Planning the centralization level in wastewater collection and treatment: A review of assessment methods

Centralized wastewater treatment has been the favorite wastewater treatment strategy until a few decades ago, in order to exploit each possible scale economy. Nowadays, water stress and resource scarcity, due to population growth and climate change, call for water reuse and resource recovery, and these goals do not often find in centralization the best solution. Today, the reuse of reclaimed water can take place at different levels and rep-resents an option of primary importance; therefore, in some cases, centralized systems may be economically and environmentally unsustainable for this additional purpose, and the search for the optimal infrastructure centralization degree must take into account these goals. This review analyzes studies that investigated the search of the best centralization level of wastewater collection and treatment, focusing on the methodologies applied to take the decision and highlighting strengths and weaknesses of the different approaches and how they have evolved over time. The final goal is to guide planners and decision-makers in choosing and handling the most suitable method to assess the centralization level of wastewater infrastructures, based on the objectives set out. The reviewed studies cover a period of twenty years. The differences found along this time span show an ongoing paradigm shift towards hybrid systems, which combine centralized and decentralized wastewater treatments that promote the storage of treated water and various forms of local water reuse and resource re-covery. The protection of human health and the environment (which primarily promotes water reuse and resource recovery) has become the main challenge of wastewater treatment systems, that will presumably improve further their economic, social and environmental sustainability to achieve urban development in the context of the water-energy-food security nexus.