Anaerobic decomposition of organics is accomplished through a series of biochemical reactions which is very dependent on temperature. Most take place at mesophilic condition. As a core microbial component of anaerobic granules, methanogenic bacteria grow slowly in wastewater and their generation times range from 3 days at 35°C to as high as 50 days at 10°C (Bitton, 1999). When the reactor temperature is below 30°C, the activity of methanogens is seriously reduced. Although high temperature seems to increase the pace of granulation, most bacteria will lose their activity if the temperature is too high. Experiments showed that if temperature is increased suddenly from 35° to 55°C, sludge washout and lower COD removal efficiency was observed (Fang and Lau, 1996); Lepisto and Rintala (1999) further reported that effluent quality from a UASB reactor operated at 70° C was lower than that from reactors operated at 35 and 55°C.

There is an optimum range of temperature for successful functioning of anaerobic system. Most UASB reactors are operated at mesophilic range though some can be operated at a temperature as high as 70°C. However, there is seemingly no advantage to operate a UASB reactor at such a high temperature when the reactor can operate well at 35°C. High temperatures are known to encourage the growth of suspended biosolids; however, extremely high temperatures inhibit bacterial growth. Extreme thermophilic UASB reactors (i.e. temperature above 55°C) are impracticable because of the additional energy required to maintain the high temperature and the relatively poor effluent quality. This is indeed the main reason why mesophilic UASB reactors are more attractive as compared to their thermophilic counterparts. Moreover, a high-temperature operation is also difficult to control.

Recently, attention has been given to the impact of low temperature on the performance of anaerobic granular sludge reactors (Angenent et al., 2001; Lettinga et al., 2001; Lew et al., 2003; Singh and Viraraghavan, 2003). Singh and Viraraghavan (2003) showed that COD removal efficiency can be as high as 70 to 90% in a UASB reactor operated at 11°C with a hydraulic retention time of 6 h. Similarly, the expanded granular sludge bed (EGSB) reactors have been shown to be practicable systems for anaerobic treatment of mainly soluble and pre-acidified wastewaters at temperatures of 5 to 10°C (Lettinga et al., 2001). In addition, anaerobic migrating blanket reactors (AMBRs) have also been successfully applied to treat low-strength wastewaters at low temperatures (Angenent et al., 2001). Therefore, it is clear that anaerobic granular sludge systems are most suitably operated for the treatment of municipal wastewater at low and moderate temperatures.

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