Effect of Vegetable Oil Industry Effluent on Soil Properties

Abstract

The discharge of wastewater into the environment is one of the environmental problems facing industrial areas. The effect of vegetable oil industry effluent discharged from a vegetable oil processing company located in Anambra State- South East Nigeria, on soil properties was evaluated relative to Federal Environmental Protection Agency (FEPA), 1991 standard. Wastewater physicochemical properties namely: Temperature, total suspended solids (TSS), total dissolved solids (TDS), turbidity, pH, electrical conductivity (EC), biochemical oxygen demand (BOD), chemical oxygen demand (COD), dissolved oxygen (DO), oil and grease, and heavy metal (Cd, Fe, Cu, Pb, Zn, Ni, Cr) concentrations were analyzed using standard methods of American Public Health Association (APHA). Normal agricultural (control A) and contaminated soils (effluent pit sediment and control B) were subjected to soil analysis and soil enzyme assay in the laboratory. The results obtained showed that the physicochemical properties of the effluent analyzed were all below the FEPA permissible limits except turbidity (15.20 FNU) and oil and grease (100 mg/l) which are appreciably higher than the permissible limits of 5 FNU and 10 mg/l respectively. The concentrations of all the heavy metals analyzed were higher than the maximum concentration levels of each metal allowed to be disposed in water and land as recommended by FEPA; with Cu2+ (18.73 mg/l), Cd2+ (35.45 mg/l) and Zn2+ (45.60 mg/l) having the highest load of the metals. Effluent pit sediment showed higher Pb2+ content (10.82 mg/kg) than controls but lower than the permissible limit (100 mg/kg); while cadmium concentrations in both controls (11.82 mg/kg), and effluent pit sediment (47.27 mg/kg) were all higher than the permissible limit for cadmium (3.00 mg/kg) in soil. Analysis of the soil samples showed that clay (29.76 %), silt (9.28 %), fine sand (56.13 %) and coarse sand (4.83 %) of normal control A and control B did not alter but varied in the effluent pit sediment with clay, silt, fine sand and coarse sand being 9.76, 4.28, 71.75 and 8.44 % respectively. The pH of the soil samples ranged from 4.1 - 4.9 for the controls, but higher in effluent pit sediment (5.2 - 5.7). There was a high percentage of carbon (9.54 %), organic matter (16.45 %), available phosphorus (11.15 mg/g) and C: N ratio (54.53) in the effluent pit sediment. The exchangeable bases showed that control B had higher value for sodium ion (60.87 meq/100g) while normal control A had higher CEC (16.80 meq/100g). Aluminum ion was not detected in both the effluent pit sediment and the controls. When the results of soil enzyme assay were subjected to analysis of variance (ANOVA), it showed that soil urease and dehydrogenases activities of both contaminated soils, respectively increased and decreased significantly (p < 0.05) compared to normal control A, while phosphatase activity showed no significant difference (p > 0.05). There is need for proper treatment and monitoring of effluent before discharge to agricultural soil to prevent adverse effect of hazardous chemicals on soil and human health.