Wastewater Reuse for Urban Agriculture: A Path to Food Security in Colombo’s Slum Areas

A study by Japanese and Sri Lankan researchers finds strong community interest in using treated wastewater for irrigation in Colombo’s underserved settlements, but lower acceptance of biosolids as fertilizer. The research urges space-efficient farming, proper treatment, and public awareness to turn wastewater into a safe, sustainable resource.

Researchers from the Institute for Future Initiatives at the University of Tokyo, the Institute for the Advanced Study of Sustainability at the United Nations University, and the Global Research Centre for Advanced Sustainability Science at the University of Toyama have explored whether Colombo's poorest neighborhoods can transform wastewater from a liability into a resource. Their study examines the social feasibility of reusing treated wastewater and biosolids for urban agriculture in an underserved settlement of Sri Lanka's commercial capital. Colombo's canal network, 65 percent of which is rated poor or very poor in quality, is burdened by decades-old sewerage infrastructure that fails to serve nearly half the residents. The case study focuses on Borella's Saranapala Himi Mawatha, a tightly packed strip of low-income housing flanking a stagnant, refuse-filled canal. Here, untreated wastewater is routinely dumped into the waterway, yet some residents already grow plants in containers, an indicator of both need and opportunity for localized, circular solutions to waste and food insecurity.

A Community Willing, but Cautious

In April 2024, researchers surveyed 166 households living within 30 meters of the canal. Most respondents were women, and a large majority expressed interest in farming. Asked about using treated wastewater for irrigation, 62 percent were supportive, but enthusiasm dropped to 40.4 percent when it came to biosolids as fertilizer. While openness to recycling water is high, skepticism surrounds the idea of fertilizers derived from human waste. Statistical analysis revealed that smaller households, strong interest in farming, and motivations such as accessing healthy food or seeking leisure and relaxation were linked to greater willingness to use treated wastewater. For biosolids, factors such as education level, gender, and motivations like generating income or improving nutrition played a role, with women showing slightly higher acceptance than men, and willingness declining with age.

Small Spaces, Big Solutions

The study underscores that technical feasibility is only part of the challenge; social and spatial realities matter. In Colombo's underserved settlements, women and older residents often manage water, food, and nutrition, making them central to adoption. But space is scarce, and open land for cultivation is almost nonexistent. To overcome this, the researchers recommend space-efficient systems like "sack gardening," successfully used in Bangladesh and Kenya, which enables households to grow vegetables in tall sacks using compost and greywater, providing a year-round supply in tight spaces. Another opportunity lies in converting underused public or semi-public spaces, such as canal banks, edges of playgrounds, or strips under power lines, into community gardens. However, over 80 percent of Colombo's underserved settlements lack active community-based organizations (CBOs), so establishing or revitalizing them is crucial. Religious leaders and local officials could help foster trust and participation, ensuring these spaces are managed effectively.

Balancing Promise with Risk

The potential benefits of wastewater reuse are clear: reduced water pollution, lower fertilizer costs, and better food security. But the health risks cannot be ignored. Wastewater and biosolids may contain pathogens like E. coli, Salmonella, and helminths, which can survive on crops and in soil, as well as heavy metals such as cadmium, which can accumulate in soils and crops over time. Prolonged exposure can lead to serious health problems. Treatment is therefore essential. The researchers highlight constructed wetlands as a low-cost, low-maintenance option, especially suited for settlements near canals. Filtration systems using sand, crushed lava rock, and activated carbon can also effectively remove organic matter and nutrients from greywater. For fecal sludge, co-composting with organic waste or processing into fertilizer pellets could create value, but transporting sludge from dense urban settlements to farms remains a logistical hurdle.

From Awareness to Action

The authors emphasize that public health safeguards must accompany any adoption of wastewater reuse in agriculture. They recommend comprehensive awareness campaigns that teach safe handling practices, promote the use of gloves and boots, encourage post-harvest washing of crops, and clearly explain the benefits and risks. Establishing a regulatory framework adapted to low-income urban contexts would help manage environmental and health hazards while building public trust. Many residents' views may be shaped by incomplete or inaccurate information about treatment processes, so transparent communication is key.

While the research centers on a single settlement, its insights resonate far beyond Colombo. It demonstrates that urban agriculture can be more than just a food source; it can function as a decentralized, nature-based wastewater treatment strategy that addresses pollution, boosts local economies, and enhances community resilience. Yet the authors caution that social feasibility is only the first piece of the puzzle. Environmental sustainability, institutional capacity, technical viability, and public health considerations must also be evaluated before scaling up.

If communities, technology providers, and authorities can align, Colombo's polluted waterways could shift from being vectors of disease to channels of renewal. The transformation would require more than engineering; it would demand trust, participation, and locally adapted farming techniques. In that vision lies a model for other cities struggling with the intertwined challenges of urban poverty, water pollution, and food insecurity: turning waste into opportunity, and canals from dumping grounds into lifelines.