| During the past30years, discharge of municipal wastewater has been increasing rapidly along with the economy development and town expanding in China, leading to serious deterioration of natural water. The effluent of many existing wastewater treatment plants (WWTPs) could not meet the discharge standard because of drawbacks of process technologies, equipments and operation control. In order to improve nutrient removal performance of WWTPs, researchers have devoted themselves to modify traditional biological nutrient removal (BNR) processes and develop new processes via modifying process configuration, adopting new nutrient removal technologies and controlling operating parameters. However, the nutrient removal efficiencies of many processes are still low and unstable, due to problems such as complex influent composition, carbon resource insufficiency and difficulty of process control. So it is urgent to develop new BNR process with simple operation, high efficiency and low energy consumption. In view of the problems such as low C/N and C/P ratios of municipal wastewater, process complex, high energy consumption and effluent exceeding the standard, the objectives of this thesis are to modify the present BNR processes and develop new processes to remove nutrient efficiently from municipal wastewater with low C/N and C/P ratios. The main conclusions are summarized as follows:1. Influencing factors of an A/O process with multiple zones were investigated. The results showed that the nitrogen removal performance of the A/O process was mainly influenced by influent C/N ratio and the TN removal efficiency decreased from81.49±6.64%to61.93±8.55%when the influent C/N ratio was from14.25±2.66to5.72±0.47. The phosphorous removal performance of the A/O process was influenced by influent C/N and C/P ratios and sludge concentration. With influent C/N and C/P ratios of11.55±2.40and62.48±7.34, the removal efficiency of PO43--P reached99.33+0.91%when sludge concentration was controlled at3.61±0.51gMLSS/L. The metabolic characteristic of this continuous A/O process was similar to that of a typical enhanced biological phosphorous removal (EBPR) system.2. In order to improve nutrient removal and save energy consumption, a novel A/O process with low DO level was developed. With influent COD, NH4+-N and PO43--P concentrations of241.94±32.55mg/L,47.86±2.01mg/L and3.78±0.13mg/L, the removal efficiencies of COD, NH4+-N, TN and PO43--P reached91.84±4.90%,98.15±1.56%,65.53±2.21%and100%, under DO concentration of0.54±0.92mg/L. It was discovered that under low DO condition, biological phosphorous removal was improved and kept stable; ammonia removal was not affected and TN removal was improved via enhancing simultanesou nitrification and denitrification (SND) function. Aeration intensity analysis showed that during stable operation, the air needed for treating wastewater was48±3L/L, which decreased by71.2±3.8%compared with the previous A/O process with multiple zones, implying that the energy consumption of this A/O (low DO) process was significantly lower than traditional A/O process.3. Based on efficient utilization of internal carbon source, a novel continuous anaerobic/aerobic/anoxic (AOA) process was developed. In this process, part of the anaerobic mixed liquor was transferred to the post-anoxic zone. At influent COD, NH4+-N and PO43--P of400mg/L,37.65±1.64mg/L and5.63±0.14mg/L, R of1.0and F (the ratio of the flow of the transferred mixed liquor to that of influent) of0.5, the average removal efficiencies of TN and PO43--P reached74.50±1.42%and92.11±5.20%, demonstrating better nitrogen removal performance than A/O process.A mass balance model was established to analyze the nutrient removal in the AOA process and results showed that1.63±0.09gP/d of PO43--P and0.78±0.14gN/d of NO3--N were simultaneously removed in the anoxic zone at PO43--P/NOs--N ratio of2.17±0.42, and24.19±0.52%of phosphate was removed in the anoxic zone. It’s speculated that denitrifying phosphorous removal occurred in the anoxic zone of the AOA process. Batch tests revealed that22.20±1.97%of polyphosphate-accumulating organisms (PAOs) in the sludge were denitrifying polyphosphate-accumulating organisms (DNPAOs). Model analysis revealed that TN was removed via heterotrophic denitrification, SND and denitrifying phosphorous removal.4. Influencing factors of the AOA process treating wastewater with low C/N and C/P ratios were investigated. It was found that with influent C/N and C/P ratios of7.41±0.26and52.36±1.25, the removal efficiencies of TN and PO43--P reached65.86±2.06%and90.00±3.97%, respectively, when SRT was increased from10d to16d. With influent C/N and C/P ratios of6.14±0.32and43.40±1.37, the removal efficiencies of TN and PO43--P increased to69.76±3.36%and98.73±1.82%when the A/O/A ratio was changed from1/3/1to1/2/2. Results showed that when the A/O/A ratio was1/2/2and SRT was16d, nutrient was removed efficiently from wastewater with low C/N and C/P ratios.Analysis of polyhydroxyalkanoate (PHA) content in biomass at different stages showed that PHA content decreased at lower influent COD level and the properties of SND and denitrifying phosphorous removal decreased, leading to deterioration of nutrient removal. After lengthening SRT, both sludge concentration and PHA content in sludge increased; with shorter aerobic HRT and longer anoxix HRT, PHA content also increased and the nutrient removal performance of the AOA process was enhanced. So it is concluded that the nutrient removal performance of the AOA process can be kept stable by controlling SRT as well as aerobic and anoxic HRTs when treating wastewater with low C/N and C/P ratios.5. For improving nutrient removal with simpler process, a novel continuous AOA (low DO) process without anaerobic mixed liquor conversion was developed. Under DO concentration of1.19±0.16mg/L and the HRTs of the aerobic and anoxic zones at2h and4h, the removal efficiencies of NH4+-N, TN and PO43--P reached95.07±1.57%,93.26±1.30%and97.83±2.95%, respectively.Model analysis showed that the TN removal loading of the aerobic and anoxic zones were0.112±0.013gN/gVSS/d and0.055±0.015gN/gVSS/d, respectively, while the TN removal in the aerobic and anoxic zones contributed41.76±1.03%and39.93±6.11%of total TN removal. In the anoxic zone,0.76±0.14gP/d of PO43--P and0.80±0.16gN/d of N03--N were removed simultaneously. It is speculated that0.36±0.07gN/d of NO3--N was removed through denitrifying dephosphatation and0.44±0.09gN/d of N03--N was removed through other way, possibly by denitrifying glycogen-accumulating organisms (DNGAOs).Nutrient removal mechanisms were analyzed and results showed that, when aerobic HRT was shortened, less PHA was degraded in the aerobic zone and PHA content in biomass incresed significantly; as a result, the effects of SND and denitrifying dephosphatation were enhanced and nutrient removal was improved. Compared with the previous AOA process with conversion of anaerobic mixed liquor, the air needed for treating wastewater of this AOA (low DO) process decreased by11.6±0.6%, showing that its energy consumption was lower.To sum up, the characteristics of this novel process include simple operation, high nutrient removal efficiency and low energy consumption. The AOA process developed in this research provided new technologies for treatment of wastewater with low C/N and C/P ratios. |
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