Co-composting: New take on traditional technology for better soils and sanitation

About 1.3 billion tons – that’s how much solid waste is generated in cities globally each year. By 2025, the number is expected to have almost doubled, reaching an estimated 2.2 billion tons. A lot of this waste is never collected and even less is treated and reused.

Growing cities, especially those in developing countries, also struggle to put in place sanitation systems. About 1.77 billion people depend on pit latrines. The lack of waste collection and treatment combined with growing sanitation challenges present serious threats to human well-being and the environment.

Therefore, researchers are investigating if this double challenge could be addressed by finding ways to reuse or recycle waste. Low-cost technologies and business models hold promise for turning waste into fertilizer, for example in the form of enriched compost, and can create value in terms of waste treatment savings, improved sanitation, soil rehabilitation, and job opportunities.

Combining waste streams can solve multiple challenges

A recently published paper, authored by researchers from the International Water Management Institute (IWMI), proposes co-composting as a relevant option for enhancing waste management in developing countries.

Composting is, of course, in itself not a new technology. For generations, families have composted kitchen scraps as a way to feed nutrients back to vegetables gardens, and in recent years, composting has even become a bit of a movement. The question is how to make this technology go to scale, while yielding the greatest possible environmental and social benefits. Co-composting may be part of the answer.

About 40-70% of municipal solid waste is comprised of organic matter, such as food waste, biowaste and garden waste. This organic waste can be co-composted with fecal sludge, collected from for example pit latrines, and with livestock manure. The result is a rich compost that returns nutrients and organic matter to the soil, providing a valuable material for agriculture, horticulture and landscaping.

“In low-income countries, where intensive agriculture provides the main source of income, co-composting has clear advantages for municipalities and farmers,” explains Olufunke Cofie, lead author and senior researcher at IWMI. “But, it may not automatically offer mutual win-win options for both parties unless it is properly planned and well coordinated.”

In addition, using compost to enrich soils improves organic carbon storage and hence overall functioning and productivity of an agro-ecological system.

Context matters for technology and business model choice

Co-composting of solid waste and fecal sludge is not yet widely adopted at scale, and uptake anywhere will depend on matching the country-specific context and socio-cultural conditions.

The best approach for developing a co-composting scheme can be determined by considering a multitude of factors, including demand, market, technology options as well as legal and institutional frameworks.

It is also in the context of selecting the most appropriate technology that value-adding activities can be explored. The quality of compost can be improved by adjusting the combination of compost feedstock or by adding other nutrient sources to best meet the demands of the crops and soils it is intended for. 

“The compost generated from waste and fecal sludge must compare well with other fertilizer materials,” says Cofie. “Otherwise, there is no market demand, and the model will collapse.”

In Ghana, Cofie and colleagues have supported the establishment of a public-private partnership to produce fertilizer pellets from fecal sludge and solid waste. Trials have proved that the resulting compost product, named ‘Fortifer’, is safe to use and generates yields that are comparable to or higher than inorganic fertilizers, while also improving soil health.

Public and private sector support required

The biggest challenge related to uptake of co-composting of solid waste and fecal sludge at scale is related to its economic viability.

Revenues from compost sales alone usually cannot cover a plant’s capital and operation costs, which means that subsidies from the municipality are needed, especially in setting up a plant. Composting, however, does reduce the time and money a municipality spends on waste disposal.

Finding ways to incentivize the private and public sector to collaborate on composting efforts, such as is the case in the public-private partnership for  ‘Fortifer’, may be the most promising strategy for wider adoption. One place to start would be to amend public policies and legal frameworks in ways that attract private sector investment.

The research paper “Co-Composting of Solid Waste and Fecal Sludge for Nutrient and Organic Matter Recovery” provides practical guidance to planners, researchers, development experts and practitioners seeking to advance co-composting initiatives in developing countries.