Landfill leachate is a kind of organic wastewater with high concentration, which comes from the water contained in the landfill itself, the rain and snow water and other water that enters the landfill, deducts the saturated water holding capacity of the garbage and the overlying soil layer, and goes through the garbage layer and the overlying soil layer.
In view of the above problems, combined with the current common treatment process, that is, "regulating tank + anaerobic system +MBR system + deep membrane treatment system (nanofiltration + reverse osmosis)" as the core of the treatment process. With reference to the operation situation of actual engineering cases, the response measures considered by comprehensive design experience are summarized as follows:
(1) Discuss the strain capacity of water fluctuation
The amount of leachate fluctuates with the change of season or weather. Generally, the amount of leachate is less in the dry season in winter and the concentration of pollutants is high. Summer rainy season water is more, the concentration of pollutants is lower. Therefore, in the project design, all process units and processing equipment of the whole process flow have a certain amount of surplus, which can meet the impact of water volume within a certain range and meet the requirements of seasonal or weather changes of water volume.
(2) Discuss the strain capacity of water quality fluctuation
1) In the process, the MBR system adopts an external tubular ultrafiltration membrane for sludge and water separation. Compared with ordinary MBR, the biochemical tank can maintain a higher activated sludge concentration (greater than 15g/L), which undoubtedly enhances the impact resistance of the system to water quality changes. The decrease of the organic load caused by the rainy season can adjust the sludge concentration of the system by changing the tubular membrane reflux to ensure the stable operation of the system.
2) run against sudden deterioration of water quality (waste leachate pollutants resulting from seasonal changes of the anaerobic effluent may occur insufficient carbon nitrogen ratio, etc.) leads to abnormal growth, biochemical pond sludge denitrification effect is poor, set the anaerobic tube, ensure the carbon and nitrogen in the biochemical pool than meet the requirements of biological denitrification, biochemical indicators meet the process unit target water production.
3) The MBR biochemical section adopts A/O process, and the nitrification liquid reflux ratio is more than 10 times, which strengthens the denitrification effect. At the same time, the full mixing of biochemical influent and reflux nitrification fluid can also effectively buffer the influent pollution load change and reduce the instantaneous impact.
4) Cooling system is set to strictly control the operating water temperature of each process section in view of the situation that excessive temperature of biochemical pool caused by biochemical reaction affects the normal operation of the reactor.
5) A defoaming system is set for the deterioration of sludge properties when the system is impacted and aeration produces a large amount of foam, including the addition of defoaming agent.
6) The aeration fan of membrane biochemical reactor is designed for frequency conversion control, which can effectively deal with water quality fluctuation and avoid excessive aeration to accelerate sludge aging, and too small aeration to lead to insufficient nitrification reaction.
(3) Discuss the removal ability of high concentration organic pollutants
The high concentration of organic pollutants in the leachate, namely COD and BOD, is one of the difficulties in its treatment. It is difficult to achieve good and stable effluent water quality by traditional treatment technology.
In view of the water quality characteristics of leachate with high COD and BOD, a high-efficiency anaerobic reactor with high volume load rate, mature process and stable operation was selected to ensure the efficient anaerobic removal of organic matter. At the same time, the common problems in anaerobic reactor treatment of landfill leachate were solved to ensure that 85% organic matter could be effectively degraded in the anaerobic stage.
At the same time, the external membrane biochemical reaction process uses the combination of biochemical and ultrafiltration membrane, so that the microbial community is completely trapped in the bioreactor, the biochemical tank can maintain a higher activated sludge concentration, greatly improving the removal effect of ammonia nitrogen and total nitrogen. Ensure better effluent water quality, and water quality is stable.
(4) Discuss the removal ability of high concentration ammonia nitrogen
Biochemical process for high concentration of ammonia nitrogen compounds to choose A/O as the main process to ensure that the biochemical stage to retain enough residence time.
Nitration system for denitrification of denitrifying microorganism (denitrifying bacteria) belong to the autotrophic microbiology, the microbial propagation speed slower, namely generation cycle is long, in actual design and engineering application of nitrifying sludge age must be very long, the traditional denitrifying and nitrifying process is subject to the size of the reactor, sludge erosion factors, such as in the treatment of high concentration of ammonia nitrogen wastewater often can't nitration completely, and the MBR membrane biochemical reactor process because of its microbial completely trapped, make the microorganisms of the sludge age is far more than the time needed for denitrifying microorganism growth, and be able to reach the needed for the complete nitrification microbial breeding, aggregation concentration, This allows complete nitrification of ammonia nitrogen. The engineering example shows that the removal efficiency of ammonia nitrogen by two-stage A/O+ external membrane biochemical reaction process can reach more than 95%.
(5) Discussion on slag removal capacity of pretreatment
The content of sediment, suspended matter and fiber in the landfill leachate is high, and if it is not effectively controlled during the pretreatment period, the cross section of the ultrafiltration will be blocked and the membrane flux will be affected after it enters the subsequent membrane system. The solid-liquid separation and slag removal machine equipped with automatic high-pressure backwashing and slag scraping system is used in the design. The grid distance is less than 1mm, which can effectively block the flow of silt, hair and fiber, so as to ensure the stable operation of the follow-up biochemical and membrane system.
(6) Coping measures with different climate characteristics in summer and winter
1) Temperature control
The leachate is heated by steam before anaerobic influent, and the temperature is controlled at 35~38℃.
Summer high temperature on membrane biochemical reactor main impact is bigger, when the reactor temperature is above 40 degrees Celsius, aerobic microbes will die, oxygen utilization become low, so the membrane biochemical reactor with the cooling system of form a complete set, when the reaction temperature in the reactor cooling system start to cooling of biochemical, temperature drop to 30 ~ 35 degrees Celsius.
When the temperature is low in winter, because the membrane biochemical reactor is a high-load biochemical reaction, in the process of biochemical degradation, part of the chemical energy of organic matter and ammonia nitrogen is converted into heat energy, and the temperature rises. The mechanical energy of the power equipment fan and water pump is converted into heat energy in the operation process, which also causes the temperature to rise. The ultrafiltration mixture is recycled to the biochemical pool to maintain a relatively stable temperature of the liquid. According to the calculation of heat balance and some engineering examples, it is shown that the temperature of biochemical reaction can be maintained above 30 ℃ after the membrane biochemical reactor is designed with heat preservation, and there is no need for additional heating measures.
Membrane treatment equipment installed indoors, basically not affected by temperature changes.
2) Control measures for water quality and water quantity transformation in summer and winter
Leachate water quality fluctuates with the change of season or weather, under normal circumstances, the summer rainfall is large, large amount of leachate, relatively low concentrations, anaerobic concentration is relatively low, the water is less than 40000 mg/L, winter rainfall is less, the small amount of leachate and concentration is higher, when leachate reduction can only open a group run, save operating cost.
