|By Pinoy Farmer | May 2, 2008|
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 which is 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.
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 which can 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.
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.
Methods of Processing Organic Waste
As mentioned in the introduction, there are three main ways in which organic waste can be used:
- soil improvement
- animal raising and
- to provide a source of energy
Differing levels of processing are required for achieving the above and in this section we will take a brief look at just some of the common approaches to using organic waste.
Composting is simply the method of breaking down organic materials in a large container or heap. The decomposition occurs because of the action of naturally occurring microorganisms such as bacteria and fungi. Small invertebrates, such as earthworms and millipedes, help to complete the process. Composting can convert organic waste into rich, dark coloured compost, or humus, in a matter of a few weeks or months. There is nothing mysterious or complicated about composting. Natural composting, or decomposition, occurs all the time in the natural world. Organic material, the remains of dead animals and plants, is broken down and consumed by micro-organisms and eaten by small invertebrates. Under controlled conditions, however, the process can be speeded up.
Composting has many benefits;
- It provides a useful way of reclaiming nutrients from organic refuse
- Saves valuable landfill space and possible contamination of land and water due to landfill ‘leachate’
- Can be used as fertiliser on farmland or in the garden
- Improves the condition of soils
In composting, provided the right conditions are present, the natural process of decay is speeded up. This involves controlling the composting environment and obtaining the following conditions:
- The correct ratio of carbon to nitrogen. The correct ratio is in the range of 25 to 30 parts carbon to 1 part nitrogen (25:1 to 30:1). This is because the bacteria which carry out the composting process digest carbon twenty five to thirty times faster than they digest nitrogen. This is often seen as being a roughly equal amounts of “greens” and “browns”. Carbon to nitrogen ratio will be referred to hereafter as the C:N ratio. The C:N ratio can be adjusted by mixing together organic materials with suitable contents.
- The correct amount of water. Plants have a liquid rather than a solid diet and therefore the compost pile should be kept moist at all times. On the other hand, a wet compost pile will produce only a soggy, smelly mess. · Sufficient oxygen. A compost pile should be turned often to allow all parts of the pile to receive oxygen.
- The optimum pH level of the compost is between 5.5 and 8
In these conditions, bacteria and fungi feed and multiply, giving off a great deal of heat. In well managed heaps, this temperature can reach as high as 60 C, which is sufficient to kill weed seeds and organisms that cause disease in plants and animals. While the temperature remains high, invertebrates are not present in compost heaps, but when the temperature drops, the invertebrates enter the heap from the surrounding soil and complete the process of decomposition.
Forms of Decomposition
Anaerobic. In anaerobic decomposition, the breakdown of the organic material is caused by bacteria and fungi that thrive in low or no-oxygen conditions. It is the type of decomposition that takes place in closed containers. This type of system is more complex and difficult to control and requires complex equipment for larger scale composting.
Aerobic. In aerobic decomposition, bacteria and fungi which thrive in high oxygen conditions are responsible for the decomposition. This form of decomposition occurs in open heaps and containers that allow air to enter. With open heaps and more ventilated containers, compost can be formed in a matter of a few months, and even faster if the organic material is turned regularly. In heaps or bins where aerobic decomposition is occurring, there should be no unpleasant odours.
Some Methods of Composting
Composting systems can be opened or closed, that is the organic matter will either be placed in open piles or rows or in a closed container or reactor. The open system is rarely used in low-income countries due to its technical complexity, so we look at some of the open systems in use.
Backyard composting at the household level is a simple technique. It requires only suitable organic waste, space to construct the heap and time to carry out the necessary work. The waste can be placed in a pit (say 2m x 2m x 1m deep) and left to decompose for 2 – 3 months. Alternatively, the waste can be piled up within an enclosure of 4 poles and surrounded by boards or chicken wire and left for a similar period. This produces a rich compost which can be used as a fertiliser on fields or gardens.
Neighbourhood composting. A commonly used technique for neighbourhood composting is the use of windrows. Here waste is simply laid out in long rows and turned occasionally. Another method is the rotating bin method which uses a series of closed, aerated bins.
