Sanitary landfill is the most common domestic waste treatment technology in China. Through aerobic and anaerobic reaction, the inorganic substances and organic matter produced by decomposition in the waste will form the landfill leachate with rainwater, water brought by the garbage itself and the water generated by decomposition. Landfill leachate is a kind of high concentration wastewater with complex composition. If it is not treated properly, it will seriously pollute the groundwater, surface water and soil in the landfill area, which will bring great harm to the ecological environment and human health. Therefore, the leachate of municipal landfill must be properly treated.

1、 Project overview

For example, the landfill waste in a county is mainly residential waste. According to the investigation and analysis of the population size, the number of garbage collection stations and the transport capacity of the county, the production scale of the leachate in this project is determined to be 100t / d. The landfill landfill landfill landfill leachate mainly has two sources, most of which are filtered out of fresh waste after landfill, and the other part is from the transformation of rainfall from the landfill operation surface and the middle cover surface. The leachate is mostly the product of acid fermentation stage of garbage, which belongs to the typical primary filtrate, and the concentration of pollutants is high. See Table 1 for the water quality of inlet and outlet water of the project. The effluent quality requirements meet the discharge standards specified in Table 2 of gb16889-2008 pollution control standard for domestic waste landfill.

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2、 Process design

2.1 process flow

The quality of landfill leachate is affected by the factors such as waste composition, precipitation, landfill process and landfill time, which has the characteristics of complex composition, high organic concentration and large water quality change. It is difficult to meet the standard by conventional biochemical treatment. Compared with biochemical method, membrane separation technology is less affected by the change of raw water quality, and can keep the water quality stable, and it has obvious advantages in the treatment of landfill percolation liquid. The process flow of Jinzheng environmental protection is as follows: the waste leachate is treated by DTRO membrane

The secondary DTRO process is shown in Figure 1.

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The leachate is collected to the regulating tank for water quality and water quantity regulation. The effluent of the raw water storage tank is adjusted to pH value by adding acid to prevent the scaling of carbonate inorganic salt, and then the SS concentration is reduced by sand filter and core filter. The pre-treatment leachate enters the first stage system, and reverse osmosis is carried out in the membrane module. The generated liquid enters the second DTRO system, and the first stage DTRO concentrate is discharged into the concentrate storage tank for reinjection; The second DTRO system is discharged into degassing tower through liquid, and the water carbon dioxide and other gases are blown off to make the pH value reach 6-9, then it enters the clean water tank, and then discharge after reaching the standard. The second stage DTRO concentrate flows back to the inlet end of the first DTRO.

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The inevitable product of membrane treatment is concentrated liquid. The project adopts the method of reinjection. There are many methods for the treatment of concentrated liquid, including incineration, curing, distillation and drying, but compared with the reinjection method, the equipment investment and operation cost of other methods are very huge. Landfill waste dump is a huge bioreactor and storage body. A large number of organic pollutants in the waste are digested and stabilized here. Although the concentration of pollutants in the concentrate is very high, the amount of pollutants is less than the total amount of pollutants in the waste, accounting for about 2.4%. Landfill is used as a biofilter with garbage as filler, and concentrated liquid is recycled in a controlled way, and pollutants are degraded by physical, chemical and biological functions.

2.2 reinjection of concentrate

The concentrated liquid of the project adopts shallow reinjection method, that is, control the distribution well point and reinjection water amount of the reinjection pipeline system, so that the concentrate can be accepted within 2-3m thickness of the surface of the landfill body, so as to prevent the formation of saturated column in the waste body of reinjection due to the concentration of the reinjection amount, and reduce the effect of reinjection treatment.

Because of the high load of organic pollutants in the concentrated liquid, the reinjection rate should be controlled at 1.6-2.5l/ (H · m2). The daily treatment of leachate in the project is 78% and the total production of concentrated liquid is 22T / d. The area of reinjection is calculated as per the reinjection rate of 1l / h · m3. 5 circular reinjection points are designed, and each service area is not less than.

3、 Calculation parameters of main structures

3.1 pretreatment.

The leachate is lifted from the regulating tank to the raw water tank of the reverse osmosis system, and H2SO4 is added. The amount of the leachate is 1.0-1.5l/m3 to remove the insoluble carbonate inorganic substances and eliminate the membrane pollution.

The water from the raw water tank is pressurized by the water pump and then enters into quartz sand filter, with the filtration accuracy of 50 μ m。 The backwash procedure shall be carried out when the pressure difference between inlet and outlet water of sand filter exceeds 25kpa. The backwash period is about 100h. For raw water with low SS value, if the pressure difference is not more than 25kpa after 100h of sand filter operation, backwashing shall be carried out to avoid excessive compaction and sheet forming of quartz sand. The backwash procedure shall be activated automatically in time first. Sand filter backwash adopts the combination of gas and water, first air washing and then water washing. The flushing time is generally 5min. It can be set separately according to the operation status, and the flushing wastewater is discharged to the regulating tank.

