Department Sanitation, Water and Solid Waste for Development

The co-composting plant developed and operated by IWMI, Sandec, KMA and KNUST
Municipal solid waste (MSW)
Drying bed for faecal sludge

Co-composting of Faecal Sludge and Municipal Organic Waste in Kumasi, Ghana

Context of the Project

The pilot project was conducted in Kumasi, the second largest city in Ghana with about 1 million inhabitants who generate 860 tonnes of solid waste and 500 m3 of faecal sludge per day. The rapid increase in urban population coupled with insufficient waste management strategies have led to the accumulation of considerable amounts of waste in and around the city. Inadequate waste management results in environmental pollution and poses health risks to Kumasi’s inhabitants.
Only 8% of the population have access to sewerage systems, the remaining inhabitants use on-site sanitation units. Since the city lacks appropriate treatment facilities for faecal sludge from onsite sanitation systems, the faecal sludge is discharges into the river without appropriate treatment.

In Kumasi, intensive irrigation of vegetables in open land spaces is practised at different sites over approximately 120 hectares of land. Backyard garden cultivation is also a common phenomenon as well as peri-urban cultivation of maize and plantain. About 90% of fresh leafy vegetables (lettuce, spring onions) consumed in Kumasi originate from production within the urban area and hence contribute to food security of the urban population.

Based on this background, a pilot project on combined composting of organic solid waste and faecal sludge was developed on the premise that:

The project was developed and is operated by a multidisciplinary team of engineers, agronomists, environmental scientist, and biologist from:

  • the International Water Management Institute (IWMI) in collaboration with
  • the Department of Water & Sanitation in Developing Countries (Sandec) of the Swiss Federal Institute for Environmental Science and Technology (Eawag),
  • Kwame Nkrumah University of Science and Technology (KNUST), Kumasi and
  • the Waste Management Department (WMD) of Kumasi Metropolitan Assembly (KMA).

Co-composting of municipal organic solid waste with faecal sludge could offer:

  • a viable and highly appropriate waste management option and in the long-term
  • a valuable alternative nutrient source and soil conditioner for urban and peri-urban agriculture.

The objective is to provide decision support to the municipal authorities on the mode of operation of a co-composting plant adapted to the opportunities and constraints of Kumasi and set an example to other African cities. Its further aim is to increase the awareness of farmers on the usefulness of co-compost as an alternative soil conditioner and nutrient source.

Download [pdf] a literature review on co-composting and results of the Kumasi case and preliminary recommendations

Specific Project Objectives and Activities

  • To acquire scientific knowledge on the technical and operational co-composting aspects
  • To study the socio-economic aspect of co-composting (market potential, perception and willingness to pay for compost by urban and peri-urban farmers)
  • To evaluate the effect of compost use on crop and soil
  • To enhance human capacity for urban waste management and related research with respect to co-composting.

In February 2002 a pilot co-composting plant was set up about 15 km from the city centre of Kumasi where various issues ranging from compost production to utilisation were studied. Research components include:

  • Efficiency of faecal sludge dewatering in drying beds;
  • Suitable mixing ratios of solid waste and faecal sludge and their impact on the composting process and compost quality;
  • Inactivation of pathogens in the co-composting process;
  • Utilisation of compost in urban agriculture and other market sectors,
  • Demand and willingness-to-pay for compost;
  • Economic analysis of co-composting as an option for sustainable solid waste management in Kumasi.

Results Obtained

Sludge Drying Beds

Faecal sludge needs to be dewatered before it is co-composted with organic solid waste. Ideally, the content of total solids should rise up to 20% within 7-10 days. Sludge drying beds constitute an effective option to obtain dewatered biosolids. However, the dewatering process lacks continuity of performance of the drying beds. This performance is dependent on several factors:

  • Heavy rainfall in the early stages of dewatering cycles
  • Depth of sludge layer
  • Structure and grain size of filter layer
  • Age and biochemical stability of delivered faecal sludge

Some cycles required a dewatering time of 27 days. Hence, further improvements of the process and tests are necessary. However, preliminary recommendations [pdf] on the design of the drying beds are given and can be downloaded as pdf-file.

Co-composting

After dewatering, the sludge is mixed with the organic fraction of municipal solid waste at a 1:2 or 1:3 ratio (faecal sludge:solid waste) for co-composting. Since the thermophilic windrow composting technique is well understood, the influence of faecal sludge on co-composting can be studied in detail. The co-composting period lasts between 10-12 weeks and undergoes a thermophilic phase (50-70 °C) of about 3 weeks. The high temperature ensures hygienisation of the compost (e.g. destruction of helminth eggs and pathogenic bacteria.) Thereafter, the temperature gradually decreases until the compost is mature.

The mature compost is tested for its nutrient content and hygienic properties: The few experiments monitored to date reveal that its nitrogen content is low (1%). This is not surprising as nitrogen losses are likely to be substantial due to the long retention time of the FS in the drying beds and as a result of the composting process itself (duration, high temperatures). Further studies will be conducted on improvement of nutrient contents.

The duration of the thermophilic phase and a sufficient maturation period influence the degree of compost hygienisation. The final products show and impressive inactivation of pathogens such as coliforms and helminth eggs. The results obtained have to be confirmed by further test cycles, as the composting process still has potentials for improvement and varies in temperature and duration.

Lettuce growing experiment
Use of Compost in Agriculture

A study on the perception of compost and willingness-to-pay was conducted among farmers. About 70% of the farmers had a positive attitude towards compost but are only willing to pay a low price for the product. However, the study also revealed that the farmers lack experience in the benefits and application of compost. Their willingness-to-pay is dependent on the alternative fertiliser in use. Furthermore, since alternative products such as poultry manure are currently used as fertiliser in Kumasi, they compete with compost. Therefore, additional measures like awareness building and training on demonstration fields seem to be necessary.

Preliminary field investigations show that the compost may be used at an application rate of 40 t/ha for intensively irrigated vegetable production, particularly on sandy soils lacking organic matter.