TAD uses the metabolic heat generated from microbial oxidation of organic compounds to raise and maintain the temperature of digesting biomass at thermophilic levels. To achieve this, the metabolism of the microbial population has to be strongly oxidative, and rapid. Heat evolution is enhanced as the reaction temperature increases, particularly if uncoupling of metabolism occurs, leading to reduced cell yield, and dissipation of a greater amount of the energy content of the substrate as heat (Birou et al., 1987; Hamer and Bryers 1985). The process requires efficient transfer of oxygen into solution, to be able to sustain the rapid metabolism of thermophiles (Burt et al., 1990a). Conventional sparged, impeller agitated reactors are considered unsatisfactory for the level of aeration required for auto-thermophilic aerobic digestions (Messenger et al., 1990), particularly as oxygen solubility decreases with increase in temperature.
The problem of oxygen transfer has had profound impact on the economics, applicability, and development of this process. The result is that early development works on TAD were centred on the development of aeration processes, with little attention paid to the potentially versatile application of the process and the associated population in waste biomass handling and processing. Another implication of this was that most of the early studies on the process were directed at its application in the pasteurisation of sewage sludge prior to treatment by anaerobic digestion and disposal to agricultural land. In the early stages of development of TAD, it was believed that thermophilic temperatures could only be achieved if pure oxygen (Gould and Drnervich, 1978) or oxygen enriched air (Bruce 1989) was used. Aeration with pure oxygen was employed in pilot plant studies for thermophilic treatment of sewage sludge at Ponthir Water Works UK, where a temperature of 63 °C was easily obtained (Morgan and Gumson, 1981). Although the process was uneconomic, it provided a basis for studies on TAD leading to the development of alternative aeration system to achieve thermophilic temperatures with air (Jewell et al., 1982). Unfortunately, some of the processes were considered uneconomic because significant proportion of the heat was of mechanical origin from agitators.
A variety of aeration devices have been developed for use in TAD. The Venturi Aerator was shown to aerate sewage sludge to thermophilic temperature inexpensively (Morgan et al., 1986). It was based on a nozzle type aerator which has been applied to a variety of biochemical processes (Jackson, 1964), and achieves aeration (and mixing) by having the digesting mass pumped through an external recirculation loop and back into the reactor, through a nozzle. The air/ liquid mixture emerging from the nozzle is then discharged at the base of the reactor vessel, where large quantities of oxygen are forced into solution in a well-mixed mass. These self-aspirating aerators were used to achieve over of 20% oxygenation with air. Oxygenation efficiency equal to or greater than 10% (i.e., capable of solubilising at least 10% of the available oxygen) has been recommended as the minimum needed to maintain thermophilic temperatures (Jewell, 1991). It has also been suggested that the reduced amount of nitrification (if any), taking place at high temperatures, means that TAD may have significantly lower oxygen requirement than aerobic mesophilic digestion for the same amount of solid material destroyed (Hawash et al., 1994; Jewell and Kabrick, 1980).
Additional problems associated with the aeration of TAD are the limited solubility of oxygen at elevated temperatures (50°C and above), and the high solids content often desired in such operations. However, in spite of the low solubility of oxygen at high temperatures, the molecular diffusivity of oxygen increases with temperature (Surucu et al., 1975). This compensates for the low solubility, leading to comparable or better oxygen transfer at the higher temperatures. In fact, oxygen-transfer efficiencies comparable to, or better than that obtainable at mesophilic temperatures have been demonstrated during the fermentation of some caldoactive organisms, including Pyrococcus furiosus, and Sulfolobus shibatae at 90oC (Krahe et al., 1996). Oxygen transfer at high temperature is also aided by the decrease in viscosity of the reaction medium with increase in temperature.
Various levels of aeration have been applied to TAD with different results. Following pilot trials in Canada, Kelly (1991) and Kelly et al. (1993) recommended that aeration rate in the digester should be maintained at 0.5 to 2.0vvh (volume air per volume medium per hour) depending on the organic load. Hawash et al. (1994) reported increase in efficiency of TAD (destruction of volatile suspended solids (VSS) and reduction of biochemical oxygen demand (BOD)) with increase in aeration rate up to 0.5vvm (the highest employed to minimise evaporation and frothing). Frost et al. (1990) reported oxygen consumption rates between 1-2 kg-1 kg-1 VSS destroyed in sewage sludge. Vismara (1985) proposed that the aeration rate required for TAD would be midway between that needed in classical aerobic systems, and that of typical anaerobic mesophilic digestions. Sonnleitner and Fiechter (1983ab) reported aeration rates of 0.02 to 0.3vvm (volume air per volume medium per minute), leading to stable final temperatures between 50oC and 67oC, during a two-year study of TAD. In studies that employed CSTR, Ugwuanyi et al. (2004ab) demonstrated that destruction of VSS and soluble COD, as well as development of critical microbial enzymes increased with the aeration rate of TAD process from 0.1vvm to a peak at 0.5vvm, before declining with further increase in the aeration rate applied to the process. Microbial population however, did not decline with the increase in aeration rate to 1.0vvm.
As the main cost of TAD is that of aeration, running it at microaerophilic rates should make it more economical (Ugwuanyi et al., 2004ab; Carlson 1982). However, a highly aerobic system will be desirable if TAD is to find a role in highly aerobic oxidations such as may be required in detoxification of aromatic and haloaromatic chemicals. Since heat generation in microbial systems is a function of oxygen consumption, at least to a point, it follows that highly aerobic systems will be more efficient in heat generation than less aerobic ones (Messenger et al., 1990; Williams et al., 1989; Birou et al., 1987; Cooney et al., 1967). An additional advantage of extensive aeration could be located in increased evaporation of waste liquid, if this is desired to reduce the amount of waste available for disposal, provided of course, that it does not lead to evaporative cooling of the waste. Achievement and maintenance of thermophilic conditions should also benefit from effective insulation, and where possible the use of heat exchangers between effluent and influent waste, particularly in very cold climates. This has led to various modification being made to different digesters that have been employed for TAD (Messenger et al., 1990; Morgan et al., 1986). These modifications include the use of stainless steel tanks with insulation cladding, as well as concrete tanks with plastic or fibre glass interior. Insulation of digesters will enable the operation of TAD with the minimal amount of aeration necessary to achieve thermophilic temperature.
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