Closing the rural-urban nutrient cycle?

Fast-growing cities in sub-Saharan Africa challenge rural food production. Consumption-related waste, however, ends up in urban latrines, drains or landfills, transforming cities into vast nutrient sinks. Composting this waste for agriculture could bring 'rural' nutrients back to the farmer. But how might this work?

The potential for recycling nutrients from organic city waste for use in agriculture is great. Estimates for Bangkok, Thailand, and Kumasi, Ghana, show that up to only 10 percent of the major plant nutrients entering the city is currently being reclaimed. The recycling of nutrients from urban areas and their channeling into rural agriculture would reduce on-farm soil nutrient mining and land degradation and enhance the life span of urban landfills. Although this is a win-win scenario for municipalities and farmers, there are still few success stories and negligible impact on a larger scale.

Most compost stations have encountered numerous technical, institutional and marketing problems. Often the products are too costly, even for most commercial farmers. Thus, successful compost planning involves not only technical, but also social and economic considerations, as shown in the diagram.

An International Water Management Institute (IWMI) project is using this approach in several municipalities in Ghana. Findings so far have revealed that there is sufficient good quality organic waste for composting, as well as options for successful community involvement in the operation of compost stations. In a pilot station, the production of a safe, high quality product from solid waste and faecal matter is being tested. Different processing and distribution methods to minimise running costs and enhance marketability will be studied. This is a crucial exercise as farmers' ability to pay for compost is still several times lower than the operational cost, and additional transport costs can put municipal compost beyond rural farmers' reach.

Willingness to pay for compost depends on the farmers' experience with compost and the availability of alternative soil inputs. Where cheap poultry manure is available in Kumasi, compost cannot compete. Peri-urban pineapple exporters around Accra, on the other hand, have no alternatives to inorganic fertiliser. The same applies to farmers in Tamale due to the lower availability of organics in the Savannah zone.

Alternative approaches for cost reduction require policy support:

• Subsidised composting Municipal subsidies could support compost production in peri-urban areas not connected to urban waste collection. Composting would be closer to farmers, local waste dumps would be given significant relief and funds otherwise spent on the transport of waste would be saved.
• Dump separation Certain areas of landfill sites could be designated to receive the least contaminated waste with the most organics allowing controlled mining by estate developers or farmers after natural composting. Although this is the cheapest method, quality control is needed.
• Source separation About 80 percent of the labour costs for sorting organic from inorganic waste are handed over to the household. Continuous education and a separate waste collection system are required.
• Straight dumping Dumping of fresh septic sludge on exhausted cereal fields after harvest allows in-situ mineralisation during the dry season or fallow period. While microorganisms will be killed, care has to be taken to monitor heavy metals. This technique could also be used in peri-urban reforestation projects or for land rehabilitation.

Pay Drechsel, Olufunke O. Cofie, and George Danso
UPA Project, International Water Management Institute
IWMI Ghana Office
c/o UST
Kumasi
Ghana

T/F + 233 51 60206

iwmi-ghana@cgiar.org

See also
'Waste composting for urban and peri-urban agriculture - closing the rural-urban nutrient cycle in sub-Saharan Africa', edited by P. Drechsel and D. Kunze, IWMI/FAO/CABI, Wallingford, 2001
'Low-cost, waste-derived organics for urban and peri-urban farmers: dilemmas of a transition period', Urban Agriculture Magazine, Vol. 6, by C. Furedy, 2002 (forthcoming)

Views expressed on these pages are not necessarily those of DFID, IDS, id21 or other contributing institutions. Copyright remains with the original authors but (unless stated otherwise) articles may be copied or quoted without restriction, provided id21 and originating author(s) and institution(s) are acknowledged. Copyright © 2005 id21. All rights reserved.