Landfill leachate refers to a kind of high concentration organic wastewater that comes from the water contained in the garbage in the landfill, the rain and snow water entering the landfill and other water, deducting the saturated water holding capacity of the garbage and the overburden, and passes through the garbage layer and the overburden. There is also water leakage from the accumulated garbage ready for incineration.
The properties of landfill leachate change with the operation time of the landfill, which is mainly determined by the stabilization process of garbage in the landfill. The stabilization process of landfill is usually divided into five stages, namely initial adjustment phase, transition phase, acid phase, methane fermentation phase and maturation phase.
Landfill leachate refers to a kind of high concentration wastewater that comes from the water contained in the garbage itself, rain, snow and other water entering the landfill, deducting the saturated water holding capacity of garbage and overburden, and passes through the garbage layer and overburden.
The water quality of landfill leachate is quite complex, which generally contains high concentrations of organic matter, heavy metal salts, SS and ammonia nitrogen. Landfill leachate not only pollutes soil and surface water sources, but also pollutes groundwater. There have been many studies on the removal of CODCr in landfill leachate, which are generally treated by biological methods, but the treatment effect is not very ideal, and the operation cost is relatively high.
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The treatment of leachate from municipal solid waste landfills has always been a very thorny problem in the design, operation and management of landfills. Leachate is the product of the gravity flow of liquid in the landfill, which mainly comes from the precipitation and the water contained in the garbage itself. Because there are many factors that may affect the properties of leachate in the flow process, including physical factors, chemical factors and biological factors, the properties of leachate change in a considerable range. Generally speaking, its pH value is between 4 ~ 9, cod is in the range of 2000 ~ 62000mg/l, BOD5 is from 60 ~ 45000mg/l, and the concentration of heavy metals is basically the same as that in municipal sewage. Urban landfill leachate is a kind of high concentration organic wastewater with complex components. If it is directly discharged into the environment without treatment, it will cause serious environmental pollution. In order to protect the environment, it is essential to treat leachate.
1. Current situation of leachate treatment process
The treatment methods of landfill leachate include physical and chemical methods and biological methods. Physicochemical methods mainly include activated carbon adsorption, chemical precipitation, density separation, chemical oxidation, chemical reduction, ion exchange, membrane dialysis, gas lift wet oxidation and other methods. The COD is 2000 ~ 4000? At mg/l, the COD removal rate of physicochemical method can reach 50% - 87%. Compared with biological treatment, physicochemical 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 (0.07 ~ 0.20) which is difficult to be biologically treated. However, the physicochemical method has high treatment cost and is not suitable for the treatment of large amount of landfill leachate. Therefore, the landfill leachate is mainly treated by biological method.
Biological methods are divided into aerobic biological treatment, anaerobic biological treatment and the combination of the two. Aerobic treatment includes activated sludge process, aerated oxidation tank, aerobic stabilization pond, biological rotating disc and trickling filter, etc. Anaerobic treatment includes upflow sludge bed, anaerobic immobilized bioreactor, mixed reactor and anaerobic stabilization pond.
2 Introduction to leachate treatment
Garbage leachate has characteristics different from general urban sewage: high concentration of BOD5 and COD, high metal content, large changes in water quality and quantity, high content of ammonia nitrogen, imbalance of microbial nutrient elements, etc. Among the treatment methods of leachate, the combined treatment of leachate and municipal sewage is a simple method. However, landfills are usually far away from cities and towns, so there are certain specific difficulties in the combined treatment of leachate and urban sewage, and they often have to be treated separately. Common treatment methods are as follows.
2.1 aerobic treatment
There are successful experiences in treating leachate with aerobic methods such as activated sludge method, oxidation ditch, aerobic stabilization pond and biological turntable. Aerobic treatment can effectively reduce BOD5, COD and ammonia nitrogen, and also remove other pollutants such as iron, manganese and other metals. In the aerobic method, the delayed aeration method is widely used, as well as the aeration stabilization pond and biological rotating disc (mainly used to remove nitrogen). The following will be introduced respectively.
