Microbiology of Aerobic Thermophilic Digestion

There have been only a few studies on the microbiology of TAD, mainly because the process is relatively new. Besides, application of thermophilic organisms in bioprocesses was essentially unknown before the 1970s (Brock, 1986). And studies with thermophiles have since concentrated on the caldoactive thermophiles (mostly Archaea), with a view to pure culture biotechnological application (Krahe et al., 1996), particularly the extraction of high value biochemicals, rather than biotransformation. This is in addition to their limited use in biogasification of organic wastes. As a result, the microbiology of TAD is poorly understood, and its potentials have remained largely unexploited (Fiechter and Sonnleitner, 1989). As in the case with composting, the micro-organisms responsible for TAD develop from the proliferation of thermophiles and facultative thermophiles indigenous in the waste, whose growth would have been suppressed at the mesophilic or ambient temperature of the influent waste.

The stability, or otherwise, of microbial populations during the operation of TAD is not well understood, neither is the effect of different substrate types and waste load on the (selection of) populations, since the few studies on microbiology of TAD have employed sewage sludge. An unstable thermophilic population, or one with a long doubling time, would require long retention times for waste processing, as in the case with thermophilic anaerobic digestion (Verstrate et al., 1996). On the other hand, a rapidly metabolising population with short doubling time will have the advantage of rapid waste stabilisation, short retention time and greater process stability. Sonnleitner and Bomio (1990), Sonnleitner and Fiechter (1983ab) studied the microbiology of TAD of sewage sludge at temperatures ranging from 50° to 67°C. They characterised at least 95% of the isolates as members of the genus Bacillus with maximal growth temperatures in excess of 70°C. The balance of 5% would have been so classified but for their inability to produce endospores in culture. The isolates showed very rapid growth rates (|m =0.7-2.2 h-1), but low final biomass yield (0.2 to 0.3g g-1) when grown in carbohydrate medium in shake flask cultures. In pilot studies they exhibited rapid adaptation at various retention times. The authors concluded that the thermophilic population responsible for heat generation in TAD consists entirely of members of the extremely thermophilic B. stearothermophilus group. Loll (1976, 1989) also reported that thermophilic and thermotolerant Bacillus spp. were responsible for the stabilisation of continuously treated model wastewater.

During TAD of swine waste, Beaudet et al. (1990), counted microbial populations at 55°C, varying from 104 to 10' ml-1 of waste. The organisms were identified as Bacillus spp., including B. licheniformis. Peak population was shown to vary with the final pH of the digesting waste, which seemed to vary with the COD load. During TAD of sewage sludge, 65° and 55°C thermophiles in excess of 106 and 108 ml-1 respectively were reported (Burt et al. 1990b). These were approximately 102 fold greater than the thermophilic population in feed sludge. It was considered that the 65°C population was made up of obligate thermophiles since their population did not rise significantly above that of feed sludge until waste temperature exceeded 54°C, unlike the 55°C population. Malladi and Ingham (1993) reported thermophilic aerobic spore-former population of up to 109 ml-1 during TAD of potato process waste water. They also identified Lactobacillus spp. during digestion at 55°C.

The paucity of information on the microbiology of TAD has left room for speculations on the diversity of microorganisms in the process. Even the limited information that exists has been based on reactions that employed sewage sludge (Sonnleitner and Fiechter, 1983ab). This waste type has limitations as the basis for projection to other more diverse and potentially reusable (particularly in animal nutrition) wastes (Ugwuanyi et al., 2006; Couillard and Zhu 1993). The possibility of upgrading and recycling of wastes by TAD requires that further studies on the microbiology of the process be implemented, particularly using such wastes that have potential for reuse in animal nutrition. In a study on the microbiology of TAD using potato process wastes, Ugwuanyi et al., (2008b) reported that the principal populations that drive the process include facultative thermophiles identified as B. coagulans and B. licheniformis (which predominated when the temperature was below 55 °C) as well as obligate thermophiles identifies as B. stearothermophilus. These populations developed rapidly and fluctuated with changes in temperature, with the obligate thermophiles predominating as the temperature increased to beyond 55 °C, such that at above 60°C they were present as nearly pure cultures. The predominant populations did not change in response to waste type, load, operational pH or aeration rate but changed in response to temperature. It is safe to state then, that a eurythermal population of thermotolerant and thermophilic organisms carry out TAD, with selection and succession responding to the local environment, particularly temperature.

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