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Ensuring Stable Wastewater Treatment Standards in Winter

2025-01-21
  1. Key factors for stable and standard-compliant sewage treatment in winter

(I) Impact of temperature on sewage treatment

The temperature is low in winter, and the water temperature also decreases accordingly, which has many effects on sewage treatment. First of all, microorganisms play a core role in the biological treatment of sewage, such as bacteria and other microorganisms in the activated sludge method. However, their activity is sensitive to temperature. The suitable temperature for nitrification reaction is 20-30℃. When it is below 15℃, the nitrification rate decreases and it stops completely at 5℃; the suitable temperature for denitrification reaction is 20-40℃. When it is below 15℃, the proliferation rate of denitrifying bacteria decreases and the metabolic rate also decreases. In the northeast region, the sewage temperature in winter is around 10℃ or even lower, which is far from the optimal temperature for nitrifying and denitrifying bacteria. This seriously affects the nitrogen removal efficiency, especially nitrifying bacteria are more susceptible to low temperatures than denitrifying bacteria, resulting in insufficient nitrification reaction. If it is not properly controlled, NH3-N instability is very likely to occur.

(II) Changes in influent water quality

The influent volume is relatively small in winter, and the proportion of industrial wastewater has increased, which has led to a relatively higher concentration of influent pollutants. Higher pollutant concentrations put a greater load on the sewage treatment system, making it more difficult to treat. In addition, industrial wastewater has a complex composition and may contain some substances that are toxic to microorganisms or difficult to biodegrade, which is a challenge to the stability of the sewage treatment process. For example, heavy metal ions contained in some industrial wastewater may inhibit the activity of microorganisms, thereby affecting the sewage treatment effect.

(III) Reduction of sludge activity

The sludge activity is weak at low temperatures, and the enzymatic reaction rate of microorganisms will decrease. When the temperature drops by 10 degrees, the enzymatic reaction rate of microorganisms will decrease by 1-2 times, which slows down the reaction rate and reduces the sewage treatment efficiency. At the same time, sludge swelling is very likely to occur during low-temperature operation in winter due to reduced sludge activity and abnormal process operation. Sludge swelling has the characteristics of extremely high incidence (60% of urban sewage treatment plants experience sludge swelling every year), universality (existing in various types of activated sludge processes), and serious harm (not only causing sludge loss and excessive suspended solids (SS) in effluent, but also greatly reducing the treatment capacity). Once sludge swelling occurs, it is difficult to control or it takes a long time to recover.

(IV) Operational stability of equipment and processes

The low temperature environment in winter places higher requirements on the operational stability of sewage treatment equipment and processes. Equipment may malfunction at low temperatures, such as freezing of pipes and icing of equipment. Some open-air equipment, such as the bars of the coarse grid, may freeze, resulting in smaller spacing and deformation and breakage of the rake teeth, which in turn reduces the water intake and even blocks the pipeline; the transmission scrapers of the primary and secondary sedimentation tanks may slip after snowing, affecting the scraping and sludge return; the equipment of the sludge thickening tank is easy to freeze, and the ice on the pool surface will cause the thickening tank to lose its thickening effect and increase the risk of damage to the structure; the mud cake in the dehydration room cannot be transported after freezing, the concentration of toxic gases is too high to endanger health, and the distribution cabinet short circuit may also occur. In terms of technology, if adjustments cannot be made according to the special conditions in winter, such as reasonable control of aeration and adjustment of sludge concentration, it is difficult to ensure that the sewage treatment is stable and meets the standards.

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  1. Common technologies and methods for winter sewage treatment

(I) Insulation and heating measures

①Pool insulation

- For the aeration tank, secondary sedimentation tank and other pool walls, foam insulation board insulation, external brick enclosure (filled with slag, expanded perlite, etc.) structure, pool top cover and other insulation measures can be used. This can reduce the heat loss of the pool and maintain the water temperature. For example, after some sewage treatment plants in the north adopted this insulation method, the rate of water temperature drop was significantly slowed down, which is conducive to microorganisms to carry out sewage treatment work in a relatively stable temperature environment.

- For sludge pools and sludge thickening pools, a solar panel insulation cover can be installed above the pool body. The principle is similar to that of agricultural vegetable greenhouses, ensuring that the temperature in the pool is always kept above 0℃, effectively preventing the occurrence of surface icing. If there is no condition to install an insulation cover, you can also temporarily use plastic film or insulation quilts to cover the surface of the sludge pool and thickening pool for insulation.

②Air preheating and heating

- An air preheating chamber is set up on one side of the blower to preheat the cold air of -10 to -20℃ in winter to 5 to 8℃. The air duct is set up with a pipe gallery for heat preservation. This can increase the temperature of the air entering the aeration tank, which helps to maintain the water temperature and provide a suitable oxygen supply environment for microorganisms, which is conducive to the metabolic activities of microorganisms.

