludge and the form of water binding. Methods of sludge conditioning that are currently widely applied include reagent treatment and heat treatment of sludge. Reagent treatment of sludge is the most widely used conditioning method. Reagent treatments use inorganic and organic compounds, namely coagulants and flocculants. The most commonly used coagulants are metal salts (iron and aluminum salts) and acidic reagents (acids), which can be mixed at any time or at any time depending on the composition of the sludge and other conditions. used separately. The flocculant is a cation,
It is divided into nonionic and anionic flocculants. Continuous sludge treatment with coagulants and flocculants has been found to be most useful for sludge consolidation. Furthermore, the dewatering of the sludge is carried out under natural conditions (on sludge drying beds and in sludge retention ponds) and under artificial conditions (in vacuum filters, pressure filters, centrifuges, vibratory filters and separators). Sludge treatment technology depends on a number of factors such as sludge nature, sludge quantity, climatic conditions, land availability, etc. For example, methods of treating sewage sludge using organic flocculants are known in the prior art (see UK Application Nos. 1460318 and 1460319). This method of sludge treatment requires substantial amounts of flocculant and the dewatered sludge has a high water content, which increases the cost of sludge treatment and utilization in sewage disposal plants. consisting of concentration of the primary sewage sludge and activated sludge, heating of the sludge, treatment of the sludge with an organic flocculant and subsequent mechanical dewatering steps;
More developed methods for the treatment of sewage sludge (see German application No. 2103970) are also known to those skilled in the art.
The consumption of organic flocculant in this process is somewhat reduced compared to that of the previously described process due to the partial coagulation of the sludge upon heating. However, the consumption of flocculant is still considerable and the sludge has a relatively high water content even after mechanical dewatering. The prototype of the present invention is a method for treating sewage sludge (hereinafter referred to as the conventional method) according to West German Application No. 2317673. This conventional method consists of the following continuous operations: digestion of the sludge in a methane tank, concentration of the digested sludge, and supply of low-pressure steam for 70-100 hours.
heating the sludge to 30-60°C, measuring the PH of the sludge, treating the sludge with a coagulant to reduce the PH value to 3-6, neutralizing the sludge, treating with a flocculant, and including mechanical dehydration. In this method, sulfuric acid or iron chloride, or a compound thereof is used as a coagulant. The consumption of flocculant in this conventional method is somewhat reduced compared to that of the previously described method due to the use of coagulant. However, sludge digestion requiring the use of methane tanks complicates the sludge treatment process and increases its cost. Heating the sludge to 70-100°C by supplying low-pressure steam following sludge concentration and subsequent cooling of the sludge causes an increase in water content and an increase in the volume of sludge due to steam condensation, which is time consuming. It becomes a method. This complicates the process of dewatering the sludge. In doing so, such methods of sludge treatment transform some of the colloidal materials of the sludge into a dissolved state, so that the liquid phase after dewatering the sludge (centrifuge effluent or filtrate) is mainly Before introduction into the sewage waste plant, it is contaminated with dissolved organic and inorganic contaminants, which require additional treatment, which complicates and increases the complexity of sewage and sludge treatment. be. The reduction of the PH of the sludge after heating to 3-6 by treatment of the sludge with a coagulant, followed by neutralization (increase in PH) and treatment of the sludge with an organic flocculant after neutralization, reduces the PH This will be done while the changes occur. As a result, shear-resistant flocs cannot be formed. That is,
The main requirements placed on the dewatered sludge are not met and therefore the centrifuge effluent or filtrate is contaminated by crushed flocs.
The efficiency of further mechanical dewatering of the sludge is reduced. On the other hand, coagulation and flocculation of the sludge is carried out at 30-60° C. with media of various PHs that do not give irreversible coagulation and flocculation of protein substances.
