The treatment methods of landfill leachate include physical and chemical methods and biological methods. Physical and chemical methods mainly include activated carbon adsorption, chemical precipitation, density separation, chemical oxidation, chemical reduction, ion exchange, membrane dialysis, gas stripping wet oxidation and other methods. When the COD is 2 ~ 4 mg/L, the COD removal rate of physical and chemical methods can reach 5% ~ 87%. Compared with biological treatment, physical and chemical treatment is not affected by the change of water quality and quantity, and the effluent quality is relatively stable, especially for landfill leachate with low BOD5/COD ratio (.7 ~ .2), which is difficult to be biologically treated. However, physical and chemical methods are expensive to treat, which is not suitable for the treatment of large amount of landfill leachate. Therefore, biological methods are mainly used for landfill leachate at present.
biological methods can be divided into aerobic biological treatment, anaerobic biological treatment and their combination. Aerobic treatment includes activated sludge process, aeration oxidation pond, aerobic stabilization pond, biological turntable and trickling filter. Anaerobic treatment includes upflow sludge bed, anaerobic immobilized bioreactor, mixing reactor and anaerobic stabilization pond. Leachate can be treated by biological method, chemical flocculation, carbon adsorption, membrane filtration, lipid adsorption and gas stripping 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, Fall Town Sewage Treatment Plant in Pennsylvania, USA, the influent CODCr of landfill leachate is 6 ~ 21mg/L, BOD5 is 3 ~ 13mg/L, and ammonia nitrogen is 2 ~ 2mg/L.. The sludge concentration (MLVSS) of aeration tank is 6 ~ 12 mg/l, which is 3 ~ 6 times of the general sludge concentration. When the volume organic load is 1.87 kg BOD5/(m3 d), the F/M is .15 ~ .31 kg BOD5/(kg mlss d), and the removal rate of BOD5 is 97%. When the volume organic load is .3 kg BOD5/(m3 d), the F/M is .3 ~ .5 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 .3 and .31 kg BOD 5/(kg MLSS d) (not higher).
many scholars have also found that activated sludge can remove 99% of BOD5 from leachate, and more than 8% of organic carbon can be removed by activated sludge. Even if the organic carbon in influent is as high as 1mg/L, the sludge biological phase can quickly adapt and degrade. The activated sludge system running under low load can remove 8% ~ 9% of COD from leachate, and the effluent BOD5 <: 2mg/L。 For leachate with COD of 4 ~ 13 mg/L, BOD of 516 ~ 11 mg/L and NH3-N of 87 ~ 59 mg/L, the removal rate of COD by mixed aerobic activated sludge method can be stabilized at over 9%. Many practical landfill leachate treatment systems show that the treatment effect of activated sludge method is better than other methods such as chemical oxidation method. Improved activated sludge processes, such as low oxygen and aerobic activated sludge process and SBR process, are more effective than conventional activated sludge process because of their characteristics of maintaining high operating load and short time consumption. Xu Dimin, Tongji University, etc. used the method of low-oxygen and aerobic activated sludge to treat landfill leachate. The experiment proved that under the controlled operation conditions, the method of low-oxygen and aerobic activated sludge was effective. The average CODCr, BOD5 and SS of the final effluent decreased from the original 6466mg/L, 352mg/L and 239.6mg/L respectively to CODcr <: 3mg/L、BOD5< 5mg/L (average 13.3mg/L) and SS <: 1mg/L (average 27.8mg/L). The total removal rates were 96.4% for CODcr, 99.6% for BOD5 and 83.4% for SS.
if the treated effluent is further treated by chemical coagulation with basic aluminum chloride, the CODCr of the effluent can be reduced to below 1 mg/L ..
