Activated Sludge

An activated sludge process refers to a multi-chamber reactor unit that makes use of highly concentrated microorganisms to degrade organics and remove nutrients from wastewater to produce a high-quality effluent. To maintain aerobic conditions and to keep the activated sludge suspended, a continuous and well-timed supply of oxygen is required.

Different configurations of the activated sludge process can be employed to ensure that the wastewater is mixed and aerated in an aeration tank. Aeration and mixing can be provided by pumping air or oxygen into the tank or by using surface aerators. The microorganisms oxidize the organic carbon in the wastewater to produce new cells, carbon dioxide and water. Although aerobic bacteria are the most common organisms, facultative bacteria along with higher organisms can be present. The exact composition depends on the reactor design, environment, and wastewater characteristics.

The flocs (agglomerations of sludge particles), which form in the aerated tank, can be removed in the secondary clarifier by gravity settling. Some of this sludge is recycled from the clarifier back to the reactor. The effluent can be discharged into a river or treated in a tertiary treatment facility if necessary for further use.

Design Considerations

Activated sludge processes are one part of a complex treatment system. They are usually used following primary treatment (that removes settleable solids) and are sometimes followed by a final polishing step (see tertiary filtration and disinfection). The biological processes that occur are effective at removing soluble, colloidal and particulate materials. The reactor can be designed for biological nitrification and denitrification, as well as for biological phosphorus removal.

The design must be based on an accurate estimation of the wastewater composition and volume. Treatment efficiency can be severely compromised if the plant is under- or over-dimensioned. Depending on the temperature, the solids retention time (SRT) in the reactor ranges from 3 to 5 days for BOD removal, to 3 to 18 days for nitrification.

The excess sludge requires treatment to reduce its water and organic content and to obtain a stabilized product suitable for end-use or final disposal. It is important to consider this step in the planning phase of the treatment plant). To achieve specific effluent goals for BOD, nitrogen and phosphorus, different adaptations and modifications have been made to the basic activated sludge design. Well known modifications include sequencing batch reactors (SBR), oxidation ditches, extended aeration, moving beds and membrane bioreactors.


An activated sludge process is only appropriate for a Centralized Treatment facility with a well-trained staff, constant electricity and a highly developed management system that ensures that the facility is correctly operated and maintained.
Because of economies of scale and less fluctuating influent characteristics, this technology is more effective for the treatment of large volumes of flows .
An activated sludge process is appropriate in almost every climate. However, treatment capacity is reduced in colder environments. 

Health Aspects/Acceptance

Because of space requirements and odours, Centralized Treatment facilities are generally located in the periphery of densely populated areas. Although the effluent produced is of high quality, it still poses a health risk and should not be directly handled. In the excess sludge pathogens are substantially reduced, but not eliminated.

Operation & Maintenance 

Highly trained staff is required for maintenance and trouble-shooting. The mechanical equipment (mixers, aerators and pumps) must be constantly maintained. As well, the influent and effluent must be constantly monitored and the control parameters adjusted, if necessary, to avoid abnormalities that could kill the active biomass and the development of detrimental organisms which could impair the process (e.g., filamentous bacteria).


Further Readings

  • Cover image of a reference book or miscellany.

    TILLEY, E.; ULRICH, L.; LUETHI, C.; REYMOND, P.; SCHERTENLEIB, R.; ZURBRUEGG, C. (2014): Compendium of Sanitation Systems and Technologies (Arabic). 2nd Revised Edition. Duebendorf, Switzerland: Swiss Federal Institute of Aquatic Science and Technology (Eawag). PDF

    This is the Arabic version of the Compendium of Sanitation Systems and Technologies. The Compendium gives a systematic overview on different sanitation systems and technologies and describes a wide range of available low-cost sanitation technologies.

  • Cover image of a reference book or miscellany.

    CRITES, R.; TCHOBANOGLOUS, G. (1998): Small and Decentralized Wastewater Management Systems. New York: The McGraw-Hill Companies Inc.

    Decentralised wastewater management presents a comprehensive approach to the design of both conventional and innovative systems for the treatment and disposal of wastewater or the reuse of treaded effluent. Smaller treatment plants, which are the concern of most new engineers, are the primary focus of this book.

