Research Highlights: Fuel from Waste. The Microbial Community Approach

contributed by Priscilla Carrillo-Barragán

In a previous post I left you thinking of an ideal world, one where it was possible to fuel your car with ethanol (EtOH) that comes from organic waste, and not from crops that could have been eaten. Not coincidentally, my research focusses in trying to turn this possibility into a reality.

Today the transport sector worldwide is almost totally dependent on fossil fuels, being responsible for 60% of the world oil consumption and generating more than 70% of the CO and 19% of CO2 emissions (Balat, 2011).

The mitigation of these greenhouse gas (GHG) emissions is not an easy task, as it requires an integral –and ideally, global- strategy that considers transport technologies, service infrastructure and low-carbon fuels.

fig 1Different strategies to reduce the carbon intensity of transportation. Source: Sustainable Transport. NREL, 2015

Crop-based ethanol, which is produced by the U.S.A. and Brazil, provides a degree of independence from petroleum products in an economically “viable” way. But when considering its Life Cycle, it often fails to be a sustainable alternative to fossil fuels. This is due to the fact that its production can generate higher levels of greenhouse gases, can involve deforestation, damage of biodiversity, erosion and stress of soils, high level of water demand and contamination, competition with food production and rise in food prices, among others (Fargione et al.(2008); Goldemberg et al.(2008)). Extensive research is being carried out to overcome these drawbacks, but to date not a single feedstock or technology represents the panacea to this situation, and if the lesson with fossil fuels is to be learned, this issue should be addressed from a holistic, multi-disciplinary approach, leading to diversification.

Organic Municipal Solid Waste: the problem and the opportunity

According to the World Bank (Hoornweg and Bhada-Tata, 2012), the 1.3 billion tonnes of Municipal Solid Waste (MSW) produced annualy will double by 2025, of which around 30%-60% is organic matter. In countries with a strategy for waste management, these wastes will be diverted according to the waste hierarchy, but in the bast majority the organic fraction will be landfilled.

MSW is then an attractive feedstock for EtOH production due to its availability, high cellulosic content and particularly, to address the international concern to reduce the quantity of waste going to landfill.

fig 2Food waste can represent more than 50% of the organic francion of MSW in some countries (Hoornweg and Bhada-Tata, 2012). Image source: Lower Reule Bioenergy Ltd.

Sure, anaerobic digestion along with incineration are promising ways to treat the organic fraction of MSW at large, and undoubtedly these, together with solar and fuel cells, geothermical, eolic, tidal and nuclear technologies, will help to overcome the threat to energy security in the future. However, what will substitute the petrol and diesel required for the over 2 billion vehicles worldwide expected to be on the roads by 2050 (IEA, 2014), when most of them will continue to be powered by internal combustion engines?

Holtzapple et al. (1992) estimated that if all the lignocellulose in MSW were fermented, over 38 billion litters of EtOH would be produced in the US annualy, which would account for around 8% of the annual total demand!

Now that I have made my point, let’s face it: lignocellulosic biomass is hard to transform, and to affordably produce any sustainable fuel out of it, a complex treatment would be required, so…

How do I plan to do it?

The current approach to bioethanol production from lignocellulosic materials can be generalized as:

 

Picture1

Diagram of ethanol production from lignocellulosic biomass. Each step takes place in a different reactor. The dotted line exemplifies Simultaneos Saccharification and Fermentation (SFF), where both steps ocurr in the same reactor.

Alternatively, in Consolidated Bioprocessing (CBP) four basic biochemical events take place simultaneously: Hydrolytic enzymes production, saccharification or substrate hydrolysis, hexose fermentation and pentose fermentation, all in the same reactor (Lynd, 1996).

An approach to achieve this is to engineer a “super bug” expanding the metabolic capabilities of well-known “industrially friendly” microorganisms (S. cerevisiae, Z. mobilis, or E. coli).

Or to try to imitate nature…

Using a microbial community where different species would perform the different steps required for EtOH production. Assuming that species integrate an overlapping set of relationships, spreading risks and benefits imposed by the environment, the system would be able to resist fluctuation as a community. Thus, the expensive drawbacks of the conventional process, as are enzyme production and product inhibition, the instability, narrow operational conditions, susceptibility to contamination and catabolite repression could be overcome (Zuroff and Curtis, 2012).

fig 4

As a biochemist-turned-microbiologist, this has been the first stage of my project, sampling different environments where lignocellulose degradation naturally occurs such as composting piles, where OMSW (what does this stand for? Organic Municipal Solid Waste?) is already being aerobically degraded, forest soil, where the leaves-mat is degraded by soil microorganisms, rumen and cattle dung, to obtain highly specialized cellulose degraders and anaerobic digestate for fermenters.

Taking compost and the less-attractive “very fresh” manure samples at Nafferton Park Farm, one of the farms managed by Newcastle University.

Imitate nature? Lignocellulose degradation, sure, but you may be wondering if EtOH is naturally produced at all.

The short answer is yes. EtOH is a fermentation product of numerous fungi, bacterial and even archaea species. But your suspicions are also right, the high efficiency of the interactions in a community allows the capture of most of the free energy available in the system, giving structure and stability (Zuroff and Curtis, 2012), which is good for the community, but represents one of the major challenges in EtOH production, as EtOH is produced in really small amounts –other more energetically favourable compounds like acetate are produced-, and whatever is excreted in the environment do not accumulate, as in certain conditions EtOH can be further oxidised by another community member to obtain energy, or simply not being produced, as result of inhibition.

The good news is that up to date all the environments sampled were effectively capable of lignocellulose transformation and EtOH production using an OMSW analogue as substrate. However, the yields obtained are still far from being ideal.

The chemist in me suggest that thermodynamic studies integrated with molecular biology tests could allow “simple”, but informed environment manipulation (pH, oxygen concentration, temperature) that can be used to direct the metabolism towards EtOH production in an efficient and robust way. Although this remains a challenge, the first results of my research keep me believing in the old Nature’s lesson: Union is strength.

References

  • Balat, M. (2011) ‘Production of bioethanol from lignocellulosic materials via the biochemical pathway: A review’, Energy Conversion and Management, 52(2), pp. 858-875.
  • Fargione, J., Hill, J., Tilman, D., Polasky, S. and Hawthorne, P. (2008) ‘Land Clearing and the Biofuel Carbon Debt’, Science, 319(5867), pp. 1235-1238.
  • Goldemberg, J., Coelho, S.T. and Guardabassi, P. (2008) ‘The sustainability of ethanol production from sugarcane’, Energy Policy, 36(6), pp. 2086-2097.
  • Holtzapple, M., Lundeen, J., Sturgis, R., Lewis, J. and Dale, B. (1992) ‘Pretreatment of lignocellulosic municipal solid waste by ammonia fiber explosion (AFEX)’, Applied Biochemistry and Biotechnology, 34-35(1), pp. 5-21.
  • Hoornweg, D. and Bhada-Tata, P. (2012) What a waste: a global review of solid waste management (68135). Bank, T.W. [Online]. Available at: http://documents.worldbank.org/curated/en/2012/03/16537275/waste-global-review-solid-waste-management# (Accessed: 30/07/2014).
  • IEA (2014) FAQs: Transport. Available at: http://www.iea.org/aboutus/faqs/transport/ (Accessed: 05/08/2014).
  • Zuroff, T. and Curtis, W. (2012) ‘Developing symbiotic consortia for lignocellulosic biofuel production’, Applied Microbiology and Biotechnology, 93(4), pp. 1423-1435.