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6 Volume 9 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 mesophili 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 favorable process stability. These systems are commonly found in the form of a covered lagoon and, as such, they are usually subject to fluctuations in temperature. 3. 2. 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 11.0 kg VS/m3/d for various types of digesters and manure sources (Persson et al, 1979). While high loading rates use the digester volume more efficiently, they also increase solids concentration, retention time and alkalinity, which must be taken into consideration. 3. 3. Retention Time The Hydraulic Retention Time (HRT) and Solids Retention Time (SRT) are the average lengths of time the liquid or solid portion of manure remains in the digester. Generally, the lower the operating temperature (e.g. psychrophilic digestion) the higher the retention time that is needed. 3. 4. Solids Concentration Normally reported as the percentage dry matter and the volatile solid percentage of that dry matter, the solids concentration is necessary to determine the loading rate. The solids concentration also helps to determine the most suitable type of digester. 3. 5. Alkalinity and pH Optimum pH conditions for methanogenic bacteria range from 6.4 to 7.6 (Price and Cheremisinoff, 1981). Other bacterial species are more tolerant to pH levels outside of this range.PDF Image | Biogas as Resources of Energy
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