The methods of sludge concentration mainly include gravity concentration, air flotation concentration, belt gravity concentration and centrifugal concentration, as well as microporous concentration, diaphragm concentration and biological flotation concentration. Natural sedimentation and gravity separation are the most energy-saving sludge concentration methods, and no additional energy is needed. Gravity concentration is only a process of sedimentation and separation, and it is the main method of sludge concentration by forming a high concentration sludge layer in sedimentation. Independent gravity concentration is completed in an independent gravity concentration tank, and the process is simple and effective, but long residence time may produce odor, which is not suitable for all sludge; If it is applied to the concentration of excess sludge from biological phosphorus removal, a large amount of phosphorus will be released, and its supernatant needs chemical phosphorus removal. Gravity concentration method is suitable for primary sludge, chemical sludge and biofilm sludge.
Sludge treatment: the principle of centrifugal concentration method is to concentrate by using centrifugal force with different solid-liquid specific gravity in sludge. Centrifugal concentration method is characterized by its own system, good effect and simple operation; But the investment is large, the power cost is high, and the maintenance is complicated; Suitable for biological and chemical sludge in large and medium-sized sewage treatment plants.
2) Sludge treatment
The purpose of stabilization treatment is to degrade the organic matter in sludge, further reduce the water content of sludge, kill bacteria and pathogens in sludge and eliminate odor, which is the key step of effective utilization of sludge. Sludge stabilization methods mainly include composting, drying and anaerobic digestion. Anaerobic digestion: Anaerobic digestion is a commonly used stabilization technology in sludge treatment. Sludge anaerobic digestion is also called anaerobic biological stability of sludge. Its main purpose is to reduce high-energy substances in the form of carbohydrate, protein and fat in raw sludge, that is, to transform high-molecular substances into low-molecular-weight substance oxides through degradation. Anaerobic digestion is an anaerobic biochemical reaction to decompose organic matter in sludge under anaerobic conditions, which is an extremely complicated process. Aerobic digestion of sludge appeared in 1950s, which is very similar to activated sludge process. When foreign nutrients are consumed, microorganisms consume their own bodies to generate energy to maintain life activities. This is the endogenous metabolic stage of microorganisms. Endogenous metabolites of cell tissue are CO2, NH3 and H2O under aerobic conditions, while NH3 will be further oxidized to nitrate under aerobic conditions. The reaction of aerobic digestion of sludge can be expressed by the following equation:
C6H7NO2+7O2→5CO2+NO3-+3H2O+H+
In the above formula, C6H7NO2 is the element composition of cell tissue.
The method has the advantages of high degradation degree, odorless stability, easy dehydration, high fertilizer content, simple operation and management, low capital construction cost and the like. However, the operation cost is high, the amount of digested sludge is small, and the degradation degree fluctuates greatly with temperature. The discussion of composting technology began at 1920. Composting systems can be divided into three categories: strip composting system, static aerobic composting system and equipment composting system. The sludge of urban sewage treatment plant contains a lot of nutrients such as nitrogen, phosphorus and potassium, which can promote the growth of plants and crops and has good fertilizer efficiency. After composting, it can achieve the purpose of stabilization, harmlessness and resource utilization. Composting is a stable process of aerobic decomposition of organic matter by psychrophilic bacteria and thermophilic bacteria. Its characteristics are that it can generate a certain amount of heat, the high temperature lasts for a long time, and it can be harmless without external heat source. The general technological process of composting is mainly divided into four processes: pretreatment, primary fermentation, secondary fermentation and post-treatment. After composting, the properties of sludge are improved, the water content is reduced (below 40%), and it becomes loose, dispersed and fine-grained, which can kill pathogenic bacteria and parasites (eggs) and is convenient for storage, transportation and use.
Lime stabilization technology Lime stabilization technology began in 1950s. Under the condition of adding lime and keeping a certain pH value for a certain time, infectious bacteria can be killed, and odor can be prevented and suppressed. This technology is simple in operation, low in cost and easy to dehydrate after treatment. The final disposal of sludge can be agricultural or sanitary landfill.
