Operational Temperatures Employed in TAD

There is as yet, no consensus on what constitutes or optimal temperatures, for the operation of TAD. The fact of the fluidity of definition of thermophilic temperatures as applied to microbial processes has complicated its application in aerobic thermophilic digestion. Difficulties in setting temperature standards arise from the subjective and imprecise definition of thermophily (Brock, 1986), and also from the heterogeneity of microbial population likely to operate in TAD, which will result in a wide band and overlaps of growth temperature optima. Thermophily applied to waste treatment has also been considered in relation to mesophilic treatment, or the differential between the reactor and feed temperatures. In temperate countries feed temperatures may vary from under 5°C to more than 20°C depending on the season (Vismara, 1985), while approaching 40°C in some tropical countries. The implication of this for the definition of thermophilic digestion as applied to TAD is considerable. Surucu et al. (1976, 1975) consider that TAD would be a process that operates between 50° and 60°C. Vismara (1985) considered a process that operates in a range of 40°-50°C as TAD, while Frost et al. (1990), defined TAD as a digestion which operates above 35°C, and up until temperature becomes the limiting factor (at about 70°C). Matsch and Drnevich (1977) consider TAD as that operating above 45°C, while Jewell (1991), defined it as that which achieves 43°C or above.

Notwithstanding the seeming requirement for temperatures of up to 45°C and above, the relationship, if any, between operational temperature and stabilisation efficiency in TAD is subject of considerable controversy. This has led to the imposition of mesophilic digestion standards on TAD (Koers and Marvinic, 1977; Vismara, 1985). In their simulation studies, Kambhu and Andrew (1969) considered 45°-60°C as acceptable range for TAD, and believed that the highest reaction rate constant would be achieved at 55°C while decreasing to zero at 75°C. Carlson (1982), reported increase in sludge degradation, as the temperature increased to 57°C. Hawash et al. (1994) also reported increase in the rate of waste stabilisation with temperature, with the kinetic parameters of stabilisation nearly doubling with each 10°C increase in temperature within the permissible range. Tyagi et al. (1990) reported an increase in digestion efficiency between 45°and 55°C, followed by a gradual decline thereafter. Ugwuanyi et al. (2004b) reported increase in the digestion and stabilization efficiency during TAD as the temperature increased from 45°C to 55°C, followed by slight decline thereafter. However, above 55 °C, waste pasteurisation and consumption of soluble COD increased up to 60°C before declining drastically thereafter. On the other hand the degradation and solubilization of particulate waste matter increased as the temperature decreased to 45°C. The implication of these variations in the response of different waste components to changes in the temperature of the process is significant, and suggests that the choice of operation temperature will vary with the nature of the waste and the principal reason for the operation. It has also been demonstrated that the level of accumulation and quality of protein achieved during the protein enrichment of agro-food waste by TAD vary with the temperature of digestion of waste

(Ugwuanyi et al., 2006; Ugwuanyi, 1999). This is expected to impact significantly on the choice of temperature for digestion, particularly if protein enrichment is intended in the process.

Temperatures so far reported as optima for TAD correspond to the approximate optimum for growth of a range of common aerobic thermopiles (Brock, 1986). By comparison, in various studies of thermophilic anaerobic digestion, different optimum temperatures have been reported in the range of 50°-65°C, and these appear to also vary considerably with the waste type (Verstreate et al., 1996; Lettinga, 1995; Ahring, 1994). Sonnleitner (1983), Sonnleitner and Fiechter (1985, 1983bc), in some of the few studies on the microbiology of TAD to date, consider that extreme thermopiles (caldoactive organisms) are relatively fastidious with respect to their nutritional requirements for growth, and hence are poorly suited for application in TAD. Thus, only thermotolerant and moderately thermophilic organisms are likely to play a role in TAD. These organisms have been classified by Hamer and Bryers (1985) as those growing optimally at 40°-50°C and 50°-65°C respectively, thus giving a wide temperature range of 40°-65°C within which TAD may be operated efficiently. This has been demonstrated to be the case during the digestion of vegetable waste (Ugwuanyi et al., 2007).

Although there are only few full or even pilot scale experiences with TAD to check these projections, Kelly et al. (1993) and Edginton and Clay (1993) successfully operated pilot scale digesters within this range for over one year. Ponti et al. (1995ab) reported that TAD could be efficiently run at 65 °C, particularly if the primary motive is to achieve waste pasteurisation. The requirement for pasteurisation is also the basis upon which Messenger et al. (1993ab) and Messenger and Ekama (1993ab) operated dual TAD-AD system at 65°C and above. Since the TAD was designed principally for pasteurisation, the waste was subsequently stabilised in the mesophilic AD system. They however, reported progressive loss of stabilisation efficiency in the TAD phase as the temperature approached and exceeded 65°C. Although no reason was advanced for the decline in efficiency, the high temperature could have robbed the system of its metabolic versatility by restricting the variety of active microbial population. This has been demonstrated to be the case (Ugwuanyi et al., 2008b). The implication of this population selection and restriction are considerable and dependent on the target of the digestion process. For instance, very high temperatures may not be an asset if TAD is to be used as a stand alone waste treatment process. In self heating systems, precise control of temperature is unlikely, and the emphasis will continue to be on a range which gives acceptable performance. The preferred range of temperature within the band at which efficient treatment have been reported will depend on a variety of factors, such as system design and treatment target, type and organic loading of waste, pathogen content and the need for pasteurisation among others.

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