Wastewater reuse effects on soil hydraulic conductivity

The wastewater total suspended solids (TSS) concentration effects on the saturated hydraulic conductivity, K-s of a clay and a loam soil were investigated on laboratory repacked soil cores by a constant head permeameter. Both municipal wastewater (MW) and artificial wastewater (AW) with, different TSS concentrations were used, with the aim to evaluate, by comparison, the effects of biological activity. The development of a surface scaled layer was investigated in loam soil columns supplied with AW and equipped with water manometers at different depths to detect the hydraulic head gradient changes. In the loam soil, K-s reduced to about 80% of the initial value after infiltration of 175 mm of MW with TSS = 57-68 mg L-1. Reductions in K-s were more remarkable in the clay soil. An empirical relationship was proposed to predict the relative hydraulic conductivity, K-r i.e., the ratio between actual and initial hydraulic conductivity versus the cumulative density loading of TSS. Hydraulic head gradients in the top layer (0-20 mm) of the soil columns increased during application of AW, as a consequence of the formation of a sealed layer, denoting that the surface pore sealing was the main mechanism responsible for the observed K-s reductions. Laboratory data were gathered in a numerical simulation code specifically created to assess the consequences of K-s reduction on water movement through the soil profile. Simulation of both ponded and sprinkler irrigation with MW resulted in reduced infiltration and increased surface ponding condition compared to the application of fresh water (FW).