Co-composting is technique whereby organic food waste is mixed with human or animal excreta and composted Similar techniques are used to those described above.
Large-scale, centralised composting has tended to be unsuccessful in developing countries for a number of technical and organisational reasons.
Medium scale biogas and compost production from market garbage in Colombo, Sri Lanka. A pilot project being implemented by the Colombo Municipal Council uses organic waste from local city vegetable markets to produce biogas and compost. The digesters were developed by the National Energy Research and Development Centre and accept dry batches of organic waste. There are four 20 foot diameter floating dome digesters each with a capacity of 40 tonnes dry waste. The residence time for the organic matter is 4 months and thus the four tanks are able to deal with a total of 480 tonnes of market waste each year.
The waste produces approximately 1 cubic metre of biogas per tonne per day and this translates to a total of 7500 kilowatt hours of electricity each year. The system also yields 300 tonnes of saleable fertiliser each year. Before this, all the waste had to be landfilled outside the city.
The digester is made from concrete with a floating fibreglass cover. The gas is piped from the digester and is used to power a 220 volt, 5 kilowatt converted engine. There is also a baker’s oven and a catering size gas burner at the site to demonstrate the uses of the gas. Now we will look at an example of animal rearing using organic food scraps. This is a typical example of waste being put to good use and benefiting a number of groups.
Pig-feeding in Metro Manila
In the outlying urban areas of Manila, backyard pig- rearing has long been a traditional source of income. Commercially produced feed for this activity is expensive and pig raisers often turn to organic scraps to supplement or replace the commercial product. A network of collectors has developed that collects organic waste from restaurants in the city centre, and then distribute it amongst the backyard farmers. The farmers can purchase the scrap at about half the price of the commercial feed. A cost comparison carried out under the WAREN project (cited in a report titled ‘Recycling activities in Metro Manila’) shows that profit is more than doubled by feeding the pigs on organic scraps, even after all other costs, such as veterinary costs, transport, fuel, etc., are taken into consideration. Such ventures are beneficial not only to the pig raisers, but also to the municipality who would otherwise have to dispose of the waste in a landfill.
Biogas is produced by means of a process known as anaerobic digestion. It is a process whereby organic matter is broken down by microbiological activity and takes place in the absence of air (anaerobic means ‘in the absence of air’). It is a phenomenon that occurs naturally at the bottom of ponds and marshes and gives rise to ‘marsh gas’ or methane, which is a combustible gas. It also takes place naturally in landfill sites and contributes to harmful greenhouse gases. Biogas can be produced by digesting human, animal or vegetable waste in specially designed digesters . Animal waste is particularly suitable for biogas production because it is often available is large quantities and also has a suitable C:N ratio. The scale of simple biogas plants can vary from a small ‘household’ system to large commercial plants of several thousand cubic metres. The process is sensitive to both temperature and feedstock (the correct C:N ratio is required as with composting) and both need to be controlled carefully for digestion to take place. Digestion time varies from a couple of weeks to a couple of months.
The digestion of waste yields several benefits:
- the production of methane for use as a fuel.
- the waste is reduced to slurry which has a high nutrient content which makes an ideal fertiliser; in some cases this fertiliser is the main product from the digester and the biogas is merely a by-product.
- during the digestion process pathogens in the manure are killed, which is a great benefit to environmental health.
Two popular simple designs of digester have been developed for use in developing countries; the Indian ‘floating cover’ biogas digester and the Chinese fixed dome digester. Both operate in the same way but the storage chambers have a slightly different design.
The residual slurry is removed at the outlet and can be used as a fertiliser. Biogas can be used for a number of applications, including lighting, cooking, electricity generation and as a replacement for diesel in diesel engines. Some countries have initiated large-scale biogas programmes, Tanzania being an example. The Tanzanian model is based on integrated resource recovery from municipal and industrial waste for grid-based electricity and fertiliser production (Karekezi 1997).
Source : http://www.practicalaction.org