Abstract
Abstract: Increasingly stringent discharge standards make it necessary to research and develop a new processes that have the advantages of relatively high efficiency, less land occupation, investigated savings, easy management, and maintenance for nutrient removal from the domestic sewage. An enhanced denitrifying phosphorus removal system was developed to treat domestic sewage, especially for the sewage with comparatively low carbon source and high nitrogen and phosphorus concentration. The system included an integrating anaerobic/anoxic/oxic (A2/O) reactor and a biological contact oxidation (BCO) (A2/O - BCO for short). The A2/O unit consisted of four compartments in sequence with a working volume of 40L. It was mainly used for the removal of organic compounds and nutrients without ammonium oxidation, while the BCO unit was mainly responsible for nitrification. The BCO unit, with 24L working volume, consisted of three compartments in sequence, where suspended carriers were filled in it, with packing rates of 45% in each zone. The removal of biological nitrogen (N) and phosphorus (P) were investigated in an A2/O - BCO nitrogen and phosphorus removal process when treating domestic sewage. The experiment was carried out with the influent flow at a rate of 5L/h, total hydraulic retention time (HRT) of A2O about 8 h, sludge reflux ratio of 100%, sludge retention time (SRT) of 12d, MLSS maintained at 3500 mg/L, total HRT of BCO about 1.9 h, DO should be controlled within a reasonable range, and nitrate recycling ratios were set as 100%, 200%, 300%, and 400% respectively. Based on the experimental data under steady state operating conditions of different nitrate recycling ratios, the equations for calculating material balances of COD, nitrogen, and phosphate were established. These three material distributions in the system were also evaluated. The result indicated that the process has an advantage of making full use of the raw water carbon and removing biological nitrogen and phosphorus deeply. With the nitrate recycling ratios of 100%, 200%, 300%, and 400% conditions, the system COD was mainly utilized in the anaerobic stage in A2/O reactor, and removal percentages of total COD were 78.5%, 71.8%, 57.9%, and 71.1% respsectively. Nitrogen removal was mainly achieved by denitrifying in the phosphorus removal process by denitrifying phosphorus bacteria. The denitrification removal ratios of the total amount were 28.0%, 35.7%, 48.5%, and 33.9%, respectively. Phosphorus removal was mainly achieved by the discharge of excess sludge, where the amount of phosphorus percentages of total were 78.0%, 88.4%, 84.3%, and 85.4% respectively. Under the different nitrate recycling ratios, COD balance ratios were 96.4%, 99.6%, 98.7%, and 98.3%, nitrogen balance ratios were 99.7%, 98.2%, 99.2%, and 96.5%, and phosphorus balance ratios were 92.0%, 98.1%, 93.3%, and 90.4%. Fluorescence in situ hybridization showed that the anammox bacteria existed in the biofilm of BCO, and its proportion of the total bacterial number was 0.6~2.7%. The nitrogen loss may be due to the occurrence of the anammox reaction. The amount of denitrifying phosphorus increased slightly with the nitrate recycling ratio improved. But it is essential to control nitrate recycling ratio in the appropriate range to ensure nitrogen removal efficiency. The system achieved the best removal efficiencies of nitrogen and phosphorus when the nitrate recycling ratio was set as 300%, in which the average concentrations of TN and TP were 14.96 mg/L and 0.49 mg/L respectively in the effluent, which met the class of a limit (GB18918-2002) of the discharge standards. This study not only contributes to a better understanding and analysis of distribution and changes about organics, nitrogen and phosphorus in process systems, but also provides a theoretical basis and guidance for the reliability evaluation of the of the experimental data by building a mathematical model. The combined A2/O - BCO process will get a better promotion in treating domestic wastewater, especially the rural domestic wastewater with the universal features of small quantity, scattered distribution, and water fluctuation.
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