JPS5861899A - Treatment for sewer sludge - Google Patents

Abstract

PURPOSE:To improve the solid-liquid sepn. of digested sludge within the range of a digestion process by digesting the initial settling basin sludge produced in sewage treatment facilities anaerobically then mixing the sludge with excess activated sludge and subjecting the sludge to solid-liquid sepn. CONSTITUTION:The initial settling basin sludge produced in sewage treatment facilites are digested anaerobically until 60-75% the org. materials thereof are decomposed. Thereafter, the sludge is mixed with excess sludge and the mixture is subjected to solid-liquid sepn. A dehydration treatment and a combination of a concn. treatment and the dehydration treatment are suitable as the operations for solid-liquid sepn. A settling method, a floating method, and centrifugal concn. can be utilized for the concn. treatment and mechanical dehydration, etc. for the dehydration treatment. This method may cause a decrease in the volume of gases generated as compared to the prior art by as much as the digestion of the excess activated sludge is omitted, but the flocculating properties that the excess activated sludge possesses are acted effectively in the stage of the solid- liquid sepn. by mixing the excess activated sludge as it is with the digested sludge of the initial settling basin sludge.

Description

[Detailed description of the invention] The present invention relates to a method for treating sewage sludge.

With the energy-saving and oil-saving boom that has been going on for the past few years, many plans are being made in the field of sewage treatment to anaerobically digest the sludge generated at treatment plants and recover the gas generated as some kind of energy source. I've come to Kfi. In addition to the conventional anaerobic digestion system for sewage sludge, various efforts have been made to recover as much energy as possible. However, when anaerobic digestion is not evaluated simply based on energy recovery, but is viewed as a sludge treatment method, the solid-liquid separability of KFi must be taken into consideration.

It seems that #C in books etc. has been written for some time that anaerobic digestion of sludge improves solid-liquid separation, but in reality this is not the case, and the sludge before anaerobic digestion (hereinafter referred to as mixed raw sludge) solid-liquid separation is not better than that of This fact is clearly expressed numerically in foreign books as well. The quality of solid-liquid separation affects the amount of chemicals and electricity consumed in the solid-liquid separation process, and even if the solid-liquid separation of busy anaerobic digested sludge is worse than that of mixed raw sludge, For example, when evaluating anaerobic digestion methods, it is necessary to comprehensively consider the advantage of energy recovery and the disadvantage of consuming a large amount of energy in the solid-liquid separation process.

However, what has been accomplished so far regarding anaerobic digestion -3- One of the ideas that has been developed is energy recovery.

Little thought was given to improving solid-liquid separation. When trying to re-evaluate anaerobic digestion, improving solid-liquid separation cannot be ignored and can be said to be the most important issue.

In terms of improving the solid-liquid separation of digested sludge, the present invention does not attempt to improve the solid-liquid separation by applying some external factor to the digested sludge discharged from the digestion tank, but rather improves the solid-liquid separation within the scope of the digestion process. The purpose of the present invention is to provide a method for treating sewage sludge that can obtain digested fij mud with improved solid-liquid separation compared to conventional methods, in which sedimentation tank sludge is first subjected to anaerobic digestion. It is characterized in that it is then mixed with surplus activated sludge and subjected to solid-liquid separation.

In general, mixed raw sludge in a sewage treatment plant is a mixture of the initial settling tank sludge and surplus activated sludge in the ratio of generation amounts. However, not only are the original properties of the initial settling tank sludge and surplus activated sludge very different, but also the conditions of digestion when they are anaerobically digested, and the properties of the digested sludge obtained. im 1982-61899 (241-Digesting two types of sludge with different properties, which show considerable differences, in the same digestion tank is important from the standpoint of efficient energy use, and also because of the waste of digested sludge. It is not preferable from the viewpoint of solid-liquid + fermentation, and a digestion method that takes advantage of the properties of each, especially the digestion conditions, is desired.

Describing the digestion status of the two types of sludge and the properties of the digested sludge, in general, the total solids (hereinafter referred to as 0%) in the initial settling tank sludge.

