1 Leachate water quality characteristics and treatment process
Leachate is a liquid containing organic or inorganic components produced during the storage and landfilling process due to physical, biological, and chemical actions such as compaction and fermentation, as well as under the action of precipitation and other external incoming water . Leachate generally has the characteristics of high pollutant content, high ammonia nitrogen content, high chroma, strong toxicity, long pollution time, etc. It is a kind of high-concentration organic wastewater with complex composition.
With the increase of landfill age, the degradation rate of microorganisms, the water-holding capacity and the water permeability of the garbage will change. Most of the organic compounds in the leachate of the middle-aged and old landfill are long-chain carbohydrates that are difficult to degrade Or humus, and generally has the characteristics of low biodegradable substances and high ammonia nitrogen concentration . At present, the commonly used treatment method for domestic leachate is a combined treatment process with MBR as the core. The main treatment processes are usually as follows:
(1) Pretreatment. Including grille, regulating pool and other devices. The leachate to be treated can retain coarse suspended matter and homogenize water quality and quantity through pretreatment.
(2) Pre-treatment. Including physicochemical treatment such as ammonia stripping, adding adsorbent, coagulation and sedimentation. This treatment stage needs to select a specific process based on the water quality in the leachate. If high concentrations of ammonia nitrogen are present in the water, ammonia stripping must be considered for pre-treatment; if there is a certain color in the water, difficult to degrade organic matter, heavy metal ions, You can consider using activated carbon adsorption for pre-treatment to reduce the load on subsequent processing facilities.
(3) Main processing. For MBR combined process processing technology. Through the process combination, better ammonia nitrogen and organic matter treatment results can be achieved.
(4) Post-processing. Adopt membrane treatment or physical and chemical treatment for advanced treatment. The membrane treatment process can further treat heavy metal ions and non-biochemical organic matter, and improve the quality of the effluent. The selection of the post-treatment process should be based on the engineering cost and the water quality standards to be achieved.
2 Leachate combined treatment process with MBR as the core
2.1 Biochemical treatment + MBR + membrane treatment
Most of the landfill leachate
needs to be processed through a combined process to reach the pollutant emission concentration limit required in GB16889-2008 "Pollution Control Standards for Domestic Waste Landfill Sites". The combined process commonly used in leachate treatment is a combination of biochemical treatment + MBR + membrane treatment. The upflow anaerobic sludge bed reactor (UASB) is suitable for the treatment of high-concentration organic wastewater due to its large load capacity, so many Scholars combined this process with MBR, and explored the treatment effect of the combined process on leachate through experiments or engineering applications. The research results show that the UASB + MBR combined process can give full play to the combination of anaerobic, aerobic biochemical treatment and membrane treatment. The technical advantages of the leachate effluent treated by this process are stable and the main technical indicators CODCr and ammonia nitrogen can meet the discharge requirements [3-8].
The UASB anaerobic treatment process can reduce the operating cost to a certain extent, but it is easy to cause the imbalance of carbon-nitrogen ratio in the subsequent A / O biochemical pool. Therefore, care should be taken to control the operation process to reduce the imbalance of carbon-nitrogen ratio in the effluent.
Due to the extremely low operating load of the UASB process in cold regions, it should be considered that this process is not suitable for use in the Northeast and other regions. Later, as the technology matured, the third-generation anaerobic reactor UBF appeared. UBF combines the advantages of UASB and AF and integrates granular sludge and biofilm into one. When treating landfill leachate with large changes in water quality and high pollutant concentration It has advantages and is often used in combination with the MBR process to treat the leachate from the incineration plant. UBF + MBR combined process is used to treat the leachate of the incineration power plant. The project operation results show that the comprehensive benefits are good, but the nitrogen compounds in the leachate are high. The two-stage UBF + MBR + NF combined process is used to treat the leachate from the waste incineration plant. The operation results show that the removal rates of CODCr, BOD5, SS, and ammonia nitrogen are all greater than 99.8%. The technology is feasible and economically reasonable.
The mass concentration of ammonia nitrogen in the leachate of aging landfills with a landfill age of more than 10a is usually as high as 3000-4000 mg / L. Therefore, anaerobic treatment technology is often used as a pre-treatment process in the treatment of aging leachate in engineering. .
An anaerobic aerobic membrane bioreactor and anaerobic + GAC membrane bioreactor were used to treat the aged landfill leachate in a landfill in Taizhou, and the MBR effluent was further processed in combination with the NF / RO process. The results showed that The effluent basically meets the requirements of industrial reuse water.
Taking the late landfill leachate of the landfill as the treatment object, it was verified that the effluent treated by the hydrolytic acidification + hypoxia + MBR combined process has a stable and efficient water quality. Although nitrification and denitrification biological denitrification can achieve good denitrification effect and stable operation, it often needs to add a large amount of carbon source, which is not enough in economic benefits. However, anaerobic ammonia oxidation has a good denitrification effect. Compared with traditional denitrification methods, it can effectively reduce energy consumption and does not require an external carbon source. The CODCr concentration is also low. Therefore, some scholars have combined it with MBR to discuss treatment of infiltration. The effect of the filtrate.
