Activated sludge

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. As the microorganisms are mixed, they collide with other microorganisms and stick together to form larger particles called floc. The large flocs that are formed settle more readily than individual cells. These flocs also collide with suspended and colloidal materials (insoluble organic materials), which stick to the flocs and cause the flocs to grow even larger. The microorganisms digest these adsorbed materials, thereby re-opening sites for more materials to stick.

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.

Modifications to the conventional activated sludge process include:

• Extended aeration. The mixed liquor is retained in the aeration basin until the production rate of new cells is the same as the decay rate of existing cells, with no excess sludge production. In practice, excess sludge is produced, but the quantity is less than that of other activated sludge processes. This process is often used for the treatment of industrial wastewater that contains complex organic materials requiring long detention times for degradation.
• Contact stabilization. A process based on the premise that as wastewater enters the aeration basin (referred to as the contact basin), colloidal and insoluble organic biodegradable materials are removed rapidly by biological sorption , synthesis, and flocculation during a relatively short contact time. This method uses a reaeration (stabilization) basin before the settled sludge from the clarifier is returned to the contact basin. The concentrated flocculated and adsorbed organic materials are oxidized in the reaeration basin, which does not receive any addition of raw wastewater.
• Plug flow. Wastewater is routed through a series of channels constructed in the aeration basin; wastewater flows through and is treated as a plug as it winds its way through the basin. As the "plug" passes through the tank, the concentrations of organic materials are gradually reduced, with a corresponding decrease in oxygen requirements and microorganism numbers.
• Step aeration. Influent wastewater enters the aeration basin along the length of the basin, while the return sludge enters at the head of the basin. This process results in a more uniform oxygen demand in the basin and a more stable environment for the microorganisms; it also results in a lower solids loading on the clarifier for a given mass of microorganisms.
• Oxidation ditch. A circular aeration basin (racetrackshaped) is used, with rotary brush aerators that extend across the width of the ditch. Brush aerators aerate the wastewater, keep the microorganisms in suspension, and drive the wastewater around the circular channel.

[Judith Sims ]

RESOURCES

BOOKS

Corbitt, R. A. "Wastewater Disposal." In Standard Handbook of Environmental Engineering, edited by R. A. Corbitt. New York: McGraw-Hill, 1990.

Junkins, R., K. Deeny, and T. Eckhoff. The Activated Sludge Process: Fundamentals of Operation. Boston: Butterworth Publishers, 1983.