Study On The Soil Treatment Effect Of A Domestic Wastewater Septic System In Xi'an, China

In this study, an experimental system of septic system for domestic wastewater treatment was set up. Under different hydraulic loading rates, daily effluent flow rate was recorded and water samples were taken for analysis of COD, ammonia and total nitrogen concentrations. The influence of rainfall on the treatment effect of the system was analyzed for different rainfall intensity and hydraulic loading rate. The Hydrus-1D model was used to simulate soil water content in the disposal field and daily outflow from the system. Major findings are as below:(1) With a hydraulic loading rate of 2.4cm/d, the system reached the mature state after 30 days. The average removal efficiencies was 91.4% for COD, 29.5% for NH⁺₄-N and 98.6% for total nitrogen. The daily outflow concentration of COD and NH+4-N fluctuated with a generally decending trend, and the daily average removal efficiencies of the system were 89.0% for COD and 93.6% for ammonia on the mass basis.(2) Under different hydraulic loading rates of 2.4cm/d, 1.8cm/d and 1.3cm/d, it was found that when the loading rate increases, the system removal efficiency increased for COD, changed little for ammonia, and decreased for the total nitrogen..(3) Under a hydraulic loading rate of 1.8cm/d, with a 2 day rainfall of 1.8mm and 40mm, the concentration and the mass based system removal efficiencies for COD, NH⁺₄-N and total nitrogen all decreased, indicating that the rainfall intensity has significant impact on the treatment effect of the system. After the rainfall, thesystem recovered its waste removal capability. Under a hydraulic loading rate of 1.3cm/d, with a 3 days rainfall of 3.8mm, 0.2mm and 18.8mm, the system removal efficiencies for COD, NH⁺₄-N and total nitrogen were little affected.(4) The simulation results with the Hydrus-1D model showed that when the movement of soil water reached a stable condition in the system, saturated and unsaturated flow appeared alternatively within a depth of 0-20cm; below the depth of 20cm there occurred only unsaturated flow. The simulated daily outflow process is in good agreement with the observed values. The outflow rate reaches the peak value within 5 hours.