Waste treatment by microbiological methods is probably as old as the generation of waste itself. In spite of this, scientific knowledge of the processes involved in biological waste treatment is relatively recent. Traditionally, wastes have been treated to remove them from areas in which they are not wanted (Bewely et al., 1991). Hence, early methods of waste treatment were essentially waste relocation or disposal processes, inspired by man's desire to protect his immediate environment. The processes of disposing of wastes applied to both human and agricultural wastes, and therefore did not discriminate between wastes that were potentially reusable with only minor reprocessing and, those that needed to be disposed of on account of their limited reuse value. An increase in the scale of the problems of environmental pollution, as well as changes in social attitude, have led to multidisciplinary approaches to the problems of waste management and pollution control (Grainger, 1987a). Increasing pressures on resources also mean that the vast quantities of organic materials that remain from human productive activities can no longer be seen strictly as wastes that need disposal.
The ever-increasing world population, with the attendant food supply problems, as well as the lessons of the energy crisis of the 1970s, combined to force a change in the global attitude towards waste. Wastes, particularly organic and agricultural wastes, are progressively being seen as resources in the wrong location and form that should be recycled, rather than as refuse that must be disposed of. Thus, biomass, reprocessing, recycling and reuse are progressively gaining increased importance in discussions on waste management, at the expense of refuse, disposal and waste treatment, which can no longer be seen solely as exercises in pollution control.
Conventional treatment processes are rarely linked to any form of reprocessing, recycling or reuse. Where reuse occurs, it is almost solely as organic fertiliser, as a means of disposal rather than the target. The need to recycle waste is most relevant in the food, agricultural and related industries (Grainger, 1987a,b), where biomass estimated to be in excess of 1017 tonnes are generated annually, on a worldwide basis (Bath, 1991). Refuse amounting to up to 40% of world food production is generated following diverse agricultural and food industry processes. These are very important energy-rich resources, the reprocessing and recycling of which offer the possibility of returning them to more beneficial use than is currently possible. The key to successful processes of this nature is an economical "no loss" process, in which the cost of processing should be offset, at least in part, by the possibility of producing a valuable product, with the added benefit of stabilising the otherwise environmentally hazardous waste. Such a process should be satisfactory if it causes the overall cost of waste management to be significantly less than the classical approach (Loehr, 1977). Today, agricultural and food industry residues, refuse and wastes constitute a significant proportion of worldwide agricultural productivity. The continued disposal of these "wastes" constitutes significant loss of agricultural productivity and calories which, if properly harnessed, can impact positively on worldwide food supply, animal production and global food security.
Traditional subsistence agriculture led to the production of only limited agricultural refuse, which was generally disposed of untreated to land at zero cost, or actually at a credit because of its use as farmyard manure and soil conditioner. Introduction of cheap inorganic fertiliser obviated much of the need for farmyard manure. Increasing world population and industrialisation led to the introduction of intensive agricultural practices, the latter resulting in accumulation of large quantities of agricultural refuse in small land areas. Recent advances in the field of food technology, and the increasing demand for factory processed foods have led to a considerable increase in the quantity of food industry / processing waste. Additionally, the tendency to locate food processing factories close to food producing areas has led to a concentration of agricultural refuse disposal problems in limited land areas.
An associated problem of waste disposal on land is concern over the survival of pathogens on pasture land, and pollution of underground waters at areas of high water tables. The introduction of various noxious/man-made chemicals into agriculture has also introduced new requirements and pressures that conventional treatments are not able to handle. This is in addition to new needs to utilise agricultural products that are not conventionally used due to their natural content of toxins. Thus, waste management has progressed through the stages of disposal and treatment, beginning with the time when the only wastes that attracted attention were municipal solid wastes and sewage, to one of reprocessing and recycling.
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