Wangbaozhen

Some technologies and processes of sludge reduction at home and abroad are introduced, such as: the method of feeding bacteria to protozoa and metazoa can reduce the sludge output by more than 60%, and the method of fixed submerged biofilm does not even produce excess sludge; Microbial enhancement method, using optimized strains to reduce sludge discharge16%; Adding enzyme method can decompose insoluble macromolecular organic pollutants into small molecular soluble organic substances that can be easily absorbed and utilized by microorganisms, which is not only beneficial to the degradation of organic pollutants, but also can promote the proliferation of bacteria and reduce the sludge output by 50%. In addition, a series of methods such as ultrasonic technology, ozone oxidation, Cambi process and biological cell lysis system are also introduced.

Keywords: activated sludge process to reduce the biomass of secondary sludge

Preface of 0

? The purpose of activated sludge process is to reduce BOD and sludge production at the same time. The traditional activated sludge process is not easy to meet the emission standards of nitrogen and phosphorus, and another main disadvantage is the large sludge output. In the process of biological treatment of sewage, a large amount of biological sludge will be produced, which needs to be separated, stabilized, digested, dehydrated and disposed. This requires a lot of infrastructure investment and high operating costs; From treatment to final disposal, the operating cost of sludge accounts for about 40% (drying) ~ 65% (incineration) of the total operating cost of sewage treatment plants. The final disposal of sludge often adopts landfill, reclamation or agriculture. However, with the decrease of available land, considering human health, sludge must be further treated before being used in agriculture, and the final disposal of sludge becomes more and more difficult, which makes people more interested in biological treatment processes that can reduce sludge production.

? Any sludge reduction technology should reduce sludge production without affecting process efficiency or effect. At present, some research focuses on the reduction of excess sludge from activated sludge. Activated sludge process is a mature process, and its further improvement should meet the following requirements: economic feasibility, easy operation and management, and long-term benefits. This paper summarizes the sludge reduction technologies currently developed as follows. ?

Grazing/bacterial predation of 1

? Sewage provides an ideal medium for the survival and reproduction of various microorganisms. Because no single microorganism can use all the compounds in sewage as substrates, it is best to build a complex ecosystem composed of various microorganisms, including many longer food chains, such as bacteria → protozoa → metazoa. Among them, protozoa, such as ciliates, metazoa, such as rotifers, oligochaeta, annelids and nematodes, play the role of predators at the highest end of the food chain. They prey on bacteria and convert sludge into energy, water and carbon dioxide, thus reducing the amount of sludge.

? Protozoa and metazoa of feeding families, especially oligochaetes, are well-known populations that can reduce sludge production. Oligochaeta is located at the top of the food chain, which is more in trickling filter, but less in activated sludge treatment plant, and the flora is different with different environment. This physical and ecological environment is very important for maintaining the number of oligochaeta. In order to maintain the population of planktonic worms in the activated sludge treatment plant, the worm generation time must be less than the hydraulic retention time, otherwise it will flow out with the water; For sessile worms, the above conditions are not very important when there are fillers and other attached media. Through the comparative study of adding metazoa and not adding metazoa, adding filler carrier and not adding filler carrier in the conventional activated sludge process, the sludge yield is 0. 15gMLSS/gCOD in the presence of worms, while it is 0.40 gmlss/gcod [1] under normal operating conditions.

? In aerobic process, protozoa and metazoa feed on dispersed bacteria to maintain life and promote the growth of flocculent or film-forming bacteria, while most microorganisms formed in conventional process cannot be consumed by predators. In order to overcome the selectivity of predation and promote the growth of dispersed microorganisms, Lee and Welander and others studied the treatment of synthetic wastewater in Sweden in 1996. The first stage of the system is a complete mixing reactor, with no biomass retention and short sludge retention time, which can prevent the growth of predators and promote the growth of fast-growing dispersed microorganisms. The second stage is biofilm process to ensure a long sludge age and suitable conditions for predator growth. Under these conditions, the sludge yield of the system is only 0.05 ~ 0. 1.7 gtss/gcod, which is 30% ~ 50% of that of the conventional process [2]. The new submerged biofilm wastewater treatment technology developed by Wang Baozhen of Harbin Jianzhu University uses fixed carrier filler, which increases the number of protozoa and metazoa in the aeration tank and effectively reduces the output of excess sludge. The pilot study confirmed that the surplus sludge output was only110 ~1/5 [3] of the conventional activated sludge process. Panyu Qifu New Village Sewage Treatment Plant (with a daily treatment capacity of 8000m3/d) has not produced excess sludge for more than one year since it was put into operation in May 1998, thus achieving zero sludge discharge. ?

