Summary Natural water and wastewater are characterized in terms of their physical, chemical, and biological composition. It is important to treat wastewater to remove bacteria and chemical pollutants, deplete natural oxygen levels in receiving water, and pose risks to animals and wildlife. Wastes may be subcategorized into major and minor elements, but it should be noted that in specific instances particular minor waste components may exercise significant polluting effects. This chapter presents the guidelines that should be taken into consideration for the selection of treatment technologies. It describes application and definition of the various stages of treatment and methods to perform specific functions. The chapter discusses the sources of pollution and control methods. It also discusses the general outline of pollutant specific for management in the siting and design of a refinery or terminal. The chapter also describes the background of the chemical oxygen demand method and the method used to calculate the theoretical oxygen demand.