TM 5-814-3/AFM 88-11, Volume III
ACTIVATED SLUDGE PLANTS
13-1. General considerations.
The activated sludge process has been employed extensively throughout the world in its conventional form
and modified forms, all of which are capable of meeting secondary treatment effluent limits. This chapter
presents the different modifications of the conventional activated sludge process, including general bases for
design, methods of aeration, and design factors for aeration tanks, final sedimentation units and sludge
handling systems. Figures 13-1 through 13-4 are schematic diagrams of the conventional and modified
processes. The characteristics and obtainable removal efficiencies for these processes are listed in table 3-3.
All designed processes will include preliminary treatment consisting of bar screen as a minimum and, as
needed, comminutor, grit chamber, and oil and grease removal units. (See Winkler, 1981; Metcalf and Eddy,
13-2. Activated sludge processes.
a. Conventional activated sludge. In a conventional (plug-flow) activated sludge plant (fig 13-1), the
primary-treated wastewater and acclimated micro-organisms (activated sludge or biomass) are aerated in a
basin or tank. After a sufficient aeration period, the flocculent activated sludge solids are separated from the
wastewater in a secondary clarifier. The clarified wastewater flows forward for further treatment or
discharge. A portion of the clarifier underflow sludge is returned to the aeration basin for mixing with the
primary- treated influent to the basin and the remaining sludge is wasted to the sludge handling portion of
the treatment plant (chap 16). The portion recirculated is determined on the basis of the ratio of mixed liquor
volatile suspended solids (MLVSS) to influent wastewater biochemical oxygen demand which will produce
the maximum removal of organic material from the wastewater. Recirculation varies from 25 to 50 percent
of the raw wastewater flow, depending on treatment conditions and wastewater characteristics.
b. Step aeration. In this process (fig 13-2), the influent wastewater is introduced at various points along
the length of the aeration tank. Sludge return varies between 25 and 50 percent. Aeration or the oxygen
requirement during step aeration (3 to 7 hours) is about half that required for the conventional process. This
results from a more effective biomass utilization in the aeration basin, allowing organic loadings of 30 to 50
pounds biochemical oxygen demand per 1,000 cubic feet per day as compared to loadings of 30 to 40 pounds
biochemical oxygen demand per 1,000 cubic feet per day permitted for conventional systems.