Studies have shown that the two-stage anaerobic + anaerobic ammonia oxidation + MBR combined process can treat the high-concentration organic wastewater such as incinerator leachate to fully utilize the advantages of anaerobic treatment, remove organic matter and ammonia nitrogen, and reduce pollutants in subsequent MBR systems. load. This process can remove ammonia nitrogen greatly, only need to design first-level nitrification and denitrification, and it is a stable and sustainable biological treatment technology without additional carbon source.
2.2 MBR + membrane treatment / physical and chemical treatment
2.2.1 MBR + film processing
Membrane treatment technology mainly uses a membrane to separate the solvent from solutes and particulates. Microfiltration (MF) membranes and ultrafiltration (UF) membranes have larger pore sizes and lower removal rates of pollutants. They are generally used as pretreatment technologies for leachate; Nanofiltration (NF) membrane and reverse osmosis (RO) membrane have high removal rate of pollutants in leachate, and are generally used as advanced treatment technology for landfill leachate . Membrane treatment technology is used for the final treatment of leachate, and many scholars have conducted in-depth research on its effect.
The external MBR + NF process combination was used to treat the landfill leachate with a mass concentration of 4760, 1840, 835, and 690 mg / L of influent CODCr, BOD5, ammonia nitrogen, and SS, and the results of the project operation showed that the above indicators can achieve emissions Standard, the removal rate can reach more than 96%. The MBR + RO combined process is used to treat the leachate of Chengdu Changan landfill. The project operation results show that the process can effectively remove most of the organic matter, ammonia nitrogen and SS.
Productive tests were conducted using the MBR + NF + RO combined process. The results show that the process is simple, has strong impact load resistance, and high pollutant removal rate. Advanced treatment can further trap ammonia nitrogen through membrane treatment technology, but it will produce a concentrated solution containing high concentrations of salts, organic pollutants and heavy metal ions. Therefore, when choosing a membrane treatment process, consideration should be given to the subsequent treatment of the concentrated solution, not only Only diluted emissions. In the membrane treatment process, the membrane is easily contaminated, blocked, and the short life is the biggest disadvantage of the process. Therefore, the disc-type reverse osmosis membrane (DTRO) with strong anti-pollution ability, high operating pressure and strong pollutant retention ability is also Aroused the attention of scholars.
The MBR + DTRO + aerated zeolite biological filter process was used to explore the effect of treating landfill leachate in a project with a design water volume of 900m3 / d and a total nitrogen mass concentration of up to 2400mg / L. The project operation results show that The quality of the effluent water can reach the level A discharge standard. Although DTRO has the advantage of a longer membrane life compared to other membranes, and is also widely used in leachate treatment, there is also a phenomenon of waste of energy consumption. Therefore, it is necessary to explore related transformation schemes to save costs.
2.2.2 MBR + physical processing
In combination with MBR, the adsorption method is commonly used as the adsorption method. Activated carbon is often used in the adsorption treatment process because it has a good effect in removing difficult-to-degrade organics, heavy metal ions, and chromaticity . The combined membrane-bio-activated carbon process was used to treat the landfill leachate. As a result, it was found that the pollutants were effectively removed during the process of activated carbon adsorption + microbial degradation + membrane filtration, and the removal rate of TOC was as high as 95% or more. Because membrane pollution is the most important issue when membrane treatment is used, many scholars have also conducted in-depth experimental research on whether the addition of activated carbon will cause membrane pollution.
Studies have shown that powdered activated carbon and activated sludge together form a porous membrane with an adsorption function on the membrane surface, which improves the removal rate of organic matter in the system. In addition, this layer of PAC film can be removed during backwashing. Therefore, the pore blocking rate of the film can also be reduced, and the membrane pollution can be slowed down. Researchers have studied the combined process of PAC + MBR and found that the membrane flux recovery rate is higher after adding PAC, which can effectively reduce membrane pollution and improve the quality of effluent water [23-24]. In addition to activated carbon, other methods can be used in the physical and chemical treatment methods, such as adding coagulant to reduce the turbidity and color of wastewater, removing a variety of polymer materials, http://euhowblog.com
organic matter, certain metals Ions and soluble inorganic substances such as nitrogen and phosphorus, or the formation of insoluble salt precipitation to remove ammonia nitrogen and heavy metal ions in the exuded water.
However, the selection of the coagulant and the amount of the coagulant in such a treatment method have a great influence on the treatment effect and the cost. Therefore, attention should also be paid to the type and amount of the coagulant. Leachate with a high ammonia nitrogen content is usually pretreated with ammonia stripping to reduce the load of subsequent biological nitrogen removal and ensure that the leachate treatment meets the standard discharge. A large amount of lime needs to be added during the ammonia stripping process. The transportation, storage, and use of lime will pollute the surrounding environment, and the ammonia stripped out will need to be recovered. The problem of disposal of recovered ammonium sulfate is also a difficult point. Therefore, how to effectively balance the environmental impact and treatment cost brought by ammonia stripping treatment.