That is, under these conditions some protein substances will return to the colloidal phase and their coagulation and aggregation will therefore require additional amounts of coagulant and coagulant. Coagulation and flocculation of the sludge due to non-optimal PH values cannot provide the maximum possible extraction of bound water from the sludge. As a result, the concentration of the dewatered sludge is quite low. From the above, it can be said that the conventional method is not sufficiently effective, consumes an increased amount of reagents, and is difficult to implement. The main object of the present invention is to improve the efficiency of the above-mentioned method, to simplify the method and to carry out the process under optimal conditions, which reduces the consumption of the reagents used. made in order,
The object of the present invention is to provide a sewage sludge treatment method having such parameters. To achieve this objective, according to the invention, the sewage sludge is charged with an amount of acidic reagent such that its pH is reduced to a value corresponding to the isoelectric state of the sludge,
A method for treating sewage sludge is provided, which is characterized in that it is heated to a temperature of 75° C., then a polymer flocculant is introduced, and then the sludge is dewatered. Immediately after concentrating the sludge, the pH of the sludge is lowered to a value such that an isoelectric state of the sludge is obtained, and then the sludge is heated to 65-75°C and the heated sludge is treated with a flocculant. This makes it possible to obtain compact hard floes of large size, ie to meet the main requirements placed on conditioned sludge. This improves the efficiency of subsequent sludge processing, giving the dewatered sludge maximum consistency and free structure. When the sludge is heated to 65-75°C, irreversible coagulation of the protein material of the sludge occurs near its isoelectric point, which prevents the protein material from entering a colloidal state. This improves the quality of the liquid phase of the sludge after dewatering. Even when the sludge is treated with a flocculant, the consumption of flocculant is reduced because the particles of the sludge are near the isoelectric point. The sludge treatment according to the invention does not require digestion of the sludge, which simplifies the method. Moreover, the raw sludge contains less colloidal particles and has a lower value of PH than that of the digested sludge. This reduces the consumption of reagents (coagulants and flocculants) used for sludge treatment. It is expedient to heat the sludge with an immersion gas burner. This allows the sludge to maintain its concentration within the range of the concentration it had after concentration,
That is, heating in this case does not reduce the sludge concentration. This also simplifies the method (compared to conventional methods) and improves its efficiency.
Furthermore, by using an immersion gas burner, the carbon monoxide contained in the calcining gas dissolves into the sludge to form carbon dioxide, which reduces the consumption of coagulant. All these troublesome children simplify the method and
Gives high efficiency and reduces consumption of used reagents. The main objects of the invention will become more apparent upon consideration of the following detailed description of embodiments thereof, taken in conjunction with the accompanying drawings. According to the method, primary sewage sludge is mixed with excess activated sludge and the mixture is concentrated in a gravity thickener with a rod-like agitator. Then the PH value is measured. After that, the value of PH corresponding to the isoelectric state of the sludge and the amount of coagulant required to change the original value of PH to a value corresponding to the isoelectric state of the sludge are shown in Figures 1 and 2. found from the curve. Agents such as sulfuric acid, metal salts or compounds thereof can be used as coagulants. The amount of coagulant given by the curve shown in FIG. 2 is added to the sludge. The pH of the sludge reaches a value corresponding to the isoelectric state of the sludge, and the sludge is heated to 65-75℃ by an immersion gas burner or a spiral heat exchanger.
The process of coagulation of coagulant and sludge is rapidly enhanced when heated to . The temperature for heating the sludge was found experimentally. Immediately thereafter, a flocculant is added to the heated sludge and the sludge is dewatered in a centrifuge or pressure filter. Then,
Quicklime or alkali is added to the dewatered sludge to neutralize the sludge and provide additional reduction in the moisture content of the sludge. Here, the graph in FIG. 1 was obtained as follows. That is, by adding various amounts of coagulant into the concentrated sludge, sludge samples with different pH values are created. The capillary absorption time for each sludge sample was measured using a capillary absorption time measuring device, and the graph shown in FIG. 1 was constructed from the obtained results. In the graph thus obtained, the PH value at the point where the capillary absorption time is the shortest corresponds to the isoelectric point of this sludge. The graph in FIG. 2 was obtained as follows.
That is, various amounts of coagulating agents (iron chloride and/or sulfuric acid) are added to the initially concentrated sludge, and the pH of the sludge is measured. From the PH of the sludge and the amount (wt%) of the coagulant relative to the solid content of the sludge,
The graph of FIG. 2 is constructed. Therefore, the PH value that gives the isoelectric point is determined from FIG. 1, and the amount (wt%) of iron chloride and/or sulfuric acid required to give this PH value is read from the graph in FIG. 2. The following are specific experimental examples conducted in accordance with the sludge treatment method of the present invention. Three similar sludge sections were prepared to conduct the experiment. Each portion consisted of a mixture of primary sewage sludge in an amount of 18 m ³ /hr with a concentration of 0.6% and excess activated sludge in an amount of 27 m ³ /hr with a concentration of 0.4% (based on solids content). (1:1 ratio). Example 1 A portion of the sludge was concentrated to a concentration of 4.5% in a gravity thickener with a rod-like stirrer.
After concentration, the PH value of the sludge was measured and found to be 6.95. Next, the value of PH corresponding to the isoelectric state of the sludge is calculated using the capillary absorption time and the PH of the sludge.
Determined from the curve in Figure 1 showing the relationship with the value of
It turned out to be 3.0. Then Suratji's PH
The amount of sulfuric acid required to lower the pH from 6.95 to 3.0 was determined from the curve in FIG. It was found to correspond to 6.7% by weight of the dry matter of the sludge.
This amount of sulfuric acid was added to the concentrated sludge.
