The Influence Factors Of N₂O Emission From Subsurface Infiltration System And Control Technology

As a type of wastewater land-treatment technology, the subsurface infiltration system is applied to deal with disperse sewage widely due to its advantages, such as, simple management, good effluent quality, low cost and unaffected landscape.Biological nitrification-denitrification is the main way to nitrogen removal in subsurface infiltration system. Nitrous oxide(N2O) is the by-product of biological denitrification process, and its greenhouse effect can not be ignored.Now the studies about subsurface infiltration system are focused on matrix matching, strengthen denitrification and operation parameter regulation, but there have fewer studies on the generation mechanism, influencing factors and control technology of N2 O in the system. The impact of matrix structure and operating conditions on the emission of N2 O needs further research.The effect of matrix layer structure in filtration bed and operation conditions(including influent loads, pollution load, operation mode) on emission features of N2 O is analyzed in the paper, by the combination of simulation experiment and in-situ test. Under the precondition of effluent with good quality, the paper try to explore the proper preparation of substrate and operating parameters to reduce the emission of N2 O, and verify the results by in-situ test. The control technology of N2 O emission from subsurface infiltration system is put forward. The main conclusions are:(1)When the organic content of aerobic zone matrix is between 2% to 9%, with the rise of organic content, the removal efficiency of Chemical oxygen demand(COD), Ammonia nitrogen(NH4+-N) and Total nitrogen(TN) is reduced, and compared with the organic content of 2.0%, when the organic content is 9.5%,the removal rate of COD, NH4+-N, TN is reduced by 10.9%, 19.5%, 24%;(2) With the increase of depth of filtration bed, the number of nitrifying bacteria in substrate reduce gradually, while the number of denitrifying bacteria is increased. With the rise of organic content, the number of nitrobacteria in different depths drop off, but the number of denitrifying bacteria shows increasing trend; and the emission of N2 O reduce with the increase of organic content. With the organic content increasing from 2.0% to 9.5%, the productivity of N2 O is reduced by 2.77 mg N2O·m-2·d-1.(3)The conclusion of simulation experiment was validated and amended through in-situ test, considering the results of simulation experiment and in-situ test, suggest that the influent hydraulic load should be(0.10~0.15) m3·m-2·d-1, influent nitrogen load should be(3.9~4.6)g N·m-2·d-1, the psychrometric ratioshould be 1:1~1:2.(4) The significant degree sequence of variation of different operation conditions on N2 O emission by multi-factor analysis of variance is: influent hydraulic load > influent nitrogen load >psychrometric ratio. So, when ensure the effluent quality, in order to reduce the release of N2 O, the nitrogen load and hydraulic load should be controlled first.