Little information is available concerning the economics of the bioconversion process. Only rough estimates of costs were given by Maiorella et al. (1983) and Tomlinson (1976b). The main costs are high capital investment (Tomlinson 1976b), energy needed for aeration, agitation and cooling (Maiorella et al. 1983), biomass separation (Tomlinson 1976b), nutrient supplementation, sterility problems, limited production, and irregularities in raw material composition. However, capital cost may not be too high, as was shown in the cost evaluation for molasses stillage treatment methods, where aerobic yeast growth was estimated to require the lowest capital investment (Maiorella et al. 1983). Separation of cell materials from the broth would be easy with the application of filamentous fungi. The addition of nutrients is not always required. Sterility problems could be managed or at least reduced by maintaining a low pH (Cabib et al. 1983; Huyard et al. 1986; Matsuo et al. 1966), and the use of mixed cultures (Barker et al. 1982). The problem with large-scale plants is the need for a continuous and relatively homogeneous supply of stillage. Comparison of cost with that for conventional biological treatment has shown that annual savings could be increased by applying fungal bioconversion of stillages (Tomlinson 1976b). Similarly, rough estimates of capital and operating costs for molasses stillage treatment by fungi show that savings could be gained, even though wastewater would still have to be purified. Comparison of different treatment methods for molasses stillage showed that aerobic Candida yeast growth was the most suitable option. The same method was proposed for sulfite waste liquor and wood hydrolysis stillages (Maiorella et al. 1983).
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