MANAGING ORGANIC MUNICIPAL WASTE
Introduction Organic waste often forms as much as 75% of household waste generated in developing countries, compared with just 30% in industrialised countries. In many cities in developing countries, per capita waste generation rates are in the order of 500g/day, some 300g of which may be organic. Thus a city of 1 million population may produce 300 tonnes of organic waste daily. Organic waste is a major issue! This technical brief begins by describing the characteristics of organic waste, its sources and the particular hazards, challenges and opportunities it presents. It goes on to present a number of options for processing organic waste, including use as animal feed, biogas digesting, and composting. Many composting techniques are simple, and compost is relatively easy to make. However, sourcing uncontaminated raw materials, making a high-quality product, and making composting viable, can be difficult. This technical brief discusses some of the challenges and how to overcome them. This brief would be useful for anyone facing the challenge of managing organic waste. It is particularly intended for project engineers, planners or managers in municipalities, NGOs and businesses. Figure 1: Composting in Colquencha, Bolivia.
Photo: Alfredo Quezada / Practical Action
Organic waste Organic waste in towns and cities is generated by households, businesses, industries and local authorities. It consists of kitchen waste (e.g. potato peelings), waste food (e,g, leftovers in restaurants, spoiled fruit and vegetables from markets), garden waste (e.g. grass clipp ings and hedge trimmings) and industrial waste (e.g. from agricultural and food processing factories). Of course agriculture produces vast quantities of organic waste such as rice husk, straw and manure. However, this rarely becomes mixed with domestic or commercial organic waste so is not discussed in this brief. In addition, most farmers compost it themselves, as do many urban and peri-urban nurseries. Unlike other components of household waste such as metals, glass and paper, organic waste is considered low-value and is rarely collected from recycling or processing by the informal sector or businesses. This can be explained by its density (it is composed predominantly of water), the cost and difficulty of transportation, the land required for processi ng, and the relatively low-value of resultant products. Particularly in warm climates organic waste tends to begin decomposing quickly -- within a day or so. Rotting organic waste is often responsible for the foul smell in bins, vehicles and
Practical Action, The Schumacher Centre, Bourton on Dunsmore, Rugby, Warwickshire, CV23 9QZ, UK T +44 (0)1926 634400 | F +44 (0)1926 634401 | E firstname.lastname@example.org | W www.practicalaction.org ______________________________________________________________________________________________ Practical Action is a registered charity and company limited by guarantee. Company Reg. No. 871954, England | Reg. Charity No.247257 | VAT No. 880 9924 76 | Patron HRH The Prince of Wales, KG, KT, GCB
Managing organic municipal waste
disposal facilities. The products of decomposition are corrosive, and containers and vehicles need to be designed with this in mind, and cleaned fr equently to reduce this problem. In industrialised countries much organic waste is disposed of in landfills where it de composes anaerobically, producing methane. It also produces leachate: the liquid which filters down through the layers of waste picking up soluble chemicals and metals on its way. It can be highly toxic and poses a serious environmental and health risk u nless carefully confined and treated. In developing countries, organic waste is often left to rot on streets where it is eaten by animals and birds, blocks drains, and generally causes a public nuisance and health hazard.
In view of the quantities of organic waste produced and the problems associated with transporting and disposing of it, finding alternative solutions is a high priority.
Dealing with organic waste
There are three main ways of dealing with the organic portion of municipal waste: Feed for animals; Feedstock for anaerobic digestions (i.e. biogas plants); Aerobic composting. This technical brief focuses primarily on composting, which is often the most straightforward and lowest-cost option. However we will briefly discuss the first two opt ions. Feed for animals Using organic waste as animal feed is outlawed in many industrialised countries because of concerns of disease transmission, and the risk of introducing toxic chemicals into the human food chain. However, some countries do allow this, and it may be a suitable solution, for example, for using waste from a food processing factory or a vegetable market where quality is relatively easy to control. Check with national regulations. The following box presents an example of the use of organic waste to feed pigs in the Philippines.
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 collects organic waste from restaurants in the city centre, and then distributes it among backyard farmers. The farmers can purchase the waste at about half the price of the commercial feed. A cost comparison showed that profit was 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. Animal feed ing happens in many other countries such as Egypt, Turkey, India and Pakistan. 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.
