Study On The Treatment Of Rural Domestic Wastewater By Combination Of Activated Sludge-Constructed Wetland Hybrid System

The water pollution has become one of the serious environmental problems faced in the world nowadays. In recent years, various sewage treatment technologies have been studied and applied to practical wastewater treatment, in which the activated sludge process has gained much attention due to its developed technology, stablely running and good performance. Meanwhile, as an ecological treatment technology, constructed wetland treatment technology also receives great popularization because of its low-cost, easily-managed and effective characteristics. The major problems met in practical application of constructed wetlands for wastewater treatment are: (1) From view point of technology design and performance, effects of running parameters on performance of wetland system were not understood well, for example, the interactions between hydraulic loading, wastewater residence time and treatment efficiency were not clear, which leads to the difficulty to obtain general guideline for design and run of constructed wetland. (2) Big area needed for constructed wetland and small hydraulic loading limited its large-scale application to some extent. (3) There are limited literatures dealing with the combination of constructed wetland system and other wastewater treatment technologies, the effect of high hydraulic loading on performance of the combined hybrid system. Hence, the key problem for large-scale application of constructed wetland is relying on the development of highly effective hybrid system with samll area.In this study, a pilot-scale wastewater treatment system combined by activated sludge and four subsurface flow constructed wetlands in series was sampled for two years to study the system performance for common contaminants treatment in rural wastewater with focus on the following aspects:(1). Long term performance of the combined system for rural wastewater under high hydraulic loading;(2). Trends of pollutants concentrations along with treatment unit;(3). Factors affecting the performance of the combined system, including treatment unit, hydraulic loading, and season;(4). Organic pollutant removal mechanism study by investigating the concentration trends of various electronic acceptors such as sulfate, nitrate, and nitrite in the constructed wetlands system.The major conclusions through two years (from Dec.2004 to Dec.2006) study are summarized as follows:(1). The combined hybrid system can effectively purify domestic wastewater under high hydraulic loading rate. Indexes such as AN (ammonium), COD and BOD₅ in the effluent of the combined system could meet the I-grade discharge standard of GB8978-1996. When the average wastewater flow was 81 T d^-1 (17-159 T d^-1) corresponding to hydraulic loading rate of 0.24 - 2.28 m d^-1, the combined system removed 80% of the TSS (total suspended solids), 75% of the AN, 60% of the TN (total nitrogen), 33% of the TP (total phosphorus), 77% of the COD, 85% of the BOD₅, respectively. The constructed wetlands system removed 48%, 12%, 28%, 13%, 33% and 48%, of TSS, AN, TN, TP, COD and BOD₅, respectively. The contaminants removal increased with the increase of contaminants loading in the influent of constructed wetlands system.(2). In comparison with activated sludge pond, the contaminant removal efficiency of four wetlands in serials was low. However, it still showed obvious pollutant removal for wastewater with low concentration wastewater from the activated sludge system.(3). ANOVA statistical analysis performed on the data set of 2 years of operation showed that the hydraulic loading rate (HLR) and season were major factors that controlling the performance of constructed wetlands system for the target analytes. The effluent water quality was not good under low hydraulic loading rate (0.35 m d^-1 0.55 m d^-1) compared with that under high hydraulic loading rate (0.80 - 2.20 m d^-1). The treatment performance of constructed wetlands system effluent was better in summer and autumn than winter in 2005. However, the treatment performance of combined system became poor in summer in 2006 for the lasting high temperature and drought in Chongqing.(4). A survey of electron acceptors such as SO₄^2- , NO₃^- and NO₂^- in the constructed wetlands system indicated that denitrification was estimated to be significant for organic matter removal in constructed wetland beds. In contrast, the organic matter removal by sulfate reduction was minor. Large amount of the NO₃^- in the effluent of activated sludge pond was helpful for the enhancement of the organic contamination removal and resistance to shock-loading in subsurface constructed wetlands system.This research was supported by the National Natural Science foundation of China "Study on the degradation of organic contaminants in the constructed wetlands treating wastewater" (No.20477033) and the National Natural Science foundation of Chongqing (No. 2004BA7019)...