| In recent years, with the improvement of urbanization and industrialization, wastewater with low C/N ratio is increasing. However, due to both denitrifiers and phosphate accumulating orgasims (PAOs) require carbon source, the effluent could not reach the discharge standards when conventional biological nutrient removal (BNR) was used to treat the wastewater with low C/N ratio. Denitrifying phosphorus removal which could remove nitrogen and phosphorus simultaneously using the same carbon source is a new prominent biological wastewater treatment technique, and it is particularly suitable for treating low COD/N wastewater. However, several studies have reported that denitrifying phosphorus removal process could produce a lot of nitrous oxide (N2O). Therefore, effective control of N2O emission from denitrifying phosphorus removal process is of great importance and attracts increasingly more attention.In this study, the denitrifying phosphorus removal was achieved in a two-sludge system which was used to treat municipal sewage with low C/N ratio, and the N2O production characteristics during this system were systemically studied, and the main N2O production phase was clear; the mechanism and influence factors of N2O production during the aerobic nitrification stage and anoxic denitrifying phosphorus removal stage in two-sludge system was investigated, and the best operating conditions during aerobic and anoxic stages was clear; feasible N2O reduction strategies in two-sludge system were proposed, and the reduction effects of the proposed strategies were evaluated. The main research conclusions are as follows:(1) The nitrogen and phosphorous removal rates and N2O production characteristics in two-sludge system was clear. It was found that the TN and TP removal rates in two-sludge system reached92.13%and95.18%respectively. A lot of N2O was produced in two-sludge system, N2O production during one cycle accounted for1.47%of the total nitrogen removal, and the production amounts during aerobic, anoxic and post-aerobic stages were0.86%,0.41%and0.20%of the TN removal.(2) The sources of N2O production and the best DO concentrations in aerobic stage of two-sludge system were clear. Autotrophic nitrification which accounted for97.1%of the N2O production was the main source due to no heterotrophic denitrification occurred in aerobic stage of two-sludge system. DO concentrations significantly affected N2O production in aerobic stage. When DO concentration was2.5mg/L, the N2O production was lowest, and the amount during one cycle was only0.20mg/L.(3) The mechanism of N2O production and the best operating conditions which could decrease N2O production during anoxic denitrifying phosphorus removal stage were clear. The main reason for N2O production during denitrifying phosphorus removal process was that denitrifying phosphorous accumulating organisms DPAOs used poly-β-hydroxyalkanoates (PHA) as carbon source for denitrification, and the limited electrons provided by PHA could not satisfy the requirement of denitrifying enzymes, and this resulted high nitrite accumulation. Accordingly, the reduction of N2O production was successfully achieved via two control measures:the use of continuous nitrate addition and the use of propionate as the carbon source.(4) The mechanisms of carbon sources affected N2O production during denitrifying phosphorous removal process were studied, and the causes of low N2O production when using propionate as carbon source was clear. Carbon sources significantly affected N2O production during denitrifying phosphorous removal process. When using acetate as the sole carbon source, the N2O production amount was up to16.3%of the TN removal; whereas, when using the mixture of acetate and propionate or propionate as carbon source, the N2O production amount were1.92%and0.43%of the TN removal. The main reason for nearly no N2O production in propionate system was that:when using propionate as the sole carbon source, the electrons provided by PHA could satisfy the requirement of denitrifying enzymes, and the nitrite and N2O reductase activities were not inhibited, as a result, no nitrite was accumulated.(5) The two-sludge system and optimization strategies which could improve nitrogen and phosphorous removal ratio and decrease N2O production were established. When using propionate as the sole carbon source and the DO concentrations were controlled at2.5mg/L, not only low N2O production but also high nitrogen and phosphorous removal were achieved in two-sludge system. The TN and TP concentrations in effluent were only3.16and0.39mg/L, respectively. N2O production amount during one cycle was only0.75%of TN removal, which was much lower than the conventional wastewater treatment (1.72%) and other processes reported in the literatures. |
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