Analysis of 6 types of biological treatment processes for leachate from garbage!

Garbage leachate refers to a high concentration organic wastewater formed by the moisture contained in the garbage itself, rain, snow, and other moisture entering the landfill site, deducting the saturated water holding capacity of the garbage and covering soil layer, and passing through the garbage layer and covering soil layer. There is also accumulated water leaked from the garbage prepared for incineration.

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1、 The Generation Stage of Garbage Leachate

The properties of leachate from garbage vary with the operating time of the landfill, mainly determined by the stabilization process of garbage in the landfill. The stabilization process of landfill sites is usually divided into five stages, namely the Initial adjustment phase, Transition phase, Acid phase, Methane fermentation phase, and Maturation phase.

1. Initial adjustment stage: Garbage is filled into the landfill, and the stabilization stage of the landfill enters the initial adjustment stage. During this stage, the easily degradable components in the garbage quickly undergo aerobic biodegradation reactions with the oxygen carried in the garbage, generating carbon dioxide (CO2) and water, while releasing a certain amount of heat.

2. Transition stage: During this stage, the oxygen in the landfill is depleted, and anaerobic conditions begin to form within the landfill. The degradation of garbage transitions from aerobic degradation to facultative anaerobic degradation. At this stage, nitrate and sulfate in the garbage are reduced to nitrogen (N2) and hydrogen sulfide (H2S), respectively, and the pH of the leachate begins to decrease.

3. Acidification stage: When hydrogen gas (H2) is continuously produced in the landfill, it means that the stabilization of the landfill enters the acidification stage. At this stage, the microorganisms that play a major role in the degradation of garbage are facultative and specialized anaerobic bacteria. The main components of landfill gas are carbon dioxide (CO2), leachate COD, VFA, and metal ion concentrations, which continue to rise until reaching high values in the middle stage and gradually decrease thereafter; The pH continues to decrease to a low value and gradually increases thereafter.

4. Methane fermentation stage: When the H2 content in the landfill decreases to a low point, the landfill enters the methane fermentation stage. At this time, methanogenic bacteria convert organic acids and H2 into methane. The concentration of organic matter, metal ions, and conductivity all rapidly decrease, with a decrease in BOD/COD and a decrease in biodegradability. At the same time, the pH value begins to rise.

5. Maturity stage: When the easily biodegradable components in the landfill waste are basically degraded, the landfill enters the maturity stage. At this stage, due to the fact that the vast majority of nutrients in the garbage have been discharged with the leachate, only a small amount of microorganisms degrade some difficult to degrade substances in the garbage. At this time, the pH remains in a slightly alkaline state, and the biodegradability of the leachate further decreases. The BOD/COD will be less than 0.1. But the concentration of the filtrate is already very low.

2、 Comparison and Selection of Leachate Treatment Processes

The treatment of leachate from urban landfill sites has always been a very challenging issue in landfill design, operation, and management. Leachate is the product of gravity flow of liquids in landfills, mainly derived from precipitation and the water contained in the garbage itself. Due to the many factors that may affect the properties of the filtrate during the flow process, including physical, chemical, and biological factors, the properties of the filtrate can vary over a considerable range. Generally speaking, its pH value ranges from 4 to 9, COD ranges from 2000 to 62000mg/L, and BOD5 ranges from 60 to 45000mg/L. The concentration of heavy metals is basically consistent with that in municipal wastewater.

3、 The current status of leachate treatment technology

The treatment methods for landfill leachate include physicochemical and biological methods. The physical and chemical methods mainly include activated carbon adsorption, chemical precipitation, density separation, chemical oxidation, chemical reduction, ion exchange, membrane dialysis, and gas lift wet oxidation. When the COD is 2000-4000mg/L, the COD removal rate of the physical and chemical methods can reach 50-87%. Compared with biological treatment, physical and chemical treatment is not affected by changes in water quality and quantity, and the effluent quality is relatively stable, especially for leachate with low BOD5/COD ratios (0.07-0.20) that are difficult to biotreat. It has a good treatment effect. However, the physical and chemical methods have high processing costs and are not suitable for the treatment of large amounts of leachate from garbage. Therefore, biological methods are mainly used for the treatment of leachate from garbage.

Biological methods are divided into aerobic biological treatment, anaerobic biological treatment, and a combination of the two. Aerobic treatment includes activated sludge process, aeration oxidation tank, aerobic stabilization pond, biological rotary table, and drip filter. Anaerobic treatment includes upflow sludge beds, anaerobic immobilized bioreactors, mixed reactors, and anaerobic stabilization ponds.

4、 Introduction to Leachate Treatment Process

Garbage leachate has characteristics different from general urban sewage, such as high concentrations of BOD5 and COD, high metal content, significant changes in water quality and quantity, high content of ammonia nitrogen, and imbalanced proportion of microbial nutrients. Among the treatment methods for leachate, combining leachate with urban sewage is the simplest method. However, landfills are often far from cities, so there are specific difficulties in combining their leachate with urban sewage for treatment, and they often have to be treated separately. The commonly used processing methods are as follows

4.1 Aerobic treatment

There are successful experiences in using aerobic methods such as activated sludge method, oxidation ditch, aerobic stabilization pond, and biological rotary table to treat leachate. Aerobic treatment can effectively reduce BOD5, COD, and ammonia nitrogen, and can also remove other pollutants such as iron, manganese, and other metals. Among aerobic methods, delayed aeration is the most commonly used method, as well as aeration stabilization ponds and biological turntables (mainly used for nitrogen removal). Below will be introduced separately.

