The VFAs are short and long-chain fatty acids produced from biomass with low lignin content using anaerobic digestion process by mixed anaerobic bacteria, which does not need sterilization, additional hydrolysis enzymes (cellulase and xylanase) and high cost pre-treatment. Volatile fatty acids (VFAs) are used in the production of bio-energy. Cost of raw materials are responsible for the majority of the biofuel production costs, therefore, biofuels derived fromVFAs have an economic advantage by lowering the cost of raw materials. VFAs can be produced from all the biomass species (terrestrial, aquatic and marine biomass) with high yield, and they can be converted to various biofuels and biochemicals.
VFA’s are produced by anaerobic fermentation of lignocellulosic substrates. VFA’s are used to produce fuels by thermally decomposing their salts to high-octane ketones and condensed ketone products. Advantages of fermenting lignocellulosic materials to Acetic, Propionic and Butyric acids are the substrates used do not compete with human food and are cheap. A separate chemical or enzymatic hydrolysis step is not required. Mixed cultures of anaerobic microorganisms eliminate the sterilization cost. Disposal of organic wastes produced by agricultural, municipal and food processing activities can be done effectively. All carbohydrate components can be used, including pentoses.
VFA has low Capital Requirements, low Transport Cost, high Value of Product, low Carbon Footprint and high Biofuel Production. VFA when converted to transport fuel through direct hydrogenation in which vapour phase hydrogenation of VFA yields alcohols. Greater VFA yield needs greater VS destruction rates. By reducing the bioreactor minimum retention time and maintaining a low pH, the methanogenic reactions can be suppressed and VFA can be recovered as the product of choice.Acetic and butyric acids are used as materials for the development of biodegradable polymers and plastics production industry. Acetic acid is also used in the synthesis of acetyl lignocellulose and plastics. Formic acid is used in antibacterial and preservative agent in livestock feed. Propionic acid is also used as a chemical intermediate, mainly for the modification of synthetic lignocellulose fibres. Various biomass-derived VFAs are also converted to esters in an uncatalyzed esterification system. Ethyl lactate, ethyl levulinate, ethyl acetate, ethyl formate, and ethyl linoleate are used as intermediates for fuels and chemicals and biofuel. Major bottleneck is the cost of separation of the VFAs from the fermentation solution before converting into any product.Therefore, the VFA recovery from dilute aqueous stream is a critical step which is both a technical and an economical challenge. VFA production rate decreases when temperature decreases. Temperature is responsible for affecting VFA formation.Lack of mixing reduces the amount of VFA generated at low temperatures.Overloading and inhibitors, create obstacles in the removal of volatile organic acids as quickly as they are produced.Increased solid loading rate and removal of fermentor solids are also bottlenecks in the production of VFA. Agitation also effects VFA production from fermentation because microbes detach from substrate under excessive mixing.Although a higher concentration of VFAs can be achieved at a high OLR but high OLR rapidly decreases VFA concentration.Although acidogenic bacteria have tolerance towards the low pH and short hydraulic retention time but low pH does not help the acidogenesis process.