The proportion of volatile solids (hereinafter referred to as VS) in the T8 (hereinafter referred to as VS) is almost the same as that of surplus activated sludge, or slightly lower than that of surplus activated sludge, but in the constituent substances of VS, carbohydrates and fat are higher than surplus activated sludge. Since it contains more protein and less protein, the decomposition rate of VS is quite fast, and the decomposed unit weight of VS
There is also a large amount of gas generated. It was long. In terms of solid-liquid separation of digested sludge, the first digested sedimentation tank sludge is busy with settling and thickening, the chemical injection rate during dewatering is low, and the amount of solids processed is large. In comparison, for the same digestion time, digested surplus activated sludge lacks sedimentation properties, has a high chemical injection rate during dewatering, and has a small throughput of solids.

5- Initial settling tank sludge and surplus activated sludge show such differences, and it is not preferable to digest them together; it is necessary to design a digestion process that takes advantage of the characteristics of each digestion situation. Compared to the conventional method of digesting primary sedimentation tank sludge and surplus activated sludge together without distinguishing them, this method does not significantly impair the advantage of anaerobic digestion, which is the recovery of energy through gas generation, and is much more efficient. It is possible to obtain a digestion method that satisfactorily overcomes the problem of solid-liquid separation of digested sludge.

These effects become greater as the ratio of the amount of initial settling tank sludge generated to that of surplus activated sludge is higher.

The process of the present invention was completed based on this knowledge. In other words, the digestion status of surplus activated sludge is much worse than that of primary settling tank sludge;
Digesting the surplus activated sludge actually impairs the solid-liquid separation of the digested primary sedimentation tank sludge, so the surplus activated sludge is not digested at all and is mixed with the digested sludge of the primary sedimentation tank sludge. The process of the present invention involves solid-liquid separation. In addition, in anaerobic digestion, the sedimentation tank sludge is initially
It is preferable to carry out until 75% is decomposed.The solid-liquid separation operation is preferably a combination of dehydration treatment, thick ring and dehydration treatment, and the concentration treatment is performed by sedimentation method, flotation method, centrifugal concentration. Dehydration methods such as hajo dehydration and other dehydration methods can be used.

In the process of the present invention, the amount of gas generated may be reduced compared to the conventional method due to the omission of digestion of excess activated sludge; By mixing with the digested sludge, the flocculating properties of the surplus activated sludge work effectively in the solid-liquid separation operation, resulting in a more efficient solid-liquid separation compared to conventional methods.

In solid-liquid separation, the cohesiveness of sludge is an important indicator,
Even if other factors related to cohesiveness are the same, solid-liquid separability shows considerable differences depending on the presence or absence of cohesiveness or the strength of cohesive force. By not anaerobically digesting the surplus activated sludge and mixing it after the initial settling tank sludge has been digested in a state that retains its flocculating properties, -7- the solid-liquid separability of the digested sludge of the initial settling tank sludge will be impaired. Instead, they work to promote it.

Originally, the amount of surplus activated sludge generated is smaller than that of the initial settling tank sludge (the value of vs for TS also changes).
) The decomposition rate is low, and the amount of gas generated per unit weight of decomposed vB is also small, so the amount of gas generated, VB(T
Since the amount of reduction in S) is considerably smaller than that in the initial settling tank sludge, even if the amount of gas generated is smaller than in the conventional method, the difference is small and is rather a long-standing issue. This disadvantage is more than offset by the fact that the poor solid-liquid separation is overcome.

In addition, because the initial settling tank sludge vB decomposition is fast, the number of days needed for digestion to decompose 60 to 75% of the organic matter is short, and since only the initial settling tank sludge is digested, the digestion tank does not need to be heated. The energy required for this method is also significantly lower than that of conventional methods. Therefore, if this point is taken into consideration, the fact that the amount of gas generated may be smaller than that of the conventional method is not a disadvantage. 1. ! 58-61899
(3) As mentioned above, since the basic process is to anaerobically digest only the initial sedimentation tank sludge, mix it with excess activated sludge, and perform solid-liquid separation, the present invention Not only can this method sufficiently overcome the poor solid-liquid separation, which was the biggest problem with anaerobically digested sludge, but it can also perform energy recovery, which is an advantage.