Two-stage treatment of nitrogen and phosphorus in leachate is also better than general biological method. The average removal rate of phosphorus is 9.5%; The average removal rate of nitrogen is 67.5%. In addition, the operation of this method makes up for the shortcomings that the first stage of anaerobic-aerobic two-stage biological treatment method forms more NH3-N, which makes the second stage difficult to carry out and the two aerobic treatments take too long. Compared with activated sludge process, aeration stabilization pond has a 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. The small-scale, pilot-scale and production scale studies in the United States, Canada, Britain, Australia and Germany all show that the aeration stabilization pond can achieve better treatment effect of landfill leachate.
For example, Britain invested 6, pounds in Bryn Posteg Landfill to build a 1m3 aerated oxidation pond with two surface aeration devices, and the minimum hydraulic retention time was 1 days. After sedimentation, the effluent from the oxidation pond flowed through a 3km long pipeline and entered the urban sewer. This system began to operate in 1983. The maximum CODCr of leachate is 24, mg/L, the maximum BOD5 is 1, mg/L, f/m = .5 ~ .3 kg COD/(kg mlss d), the water volume varies from to 15 m3/d, and the average BOD5 of effluent is 24mg/L, but occasionally it exceeds 5 mg.
the Water Research Center in Britain has made a survey of CODcr >: The 15mg/L leachate was also used in the pilot-scale aeration stabilization pond. When the load was .28 ~ .32 kg COD/(kg mlss d) or .4 ~ .64 kg COD/(kg mlss d) and the sludge age was 1 days, the removal rates of COD and BOD5 were above 98% and 91% 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 2 years, with the development of microbiology, biochemistry and other disciplines and the accumulation of engineering practice, a new anaerobic treatment process has been continuously developed, which has overcome the characteristics of long hydraulic retention time and low organic load of the traditional process, made great progress in theory and practice, and achieved good results in treating high-concentration (BOD5 ≥2mg/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 costs are low. Moreover, due to the small amount of excess sludge produced, less nutrients are needed. For example, its BOD5/P ratio is only 4∶1. Although the content of P in leachate is usually less than 1mg/L, it can still meet the requirements of microorganisms for P. With ordinary anaerobic nitrification, the removal rate of COD in leachate can reach 9% at 35℃, with a load of 1kg COD/(m3 d) and a residence time of 1d.
In recent years, the developed anaerobic biological treatment methods include: anaerobic biofilter, anaerobic contact tank, upflow anaerobic sludge bed reactor and staged anaerobic nitrification. Anaerobic filter is suitable for treating dissolved organic matter. The average COD of leachate from Halifax Highway11 landfill in Canada is 1285mg/L, BOD5/COD is .7, and pH is 5.6. The leachate was first adjusted to pH=7.8 by limewater, and then precipitated for 1 hour in anaerobic filter (this process can also remove 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 Toronto University in Canada also successfully treated landfill leachate aged 1.5 years and 8 years with anaerobic filter at room temperature, with their COD of 14mg/L and 4mg/L, BOD5/COD of .7 and .5, respectively, when the load was 1.26 ~ 1.45 kg COD/(m3 d). 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 ~ 2 kg 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 COD >: When the load of 1mg/L leachate is 3.6 ~ 19.7 kg COD/(m3 d), the average sludge age is 1.～4.3d, and the removal rate of COD and BOD5 is 82% and 85% respectively at 3℃, their load is much larger than that of anaerobic filter.
during anaerobic decomposition, organic nitrogen is transformed into ammonia nitrogen, and there is NH4+NH3+H+ reaction. If pH> At 7 o'clock, NH3 in equilibrium is dominant, which can be removed by blowing off. However, during anaerobic decomposition, the pH is approximately equal to 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. It is economical and reasonable to adopt anaerobic aerobic treatment process for high concentration landfill leachate, and the treatment efficiency is high. The removal rates of COD and BOD were 86.8% and 97.2% respectively.
6.3.1 anaerobic aerobic biological oxidation process (anaerobic nitrification and biological oxidation pond)
The Department of Biology of Southwest Normal University treated the leachate with pH of 8. ~ 8.6, COD of 16,124 mg/L, BOD5 of 214 ~ 46 mg/L and NH3-N of 475mg/L, and obtained the effluent pH of 7.
