Anaerobic co-digestion of petroleum hydrocarbon waste and wastewater treatment sludge

Abstract

The development of efficient treatment processes for petroleum hydrocarbon waste (PHW) can lead to greater impact in the petroleum industry which has been generating disconcerting amounts of hazardous oily sludge. Traditionally, PHW is thermally remediated to stable non-hazardous soil. In this work, PHW in the form of a semi-solid sludge (hydrocarbons, soil, and water) is treated biochemically in a batch reactor. Contrary to municipal solid waste (MSW), biomass or wastewater treatment sludge (WWTS), PHW lacks the necessary nutrients to biodegrade and may include some heavy metals inhibitors. Therefore, co-digestion, addition of inoculum, and using other additives anaerobically and concurrently with adjustment of the reactor conditions is necessary to stimulate the microbial environment and thereby produce higher specific landfill gas (SLFG) generation. Additionally, hydrogen can also be produced in two stage anaerobic digestion. In this work, multiple samples of PHW and WWTS were collected and analyzed per their proximate/elemental composition and heating values. Then several bench-scale batch reactors were incubated with appropriate PHW and WWTS inoculum to assess their biodegradation. The SLFG generation were the maximum when 60% PHW is co-digested with 40% WWTS at 35 °C under additional hydration in a larger reactor. SLFG in this case reached as high as 130 ml/kg which is in reasonable comparison with the stoichiometric model (153 ml/kg), while inferior to the EPA model (180 ml/kg). Thermogravimetric analysis (TGA) of the digestate before and after the incubation explains the lower SLFG generation as more remaining organic fraction. These experiments set the baseline metrics for future two stage conversion, in which more favorable conditions (higher temperature and water contents) or remediation of inhibitors are pursued for near complete biodegradation.