The activated sludge process is an aerobic (oxygen-rich), continuous-flow biological method for the treatment of domestic and biodegradable industrial wastewater, in which organic matter is utilized by microorganisms for life-sustaining processes, that is, for energy for reproduction, digestion, movement, etc. and as a food source to produce cell growth and more microorganisms. During these activities of utilization and degradation of organic materials, degradation products of carbon dioxide and water are also formed. The activated sludge process is characterized by the suspension of microorganisms in the wastewater, a mixture referred to as the mixed liquor. Activated sludge is used as part of an overall treatment system, which includes primary treatment of the wastewater for the removal of particulate solids before the use of activated sludge as a secondary treatment process to remove suspended and dissolved organic solids.

The conventional activated sludge process consists of an aeration basin, with air as the oxygen source, where treatment is accomplished. Soluble (dissolved) organic materials are absorbed through the cell walls of the microorganisms and into the cells, where they are broken down and converted to more microorganisms, carbon dioxide, water, and energy. Insoluble (solid) particles are adsorbed on the cell walls, transformed to a soluble form by enzymes (biological catalysts) secreted by the microorganisms, and absorbed through the cell wall, where they are also digested and used by the microorganisms in their life-sustaining processes. The microorganisms that are responsible for the degradation of the organic materials are maintained in suspension by mixing induced by the aeration system.

The aeration basin is followed by a secondary clarifier (settling tank), where the flocs of microorganisms with their adsorbed organic materials settle out. A portion of the settled microorganisms, referred to as sludge, are recycled to the aeration basin to maintain an active population of microorganisms and an adequate supply of biological solids for the adsorption of organic materials. Excess sludge is wasted by being piped to separate sludge-handling processes. The liquids from the clarifier are transported to facilities for disinfection and final discharge to receiving waters, or to tertiary treatment units for further treatment.

Activated sludge processes are designed based on the mixed liquor suspended solids (MLSS) and the organic loading of the wastewater, as represented by the biochemical oxygen demand (BOD) or chemical oxygen demand (COD). The MLSS represents the quantity of microorganisms involved in the treatment of the organic materials in the aeration basin, while the organic loading determines the requirements for the design of the aeration system.

The sequencing batch reactor (SBR) is a fill-and draw activated sludge system for wastewater treatment. In this system, wastewater is added to a single “batch� reactor, treated to remove undesirable components, and then discharged. Equalization, aeration, and clarification can all be achieved using a single batch reactor, while the steps are performed after another. Aerobic decomposition, settling, and return occur in the same chamber. Air is bubbled through the liquid during the decomposition cycle. The bubbler shuts off, and the wastewater goes through a settling cycle (see figure 2). Once the bubbler turns back on, the tank re-enters the decomposition cycle, and settled bacteria mixes back into the aerobic Environment. After settling of bacteria and solids, the treated effluent is discharged to the soil treatment system. Bacteria settle out more consistently in this kind of tank, but since it has more moving parts and requires a controller, it has more potential for mechanical and electrical failure.

To optimize the performance of the system, more batch reactors can be used in a predetermined sequence of operations. SBR systems have been successfully used to treat both municipal and industrial wastewater. They are uniquely suited for wastewater treatment applications characterized by low or intermittent flow conditions with low ground space demand, while conventional activated sludge technologies have a high demand on area.

Figure 2: Sequencing batch reactor during aeration (left) and settling (right)- (Source University of Minnesota)