Introduction
Organic waste is produced wherever there is human habitation. The main forms of organic waste are household food waste, agricultural waste, human and animal waste. In industrialised countries the amount of organic waste produced is increasing dramatically each year. Although many gardening enthusiasts ‘compost’ some of their kitchen and garden waste, much of the household waste goes into landfill sites and is often the most hazardous waste. The organic waste component of landfill is broken down by micro-organisms to form a liquid ‘leachate’ which contains bacteria, rotting matter and maybe chemical contaminants from the landfill. This leachate can present a serious hazard if it reaches a watercourse or enters the water table. Digesting organic matter in landfills also generates methane, which is a harmful greenhouse gas, in large quantity. Human organic waste is usually pumped to a treatment plant where it is treated, and then the effluent enters a watercourse, or it is deposited directly into the sea. Little effort is made to reclaim the valuable nutrient or energy content of this waste.
In developing countries, there is a different approach to dealing with organic waste. In fact, the word ‘waste’ is often an inappropriate term for organic matter, which is often put to good use. The economies of most developing countries dictates that materials and resources must be used to their full potential, and this has propagated a culture of reuse, repair and recycling. In many developing countries there exists a whole sector of recyclers, scavengers and collectors, whose business is to salvage ‘waste’ material and reclaim it for further use. Where large quantities of waste are created, usually in the major cities, there are inadequate facilities for dealing with it, and much of this waste is either left to rot in the streets, or is collected and dumped on open land near the city limits. There are few environmental controls in these countries to prevent such practices.
There are a variety of ways of using organic waste and in this technical brief we hope to outline a few of the principle methods used for putting it to good use. The three main ways of using organic waste that we will look at are for soil improvement, for animal raising and to provide a source of energy.
Organic waste – types, sources and uses
As mentioned earlier, there a number of types of organic waste which are commonly discarded. Below we will look at the types and sources of organic waste and some examples of common uses for this waste.
Domestic or household waste
This type of waste is usually made up of food scraps, either cooked or uncooked, and garden waste such as grass cuttings or trimmings from bushes and hedges. Domestic kitchen waste is often mixed with non-organic materials such as plastic packaging, which cannot be composted. It is beneficial if this type of waste can be separated at source – this makes recycling of both types of waste far easier. Domestic or household waste is usually produced in relatively small quantities. In developing countries, there is a much higher organic content in domestic waste. From Figure 1 we can see that up to 60% (or more in some cases) of all municipal waste is organic matter, much higher than the figure for an industrialised country. It is therefore well worth intercepting this supply of useful material where it can be used effectively.
Commercially produced organic waste.
By this, we mean waste generated at institutional buildings, such as schools, hotels and restaurants. The quantities of waste here are much higher and the potential for use in conjunction with small-scale enterprise is good (see box 2). Animal and human waste.
It is worth mentioning at the start of this section that there are serious health risks involved with handling sewage. Raw sewage contains bacteria and pathogens that cause serious illness and disease. It should be stressed that health and safety procedures should be followed when dealing with sewage and that people involved with its handling should have a clear understanding of the health risks involved. Raw sewage should never be applied to crops which are for consumption by humans or animals.
• Human faecal residue is produced in large quantities in urban areas and is dealt with in a variety of ways. In the worst cases, little is done to remove or treat the waste and it can present enormous health risks. This is often the case in the slum districts or poor areas of some large cities. Sewage is often dealt with crudely and is pumped into the nearest water body with little or no treatment. There are methods for large-scale treatment and use of sewage as a fertiliser and a source of energy. The most commonly used method is anaerobic digestion to produce biogas and liquid fertiliser. Composting toilets (see later section) facilitate the conversion of human faecal waste into rich compost.
• Animal residue is rarely wasted. This fertile residue is commonly used as a source of fertiliser, being applied directly to the land, or as a source of energy, either through direct combustion (after drying) or through digestion to produce methane gas.
Agricultural residue
This is the ‘waste’ which remains after the processing of crops (e.g. maize stalks, rice husks, foliage, etc.). There are a wide variety of applications for this residue, ranging from simple combustion on an open fire to complex energy production processes that use this waste as a fuel stock. It is not within the scope of this paper to deal with the many and varied uses of agricultural residues.