Biological methods are divided into aerobic biological treatment, anaerobic biological treatment and their combinations. Aerobic treatment includes activated sludge process, aeration oxidation tank, aerobic stabilization tank, biological turntable and trickling filter. Anaerobic treatment includes upflow sludge bed, anaerobic immobilized bioreactor, mixing reactor and anaerobic stabilization tank. Leachate can be treated by biological method, chemical flocculation method, activated carbon adsorption method, membrane filtration method, lipid adsorption method and gas stripping method alone or in combination, among which activated sludge method is widely used because of its low cost and high efficiency. The operation results of several activated sludge wastewater treatment plants in the United States and Germany show that the activated sludge process can achieve satisfactory landfill leachate treatment effect by increasing sludge concentration and reducing sludge organic load. For example, the sewage treatment plant in Fall Town, Pennsylvania, USA, the influent CODCr of landfill leachate is 6000 ~ 2 1000mg/L, BOD5 is 3000 ~ 13000mg/L, and ammonia nitrogen is 200 ~ 2000 mg/L. The sludge concentration (MLVSS) of aeration tank is 6000 ~/L. When the volume organic load is1.87kg bo D5/(m3 d), the F/M is 0.15 ~ 0.31kg BOD5/(kg mlss d), and the removal rate of BOD5 is 97%. When the volume organic load is 0.3 kg bo D5/(m3 d), the F/M is 0.03 ~ 0.05 kg BOD5/(kg mlss d), and the removal rate of BOD5 is 92%. The data of this factory show that the activated sludge process can effectively treat landfill leachate as long as the concentration of activated sludge process is properly increased and the F/M is between 0.03 ~ 0.31kgbod5/(kg mlss d) (not higher).
Many scholars also found that activated sludge can remove 99% BOD5 from leachate, and more than 80% of organic carbon can be removed by activated sludge. Even if the influent organic carbon is as high as 1000mg/L, the sludge biological phase can adapt and degrade quickly. The activated sludge system running at low load can remove 80% ~ 90% of COD from leachate and produce BOD5.
If the treated wastewater is further treated by chemical coagulation with basic aluminum chloride, the CODCr of the wastewater can be reduced to below1000 mg/L.
The two-stage treatment of nitrogen and phosphorus in leachate is also better than the general biological method. The average removal rate of phosphorus is 90.5%; The average removal rate of nitrogen is 67.5%. In addition, the operation of this method makes up for the shortcomings of the anaerobic-aerobic two-stage biological treatment method that the first stage forms more NH3-N, which makes the second stage difficult to carry out and the two-stage aerobic treatment time is too long. Compared with activated sludge process, aeration stabilization pond has large volume and low organic load. Although the degradation progress is slow, it is the most economical aerobic biological treatment method for landfill leachate in areas where land is not expensive because of its simple engineering. Small-scale, pilot-scale and production scale studies in the United States, Canada, Britain, Australia and Germany all show that aeration stabilization pond can achieve better treatment effect of landfill leachate.
For example, Britain invested 60,000 pounds in Bryn Posteg landfill to build an aeration oxidation pond with 1000m3, with two surface aeration devices, and the minimum hydraulic retention time was 10d. After precipitation, the effluent from the oxidation pond flows into the urban sewer through a 3 km long pipeline. The system started to operate in 1983, and the maximum CODCr of leachate was 24,000 mg/L, the maximum BOD5 was 10000mg/L, f/m = 0.05 ~ 0.3 kg COD/(kg mlss d), and the water volume was 0 ~150. Considering the daily operation cost, investment return and interest, compared with leachate directly discharged into the municipal pipe network, it can save 750 pounds per year.
The British Water Research Center conducted a survey of CODcr >: 15000mg/L leachate was also tested in the pilot aeration stabilization pond. When the load is 0.28 ~ 0.32 kg COD/(kg mlss d) or 0.04 ~ 0.64 kg COD/(kg mlss d) and the sludge age is 10d, the removal rates of COD and BOD5 are 98% and 98% respectively. Phosphoric acid also needs to be added during operation. The purposeful application of anaerobic biological treatment has a history of nearly a hundred years. However, in recent 20 years, with the development of microbiology, biochemistry and other disciplines and the accumulation of engineering practice, the new anaerobic treatment process has been continuously developed, which overcomes the characteristics of long hydraulic retention time and low organic load of the traditional process, and has made great progress in theory and practice, and achieved good results in treating high-concentration (BOD5 ≥2000mg/L) organic wastewater.
Anaerobic biological treatment has many advantages, the most important of which is low energy consumption and simple operation, so the investment and operation cost are low. Moreover, due to the small amount of excess sludge, less nutrients are needed, such as its BOD5/P ratio is only 4000:1. Although the content of P in leachate is usually less than 1mg/L, it can still meet the requirements of microorganisms for P. Under the conditions of 35℃, load1kg COD/(m3 d) and residence time 10d, the removal rate of COD in leachate can reach 90%.
