Much previous research shows that safe disposal of human waste has a positive impact on human wellbeing, whilst preventing the degradation of ecosystems. However, to date, the role that ecosystems themselves play in treating human waste has been largely neglected. We conceptualise the role nature plays in treating human waste – acting as a pipeline and/or treatment plant. We estimate that nature is treating ~41.7 million tonnes of human waste per year worldwide, a service worth at least 4.4±3.0 billion USD yr-1. We demonstrate the opportunities and challenges of quantifying these ‘sanitation ecosystem services’, using 48 cities across the globe as a worked example. In highlighting this, we are not marginalising the vital role of engineered infrastructure, but instead are promoting better understanding of how engineered and natural infrastructure interact within a circular economy. This is a promising route for further research and may allow adaptive design and management, reducing costs, and improving effectiveness and sustainability.