The nature of the waste determines the amount of biodegradable chemical oxygen demand (COD), which directly affects heat evolution. As the biodegradable organic load increases, the amount of material removed increases and so does heat evolution, not least because the amount of ballast water present per unit mass of organic matter decreases. Jewell and Kabrick (1980), and Jewell (1991), have related heat evolution to COD removal by the expression: AF= 3.5ACOD;
• Where AF is the heat released in kilocalories per litre and
• ACOD is the measured change in COD in gl-1.
• One kilocalorie per litre is equal to 1°C change in temperature.
The maximum temperature reached depends on the balance between heat loss and heat input. This equilibrium state is facilitated by two prominent factors, a) as the biodegradable organic load increases the system soon becomes mass transfer limited, a situation accelerated by reduction in oxygen solubility with increase in waste (solid) load, and rapid consumption of oxygen by the proliferating microbial populations, putting more pressure on the available oxygen.
b) as the temperature of the digesting mass increases to 60°C and above, the number of viable thermopiles gradually starts to decline leading to decrease in organic matter removal, and consequently a decrease in heat production (Matsch and Drnevich, 1977; Ugwuanyi et al., 2004b). This self-regulation is very similar to what obtains in classical composting.
Within the permissible range, increase in temperature will be accompanied by an increase in digestion rate, following the classical van Hoff Arrhenius relationship, provided that care is taken to account for transition from growth of mesophiles to that of thermophiles if such temperature transition is involved. Hisset et al. (1982) reported less respiration in piggery slurry at 50°C than at 35-40°C. And Ponti et al. (1995ab) reported that respiration is greater at 60°C than at 50°C. Also, Surucu et al. (1975) reported greater activity at 55° and 58°C than at 50°C. This supports the presence of transition temperature (range) between upper limits of thermotolerant mesophiles and the lower limits of facultative thermophiles at which neither of the two groups of organisms is at a metabolic advantage (Ugwuanyi et al., 2004b). This particular range does not take account of the behaviour of the more obligate thermophiles. As of now however, the choice of temperature seems to be a matter of trial and error, and varies widely with the process and the operator.
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