Composting: a win-win way to reduce urban waste?

Sustainable management of urban waste is a major challenge for municipal authorities. Much urban waste is biodegradable and recyclable. A large proportion consists of organic matter that can be recycled into compost for urban agriculture. Promising new composting techniques are available but planners must learn why previous projects have failed.

Research from theInternational Water Management Institute explores the benefits and constraintsto composting for agriculture in cities in developing countries.

Municipal authorities lack capacityto collect, transport and safely treat waste. In many cities household wastecollection is restricted to wealthy neighbourhoods and most waste is dumpedalong road sides, in illegal dumps and in storm water drains.

Solid waste is a collectionof different materials including organic material, plastics, metal andtextiles. These can be handled in different ways to maximise recovery. Overhalf the waste generated in cities in low- and middle-income countries is bio-degradable.In African cities there is a very high proportion of organic matter. In theRwandan capital, Kigali, this reaches 94 per cent. It includes vegetable andfruit peelings, charcoal ash, garden refuse, leaf litter, animal dung, humanfaecal matter, sludge from septic tanks and waste generated by abattoirs,breweries and agro-industries.

Composting involves breakingdown organic waste materials by micro-organisms such as bacteria, protozoans,fungi and invertebrates. Composting complements chemical fertilisers for cropproduction and reduces the need for large imports of fertilizer. It reduces amountof waste for landfill and methane generation at landfills. When urban organicwaste recycling is decentralised there is reduced need for external inputs likeequipment, fuel and transportation. Centralised composting has not provedsustainable.

When humanexcreta is included in compostingmaterials, extra care is required in agricultural application. If compost pilesare badly managed dangerous organisms like nematodes and parasite eggs may survivethe decomposition process. They may then be carried to farmers’ fields whencomposts are applied to soils.

Urban composting projectshave often failed because of:

• insufficientknowledge and care in carrying out composting, resulting in unacceptable odours,attraction of rodents and heavy metal contamination
• lack of marketsfor compost where customers have been put off by low quality compost
• limited supportby municipal authorities who prioritise centralised waste collection servicesrather than promoting local recycling
• lack ofcoordination between the various institutions and sectors  concerned
• over-dependence on external funding.

Many urban and peri-urban areasare potential users of recyclable nutrients from municipal waste. In order toreap the potential benefits of composting and restore the natural nutrientrecycling loop, planners must:

• do much more thansimply choose the right locally appropriate technology
• focus on institutionaland management issues, delivery of feedstock (raw organic material),distribution of compost products and extension and education services
• invest inimproved segregation and sorting procedures to ensure purer organic waste forcomposting
• reduce health risks through provision of protective clothingand first aid facilities for compost workers, training in waste management and recycling.