This book deals with natural treatment systems and the challenges the water industry faces in dealing with sustainability and the realisation of reaching Net Zero by 2030. Surface waters are all under threat, with freshwater ecosystems now facing unprecedented levels of contamination, even after a century of ever stricter legislation and regulation. The increase in population and especially in urbanization without sufficient planning and investment to ensure adequate wastewater collection and treatment coupled with the need to reduce greenhouse gas emissions associated with wastewater treatment is leading to a crisis in wastewater treatment in many countries. Natural treatment systems which use plants and soil micro-organisms are very much nature-based solutions and wherever applicable might offer sustainable and low emissions options for a range of wastewater problems protecting surface waters as well as creating new habitats to support and enhance wildlife diversity. In terms of circularity, natural treatment systems have the potential to produce a staggering array of useful and valuable by-products, including high-value compounds for the pharmaceutical industry.