ABSTRACT
Secondary treatment usually consists of biological treatment of primary ef€uent wastewater. The objectives of secondary treatment are to reduce the biochemical oxygen demand (BOD) and suspended solids of the ef€uent to acceptable levels. In some cases, nutrient removal may also be an objective. Depending on discharge limits, the secondary ef€uent may be discharged to surface waters after disinfection, or may proceed to tertiary treatment. Two major categories of biological treatment processes are (1) suspended growth and (2) attached growth processes. In this chapter, aerobic suspended growth processes for BOD removal will be described in detail, with major emphasis on the activated sludge process. Membrane biological reactors (MBRs) and land-based systems, such as ponds and lagoons, are presented toward the latter part of this chapter. Attached growth processes are discussed in Chapter 9. Biological processes used for nitrogen and phosphorus removal are presented in detail in Chapter 13.
