Controlled Field Experiment for Performance Evaluation of Septic Tank Effluent Treatment during Soil Infiltration

Decentralized systems are responsible for treating approximately 25% of the wastewater generated in the United States. The most common decentralized system involves onsite treatment using a septic tank unit followed by dispersal to a subsurface soil infiltration unit where percolation to groundwater occurs. To evaluate the hydraulic and purification processes occurring during soil treatment of septic tank effluent (STE), a field experiment was initiated in the Spring of 2003 with continued operation and monitoring for 2 years. A replicated factorial design $\left(2^2\right)$ was employed to evaluate three infiltrative surface architectures (ISAs) (open, stone, and synthetic) and two daily hydraulic loading rates (HLRs) (4 and $8\mathrm{cm}∕\mathrm{day}$ ). Pilot-scale test cells were established in native sandy loam soils at the Mines Park Test Site located on the Colorado School of Mines campus in Golden, Colo. STE was obtained from a nearby multifamily apartment building and applied to the test cells daily. Field monitoring included baseline characterization of soil and site properties, routine characterization of the STE applied, observations of STE ponding on the infiltrative surface, periodic measurement of constant-head infiltration rates, and periodic sampling and analyses of the soil pore water at 60- or 120-cm depths below the infiltrative surface. Monitoring revealed that the ISA and HLR influenced the rate and extent of hydraulic capacity loss during soil treatment. For example, an open horizontal infiltrative surface maintained an infiltration capacity that was 40–80% higher than one covered with either washed stones or synthetic aggregate. Purification of STE during infiltration and percolation through the sandy loam soil was very high. The cumulative mass removed during 2 years of operation for dissolved organic carbon, total nitrogen, and total phosphorus averaged 94, 42, and 99%, respectively. While there was no significant difference in the purification performance based on ISA or HLR, an increase in the vadose zone depth slightly increased purification.