Chapter 3 - Sewage treatment

Abstract

This chapter focuses on the process of sewage treatment. The waste disposed of by domestic households and industry is conveyed to the treatment works by means of pipes (sewers). In a traditionally combined sewer, all sewage, both foul and surface water, is conveyed in a single pipe. In case of separate systems, two pipes are laid in the trench for the sewerage system; one for the foul sewer, and the second for the surface water. Typically, raw sewage contains 99.9% water and 0.1% solids and the sewage treatment process fundamentally aims to separate solids from the water. The impurities in the sewage can be categorized as floating or suspended solids, colloidal solids, dissolved solids, and dissolved gases. The treatment of solids and sludge forms an important and costly area of sewage treatment and the impurities are removed from the sewage by using operations or processes, which are either physical, chemical, or biological in nature. The sewage treatment process has several stages, namely, preliminary treatment (physical); primary treatment (physical and/or chemical); secondary treatment (biological and/or chemical); and tertiary treatment (physical and/or chemical and/or biological). Wetland systems can be designed for each engineering stage, and for sludge treatment. However, constructed treatment wetlands are usually applied for secondary or tertiary treatment stages. Wetlands integrated in sustainable urban drainage systems (SUDS) are frequently used for preliminary and primary treatment purposes.