摘要：Humans and domestic animals produce large quantities of fecal waste, projected to be at least
4.6
×
10
12
kg
globally by 2030, with animals producing six times the waste of humans (
Berendes et al. 2018). Much of this waste is poorly managed in low- and middle-income countries (LMICs), where sanitation access remains limited and humans and animals may live in close proximity. This contributes to a high burden of sanitation-related disease (
GBD 2019 Risk Factors Collaborators 2020), including diarrheal diseases and a range of other outcomes.
Productive use of human and animal waste presents a compelling option for safe management of excreta. Such strategies create a potential “win–win” through valorization: If value-added products can be reclaimed from nutrient- and energetically rich “waste,” these materials become an economic resource, incentivizing their removal from the local environment (
Harder et al. 2020;
Trimmer et al. 2020). Creating value from fecal waste has the potential to support livelihoods, primarily via agricultural use or energy production (e.g., biogas or biomass fuels). However, because typical LMIC settings are also characterized by high burdens of enteric infections (
Kotloff et al. 2013), any use of excreta carries risks of exposure to those contacting the material if pathogens are not inactivated. Enteric pathogens are highly transmissible through well-known direct and indirect pathways (
Ginn et al. 2021;
Penakalapati et al. 2017;
Wagner and Lanoix 1958).
In Amato et al. (
2021), the authors describe a large-scale, longitudinal study of domestic biogas production in Nepal, an increasingly common intervention to reduce household reliance on wood-burning stoves and the attendant risks of acute lower-respiratory infections (ALRIs) while also delivering sanitation benefits (
Manyi-Loh et al. 2013). These systems use human and animal excreta as feedstocks, and operation requires close and frequent contact with these wastes. The research team observed a clear increase in self-reported diarrhea in children whose households were actively using biogas digesters, which they attribute to increases in excreta handling by members of the household. The authors further observed a greater risk of diarrhea attributable to household biogas use among children who were exclusively breastfed, a finding that warrants further exploration of transmission pathways related to caregiver–child interactions. Repeated exposures to fecal pathogens and frequent diarrheal episodes in very early childhood can result in malnutrition, stunted growth (
Robertson et al. 2019), and cognitive deficits (
MAL-ED Network Investigators 2018). The authors rightly conclude that strategies for productive use of fecal wastes must be accompanied by a careful assessment of risks attendant on direct users, household members, and the community at large.
Previous studies have highlighted risks of sanitation innovations that involve contact with excreta. For example, in a study from El Salvador (
Corrales et al. 2006), investigators reported that contact with posttreatment waste from dry “Eco-san” composting latrines resulted in increased infection with
Ascaris and
Trichuris—both relatively resistant to inactivation in composting—compared with pit latrine users who did not handle these materials. Excreta must either be sequestered from human contact altogether or effectively treated before handling to be safely used (
Humphries et al. 1997).
Since the Victorian era, sanitation planning has mainly focused on removing excreta from where people live, and this has resulted in substantial long-term gains in global public health (
Bartram and Cairncross 2010;
Cutler and Miller 2005). But the future of excreta management may include a greater emphasis on productive use, which will necessarily entail a paradigm shift away from the existing practice of moving excreta as far away from people as possible. It may well be advantageous to use excreta productively on-site—where wastes are generated—so that the benefits may be captured locally and the high costs for transportation (i.e., pipes, pumps, water, and desludging), treatment, and disposal are avoided. But this is a tall order: Excreta is hazardous waste, and treating it effectively and safely takes substantial energy and time, requiring costly infrastructure for containment and treatment.
Decentralized waste management cannot capture the economies of scale that make large-scale wastewater treatment possible, including in capital costs but also in operations and maintenance. There are limits to devolving environmental health responsibilities to households (
Ray and Smith 2021), often very poor households that may not be willing or able to spend the time and resources to manage excreta safely and effectively. Technologies may also originate with researchers far removed from the lived experience of intended beneficiaries and who are therefore poorly placed to determine what is and is not workable (
Kearns and Mulhern 2021). Innovation in this space is desperately needed to meet massive unmet needs in sanitation (
UNICEF and WHO 2021;
Berendes et al. 2017;
Jones et al. 2020), but it is important not to underestimate the costs of failing to treat fecal waste as the conveyor of disease that we know it to be. Sanitation’s original purpose and first responsibility—and its greatest legacy in human development—is to support public health and well-being. Proponents of biogas systems should carefully weigh the potential risks alongside the expected benefits, and proceed with caution: Public health interventions should firstly do no harm.
