Field-compatible scalable microcosm-based biostimulation-bioaugmentation for TPH degradation in refinery tank bottom sludge: optimization, kinetics, and toxicity assessment

Refinery tank bottom sludge (TBS) is a complex and hazardous petroleum waste, and the development of sustainable, field-compatible remediation strategies for its treatment remains a significant challenge. The study employed a multifunctional bacterial consortium possessing hydrocarbon-degrading, biosurfactant-producing, and plant growth-promoting traits. Inhibition assays were performed to select compatible isolates, resulting in the development of a consortium comprising non-antagonistic strains. Biodegradation experiments were conducted in a soil microcosm system, and independent variables were optimized using a Box-Behnken Design under Response Surface Methodology to evaluate their effects on TPH degradation. NPK fertilizer, cow manure, and oily sludge were considered as independent variables, while TPH degradation was the response. The interaction of these variables significantly influenced degradation efficiency, as reflected by a high coefficient of determination (R2 = 0.99). The RSM-predicted optimum conditions were 2.896 g NPK, 35.861 g cow manure, and 18.914% sludge per kg of soil, yielding a predicted TPH removal of 63.68%, which closely matched the observed value of 64.32%. GC-MS analysis over a 60-day period revealed the degradation of halogenated, aliphatic, aromatic, and cyclic hydrocarbons. Concurrently, the emergence of oxygenated intermediates and a shift in carbon chain distribution confirmed active microbial metabolism. Kinetic analysis indicated that TPH degradation predominantly followed first-order kinetics (k1: 0.0101–0.0215 d−1; t1/2: 32.19–68.57 days), while second-order kinetics (k2: 3.8 × 10−7–1.1 × 10−5 g·kg−1·d−1; t1/2: 9.23–61.55 days) better described high-sludge treatments, suggesting microbial-substrate interaction effects. Phytotoxicity assessment using mung bean (Vigna radiata) demonstrated a clear concentration and time-dependent toxicity pattern. This approach offers a sustainable and scalable strategy for the treatment of petroleum waste and the ecological restoration of impacted environments.