A conventional activated sludge plant (CAS) is the precursor to the extended aeration plants that are favored by smaller facilities (less than 15 MGD) in the Commonwealth today.
The CAS normally begins with the following preliminary treatment processes: a screening device to remove trash, debris and other untreatable objects; grit removal to remove the heavier inorganic material and comminutation to cut or grind any material that passes through the screening device.
Following preliminary treatment, primary clarification is used to create a relatively quiescent area where solids with a higher specific gravity than the liquid will tend to settle; and those with a lower specific gravity will tend to rise so that readily settleable solids, such as food particles and floating material like grease, can be removed and reduce the load on the following biological treatment units. The primary clarification process may remove up to 40 percent of the BOD and up to 80 percent of the TSS from the influent. That sludge is normally sent to an anaerobic digester for further treatment. The detention time in a primary clarifier at design flow is usually one-and-a-half to two hours.
After preliminary and primary treatment, the primary effluent (the influent to the aeration basin) normally has a reduced BOD and TSS load. The normal designated detention time for the CAS’s aeration basin is eight hours, instead of the 24 hours used in extended aeration systems. The return sludge rates are usually between 50 percent and 150 percent of the influent flow, which should keep the F/M to between .05 and .15. The recommended dissolved oxygen concentration is between three to five Mg/L.
Following aeration, the flow then moves to final clarification where the residual solids settle and a percentage of these solids are mixed with the sludge from the primary clarifier. These are then sent to an anaerobic digester and a percentage is returned to the head of the aeration basin as return sludge. In some cases, there is tertiary filtering or settling after final clarification, but before disinfection and discharge. Tertiary filtering can be accomplished using either a cloth filter “sock” for smaller facilities or a backwashable sand filter or a slow sand filter for larger facilities. It may also be accomplished using a polishing lagoon designed and built for the purpose.
Although invented in 1914, the activated sludge system is the basis for most of the treatment works in the United States today. They can be difficult to operate and maintain correctly, but with proper funding, maintenance and operation, they can produce an effluent that will keep the facility in compliance.