While the crucial health and environmental protection benefits of sanitation are generally understood among urban decision-makers and planners, most miss out on the fact that many components of sanitation waste streams are potentially recoverable and reusable, for example in the agriculture or energy sectors. Doing so can provide opportunities for new businesses and job creation, as well as making urban systems more resource-efficient.
Figure 2 shows typical sanitation waste streams, their resource content, and how these resources might be utilized. The availability and marketability of the resources in a given city requires a context-specific analysis, but the figure communicates the diverse opportunities. Some of these resource recovery schemes are already being implemented in several cases (e.g., irrigation with wastewater), while others are less common (e.g., excreta-based fertilizers) or still at the experimental level (e.g., protein feed production from insect larvae grown on human faeces).
In particular, human excreta as an alternative fertilizer source shows great potential. An adult on average produces 500 L of urine and 50 L of faeces per year, containing nutrients equivalent to about 10 kg of synthetic fertilizer. As a fertilizer its value is about USD 10, but its application represents an increase in agricultural yield of approximately USD 50, compared to not adding any fertilizer at all [
54]. For poor smallholder farmers this may represent an important addition to income. For example, in Dakar, safe reuse of nutrients contained in urine and faecal sludge from on-site systems (from 76% of the 2.5 million inhabitants) and organic municipal waste is estimated to be enough to fertilize 50,000 hectares of cultivated rice. This yields approximately 200,000 tons of rice per year, corresponding to a quarter of the annual rice imports for Senegal, significantly contributing to national food security and food sovereignty [
53]. In addition, the faecal sludge together with other organic solid waste (e.g., kitchen waste) can be pre-treated through anaerobic digestion generating biogas, with an annual net value in the case of Dakar corresponding to about 3000 m
3 of diesel or 16,000 km/day of bus journeys, also contributing to a more sustainable urban transport sector. There are many other emerging reuse cases to learn from around the world [
53]. Processed sewer and faecal sludge is being reused at various scales in different countries around the world [
55,
56]; likewise, biogas production through anaerobic digestion has become a widespread method in sludge processing in urban wastewater management, especially in high-income settings [
57]. To ensure sustainability, it is crucial to recognize potential health risks based on an understanding of potential exposure pathways [
53,
58]. Mitigating risks to human health can be achieved both through treatment of waste and non-technical measures (e.g., improved hygiene habits) in combination.
Thus, in planning sustainable sanitation, this approach should not be centred on technology or on the imperative of waste disposal, but could instead start from a focus on resources and their management.
Figure 3 shows how this novel order of logic for sanitation planning and design could be framed. Accordingly, the key questions that need to be raised are: (1) what resources are available in the waste streams; (2) what demand there might be for them; and (3) how they could be recovered [
53].