Thermophilic Anaerobic Digestion

Thermophilic anaerobic digestion evolved as an attempt to harness the intrinsic advantages of AD on the one hand, and of bioprocessing with thermophiles on the other (Ahn and Forster 2000; Mackie and Bryant, 1995). Thermophilic anaerobic digesters are relatively new (Zinder, 1990; Rimkus et al., 1982). However, a lot of research efforts have been made to understand and optimise the processes, in studies that have employed a variety of wastes, mostly in laboratory and pilot processes (Espinoza-Escalante et al., 2008; Forster-Carneiro et al., 2008ab; Kaparaju et al., 2008; Montero et al., 2008; Linke 2006; Angelidaki et al., 2006; Bouallagui et al., 2004; Ahring et al., 2001; Solera et al., 2001; Ahn and Forster 2000; 2002ab; Lier, 1996; Lier et al., 1993a,b; 1994; 1995;). More recently, attempts are being made to apply it to digestion of real wastes in full scale processes, particularly wastes discharged at elevated temperatures. In general, wastes that have been subjected to this form of treatment include sugar beet waste, coffee, brewery, distillery and other beverage wastes as well as slaughter house and fish process effluents, vegetable / potato wastes and paper pulping wastes as well as organic fraction of municipal wastes and household wastes (Forster-Carneiro et al., 2008abc; Koppar and Pullammanappallil 2008; Lee et al., 2008; Ortega et al., 2008; Yilmaz et al., 2008; Zupancic et al., 2007; Linke 2006; Angelidaki et al., 2006; Hartmann and Ahring 2005; Bouallagui et al., 2004).

Temperatures that have been investigated range from 35° to 65oC (Lee et al., 2008; Zinder, 1986), and various temperature optima have been reported during thermophilic digestions (Perez et al., 2001; Ahring et al., 2001; Lier, 1996;). For instance, whereas 70°C is the highest temperature recorded for anaerobic fermentation of cellulose, anaerobic metabolism of soluble sugars have been reported at 80°C (Maden, 1983; Zeikus et al., 1979). So far, 65°C is the highest temperature at which stable methanogenesis has been reported in waste treatment (Zinder, 1990), even though methanogenesis has been reported at 100°C in microbial communities sampled from super heated deep sea waters (Baross et al., 1982; Baross and Deming, 1983). Ahring (1995), reported methanogenesis during waste treatment at up to 75°C, though most of the processes were adjudged unstable, above 60°C. Recovery of thermophilic process following temperature shift-up induced deterioration often involve slow re-growth of a new populations, with growth optima in the new temperature range (Ahring, 1994). However, following temperature shift-downs the thermophiles retain considerable activity, often enough to drive the process, albeit sub-optimally, until a new population with optima at the new temperature is established (Lettinga, 1995; Waigant et al., 1986). By far the most thermophilic processes have been run in the temperature range 55°- 65°C (Kaparaju et al., 2008; Lee et al., 2008; Ortega et al., 2008). In comparison to mesophilic process, thermophilic AD has the advantage of pathogen destruction and reduced retention time (Aitken et al., 2007). However, it is energy intensive, being incapable of self-heating. Its products also require aerobic polishing prior to disposal (Lier, 1996). The reported instability of the process is aided by poor granulation of thermophiles, due to their often low content of extracellular polysaccharides and low viscosity (Schmidt and Ahring, 1994), leading to dispersed growth. Thermophilic processes are also easy to washout due to high turbulence during active gas formation (Uemera and Harada, 1993, 1995; Soto et al., 1992).

Thermophilic anaerobic waste processing has proved to be sensitive to slight upward shifts in temperature and requires precise temperature and pH control (Cecchi et al., 1993 ab). Its temperature sensitivity is such that even slight upward shifts can cause death of active microbial populations. If acid consumers are affected, the process may take many days to recover due to their slow growth rate (Zinder, 1986) or fail completely, due to significant drop in pH. It is also adversely affected by slight increases in the concentration of ammonia, hydrogen sulphide and other toxic reaction by-products, particularly at the upper ranges of temperature (Schink, 1997; Ahring, 1994; Angelidaki and Ahring, 1993, 1994). For these reasons, thermophilic anaerobic digestions have remained slow in gaining acceptability for use in rural communities since successful operation of reactors will require the services of highly trained and often expensive labour.

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