A study of metagenomics-informed biochemical functionality of microbial fuel cells using DDGS as a substrate

This proposal addresses a BBSRC initiative that aims to enhance the value of Dried Distillers Grains with Solubles (DDGS), a byproduct of grain-to-bioethanol and whisky production. DDGS will become increasingly abundant in the UK as bioethanol production develops. It is currently mainly used as a cattle feed but there is also interest in developing it as an industrial feedstock

A microbial fuel cell (MFC) is a device that contains an anerobic culture of microorgnaisms, capable of directly converting chemical energy to electrical energy. A typical microbial fuel cell consists of anode and cathode compartments separated by a cation (positively charged ion) specific membrane. In the anode compartment, nutrients are oxidized by microorganisms, generating electrons and protons. Electrons are transferred to the cathode compartment through an external electric circuit, while protons are transferred to the cathode compartment through the membrane. Electrons and protons are consumed in the cathode compartment, combining with oxygen to form water.

We will develop a microbial fuel cell that will process DDGS prior to drying and use as an animal feed. The MFC will generate electricity (to reduce consumption by the biorefinery) and enhance the protein content of the animal feed product.

The species of micro-organisms added to the MFC will be determined by analysing all of the genes present in whole populations of micro-organisms (metagenomics) under a range of conditions and using a computer simulation which highlights the most important genes to carry out the desired functions of the MFC. The population composition will be further fine-tuned by feeding the microbes with nutrients as rewards for achieving the desired characteristics, forcing it to evolve to the most effective distribution of species.

We will study the use of DDGS as a substrate for electricity generation using Microbial Fuel Cells. We have already tried this out and proved that it is possible and studies elsewhere using similar substrates provide confidence that this is a viable project with a high probability of success. In addition to addressing the pragmatic objective, implicit in this initiative, we will address important scientific questions, that will lead to publications in high-impact journals. We will test the hypothesis that maximum electrical output from a MFC is dependent on the biochemical capability of the population rather than on the identity of the individual species present in the MFC community. In doing so we will employ metagenomic analysis of the microbial community within the MFC, both in the anodic biofilm and in the anodic suspension, in order to relate the presence of critical genes in the population to the electrical output of the bioelectrochemical system. Extending this philosophy, we will carry out forced evolution of the microbial population by using the power output to modulate the nutrient feed rate to the MFC.

The metagenomic study will be used to construct a metagenome-scale metabolic model, a novel development in the field of metagenomics that is likely to lead to a high profile publication. The model will be used to investigate the effect of changes in the population during forced evolution and to predict the optimal metagenome needed to carry out this particular function

In addition to generating electricity, we plan to evaluate the production of hydrogen using a variation of the MFC concept (microbial electrolysis cell).

Our approach could be applied equally to raw DDGS or DDGS that has undergone any form of secondary processing.

Potential impact of microbial fuel cells on biorefinery operation

(Many of these issues were raised by the Steering Group during assessment of the preliminary application.)

A suitable bench-mark objective would be to produce sufficient electricity to power the electric stirrer motor for the process bioreactor.

The power requirement for mixing a bioreactor is1-2kWm-3 (Doran PM (1995) Bioprocess Engineering Principles, AP). Hitherto, the highest reported MFC output is 1.55kWm-3 (Fan et al, 2007Env Sci Tech 41:8154-8) so this is an achievable objective for this technology. Our study will show the extent to which this ideal scenario can be met with DDGS and provide a benchmark for the utility of bioprocess MFC waste treatment that can be employed throughout the industry.

We also intend to study hydrogen generation by operating the MFC vessel as a microbial electrolysis cell. A recent study has concluded that, at a cost of $4.51/kg H2 for winery wastewater (a similar substrate to DDGS) and $3.01/kg H2 for domestic wastewater the cost is less than the estimated merchant value of hydrogen ($6/kg H2) (Cusick et al, 2010, Int J Hydrogen Energy.35 8855-61). Although electricity generation is our primary objective, hydrogen production could offer a useful alternative application for this type of technology, without compromising the value of DDGS as an animal feed. Efficient electricity production and hydrogen production are, however, mutually exclusive.