Water and wastewater treatment normally take place in a series of continuous flow units, each designed to perform a step of the intended purification process - typically involving coagulation or flocculation, sedimentation or filtration, and disinfection. The flow pattern governs the residence/contact time, turbulence levels, collisions and shear to which different fluid portions are subjected in their passage through the unit. The efficiency of a given unit depends as much on the relevant physical, chemical or biological reaction as on the flow pattern taking place inside. This combined effect of flow features on process efficiency is often overlooked in teaching the design of water and wastewater treatment units, and so it is not uncommon to find treatment units in operating in a cost-ineffective way, causing health and environmental problems. This book introduces engineering students to concepts and practical measures associated with the rational design of treatment units, leading to more realistic and potentially optimal solutions for new units as well as for retrofitting existing units. Key basic concepts and suitable analytical tools are described, illustrated and worked through using practical examples. Engineering undergraduates and graduates should benefit from the book while undertaking standalone modules on the topic and/or supplementary classes of existing courses on unit treatment processes. The book may also be useful for technical and engineering staff involved in designing and/or retrofitting units for better cost-effectiveness and footprint reduction of the water and wastewater treatment sector.