Recovery of Bioenergy
Methane gas is the major by-product of the anaerobic degradation of organic solid and liquid wastes. Methane gas has an energy content of 55,525 kJ/kg at 25°C and 1 atmosphere pressure. Biogas is currently being implemented for the production of electricity and heat using combined heat and power (CHP) units, is upgraded to natural gas quality for injection into gas pipelines, and is compressed for use in compressed natural gas (CNG) engines for transportation. Europe alone produced 3,000 MW of electricity from biogas in 2007 and nearly half of that came from Germany, a global leader in biogas technology.
In anaerobic fermentation, hydrogen is produced during acidogenic phase. From a global environmental perspective, production of hydrogen from renewable organic wastes represents an important area of bioenergy production and research.
Butanol is also a potential substitute for fossil fuel and is considered to be a superior fuel to ethanol for several reasons: more favorable physical properties; better economics and safety. In addition, the butanol also eliminates the need for engine modifications since it behaves similarly to gasoline. Butanol is produced by fermentative bacteria including Clostridium acetobutylicum and Clostridium beijerinkii. The ratios of acetone, butanol and ethanol (ABE) in typical butanol fermentations are 3:6:1, with butanol being the major product.
Biodiesel Production from Biogas
Biogas generated during the AD of organic waste can be converted into liquid fuel – biodiesel. The biogas is first converted into liquid methanol using a thermal catalytic process. Biodiesel or methyl ester is then produced by transesterification of fats or oil with methanol in the presence of a base catalysis (e.g. sodium or potassium hydroxide).
Electricity Generation using Microbial Fuel Cell
A microbial fuel cell (MFC) is a device that directly converts biochemical energy stored in the carbohydrates and other organic matter (e.g. from wastewater) into electricity. A MFC contains two chambers, consisting of an anode and cathode similar to hydrogen fuel cell, separated by a proton (cation) exchange membrane (PEM). The organic matter is oxidized by anaerobic microbes in the anode chamber and electrons are released. These electrons are then transferred to the anode and flow toward the cathode through a conductive material. The electrons reaching the cathode combine with protons that diffuse through the PEM and oxygen (from air). The oxygen is reduced to water. In a MFC, the driving force is the redox reaction of substrates (wastewater) mediated by anaerobic microorganisms. Thus, MFC research has a potential to treat the wastewater and produce electricity. MFC studies, however, have been mainly confined to the laboratory-scale level, and their full-scale application is still yet to be determined.