Ferment sludge into organic fertilizer. If you add some cow dung, it will ferment into high-quality organic fertilizer. The specific operation method is as follows: 1, adding bacteria. 1 kg Gymboree fertilizer starter can ferment about 4 tons of sludge+cow dung. It is necessary to add about 30-50% organic matter such as cow dung, straw powder, mushroom residue, peanut shell powder, rice husk and sawdust according to the weight ratio in order to adjust ventilation. If rice husk and sawdust are added, the fermentation time should be prolonged because of their high cellulose lignin. Strain dilution: Add 5- 10 kg of rice bran (or substitutes such as bran and corn flour) to each kg of starter, stir and dilute evenly, and then spread evenly into the pile, the use effect will be better. 2. Stacking: After the materials are prepared, pile them while spreading bacteria. The height and volume of piles should not be too short or too small. Requirements: pile height 1.5-2m, width 2m and length 2-4m. 2. Stir well and ventilate. Gymboree fertilizer starter needs good oxygen fermentation, appropriately increase oxygen supply measures, stir evenly, frequently turn over and ventilate. Otherwise, it will lead to anaerobic fermentation, produce odor and affect the effect. 4. Moisture. The water content of fermented materials should be controlled at 60 ~ 65%. Moisture judgment: Hold a handful of materials tightly, see a watermark between your fingers, do not drip, and it is appropriate to spread it when landing. Less moisture leads to slow fermentation, and more moisture leads to poor ventilation, which also leads to the work of "spoilage bacteria" and produces odor. 5. temperature. The initial temperature should be above 15℃ (it can be operated in all seasons and is not affected by seasons, so it is suitable for indoor or greenhouse fermentation in winter), and the fermentation temperature should be controlled below 70-75℃. 6. done. From the second day to the third day, when the temperature reaches above 65℃, the boat will capsize. Generally, fermentation can be completed within one week. The substance was dark brown, and the temperature began to drop to normal temperature, indicating that the fermentation was completed. If there are too many auxiliary materials such as sawdust, sawdust and rice husk, the fermentation time should be extended until it is completely decomposed. Fermented organic fertilizer is effective, safe and convenient to use, resistant to diseases and promoting growth, and can also improve soil fertility. Sludge dewatering is an important link in the whole sludge treatment process. Its purpose is to enrich solids, reduce sludge volume and create conditions for the final disposal of sludge. In order to separate the liquid phase and solid phase of sludge, the binding force between them must be overcome, so the main problem encountered in sludge dewatering is energy. According to different forms of binding force, different external measures can be taken purposefully to obtain different dehydration effects. Sludge dehydration and drying include natural dehydration, mechanical dehydration and heat treatment drying.
After concentrated digestion, the sludge still has a moisture content of 95%~97%, which is easy to rot and stink and needs to be dried and dehydrated. The commonly used dehydration methods are natural drying and mechanical dehydration. Biogas plants such as reeds can also be used for better dehydration. This technological innovation adopts sludge washing process, which washes out the organic matter in the sludge first, separates the inorganic matter from the sludge soil, and then concentrates the organic sludge for high-temperature anaerobic digestion. Washing the precipitated sludge to wash away half of the solid inorganic sludge soil in the sludge, so as to reduce the biological treatment capacity by half and save the engineering investment and treatment cost; Separate treatment of organic sludge removes the precipitation of inorganic sludge soil in the reactor, reducing equipment wear and reactor maintenance; Washing the precipitated sludge, washing away most of the heavy metals and inorganic sludge soil that are easy to precipitate, and improving the quality of organic fertilizer; The washed sludge soil can also be used to produce colored pavement bricks and permeable bricks. Other innovative processes: ultra-high temperature anaerobic digestion, multi-stage anaerobic digestion, biogas residue floating, etc. The speed of sludge biological treatment has increased several times, and the biogas production has increased by more than 20%.
In the biological treatment system of precipitated sludge, the engineering design innovation adopts the design of buried, compact and multistage digestion reactors, integrates multiple independent anaerobic digestion reactors, saves building materials, and adopts concrete structure with low cost. The existing anaerobic digestion reactors at home and abroad generally adopt the above-ground structure, which can make the equipment easy to maintain, facilitate the discharge of biogas residue and prevent the precipitation of biogas residue. The engineering design of biological treatment system has well solved the maintenance of supporting equipment and the precipitation of biogas residue. The system is equipped with few equipment, only a few pumps are needed, that is, the pump is broken and can be replaced in less than 20 minutes to ensure that the equipment maintenance will not stop production. After the precipitated sludge is washed, the inorganic sludge soil which is easy to precipitate is removed, and the organic sludge floats completely without precipitation under the action of the blowing-floating system. Buried anaerobic digestion reactor not only needs less investment and does not occupy land, but also can prevent earthquakes and lightning strikes, prolong service life and reduce heat loss of digestive system.