Anaerobic microorganisms thrive in environments with no oxygen. Many such microorganisms occur naturally; in the absence of air these will prevail and decompose the organic material. Anaerobic decomposition gives rise to methane. Methane is a potent green house gas which over a period of 100 years is thought to be 23 times more harmful to the environment than carbon dioxide (CO2). Therefore, where anaerobic digestion is employed as a treatment method, it is vital that the methane is captured and used. One such example of a controlled anaerobic digestion system for organic waste is biogas digester. These are most often used
Managing organic municipal waste
for human and animal waste, but there are examples of their successful use with organic waste. Innovative small-scale biogas plant in India The Appropriate Rural Technology Institute (ARTI) in Pune, India recently won an Ashden Award for its design of a biogas digester which can be fed with household organic waste. It can also accept spoiled grain, fruit, oil cake and so on. Costing around $350, the plant is made using two plastic water containers. One contains the digesting materials, the other is inverted to capture the gas. Users apply 1Kg of organic waste daily and add 10 litres of water. In return the plant will produce around 250 g of methane per day, enough to cook a full meal for a family of five. This is an impressive input: output ratio. The gas could also be fed into a generator to provide around 1 kWh of electricity. One of the main advantages of using organic waste as the feedstock compared with dung or excreta, is that its calorific value is considerably higher. After all, it has not already been digested using microorganisms in a cow’s intestine! See www.arti-india.org Biogas is a source of energy with one of the lowest relative carbon footprints of all. Methane can be burnt cleanly on simple stoves, producing mainly carbon dioxide and water, making it a very clean household fuel. As with all organic waste processing t echniques, one of the most significant challenges of using digesters is ensuring the quality of raw materials. Contamination from plastic, sand and soil can reduce the effectiveness of the plant, and chemical contamination could compromise the microorganisms, as well as contaminate the resultant compost. Compost and composting Compost is a stable, dark brown, soil-like material which can hold moisture, air and nutrients. Contrary to popular belief compost does not smell rotten: often it will smell as fre sh as a forest floor (which is, of course, naturally-made compost). Compost contains some plant nutrients including nitrogen, phosphorus and potassium (NPK), though not as much as animal manure or chemical fertilisers. Compost can also contain a range of minerals and microorganisms beneficial to plant growth. However, its main benefit is as a soil conditioner. Soil is made up of sand and 'humus’: stable organic matter which retains nutrients and water. Adding compost to soil can lessen the need for c hemical fertilisers because it holds nutrients in the soil, it can also help reduce soil erosion, and improve the structure of the soil thus benefiting drainage and plant roots. Compost is a product of controlled aerobic decomposition of organic matter m ade using aerobic microorganisms, insects and worms. Microorganisms thrive in a moist, warm environment with an abundance of organic matter and air. If conditions are too hot, cold, wet or dry, the composting process will be compromised. The activity of the microorganisms generates heat which can act to kill pathogens and denature seeds. The composting process can take as little as two months. Ideally compost is matured for 3 – 4 months before use. In cold weather, high altitudes or very dry conditions, the composting process may slow or even stop.
Managing organic municipal waste
Compost - technical data The ideal moisture level of raw materials for compost is 40%. If they are much drier, microorganisms cannot access nutrients and composting will slow or stop. If it is wetter anaerobic microorganisms will prevail. The centre of a compost heap should reach temperatures of around 65°C. Turning compost, although not always necessary, can help ensure that all organic matter has been exposed to high temperatures during production. The carbon: nitrogen (C:N) ratio is important for microorganisms to thrive. Generally 'brown materials' such as wood chips and sawdust are high in carbon, while 'green materials' such as leaves and grass are high in nitrogen. The ideal ratio is between 25:1 and 40:1. This can often be achieved by a 50:50 mixture of green and brown materials, though this differs according to the exact types of waste.
There are many methods of making compost, ranging from small-scale home composting techniques to large-scale industrial plants requiring significant capital investment. The following box presents some of those most commonly used. Technologies may be selected according to a number of criteria, including the volume of raw materials available, budget, land availability, the cost and availability of water and electricity, and costs of labour. The nature of the market for compost may also affect technology choice.
Methods of making compost
Barrel composting (Dhaka, Bangladesh) This barrel is installed in a low-income area in Dhaka, Bangladesh. It receives organic waste from around four families. High-quality compost is made because the waste is uncontaminated. It is sold to a local NGO. Each barrel can produce around 160 kg of compost before requiring emptying, from around 600 kg of organic waste. Compost is sold at Tk2/ Kg (around US$0:03). In view of generation rates, this could generate an income of around Tk30/ family/ month. Barrels cost around Tk2000, meaning relatively long payback periods. Vermi composting (Bais City, Philippines) Vermi composting predominantly uses worms to digests the waste, rather than microorganisms. Raw materials are spread daily in thin layers and cannot be piled very high, so the technique requires much more space than other methods. Worms are also more vulnerable to extreme temperature and contamination than microorganisms. One of the advantages of vermicomposting is the high nutrient content of the product.
Managing organic municipal waste
Pit composting (Pune, India) Biodegradable waste is placed in shallow pits and left to decompose for several months. This method is very simple, often practiced in public parks or domestic gardens. In rainy conditions it is susceptible to water logging.
Silke Rothenberger Jonathan Rouse Silke Rothenberger Silke Rothenberger David Kuper
Manual windrow composting (Dhaka, Bangladesh) A windrow is a convenient way of piling organic matter for composting in long rows with a triangular cross section. Windrows can make fairly efficient use of space, and turning compost relatively easy. In this case they are turned manually to allow sufficient air supply. The aerobic condition allow the compost to mature within three months. Mechanical windrow composting (Luxor, Egypt) This system is comparable to the manual windrow composting but is applied at larger scale as mechanical equipment is used. Mixed waste is sieved prior to composting. The organic waste is piled onto long windrows, which are frequently turned mechanically with a turning machine.
Compost chute (Kandy, Sri Lanka) This chute composting plant is basically a long tube. Waste is fed in at the top. As more waste is added, over a period of a few months, mature compost emerges at the bottom. Gravity drives this process, which involves minimal mechanisation. Chimneys draw air up through the compost. This chute was developed by a partnership between a university, NGO and municipality. High-tech aerated static pile composting (Bali, Indonesia) Instead of a manual or mechanical turning of the windrow, in this example the pile remains unturned. Air is pressed through the material through valves using a motor driven ventilator. In Europe, the piles are additionally covered by a geo-textile, reducing moisture losses.
Adapted from 'Marketing Compost’, Sandec 2008
By Jonathan Rouse, Published by Practical Action on 02/02/02
Let us know which of the options below best describes you and we'll direct you to the most relevant content.
Practical Action uses technology to challenge poverty, working with poor women and men around the world.
Explore our work by Country
Explore our work by Technology
+44 (0)1926 634400 email@example.com
© Practical Action