Landfill leachate is a kind of high-concentration organic wastewater, and its chemical characteristics are closely related to the composition of garbage, landfill methods, landfill years and landfill environmental conditions. The general indexes for evaluating leachate are: CODCr, BOD5, SS, total nitrogen, ammonia nitrogen, heavy metals, etc. The main pollutants are high content of dissolved organic matter, ammonia nitrogen, inorganic salts and low concentration of heavy metals.
According to the landfill time, leachate can be simply divided into young leachate (5 10= 10 years). Young leachate generally has a high BOD 5/COD Cr, and biological treatment can achieve good results, but the residual COD Cr is still as high as 500- 1500mg? L- 1 is toxic and difficult to biodegrade, which is similar to mature leachate after methane production stage.
Residual COD is mainly refractory dissolved organic matter (DOM), which contains many substances with complex physical and chemical characteristics, and shows great differences with the change of landfill time and other conditions. Humus (HS) can account for about 60% of DOM, and it is the main component of refractory organic matter in leachate, and it is also the main substance that causes the chroma of leachate. Its relative molecular mass can range from several million to several million, and it is a complex and stable substance.
Therefore, how to effectively remove HS from leachate is the key to achieve the standard discharge. Physical-chemical method is paid more and more attention because of its simple process, high pollutant removal efficiency, little influence by water quality and quantity change, and stable effluent quality.
Advanced oxidation can destroy the stable structure of macromolecular substances which are difficult to degrade, such as humic acid, and transform them into biodegradable compounds with small molecules. It can also play the role of decolorization and deodorization, and has a good application prospect in treating high-concentration landfill leachate. The research on micro-electrolysis treatment of landfill leachate is still in its infancy. The characteristics and removal technologies of DOM in landfill leachate are summarized as follows.
1 characteristics of dissolved organic matter in landfill leachate
Table 1 Variation of leachate characteristics with landfill years
Table 2 Application of Advanced Oxidation Process to Remove DOM from Landfill Leachate
3 Conclusion and prospect
There are many kinds of DOM in landfill leachate with complex physical and chemical characteristics, which show great differences with different landfill time. The treatment technologies of landfill leachate mainly include enhanced coagulation, activated carbon adsorption, membrane separation, Fenton oxidation, ozone oxidation, photocatalytic oxidation, persulfate oxidation and micro-electrolysis. However, these methods can only effectively remove one or more organic substances in DOM of leachate.
The removal of different kinds of DOM in leachate with different physical and chemical properties needs further study in order to develop a more effective combined process for advanced treatment of DOM in landfill leachate. The development trend of advanced treatment technology of landfill leachate is as follows:
After the implementation of the new standard (1)GB 16889—2008, the membrane separation technology of "nanofiltration+reverse osmosis" was adopted to treat the DOM in the effluent of MBR membrane bioreactor, which was widely used in the field of landfill leachate treatment in China. In fact, the COD Cr of nanofiltration effluent is close to the standard in Table 2 of GB16889-2008. The follow-up reverse osmosis process not only has large investment and high operating cost, but also brings difficulties to the treatment and disposal of reverse osmosis concentrated solution. Therefore, it is the development direction of landfill leachate treatment technology to research and develop a new advanced treatment process instead of reverse osmosis to treat DOM in nanofiltration effluent.
(2) The removal effect of DOM in landfill leachate by single ozone oxidation process is not good, and the combined process of ozone oxidation, catalytic flocculation and micro-electrolysis has broad application prospects in advanced treatment of leachate.
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