WASTES: Solutions, Treatments and Opportunities IV contains selected papers presented at the 6th edition of the International Conference Wastes: Solutions, Treatments and Opportunities, that took place on 6-8 September 2023, in Coimbra, Portugal. The Wastes conference, which takes place biennially, is a prime forum for sharing innovations, technological developments and sustainable solutions for waste management and recycling sectors worldwide, with the participation of experts from academia and industry. The papers included in this book cover a wide range of topics, including: - Management of waste streams - Environmental, economic and social aspects in waste management - Logistics, policies, regulatory constraints and markets in waste management - Waste-to-energy technologies - Life cycle assessment and carbon footprint - Biological treatment techniques - Waste treatment and valorization technologies - Circular economy and industrial symbioses - Smart technologies and digital tools in waste management - Recycling of wastes and resources recovery - Wastes refineries - Food waste management and bioeconomy - Plastic waste impacts, management strategies and solutions - Wastes as critical raw materials resources WASTES: Solutions, Treatments and Opportunities IV is aimed at academics and professionals involved in waste management and recycling sectors globally.