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Municipal sewage treatment

Urban sewage treatment is generally targeted at removing BOD (biochemical oxygen demand) substances. In large-scale sewage treatment plants, sediment-centered sewage primary treatment and biological treatment-centered sewage secondary treatment are often used. In order to remove substances such as nitrogen and phosphorus, the sewage is also subjected to tertiary treatment after biological treatment. In the future development of urban sewage treatment plants, in addition to the increasing number, the proportion of secondary treatment plants will gradually increase, and the construction of tertiary treatment plants will begin.
When the municipal sewage is treated, the substances to be treated are mainly organic substances and solid substances represented by BOD and COD, and the treatment method is a biological treatment method such as an activated sludge method or a biological rotary method. Moreover, in recent years, nitrogen and phosphorus have been partially eliminated due to the marked increase in eutrophication in closed waters such as lakes and water bays.
In addition to domestic sewage, the sewage treatment plant also accepts part of the production wastewater and treats it at the same time. Most of the treatment methods use the activated sludge method.
Sludge properties and treatment methods
1. Sludge treatment process
The sludge treatment process is roughly divided into: (1) concentration, (2) stabilization (anaerobic digestion of sewage sludge), (3) pretreatment (dosing, heat, freezing, etc.) (4) dehydration, (5) Drying, incineration, and (6) six processes such as treatment and utilization. The common process is as follows:
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(1) Concentration
Concentrated by gravity, the concentrated SS concentration can range from 1% to 2~3%. Recently, in order to further increase the concentration, a pressurized flotation and a centrifugal separator are employed.
(2) Stabilization
The anaerobic digestion of sewage sludge (raw sludge) can reduce the volume by decomposition of organic matter, and has the effect of killing pathogenic bacteria.
(3) Pretreatment
It is generally pretreated with a polymeric dehydrating agent or ferric chloride and hydrated lime.
The polymer dehydrating agent is suitable for a centrifugal dewatering machine which requires formation of coarse flocs, that is, a belt pressure type dewatering machine. It can be applied to ferric chloride and slaked lime by a vacuum dehydrator and a filter press, but it can also be treated with a polymer dehydrator.
(4) Sludge dewatering
In the treatment of excess sludge, the ratio of belt presses and centrifugal dewatering machines is increasing in recent years. Most of the centrifugal sludge dewatering machines are used in the treatment of manure sludge. However, belt presses also have an increasing trend. From the current and future developments, it is desirable to reduce the water content without increasing the ash content in the filter cake. This requires attention to the use of belt pressure type dewatering machines and filter presses. Polymer dehydrator.
(5) Drying and incineration
In the wastewater sludge treatment, a continuous drying, multi-stage incinerator, fluidized bed incinerator, and rotary furnace are used. The moisture content of the fecal sludge is relatively high, the amount of cake in each treatment plant is small, and the odor treatment (incineration and deodorization) is required, and drying, incineration (mixing with feces, burning with a burning furnace) is often performed. Deodorization three processes.
(6) Utilization (resources)
As a method of utilizing and recycling the sludge of the domestic sewage treatment plant, it is mainly considered to be reduced to the soil. Now, there have been examples in which the dewatered filter cake of fecal sludge is directly reduced to farmland after drying. However, in the future, methods for producing stable organic fertilizers should be studied. In addition, as a result of the study, we are carrying out research on the incineration ash mixture and the new attempt to recover valuable substances (methanol) in the sludge.
2. The nature and impact of sludge
The sludge obtained after the domestic sewage treatment is mainly composed of microbial flocs separated by the activated sludge treatment method, and is composed of undecomposed organic matter such as cellulose, sand, metal hydroxide, etc., and the ratio thereof is different, which affects the sludge. The dehydration effect and the adaptability of the polymer dehydrating agent. In order to determine the nature of the sludge, the following analysis items are required.
PH, conductivity, total solids, SS, VSS/SS (the proportion of organic matter in SS), fiber composition / SS, ash / SS, metal hydroxide / SS (especially the AL2O3 component of the flocculating sludge mixed) .
(1) VSS/SS (the proportion of organic matter in SS)
The main factors determining the type of dehydrating agent are determined, as well as the dosage and dewatering effect (filter cake moisture content). When the VSS/SS ratio is high, it has the following effects:
a, sludge SS concentration decreased.
b. Increase the moisture content of the dewatered filter cake.
c. Affect the effect of the polymer dehydrating agent.
(2) SS concentration
In the same sludge, the higher the concentration, the higher the dehydration.
(3) Fiber/SS ratio
If there is a large amount of fiber, the dehydration property is also high. Especially more obvious on belt filter presses.
(4) Metal hydroxide
Due to the metal hydroxide generated by flocculation treatment of sludge mixing and factory drainage, the sludge concentration is lowered, which seriously affects the dehydration process. However, in the case of fecal sludge, there is a case where the effect is improved by the incorporation of metal hydroxide.
