This book begins by providing information about wastewater treatment and its various application, especially in agriculture sectors. Some information about wastewater use and regulation in Saudi Arabia is also discussed. The main characteristics of the natural oxidizing pond system and the activated sludge procedure are described and their performance in the abatement of physico-chemical, bacteriological and virological pollution is discussed. Next, the authors describe and discuss the most common wastewater treatment processes and the importance of the activated sludge process in wastewater treatment, as well as introduce the idea of implementing microbial fuel cells into the procedural design of wastewater treatment for resource recovery. This compilation also covers strategies aimed at minimising the expense of water during every stage of energy and biofuel production, as well as forms of reuse and recycling that guarantee the utilization of wastewater in order to develop the circular economy in biogas and bioethanol plants. Characteristics of cultivated bio-granules including the number of granules, density, sludge volume index, settling velocity, and physical strength are studied and their performance in treating petroleum and other types of wastewater is investigated. Following this, the authors provide an overview of the achievements of studies in which anaerobic sequencing batch biofilm reactors have been used to co-digest agro-industrial wastes for the production of methane, with a focus on operational strategy and perspectives for energy estimations. The closing study discusses the characteristics of grey water and available methods for its recycling and reuse. Grey water is the wastewater from homes, excluding black water, which typically makes up 50 to 80% of wastewater.