Abstract
The utilisation of the organic fraction of municipal solid waste as feedstock for bioethanol production could reduce the need for disposal of the ever-increasing amounts of municipal solid waste, especially in developing countries, and fits with the integrated goals of climate change mitigation and transport energy security. Mixed culture fermentation represents a suitable approach to handle the complexity and variability of such waste, avoiding expensive and vulnerable closed-control operational conditions. It is widely accepted that the control of pH in these systems can direct the fermentation process toward a desired fermentation product, however, little empirical evidence has been provided in respect of lignocellulosic waste substrates and different environmental inocula sources. We evaluated ethanol production from the organic fraction of municipal solid waste using five different inocula sources where lignocellulose degradation putatively occurs, namely, compost, woodland soil, rumen, cow faeces and anaerobic granular sludge, when incubated in batch microcosms at either initially neutral or acidic pH and under initially aerobic or anaerobic conditions. Although ethanol was produced by all the inocula tested, their performance was different in response to the imposed experimental conditions. Rumen and anaerobic granular sludge produced significantly the highest ethanol concentrations (∼30 mM) under initially neutral and acidic pH, respectively. A mixed-source community formed by mixing rumen and sludge (R + S) was then tested over a range of initial pH. In contrast to the differences observed for the individual inocula, the maximal ethanol production of the mixed community was not significantly different at initial pH of 5.5 and 7. Consistent with this broader functionality, the microbial community analyses confirmed the R + S community enriched comprised bacterial taxa representative of both original inocula. It was demonstrated that the interaction of initial pH and inocula source dictated ethanologenic activity from the organic fraction of municipal solid waste. Furthermore, the ethanologenic mixed-source community enriched, was comprised of taxa belonging to the two original inocula sources (rumen and sludge) and had a broader functionality. This information is relevant when diverse inocula sources are combined for mix culture fermentation studies as it experimentally demonstrates the benefits of diversity and function assembled from different inocula.
Highlights
Research into bioethanol production from lignocellulosic biomass has focussed on agricultural (Talebnia et al, 2010; Sarkar et al, 2012; Agrawal et al, 2015) and industrial (Kádár et al, 2007; Frankó et al, 2016) products and wastes as feedstocks
Τ, Kendalls’s τ . ∗Not significant at 95% confidence level. ∗∗Data did not meet the linear covariation assumption for correlation test validity. With both initial and pH at [EtOH]max, whereas sludge had a negative correlation with initial pH and pH at max [EtOH]. These results revealed that similar ethanologenic activity could be achieved at either initial acidic and neutral pH values when using different inocula sources, namely rumen and sludge, leading to the hypothesis that a novel community enriched by mixing these inocula sources could benefit from specialised functional redundancy, allowing EtOH production at a range of pH
In this work the interaction of both initial pH and inoculum, was shown to drive in opposite directions the ethanologenic activity of rumen and sludge inocula, where maximal EtOH production was achieved at an initial neutral pH or at an initial acidic pH, respectively
Summary
Research into bioethanol production from lignocellulosic biomass has focussed on agricultural (Talebnia et al, 2010; Sarkar et al, 2012; Agrawal et al, 2015) and industrial (Kádár et al, 2007; Frankó et al, 2016) products and wastes as feedstocks. OMSW is an attractive substrate due to (i) its high cellulose content (Sun and Cheng, 2002; Li et al, 2012) and because (ii) it has already been discarded The latter contrasts with feedstocks produced for bioethanol production, which utilise productive agricultural land, or with feedstocks which have alternative uses such as compost (Blake et al, 2017). Internationally there is a drive to reduce the quantities of waste going to landfill. This is of special relevance in the rapidly growing urban regions of developing countries, where overpopulation, lack of landfill space and MSW treatment/disposal associated negative costs collude (Kalogo et al, 2007; Bozorgirad et al, 2013; Leme et al, 2014)
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