3. General properties of sewage and manure sludge
A, sewage sludge
The sewage sludge is roughly classified into raw sludge (primary sludge and excess sludge) and digested sludge. Due to the digestion treatment, the organic component is lowered and the pH is raised. In addition, the fibrous component is reduced.
B, manure sludge
The fecal sludge is mixed according to the treatment method (1 time, 2 times treatment method), whether there is three treatment sludges, and the removal rate of the feces during pretreatment, and the nature and dehydration property are different.
(1) Anaerobic digestion
The secondary treatment sludge (excess sludge) is characterized by high pH (7.0 to 8.0) and slightly lower VSS/SS (65 to 80%). The sludge once is often different in 2 sludge treatments. At this time, it is generally not administered. For this reason, the recovery rate of SS is relatively low, about 70%, and the water content is about 75%.
(2) aerobic treatment
The sludge is often mixed for 1 time and 2 times (the sludge is not excluded as the case may be). The sludge pH is neutral and the VSS/SS is as high as 75-90%. The moisture content of the dewatered filter cake is slightly higher than that of the anaerobic digestion method. However, in the pretreatment, when the removal rate of the feces is low, the fiber content is increased and the dehydration is easy.
The sludge was treated three times, or treated separately or mixedly, and the moisture content of the filter cake tends to increase as compared with the treatment of 1 or 2 times. However, the mixing process cannot be generalized.
4. Types and methods of use of dehydrating agents
(1) Properties of the dehydrating agent
The main component of the sludge, the microbial floc, has a negative charge, and in order to flocculate it, a cationic dehydrating agent must be used. The centrifugal dewatering machine and the belt pressure type dewatering machine form coarse and unbreakable flocs with a molecular weight of 1 million or even more than 10 million. There are a variety of commonly used cationic polymers, but mainly polymers of amino methacrylate and amino acrylate, and Mannich denatures of polyacrylamide (aminomethylated denatured products).
The reason is summarized as the following two points.
a) High molecular weight products are readily available (and there are also more than 15 million molecular weights).
b) The former is a copolymer with acrylamide, and the latter can obtain products of different cationic degrees by adjusting the cationic denaturation conditions.
A method for determining the cationicity of a dehydrating agent is usually a colloid titration method. According to the degree of cationicity, it is divided into high, medium and low cations.
(2) Relationship between sludge properties and dehydrating agent
As described above, the value of VSS/SS in the sludge property item has a great influence on the type and effect of the dehydrating agent. That is, when VSS/SS is large,
a) A dehydrating agent with a high degree of cationicity is required.
b) The amount of dehydrating agent should be increased.
When the sludge is mixed and produced by sludge, VSS/SS is not considered, and the Mannich denature of polyacrylamide can be used.
When the SS concentration is high, the high molecular weight dehydrating agent is effective, and when the SS concentration is low, the lower molecular weight dehydrating agent is effective.
The sludge with a large amount of fiber/SS has a low moisture content of the filter cake, but it has nothing to do with the dehydrating agent used.
The dehydrating agent used when mixing flocculated sludge (polyaluminum hydroxide-containing) moves to the low-cationic side, and is effective using a Mannich-type dehydrating agent.
The pH of the sludge is independent of the cationicity and molecular weight of the dehydrating agent. However, it should be noted that when the pH is high, the functional group is a dehydrating agent having a structure of a tertiary amine or a tertiary amine, and the effect is lowered. The amino methacrylate and the amino acrylate dehydrating agent usually have a quaternary ammonium functional group.
(3) Relationship between type of dehydrator and dehydrating agent
Because the centrifugal dewatering machine performs solid-liquid separation under high centrifugal force of 2000~3000G, it can form stronger flocs with high molecular weight dehydrating agent compared with belt press. In the case of a belt press, when the molecular weight is too high, the viscosity of the dehydrating agent remains on the floc, so that the peeling property of the filter cake on the filter cloth is poor.
If observed with a cation, the belt press requires a high degree of cationicity for the same VSS/SS. In addition, the centrifugal dewatering machine is used in a large amount for the amount of the dehydrating agent to be dosed.
(4) Effective use of dehydrating agent
a, should understand the quality and changes of the treated sludge.
b. Dehydrating agents suitable for the nature of the sludge and the type of dewatering machine should be selected.
c. Pay attention to the injection point, reaction conditions and dosage of the dehydrating agent.
d. Dissolve and store the dehydrating agent carefully.
(5) Dissolved water quality of dehydrating agent and preservation of solution
Some water quality projects that seriously affect the dehydrating agent as dissolved water quality include: M alkalinity (100 ppm or more), phosphate ion, and the like. As a dehydrating agent, the tendency is that the lower the cation, the more susceptible it is to the influence. Recently, in sewage treatment plants, there is a case where treated water is used as dissolved water, but this requires consideration of shortening the storage time of the solution.
The storage time of the solution is, in principle, controlled within one day. However, when using low cationicity, the longest time should be controlled within two days, and the high cationicity should be controlled within seven days.