This Third Edition of the book is thoroughly revised to present a detailed understanding of the principles of operation and design of domestic wastewater treatment plants. The book opens up with clearly stating the basic concepts of treatment of wastewater and the design considerations required for an efficient treatment plant. Thereafter, the design criteria for domestic wastewater treatment units are discussed which forms the basis of sizing of the treatment plant units. In essence, the text is strengthened to give detailed procedures for design computations of all units of a wastewater treatment plant with many solved numerical. Most common types of reactors used for physical operations and biological processes in wastewater treatment plants are also discussed in detail. The present edition includes a new chapter on “Biological Nutrient Removal” covering the aspects of nitrification and denitrification. This is now essentially legally required The book is intended for the undergraduate and postgraduate students of Civil and Environmental Engineering. It will also be useful to the practising and consulting engineers involved in the design of wastewater treatment plant and municipal corporation and pollution control authorities.