2 Microbial reinforcement

? Sewage treatment is to use natural microorganisms to oxidize organic matter into usable food elements. For example, in the system of 1000g nutrients, 600g is used to synthesize microorganisms, and 400g is oxidized into carbon dioxide and water. This shows that the sludge can be reduced by 40% in the bacterial growth stage. If the activated sludge of a sewage treatment system can run indefinitely, there will be no sludge, which is actually impossible.

? Microbial Enhancement Not all microorganisms based on natural systems are the most effective microorganisms. In order to improve the efficiency of the sewage treatment plant, specially selected microbial strains or transgenic strains should be put into the sewage treatment plant, and the selected strains should be able to maintain and strengthen the activity of naturally occurring strains, so as to optimize and control the balance of microbial population.

? Microorganisms give priority to the use of dissolved organic matter in water before degrading insoluble organic matter. When degrading insoluble organic matter, microorganisms secrete extracellular enzymes to decompose insoluble organic matter. By selectively adding external bacteria to the system, the concentration and metabolic activity of bacteria in the system are increased. This requires that the added strains can not only provide the existing flora and promote its growth, but also inhibit the growth of a few unfavorable organisms, thus increasing the treatment efficiency of the unit.

? Exogenous microorganisms can adapt to different ranges of sewage. From the knowledge of sewage and protozoa, only the most suitable microorganisms can survive if put into the system. For example, fat cannot be used by microorganisms, but adding microorganisms that can produce lipase can quickly decompose fat and form microbial tissue components. A basic environmental microbiology company in Switzerland uses microorganisms to treat different types of sewage, and its sewage treatment plant can reduce the amount of excess sludge 16% [4]? . ?

3 metabolic termination

? In the process of sewage treatment by activated sludge, bacteria metabolize and degrade organic matter, form carbon dioxide and water at the same time, and complete the growth and replication of cells. This process is controlled by complex metabolic pathways, including catabolism (decomposing organisms) and anabolism (releasing energy for the growth of new cells). Metabolic termination is to terminate this process after catabolism is completed and before anabolism begins, thus blocking the formation of new cells.

? It is reported that many chemicals can prevent catabolism and anabolism. For example, bacteria-specific cycloserine contained in antibiotics can prevent the synthesis of cell walls. Other chemicals, such as 2,4-dinitrophenol and dicoumarin, can prevent oxidative phosphorylation by interfering with the gene chain transfer of ATP from catabolism to anabolism? 〔2〕? . Can rotenone terminate the oxidation of substrate and the connection of oxidative phosphorylated nicotinamide adenine dianhydride? 〔6〕? Many metabolic pathways depend on DNA steps, so the melting of rotenone can be widely used in different pathways. The advantage of chemical melting is that the existing equipment does not need to be greatly modified, but the disadvantage is that some chemicals will be released into the environment. ?

4. Add enzyme

? The function of enzyme is to promote the decomposition of macromolecular compounds in sewage into small molecular compounds and release bound oxygen. These simple compounds are easily utilized by many microorganisms. This is conducive to the diversity of bacteria, and can improve the activity and reproductive capacity of bacteria, which is conducive to the formation of a large number of higher organisms and promote the proliferation of higher photosynthetic organisms, thus providing a large amount of dissolved oxygen for sewage. Many biological treatment systems in the United States have applied the method of adding enzymes to control odor, treatment efficiency and sludge reduction.

? Enzymes can also be added to the water inlet pipe of the sewage treatment plant to turn the anaerobic population in the sewage into aerobic population and produce carbon dioxide instead of hydrogen sulfide. This helps to improve the performance of the primary sedimentation tank, and the growth of photosynthetic organisms and aerobic organisms also changes the ecology of the system [7].

? Algae and bacterial micelles in the trickling filter tower become photosynthetic organisms, which reduces the problem of flying insects and enables the aerobic mixing system to work under the conditions of high sludge age, high mixed liquor concentration and less dissolved oxygen. Low dissolved oxygen concentration promotes facultative bacteria reproduction, and nitrogen and phosphorus are easily converted into available nutrient sources. American BIOCOPE Company reported that using the enzyme solution developed by them, the dissolved oxygen in conventional treatment can be reduced to 0.8 ~ 1.2 mg/L, while the MLSS is 4500 ~ 8500 mg/L and the sludge age is 30 ~ 70 days, which can reduce the sludge output by 50% and cancel the sludge digestion equipment. ?

5 ultrasonic wave

? Ultrasonic wave was applied earlier in water industry. Low-intensity ultrasound is usually used to measure flow, and it is a brand-new field to apply ultrasound to sludge reduction. Ultrasonic wave produces cavitation through the alternating action of compression and expansion, which is reflected in the formation, growth and rupture of microbubbles in solution, thus crushing the cell wall and releasing the components and cytoplasm contained in the cells for further degradation. Ultrasonic cell processor can accelerate cell lysis, and when used in sludge return system, it can strengthen cell biodegradability and reduce sludge output. Used for sludge dewatering equipment, which is beneficial to sludge dewatering and sludge reduction.

