Study On Purification Effect And Its Mechanism Of The Aquatic Plant Stepping-shallow Pool System On The Rural Domestic Sewage

At present,aquatic plants are the preferred plants for the treatment of rural domestic sewage.They can not only beautify the rural environment,but also effectively treat and purify rural domestic sewage.They have broad application potential.In this paper,the aquatic plant stepping-shallow pool system was chosen as the research objective in the natural stream,in contrast with the non-stepped shallow pool system,based on the experimental data,the removal and purification effect of the aquatic plant stepping-shallow pool system on the rural domestic sewage was in-depth analyzed in a growing cycle using the both field and indoor experiments.It mainly includes the impacts of the aquatic plant stepping-shallow pool system on the dissolved oxygen,the conductivity,the oxidation-reduction potential,the pH,the transparency,the flow rate in water,and the removal effects of that on pollutants,such as sediment,COD,total nitrogen,total phosphorus,ammonia nitrogen,nitrate nitrogen.On the basis,the removal mechanism is discussed.This study can provide the theoretical proof for the application and management of the aquatic plants for the purification of rural domestic sewage in constructed wetlands.Moreover,it can also lay a theoretical foundation for the implementation of the low-cost and easy-to-manage rural sewage treatment technique so as to exert the ecological function of the aquatic plant.The main conclusions are as follows:(1)the aquatic plant stepping-shallow pool system can increase the content of dissolved oxygen in the water body and its average growth rate is 5.9%,which is greatly higher than that of the non-stepped shallow pool system;it can intercept the suspended matter in the water body and increase the transparency and its average increase rate is 6.9%,which is greatly higher than the transparency increase rate(3.23%)of the non-stepped shallow pool system.However,it has no effect on the conductivity,the oxidation reduction potential and the pH of the water body.(2)The aquatic plant stepping-shallow pool system has a certain intercepting effect on sediment,and the intercepting effect on coarse sediment(mesh size larger than 0.45mm)is the best,which is approximately above 70% of the total sediment retention.It is thereby greatly higher than that of the non-stepping shallow pool system(6.45%).(3)The aquatic plant stepping-shallow pool system has a greater pollutant-removal effect than the non-stepped shallow pool system on the rural domestic sewage.The average COD removal by the stepping-shallow pool system is 39.65mg/L and the reduction rate is over 54%,and the maximum reduction rate is 64.80% in August.Moreover,the removal process mainly occurred in the range of 0~13m.The average reduction rate of TN is 21.21%,and the TN reduction rate by the shallow pool system is the highest in August(25.96%),and the TN reduction rate is the lowest(13.10%)in June,furthermore,the removal process primarily occurred in the range of 32~50m.The average reduction rate of ammonia nitrogen and nitrate nitrogen is 35.97% and 43.67%,respectively,furthermore,the removal process primarily occurred in the range of 13~32m.The average reduction rate of TP is 18.95%,and the TP concentration of the outflow was lower and relatively stable between 0.15 mg/L and 0.19 mg/L,however,the removal process mainly occurred in the range of 0~13m.(4)The main trapping function of the stepping-shallow pool system on the coarse-grain sediment is because of the “mesh structure” formed by the root system of aquatic plant in the water;the nitrogen removal in the sewage is mainly caused by the synergic function of the physical,chemical and biological processes,particularly being the nitrification and denitrification functions;the phosphorus removal is mainly because of the adsorption filtration of the substrate,settling and the absorption of the aquatic plants,and the assimilation of microorganisms.