The anaerobic biological treatment methods developed in recent years include: anaerobic biofilter, anaerobic contact tank, upflow anaerobic sludge bed reactor and staged anaerobic nitrification. Anaerobic filter is suitable for treating dissolved organic matter. Halifax highway101landfill leachate in Canada has an average COD of 12850mg/L, a BOD5/COD of 0.7 and a pH of 5.6. The leachate is adjusted to pH=7.8 with limewater to precipitate 1h, and then passes through an anaerobic filter (this process also plays a role in removing heavy metals such as Zn). When the load is 4 kg COD/(m3 d), the COD removal rate can reach above 92%. When the load increases again, its removal rate drops sharply.
J. G.Henry of the University of Toronto in Canada also successfully treated landfill leachate of 1.5 years and 8 years with anaerobic filter at room temperature. When the load is 1.26 ~ 650, the COD is1.4000 mg/L and BOD5/COD is 0.7 and 0.5 respectively. When the load increases again, its removal rate also drops sharply. It can be seen that although the load of anaerobic filter can reach 5 ~ 20kg COD/(m3 d) when treating high-concentration organic wastewater, the load of leachate must be kept at a low level in order to obtain ideal treatment effect. The British Water Research Center reported that the upflow anaerobic sludge bed (UASB) was used to treat leachate with COD >:10000 mg/L. When the load was 3.6 ~19.7 kg COD/(m3 d), the average sludge age was 1.0 ~ 4.3d at 30℃. Their load is much larger than that of anaerobic filters.
In the process of anaerobic decomposition, organic nitrogen is transformed into ammonia nitrogen, with NH4+NH3+H+ reaction. If NH3 in equilibrium is dominant at pH>7, it can be removed by blowing. However, in the process of anaerobic decomposition, the pH value is about 7, so the effluent may contain more NH4+, which will consume the dissolved oxygen in the receiving water. Although the practice has proved the effectiveness of anaerobic biological method in treating high concentration organic wastewater, it is rare to treat leachate by anaerobic method alone. The anaerobic aerobic treatment process for high concentration landfill leachate is economical and reasonable, and the treatment efficiency is high. The removal rates of COD and BOD are 86.8% and 97.2% respectively.
6.3. 1 anaerobic aerobic biological oxidation process (anaerobic nitrification-biological oxidation tank)
The Department of Biology of Southwest Normal University treated the leachate with pH of 8.0 ~ 8.6, COD of 16 124mg/L, BOD5 of 2 14 ~ 406 mg/L and NH3-N of 475mg/L, and the effluent pH was 7./kloc-0.
6.3.2 Anaerobic oxidation ditch-facultative pond process
The following description and analysis are combined with Guangzhou Likeng landfill. The sewage treatment plant of Likeng Landfill is designed according to the flow rate of 300m3/d, and the influent BOD5 is 2500mg/L, CODCr is 4000mg/L, NH3-N is 1000mg/L, SS is 600mg/L, and chroma is1000mg/L; The effluent BOD5 is 30mg/L, CODCr is 80mg/L, NH3-N is 10mg/L, SS is 70mg/L, and the chroma is 40 times. The selected process flow is: anaerobic oxidation ditch-facultative pond-flocculation and sedimentation. When the influent water quality is good and the effluent from the facultative tank reaches the standard, the facultative tank water can be directly discharged; However, when the influent water quality is poor and the effluent from the facultative tank cannot meet the discharge standard, the coagulation and sedimentation system is started, and then the supernatant of the sedimentation tank is discharged.
Judging from the current operation of the process, when the influent COD is high, the effluent quality is better; Once COD is reduced, especially in winter, when the temperature is low and there is little rain, it is not conducive to biochemical treatment, and the effluent water quality components all exceed the standard, and the effluent is brown. Although the flocculation and sedimentation system has been started, the effect is still not ideal. It can be seen that the effective removal of chroma and NH3-N in leachate will have a favorable impact on biochemical treatment.
6.3.3 Anaerobic-Air Flotation-Aerobic Process
This process is used to treat leachate in Datianshan Waste Sanitary Landfill. According to the detection data and simulation test of leachate from similar landfill sites by Guangzhou Institute of Environmental Health, the design parameters of leachate sewage treatment are determined in combination with the actual situation of the site. The influent water quality CODCr is 8000mg/L, BOD5 is 5000mg/L, SS is 700mg/L, and pH is 7.5; The effluent quality is CODCr 100mg/L, BOD5 60mg/L, SS 500mg/L, and pH 6.5 ~ 7.5. In view of the fact that the site is far away from the urban area, in order to facilitate management and save energy consumption, anaerobic and aerobic combined treatment processes are selected after comparison. The anaerobic stage is an upflow anaerobic sludge bed reactor, and the aerobic stage is a biological contact oxidation process. Increase chemical coagulation and sedimentation and biological oxidation ponds, and purify and discharge them up to standard. The remaining sludge is concentrated and sent back to the landfill for treatment.
