| Bioconversion of Organic Residues for Rural Communities (1979) |
|Production of feed as an objective for bioconversion systems|
The success of "wastes" such as oilseed meals, fish meal, and rendering plant by-products as well established feed ingredients may create the false impression that the use of any waste as a feed is an easy, straightforward process. In reality, the feasibility of big-regeneration of wastes for feeds requires the solution of many problems that can be divided into two groups: techno-economic factors and health problems
The production of animal feed could be the most profitable way of waste utilization in small communities. The feed route represents the highest immediate cash return because demand for feed is huge and stable, and the technologies involved are not too sophisticated. Marketing is relatively easy; the introduction of an unconventional feed is much easier than that of an unconventional food. Unfortunately, there are also many cost factors that may limit the apparent profitability of the feed approach.
Problems of acquisition
To quote Zucker (1): "Wastes could be defined as materials with disposal cost, or as products with a negative price." This is true as long as there is no use for a waste. As soon as demand or even signs of potential demand appear, the material assumes a price. Therefore, the cost of acquiring a residue is seldom zero, let alone a negative figure.
Statistics about the quantities of waste produced are certainly impressive. For every kilogram of edible plant product, five to ten kg of residues are produced. The production of one kilogram of beef creates some 25 kg of manure (2). However, wastes are often widely scattered and may be quite remote from the point of utilization or processing. Thus, a cost of collection and transportation is involved.
Lack of uniformity
Rational rearing of farm animals requires a high degree of uniformity in feeds. Most agricultural residues lack such uniformity as a result of seasonal variations, differences in agro-technical methods, and diversity of sources. The modern feedmill technology can cope with variability of ingredient composition within certain limits, but at the village level, variability of the material may become a limiting factor.
The need to process
Many wastes cannot be utilized as such without some sort of processing. For example, cellulosic residues must be partially hydrolized to improve their digestibility. Sloeneker et al. (3) advocated a fractionation process to upgrade feedlot waste. More complex processes, such as the transformation of a waste into a micro-organism biomass, may sometimes be the best way to prepare waste for utilization.
Some wastes require dehydration before they can be used as feed. Removal of water is essential for stabilization, and for the reduction of transportation and storage costs. However, dehydration can be an extremely costly process, and wet feeding, if feasible, should be considered first. A study in Israel on the economics of waste as feed indicated that the most profitable way to dispose of citrus processing waste is to feed it as such to dairy cattle. Fortunately for Israel, citrus plants and dairy farms are not too far apart. This is not the case for many citrus-producing areas in the world.
Ensilage is a process of great value in the production of feed from waste. It may be regarded as an in-storage stabilization process, resulting, if properly done, in considerable improvement of the microbiological and nutritional quality of the feed. Thermal processes or treatment with chemicals such as acids or formaldehyde may be necessary for the destruction of pathogenic organisms.
Nutritional "value" of residues
"Value" is put in quotes to emphasize its commercial rather than biological significance. In other words, how much money is a certain waste material worth in a specific application as feed? Obviously, this value is determined not only by strictly nutritional characteristics such as nutrient composition, digestibility, presence of anti-nutritional factors, palatability, and tolerance, but also by usage characteristics such as convenience, stability, effect of the feed on the acceptability of the final product (e.g., effect on colour of egg yolk or on the flavour of milk, etc.), aesthetic barriers, or plain traditionalism. For these and other reasons, the value of wastes as feed is often considerably less than the value that would be assigned to them by a computer programmed for least-cost feed formulation.
The question of health factors in feed from waste should be examined from two aspects: (i) Factors associated with the effect of the feed on the health of the animals consuming the feed. We designate these factors as primary toxicology, primary parasitology, primary microbiology, etc. (ii) Factors affecting the wholesomeness of the final animal products (meat, eggs, milk, etc.). These factors are the subject matter of secondary toxicology, parasitology, microbiology, etc.
The evaluation of primary safety is relatively simple. The customary feeding trials will show the presence or absence of acute toxicity or the danger of outbreak of infectious diseases. Chronic effects, or effects that can be detected only after many generations, are usually of no consequence, except perhaps in the case of animals raised for reproduction. Whenever we intend to use a waste material as an ingredient of the diet for only a certain portion of the life span of the target animal, health hazards should be evaluated in relation to animals of corresponding age. Thus, there is no justification for the rejection of a certain feed ingredient on grounds of toxicity to newborn animals if the feed is not intended for use as a starter, but rather as a fattening or finishing ration.
The question of secondary safety or transmitted health hazards is much more complicated. The basic philosophy and methodology of evaluation are still debated and no accepted standards are available. This is well illustrated by the vagueness of the following statement from the PAG Guideline No. 6 for Preclinical Testing of Novel Sources of Protein (4): "Products intended for incorporation into animal feeds may not require as extensive testing as is suggested here for human foods, but foods derived from such animal sources must be considered from the viewpoint of the possible presence of residues in meat, milk, or eggs, transmitted from animal feeds. Controlled tests in farm animals may contribute useful information concerning safety or nutritional value for man." A later PAG Guideline (5) is more specific as to primary safety, but as vague as the earlier one on the matter of safety criteria for the edible animal end-product.
The following is a list of health and safety factors to be considered in the evaluation of an agricultural waste as feed: pathogens, heavy metals, pesticide residues, drug residues, toxic metabolites, and foreign bodies (e.g., metal, glass).
The application of some of the general principles discussed above will now be examined in two specific cases: recycling of manure and the use of sewage-grown micro-algae.