Activated sludge is the most widely used biological wastewater treatment process globally to date, although its high energy demand makes it a major contributor of greenhouse gas emissions. Over recent decades it has been constantly modified and retrofitted to treat ever higher loads or improve effluent standards which have often resulted in even greater carbon emissions. Conventional activated sludge treatment is at a crossroads where new sustainable solutions are required if we are to protect the quality of our rivers and meet net-zero carbon targets. The book details current operation and design with special emphasis on the biological aspects of the process. From the microbial kinetics to the fascinating process of floc formation and development, the book explores the development of our understanding of the process looking at new sustainable designs, including biological nutrient removal and new aeration systems. Sludge separation problems and control options are explained, with a trouble-shooting guide to non-bulking problems. Environmental issues including noise, odor, aerosols, micro-plastics and nanoparticles are all reviewed, as is pathogen removal and the problem of antibiotic resistant genes and bacteria. The development of membrane bioreactors has increased process reliability and effluent quality, while integrated fixed-film activated sludge processes are more efficient and compact. The book concludes by exploring how activated sludge can become more sustainable, for example, by carbon harvesting and byproduct recovery. This interdisciplinary book is essential reading for both engineers and scientists whether training at university or practitioners and consultants in the wastewater industry.