Indigenous microbes survive in situ ozonation improving biodegradation of dissolved organic matter in aged oil sands process-affected waters

• 50 mg L−1 utilized ozone dose improves biodegradability of dissolved organic carbon.
• Indigenous oil sands microbial community survive moderate ozone exposure.
• Microbial community exposed to ozone performs as well as unexposed community.
• Aged oil sands process-affected waters contain <1 mg L−1 naphthenic acids.
• Naphthenic acids are removed with light ozone treatment.

The oil sands industry faces significant challenges in developing effective remediation technologies for process-affected water stored in tailings ponds. Naphthenic acids, a complex mixture of cycloaliphatic carboxylic acids, have been of particular concern because they concentrate in tailings ponds and are a component of the acutely toxic fraction of process water. Ozone treatment has been demonstrated as an effective means of rapidly degrading naphthenic acids, reducing process water toxicity, and increasing its biodegradability following seeding with the endogenous process water bacteria. This study is the first to examine subsequent in situ biodegradation following ozone pretreatment. Two aged oil sands process-affected waters from experimental reclamation tailings ponds were ozonated to reduce the dissolved organic carbon, to which naphthenic acids contributed minimally (<1 mg L−1). Treatment with an ozone dose of 50 mg L−1 improved the 84 d biodegradability of remaining dissolved organic carbon during subsequent aerobic incubation (11–13 mg L−1 removed from aged process-affected waters versus 5 mg L−1 when not pretreated with ozone). The ozone-treated indigenous microbial communities were as capable of degrading organic matter as the same community not exposed to ozone. This supports ozonation coupled with biodegradation as an effective and feasible treatment option.