Study On Advanced Treatment For Tailrace Of Rural Domestic Wastewater With Planting Constructed Wetlands

Compared with the other conventional biological treatment processes, the constructed wetland has imponderable advantages in removal of nitrogen and phosphorus because of its low operating costs and easy management. However it was influenced by some factors obviously, such as hydraulic loading rate, plants selection, seasonal variation and so on. And the economic value of traditional constructed wetland which planting landscape plants is poor. It presents difficulties for the implementation of constructed wetland technology in rural areas where the agricultural production is regarded as the main source of income. In this paper, constructed wetland, one part of combination of biological and ecological process which is called "anaerobic-anoxic-water dropping aeration bio-contact oxidation-wetland" and proposed by Southeast University, was used as the main device of the experiment. It has studied the selection principle of materials in constructed wetland and investigated the removal efficiency of NH4+-N?TN?TP in rural domestic wastewater under different hydraulic loading and combinations of plants. And it demonstrated the feasibility of rural domestic wastewater treatment by the combined ecosystems in winter. This paper aims to provide theoretical basis for the application of grain crops and economy vegetables in the combined wetlands.Freundlich equation indicated that process of phosphorus adsorption with ceramic, brick and grave are all excellent. And thermodynamics experiments illustrated adsorption of phosphorus by three kinds of materials are endothermic process and spontaneously. Removal effect of phosphorus by three different materials in simulating subsurface-flow constructed wetland is consistent with the experiment of kinetics and isothermal adsorption model. It proved the feasibility that analysis of kinetics and isothermal model of phosphorus adsorption in wastewater using in practical application. Under same hydraulic loading, contribution rate of phosphorus adsorption in gravel subsurface wetland is highest, followed by ceramic subsurface wetland. However, ceramic has large porosity and is rougher than gravel which is benefit for the growth of plant roots in the wetland. Gravel could be mixed with ceramic in practical engineering design which could increase the growth of plant roots and ensure the removal of phosphorus from rural domestic wastewater simultaneously.In summer and autumn, concentration of NH4+-N and TN in effluent water reached "Discharge standard of pollutants for municipal wastewater treatment plant" first A standard when hydraulic loading is 0.24-0.40 m3/(m2d). But concentration of TN in effluent water achieved first B standard while TP did not reach it when hydraulic loading is 0.40 m3/(m2·d) in winter and spring. Due to the height of packing in subsurface constructed wetland is low, hydraulic loading is different when removal effect of TN and TP achieved same standards. Removal efficiency of phosphorus becomes a limiting factor in hydraulic loading selection. Moreover, plants which could produce economic benefit would be the priority in this design. It brought considerable economic benefits for wetland to cultivate edible vegetables such as water spinach, leeks, crowndaisy chrysanthemum, cress and celery or grain crops like rice and wheat. The tension that lacking of plough might be alleviated in some villages if this design partly replace the traditional way of planting by soil. On the basis of economic benefits, it may appropriately reduce the hydraulic loading in order to make effluent water reach higher emission standards.Removal effect of NH4+-N is concentrated on filter bed while subsurface constructed wetland is more conducive to the removal of TN and TP. And plants with large biomass and multiple roots are more suitable for combined wetlands. Furthermore, the length of canna is only about 1 meter in this design where rice planting in combination with canna in wetland. Removal efficiency of TN in combined ecosystem which planting rice and canna is slightly better than the wetland which only planting rice.The average contribution rate of nitrogen and phosphorus are 12.9% and 25.8% due to lush plants in the combined wetlands. On the one hand, concentration of nitrogen and phosphorus in inflow water is not high. On the other hand, the height of packing in subsurface constructed wetland is low, especially there is no packing in filter bed. Yield per unit area of vegetables is impressive in combined ecosystem which the yield of water spinach are 19.37 kg/m2 and around RMB 15496 per 667 m2, and cress are 19.37 kg/m2 and RMB 14352 per 667 m2. However, grain crops such as rice and wheat have a lower yield, which the data of them are 0.78 kg/m2, RMB 14352 per 667 m2 and 0.69 kg/m2, RMB 828 per 667 m2. Results showed that vegetables with large biomass, multiple roots, big areas of leaves are suitable for combined wetlands where plough is abundant in the villages. But proportion of grain crops might be increased when some villages are lack of plough. Meanwhile, there is a possibility to mix plants with large biomass, multiple roots and grain crops in the combined ecosystem. It can increase overall efficiency when water spinach plant in summer and autumn and cress plant in winter and spring in the filter bed. In addition, further research could lay emphasis on planting grain crops such as rice and wheat in filter bed using ecological floating bed technology.The results of the study demonstrate that, not only a high purification ability towards nitrogen and phosphorus nutrients but also a great economic benefit could be achieved after introducing grain crops such as rice and wheat in plant filter bed and subsurface wetlands. Therefore, it may have potential for realizing agriculture economization of planted wetland system in rural areas and reaching the unification of environment benefit and economy benefit.