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Crude oil affects soil ecosystems, resulting in significant losses in soil quality. The objective of the present study was to evaluate the effects of hydrocarbon pollution (crude oil) on soil enzyme activities and determine the removal of total petroleum hydrocarbons (TPH) during an incubation period of 150 days. The potential use of wastewater sludge as a biostimulating agent in petroleum-contaminated soils was also evaluated. The degradation of crude oil in contaminated soil was significantly enhanced after the addition of wastewater sludge.The results also indicated that significant TPH degradation in crude oil-contaminated soils (80% for dose of 0.5% and 83% for dose of 5% at 18 °C) (94% for dose of 0.5% and 92% for dose of 5% at 28 °C) occurred after an incubation period of 150 days. Enhanced enzyme activity levels in contaminated soils indicated that crude oil (0.5% and 5%, w/w) stimulated microbial growth and enzyme activity in the soil environment. Urease activity (UA) levels in crude oil contaminated soils were generally higher than initial UA levels in soil during the incubation period. Soil UA exhibited a stronger response to petroleum treatment. Alkaline phosphatase (APA) and β-Glucosidase (BGA) activities were not affected crude oil pollution. However, contamination of crude oil had a negatively effect on soil dehydrogenase activity (DHA).

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Soil contamination by used engine oil is a common occurrence in most developing countries. This has been shown to have harmful effects on the environment and human beings at large. Used oils are considered to be hazardous waste materials. These are composed of toxic chemicals, such as heavy metals (which come from additives and wear and tear of engine parts), combustion products, light hydrocarbons, polar compounds, uninuclear and polynuclear aromatic compounds, resinous materials, and organometallic compounds. Some of these pollutants are carcinogenic in nature. The objective of this study was to evaluate the effects of used engine oil (doses of 0.5% and 5%) with hydrocarbon pollution on total petroleum hydrocarbon (TPH) removal from soil and determine the fate of TPHs at different temperatures (18 °C and 28 °C) during an incubation period of 240 days. The possible use of wastewater sludge as a biostimulating agent in used engine oil-contaminated soils was also evaluated. The results of 240 days of incubation indicated that TPH removal percentages in used engine oil-contaminated+sludge amended soils at 18 °C were 68% and 66% for doses of 0.5% and 5%, respectively. Incubation at 28 °C resulted in higher TPH removal with values of 56% (dose of 0.5%) and 74% (dose of 5%). Based on the first-order kinetics model, the high dose (5%) of used engine oil-contaminated soil amended with wastewater sludge showed the highest biodegradation rate of 0.00562/day and half-life of 123.13 days at the end of the incubation period at 28 °C. These rates were significantly higher than those of the control soil (0.00366/day and 189.01 days).