[Hongjie Water] A landfill leachate treatment process with MBR as the core

According to national statistical data, the harmless treatment rate of domestic waste in China is on the rise year by year, and the main methods of waste treatment and disposal in China are sanitary landfill and incineration. Due to both the recent landfill based waste disposal method and the future incineration based waste disposal method, non combustible waste ultimately needs to be treated through landfill.

The most concerning issue for landfill sites is the treatment of leachate. As membrane bioreactor (MBR) process is one of the commonly used leachate treatment combination processes, it is necessary to study various forms of combining MBR as the core process with other processes. This article summarizes various combination processes for treating landfill leachate with MBR as the core, summarizes their research and engineering application status, analyzes existing problems, proposes applicable scope, and puts forward some suggestions for research related to MBR technology, in order to provide reference for future landfill leachate treatment.

1. Characteristics of Leachate Water Quality and Treatment Process

Leachate is a liquid containing organic or inorganic components produced during the stacking and landfilling process of garbage due to physical, biological, and chemical reactions such as compaction and fermentation, as well as the infiltration of precipitation and other external water [1]. Garbage leachate generally has characteristics such as high pollutant content, high ammonia nitrogen content, high chromaticity, strong toxicity, and long pollution time, and is a high concentration organic wastewater with complex components.

As the age of landfilling increases, the degradation rate of organic matter in garbage by microorganisms, the water holding capacity of garbage, and the permeability of water will change. The organic matter in the leachate of middle-aged and elderly landfills is mostly difficult to degrade long-chain carbohydrates or humus, and generally has the characteristics of low content of biodegradable substances and high concentration of ammonia nitrogen [2]. At present, the commonly used treatment method for leachate in China is the combination treatment process with MBR as the core, and the main treatment process is usually as follows:

(1) Preprocessing. Including grilles, regulating tanks, and other devices. The leachate to be treated can retain coarse suspended solids and homogenize the water quality and quantity through pretreatment.

(2) Pre processing. This includes physical and chemical treatments such as ammonia stripping, addition of adsorbents, coagulation and precipitation. In this treatment stage, specific processes need to be selected based on the water quality of the leachate. If there is high concentration of ammonia nitrogen in the water, ammonia stripping should be considered for pre-treatment; If there is a certain degree of chromaticity, difficult to degrade organic matter, heavy metal ions, etc. in the water, it can be considered to use activated carbon adsorption and other methods for pre-treatment to reduce the load on subsequent treatment facilities.

(3) Main processing. MBR combined process treatment technology. By combining processes, better ammonia nitrogen and organic matter treatment effects can be achieved.

(4) Post processing. Use membrane treatment or physicochemical treatment for advanced treatment. The membrane treatment process can further treat heavy metal ions and non biodegradable organic matter, improving the effluent quality. The selection of post-processing technology should be combined with engineering costs and the water quality standards to be achieved.

2. MBR based combined treatment process for leachate

2.1 Biochemical treatment+MBR+membrane treatment

The vast majority of leachate from municipal solid waste needs to be treated through a combination process to meet the pollutant emission concentration limits required by GB16889-2008 Pollution Control Standards for Municipal Solid Waste Landfills. The commonly used process combination in leachate treatment is the use of biochemical treatment+MBR+membrane treatment combination process. The upflow anaerobic sludge bed reactor (UASB) is suitable for the treatment of high concentration organic wastewater due to its large load capacity. Therefore, many scholars combine this process with MBR to explore the treatment effect of this combination process on leachate through experiments or engineering application studies. The research results show that the UASB+MBR combination process can fully leverage the technical advantages of anaerobic, aerobic biochemical treatment and membrane treatment. The effluent water quality of the leachate treated by this process is stable, and the main technical indicators CODCr and ammonia nitrogen can meet the discharge requirements [3-8].

The UASB anaerobic treatment process can reduce operating costs to a certain extent, but it is easy to cause an imbalance in the carbon to nitrogen ratio in the subsequent A/O biochemical tank. Therefore, attention should be paid to controlling the operating process to reduce the imbalance in the carbon to 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 regions such as Northeast China. Later, with the maturity of technology, the third-generation anaerobic reactor UBF emerged. UBF combines the advantages of UASB and AF, integrating granular sludge and biofilm. It has advantages in treating leachate with large water quality changes and high pollutant concentration, and is often used in combination with MBR process to treat leachate from incineration plants. The UBF+MBR combined process was used to treat the leachate from incineration power plants. The engineering operation results showed good comprehensive benefits, but the leachate contained high nitrogen compounds. The two-stage UBF+MBR+NF combined process was used to treat the leachate from the garbage incineration plant. The operating results showed that the removal rates of CODCr, BOD5, SS, and ammonia nitrogen were all greater than 99.8%, which is technically feasible and economically reasonable.

