Reduced Disposal Area Performance Utilizing Secondary-Treated Effluent in Profile-Limiting Soils

Abstract

Onsite wastewater systems dispose of primary treated effluent by utilizing the soil for final recycling and renovation of wastewater into the environment. Soil and site limitations have become a challenge to design a wastewater system and dispose of onsite wastewater using a conventional pipe and gravel design. Using secondary-treated effluent from an advanced treatment unit applied to a reduced disposal area offers an additional alternative when developing an onsite wastewater system. The objective of this study was to determine the feasibility of hydraulically loading limiting soils with secondary-treated effluent in a reduced disposal area. A reduced disposal area was constructed at six existing residences within the same subdivision that had shallow redoximorphic features that precluded using a conventional pipe and gravel wastewater design. Each residence had an existing advanced treatment unit with a surface discharge of secondary-treated effluent. Flows were diverted from the surface discharge to the reduced disposal area. Wastewater flows were recorded at regular intervals, along with ponding depths in the disposal area and fluctuations in the seasonal water table over a 12-month period (March 2017 to March 2018). The disposal areas were hydraulically loaded at 2 to 3.8 times the rate recommended for secondary-treated effluent. Wastewater effluent was sampled throughout the study and resulted in a mean of <8.5 mg·L -1 total suspended solids, <5.3 mg·L -1 biochemical oxygen demand, and >6.3 mg·L -1 dissolved oxygen, all of which met or exceeded the minimum water quality criteria for surface discharges of secondary-treated effluent. Three of the six sites showed ponding depths between 0 and 4 cm in the trenches during the study period. The remaining three sites showed ponding between 0 and 35 cm in the trenches during the study period. Based on the results of this study, a reduced disposal area utilizing secondary-treated effluent appears to be a feasible option to surface discharging.