The leachate of MSW incineration plant mainly comes from the water holding capacity of MSW, the liquid produced by MSW decomposition and the rainwater brought in during the process of MSW collection and transportation. Leachate from waste incineration plant is characterized by high ammonia nitrogen content, high concentration of organic pollutants, high salt content, complex components, and large fluctuation of water quality and quantity, which makes treatment difficult.

Characteristics of leachate from waste incineration plant

1、High concentration of organic pollutants and good biodegradability

      The CODcr and BOD5 of leachate from MSW incineration plant are as high as 20000 ~ 60000mg / L and 10000 ~ 30000mg / L respectively, which belong to high concentration organic wastewater. Most of the organic compounds in the leachate are soluble organic compounds, about 90% of the soluble organic carbon is composed of short chain volatile fatty acids, the main components are acetic acid, propionic acid and butyric acid, followed by griseofulvin acid with multi carboxyl and aromatic carboxyl groups, so the biodegradability of the leachate is better.

2、High concentration of ammonia nitrogen

      The concentration of ammonia nitrogen in the leachate can be as high as 1000 ~ 3000mg / L, and the nitrogen in the leachate mostly exists in the form of ammonia nitrogen, accounting for 75% ~ 90% of the total nitrogen.

3、High salt content

       The salt content in the leachate is usually more than 10000 mg / L. if the membrane treatment is used, the water yield will be too low due to the high osmotic pressure. If only the ordinary biochemical treatment is used, the start-up will be difficult, the load will be low, and the operation will be unstable or even unable to operate because of the high salt content.

4、Water quantity and water quality fluctuate greatly

       The output of leachate is affected by the type of urban garbage collection and transportation system, the composition of garbage, rainfall and other factors. The daily output of leachate is about 5% ~ 40% of the amount of garbage. The variation range of pollutant concentration is 3 ~ 5 times.

Common technical routes of leachate treatment

1、Biochemical + Advanced Oxidation + Advanced Treatment

       Leachate has high concentration of organic pollutants and good biodegradability. Biochemical treatment process is the most thorough and economic process for treating high concentration organic wastewater. It can degrade organic pollutants in a large extent under relatively economic conditions, and give play to the effect of nitrogen and phosphorus removal, so as to save the overall treatment cost of leachate. Because there are many refractory macromolecular organics in leachate, some "inert COD" which can not be biodegraded and adsorbed will always be retained after biochemical treatment. The engineering practice shows that the CODcr of leachate can be reduced to less than 1000mg / L by using various biochemical treatment processes, and the removal rate is very considerable, but the effluent can not directly meet the requirements of the discharge standard.

2、Biochemical + membrane process treatment

      Membrane process is the most commonly used treatment method for leachate after biochemical treatment. The effluent quality of this process is good, which can reach the standard of reuse water. It also has high anti variation ability and high operation stability for the fluctuation of leachate quality and quantity. After membrane separation treatment, the effect of pollutants is obvious, and the separated effluent can meet the corresponding national discharge standards. Moreover, membrane technology has the advantages of continuous operation, high degree of mechanization and easy management. The instability of water quality has little impact on the effect of membrane treatment.

3、Membrane process or evaporation treatment is adopted

      The disc tube reverse osmosis DTRO membrane has the characteristics of good anti pollution, high flux and long service life. The front end of the disc tube reverse osmosis DTRO membrane only needs sand filtration protection to directly treat the leachate. Even in the case of high turbidity, high SDI value, high salinity and high COD, it can also operate economically, effectively and stably.

       MVC evaporation process for leachate treatment has the advantages of fast start-up, low energy consumption, low proportion of concentrated liquid and small floor area. The problems of evaporation process are as follows: first, the condensate contains volatile hydrocarbons, volatile organic acids, ammonia and other pollutants, which need further treatment to reach the standard, and the treatment cost is relatively high; Second, when the COD in the leachate stock solution is relatively high, it is easy to bubble in the reactor, which directly affects the effluent quality and concentration multiple, and can be solved by adding defoamer, with high cost; Most of the ammonia nitrogen is transferred to the condensate, and the resin replacement frequency is high when the subsequent ion exchange treatment is used.