2.1.1 activated sludge process
2.1.1.1 traditional activated sludge process
Leachate can be treated separately or jointly by biological methods, chemical flocculation, carbon adsorption, membrane filtration, lipid adsorption, gas stripping and other methods. Among them, 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 by increasing the sludge concentration to reduce the sludge organic load, the activated sludge process can achieve satisfactory landfill leachate treatment effect. For example, in the fall township sewage treatment plant in Pennsylvania, the CODcr of the landfill leachate is 6000 ~ 21000mg/l, and the BOD5 is? 3000 ~ 13000mg/l, ammonia nitrogen is 200 ~ 2000mg / L. The sludge concentration (MLVSS) in the aeration tank is 6000 ~ 12000mg/l, which is 3 ~ 6 times the general sludge concentration. When the volume organic load is 1.87kgbod5/ (M3 · d), the f/m is 0.15 ~ 0.31kgbod5/ (kgmlss · d), and the removal rate of BOD5 is 97%; When the volume organic load is 0.3kgbod5/ (M3 · d), the f/m is 0.03 ~ 0.05kg bod5/ (kgmlss · d), and the removal rate of BOD5 is 92%. The data of the plant shows that as long as the concentration of activated sludge method is appropriately increased and the f/m is between 0.03 ~ 0.31kgbod5/ (kgmlss · d) (no higher), activated sludge method can effectively treat landfill leachate.
Many scholars also found that activated sludge can remove 99% of BOD5 in leachate, and more than 80% of organic carbon can be removed by activated sludge. Even if the organic carbon in influent is as high as 1000mg/l, the sludge biofacies can quickly adapt and degrade. The activated sludge system operated under low load can remove 80% - 90% of COD in leachate, and the effluent BOD5 is less than 20mg / L. For COD? 4000～13000? Mg/l, bod51600 ~ 11000mg/l, NH3-N 87 ~ 590mg/l leachate, the removal rate of COD by mixed aerobic activated sludge method can be stabilized at more than 90%. Many practical landfill leachate treatment systems show that activated sludge method has better treatment effect than other methods such as chemical oxidation method.
2.1.1.2 low oxygen aerobic activated sludge process
Hypoxia? The improved activated sludge processes such as aerobic activated sludge process and SBR process are more effective than the conventional activated sludge process because they can maintain a high operating load and take less time. Tongji University xudimin and others use hypoxia? Aerobic activated sludge method is used to treat landfill leachate. The test shows that under controlled operating conditions, the landfill leachate passes through low oxygen? Aerobic activated sludge treatment, effect. The average CODCr, BOD5 and SS of the final effluent decreased from the original 6466mg/l, 3502mg/l and 239.6mg/l to codcr<300mg/l, bod5<50mg/l (average 13.3mg/l) and ss<100mg/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 with basic aluminum chloride for chemical coagulation, the CODcr of the effluent can be reduced to less than 100mg / L.
The nitrogen and phosphorus of leachate treated by two-stage method are also better than that of general biological method. The average removal rate of phosphorus is 90.5%; The average removal rate of nitrogen is 67.5%. In addition, the method runs to make up for anaerobic? More NH3-N is formed in the aerobic two-stage biological treatment method, which makes the second stage difficult to carry out and the duration of two aerobic treatments too long.
2.1.1.3 physicochemical activated sludge composite treatment system
Due to the high proportion of polymer compounds that are difficult to degrade in percolation water and the inhibition of heavy metals, biological methods and physical methods are commonly used? A composite system combining chemical methods is used to treat landfill leachate. For BOD5? Some scholars used this method to treat the leachate of 1500m g/l, cl-800mg/l, hardness (calculated by CaCO3) 800mg/l, total iron 600mg/l, organic nitrogen 100mg/l, TSS 300mg/l, so2-4300mg/l, and found that the effect was very good, and the removal rates of BOD5, COD, NH3-N, Fe reached 99%, 95%, 90%, 99.2% respectively. After the influent in the system passes through the regulating tank, the instantaneous high concentration of toxic substances can be avoided to inhibit activated sludge organisms; Adding lime to the clarifier can remove heavy metals and some organic matter; The air stripping tank (aerating, adding NaOH when the temperature is low) can remove 50% of the influent NH3-N, so that the concentration of NH3 is under the inhibition level; Since phosphorus in the wastewater is precipitated by added lime and the pH value is too high, phosphorus and acidic substances need to be added; The activated sludge system can be used in series or in parallel. During operation, the conventional method or delayed aeration method can be selected by adjusting the reflux sludge ratio, which has greater operational flexibility.
2.1.2 aeration stabilization pond
Compared with activated sludge process, aeration stabilization pond has large volume and low organic load. Although the degradation progress is slow, it is a cost-effective aerobic biological treatment method of landfill leachate in areas where land is not expensive due to its simple engineering. Small scale, pilot scale and production scale studies in the United States, Canada, Britain, Australia and Germany have shown that aeration stabilization ponds can achieve better landfill leachate treatment results.
