Phytotoxicity complements chemical assessment for re-use and re-purposing of refinery wastes for soil amendment purposes after bioremediation.

The objective of this study was to assess the suitability of onsite re-use of mature compost for landscaping and tree mulching, produced from the bioremediation of oily sludge from the refinery. Compost samples from the co-composting process were analysed for a range of contaminants, including a human health risk assessment fractionation (HRAF) of the remaining petroleum hydrocarbons, as well as a phytotoxicity test. The chemical characterisation demonstrated that the process removed more than 94% of the original petroleum hydrocarbons from the sludge, and the removal rates were high at 1155 mg/kg/day. The HRAF demonstrated no residual risks, posed by the petroleum hydrocarbons present in the compost to human health if used on-site when compared to the relevant Australian environmental investigation levels (ILs). However, the phytotoxicity assessment demonstrated that the compost was toxic to germinating lettuce. The gap in the literature this study addressed was to provide an estimate of the LD50 and no effect concentration (NEC) for the compost using a standard plant bioassay containing a range of residual (aged) and bioremediated refinery process wastes, including petroleum hydrocarbons. The values estimated for LD50 and NEC were approximately 125 and 43 mg/kg, respectively for compost containing residual petroleum hydrocarbon fractions, filling a gap in the current literature which has limited data on standard toxicity values that can be used in determining and informing commercial remediation strategies and their outcomes with aged sludges. Phytotoxicity was shown to be an important complement to conventional analyses and HRAF data when characterising the sludge, and understanding its potential for re-use. The novelty of the study is that it highlighted a gap in the complementary use of chemical and bioassay analyses for evaluating refinery waste remediation endpoints, which has potential for broader application to other projects.