Simulating Hydraulic Capacity Of A Wastewater Land Treatment System With DRAINMOD

Wastewater land treatment system utilizes the purifying effect of the soil-plant-microbe eco-system. It is an economic and effective wastewater recovering approach. In order to help solve the severe water shortage and water pollution problems in Xi'an area, this thesis study was conducted to simulate the hydraulic capacity of a wastewater land treatment system with the field hydrology model-DRAINMOD on the basis of the wastewater irrigation practices in Xi'an , China. Based the hydrologic predictions, solute transport theory was introduced to predict the concentration of pollutants in the drainage water. Main findings of this research are as follows:1. The capacity of a wastewater land treatment system varied with field drainage conditions. When the drain spacing was small, surface drainage condition had little effect on the loading rate. As the drain spacing grows, an improvement in surface drainage condition improved the loading rate. Under the same surface drainage conditions, annual loading rate decreased with an increase in drain spacing. When drain spacing is above a certain value, total wastewater loading rate became independent of the drain spacing.2. Less amount and more frequent irrigation improved treatment capacity of the system. Model simulation results of different irrigation schedules showed that more frequent and less amount irrigation scheme could more easily satisfy the requirement of air volume in soil and resulted in few cancellations of scheduled irrigations, and therefore more wastewater was applied to the field. 3. Annual treatment capacity of a wastewater land treatment system was much affected by the distribution of annual precipitation. In order to produce enough air volume in the soil for wastewater intake, water table has to be kept lower than a threshold value. The modeling study results on the relationship of water table depth and annual loading rate showed that although the drainage discharge was high in wet years, the wet soil condition of the disposal fields resulted in low treatment capacity, such conditions were reversed in dryyears.4. Sensitivity analysis on input parameters of DRAINMOD indicated that model predictions of annual treatment capacity are most sensitive to the saturated hydraulic conductivity and the depth to the impermeable layer. Therefore, selection of these input parameters should be conducted with great cautions.5. Analysis on solute transport time and decay of pollutants in artificially drained fields showed that different loading rate contributed to the major difference in outflow concentrations. With drain spacings of 20, 100 and 500m, an initial NO3-N concentration of 40mg/l in the waster water reduced to 3.5, 2 and 1.2mg/l in the drainage outflow.