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Activated sludge is a biological wastewater treatment process in which a suspended culture of aerobic microorganisms is mixed with wastewater in an aeration tank and supplied with oxygen, so that the microorganisms metabolize dissolved and particulate organic matter into carbon dioxide, water, and new cell mass [1]. The mixed liquor then flows to a secondary sedimentation tank where the microbial flocs settle by gravity; a portion of the settled biomass is returned to the aeration tank as return activated sludge to maintain the desired mixed-liquor suspended-solids concentration, while the excess biomass is wasted as waste activated sludge [2]. The process is characterized by operational parameters including the sludge age, or mean cell residence time, the food-to-microorganism ratio, and the oxygen uptake rate, and its kinetics are described by structured mathematical models that relate substrate removal, biomass growth, and nutrient transformations to the hydraulic and solids retention times [3]. Flocculation of the microbial biomass is essential for efficient solid-liquid separation in the secondary clarifier.
References
- Tchobanoglous, G.; Stensel, H.D.; Tsuchihashi, R.; Burton, F.L. Wastewater Engineering: Treatment and Resource Recovery, 5th ed.; McGraw-Hill: New York, NY, USA, 2014. ISBN: 0073401188.
- Henze, M.; van Loosdrecht, M.C.M.; Ekama, G.A.; Brdjanovic, D. Biological Wastewater Treatment: Principles, Modelling and Design; IWA Publishing: London, UK, 2008. ISBN: 9781780401867.
- Henze, M.; Gujer, W.; Mino, T.; van Loosdrecht, M.C.M. Activated Sludge Models ASM1, ASM2, ASM2D and ASM3; IWA Publishing: London, UK, 2000. ISBN: 1-90022-224-8.
