The solids content (biodegradable solids/chemical oxygen demand (COD) and biochemical oxygen demand (BOD)) required to achieve and maintain thermophilic temperatures in TAD is not well defined. In sewage sludge treatment, it has been recommended that a high solids content is essential to achieve thermophilic temperature, hence the slurry nature of TAD (Hamer and Bryers, 1985). This draws from the experience of composting, in which high temperature is aided by high solids content and reduced moisture. However, as sewage sludge consists essentially of partially degradable microbial cells (within the context of the duration of digestion), the solid content of sludge cannot be used as a direct index of biodegradable COD. Theoretically, since the principal source of energy of the process is the enthalpic content of organic matter, the temperature reached in the reactor depends on the concentration of biodegradable material in the waste (Ugwuanyi 1999; Vismara, 1985; Wolinski and Bruce, 1984; Jewell and Kabrick, 1980).
In the absence of heat loss, the minimum theoretical concentration of biodegradable solids needed to produce a 50°C rise in temperature has been reported to be 3 g l-1 (Vismara, 1985). Jacob et al. (1989) recommended a minimum solid content of 2.5% for the attainment of thermophilic temperature in sewage sludge, while Jewell and Kabrick (1980) recommended 5%, to be able to sustain thermophilic temperatures for long enough to achieve waste pasteurisation. Successful treatment of wastes whose concentrations range between 2% and 6% have been reported at different thermophilic temperatures (Ugwuanyi et al., 2004b; Hawash et al., 1994; Tyagi et al., 1990; Kelly et al., 1993; Edginton and Clay, 1993; Morgan and Gumson, 1981). Because of the importance of degradable organic load in heat evolution in TAD, Surucu et al. (1975) recommended it for treatment of high strength agro-food wastes with potential for protein recovery, especially in warm climate countries and in particular for the treatment of wastes generated at high temperatures.
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