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 Activated Sludge suspended, a continuous and well-timed supply of oxygen is required.Describes biological processes that occur in the presence of oxygen.
General term for a liquid that leaves a technology, typically after blackwater or sludge has undergone solids separation or some other type of treatment. Effluent originates at either a collection and storage or a (semi-) centralised treatment technology. Depending on the type of treatment, the effluent may be completely sanitised or may require further treatment before it can be used or disposed of.Refer to biodegradable plant material (organic waste) that must be added to some technologies in order for them to function properly. Organic degradable material can include, but is not limited to, leaves, grass and food market waste. Although other products in this compendium contain organic matter, the term organics is used to refer to undigested plant material.Mixture of solids and liquids, containing mostly excreta and water, in combination with sand, grit, metals, trash and/or various chemical compounds. A distinction can be made between faecal sludge and wastewater sludge. Faecal sludge comes from on-site sanitation technologies, i.e. it has not been transported through a sewer. It can be raw or partially digested, a slurry or semisolid, and results from the collection and storage/treatment of excreta or blackwater, with or without greywater. Wastewater sludge (also referred to as sewage sludge) originates from sewer-based wastewater collection and (semi-)centralised treatment processes. The sludge composition will determine the type of treatment that is required and the end-use possibilities.Describes technologies for on-site collection, storage, and sometimes (pre-) treatment of the products generated at the user interface. The treatment provided by these technologies is often a function of storage and is usually passive (i.e. requires no energy input), except a few emerging technologies where additives are needed. Thus, products that are ‘treated’ by these technologies often require subsequent treatment before use and/or disposal. In the technology overview graphic, this functional group is subdivided into the two subgroups: “Collection/Storage” and “(Pre-)Treatment”. This allows a further classification for each of the listed technologies with regard to their function: collection and storage, (pre-) treatment only or both.Refers to the methods through which products are returned to the environment, either as useful resources or reduced-risk materials. Some products can also be cycled back into a system (e.g. by using treated greywater for flushing).A functional group is a grouping of technologies that have similar functions. The compendium proposes five different functional groups from which technologies can be chosen to build a sanitation system:
User interface (U), Collection and Storage/Treatment (S), Conveyance (C), (Semi-) Centralised Treatment (T), Use and/or Disposal (U).
A sanitation system is a multi-step process in which sanitation products such as human excreta and wastewater are managed from the point of generation to the point of use or ultimate disposal. It is a context-specific series of technologies and services for the management of these sanitation products, i.e. for their collection, containment, transport, treatment, transformation, use or disposal. A sanitation system comprises functional groups of technologies that can be selected according to context. By selecting technologies from each applicable functional group, considering the incoming and outgoing products, and the suitability of the technologies in a particular context, a logical, modular sanitation system can be designed. A sanitation system also includes the management and operation and maintenance (O & M) required to ensure that the system functions safely and sustainably. The utilisation of products derived from a sanitation system.
Any cellular or non-cellular microbiological entity capable of replication or of transferring genetic material (e.g. bacteria, viruses, protozoa, algae or fungi).
Any substance that is used for growth. Nitrogen (N), phosphorus (P) and potassium (K) are the main nutrients contained in agricultural fertilisers. N and P are also primarily responsible for the eutrophication of water bodies.
A sanitation system in which excreta and wastewater are collected and stored or treated on the plot where they are generated.
A diverse group of unicellular eukaryotic organisms, including amoeba, ciliates, and flagellates. Some can be pathogenic and cause mild to severe illnesses.
The means of safely collecting and hygienically disposing of excreta and liquid
wastes for the protection of public health and the preservation of the quality of public water bodies and, more generally, of the environment.

Waste matter that is transported through the sewer.
An open channel or closed pipe used to convey sewage. See C.3 and C.4
An infectious agent consisting of a nucleic acid (DNA or RNA) and a protein coat. Viruses can only replicate in the cells of a living host. Some pathogenic viruses are known to be waterborne (e.g., the rotavirus that can cause diarrheal disease).
Used water from any combination of domestic, industrial, commercial or agricultural activities, surface runoff/stormwater, and any sewer inflow/infiltration.

