Processes which remove the harmful contaminants from water primarily through the use of some bacteria, protozoa or other microorganisms are studied under biological wastewater treatment. These microorganisms cause breakdown of organic pollutants in water to create a flocculation effect. This enables the organic matter to settle out of the solution. Biological wastewater treatment processes are classified into three categories, namely, aerobic, anaerobic and anoxic. Aerobic wastewater treatment technologies include activated sludge systems, fixed bed bioreactors, membrane bioreactors and biological trickling filters. Anaerobic technologies are divided into upflow anaerobic sludge blankets and anaerobic digesters. Most of the topics introduced in this book cover new techniques and the applications of biological wastewater treatment. The various studies that are constantly contributing towards advancing technologies and evolution of this field are examined in detail. As this field is emerging at a rapid pace, the contents of this book will help the readers understand the principles, modeling and design of biological wastewater treatment.