Next, examples of the present invention will be described.

Real lll1 example 1 T 84.3%, Vii 3.4%, v87T
When 877.5% of the primary sedimentation tank sludge was put into an anaerobic digestion tank with an internal temperature of 35°C for 1.411 days and digested for 10 days, 73% of the input vs was decomposed and the gas was 32.597/
B occurred. The digested primary sedimentation tank sludge has a Ta2.
1%, v s O, 9% e v8/T 848.1
% and Natsuta.

On the other hand, T 82.8%, V B 2.0%, V 8
/T 574.2% of the surplus activated sludge and the initial sedimentation tank sludge were adjusted to match the generated amount ratio per solid matter. 1. I
When mixed with the previously digested primary sedimentation tank sludge at J/B, T
The a was 2.39-% and the VS was 1.4%.

This mixed sludge was dehydrated using a vacuum dehydration tester using ferric chloride and slaked lime as coagulation aids. As a result, TS of ferric chloride
When 70% of slaked lime is added, the moisture content of the dehydrated cake is 77.1%, and the solids throughput (Net) is 21.
8 M/-h was obtained. However, when this mixed sludge is dehydrated using a cationic polymer flocculant in a belt press type pressure dehydrator, the polymer flocculant is required at 0.65% per TS;
The moisture content of the dehydrated cake was H78.5%. Note that the size of the digestion tank is 141 mm.

Example 2 TS4.3%, V 83.4%, V8/7877.5
% of the initial settling tank sludge was put into an anaerobic digestion tank with an internal temperature of 35°C for 8 days, and 6% of the input vs.
8% decomposed and gas was generated for 22,617 days.

And digested first sedimentation tank sludge #1T82.0%, VB
1.1%, V S/T B 52.5%t! =lk ivy. On the other hand, TS0.45%, VB0.35%, vs/T
When 878.0% of surplus activated sludge is mixed with the previously digested primary sedimentation tank sludge at a rate of K 7 J/day to balance the -10- generation amount per solid matter with the primary sedimentation tank sludge, T B 0.65%.

VB was 0.45%.

When this mixed sludge was concentrated by gravity sedimentation for one day, the volume of the thickened sludge became F123% and its concentration V12.8%. When this was dehydrated using a vacuum dehydration tester using ferric chloride and slaked lime as flocculation aids, the water content of the dehydrated cake was 76.3% by adding 15% ferric chloride and 70% slaked lime per TS.
The amount of filtration of 22.1 was also obtained. In addition, if this mixed sludge is dehydrated using a cationic polymer flocculant in a belt press type pressure dehydrator, the polymer flocculant will be reduced to 0 per TS.
．． 65% is required, and the moisture content of the dehydrated cake is 1178.5%.
It became. Note that the capacity of the digestion tank is Fi81.

Next, as a comparative example, a case where processing is performed using a conventional method will be described.

Comparative example? 83.6%, vs 2.6%, V 8/T B
When 76.3% mixed raw waste was fed into an anaerobic digestion tank at 35°C at 2.5 knots/day and retained for 25 days, the amount of v
49% of s was decomposed and 27.4 wts of gas was generated. As a result, the digested sludge FiT 82.3%, VB 1.4%,
”/1862.1%.

This digested sludge was dehydrated using a vacuum dehydration tester using ferric chloride and slaked lime as a coagulation aid. As a result, when 15% of ferric cyclization and 100% of slaked lime were added per TS, the moisture content of the dehydrated cake was 80.2% and the solid matter throughput (Net) was 8.1 g/h. Furthermore, when this digested sludge is dehydrated using a cationic polymer flocculant in a belt press type pressure dehydrator, the polymer flocculant is required at 0.85% per TS, and the water content of the dehydrated cake is 82.2%. became. In this case, the size of the digester is Vi62.5j.

Claims (1)

[Claims] L. A method for treating secondary sewage sludge, which comprises anaerobically digesting primary sedimentation tank sludge generated in a sewage treatment facility, and then mixing the sludge with excess activated sludge for solid-liquid separation. 2. The method according to claim 1, wherein the first settling tank sludge is anaerobically digested until 60 to 75% of its organic matter is decomposed.