Shaokui Zheng

This study developed satisfactory TN and TP removal models (R2 = 0.9976 and 0.9958, percentage deviations of £ 3% and £ 5.5%) for a high-rate anoxic/microaerobic system (H-A/M) with the ratios of chemical oxygen demand to total nitrogen (COD/TN) and total phosphorus (COD/TP) in wastewater, and the anoxic fraction as three independent variables. They were used to optimize the anoxic fraction of H-A/M system in order to achieve the highest TN and TP removals for each wastewater quality. The effluent TN, TP, NH4+-N and NOx⁻-N concentrations (2.4 ± 0.4, 0.2 ± 0.1, 0.8 ± 0.7 and 0.0 ± 0.0 mg L-1, respectively) of H-A/M system at the optimum process point (COD/TN ratio 10, COD/TP ratio 72, anoxic fraction 46%) approximated to their concentrations in the current Environmental Quality Standard IV for Surface Water in China. The remarkable change solely in influent COD levels (125 – 500 mg L-1) did not obviously influence the treatment performance. The biological nutrient removal mechanism of an optimized H-A/M system at a COD/TN ratio of 10 was anaerobic phosphorus release and microaerobic phosphorus uptake, and microaerobic simultaneous nitrification and endogenous denitrification, while that at a COD/TN ratio of 4 or 7 was anoxic denitrifying phosphorus removal, and microaerobic nitrification and anoxic denitrification. A combination of wastewater quality and anoxic fraction played a weak role in influencing the bacterial community structure and the predominant ammonia-oxidizing bacterium species (68 – 100% as Nitrosomonas europaea).

This study used the response surface methodology to design the experimental conditions, model the experimental data, and further to analyze, predict and optimize the treatment performance of a high-rate anoxic/microaerobic (H-A/M) system, when chemical oxygen demand to total nitrogen (COD/TN) ratio (4 - 10), COD to total phosphorus (COD/TP) ration (30 - 90), and anoxic fraction (20% - 55%) were selected as key process parameters. Besides complete COD (≥99%) and NH4+-N (≥96%) removals, high TN removals (80% – 90%), low effluent NOx--N levels (< 5 mg L-1), and high intracellular phosphorus contents (2.0% – 4.0%) were observed during pseudo-steady-state periods of most experimental runs. The analysis of variance highlighted the remarkable effect of wastewater quality and anoxic fraction on TN removal, TP removal, energy consumption for aeration, nitrifying activity, biomass concentration in A/M tank, and intracellular phosphorus content. The optimized response surface models demonstrated the excellent reliability and accuracy for predicting TN or TP removal and nitrifying activity, with the correlation coefficient of ³ 0.9194, the coefficient of variation of £ 8.10%, and the adequate precision of ³ 22.12, respectively. Three-dimensional response surface plots visualized the synergistic effect of COD/TN ratio or anoxic fraction on TN removal, and a parabolic relationship between COD/TP ratio and TN removal. Optimizing the anoxic fraction of H-A/M system resulted in higher TN (³ 87%) and TP (³ 65%) removals in most experimental groups, when the highest TN (91.0%) and TP (94.2%) removals were predicted and validated.