liquid-solids separation, usually in a sedimentation tank; and(3)a recycle system for returning solids removed from the liquid-solids separation unit back to the reactor. Numerous process configurations have e of the activated-sludge process is t he nction with phys. in sedimentation tanks. In ical and nemical processes treatment of wastewater, and posttreatment, including imary ent at removing settleable soluble, colloidal, and particulate ,and for biological ater from smaller-sized communities less operator-intensive he world that have hot climates eapplications, e used, including sequencing batch ed since its early conception for higher-quality effluents from nitrogen removal, and/or biological with large length considering the evolution of the activated-slud wastewater ndustrial wastes to domestic ne f the toxic effects of some of the ume allowed for greater d activate to be single-stage, complete-mix activated-slu he CMAS process ha ank and second stage for n ditch uring th f the tank substances 1 basin. The plug- eration s, so that the concentration of the reactor length. Although process configurations employing long, narrow tanks are commonly referred to as plug-flow processes, in reality, true plug flow does not exist 7-27-2 liquid-solids separation, usually in a sedimentation tank; and (3) a recycle system for returning solids removed from the liquid-solids separation unit back to the reactor. Numerous process configurations have evolved employing these components. An important feature of the activated-sludge process is the formation of flocculent settleable solids that can be removed by gravity settling in sedimentation tanks. In most cases, the activated-sludge process is employed in conjunction with physical and chemical processes that are used for the preliminary and primary treatment of wastewater, and posttreatment, including disinfection and possibly filtration. Historically, most activated-sludge plants have received wastewaters that were pretreated by primary sedimentation, as shown on Fig. 7-la and b. Primary sedimentation is most efficient at removing settleable solids, whereas the biological processes are essential for removing soluble, colloidal, and particulate (suspended) organic substances; for biological nitrification and denitrification; and for biological phosphorus removal. For applications such as treating wastewater from smaller-sized communities, primary treatment is often not used as more emphasis is placed on simpler and less operator-intensive treatment methods. Primary treatment is omitted frequently in areas of the world that have hot climates where odor problems from primary tanks and primary sludge can be significant. For these applications, various modifications of conventional activated-sludge processes are used, including sequencing batch reactors, oxidation ditch systems, aerated lagoons, or stabilization ponds. Evolution of the Activated-Sludge Process A number of activated-sludge processes and design configurations have evolved since its early conception as a result of (1) engineering innovation in response to the need for higher-quality effluents from wastewater treatment plants; (2) technological advances in equipment, electronics, and process control; (3) increased understanding of microbial processes and fundamentals; and (4) the continual need to reduce capital and operating costs for municipalities and industries. With greater frequency, activated-sludge processes used today may incorporate nitrification, biological nitrogen removal, and/or biological phosphorus removal. These designs employ reactors in series, operated under aerobic, anoxic, and anaerobic conditions, and may use internal recycle pumps and piping. Since the process came into common use in the early 1920s and up until the late 1970s, the type of activated-sludge process used most commonly was the one in which a plug-flow reactor with large length to width ratios (typically > 10:1) was used (see Fig. 7-la). In considering the evolution of the activated-sludge process, it is important to note that the discharge of industrial wastes to domestic wastewater collection systems increased in the late 1960s. The use of a plug-flow process became problematic when industrial wastes were introduced because of the toxic effects of some of the discharges. The complete-mix reactor was developed, in part, because the larger volume allowed for greater dilution and thus mitigated the effects of toxic discharges. The more common type of activated-sludge process in the 1970s and early 1980s tended to be single-stage, complete-mix activated-sludge (CMAS) processes (see Fig. 7-lb), as advanced by McKinney (1962). In Europe, the CMAS process has not been adopted generally as ammonia standards have become increasingly stringent. For some nitrification applications, two-stage systems (each stage consisting of an aeration tank and clarifier) were used with the first stage designed for BOD removal, followed by a second stage for nitrification. Other activated-sludge processes that have found application include the oxidation ditch (1950s), contact stabilization (1950s), Krause process (1960s), pure oxygen activated sludge (1970s), Orbal process (1970s), deep shaft aeration (1970s), and sequencing batch reactor process (1980). With the development of simple inexpensive program logic controllers (PLCs) and the availability of level sensors and automatically operated valves, the sequencing batch reactor (SBR) process (see Fig. 7-1c) became more widely used by the late 1970s, especially for smaller communities and industrial installations with intermittent flows. In recent years, however, SBRs are being used for large cities in some parts of the world. The SBR is a fill-and-draw type of reactor system involving a single complete-mix reactor in which all steps of the activated-sludge process occur. Mixed liquor remains in the reactor during all cycles, thereby eliminating the need for separate sedimentation tanks. In comparing the plug-flow (Fig. 7-la) and complete-mix activated-sludge (CMAS) (Fig. 7-1 b) processes, the mixing regimes and tank geometry are quite different. In the CMAS process, the mixing of the tank contents is sufficient so that ideally the concentrations of the mixed-liquor constituents, soluble substances (i.e., COD, BOD, NH4-N), and colloidal and suspended solids do not vary with location in the aeration basin. The plug-flow process involves relatively long, narrow aeration basins, so that the concentration of soluble substances and colloidal and suspended solids varies along the reactor length. Although process configurations employing long, narrow tanks are commonly referred to as plug-flow processes, in reality, true plug flow does not exist