The mass concentration of ammonia nitrogen in the leachate of aging landfill sites with a landfill age of over 10 years is usually as high as 3000-4000mg/L. Therefore, anaerobic treatment technology is often used as a pre-treatment process in the treatment of aging leachate in engineering.

Anaerobic aerobic membrane bioreactor and anaerobic+GAC membrane bioreactor were used to treat the aging leachate of a landfill in Taizhou, and combined with NF/RO process, MBR effluent was further treated. The results showed that the effluent basically met the requirements for industrial reuse.

The late stage leachate from the Asuwei landfill in Beijing was taken as the treatment object, and the combined process of hydrolysis acidification, hypoxia, and MBR was verified to have stable and efficient water quality in the effluent treatment. Although nitrification denitrification can achieve good denitrification results and operate stably, it often requires the addition of a large amount of carbon sources, which is not economically advantageous. However, anaerobic ammonia oxidation has a good denitrification effect, which is more effective in reducing energy consumption compared to traditional denitrification methods and does not require additional carbon sources. The CODCr concentration is also lower. Therefore, some scholars have explored the effect of combining it with MBR to treat leachate.

This process can significantly remove ammonia nitrogen, requiring only the design of primary nitrification denitrification without the need for additional carbon sources. It is a stable and sustainable biological treatment technology.

2.2MBR+membrane treatment/physicochemical treatment

2.2.1MBR+membrane treatment

Membrane treatment technology mainly uses membranes to separate solvents from solutes and particles. Among them, microfiltration (MF) and ultrafiltration (UF) membranes have larger pore sizes and lower pollutant removal rates, and are generally used as pretreatment technologies for leachate; Nanofiltration (NF) and reverse osmosis (RO) membranes have high removal rates of pollutants from leachate, and are generally used as advanced treatment technologies for landfill leachate [14]. Many scholars have conducted in-depth research and exploration on the final treatment of leachate using membrane treatment technology.

The external MBR+NF process combination was used to treat landfill leachate with influent CODCr, BOD5, ammonia nitrogen, and SS mass concentrations of 4760, 1840, 835, and 690mg/L, respectively. The engineering operation results showed that all the above indicators met the emission standards, and the removal rate could reach over 96%. The MBR+RO combined process is used to treat the leachate from Chengdu Chang'an landfill site. The engineering operation results show that this process can effectively remove the vast majority of organic matter, ammonia nitrogen, and SS.

The production experiment was conducted using the MBR+NF+RO combined process, and the results showed that the process flow was simple, the impact load resistance was strong, and the pollutant removal rate was high. Deep treatment can further intercept ammonia nitrogen through membrane treatment technology, but it will produce concentrated solutions containing high concentrations of salts, organic pollutants, and heavy metal ions. Therefore, when selecting membrane treatment technology, the subsequent treatment of the concentrated solution should be considered, rather than just dilution and discharge. The biggest drawback of membrane treatment process is that the membrane is prone to contamination and blockage, and its lifespan is short. Therefore, the disc tube reverse osmosis membrane (DTRO) with strong anti fouling ability, high operating pressure, and strong pollutant retention ability has also attracted the attention of scholars.

Although DTRO has the advantage of longer membrane life compared to other membranes and is widely used in leachate treatment, there is also a phenomenon of energy consumption waste. Therefore, it is necessary to explore relevant renovation plans to save costs.

2.2.2MBR+physical processing

The combined membrane biological activated carbon process was used to treat landfill leachate, and it was found that pollutants were significantly removed in the process of activated carbon adsorption+microbial degradation+membrane filtration, with a TOC removal rate of over 95%. Due to the fact that membrane fouling is the most concerning issue when using membrane treatment, many scholars have conducted in-depth experimental research on whether adding activated carbon will cause membrane fouling.

Research has shown that powdered activated carbon and activated sludge jointly form a porous membrane with adsorption function on the membrane surface, improving the removal rate of organic matter in the system. In addition, this layer of PAC membrane can be removed during backwashing, thus reducing the blockage rate of membrane pores and slowing down membrane fouling. Researchers have studied the PAC+MBR combination process and found that the recovery rate of membrane flux is higher after adding PAC, which can effectively reduce membrane fouling and improve the effluent quality [23-24]. In addition to using activated carbon, other methods can also be used in physical and chemical treatment, such as adding coagulants to reduce wastewater turbidity and chromaticity, removing various high molecular weight substances, organic matter, certain metal ions, as well as soluble inorganic substances such as nitrogen and phosphorus, or forming insoluble salt precipitates to remove ammonia nitrogen and heavy metal ions from permeated water.