6.3.2 anaerobic oxidation ditch-facultative pond process
The following description and analysis are made in combination with Guangzhou Likeng landfill. The sewage treatment plant of Likeng landfill is designed according to the flow rate of 3m3/d, and the influent BOD5 is 25mg/L, CODCr is 4mg/L, NH3-N is 1mg/L, SS is 6mg/L, and the chromaticity is 1 times. The effluent BOD5 is 3mg/L, CODCr is 8mg/L, NH3-N is 1mg/L, SS is 7mg/L, and the chroma is 4 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 pond reaches the standard, the facultative pond water can be directly discharged; However, when the influent quality is poor and the effluent from the facultative pond can't meet the discharge standard, the coagulation and sedimentation system is activated, and then the supernatant of the sedimentation tank is discharged.
From the current operation of this process, when the influent COD is high, the effluent quality is good; Once the COD is reduced, especially when the temperature is low and there is little rain in winter, which is not conducive to biochemical treatment, all the water quality components in the effluent are too high to meet the standard, and the effluent is brown. Although the flocculation and sedimentation system is activated, the effect is still not satisfactory. 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 for leachate treatment 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 8mg/L, BOD5 is 5mg/L, SS is 7mg/L, and pH is 7.5; The effluent quality is 1mg/L CODCr, 6mg/L BOD5, 5mg/L SS, and the pH value is 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 section is an upflow anaerobic sludge bed reactor, and the aerobic section is a biological contact oxidation method. Chemical coagulation sedimentation and biological oxidation pond are added, and the purification treatment is discharged after reaching the standard. The remaining sludge is concentrated and sent back to the landfill for treatment.
considering the large variation of leachate quality, air flotation process is added after anaerobic stage to improve the treatment capacity to cope with the high influent quality. At present, the design of Xiaping landfill in Shenzhen adopts anaerobic-air flotation-aerobic process to treat leachate.
6.3.4UASB- oxidation ditch-stabilization pond
In 1995, Fuzhou built the largest modern comprehensive municipal waste treatment plant in China-Hongmiaoling Waste Sanitary Landfill in Fuzhou. The amount of water for treating landfill leachate is 1m3/d; The water quality (inlet) of landfill leachate is 8mg/L for CODCr and 55mg/L for BOD5; The treatment water quality requirements (export) are CODCr removal rate of 95% and BOD5 removal rate of 97%.
The design adopts the upflow anaerobic sludge bed Aubert oxidation ditch stabilization pond process. The landfill leachate is concentrated in the storage warehouse, flows to the water collection tank and grid by relying on the high terrain of the storage site, and then flows to the distribution tank by potential energy after being metered by the Bartholian metering tank, and then flows to the upflow anaerobic sludge bed by relying on the static head pressure. After anaerobic treatment, the sewage flows to a sedimentation tank for solid-liquid separation, the supernatant flows to the Ober oxidation ditch by itself, the precipitated sludge is discharged to the sludge tank by gravity, and the sludge is regularly sent to the landfill site or composted by tanker.
The sewage is treated by aerobic biochemical treatment in Aubert oxidation ditch, which adopts three-ditch A/O process, which has advanced nitrogen removal effect. The outstanding advantages of this process are that ammonia nitrogen can be nitrated in the first ditch, and nitrate can be denitrified with BOD as carbon source, and the removal rate of total nitrogen can reach 8%. Because BOD in sewage is used as carbon source, BOD5 in sewage is removed, which reduces the oxygen demand in sewage. 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 internal reflux and denitrification in the first ditch.
the sewage treated by oxidation ditch flows into the secondary sedimentation tank for solid-liquid separation, and the clarified water flows to 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 is refluxed to the oxidation ditch for treatment, and the concentrated sludge is treated with submersible pump.