BBSRC NIBB Anaerobic Digestion (AD) Network
extracted VFA through bioelectrochemical reductions or by catalytic conversion into high value-added fuel products. VFA can also be used in the production of polyhydroxyalkonates (PHA), a family of biodegradable polymers with properties similar to polypropylene. More than 300 species of micro-organisms have been shown to be capable of accumulating PHA. While VFA offer considerable potential for further biochemical and catalytic transformation, they are themselves valuable base chemicals, with bulk markets for acetic acid, propionic and butyric acids. They also are potential reactants with acetate and propionate as electron donors in microbial fuel cells (MFCs) and this has been suggested as a basis for the development of biosensors to monitor VFA concentrations in digestion plant.

VFA Factory Working Group

Anaerobic digestion processes have been used primarily for the stabilisation of wastes (both solid and liquid), with the added benefit of biogas production as an end product. In carrying out this biotransformation, a range of intermediate products are produced, which in turn are ultimately converted to methane and carbon dioxide by means of complex syntrophic microbial interactions. There is growing interest in manipulating the process biology to target the accumulation of intermediate volatile fatty acid (VFA) products as precursors for a biorefinery. VFA arise at a crossroads in the carbon flow and can be directed towards material or energy recovery, or even used for nutrient removal. Extraction and concentration of VFA is one option that could potentially be achieved by a number of processes, such as coupled membrane systems based on supported liquid membranes and electrodialysis. On the other hand, VFA can be manipulated within the reactor system itself. One major area of interest is in increasing the energy density of the VFA, by biohydrogenation or chain elongation to give a longer- chain fatty acid: for example the conversion of acetate to ethanol and longer (C6-C8) carbon chains as refinery inputs. Both chain elongation and hydrogenation require electron donors and protons, and biochemical sources for these include hydrogen and ethanol. It may also be possible to upgrade