It is already evident from earlier sections of this chapter that the presence of high levels of particulate or soluble organic matter in water will result in potential high BODs. This is precisely what is being achieved in all large-scale fermentation processes. An initial medium rich in organic matter is converted to biomass and primary and secondary metabolites. Unfortunately, the product often represents a small proportion of the initial raw material, even in an efficiently operated
Table 11.1. Factors to investigate in a site survey
Daily flow rate
Fluctuations in daily, weekly and seasonal flow
Turbidity pH range
Odours and tastes
Presence of specific toxins or inhibitors (e.g. heavy metals, phenolics etc.)
fermentation. The spent wastes remaining after the distillation of whisky may account for 90% of the initial raw organic materials, while in an antibiotic fermentation the effluent may represent in excess of 95%.
Data for a variety of fermentation effluents are summarized in Table 11.2. The BODs of many of these samples are much higher than that of domestic sewage and some may be comparable with strong effluents such as sulphite paper mill liquor. It is evident from these data that fermentation effluents may present serious potential pollution problems and may be expensive to dispose of unless well planned processes are used. A number of steps may be taken to reduce BODs in a process. Some of these will be discussed in this chapter. Careful selection of raw materials may have a significant effect on the type and quantities of effluent being produced. The cheapest raw material which meets the nutrient requirements of the micro-organisms may not be ideal if product yield, recovery cost, effluent disposal cost and possible by-product value are considered together. The high BOD value of fungal mycelium (40,000 to 70,000 mg dm"3) would indicate that any biomass should normally be kept separate from the remainder of an effluent and some of it may be sold as a by-product. It may also be worthwhile to concentrate liquid fractions, for example, industrial alcohol and distillery stillages (10,000 to 25,000 mg dm"3) will both produce dried solubles fractions which can be sold.
Metabolites or components of some fermentation effluents may be extremely toxic and polluting and will require complete destruction, for example by chemical or thermal methods, before disposal. The need for such a treatment strategy will therefore make a significant contribution to the overall cost of the process. One such metabolite is avermectin produced by Strep-tomyces auermitilis fermentations. Here all effluent streams from the process are captured and any avermectin present chemically degraded (Omstead et al., 1989).
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