Effect of Urban Sewage Sludge Application on Soil Water Repellency and Water Retention Curve

Sewage sludge contains relatively high amounts of salts and organic matters which may affect the physical and chemical characteristics of soil. In this study the effects of adding the urban sewage sludge on soil water repellency and water retention curve were investigated. A clay loam soil was picked up from Foladshar, Isfahan, a region at the vicinity of a sewage treatment system. Different amounts of urban sewage sludge (25, 50, 65 and 80 percent) were added to the soil and designated by S25%, S50%, S65% and S80%. Then water repellency of them was determined by means of water drop penetration time (WDPT) and molarity of aqueous ethanol (MED), after 50 days of incubation. Sludge addition resulted in low to severe water repellency of the soil samples. The water retention curves were determined by means of the pressure membrane and sand box apparatuses. Finally, sewage sludge effects on van Genuchten equation's parameters were assessed by using SAS8 software and Fisher test. There was a linear and positive correlation between the logarithmic values of WDPT and organic matter quantities. Furthermore, a linear regression was established between the values WDPT and MED with R2=0.95. The S80% treatment had higher Өs while S0 had lower Өs, because of higher organic matter in S80% and lower organic matter and clay loam texture of S0. The residual water content in S80% was lower than those in the other treatments and the maximum of Өr was observed in S0. The parameter α was higher in S65% and S80% than those in the other treatments with slight water repellency which showed lower air entry value of the soil. The parameter n increased by increasing the organic matter in S25% and S50% rather than that in S0, but it decreased in S65% and S80%. This could be due to existence of higher water repellency and hydrophobic material on the surface of soil particles in S65% and S80% than those in the S25% and S50%. So intensive water repellency perhaps affected the parameter n and caused it to reduce it. Also water repellent soils had higher gravitational and available water than those in the primary wettable soil.