Sustainability is an uprising concept amongst many sectors of activity of the human society, stirred by the same motivation: a global resource crisis that affects multiple sectors of human activity and can potentially mean an overall loss of life quality for humankind. One well known way to try to achieve a higher degree of sustainability is through a paradigm shift from linear to circular economy, which allows reducing waste and increasing available resources simultaneously.

Since water is one the most essential resources, the sanitation system of the urban water cycle is a prominent strategic sector where the implementation of circular economy can be performed. Waste water treatment plants (WWTPs), as a water-energy-food nexus, can provide different recoverable resources, such as clean water, energy and nutrients through different sustainable solutions. However, proper development planning for WWTPs, coupled with such sustainable solutions, is an increasing concern due to the complexity of such task and the rapidly changing environment of the modern society. The objective of this thesis is to develop and apply to a real case study a method to allow easier WWTP development planning and implementation of circular economy related processes in the wastewater treatment sector. In the WWTP system, sewage sludge is the main waste produced. Hence, the processes regarding its production and transformation are the focus of the thesis.

As detailed in Chapter 3, the applied methods are based on the concepts of system analysis and development of Decision Suport Systems (DSS). Overall, two main key strategies of circular economy implementation are selected, and for each of them adapted methodologies are developed and successfully applied to the case study as end-user tools. The methodology is validated through its application to a real case study, which comprises a heterogeneous environment with 26 WWTPs, managed by the public entity Consorci Besòs Tordera (CBT).

In Chapter 4, a comprehensive analysis of the case study infrastructure and associated processes is performed. A benchmark exercise is executed to assess the efficiency and potential of the case study in regards to international references, with special focus on the sewage sludge line.

In Chapters 5 and 6, the two key strategies selected in Chapter 3 are developed and applied. On the one hand, a tool is developed for the generation of integrated and semiautomatic assessments about the implementation of new processes related to waste valorisation (focused on the sewage sludge of sanitation systems); this challenge is solved through the development of an integrated DSS and WWTP process simulator that uses a hierarchical set of Key Performance Indicators (KPIs) for the evaluation of each scenario. On the other hand, it is tackled the optimisation of anaerobic digestion and enhancement through co-digestion coupled with centralized treatment of non-digested sewage sludge; this is approached by developing an innovative optimisation algorithm based on the combinatorial methods of Ant[1]Colony Optimisation, which includes logistic planning related issues.

Chapters 7 and 8 summarize a general discussion over the developed toolboxes, their limitations and potential impacts and mark the path for further improvement of WWTP’s development planning and optimisation of circular economy implementation.