Assessing the compost-ability of oil refinery sludge in the presence or absence of earthworms: An eco-friendly approach of biodegradation

Abstract

Composting offers a low-cost and sustainable approach in waste management, however, knowledge about its feasibility in oil refinery sludge (ORS) bioconversion is unknown. We investigated the efficiency of three composting systems, viz., Eisenia-system (with Eisenia fetida), Eudrilus-system (with Eudrilus eugeniae), and non-earthworm system for ORS bioconversion. The earthworm-based systems recorded greater increment in available nutrients and reduction of heavy metals than non-earthworm system. The Eudrilus-system recorded an increment in total bacterial count by 10%, while P-solubilizing bacteria increased five-folds in Eisenia-system. Additionally, higher economic benefit was observed in terms of net present value (US $ 203.08), benefit-to-cost ratio (0.63) and pay-back time (1 year and 6 months) in the Eudrilus-processed ORS composting approach. The earthworm-based systems also demonstrated higher activity of urease, alkaline phosphatase and dehydrogenase. The clean index (>4) and fertility index (>3.1) of earthworm-sourced composts reflected their high manurial potential. The compost quality index (CQI >7) indicated that presence of earthworms greatly facilitated the bioconversion of ORS due to higher humification and microbial diversity. Principal component analysis identified water holding capacity, total C, NPK, and earthworm growth as the major contributors in CQI. Moreover, the strong influence of enzymes in CQI (R² = 0.64) was corroborated by the linear regression analysis. Our findings underscore the potential of earthworm-based bio-systems in ORS-remediation, offering a viable way in its management while producing fortified organic manure.