|Other Abstract||Constructed wetland is widely used in some regions of China as a kind of technology for municipal wastewater treatment, such as decentralized wastewater, drainge from non-point rural pollution, aquaculture wastewater and some non-toxic industrial wastewater, due to its low cost of construction and operation. Organic matter and suspend solid removal efficiency was normally quite high in the constructed wetlands but low nitrogen removal efficiency remained to be enhanced. Thus, the nitrogen removal efficiency was usually the most focused consideration in design of constructed wetlands. Moreover, the mechanism of nitrogen removal in constructed wetlands was disputed, and the mathematic model of nitrogen removal was difficult to build. Generally, the constructed wetlands were designed according to empirical parameters of the similar wetlands in the same latitude and latitude. Unfortunately，there was only a fat lot empirical parameters about the constructed wetland, so as to employ larger land area by the designers in order to meet the effluent standard, the construction cost increased as a result. In this study, nitrogen efficiency and seasonal variation of three constructed wetlands for three types of wastewater treatment were investgated in order to find the proper parameters for the constructed wetland design. Meanwhile, the mechanisms of nitrogen removal in five constructed wetlands were studied, and the cost-effective aeration constructed wetland hereby was worked over in development.
Main results are shown as the followings:
1) Nitrogen removal efficiency in constructe wetlands varied with season. The removal rate was closely related with temperature. The highest removal rate was found in summer and lowest in winter.
2) Nitrogen removal efficiency in constructe wetlands varied with the types of wastewater on the other hand. In the constructed wetland employed for high carbon and high nitrogen wastewater treatment, the removal rate of ammonia nitrogen was 1.18g/m-2d-1 and the total nitrogen was 1.25g/m-2d-1. In the constructed wetland for low carbon and high nitrogen wastewater treatment, the removal rate of ammonia nitrogen was 1.22g/m-2d-1 and the total nitrogen was 1.44g/m-2d-1. In the constructed wetland for low carbon and low nitrogen wastewater treatment, the removal rate of ammonia nitrogen was 0.123g/m-2d-1 and the total nitrogen was 0.169 g/m-2d-1. Thus, to the types of wastewater must be paid more attention in the constructed wetland design.
3) The constructed wetland nitrogen efficiency was also related to its operation age. Following the constructed wetland ran on the rails, the nitrogen efficiency of the constructed wetland would be enhanced.
4) The mechanisms of nitrogen removal in constructed wetlands were different according to the types of wastewater treated. When the high carbon and high nitrogen wastewater was treated, the nitrogen was removed from the wastewater by the microbial process. 23% nitrogen was removed by partial nitrification and denitrification, 77% nitrogen was removed by classic nitrification and denitrification. When the low carbon and high nitrogen wastewater was treated, 50% nitrogen was removed from wastewater in the Laoduanpu constructed wetland by partial nitrification and denitrification, and 20% nitrogen was removed from wastewater in the Beixia constructed wetland by the “novel” process. When the low carbon and low nitrogen wastewater was treated, 23%, 18% and 21% nitrogen were removed from the wastewater by the process of volatilization, plant uptake and microbial process respectively.
5) Limited aeration in the operation of constructed wetland was a cost-effective method. Compared to the traditional wetland, it was more economical and with higher pollutant removal efficiency. The BOD removal rate was 16.7g d-1m-2, the ammonium nitrogen was 4.54g d-1m-2, and the total nitrogen was 4.99 g d-1m-2. Due to less land area was used, less construction cost was needed accordingly. The electricity consumption of limited aeration system accounted for only 28.8% of the traditional biological oxidation process consumed.|