2.3 Advanced Oxidation Method + MBR + Advanced Processing
The advanced oxidation method is generally used as a pretreatment of landfill leachate. The main purpose is to improve the biodegradability of wastewater or directly remove the difficult-to-degrade organic components in the seepage water. Http://euhowblog.com
uses the UV / H2O2 + MBR combination The landfill leachate with the mass concentration of CODCr of 850-950mg / L and the mass concentration of ammonia nitrogen of 450-550mg / L was treated by the process. The test results show that MBR has a significant biochemical degradation effect on organic matter and has good nitrification in the reactor. Ozone oxidation technology converts pollutants in wastewater into low-toxic intermediate products or inorganic substances through hydroxyl radicals generated by ozone decomposition. It is a green and environmentally friendly process technology. In this process, the dosage of ozone and leachate Factors such as water quality in the original solution will affect the treatment effect of the process.
The influencing factors were analyzed through experiments, and the experiments showed that the ozonation method and the catalytic ozonation method have a high removal rate of the color and humic acid of landfill leachate, and can effectively improve the biodegradability of landfill leachate. . Using ozone advanced oxidation technology, combined with coagulation pretreatment and biochemical treatment tests, the results show that for the MBR effluent from the waste incineration plant, when the total AOP dosage is 3.0 to 3.5 units, it can meet the requirements of the new emission standard. This process is suitable for advanced treatment of landfill leachate.
Although ozone technology has the advantages of no secondary pollution, etc., the oxidizing property of ozone is selective. Therefore, it is difficult to completely remove CODCr and TOD in water, and the operation cost is high. In most cases, it needs to be combined with other processes.
The combined treatment of leachate from the waste incineration plant with ultrasonic and MBR was conducted. The test results show that the ultrasonic pretreatment can improve the removal effect of CODCr and ammonia nitrogen by MBR in the later stage, but the effluent effect is not ideal.
The Fenton oxidation method in the advanced oxidation method is also often widely used in the process of treating leachate. The effect of the combined process of Fenton oxidation process + MBR on leachate has been experimentally studied. The test results show that when the initial pH of the reaction is 4 When the dosage of H2O2 is 0.048mol / L and the ratio of the amount of H2O2 and Fe2 + is 2.5: 1, the best treatment effect can be achieved. Although the advanced oxidation method has the advantages of high efficiency, thoroughness, wide application range, and no secondary pollution, the treatment cost is generally high and the oxidant consumption is high, so it is not suitable for low concentration and large flow wastewater.
2.4 Composite membrane bioreactor (HMBR)
HMBR is a new and efficient sewage treatment system formed by organic combination of membrane separation technology and traditional wastewater bioreactor.
HMBR combines the common advantages of biodegradation and high-efficiency membrane retention in the activated sludge process, which can greatly improve the system effluent. In addition, due to the lower sludge concentration in the membrane module separation zone, it can effectively delay membrane pollution and improve Membrane use efficiency.
The HMBR process was used to study the deammonia operation environment of landfill leachate. The operation results show that the removal rate of ammonia nitrogen reaches 95% to 98%. Using HMBR to treat high-concentration ammonia nitrogen landfill leachate can not only effectively degrade large molecular substances, but also Efficient removal of ammonia nitrogen. The removal of ammonia nitrogen from landfill leachate in the middle and late stages is a difficult point in landfill leachate treatment
. The HMBR process was used to treat the old landfill leachate. The pilot test results showed that the sludge concentration in the HMBR was relatively high, which played a significant role in removing ammonia nitrogen, but the removal rate of CODCr was only 56.85%. In order to discuss the treatment of leachate by single process MBR and combined process A / O + MBR, A / O + HMBR, Cui Jia  compared experimentally the effects of three different processes on CODCr, ammonia nitrogen, and total nitrogen. The results show that the A / O + HMBR process, due to the enrichment of biofilm on the surface of the filler, not only has anoxic conditions at the macro level, but also has anoxic conditions at the micro level, which makes the denitrification reaction of nitrate in the system more fully. The removal effect is significantly better than the other two forms.
Landfill leachate generally has the characteristics of high organic pollutant content, high ammonia nitrogen content, large chroma, strong toxicity, etc., and the water quality changes greatly with different landfill ages. At present, various types of combined treatment processes with MBR as the core are mainly divided into the following: ① biochemical treatment + MBR + membrane treatment; ② MBR + membrane treatment / physical and chemical treatment; ③ advanced oxidation method + MBR + advanced treatment; ④ HMBR. They have a good effect on the treatment of landfill leachate, but at the same time there are problems that the membrane is easily polluted and the operating cost is high. Use MBR as the core of various combined process technologies to treat landfill leachate. In the future, we should focus on solving and in-depth research and development of low-energy, low-cost, high-performance, pollution-resistant membrane materials to reduce investment and operating costs; rationally design the combined process, Pay attention to pretreatment; choose appropriate membrane cleaning methods.