Key technical points and difficulties

1、Accurate prediction of design water quantity and quality

       The accurate prediction of design water quantity and quality is the basis of engineering design. The daily production of leachate in waste incineration plant should consider the residence time of garbage in aggregate pit, main components and local rainfall. The water quantity and quality of leachate in waste incineration plant can refer to the operation data of waste incineration plants in the same area.

        At present, China is vigorously promoting waste classification and promoting the construction of food waste treatment system. The composition of waste entering the waste incineration plant will change to a certain extent. The proportion of food waste will gradually decrease, and the moisture content of waste will decrease with the decrease of the proportion of food waste. It is expected that the amount of leachate generated by domestic waste entering the incineration plant will gradually decrease, and the concentration of pollutants will also show a downward trend.

2、Biochemical treatment

       The COD of leachate from MSW incineration plant is high, and the energy consumption of aeration system is too high if aerobic process is used directly. Therefore, the raw leachate should be treated by anaerobic reactor to reduce the concentration of organic pollutants before aerobic process. The concentration of ammonia nitrogen in leachate is generally 500 ~ 2500mg / L, so the process with high denitrification load and good denitrification effect should be selected for aerobic treatment unit. Due to the complete interception of microorganisms by ultrafiltration in membrane biochemical reactor (MBR), the sludge age of microorganisms reaches and far exceeds the time required for the growth of nitrifying microorganisms, and it can reproduce and gather to reach the concentration of nitrifying microorganisms required for complete nitrification, so that the ammonia nitrogen in wastewater can be completely nitrified, In the same way, the extension of sludge age and high concentration of microorganisms also greatly improve the removal of organic pollutants.

3、Selection of membrane system

        The selection of membrane system is affected by the design effluent standard. When the effluent only needs to meet the "pollution control standard for domestic waste landfill" (gb16889-2008), nanofiltration membrane can be preferred, and the proportion of concentrated liquid is low. Due to the limited removal effect of nanofiltration on monovalent ions, the monovalent salt content in the concentrated liquid is less, and the concentrated liquid can be returned to the biochemical system after proper treatment, There is no need to worry about the accumulation of univalent salt.

       When the effluent is not allowed to be discharged and needs to be reused and "zero discharge", the membrane system should adopt reverse osmosis membrane or "nanofiltration + DTRO membrane" combined membrane process because the chloride ion in the effluent of nanofiltration can not meet the requirements of reuse standard. The effluent water can meet the make-up water standard of open circulating cooling water system in water quality for industrial use of urban wastewater recycling (GB / t19923-2005) and the road cleaning, urban greening and vehicle washing standard in water quality for urban miscellaneous water for urban wastewater recycling (GB / T18920-2002). The reclaimed water can be used for make-up water of cooling system in incineration plant and road cleaning in plant area Vehicle washing and greening irrigation.

New idea of concentrated liquid treatment

       In recent years, the environmental protection requirements for new garbage incineration plants are higher and higher. Many new garbage incineration plants require the reuse of leachate after treatment and the requirement of "zero emission", which puts forward higher requirements for the design of leachate treatment system. At the same time, how to deal with the concentrated solution is the key of "zero discharge".

       The domestic waste incineration fly ash must be solidified and stabilized before it can be transported out for treatment. The stabilization treatment technology of fly ash mainly includes melting, sintering, solidification, reagent, etc. generally, the solidification method is adopted, and chelating agent is added to stabilize the fly ash. During the solidification process of fly ash, water is consumed, and leachate concentrate can be used as the water source for solidification of fly ash, At the same time of saving water, it can realize the stabilization of heavy metal ions in leachate concentrate.

Summary

       At present, the treatment effect of membrane system on leachate treatment is obvious. The disc tube reverse osmosis DTRO membrane is used to treat landfill leachate with low operation cost and good effluent effect.

        With the complex composition of landfill leachate, extremely high concentration of pollutants, the difficulty of treatment, and the improvement of treatment standards, the treatment of leachate should be considered from the whole waste incineration plant, and experts in water treatment field are also actively exploring more leachate treatment processes.

Source: ecological environment cloud service