For example, the UK invested 60000 pounds in Bryn posteg landfill to build a 1000m3 aeration oxidation pond with two surface aeration devices. The small hydraulic retention time is 10d. The water from the oxidation pond flows through a 3km long pipeline after sedimentation into the urban sewer. This system began to operate in 1983. The large CODcr of leachate is 24000mg/l, the large BOD5 is 10000mg/l, f/m=0.05 ~ 0.3kgcod/ (kgmlss · d), the variation range of water volume is 0 ~ 150m3/d, and the average BOD5 of effluent is 24mg/l. But occasionally, when it exceeds 50mg/l, the COD removal rate reaches 97%, but p needs to be added in the operation process. Considering the daily operation cost, investment repayment and interest, compared with the direct discharge of leachate to the municipal pipe network, Save £ 750 a year.
The British water research center also conducted a pilot test of aeration stabilization pond for leachate with CODcr > 15000mg/l from New Park landfill in the southeast. When the load is 0.28 ~ 0.32kgcod/ (kgmlss · d) or 0.04 ~ 0.64kgcod/ (kgmlss · d) and the sludge age is 10 days, the removal rates of COD and BOD5 are more than 98% and 91% respectively. Phosphoric acid also needs to be added during operation.
2.1.3 biofilm
Compared with activated sludge process, biofilm process has the advantages of water resistance and water quality impact load, and biofilm can grow microorganisms with long generations, such as nitrifying bacteria. C.peddie and j.atwater of the University of British Columbia in Canada used a 0.9m diameter biological rotating disc to treat weak leachate with codcr<1 000mg/l and nh3-n<50m g/l. The effluent bod5<25mg/l. When the temperature rises, the nitrification ability of microorganisms will be restored. However, it should be pointed out that the nature of this leachate is similar to that of municipal sewage, and whether this method is applicable to strong leachate remains to be studied.
2.2 anaerobic biological treatment
The purposeful application of anaerobic biological treatment has a history of nearly 100 years. However, in the past 20 years, with the development of Microbiology, biochemistry and other disciplines and the accumulation of engineering practice, new anaerobic treatment processes have been continuously developed, which overcome the characteristics of long hydraulic retention time and low organic load of the traditional process, so that it has made great progress in theory and practice, and achieved good results in the treatment of high concentration (BOD5 ≥ 2000? Mg / L) organic wastewater.
Anaerobic biological treatment has many advantages. The main ones are less energy consumption and simple operation, so the investment and operation cost are low. Moreover, due to the small amount of excess sludge generated, the required nutrients are also less. For example, bod5/p only needs to be 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. With ordinary anaerobic nitrification, 35 ℃, load of 1kgcod/ (M3 · d), residence time of 10d, the COD removal rate in leachate can reach 90%.
The developed anaerobic biological treatment methods include: anaerobic biofilter, anaerobic contact tank, upflow anaerobic sludge bed reactor and segmented anaerobic nitrification.
2.2.1 anaerobic biofilter
Anaerobic filter is suitable for treating dissolved organic matter. The average COD of leachate from Halifax highway101 landfill in Canada is 12850mg/l, bod5/cod is 0.7, and pH is 5.6. The leachate is first adjusted to ph=7.8 by lime water, and then precipitated for 1H before entering the anaerobic filter (this process also plays the role of removing heavy metals such as Zn). When the load is 4kgcod/ (M3 · d), the COD removal rate can reach more than 92%; When the load increases again, the removal rate decreases sharply.
J.g.henry of the University of Toronto in Canada also successfully treated landfill leachate aged 1.5 and 8 years with anaerobic filters at room temperature, with COD of 14000mg/l and 4000? Mg/l, bod5/cod are 0.7 and 0.5 respectively. When the load is 1.26 ~ 1.45kgcod/ (M3 · d) and the hydraulic retention time is 24 ~ 96h, the COD removal rate can reach more than 90%. When the load increases again, the removal rate also decreases sharply. It can be seen that although the load of anaerobic filter can reach 5 ~ 20kgcod/ (M3 · d) when treating high concentration organic sewage, the load of leachate must be kept at a low level in order to obtain the ideal treatment effect.
2.2.2 upflow anaerobic sludge bed
The UK Water Research Center reported that the upflow anaerobic sludge bed (UASB) was used to treat leachate with cod>10000mg/l. When the load was 3.6 ~ 19.7kgcod/ (M3 · d), the average sludge age was 1.0 ~ 4.3d, and the temperature was 30 ℃, the removal rates of COD and BOD5 were 82% and 85% respectively.