- Hot steam can also be used to heat the sewage entering the aeration tank. However, this method will increase the operating cost of sewage treatment and needs to be weighed according to actual conditions.

③Sludge heating

- Appropriately heat the sludge, including the return sludge. This helps to increase the activity of microorganisms in the sludge, because the microorganisms in the sludge can better decompose pollutants at a suitable temperature. The heated sludge re-enters the treatment system, which can enhance the treatment capacity of the entire sewage treatment system.

(II) Operation parameter adjustment

①Sludge concentration adjustment

- Appropriately increase the sludge concentration MLSS, so that the total amount of sludge metabolic capacity can remain stable under the premise of reduced bacterial metabolic capacity. For example, for plants that mainly use domestic sewage, a slightly lower F/M can be controlled, and for plants that mainly use industrial wastewater, a lower F/M should be controlled, preferably at 0.03 - 0.08kgBOD5/kgMLSS·d. At the same time, increasing the sludge age (preferably controlled at 15 - 25 days) will help improve the ability of microorganisms to decompose pollutants, especially for nitrogen removal, because nitrifying bacteria can grow and function better in an environment with a longer sludge age.

②Aeration control

- According to the characteristics of its own process, appropriate aeration control should be carried out. Under the premise of ensuring that all cells are adequately aerated, the DO value should be controlled at 2.0 - 3.5mg/L, which should not be too high. Properly increasing aeration can maintain water temperature to a certain extent and increase DO. It is a commonly used method to control the treatment effect of NH3 - N. However, excessive aeration may also cause problems such as weak sludge system activity, poor properties, and poor sedimentation performance, and increase operating costs.

③Adjustment of flow and return flow

- Ensure the normal operation of the pretreatment unit and ensure that the MLVSS/MLSS, SV30, and SVI of the sludge in each cell of the biochemical pool are within the normal range. According to the specific process operation, adjust the internal and external return flow, return ratio and other parameters. For example, reasonable adjustment of the return sludge volume can optimize the distribution of sludge in the treatment system, increase the contact opportunity between microorganisms and pollutants, and thus improve the treatment efficiency.

(III) Adjustment of reagent addition

①Strengthen the biochemical system as the main

- For sewage projects with high-efficiency sedimentation tanks or chemical treatment units during the treatment process, the first consideration during operation should be to strengthen the biochemical system to treat pollutants, and then adopt chemical treatment to check. Avoid excessive reliance on chemical treatment to maintain water stability, because chemical treatment will produce a large amount of chemical sludge, which will lead to a vicious cycle of the system if it is not treated in time.

②Standardize the dosing process

- The dosing process and system must be standardized for the dosing of reagents, and a dedicated person should be responsible for dosing management. The dosage of the medicine added at different times of the day must be reasonably adjusted in combination with the water conditions of the secondary sedimentation tank, the beaker test data, and the online data of the effluent, so as to avoid the waste of medicine. For example, according to the effluent water quality of the secondary sedimentation tank, such as the content of suspended solids and ammonia nitrogen, the dosage of the flocculant, disinfectant and other medicines is adjusted to ensure that the treatment effect is guaranteed while reducing the unnecessary use of medicines and reducing the operating cost.

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  1. Maintenance points of sewage treatment equipment in winter

(I) Equipment antifreeze measures

①Antifreeze of pipes and valves

- Before winter, the valves of the process pipelines of the water plant should be insulated. They can be wrapped with rock wool, foam board, polyurethane, etc., or steam or electric heating can be used; the sampling valve can also be kept at a small opening to allow it to flow water for a long time to avoid freezing and cracking of valves and pipes (at the same time, it is necessary to avoid the water flowing directly on the ground to freeze). The decommissioned pipelines should be drained of water before winter, and the sewer pipelines and slag wells should be thoroughly dredged and cleaned before winter. Other pipelines should also consider antifreeze. For valves, first check and make sure the valve is leak-proof, then put straw mats into the valve well to keep the valve warm.

②Antifreeze of the equipment body

- Comprehensive inspection and maintenance of sewage treatment equipment should be carried out, including replacement of equipment lubricating oil and buttering. After entering winter, all sewage treatment areas and sludge treatment areas must be kept in continuous operation. After entering winter, all structures are not allowed to be emptied to avoid water freezing and thawing in the pool body. For example, for some equipment parts that require lubrication, such as fan bearings, etc., lubricating oil suitable for low temperature environment should be replaced to prevent the lubricating oil from solidifying at low temperature and affecting the operation of the equipment.

- For intermittently running pumps and valves and connected pipes, the valve opening can be reduced or the pump operation frequency can be adjusted to make it run continuously, such as the mud pump of the self-priming scraper, the air flotation feed pump, the dosing pump, etc. For the pumps and valves that are stopped (or standby) and the connected pipes, they must be emptied through the exhaust valve, pressure gauge, vent valve, etc. at the pump port, such as the standby feed pump, recycling water pump, sewage pump, etc.