Considering the great change of leachate quality, air flotation process was added after anaerobic stage to improve the treatment capacity and cope with the high influent quality. At present, the design of Shenzhen Xiaping Landfill adopts anaerobic-air flotation-aerobic process to treat leachate.
6.3.4UASB- oxidation ditch-stabilization pond
1995 Fuzhou has built the largest modern comprehensive municipal waste treatment plant in China-Fuzhou Hongmiaoling Waste Sanitary Landfill. The amount of water for treating landfill leachate is1000 m3/d; The water quality (inlet) of landfill leachate is CODCr 8000mg/L and BOD5 5500mg/L; The treatment water quality requirements (export) are CODCr removal rate of 95% and BOD5 removal rate of 97%.
Aubert oxidation ditch stabilization pond process with upflow anaerobic sludge bed was designed. Landfill leachate is concentrated in the storage bin, flows to the water collecting basin and grid by the high terrain of the storage yard, flows to the distribution basin by potential energy after being metered by the Bartholian metering box, and then flows to the upflow anaerobic sludge bed by the static head. After anaerobic treatment, the sewage flows into the sedimentation tank for solid-liquid separation, the supernatant flows to the Ober oxidation ditch by itself, the precipitated sludge is discharged into the sludge pool by gravity, and the sludge is regularly sent to the landfill or composted by a tanker.
Aubert oxidation ditch is used for aerobic biochemical treatment of sewage, and three ditch A/O process is adopted, which has advanced nitrogen removal effect. The outstanding advantage of this process is that the first ditch can nitrify ammonia nitrogen, and BOD can denitrify nitrate, and the total nitrogen removal rate can reach 80%. Because BOD in sewage is used as carbon source, BOD5 in sewage is removed and oxygen demand in sewage is reduced. In order to improve the denitrification effect of oxidation ditch, the effluent from the third ditch was pumped to the first ditch by submersible pump for reflux denitrification in the first ditch.
Sewage treated by oxidation ditch flows into the secondary sedimentation tank for solid-liquid separation, and clarified water flows into the stabilization pond for biological treatment. Excess sludge from the secondary sedimentation tank is discharged to the concentration tank by gravity. The supernatant in the concentration tank flows back to the oxidation ditch for treatment, and the concentrated sludge is pumped to the tanker by submersible pump and transported to the landfill for landfill or composting. Land treatment method, namely soil irrigation method, is the earliest sewage treatment method adopted by human beings, but the application of land treatment system in urban sewage treatment is more common. For the treatment method of leachate, the leachate is collected by sprinkler irrigation and returned to the landfill. The leachate from the circulating landfill increases the humidity of the garbage, thus improving the biological activity and accelerating the generation of methane and the decomposition of the garbage. Secondly, because of evaporation in sprinkler irrigation, the volume of leachate is reduced, which is beneficial to the operation of sewage treatment system and can save energy costs. Part of Seamer Carr landfill in northern England uses leachate for recycling. After 20 months, the COD value and metal concentration of leachate in the circulating area decreased significantly, while the concentrations of NH3-N and Cl- changed little. It shows that the reduction of metal concentration is not only caused by dilution, but also by the adsorption of inorganic components in garbage.
Because of the advantages of circulating leachate, the top of landfill should not be completely closed in design, but ditches should be arranged regularly to avoid pollution to surrounding water sources. Low-concentration leachate cannot be directly discharged, because the concentrations of NH3-N and Cl- are still high, the seasonal temperature is low, the evaporation is low, and the biological activity is weak, so the reuse effect of leachate needs further study. The British landfill leachate treatment plant adopts the disc-tube reverse osmosis system patented by Rochem to treat the primary leachate. This treatment technology is a Rochem separation membrane system designed and produced by Winterton Landfill in South Humberside.
The core of this system is Rochem's patented disc tube. The cylinder is composed of plate, octagonal steel and wear-resistant membrane pad in a circular tube, which can treat leachate that quickly blocks the common reverse osmosis membrane system. Under the pressure of membrane, leachate enters the treatment system of Rochem for aeration and pH correction. When the leachate containing pollutants flows through the inner surface of the cylinder, the pollutants in the leachate are separated by reverse osmosis and discharged through the membrane. The cleaning operation of the whole system is automatic. When the system needs chemical cleaning, the control indicator will display information, and the automatic cleaning system will use programmed chemicals to clean the system internally to restore its original function. Because the leachate forms turbulence on the membrane surface under closed conditions, reducing oxidation and producing stench, it needs to be cleaned internally within a certain period of time, but the interval of such cleaning is long. Rochem's separation membrane system can remove heavy metals, solid suspended solids, ammonia nitrogen and harmful refractory organic substances, and the treated water meets strict discharge standards.
At present, Rochem's treatment system has been installed and put into use in Ihlenbery landfill in Germany, with a treatment capacity of 50m3/h and a water recovery rate of 90%.