Beyond removal of total petroleum hydrocarbons: An evidence-tiered mass-balance framework and critical review of priority and emerging contaminants in oily-sludge valorization.

Oily sludge is a hazardous petroleum-industry waste whose risk is shaped by the heterogeneity of its oil, water and solid fractions rather than by total petroleum hydrocarbon (TPH) content alone. This critical review develops an evidence-tiered mass-balance framework for evaluating priority and emerging contaminants in oily sludge, treatment products and related receiving environments. The framework distinguishes Tier 1 direct evidence from oily sludge and treatment-derived matrices; Tier 2 source-linked receiving-medium evidence from petroleum-impacted soil, groundwater, surface water, sediment and biota; and Tier 3 adjacent or mechanistic evidence from produced water, oil-sands process-affected water, landfill leachate, municipal or sewage sludge, reactors, columns and reviews. An illustrative Evidence Confidence Index (ECI) is used to compare evidence maturity across contaminant groups, but it is explicitly not a regulatory threshold, statistical confidence interval or validated risk score. Direct studies establish PAHs/PACs and selected metals as comparatively mature source-term groups, whereas petroleum polar compounds/naphthenic acid fraction compounds, oilfield chemical residues, phthalate esters and microplastics/nanoplastics remain monitoring-priority groups because matrix-specific source, leachate and product-stream data are sparse. For valorization, raw sludge is defined as the input to an input-plus-five-destination framework comprising recovered oil or organic condensate, process water/leachate or aqueous condensate, non-condensable off-gas and entrained/captured particles, solid products or treated solids, and residual ash or stabilized residues, with non-generated destinations recorded as not applicable. A worked calculation demonstrates inventory allocation, a numerical uncertainty budget and mass-balance closure without presenting hypothetical values as empirical evidence. Oily-sludge management should therefore shift from removal-rate acceptance to evidence-based risk-reduction acceptance.