Anaerobic Processes for Waste Treatment and Energy Generation

Abstract

As global population increases and developing countries industrialize, energy demand around the world is increasing markedly. World energy consumption is expected to increase by 50% to 180,000 GWh/year by 2020 (Fernando et al., 2006), due primarily to increases in demand from rapidly growing Asian countries such as China and India (Khanal, 2008). According to the Intergovernmental Panel on Climate Change (IPCC, 2007), fossil fuel combustion already contributes 57% of emissions that cause global warming. Thus, to address future energy needs sustainably, renewable sources of energy must be developed as alternatives to fossil fuels. To aid in developing such renewable energy alternatives, environmental scientists and engineers should consider anaerobic processes for waste treatment as alternatives to aerobic processes. When aerobic processes are used for waste treatment, the low energy compounds carbon dioxide and water are formed; much energy is lost to air – about 20 times as much as with an anaerobic process (Deublein and Steinhauser, 2008). Anaerobic processes produce products of high energy like methane. Methane can be captured and burned as an energy source, and used to power gas-burning appliances or internal combustion engines, or to generate electricity. Anaerobic processes have been applied for decades in developed countries for wastewater treatment plant sludge stabilization. In recent years, considerable interest has developed in use of anaerobic treatment for a variety of other applications, due to the potential to generate renewable energy. Methane from anaerobic processes is being increasingly utilized as an alternative energy source in developed countries, via large projects that extract methane from landfills or wastewater treatment plants. Smaller plants, on the scale of an individual household or village, can also be a particularly important energy source in rural sectors of developing countries; transportation costs in these locations may limit use of fossil fuels, and lack of cheap and adequate energy hampers rural development. When generated from biomass, especially at a small scale, methane is often called biogas (FAO, 1984; Deublein and Steinhauser, 2008). In addition to providing a renewable source of energy, anaerobic processes provide some of the simplest and most practical methods for minimizing public health hazards from human and animal wastes – pathogens are destroyed or greatly reduced. Anaerobic processes have been proven for treatment of a variety of organic wastes: solid wastes at landfills, industrial wastewater, human excrement and sludges at wastewater treatment plants, human excrement in rural areas, animal manure, agricultural wastes, and forestry wastes. The