Water and Wastewater Engineering

Part II of the book deals with the treatment processes that are used in water or wastewater treatment plants. Water for domestic or municipal use is drawn from surface or groundwater sources and depending on the quality desired for drinking purposes, it is treated in a water treatment plant. After treatment, the treated water is supplied to consumers through a distribution network. Wastewater is generated by consumers after use and is collected in sewer systems from where it is transported to a wastewater treatment plant. The treated wastewater is then discharged on land, into a surface water body or into the sea. However, due to water scarcity and costs of wastewater treatment, water is now increasingly reused for irrigation or industrial purposes or recycled for other purposes. In some cases, wastewater that has been treated to tertiary or even higher levels can be used for drinking purposes. Two examples of such complete recycling of water are the public water supply systems in Singapore since 1998 and Namibia since 1968 (Pruden 2014).

Water treatment processes were mainly physico-chemical processes until a few decades ago. This was done based on an intuitive understanding that one of the major objectives of water treatment was elimination of microbes, especially pathogens, and therefore, all biological processes were kept out. However, increase in water quality requirements (removal of TOC was not necessary until regulations for disinfection by-products became effective), and the recognition that processes like filtration (porous media and membrane) and disinfection are inherently a mix of physical, chemical and biological processes has led to an understanding that any categorization is necessarily arbitrary. A specific example is the use of biofiltration for enhanced removal of TOC along with turbidity in water treatment and the recognition that biological growth occurs naturally in porous media filters and can be controlled and used to advantage. It should also be noted that physico-chemical processes are generally considered to be either primary or tertiary levels of treatment for wastewater. A list of the treatment processes, included in Chapters 5 and 6, is provided in the following table along with their points of application in water and wastewater treatment.