| Recent attempts to control nitrogen in domestic wastewater have attracted more reachers with the increase of nitrogen pollution. There are some disadvantages of traditional nitrogen treatment technologies, such as low nitrogen removal efficiency, high energy requirement, high construction cost and so on. In recent years, many researchers have focused on the study of ecological engineering, especially soil trench technology. Some investigation such as development of materials used as infiltration layers, system design, comparison of plant species, simulation of treatment processes and so on were studied, but model of nitrogen transformation, relationship between nitrous oxide (N2O) flux and nitrogen removal and mechanisms need further investigation. Moreover, higher performance of operation stability and removal efficiencies should be improved.In this study, the soil trench systems were established using a subsurface flow design for treating artificial domestic wastewater. The treatment cells are 88 cm deep and 56 cm in diameter with a substrate of an 18 cm gravel layer as the supporting medium and 43 cm sand being covered. The fluxes of N2O in the soil trench systems were assessed and a dynamic system was constructed to simulate nitrogen transformation. To better understand the mechanism of N2O emission and relationship between N2O flux and nitrogen removal, spatial distribution of ammonia-oxidizing bacteria (AOB) in the soil without previous cultivation were compared using fluorescence in situ hybridization (FISH).The results of the present study show that nitrogen removal efficiencies in the soil trench systems were greatly influenced by different plant species. For the monoculture systems, Phragmites australis (PA) contributed greatly to TN removal, compared with Zizania latifolia (ZL) and Typha latifolia (TL) under both influent loads. For the polyculture systems, TN removal efficiency of P1 cells was the highest and that of P2 cells was the lowest, reflecting that PA was the typical plant specie inhabiting the local district for removal of excess nitrogen in the domestic wastewater. The microbial reactions drive the nitrogen cycle. Aerobic and anaerobic microsites can develop within the same aggregate, supporting nitrification and denitrification, respectively. Results indicated that enhanced TN removal efficiencies were obtained in vegetation beds, which was affected greatly by seasonal fluctuations.The properties of N2O fluxes showed significant differences with seasonal fluctuations, influent load and plant species in vegetation systems. The conversion ratio of N2O-N ranged from 0.2% to 1.26%, contributing much lower to TN removal. For the monoculture systems, the highest flux rate of N2O was observed in two ZL cells, up to 2.2mg/m2/h. The emission peak with both influent loads appeared in July.The N2O flux rate was positively correlated with influent load. For the polyculture systems, the highest N2O emission occurred in the cell planted with PA and ZL. Whereas, the lower emission rate were obtained in the cell planted with PA and TL. These revealed that ZL stimulated the N2O emission. Both the pollutants removal performance and the N2O emission in TL system were the lowest.The multiple linear regression model has the important prediction value for the long-term forecast. In this study, a prediction model was constructed to simulate the N2O flux in soil trench systems. Some factors, such as TN concentration, pH, water temperature and so on were considered as the environmental parameters. The results of linear regression model indicated a good fit between the simulated and observed data. It appears that the model can give a reliable prediction for N2O flux in soil trench systems. Several environmental parameters that affected the strength of observed N2O emission were identified, such as water temperature, water quantity, plants cover, NH4+-N and TN concentrations.Direct cell counting using FISH images revealed that AOB mainly resided on the plants rhizosphere. The AOB amounts in growth seasons ranged from 10- to 30-fold higher than those in senescence seasons. Aerobic microsite and adequate NH4+-N were favored by AOB growth in the processes of N2O emission. Results indicated that the oxidation-reduction potential (ORP) values remained high in the upper and plant rhizosphere, reflecting oxidation status in these zones. The NH4+-N concentration was low in the upper, which can explain the lower AOB amounts in this layer. Under plant rhizosphere, anaerobic status was monitored with increased depth in the subsurface flow systems. The AOB amounts were also lower in these zones. Different plants species affected the AOB amounts and N2O emission greatly. The root structure of ZL was more favored by AOB for N2O formation than that of PA and TL. Comparison of three plants species in P1 (PA, ZL and TL) polyculture system showed that the rhizosphere of ZL supported higher amounts of AOB, reflecting that ZL stimulated the nitrification reaction. It is also clear that enhanced biogeochemical activity in vegetation wetland systems in growth seasons resulting in large fluxes of N2O. The characteristics of AOB distribution in the soil trench systems supported the results of N2O flux, reflecting that nitrification may be of importance in the N2O emission. It was therefore concluded that the rhizosphere of plants provided much more oxygen and then enhanced nitrification reaction.In the pilot-scale experiment, to improve the oxygen transportation, three methods were investigated, such as infiltration materials, plants species and ventilation by vertical pipes. Sand, compost and so on as the soil amended materials were used as infiltration layers in the soil trench to enhance the adsorption capability, ion exchange capability and oxygen concentration of soil. A ventilated system to maintain a high ORP in the soil was performed without power. The ventilation was provided by installing vertical pipes in the bottom of inlet and outlet channels in the trench. The conduction pipes were connected with the soil surface to ensure the oxygen transportation.The demonstration projects in Taihu lake basin for the treatment of domestic wastewater were constructed based on the lab-scale and pilot-scale experiments. The removal efficiencies of BOD5, TN, TP and SS of demonstration projects were up to 96%, 78.6%, 91.9% and 97.5%, respectively. The 1.098×103 kg of nitrogen and 69 kg of phosphorus were removed per year. Results of algal growth potential (AGP) confirmed that the maximum growth potentials of S. Capricornutum were approximately 106cells/ml for the STS effluent and 107cells/ml for the WWTP effluent, reflecting that algal bloom would occur much more easily in the effluents from bio-treatment than those from eco-treatment.The soil trench technology was suitable for the treatment of domestic wastewater. The study of nitrogen transformation and characteristics of N2O emission provides the information for the design of soil trench system and optimizing the plant community. This technology has the significant value for further application of soil trench system to treat rural domestic wastewater in different areas and conditions.
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