? Ultrasonic waves are generated by the transducer and introduced into the sewage through the probe. The design frequency band of ultrasonic wave is 25 ~ 30 kHz. Below 25kHz, it is within the hearing range of people, causing noise problems; And when it exceeds 35kHz, the energy utilization rate is low? 〔8〕? .

? The function of ultrasonic wave is influenced by many parameters of liquid, such as temperature, viscosity and surface tension. In addition, the contact time between ultrasound and various liquids, the geometry and material of the probe are also influencing factors for the application of ultrasound. ?

6 biological cell lysis system

? Biological cell cracking system is similar to ultrasonic technology, which exerts mechanical pressure on sludge reflux system to break cell wall and release substances contained in cells. Generally, this crushing action can reduce the size of granular sludge and increase the specific surface area of organisms, which is beneficial to further decomposition. Applying this method to the endogenous respiration section of activated sludge can reduce the output of excess sludge. The secondary sedimentation tank adopting this method can reduce 50% of sludge and filamentous flora, and greatly improve the sedimentation performance and dewatering performance of sludge? 〔8〕? . ?

Ozone reduces excess sludge.

? Shima Sewage Treatment Plant in Japan used ozone technology for 9 months, and there was no residual sludge. This operation effect is due to the ozonation of part of the returned sludge by ozone. So as to improve its biodegradability and strengthen its biological oxidation degradation in the aeration tank.

? The sewage treatment capacity of this plant is 450m3/d, and the experiment shows that the sludge reduction is directly proportional to the ozone dosage and sludge treatment capacity. The experiment in this factory proved that the ozone dosage needed to completely eliminate the residual activated sludge was 0.034kg/kgSS, and the amount of the returned sludge needed to be treated was four times that of the conventional sewage treatment plant? 〔9〕? . ?

8 Cambi process: simultaneously realize sludge reduction and biogas production? 〔 10〕?

? In Cambi process developed by Purac, the organic components in sludge are transformed from insoluble state to dissolved state through hydrolysis, so that the organic matter can be used for biodegradation, that is, anaerobic digestion. This process is completed by hydrolysis at high temperature. After hydrolysis, the organic matter in the solid is greatly reduced, and then anaerobic treatment promotes the increase of biogas production, and the released carbon can also be used as a carbon source for biological nitrogen removal. The total solid content of dehydrated sludge produced by Camby process reaches 30% ~ 40%, which can be directly burned. According to the source and composition of sludge, the process can be operated intermittently or continuously.

? The hydrolysis process includes four basic steps: uniform mixing, preheating, hydrolysis and flash evaporation. Sludge is pre-dehydrated by centrifugal dehydrator or filter press, and the sludge is circulated in the uniform mixing tank by immersion pump. In addition, the sludge is stirred evenly with a blender. The main part of hydrolysis takes place in the hydrolysis tank, which works at 10 bar, at 180℃ and is heated by steam. For the sludge produced after municipal sewage treatment, the reaction time of hydrolysis reactor is generally 30 minutes. After hydrolysis, protein, fat, cellulose and other complex organic substances in the sludge are transformed into simple organic substances which are easy to biodegrade. The last step is flash evaporation, which uses the pressure difference between the hydrolysis tank pressure (10bar) and the working pressure of the flash tank (1 ~ 3 bar) to send the sludge in the hydrolysis tank to flash evaporation, and the steam and high-temperature supernatant generated by the sludge in the flash tank flow back to the preheating tank to be mixed and diluted with the new sludge, and the new sludge is heated to 80-90℃. The working pressure of preheating tank is 2 ~ 3 bar to achieve high heat recovery rate. The sludge in the flash tank is pumped to the anaerobic treatment tank or other biological treatment equipment. With the help of anaerobic treatment, 60% ~ 80% of the energy in sludge is converted into biogas.

? A Camby process sludge treatment plant was established in Hamar, northern Oslo, Norway, which consists of Camby process (hydrolysis and anaerobic digestion), chemical recovery and drying. The amount of sludge sent to the factory is 1000t/ month (20%TS), which is reduced to 290t/ month after dehydration and 66t/ month after drying and extraction, that is, the amount of sludge is reduced by 93%. TS decreased by 70% after drying and extraction. The system is completely closed, and the sludge does not release odor when heated. The plant also provides Hamar with heat energy generated by biogas and recycled chemical products, such as heavy metals extracted by extraction, thus reducing operating costs and heavy metal content in sludge. The final sludge residue will be used for landfill restoration and vegetation. The biogas produced in the hydrolysis process only accounts for 10% of the biogas production, and the estimated biogas production is 2950 m3/d[ 10]? .

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