Petroleum refinery effluent contribution to chemical mixture toxic pressure in the environment

Report no. 7/22: Petroleum refinery effluents (PRE) are wastewaters from industries associated with oil refining. Within EU, PREs are regulated through local discharge permits and receive significant treatment before emission. After treatment, PREs can still contain various pollutants potentially toxic to organisms. Earlier work, including whole-effluent toxicity assessments, has shown that toxicity of PREs is often limited. However, the extent to which PREs contribute to mixture pressure in the receiving environment is unknown.

Therefore, our study aimed to assess the contribution of PREs to mixture effects in the environment, using the multisubstance potentially affected fraction of species (msPAF) as an indicator. Based on measured chemical concentrations, compiled species sensitivity distributions (SSD) and dilution factors, msPAF levels were computed for undiluted effluents at discharge points and diluted effluents downstream in receiving waters. Average msPAF-chronic and msPAF-acute levels of PREs at discharge points were 69% (P50) and 40% (P95), respectively. Levels were reduced substantially <5% downstream, indicating low to negligible toxicity of PREs in receiving environments. Regardless of differences in endpoints and locations, hydrocarbons (mainly total petroleum hydrocarbons) and inorganics (mainly ammonia) explained at least 85% of the mixture toxic pressure.

The msPAF levels of PREs were on average 2.5-4.5 orders of magnitude lower than background levels, suggesting that PREs were minor contributors to the toxic pressure in the environment. Our results provide effluent and substance rankings, helping identify hotspots and take effective targeted action to remediate potential risks. We explicitly discuss the uncertainties for further refinement and development of the method.