  • Cover image of a reference journal article.

    MAURER, M.; SCHWEGLER, T.; SCHWEGLER, P.; LARSEN, T.A. (2003): Nutrients in urine: energetic aspects of removal and recovery. In: Water Science and Technology 8 1, 37. PDF

    The analysis of different removal and recovery techniques for nutrients in urine shows that in many cases recovery is energetically more efficient than removal and new production from natural resources.

  • Cover image of a reference book or miscellany.

    HELMER, R. (Editor); HESPANHOL, I. (Editor) (1997): Water Pollution Control - A Guide to the Use of Water Quality Management Principles. World Health Organization (WHO), Water Supply and Sanitation Collaborative Council (WSSCC) and United Nations Environment Programme (UNEP). URL [Accessed: 21.04.2010]. PDF

    This document is rather old, but its publication was a milestone as it demonstrates WSSCCs capacity to bring together water and sanitation professionals from industrialised and developing countries to formulate practical guidance on a key issue of the day. Mainly regulatory, financial and technical aspects are discussed and illustrated with an extensive collection of case studies from the developing world.

  • Cover image of a reference book or miscellany.

    WSP (Editor) (2007): Philippines Sanitation Source Book and Decision Aid. pdf presentation. Washington: Water and Sanitation Program. PDF

    This Sanitation Sourcebook distils some of the core concepts of sanitation in a user-friendly format so that the book can serve as a practical reference to sanitation professionals and investment decision-makers, particularly the local governments. The annexe contains a practical collection of factsheets on selected sanitation system options.

  • Cover image of a reference book or miscellany.

    WSP (Editor) (2008): Technology Options for Urban Sanitation in India. A Guide to Decision-Making. pdf presentation. New Delhi: Water and Sanitation Program (WSP). URL [Accessed: 26.03.2010]. PDF

    These guidance notes are designed to provide state governments and urban local bodies with additional information on available technologies on sanitation. The notes also aid in making an informed choice and explain the suitability of approaches.

  • Cover image of a reference book or miscellany.

    SPERLING, M. von (2007): Basic Principles of Wastewater Treatment. London: International Water Association (IWA) Publishing. URL [Accessed: 01.11.2013]. PDF

    Basic Principles of Wastewater Treatment is the second volume in the series Biological Wastewater Treatment, and focusses on the unit operations and processes associated with biological wastewater treatment. The major topics covered are: microbiology and ecology of wastewater treatment, reaction kinetics and reactor hydraulics, conversion of organic and inorganic matter, sedimentation, aeration.

  • Cover image of a reference book or miscellany.

    SPERLING, M. von (2007): Activated Sludge and Aerobic Biofilm Reactors. London: International Water Association (IWA) Publishing. URL [Accessed: 01.01.2013]. PDF

    Activated Sludge and Aerobic Biofilm Reactors is the fifth volume in the series Biological Wastewater Treatment. The first part of the book is devoted to the activated sludge process, covering the removal of organic matter, nitrogen and phosphorus. A detailed analysis of the biological reactor (aeration tank) and the final sedimentation tanks is provided. The second part of the book covers aerobic biofilm reactors, especially trickling filters, rotating biological contractors and submerged aerated biofilters. For all the systems, the book presents in a clear and informative way the main concepts, working principles, expected removal efficiencies, design criteria, design examples, construction aspects and operational guidelines.

  • Cover image of a reference book or miscellany.

    SPERLING, M. von; LEMOS CHERNICHARO, C.A. de (2005): Biological Wastewater Treatment in Warm Climate Regions Volume 2. London: International Water Association (IWA) Publishing. URL [Accessed: 01.11.2013]. PDF

    Biological Wastewater Treatment in Warm Climate Regions gives a state-of-the-art presentation of the science and technology of biological wastewater treatment, particularly domestic sewage. The book covers the main treatment processes used worldwide with wastewater treatment in warm climate regions given a particular emphasis where simple, affordable and sustainable solutions are required. The 55 chapters are divided into 7 parts over two volumes: Volume One (also available in the SSWM library): Introduction to wastewater characteristics, treatment and disposal; Basic principles of wastewater treatment; Stabilisation ponds; Anaerobic reactors; Volume Two: Activated sludge; Aerobic biofilm reactors; Sludge treatment and disposal.

