Effect of Sewage Sludge Application on Hydrophobicity and Water Retention Curve of Soil

Abstract

Sewage sludge contains relatively high amounts of salts and organic matter, which may favorably affect the physical and chemical characteristics of the receiving soil. In this study, the effects of adding urban sewage sludge on soil water repellency and water retention curves were investigated. A sample of clay loam soil was collected from a region near Foladshar's sewage treatment plant. Different amounts of urban sewage sludge including 25, 50, 65, and 80 percent were added to the soil and named as S25%, S50%, S65%, and S80%, respectively. In addition, a control treatment applied using no organic matter (S0). Then, soil water repellency was determined after 50 days of incubation using water drop penetration time (WDPT) and molarity of aqueous ethanol (MED), methods. Sewage sludge caused slight to severe degree of water repellency in soils. The water retention curves were determined using the pressure membrane and sand box apparatuses. Finally, the data were analyzed using SAS8 software and Fisher test. The results show that there were a linear and positive correlation between the logarithmic values of WDPT and organic matter quantities. Furthermore, a linear regression was established between WDPT and MED (R 2 =0.95). It shows that S80% had a highest water content (θ s ) while S0 had lowest θ s due to higher amounts of organic matter in S80% and lower amounts of organic matter and clay loam texture of S0. The residual water content in S80% was lower than the other treatments and the maximum of θ r was observed in S0 (0.101 cm 3 /cm 3 ). The parameter was higher in S65% and S80% than the other treatments with slight water repellency which showed lower air entry value of the soil. The parameter n increased with increasing amounts of 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 that in the S25% and S50%. Preliminary results indicate that intensive water repellency may affect the parameter n, and data collected showed a decreasing trend for it.