However, the selection and dosage of coagulants in such treatment methods have a significant impact on the treatment effect and cost. Therefore, attention should also be paid to the types and dosage of coagulants. Ammonia stripping is usually used for pre-treatment in leachate with high ammonia nitrogen content to reduce the load of subsequent biological denitrification and ensure that the leachate treatment meets the discharge standards. During the ammonia stripping process, a large amount of lime needs to be added. The transportation, storage, and use of lime can cause pollution to the surrounding environment, and the ammonia blown out needs to be recovered. The disposal of recovered ammonium sulfate is also a difficult problem. Therefore, how to effectively balance the environmental impact and treatment cost brought about by ammonia stripping treatment should be considered.

2.3 Advanced oxidation method+MBR+deep treatment

The advanced oxidation method is generally used as a pre-treatment method for landfill leachate, with the main purpose of improving the biodegradability of wastewater or directly removing difficult to degrade organic components from the leachate. The UV/H2O2+MBR combined process is used to treat landfill leachate with a mass concentration of 850-950mg/L CODCr and 450-550mg/L ammonia nitrogen. The experimental results show that MBR has a significant biochemical degradation effect on organic matter, and the nitrification effect in the reactor is good. Ozone oxidation technology is a green and environmentally friendly process that converts pollutants in wastewater into low toxic intermediate products or inorganic substances through hydroxyl radicals generated by ozone decomposition. In this process, factors such as the amount of ozone added and the water quality in the filtrate will affect the treatment effect of the process.

Using advanced ozone oxidation technology, combined with coagulation pretreatment and biochemical treatment, experiments were conducted. The results showed that for MBR effluent from garbage incineration plants, the new emission standards can be met when the total AOP dosage is 3.0-3.5 units. This process is suitable for deep treatment of garbage leachate.

Although ozone technology has the advantages of no secondary pollution, its oxidizing ability is selective, making it difficult to completely remove CODCr and TOD from water. Moreover, the operating cost is high, and in most cases, it needs to be combined with other processes for application.

The combined treatment of ultrasound and MBR was used to treat leachate from garbage incineration plants. The experimental results showed that ultrasound pretreatment can improve the removal efficiency of CODCr and ammonia nitrogen by MBR in the later stage, but the effluent effect is not ideal.

The Fenton oxidation method in advanced oxidation methods is also widely used in the treatment of leachate. The treatment effect of the Fenton oxidation process+MBR combination process on leachate was studied through experiments. The experimental results showed that the optimal treatment effect can be achieved when the initial pH value of the reaction is 4, the amount of H2O2 added is 0.048mol/L, and the ratio of H2O2 to Fe2+substances is 2.5:1.

2.4 Composite Membrane Bioreactor (HMBR)

HMBR is a new and efficient wastewater treatment system that combines membrane separation technology with traditional wastewater bioreactors.

HMBR combines the common advantages of biodegradation and efficient membrane retention in activated sludge process, which can significantly improve the effluent of the system. In addition, due to the low sludge concentration in the separation zone of membrane components, it can effectively delay membrane fouling and improve membrane utilization efficiency.

The HMBR process was used to study the ammonia removal operating environment of landfill leachate. The results showed that the removal rate of ammonia nitrogen reached 95% to 98%. Using HMBR to treat high concentration ammonia nitrogen landfill leachate can not only effectively degrade macromolecular substances, but also efficiently remove ammonia nitrogen. The removal of ammonia nitrogen from landfill leachate in the middle and later stages is a challenge in landfill leachate treatment. The HMBR process was used to treat the leachate of aged garbage. The pilot test results showed that the sludge concentration in HMBR was high, which played a significant role in removing ammonia nitrogen, but the removal rate of CODCr was only 56.85%. In order to explore the treatment of leachate by single process MBR and combination process A/O+MBR, A/O+HMBR, Cui Jia [32] compared the effects of three different processes on CODCr, ammonia nitrogen, and total nitrogen through experimental research. The results showed that the A/O+HMBR process, due to the enrichment of biofilm on the surface of the filler, not only has anaerobic conditions at the macro level, but also has anaerobic conditions at the micro level, making the denitrification reaction of nitrate in the system more complete, The removal effect is significantly better than the other two forms.

3 Conclusion

Garbage leachate generally has characteristics such as high organic pollutant content, high ammonia nitrogen content, high chromaticity, and strong toxicity, and the water quality changes significantly with different landfill ages. At present, various combination treatment processes with MBR as the core are mainly divided into the following types: ① biochemical treatment+MBR+membrane treatment; ② MBR+membrane treatment/physicochemical treatment; ③ Advanced oxidation method+MBR+deep treatment; ④ HMBR. They all have good effects on the treatment of landfill leachate, but at the same time, there are also problems of membrane contamination and high operating costs. Adopting various combination process technologies with MBR as the core to treat landfill leachate, we should focus on solving and in-depth research and development of membrane materials with low energy consumption, low cost, high performance, and anti pollution, in order to reduce investment and operating costs; Reasonably design the combination process and attach importance to pretreatment; Choose the appropriate membrane cleaning method.

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