Process residuals, sludge with high oil content, were treated by composting. Inlab-scale (100 litre) trials initial oil con-centrations (30 to 50 gkg-1 dry matter) were, depending on composting conditions, reduced 55 to90% during a period of 60 to 120 days. Besides carbon dioxide, asignificant amount of oil was converted to stabilized residuals, Aminor volatile fraction (5%) evaporated. High numbers (approximately1x108 per gram dry compost) of oil degrading bacteria were detected during thehigh rate phases. Genetic fingerprinting (randomly amplified polymorphicDNA (RAPD) indicated groups of related and potentially interestingisolates from these periods. RAPD also indicated that successions of themicroflora took place over time. Initial oil contents (40 to 80 g kg-dry compost) in outdoor pilot composts (15000 to 20000 kg), were reduced86 to 90% in 10 months. Prolonged treatment (5 months) resulted infurther decreases, in total a 95 to 97% reduction. In spite ofincreased biological activity, neither the addition of organic(manure) nor inorganic (nitrogen, phosphorus and potassium) nutrientsincreased the speed or amount of oil degraded during the prolongedtreatment. Among potentially hazardous organics, elevatedlevels of polyaromatic hydrocarbons were found in the original oilysludge. Composting with adequate substrate reduced most of them. With properconsiderations, composting is suggested as a cost-and treatment-effective wayof handling these sludges.
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