Thermophilic aerobic Digestion

TAD is a process in which the metabolic heat of aerobically growing microbial cells is conserved in an insulated system, leading to the elevation of the temperature of the digesting mass to thermophilic range (Gomez et al., 2007; Coulthard et al., 1981; Surucu et al., 1975, 1976; ). It arose as a modification of the conventional activated sludge process (Adav et al., 2008). Given efficient aeration, microbial metabolism leads to production of sufficient heat to cause a rise to, and maintenance of thermophilic temperatures in the digesting mass, provided that heat loss is minimised by good insulation of the digestion vessel and control of evaporative cooling. It is believed that achievable temperature may be manipulated by varying the concentrations of oxygen, and biodegradable organic materials in the waste (Messenger et al., 1990 Hamer and Bryers, 1985).

Like all waste treatment processes, TAD is a mixed culture process. During start-up, a variety of mesophilic and thermotolerant bacteria interact, metabolising readily available waste components to generate heat (Yun et al., 2000). As the temperature increases, microbial succession and selection takes place until, at thermophilic temperatures, only a few species of micro-organisms remain active. The selection process results in the inactivation of sensitive mesophiles (including most vegetative and non spore-forming organisms which include, luckily, all important animal pathogens). And, at thermophilic temperatures, hydrolysis of complex and otherwise recalcitrant molecules (hopefully, including noxious and xenobiotic compounds) is enhanced. Considering the limitation of competing biomass reprocessing technologies, particularly anaerobic digestion in the handling of noxious compounds, the ability of TAD to achieve the degradation of xenobiotics and noxious compounds makes it attractive for the treatment of a variety of wastes that have recycling potentials (Adav et al., 2008). TAD resembles composting in many respects and has been described as liquid (slurry) composting (Yun et al., 2000; Jewell, 1991).

The process is considered to be very flexible and versatile, for which reason it may be adaptable, not just to waste treatment, but also as a means of generating useful by-products (as animal feed supplements and biochemicals) from wastes. The use of TAD in the protein enrichment of a variety of agricultural and food industry refuse for animal feed supplementation has been studied and continues to attract a lot of research interest (Ugwuanyi et al 2009; Ugwuanyi 2008; Ugwuanyi et al., 2006; 2008ab; Couillard and Zhu, 1993). The high temperature of operation of TAD, and metabolic versatility of thermophiles, ensures that the processes take place very rapidly, and under conditions that may require less control than conventional bioprocessing (Lee and Yu, 1997). A striking advantage of TAD is its ability to achieve the pasteurisation of processes waste. This has implication for the use of the technology for the reprocessing of a variety of biomass intended for reuse in animal nutrition and also for land application (Ugwuanyi et al., 1999).

Compared to older and more established biomass handling and processing techniques TAD is still poorly understood as a biotechnological process. However, work has continued into the optimization of various aspects of this process including aeration efficiency of equipment used in TAD, and its use in replacement of conventional waste biomass management methods. Considerable efforts are also being expended to achieve improved understanding of the microbiology and biochemistry of the process to enhance its ultimate applicability (Cibis et al., 2006, 2002; Heinonen-Tanski et al., 2005; Agarwal et al., 2005;

Ugwuanyi et al., 2004ab; 2005ab; Kosseva et al., 2001;). Although thermophilic microbiology has a fairly long history, the application of thermophiles in biotechnology is only just beginning to attract attention, and application of thermophiles in biomass management such as in TAD, has received even less attention. Consequently few large scale operations currently employ this potentially versatile process.

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