Marchaim (1992) noted that there is a close relationship between the biogas fermentation process and the temperature of the reactor. The higher the temperature, the more biogas is produced but when the temperature is too high, this can cause metabolic process to decline. Hobson et al., (1981) found biogas production to be greatest when the digester temperature was in the range of 32 to 400C. Hill (1982) also stated that digestion temperatures for optimum design all occur in the mesophilic range of 320C to 400 C. This work suggested that temperature beyond 400C has little effect on digester performance since the higher volumetric methane productivity is offset by the smaller digestion volume. As observed by the paper these lower temperatures also represent major savings in energy requirements when compared to thermophilic digestion (i. e. 600C). During the process of anaerobic biodigesiton in order to reach optimum operating temperatures (30-370C or 85-1000F), some measures must be taken to insulate the digester, especially in high altitudes or cold climates (VITA, 1980). Straw or shredded tree bark can be used around the outside of the digester to provide insulation. According to Carcelon and Clark (2002), anaerobic bacteria communities can endure temperatures ranging from below freezing to above 57.20C (1350 F), but they thrive best at temperatures of about 36.7 0C (980 F) (mesophilic) and 54.40C (1300 F) thermophilic. Bacteria activity, and thus biogas production falls off significantly between about 39.40C and 51.70C (103 0F and 125 0F) and gradually from 35 0C to 00 (95 0F to 32 0F). To optimize the digestion process, the digester must be kept at a consistent temperature as rapid changes will upset bacterial activity.

The potential of thermophilic digester operating temperatures (> 550C) for anaerobic biogestion of livestock waste has been investigated by several researchers (Converse et. al., 1977; Hashimoto, et. al., 1979; Hashimoto, 1983; Hashimoto, 1984; Hill, 1985; Hill and Bolte, 1985; Hill et. al., 1986) with the technical feasibility being decided in favour of the process. Hill (1990) identified the advantages of thermophilic digestion over conventional mesophilic digestion as reduced hydraulic retention time (HTR), increased loading rate, and smaller physical reactors for identical waste amounts. The major disadvantage identified is the increased use of energy required to heat the feedstock and maintain digester operating temperature. Chen and Hashimoto (1981) however suggested that the development of heat exchangers to recover energy in the effluent somewhat alleviated this advantage.

In cold climates, or during cold weather, optimal temperatures become very expensive to maintain, thus reducing the economic feasibility of the process of anaerobic biodigestion (Cullimore et al., 1985). In view of this, investigations have been conducted into the feasibility of anaerobic biodigesiton at lower temperatures. Stevens and Schulte (1979) thoroughly reviewed the literature regarding low-temperature digestion and found that methanogenesis occurs at temperatures as low as 40C, and that an increase in temperature from 40C to 250C dramatically increased the rate of methanogenesis. Cullimore (1982) reported results which indicated that as digester temperature was reduced from optimal levels, biogas production decreased linearly to extinction at between 0 and 80C. Ke-Xin and Nian-Guo (1980) successfully ran several rural digesters at ambient winter temperatures of 12 to 130C, and obtained gas yields which were 23 to 40 percent that of the optimal temperature production. Pos et al., (1985) suggested that if the anaerobic digestion process was found to function efficiently at lower temperatures, the use of large digestion units at longer retention times and without heating might be considered. It might then be possible to run full scale digesters at less than optimal temperature in order to increase their economic feasibility.

Safley Jr and Westerman (1990) reported satisfactory digester performance for both winter and summer conditions. However, biogas production was found to fluctuate seasonally with reduced biogas production being noted during the winter. Mean methane yield was found to be 0.34 m3 CH4 kg of volatile solids (VS) added. Mean biogas concentration was 69.5% CH4 and 26.8% CO2. The loading rate during the 17-month period of study was 0.12 kg VS/m3-day. Typically, anaerobic digesters are designed to operate in either in the mesophilic (200C — 450C) or thermophilic (450C — 600C) temperature ranges. However, as pointed out by Safely Jr and Westerman (1990) the production of methane (called methanogenesis) has been observed at temperatures approaching 0OC. The anaerobic decomposition of organic matter at low temperature (< 200C) is referred to as psychrophilic anaerobic digestion.