Type of Document	Thesis
Author	Harmon, Virginia P.
URN	etd-11092007-144820
Title	Numerical Simulations of Nutrient Removal by Slow-Rate Wastewater Irrigation and Conventional Septic Systems
Degree	Master of Science
Department	Civil and Environmental Engineering, Department of
Advisory Committee	Advisor Name Title Tarek Abichou Committee Chair Amy Chan Hilton Committee Member Gang Chen Committee Member
Keywords	Wastewater Land Application Modeling/Simulations Septic System Simulations
Date of Defense	2007-11-01
Availability	unrestricted
Abstract Land application is one of the natural treatment processes for raw or partially treated domestic wastewater. Examples include application of septic tank effluent to soil for subsurface infiltration, slow-rate (SR) wastewater irrigation, overland flow wastewater application, and soil aquifer treatment (SAT), also called rapid infiltration (RI). These processes can remove nitrogen, which usually is the limiting design parameter for a land application system. Wastewater or septic tank effluent applied to the land in this study contains organic nitrogen and ammonia, plus nitrate and nitrite in the wastewater. In this study, HYDRUS-1D software, version 3.0, was used to conduct numerical simulations of one-dimensional water flow and nitrogen transport in vertical profiles of unsaturated soil to simulate the following scenarios: 1) slow-rate wastewater irrigation and 2) the application of septic tank effluent to soil for subsurface infiltration. The total depth of each soil profile was 300 cm with one soil type (Kershaw Sand) in each profile. The slow-rate simulation profiles consisted only of soil; the septic simulation profiles contained a total of 270 cm of soil with a 30 cm space containing drainrock (no effect on simulation) around the effluent distribution pipe. It was assumed that all applied nitrogen was converted to nitrate within the same time period assumed for application, and that there was no adsorption of nitrate. Water balance outputs included cumulative transpiration, hydraulic flux across the bottom of the soil profile, evaporation, infiltration, and runoff per square centimeter of soil. Nitrogen balance outputs included cumulative amount of nitrate removed by denitrification, root nitrate uptake, and nitrogen flux across the bottom of the soil profile. In the slow-rate simulations 4.30 mg N per cm2 of soil surface was applied per year, and 96.0% of the applied nitrogen (4.13 mg per cm2) was removed by denitrification and root uptake. In the septic simulation, 50.5 mg N per cm2 of soil surface was applied per year for subsurface infiltration, and 63.6% of the applied nitrogen (32.1 mg/cm2) was removed by denitrification. The objective of this study was to compare the nutrient loading from two hypothetical nitrogen contributors to the environment analogous to, and with only a general resemblance to, 1) the City of Tallahassee Southeast Farm (SEF), which practices SR wastewater irrigation and 2) conventional septic systems. The modeling conditions for each contributor were set to be as similar as possible to those for the other contributor. Flux results from the above simulations (per square centimeter of soil during the second year of the simulation) were multiplied by appropriate factors to yield the total mass of nitrogen applied to the surfaces of and the total mass of nitrate exiting the bottoms of the 300-cm soil profiles for sources 1) and 2) during one year. The estimated total mass of nitrogen applied to the simulated Southeast Farm was 3.48 x 105 kg N/yr. The estimated total mass of nitrogen in the effluent from 28,217 septic tanks from conventional septic systems added to drainfields in the unconfined areas of Leon and Wakulla counties and in the semi-confined area of Leon County was 3.56 x 105 kg N/yr. The total mass of nitrogen as nitrate exiting the bottom of the soil profile of the simulated SEF (2,000 acres) was estimated to be 1.67 x 104 kg/yr. The total mass of nitrogen as nitrate exiting the bottoms of the soil profiles for the estimated 28,217 conventional septic systems in the geographical areas listed in the previous paragraph was estimated to be 1.31 x 105 kg/yr. Additional simulations of slow-rate wastewater irrigation were conducted to meet the objective of modeling the nitrogen removal while varying the wastewater loading rate and the denitrification rate constant (K). The inputs were chosen so that the nitrate concentration ranged from 0 to 1.6 mg/L N as nitrate in plots of simulated nitrate concentrations at the bottom of the soil profile versus wastewater loading rate. When K was 0.038/day, it was possible to maintain a nitrate concentration at the bottom of the soil profile at or below 1.0 mg/L as long as the daily hydraulic wastewater loading was less than or equal to 1.5 cm/day. The corresponding wastewater loading values for K of 0.025/day and K of 0.012/day were about 1.0 cm/day and 0.5 cm/day, respectively. The nitrate concentrations at bottom of soil profile corresponding to the current wastewater loading rate of 0.841 cm/d used for the simulated “Southeast Farm” were approximately 0.65 mg/L for K = 0.025/day and 0.15 mg/L for K = 0.038/day. The results of this study may be usable to compare the yearly amounts of nitrogen loading for the above facilities. However, the overall accuracy of the results in describing the real SEF or a real conventional septic system is low mainly because of the method of modeling denitrification, the simplified method of determining input values, and perhaps the complexity of the processes modeled. Future modeling could be conducted so as to improve upon the above limitations. Also, the simulation(s) could be set up to take into account the effect, if any, of the water table elevation on flow. Further studies are needed to better understand and predict denitrification, and other types of denitrification models are available. At real sites, soils with improved water-holding properties could be added via soil mixing, thus improving wastewater treatment performance.
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