Do you know the value of your city's sewage?
As the idea of the circular, resource-efficient economy gains ground, urban planners are increasingly asking not just how much it will cost to clean up "human waste", but also how much that "waste" might be worth. As part of our Initiative on Sustainable Sanitation, we've been developing a tool that can help city planners understand how they could profit from productive reuse of a wide range of urban organic waste streams.
Every day, an estimated 9.5 million m3 of human excreta and 900 million m3 of municipal wastewater are generated around the world – enough to fill 363,800 Olympic swimming pools. A large share of these goes into lakes, rivers and other waterways with little or no treatment – a major health risk and a relentless pressure on vital ecosystems.
But these waste streams are full of resources that could help meet many of the defining challenges we face: water scarcity, energy access, climate change, the shortfall in food production, while also creating green jobs in the process.
Sieving compost in preparation for sale at a composting facility. Photo by SOIL Haiti.
From nice ideas to numbers
Investment in the kind of technologies and services that could make this possible demands some credible numbers. To date, urban planners and managers have lacked tools to estimate the potential for resource recovery at a city scale. The few that have been available have focused on a single waste stream or resource reuse option, such as energy generation. Also, they have generally been designed for the large-scale waterborne (sewerage) systems typically found in the Global North; not so useful for the fast-growing cities of the Global South, where on-site sanitation systems (such as pit latrines) are common and likely to remain so.
Our Resource Value Mapping (REVAMP) tool seeks to fill these gaps. The user (probably a city planner, perhaps an entrepreneurial investor, maybe a researcher) only needs to fill in the amounts and characteristics of the different waste streams in the city, and REVAMP will estimate how much energy (biogas or solid fuel), fertilizer and animal protein feed (from insects grown on sludge) can be generated from that waste. It can also give approximate revenues that could be expected from the different reuse options, reflecting local market prices.
Testing in Kampala: a new way to save the forests?
REVAMP was tested in Kampala, Uganda; a city of 1.5 million people, 90% of whom use on-site sanitation systems like pit latrines and septic tanks. Around 78% of Kampalans rely on firewood for their household energy needs, which is a major cause of deforestation in the area.
According to REVAMP calculations, nearly 100% of this firewood could be replaced with the fuel pellets made from the city's faecal sludge, sewage sludge, and organic municipal solid waste. Alternatively, the city's waste could produce enough biogas (361,231 Nm3 annually) to substitute well over half of firewood use, and around 370 tonnes of nutrient-rich residues that can be used instead of synthetic fertilizer – which many farmers cannot afford. Other alternatives could be to create animal feed ingredients (198 tonnes of black soldier fly larvae, plus residues for fertilizer).
As well as helping to build a business and development case for reuse-oriented, citywide systems, this kind of data also provides an incentive for investing in more efficient collection covering the entire city.
An open invitation
REVAMP is still in its infancy. While the calculations are based on the best scientific literature, and it already has a uniquely useful set of capabilities, we believe it could be even better. We hope to keep refining the tool by testing it in other cities. We also aim, for example, to find ways to factor in the specifics of different treatment and resource recovery technologies to better support real-world investment decisions. And we hope to develop the user interface (currently based on spreadsheets) to make it even more useful. We welcome partnerships and collaborators who can help us do it.
Urban trash and sewage are too valuable to waste. It's time for a REVAMP.
Authored By: Daniel Isaac Waya Ddiba is a master's student at KTH, the Royal Institute of Technology in Stockholm. Kim Andersson is a research fellow at SEI, and Arno Rosemarin is a senior research fellow at SEI.