Abstract
Hydrocarbon-containing wastes and wastewaters are produced worldwide by the activities of the oil and gas industry. Anaerobic digestion has the potential to treat these waste streams, while recovering part of its energy potential as biogas. However, hydrocarbons are toxic compounds that may inhibit the microbial processes, and particularly the methanogens. In this work, the toxicity of hexadecane (0–30 mM) towards pure cultures of hydrogenotrophic methanogens (Methanobacterium formicicum and Methanospirillum hungatei) was assessed. Significantly lower (p < 0.05) methane production rates were only verified in the incubations with more than 15 mM hexadecane and represented up to 52% and 27% inhibition for M. formicicum and M. hungatei, respectively. The results obtained point out that 50% inhibition of the methanogenic activity would likely occur at hexadecane concentrations between 5–15 mM and >30 mM for M. formicicum and M. hungatei, respectively, suggesting that toxic effects from aliphatic hydrocarbons towards hydrogenotrophic methanogens may not occur during anaerobic treatment. Hydrocarbon toxicity towards hydrogenotrophic methanogens was further assessed by incubating an anaerobic sludge with H2/CO2 in the presence of a complex mixture of hydrocarbons (provided by the addition of an oily sludge from a groundwater treatment system). Specific methanogenic activity from H2/CO2 decreased 1.2 times in the presence of the hydrocarbons, but a relatively high methane production (~30 mM) was still obtained in the assays containing the inoculum and the oily sludge (without H2/CO2), reinforcing the potential of anaerobic treatment systems for methane production from oily waste/wastewater.
Highlights
Petroleum-derived oils are the most important primary energy source in our society, and represent an important fraction of the economic markets, as shown by the fact that five of the ten largest companies by revenue in the world are oil companies [1]
The transformation of hydrocarbons is driven by bacteria, but it is the activity of methanogens that allows the cascade of metabolic reactions to flow
The presence of hexane-extractable hydrocarbons in the oily sludge was confirmed by GC analysis (Figure 1) and corresponded to a total petroleum hydrocarbons (TPH) concentration of 80 ± 30 g kg−1
Summary
Petroleum-derived oils are the most important primary energy source in our society, and represent an important fraction of the economic markets, as shown by the fact that five of the ten largest companies by revenue in the world are oil companies [1]. The transformation of hydrocarbons is driven by bacteria, but it is the activity of methanogens ( hydrogenotrophs) that allows the cascade of metabolic reactions to flow This is the case for the majority of other biodegradable organic compounds, for which the activity of methanogens and the occurrence of syntrophic relationships is essential for their complete conversion to methane. Direct inhibition of methanogens may occur, but indirect inhibition is possible, by disrupting the microbial relationships necessary for the complete degradation of hydrocarbons to methane Other compounds such as halogenated aliphatic are very toxic due to their highly reactive halogen group (mainly Cl and Br) [23,24,25,26]. The feasibility of producing methane from H2/CO2 by a mixed anaerobic community (granular sludge) in the presence of a real complex mixture of hydrocarbons was further assessed in batch assays
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