| Based on the National High-tech Research and Development Program "the Complete Set Technology of Non-point Source Pollution Control in River Network Area"(Grant No. 2002AA601012), the quantity and quality of rural domestic wastewater were investigated in Yangzhu village and both the lab-scale high rate algal ponds(HRAP) and the pilot-scale high rate algal pond system were constructed. After the build-up of experiment facilities, following subjects were studied: start-up of high rate algal pond;the removal efficiencies and mechanism of nitrogen and phosphorus from rural domestic wastewater in the HRAP;the removal efficiencies of nitrogen, phosphorus and algae in the aquatic pond as a next facility of the HRAP;enhanced nitrogen and phosphorus removal of the HRAP system;the influence of water depth, water temperature, HRT and mixing method to the removal efficiencies and mechanism of nitrogen and phosphorus in the HRAP;the best operation parameters of the HRAP.The water quantity per person in rural area of Tai-lake basin was 100L/d. About 40-45% wastewater could be collected through pipe network system corresponding to 45L/d per person.The pilot plant located at Yangzhu Village, Jiangsu Province(30°16'42"N, 119°54'24"E). The process consisted of the septic tank, the first-stage HRAP, the second-stage HRAP and the aquatic pond in series.The algae in the the HRAP were cultivated directly in domestic sewage under 0.5m water deep, 0.35m/s flow rate and good weather conditions. After about 4-day cultivation, algae in the HRAP were dominated by Scenedesmus quadricauda and the Chlorophyll-a concentration was 0.28mg/L. It was concluded that there was a time sequence of the growth of algae, aerobic heterotrophic bacteria, nitrosomonas, nitrobacter in cultivation of algae-bacteria symbiosis system in the HRAP .The cultivation took about 25~30d. The algae cultivation in the HRAP was simple and the startup was quick. Both were good for the application of the HRAP in rural area.The annual removal efficiency of TN in the two-stage HRAPs was 29.4%. The percentage of TN removal in the first-stage HRAP and the second-stage HRAP were 70.9% and 29.1%, respectively. The annual removal efficiency of NH/-N in the two-stage HRAPs was 91.6%. The percentage of NH4+-N removal in the first-stage HRAP and the second-stage HRAP were 80.7% and 19.3%, respectively.The percentages of NH/-N transformation through nitrification, algae assimilation and ammonia volatilization in the first-stage HRAP were 61.6%, 35.7% and 2.7%, respectively. Those in the second-stage HRAP were 70.9%, 20.3% and 8.8%, respectively. After corrected by wind velocity the average ammonia volatilization rate in the first-stage HRAP in the summer was 415.01mgN/(m2-d), i.e. 0.83mgN/(L-d). The concentration variation of NOX'-N(NO3"-N+NO2"-N) in the HRAP was linear with time(h). The nitrification rate in the first-stage HRAP was 0.1362mgN/(L-h), i.e. 1.2382mgN/(gMLSS-h). The nitrification rate in the second-stage HRAP was 0.1016mgN/(L-h), i.e. 0.8835mgN/(gMLSS-h). The process of TN removal in the HRAP was mainly through the precipitation of particulate organic nitrogen which was maily consisted of algae debris. The TN removal through ammonia volatilization could be ignored.The annual removal efficiency of TP in the two-stage HRAPs was 21.8%. The percentage of TP removal in the first-stage HRAP and the second-stage HRAP were 73.5% and 26.5%, respectively. The annual removal efficiency of PO43"-P in the two-stage HRAPs was 39.0%. The percentage of PCV'-P removal in the first-stage HRAP and the second-stage HRAP were 76.1% and 23.9%, respectively.The main approach of TP removal in the HRAP was the precipitation of particulate organic phosphorus which was maily consisted of algae debris, next was chemical precipitation. Chemical precipitation dominated the diurnal variation of PO43"-P concentration in the HRAP. High pH could lead to phosphate precipitation. However, this phosphate would be resolubilized when the pH dropped at night. Therefore the phosphate removal through chemical precipitation was temporary and unstable.More than 50% phosphorus in the sediment of the two-stage HRAPs was organic phosphorus, next were Ca and Mg combined phosphorus, Fe combined phosphorusand adsorption phosphorus in turn.The annual removal efficiency of SS, Chl-a, TN and TP in the aquatic pond treating the effluent from the second-stage HRAP were 88.8%, 87.6%, 24.4% and 21.2%, respectively. The aquatic pond was a feasible supplemental facility of the HRAP and could well integrate with the HRAP. The improved lab-scale aquatic pond had a good performance on TN and TP removal, the average concentrations of TN and TP in effluent of the aquatic pond were 4.75mg/L and 0.67mg/L. The efficiencies of TN and TP removal were 82.6% and 79.5%. The TN concentration in the effluent met A level of the first standard of Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant(GB 18918-2002) and TP concentration met B level of the first standard.Based on the results of the lab-scale HRAPs research, it was concluded that when water temperature was high, the removal efficiency of Dissolved TN in the HRAP increased with the decrease of water depth;when water temperature was low, the efficiency was unaffected by water depth.When water temperature is under 15°C, HRT=8d is recommended to achieve well nitrogen and phosphorus removal efficiencies in the HRAP of 0.2-0.3m water depth. When water temperature is in range of 15~25°C, HRT=6d is recommended. When water temperature is upon 25 °C, HRT=4d is recommended.Underwater propeller is recommended as the mixing equipment of the HRAP to gain higher nitrogen and phosphorus removal efficiencies and stable operation.
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