After sand filtration, it enters the core filter with filtration accuracy of 10 μ m. Use 10 μ MP fusion jet filter element, when the inlet and outlet pressure reaches 200KPa, replace the filter element. A certain amount of scale inhibitor is added in front of the core filter to prevent fouling of membrane system. The scale inhibitor is a mixture of polymer and salt, and the dosage is 115mg scale inhibitor and 1mg silicate.

3.2 primary DTRO

The percolation liquid through core filter enters the membrane column through high pressure pump. One high pressure plunger pump is set, and one shock absorber is set behind the pump to absorb the pressure pulse generated by the pump, and to provide stable pressure to the reverse osmosis membrane column; Two high-pressure online pumps are set. Because the flow of high-pressure pump is difficult to guarantee the water demand of membrane column, a part of concentrated liquid at the outlet of membrane column is returned to the membrane column through on-line pump to ensure sufficient flow on membrane surface (each membrane column is not less than 0.8m3/h). The membrane material is organic composite membrane, with 46 membrane columns and a single branch area of 9.405 square meters. The water output of membrane column group is concentrated liquid and through liquid. One control valve is set at the end of concentration liquid to control the pressure in the membrane group. The liquid enters the secondary membrane column for further treatment, and the concentrate is discharged into the concentrated liquid storage tank for reinjection treatment.

3.3 secondary DTRO

After the treatment of the first stage DTRO membrane system, the liquid through the secondary high pressure pump is directly entered into the secondary DT membrane system. The high-pressure pump is set with frequency conversion control, so that its frequency and output flow will automatically match according to the feedback value of the first stage through the liquid flow sensor. Meanwhile, the inlet pipeline is equipped with a concentrated liquid self compensation device, so that the operation of the secondary system is not affected by the water production of the primary system.

The second DTRO is equipped with 9 membrane columns and a single branch area of 9.405m2. One control valve is also set at the secondary concentrate end to control the pressure in the membrane group. The second stage membrane column concentrate is discharged to the inlet end of the first stage system to improve the recovery rate of the system and discharge it into the degassing tower through the liquid.

3.4 clear water tank

The effluent from the secondary DTRO can meet the pollution control standard of domestic waste landfill (gb16889-2008). The effluent is stored in the clean water tank, which can be discharged up to standard or used for greening irrigation, etc.

3.5 equipment washing and cleaning

System flushing: the purpose is to prevent the pollutants in the filtrate from depositing on the membrane surface. When the system is shut down, when it is required to stop under normal operation, the flushing mode is generally adopted before shutdown mode. The system will stop automatically in case of system failure and the flushing procedure shall also be performed. The system flushing is divided into filtrate flushing and purified water flushing: when the first DTRO is shut down, the raw water of the filtrate is used for flushing, and then the clean water is used for flushing; The second DTRO only needs to be washed with water. The time of both flushing can be set on the operation interface, generally 5-10min, the water consumption for flushing is 5m3/h, and the water is discharged to the regulating tank after flushing.

Chemical cleaning: to maintain the performance of the membrane, the membrane group shall be cleaned regularly, and the tank shall be regularly added with cleaning agent and scale inhibitor, and the cleaning execution time shall be set, and the cleaning shall be performed automatically. The cleaning agent is divided into two types: acid and alkali, which are special cleaning agents. The main function of alkaline cleaning agent is to remove the pollution of organic matter such as fat and humic acid. The main function of acid cleaning agent is to remove the inorganic pollution such as iron salt and carbonate. The cleaning time is generally 1-2 hours, and the liquid discharged after cleaning is sent to the regulating tank. The cleaning cycle depends on the concentration of pollutants. When the flow rate of membrane system through liquid decreases by 10% ~ 15% or the pressure difference between inlet and outlet of membrane module exceeds the allowable setting value, the cleaning cycle is as follows:

The first DTRO system, alkali washing for 5 days (pH = 10-11), pickling for 10 days (pH = 2.5-3.5);

The second DTRO system, alkali washing for 14 days (pH = 10-11), pickling for 28d (pH = 2.5-3.5).

3.6 concentrated liquid storage tank

The production of concentrated liquid produced by membrane treatment system is 22T / D, which is discharged to the concentrated liquid storage tank and then recycled by pump. The designed residence time of concentrated liquid storage tank t = 15d.

4、 Conclusion

(1) The leachate of a county domestic waste treatment plant is treated by secondary DTRO process, and the effluent effect is good, and all the indexes can meet the requirements of the discharge standard specified in Table 2 of GB 16889-2008

(2) It is an economical and reliable process to make full use of the bioreactor characteristics of landfill waste dump and to use concentrated liquid reinjection to eliminate and stabilize pollutants.