Case Studies

  • Cover image of a reference book or miscellany.

    ROBBINS, D.; STRANDE, L.; DOCZI, J. (2012): Opportunities in Fecal Sludge Management for Cities in Developing Countries: Experiences from the Philippines. North Carolina: RTI International . URL [Accessed: 15.01.2013]. PDF

    In July 2012, a team from RTI International deployed to the Philippines to evaluate four FSM programs with the goal of reporting on best practices and lessons learned. The four cases—Dumaguete City, San Fernando City, Maynilad Water for the west zone of metro Manila, and Manila Water from the east zone of metro Manila—were chosen to highlight their different approaches to implementing FSM.

  • Cover image of a reference book or miscellany.

    WENDLAND, C. (2008): Anaerobic Digestion of Blackwater and Kitchen Refuse. (PhD Thesis). Hamburg: Institut fuer Abwasserwirtschaft und Gewaesserschutz (AWW), Technische Universitaet Hamburg-Hamburg (TUHH). URL [Accessed: 11.03.2010]. PDF

    Thesis assessing the anaerobic treatment of blackwater (toilet wastewater) from vacuum toilets without and with kitchen refuse and its potential for reuse and resources management sanitation concepts.

Awareness Material

  • Cover image of a reference chapter of a book/miscellany.

    JENSSEN, P.D.; VRAALE, L.; LINDHOLM, O. ; SENG, L. (Editor) (2007): Sustainable Wastewater Treatment. In: SENG, L. (Editor) (2007): Proceedings of the International Conference on Natural Resources and Environmental Management and Environmental Safety and Health. Norway. PDF

    Wastewater contains organic matter and the three main nutrients for plant production: nitrogen, phosphorus and potassium. Nitrogen fertilizer is energy consuming to produce and phosphorus is a limited mineral resource. Scandinavia is pioneering sustainable solutions to wastewater treatment. Source separation (blackwater/greywater) systems produce almost zero emissions and open up exiting urban applications of sanitation options in order to close the loop.

Training Material

  • Cover image of a reference book or miscellany.

    U.S. EPA (Editor) (1980): Design Manual - Onsite Wastewater Treatment and Disposal Systems. United States Environmental Protection Agency and Office of Water Office of Research and Development. URL [Accessed: 18.03.2010]. PDF

    Rather old design manual for onsite wastewater treatment options. However, valuable information on established systems such as septic tanks, sandfilters, aerobic treatment units (suspendend growth and fixed film), disinfection, nutrient removal as well as wastewater segregation and recycling are given. Additional information is given on disposal methods and appurtenances.

  • Cover image of a reference chapter of a book/miscellany.

    UNEP (Editor) (2002): 4.2.1 Activated sludge treatment. In: UNEP (Editor); MURDOCH UNIVERSITY (Editor) (2004): Environmentally sound technologies in wastewater treatment for the implementation of the UNEP/GPA "Guidelines on Municipal Wastewater Management". The Hague. URL [Accessed: 22.07.2010]. PDF

  • Cover image of a reference book or miscellany.

    U.S. EPA (Editor) (2000): Oxidation Ditches. United States Environment Protection Agency. URL [Accessed: 18.01.2011]. PDF

  • Cover image of a reference book or miscellany.

    U.S. EPA (1999): Sequencing Batch Reactors. United States Environment Protection Agency (U.S. EPA). PDF

    Technical factsheet on the design, application, performance and operation of sequencing batch reactors (SBRs), a type of activated sludge wastewater treatment plants. English, Spanish

  • Cover image of a reference book or miscellany.

    MECC (Editor) (n.y.): Course ENV149: Wastewater Treatment Plant Operation: Lesson 10: Secondary Treatment. Big Stone Gap: Mountain Empire Community College (MECC). URL [Accessed: 18.03.2010]. PDF

    The Mountain Empire Community College ‘Water and Wastewater Distance Learning Course’ makes available all its documentation on the web. This section describes secondary treatment systems, including trickling filters, rotating biological contactors, activated sludge, operation and control, stabilization ponds, aerated lagoons and intermittent sand filters.