When a PH value equal to 3.0 was reached, the sludge was heated to 70°C by means of an immersion gas burner. A flocculant, a cationic flocculant, was then added to the heated sludge in an amount of 0.150% of the dry matter of the sludge. Immediately thereafter, the sludge was mechanically dewatered using a centrifuge. The sludge had a moisture content of 72%. Sludge treated in this manner was centrifuged and the solids retention efficiency was 98-99%. The chemical oxygen demand of the centrifuge effluent is 1
g/ or less. PH of dehydrated sludge
It was mixed with lime (CaO) to restore it to its original state.
The amount of lime corresponded to 10% of the dry matter of the sludge. Example 2 A portion of the sludge was concentrated in the same manner as in Example 1 to a concentration of 4.5%. After concentration, the PH value of the sludge was measured and found to be 6.95. Next, the value of PH corresponding to the isoelectric state of the sludge was determined from the curve in Figure 1, and it was found to be 3.0. The amount of sulfuric acid and iron chloride mixture required to reduce the coagulant percentage, ie, pH, from 6.95 to 3.0 was then determined from the curve in FIG. Sulfuric acid and iron chloride were found to be 3.0% and 5.5% by weight of the dry matter of the respective sludges. in this case,
Since the pH of the sludge becomes 5.0 by adding 3.0% by weight of sulfuric acid, the pH of this sludge is further increased to 5.0.
The amount of iron chloride needed to reduce the amount from 5.5 to 3.0
% by weight (i.e. 10-4.5% by weight).
This proportion of the mixture was added to the concentrated sludge. After reaching a pH value equal to 3.0, the sludge was heated in the same manner as in Example 1. A flocculant, polyacrylamide, was then added to the heated sludge in an amount of 0.135% of the dry matter of the sludge. Immediately thereafter, the sludge was mechanically dewatered using a centrifuge. The moisture content in the dewatered sludge was equal to 70%. The sludge treated in this way was centrifuged and the effectiveness of the solids retention of the sludge was 98-99%. The chemical oxygen demand of the centrifuge effluent was less than 1 g/g.
The dewatered sludge was then treated with lime in a manner similar to Example 1. Example 3 After concentrating a portion of the sludge to a concentration of 4.5% in the same manner as in Example 1, the PH value of the sludge was measured and found to be 6.95. Next, the value of PH corresponding to the isoelectric state of the sludge was determined from the curve in Figure 1, and it was found to be 3.0. Thereafter, the coagulant percentage, ie, the iron chloride required to reduce from 6.95 to 3.0, was determined from the curve in FIG. The amount was found to be equal to 10% by weight of the dry matter of the sludge. This proportion of iron chloride was added to the concentrated sludge. As soon as a value of PH equal to 3.0 was reached, the sludge was heated in the same manner as in Example 1. A flocculant, polyacrylamide, was then added to the heated sludge in an amount of 0.135% of the dry matter of the sludge. Immediately thereafter, the sludge was mechanically dewatered using a centrifuge. The moisture content in the dewatered sludge was equal to 71%. The sludge treated in this manner was centrifuged and the solids retention efficiency was 98-99%. The chemical oxygen demand of the centrifuge effluent was less than 1 g/g. The dewatered sludge was then treated with lime in the same manner as in Example 1. Since different sludges have dissimilar properties, it is clear that the relationships represented by the curves (Figs. 1 and 2) are different and that individual curves must therefore be plotted for each type of sludge. be. To better illustrate these examples, Table 1 lists all the results of experiments performed according to the method of the invention. On the other hand, the influence of heating temperature on the effectiveness of sludge treatment was investigated. These data are given in Table 2. As shown in Table 2, the highest sludge concentrations are obtained at 65-75°C. While specific embodiments of the invention have been shown and described, various aspects thereof will be apparent to those skilled in the art. It goes without saying that it is therefore not the intention to limit the invention to the method of sludge treatment and the description of its details considered above, but that new developments may be made from the invention within the spirit and scope of the invention. stomach. The advantages of this method of sludge treatment are its simplicity, increased efficiency of the sludge treatment process, and reduced consumption of test membranes used for sludge treatment. To illustrate these advantages, Table 3 is given, which describes the data of the operation of a sewage disposal plant with a daily capacity of 1500 Km ³ of sewage according to the method of the invention, and a similar plant according to the prototype method. .

【table】

[Table] Example 1
Example 2 Example 3 Conventional method

Claims (1)

[Claims] 1. An acidic reagent is added to the sewage sludge in an amount that reduces its pH to a value corresponding to the isoelectric state of the sludge, heated to a temperature of 65 to 75°C, and then a polymer flocculant is added to the sewage sludge. A method for treating sewage sludge, characterized by charging the sewage sludge and then dewatering the sludge. 2. The sludge treatment method according to claim 1, wherein the sludge is heated by an immersion gas burner.