In the 1960s, many developed countries began to alleviate the pollution in surface water by constructing municipal sewage infrastructures and implementing phosphorus discharge restrictions on production sectors. The giant infrastructure of centralized wastewater treatment has drastically reshaped the phosphorus cycle within modern cities. Despite high economic costs, its environmental benefits with regard to removal of phosphorus from was-tewater are far less than satisfactory worldwide. However, some progress has been achieved in European countries and the United States. As the centralized control strategy just removes 'pollutants' into sewage sludge rather than promotes a recovery and recycling of resources, including phosphorus, it does not really solve the long-term ecological problem. The costly and rigid infrastructures have significantly reduced agricultural reuse of urban human excreta and contributed to a disconnect of nutrient cycles between urban areas and croplands. Unfortunately, no available technologies for stable recovery and recycling of phosphorus are likely to be successfully commercialized in the near future. Hence, most of the phosphorus in urban human wastes is not subject to efficient recycling and is permanently lost from the land.
Proposals for recovery of phosphorus via decentralized source-separated strategies have received increasing attention since the mid-1990s. This decentralized and downsized sanitation concept, focused on ecologically sustainable and economically feasible closed-loop systems rather than on expensive end-of-pipe technologies, advances a new philosophy. It departs from the one-way flow of excreta from terrestrial to aquatic environments, as introduced by the conventional flush-and-discharge sewage system. The new alternative separates nutrients and domestic used water at source and handles both components individually based on material flows approaches. Thus, it avoids the disadvantages of conventional wastewater solutions and enables and facilitates nutrient recycling. Although the reinvention and transition of urban wastewater systems poses a major challenge, it does provide a promising prospect for future phosphorus recovery and recycling in an ecological and economic efficient way. (Detailed studies are essential as a first step, inter alia, of technological, organizational, economic, and social aspects. In addition, the involvement of multi-stake-holders, such as residents, building owners, farmers, politicians, officials, and other interested parties, from the start seems essential. All these problems cannot be solved overnight, as it requires nothing less than a paradigmatic change of a large sociotechnical system.)
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