Modeling Long Term Acceptance Rates for OWMSs

Abstract

Estimates of how much wastewater infiltrates the soil via the trench sidewall and bottom are needed to design on-site wastewater management systems (OWMSs) based on their long term acceptance rate (LTAR). Our objective was to determine the steady rate of trench bottom and sidewall flow using a two-dimensional numerical computer model assuming that the trench was nearly full with a well developed biomat at the bottom and sidewall. We did this for 12 different soil textural classes to represent a full range of soils (hydraulic properties were taken from a soil database). Biomat saturated hydraulic conductivity (0.073 cm d-1) and thickness (0.6 cm) were based on an earlier study. The water level in the trench was a constant height of 27 cm (90% of the trench height). Simulations were run for 30 or 50 days, whichever was sufficient time for flow out of the trench to reach a constant value representing the LTAR. We compared our results to a simple method developed by Bouma to predict steady flow through the bottom and sidewall of OWMS trenches. Despite the wide range in saturated hydraulic conductivities of the soil textural classes (8.18 to 642.98 cm d-1), the steady flow through the bottom of the trench in these soils fell in a narrow range of 4.83 to 10.32 cm d-1. Sidewall flow ranged from 28.0 to 34.8% of the total flow out of the trench. Bouma's method for predicting flow through the trench bottom and sidewall produced remarkably accurate estimates for all soil textural classes and may provide a simple method of estimating LTAR.