description
- Wheat straw (WS) is an energy-rich, relatively inexpensive source of biomass that can be converted to biogas fuel during anaerobic digestion (AD) by microbes living in the absence of oxygen. WS is a particularly good candidate for conversion to clean energy and other useful products in industrial bioprocessing because it is a globally abundant agricultural residue and is commonly viewed as a waste product. However, challenges are associated with using WS as a feed resource for fuel production because the chemical structure is abundant in lignocellulose. Lignocellulose, a mixture of biomass polymers, is highly resistant to enzymatic breakdown (or hydrolysis) by the majority of well-characterised microbial species. Hydrolysis is an essential step that is required to derive small enough sugars from WS, for uptake by microbial cells. During conventional AD of WS, the speed of hydrolysis often limits the rate of biogas fuel production and this heavily influences the overall bioreactor size. Therefore, AD plants tend to be very large in order to allow for the time required for bioconversion of WS and this can increase capital and operational costs. Typically, industrial AD is reliant on undefined microbial communities. The hydrolysis stage can take weeks and is performed by a consortium of bacteria dominated by species of Clostridium. Several of these species can deconstruct WS into simple sugars that they ferment to provide for their metabolism. Products of clostridial fermentation include H2, CO2 and acetic acid. These chemicals become the substrates for a second group of microbes, the methanogenic Archaea, which convert them to methane fuel. The biological process of lignocellulose conversion in AD is analogous to microbial activity in the rumen of mammalian herbivores (e.g. cattle and sheep). However, lignocellulose in the rumen is converted over a much shorter time period that lasts for several days (as opposed to several weeks in industrial AD). One explanation for this discrepancy stems from the fact that anaerobic fungi native to the rumen are able to perform hydrolysis much more efficiently and effectively than clostridial bacteria. However, in comparison to clostridia, relatively little is known about the anaerobic fungi. Furthermore, no information is available concerning their growth and activity with co-culturing methanogens in continuously-fed fermentation systems. Almost all previous investigations on the anaerobic fungi have used culture volumes (typically less than 100 ml) and batch-culture methodologies that are not comparable with industrial AD, nor with the growth conditions prevalent in the rumen. In particular, information is unavailable about the ability of anaerobic fungi to survive in continuously fed bioreactors. This is due to the absence of a low-cost, lab-scale bioreactor, capable of continuously feeding particulate lignocellulose material while maintaining the aseptic, anaerobic conditions that are necessary for fungus-methanogen co-culture survival. Initially, this project aims to meet a requirement for the development of an automated lab-scale bioreactor system that is capable of continuously feeding WS to anaerobic fungus-methanogen co-cultures under aseptic conditions. The newly developed system will be used to study microbial growth in a continuous culture system analogous to their native habitat. Additionally, biomethane production will be compared between fungus-methanogen co-cultures and conventional AD consortia (dominated by clostridial species and their associate methanogens). If the fungus-methanogen co-culture can significantly outperform conventional AD, this new knowledge will facilitate the production of smaller digesters that can handle significant throughput of lignocellulose material. This will ease the cost of anaerobic digesters for decentralised production of clean energy in rural communities that exist in close proximity to cereal crop growers.