Sustainability is a utopian ideal pursued by many different ways and by many different actors 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 lineal 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. Wastewater 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 of increasing concern due to the complexity of such task and the rapidly changing environment of the modern society.

The thesis “Development of strategies for waste valorisation in WWTPs: Consorci Besòs Tordera case study” by David Palma Heredia aims at developing and applying 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. Two main key strategies of circular economy implementation re selected. For each of them, adapted methodologies are developed and successfully applied to the case study as end-user tools. Such case study comprises a heterogeneous environment with 26 WWTPs, managed by the public entity Consorci Besòs Tordera (CBT). Besides, 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.

Results obtained contribute to implement circular economy solutions within WWTPs. On one hand, the author developed a tool for the generation of integrated and semiautomatic assessments of the implementation of new processes related to sludge valorisation; this challenge was solved through the development of an integrated Decision Support System (DSS) and WWTP process simulator. On the other hand, David Palma tackled the optimisation of anaerobic digestion and enhancement through co-digestion coupled with centralised treatment of non-digested sewage sludge; this was approached by developing an innovative optimisation algorithm based on the combinatorial methods of Ant-Colony Optimisation and includes logistic planning related issues.

The thesis, directed by Dr. Manel Poch Espallargas and Dr. Miquel Angel Cugueró, was carried out at Consorci Besòs Tordera and UdG research group LEQUIA within the framework of the Industrial Doctorate Program by Catalan Research Agency AGAUR. The defence took place on Monday 8^th February 2021.