There is a trend towards decentralized source separation (DSS) for wastewater treatment and resource recovery. An assessment framework is required to assess whether implementing a DSS treatment over a conventional centralized one is advantageous. This framework needs to account for the performance of the wastewater treatment plant (WWTP) and the effect that resource recovery has on closely-linked sectors such as food and energy production. A framework is lacking that covers the economic dimension, the circularity, the nature reciprocity of resource recovery and that can be applied to real-life cases. A novel WFE framework has been developed here to compare a conventional centralized and a DSS-based WWTP. This novel WFE framework contains assessment methods that are reproducible, and applicable to real-life cases. It also accounts for the local climatic conditions that determine irrigation water requirements. The comparison results revealed that the need to construct new DSS infrastructure leads to a lower economic efficiency of water treatment. Further, chemical-intensive treatment reduces the DSS's material resource circularity and efficiency. Using heat pumps increases the energy use of the DSS WWTP, causing a reduction in water treatment energy efficiency. However, the advantages of DSS show up in the freshwater and nutrient efficiency of food production as well as in the energy self-sufficiency of the WWTP. The novel WFE framework contains indicators specific to water treatment and the food production sectors to improve inter-sectoral communication. Also, including the nature reciprocity assessment can help demonstrate the issue with treated wastewater discharge, especially in arid regions with low stream flows. It can potentially help improve the acceptance of treated wastewater-based reuse. To conclude, the novel framework helps to assess real-life case studies in a more integrated and holistic way. It can help make decisions related to decentralization and source separation by simultaneously considering the water treatment, energy production, and food production sectors.

This chapter presents a detailed description of the circularity and efficiency assessment methods to be used in the context of resource recovery solutions in the water sector. The resource recovery solutions are meant to produce multiple benefits, such as increased resource efficiency and decreased negative environmental impact, among others. The solutions need to be assessed and compared using a comprehensive set of criteria and indicators. This report discusses key concepts, explains methods, and briefly presents a spreadsheet tool developed for conducting the assessment. Some key concepts, such as circular bioeconomy and efficiency, are first introduced. Several resources may be recovered from the water sector, and thus, commonly recovered resources are classified. This is followed by an expansion of the scope of the material circularity indicator, a prevalent assessment method. This is done to apply the MCI to the resources relevant to the water sector. Equations for the resource categories are developed and presented. Efficiency is then defined as a ratio between the benefits and costs of a resource recovery solution. Two categories of cost indicators are introduced, followed by three categories of benefit indicators. By combining different cost and benefit indicators, several indicators such as simple material efficiency, service unit-based energy efficiency, and service unit ecoefficiency can be created. Circularity and efficiency assessment methods are explained with simple examples, and screenshots from the spreadsheet assessment tool are presented. Lastly, the assessment method is demonstrated on a real-life case study located in Italy. The case study involves reuse of treated wastewater (TW) for irrigation. The newly developed circularity and efficiency methods demonstrate the improvements in the circularity and efficiency resulting from TW irrigation reuse, along with pointing to the most crucial factors to be considered for such cases.

Resource recovery solutions can reduce the water sector's resource use intensity. With many such solutions being proposed, an assessment method for effective decision-making is needed. The water sector predominantly deals with biogeochemical resources (e.g., nitrogen) that are different from technical resources (e.g., industrial coagulants) in three ways: (1) they move through the environment in natural cycles; (2) they fulfil different human and environmental functions; and (3) they are subject to substantial environmental losses. Whilst several circularity assessment methods exist for technical resources, biogeochemical resources have received less attention. To address this, a well-established material circularity indicator (MCI) method is modified. This is done by redefining the terms: restoration, regeneration, and linear flows to create a new circularity assessment approach. The new approach is demonstrated in a real-life case study involving treated wastewater (TW) fertigation. The new approach reveals that using the original MCI method underestimates the circularity of resource recovery solutions involving biogeochemical resources. This is because, in the original MCI method, only the flows that are reused/recycled for human functions can be considered circular, whereas, in the new approach, one also considers flows such as N₂ emission and groundwater infiltration as circular flows. Even though these may not be reuse/recycle type flows, they still contribute towards future resource availability and, thus, towards sustainability. The modified assessment method shows that TW fertigation can significantly improve nitrogen and water circularity. However, careful planning of the fertigation schedule is essential since increasing fertigation frequency leads to lower water but higher nitrogen circularity. Additionally, collecting drainage water for reuse can improve nitrogen circularity. In conclusion, using the modified MCI approach, circularity can be assessed in a manner that is better aligned with sustainability.