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Design Parameters Following is a discussion of the most important parameters which must be considered in the design of an anaerobic digestion system: i) Temperature - Three ranges exist for anaerobic digestion: • Psychrophilic range - Between 5° and 25°C; characterized by slower methane production and longer retention times. • Mesophilic range - Between 30° and 40°C; the most widely used of the three (Poulsen, 2003), this range balances heating costs with methane production. • Thermophilic range – From 50° to 60°C; produces the most methane but is also the most sensitive, due to fewer bacterial species in existence. Once a stable temperature is reached, fluctuations should be kept within 5°C to avoid killing the desired bacteria (Pos et al, 1981). Thermophilic tolerance is generally less than that of lower temperatures (Price and Cheremisinoff, 1981). Each temperature range at which the digester can be operated has its own advantages. The thermophilic process has been found to be superior to the mesophilic process from an energy balance and, thus, “profit” point of view (Ahring, 1994). Thermophilic digesters usually achieve better degradation of long-chain fatty acids, have a shorter retention time, and require less biomass compared to the quantity of methane produced. The thermophilic process also achieves higher pathogen and weed seed destruction than the mesophilic process alone (El-Mashad et al, 2004). However, the risk of ammonia inhibition is greater and more energy is required to operate a thermophilic digester (Poulsen, 2003). Thermophilic processes are considered to be more prone to instability than mesophilic due to fluctuations in input quality (Earth Tech, 2002; El-Mashad et al, 2004). However, in a study of major centralized biogas plants in Denmark, Ahring (1994) found no significant difference in volatile fatty acid concentrations between the two processes. Ahring (1994) conceded that the start-up time of thermophilic digesters is longer than that of a mesophilic reactor due to the low numbers of thermophilic bacteria in organic waste. Most of the agricultural digesters in the United States are mesophilic (Kramer, 2002). The process is slightly more stable and adaptable to fluctuations in feedstock quality than thermophilic (Earth Tech, 2002). The lower heating requirements of mesophilic temperatures translate into lower costs. Residence time should be at least 15 days for adequate digestion (Earth Tech, 2002). Psychrophilic digesters require a solids retention time approximately twice as long as mesophilic (Price and Cheremisinoff, 1981). These digesters require the least amount of energy input. Biogas production is slow but gas quality and other parameters indicate favourable process stability (Price and Cheremisinoff, 1981). These systems are commonly found in the form of a covered lagoon and, as such, they are usually subject to fluctuations in temperature. ii) Loading Rate – This is expressed as the weight of volatile solids (VS) per unit of volume of digester capacity per unit of time. Loading rates typically range from 1.2 to 4PDF Image | Electricity and Heat Production Using Biogas from the Anaerobic Digestion of Livestock Manure - Literature Review
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