- For blowers with water cooling devices, the return water pipe and the inlet water pipe should be placed close together to allow the water temperature of the return water pipe to insulate the inlet water pipe. Rock wool, foam board, polyurethane, sand, etc. can also be used to wrap or cover the inlet pipe, or steam or electric heating can be performed.

(II) Equipment operation status monitoring

①Daily inspection

- Check whether the sewage pump and return water pump of the domestic sewage treatment equipment are frozen during operation every day, including the lubrication and refueling of the water pump, the sealing of the packing, and the sealing of the bottom valve is better than the filling of the air compressor oil. Check whether the air filter valve of the air compressor is frost-free to determine whether the air compressor is operating normally, whether there is noise or heat. Check whether the transmission part of the scraper and the scraper are hardened and broken in the cold state to avoid affecting the use. Pay attention to the coagulant mixing tank to avoid freezing, and perform small sample tests frequently. Check the valve pipes of all equipment to avoid valve pipe blockage, and put them in the open or closed position as needed.

②Fault warning and handling

- Any abnormal situation that occurs during the operation of the equipment should be warned and handled in time. For example, if the current of the sewage pump is found to be abnormally high, it may be a blockage inside the pump body or a bearing failure. The machine should be stopped and checked in time to prevent the fault from further expanding. If a part of the equipment is frozen, it should be handled according to the correct thawing method to avoid damage to the equipment due to improper thawing.

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  1. Sharing of experience in winter sewage treatment plant operation and management

(I) Personnel management and training

①Special training

- Before winter, all personnel should be trained in winter sewage treatment related knowledge and skills. The training content includes special process requirements for winter sewage treatment, equipment antifreeze and maintenance knowledge, safe operating procedures, etc. For example, for new employees or employees who are inexperienced in winter sewage treatment, detailed explanations are given on how to adjust process parameters in low temperature environments, how to operate equipment correctly to avoid equipment failures, etc.

- Strengthen the training of employee safety awareness, because there are some special safety risks in winter sewage treatment plants, such as inspection safety in ice and snow weather, safety during equipment thawing, etc. Employees should know how to ensure their own safety in different situations. For example, when dealing with frozen pipelines or valves, they should first use a small amount of hot water to heat them, and then increase the amount of hot water to prevent sudden temperature rise and damage to the equipment; outdoor stairs, corridors, platforms, etc. and inspection channels should be kept clean, and snow should be removed in time to prevent water accumulation and ice. Personnel should bring labor protection supplies when inspecting. Hands are not allowed to be put into pockets during the inspection process. When going up and down the ladder, they should hold the guardrail with both hands.

②System improvement and implementation

- Incorporate problems that are prone to occur under low temperature conditions into the management system and prepare emergency plans. For example, formulate emergency plans for emergencies such as equipment freezing and pipeline blockage, clarify the responsibilities of various departments and personnel in these situations, and stipulate the process and methods of emergency handling. At the same time, strictly implement the equipment inspection system, especially the night inspection system, and include antifreeze parts and antifreeze content in the handover content to ensure the safe operation of the sewage treatment plant in winter.

(II) Overall operation management

①Whole process management

- Start from the details to ensure that each sewage treatment unit fully exerts its due functions. Faults and problems that occur should be discovered, analyzed and solved in a timely manner. Small problems and minor faults should not be left unsolved and become big problems, affecting the stable operation of the entire system. Special attention should be paid to the abnormal operation of the screen, grit chamber, hydrolysis acidification tank, sludge dewatering machine, etc., which increases the burden on the biochemical treatment system, causes the abnormal operation of the biochemical system, and causes the unstable effluent. These conditions need to be given enough attention and improved. The sewage treatment plant should combine the specific conditions of the biochemical system such as the operation rules of its own process operation, the properties of sludge, and the degradation and change rules of pollutants; combined with the daily and monthly changes in the influent water quality and water volume, through appropriate process optimization and adjustment, ensure that the sewage is treated in sufficient quantity and the effluent water quality is stable and meets the standards, while saving energy and reducing consumption to optimize the operating cost.

②Data management and analysis

- Strengthen the collection, collation, statistics and analysis of production data. Pay special attention to strengthening the statistical analysis of data, and use the conclusions of data analysis to guide the adjustment and regulation of production operations. Each branch and sewage treatment plant should strengthen the laboratory analysis work to ensure that the laboratory data is timely, accurate and reliable; at the same time, ensure that the production-related data is effective and reliable. For example, through the long-term monitoring and analysis of the influent water quality, effluent water quality, sludge performance and other data, problems in the treatment system can be discovered in time. For example, the decrease in nitrogen removal rate may be due to the nitrification reaction being affected by low temperature, so that the process parameters can be adjusted in a targeted manner, such as increasing aeration and increasing sludge concentration.