Take the design of a washing biological treatment plant that can treat 600 tons of precipitated sludge with 80% water content every day as an example. The treatment capacity and sludge moisture content are completely the same as those of Dalian Xiajiahe Sludge Treatment Plant (the first place in the national demonstration project of 20 10), and only 20% investment is needed. The daily operation cost of the treatment plant is relatively low. Biogas, a by-product of sludge treatment, can generate income, biogas residue can be made into organic fertilizer to generate income, and sludge soil can produce colored pavement bricks and permeable bricks to generate income. The biological treatment of precipitated sludge does not need government subsidies and sewage treatment plants to pay for sludge concentration and transportation, and can obtain considerable economic benefits. Compared with Dalian Xiajiahe Sludge Treatment Plant, the daily operation cost of this treatment plant saves government subsidy 135 yuan (the lowest price in China) when treating one ton of precipitated sludge with 80% water content, and the total sludge concentration fee and transportation fee paid by the sewage treatment plant exceeds that of 200 yuan. Precipitated sludge washing and biological treatment plant covers a small area and is planned to be built in sewage treatment plant, which is suitable for sewage treatment plants of all sizes. Small sewage treatment plants can add local kitchen waste, septic tank waste, municipal sewage sludge and sludge from surrounding enterprises and small sewage treatment plants in villages and towns for treatment, so as to increase the treatment scale and realize profitability. The existing sludge treatment technologies at home and abroad have not reached the level of free sludge treatment and disposal. (WAO for short)
Sludge treatment technology
Wet oxidation is a chemical process that uses oxygen in the air as oxidant, and decomposes organic substances into inorganic substances such as carbon dioxide and water or small molecular organic substances in the liquid phase at high temperature (125℃~320℃) and high pressure (0.5~20MPa). Because the material structure of excess sludge is very similar to that of high concentration organic wastewater, this method can also be used to treat excess sludge. Wet oxidation treatment of excess sludge is the most successful application field of wet oxidation, and more than 50% of wet oxidation devices are used to treat excess sludge. This technology was put forward by Japanese scholars such as H. Yasui. In this process, the digestion of excess sludge and sewage treatment are carried out simultaneously in the same aeration tank. The process is divided into two processes, one is ozone oxidation and the other is biodegradation.
Part of the sludge settled in the secondary sedimentation tank directly flows back to the aeration tank, and the other part is treated with ozone and then flows back to the aeration tank. After the sludge is treated by ozone, its biodegradability can be improved, and at the same time it can be biologically treated with sewage in the aeration tank. Moreover, after ozone treatment, part of the sludge (1/3) will be inorganic. Therefore, as long as the operation is proper, the net increase of sludge in the process of sewage treatment can be equal to that of inorganic sludge, so as to achieve the goal of no excess sludge. High-speed bioreactor technology is developed on the basis of using soil to treat sludge. Using microorganisms in soil to treat sludge, because the system is open, it will be affected by air temperature and soil humidity, which limits the time and area of soil utilization.
American SWEC Company began to develop high-speed bioreactor in 1980s. This technology integrates sludge dehydration, digestion and drying, and puts the whole process of soil treatment in an indoor closed circulation system. After nearly 20 years of research and development, Texaco has matured and popularized the high-speed bioreactor technology. The core part of the whole operation system is bioreactor, which consists of two areas: the upper part is the area where sludge and soil are mixed, which makes the sludge load uniform, and the organic part of sludge is biodegraded in this area; The lower part is a gas-liquid separation zone, so that the liquid does not stay in the soil to increase the oxygen transfer rate. When the sludge with high load rate is treated by this system, the organic components in the sludge will be degraded by 70%~80%, and the removal rate of suspended solid concentration will reach 45%~60%. Sludge with the concentration of 5000 ~ 30000 mg/L discharged from sedimentation tank can directly enter the system without any pretreatment. Compared with other biological treatment technologies, the system needs less energy, can run continuously, and can maintain the optimum temperature for microbial degradation, especially suitable for sludge treatment processes limited by natural conditions or with high soil humidity.