4.1.1 Activated sludge method

Traditional activated sludge process

Leachate can be treated individually or in combination with biological methods, chemical flocculation, carbon adsorption, membrane filtration, lipid adsorption, and gas extraction. Among them, the activated sludge method is widely used due to 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 sludge concentration to reduce sludge organic load, the activated sludge method can achieve satisfactory results in the treatment of landfill leachate. For example, the Fall Township sewage treatment plant in Pennsylvania, USA, has a COD of 6000-21000mg/L, BOD5 of 3000-1300mg/L, and ammonia nitrogen of 200-2000mg/L. The sludge concentration (MLVSS) of the aeration tank is 6000-1200mg/L, which is 3-6 times higher than the general sludge concentration. When the volumetric organic load is 1.87kgBOD5/(m3 · d), the removal rate of BOD5 is 97% with F/M ranging from 0.15 to 0.31kgBOD5/(kgMLSS · d); When the volume organic load is 0.3kgBOD5/(m3 · d), F/M is 0.03-0.05kg BOD5/(kgMLSS · d), and the removal rate of BOD5 is 92%. The data from the factory shows that as long as the concentration of activated sludge method is appropriately increased, the F/M should be between 0.03-0.31 kgBOD5/(kgMLSS · d) (not too high), and the use of activated sludge method can effectively treat landfill leachate.

Many scholars have also found that activated sludge can remove 99% of BOD5 from leachate, and over 80% of organic carbon can be removed by activated sludge. Even if the organic carbon in the influent reaches 1000mg/L, the sludge biofilm can quickly adapt and degrade. The activated sludge system operating under low load can remove 80% to 90% of COD from the leachate, and the effluent BOD5 is less than 20mg/L. For leachate with COD 4000-13000 mg/L, BOD51600-1100mg/L, and NH3-N 87-590mg/L, the mixed aerobic activated sludge method can achieve a stable COD removal rate of over 90%. Numerous actual operating landfill leachate treatment systems have shown that the activated sludge method has better treatment effects than other methods such as chemical oxidation.

Low oxygen aerobic activated sludge process

Improved activated sludge processes such as low oxygen aerobic activated sludge process and SBR process are more effective than conventional activated sludge process due to their ability to maintain high operating load and short time consumption. Xu Dimin and others from Tongji University used the low oxygen aerobic activated sludge method to treat leachate from landfill sites. The experiment showed that under controlled operating conditions, the leachate from landfill sites was treated with the low oxygen aerobic activated sludge method with excellent results. The average COD, BOD5, and SS of the final effluent decreased from the original 6466mg/L, 3502mg/L, and 239.6mg/L, respectively, to COD<300mg/L, BOD5<50mg/L (average 13.3mg/L), and SS<100mg/L (average 27.8mg/L). The total removal rates are COD 96.4%, BOD5 99.6%, and SS 83.4%, respectively.

If the treated effluent is further chemically coagulated with alkaline aluminum chloride, the COD of the effluent can be reduced to below 100mg/L.

The two-stage method is also better than the general biological method in treating nitrogen and phosphorus in leachate. 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 compensates for the shortcomings of the anaerobic aerobic two-stage biological treatment method, where the first stage forms more NH3-N, making it difficult to carry out the second stage and the two aerobic treatments take too long.

Physicochemical activated sludge composite treatment system

Due to the high proportion of polymer compounds that are difficult to degrade in leachate and the inhibitory effect of heavy metals, a composite system combining biological and physical chemical methods is commonly used to treat landfill leachate. For the leachate with BOD5 1500mg/L, Cl-800mg/L, hardness (calculated as CaCO3) 800mg/L, total iron 600mg/L, organic nitrogen 100mg/L, TSS 300mg/L, and SO2-4300mg/L, some scholars have used this method for treatment and found that the effect is very good. The removal rates of BOD5, COD, NH3-N, and Fe are 99%, 95%, 90%, and 99.2%, respectively. After the inlet water in the system passes through the regulating tank, it can avoid the instantaneous high concentration of toxic substances and inhibit the biological activity of activated sludge; Adding lime to the clarification tank can remove heavy metals and some organic matter; The air lift tank (aeration, adding NaOH when the temperature is low) can remove 50% of the incoming NH3-N, thereby keeping the concentration of NH3 below the inhibitory level; Due to the precipitation of phosphorus in wastewater by lime added and the high pH value, it is necessary to add phosphorus and acidic substances; The activated sludge system can be used in series or parallel, and during operation, conventional or delayed aeration methods can be selected for treatment by adjusting the reflux sludge ratio, which has great operational flexibility.