High-Rate Activated Sludge Process: New Insight into Energy Recovery from Municipal Wastewater

In recent decades, population growth, declining oil resources, and high production of municipal wastewater have raised many concerns for communities. Due to the shortage and limitations of fossil fuels, renewable energy sources are of particular importance. Municipal wastewater has become a remarkable source of energy due to its continuous production and treatment worldwide. The conventional activated sludge process has been used for more than a century to treat municipal wastewater. This process, despite its advantages such as high effluent quality and reliability, due to problems such as aeration, oxidation of organic matter present in the wastewater and waste production can hardly be considered as a sustainable method for wastewater treatment. In order to recover the energy present in the wastewater efficiently, it is necessary to change the process and up-concentrate the organic matter in it. Several physical, chemical and biological processes are used to increase the up-concentration of organic matters present in the wastewater and capture them onto the sludge surface, which improve energy recovery through anaerobic digestion. Due to the special disadvantages of physical and chemical processes, attention is drawn to the biological process. This study will provide a comprehensive assessment of the energy available in municipal wastewater and the methods used to increase the concentration of organic matter. It will also assess the limitations of physical and chemical methods and examine the biological process of high rate activated sludge, mechanisms, operational parameters, and recent studies on this process.