| As compared to conventional activated sludge treating process, SWIS has more advantages, including low construction, operation costs and easy maintenance. Howerer, there remain problems with nitrogen removal usually due to the complicated interior environment of the application. Therefore, this paper focused on the nitrogen removal effects of SWIS, taking the microbial and enzyme studies as the breakthrough point. Preparation of the substrate and controlling of the operation methods were mainly studied. Kinetic equation was established to describe the nitrogen removal process. Finally, demonstration project was operated continually, and model techniques in enhancing nitrogen removal rates of SWIS were established. The main conclusions are:At first, a set of SWIS simulated system was designed. Analysis of the substrate and inner environment was easy with the side sampling points. The wastewater flowed through the "cross" distribution pipe to the infiltration area. As then, the distribution of the wastewater was uniform. At the lower part of the system was the water collection area. Between this and the upper one was a partition plate with holes on it. This design could insure the uniform of the water quality for analysis. Different parts of the system was connected with flanges, therefore, the risk of collapse of the substrate was overcomed. Adhesion method was adopted around the inner part of SWIS to lessen the possibility of flow shortcut. Therefore, the system could implement several tasks for different experimental purposes.Secondly, the experimental results of the substrate for enhancing nitrogen removal effects showed that, the bio-substrate was made of activated sludge, slag and meadow brown soil. The porosity, infiltration rate and organic matter content of the bio-substrate were raised considerably compared to the raw soil. At the same time, the number of nitrifying, nitrified and denitrified bacterial was increased. And also, the bio-substrate had higher NH4+-N adsorption, nitrifying and detrifying capacities than the meadow brown soil. In CSWIS (constructed subsurface wastewater infiltration system), the construction of the infiltration from up to down was:meadow brown soil, bio-substrate, meadow brown soil and gravel. NH4+-N removal rate was stabled on90.7%after20-25days’operation. The average NH4+-N concentration in effluent was4.4mg/L.The distribution of microbial in CSWIS showed that, ammonifying bacterial was affected little by depths or temperature. Conversely, the number of nitrifying bacterial reduced as the depths increased. The change of denitrifying bacterial number was opposite to that of nitrifying one, both of which rose with the elevating of the temperature. Following the input, output operation of CSWIS, the quantities of nitrifying and denitrifying bacterial fluctuated accordingly. Analyzing from SPSS software, the number of ammonifying, nitrifying and denitrifying bacterial was in considerable correlation with NH4+-N and TN removal rates.Studies of the enzyme distribution of CSWIS showed that, the distribution of urease was affected by depth, NH4+-N concentration of input and temperature. In the middle of the system, nearby the water distribution area, urease activity was the strongest, and in positive correlation with NH4+-N concentration and temperature. Distribution of NAR activity was affected more by depth than either changes of temperature or characteristics of input. The variation order of NAR activity with depth was20cm>40cm>60cm>80cm>100cm. Depth and temperature had profoundly influence on the distribution of NIR activity. Changes of NIR activity with depth was40cm>20cm>60cm>80cm>100cm, in positive correlation with temperature. From20cm to100cm, urease activity was in significant correlation with NH4+-N and TN removal. No significant correlation was found between NAR activity and nitrogen removal. From60cm to100cm, NIR activity was in correlation with nitrogen removal. The results implied that it is feasible to investigate urease and NIR activities as the enzyme index in determining nitrogen removal effects of CSWIS.Nitrifying process of SWIS accorded with the kinetic equation of NE=N0e-.48121. Temperature was the main factor affecting k, the relationship between k and temperature was kT=0.2218×1.035(T-20). With respect to denitrifying process, the NO3--N concentration of outflow was negative exponent correlation with the hydraulic retention time (HRT) of the system. Carbon source was the main factor causing the changes of k. With the distribution methods were applied in55cm and65cm, with1:1discharge ratio, k rose from0.0355to0.0488.The operation methods of CSWIS showed that, the infiltration rated and oxidation reduction potential (ORP) rose with the lower wet/dry ratio, while the moisture content and the ratio of volume reduced. ORP fluctuated with the inflow-outflow operation. The number of ammonifying changed little with the wet/dry ratio. Nitrifying bacterial’number raised and denitrifying’reduced accordingly. In the start-up period, the optimal wet/dry ratio was1:3; the start-up period was20days. In stable operation period, the wet/dry ratio was1:1. Nitrogen concentration was lower than the standard of GB/T18921-2002. With the increasing of hydraulic and BOD loading rates, ORP of the system decreased, and the nitrogen removal rates lowered. The optimal hydraulic and BOD loadings were0.065～0.081m3/m2·d and16.8g/m2·d。Distributary method raised TN removal rate from51.6%to68.1%.Finally, the continuous operation results of the demonstration project showed that, the start-up period of the system was20days, removal rates of NH4+-N、TN、COD、 BOD5and TP were87.7、70.1、84.8、91.7and85.1%, respectively. The average outflow concentration was2.3、6.9、19.7、5.7and0.3mg/L, respectively, lower than the standard of GB18921-2002. |
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