Different configurations can be employed to ensure wastewater is mixed and aerated. Aeration and mixing can be provided by pumping air or oxygen into the tank or by using surface aerators. Microorganisms oxidise organic carbon in wastewater to produce new cells, carbon dioxide and water. Aerobic bacteria are the most common organisms, but facultative bacteria along with higher organisms can be present. The exact composition depends on the reactor design, the environment, and wastewater characteristics. Several weeks are needed to establish the microorganisms required for a stable biological process.The flocs (agglomerations of sludge particles), which form in the aerated tank, are removed in the secondary clarifier by gravity settling. Excess sludge is partially removed and partially recycled for the biological process. In an immersed membrane bioreactor (IMBR), the activated sludge reactor is combined with a micro- or ultrafiltration membrane unit. By passing the membrane, treated water gets separated from sludge. The system can be set up as a pre-assembled solution or can be constructed on-site. The IMBR is an efficient compact technology for municipal (and industrial) wastewater treatment. The major drawback impeding wider application is membrane fouling, which significantly reduces membrane performance and lifespan, resulting in a significant increase in operation and maintenance (O & M) costs.

Design Considerations

Activated Sludge processes usually require primary treatment that removes settleable solids and are sometimes followed by a final polishing step POST . The biological processes are effective at removing soluble, colloidal and particulate materials. The reactor can be designed for biological nitrification and denitrification, as well as for 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 in the reactor ranges from 3–5 days for biochemical oxygen demand (BOD) removal, to 3–18 days for nitrification. Excess sludge requires treatment to reduce its water and organic content and to obtain a stabilised product suitable for reuse or final disposal. To achieve specific effluent goals for BOD, nitrogen and phosphorus, different adaptations and modifications can be made, which include sequencing batch reactors, oxidation ditches, extended aeration, moving beds and membrane bioreactors.

Materials

Usually the Activated Sludge reactor is made of plastic or concrete. The aerators consist of stainless steel or plastic and a membrane of rubber seal. For the potential subsequent membrane process either ceramic, polymeric, or composite membranes can be used. The material used has an impact on fouling propensity in IMBRs. Different pre-fabricated models are available.

Applicability

Activated Sludge treatment can be an appropriate solution in the stabilisation and recovery phases of a humanitarian emergency, particularly in more densely populated urban areas or larger camp contexts where water-based systems are preferred. It is a centralized treatment that needs well-trained staff, constant electricity and a highly developed management system. Because of economies of scale and less fluctuating influent characteristics, it is more effective for treatment of larger volumes. Activated Sludge processes are appropriate in almost every climate, but treatment capacity is reduced in colder environments. Given that the system is well operated the quality of the treated water can be suitable for reuse.

Operation and Maintenance

Trained technical staff are required for maintenance and trouble-shooting. Mechanical equipment (mixers, aerators and pumps) must be constantly maintained. Influent and effluent must be continuously monitored and control parameters adjusted, if necessary, to avoid abnormalities like kill-off of active biomass or development of detrimental organisms (e.g. filamentous bacteria). Access to the facility should only be allowed to trained personnel.

Health and Safety

Due to the space required and odour produced, Activated Sludge facilities are generally located on the periphery of densely populated areas. Although the effluent produced is of high quality, it still poses a public health risk and should not be directly handled. In the excess sludge, pathogens are substantially reduced but not eliminated. IMBR performance and treatment quality can be improved depending on the membrane used. Involved personnel need to be equipped with proper personal protective equipment.

Costs

Capital costs for Activated Sludge facilities are high. Costs may vary depending on availability and costs of construction material and electricity. Due to the requirements of skilled staff, continuous monitoring tasks and constant energy requirements the operational costs are high and need to be reflected in the total cost calculations.

Social Considerations

The installation of an activated sludge reactor should be carried out in areas where there is knowledge and experience with this technology and skilled personnel are available. Depending on the cultural context and existing regulations there may be barriers to re-using processed water.

Case Studies & Related Content

Anaerobic Baffled Reactor
The ABR is at the top of the hill and is operated through gravitational force. It consists of settler- thickening tank and 4 baffled reactors. Besides, there is a provision of planted filtration unit using different sizes gravel and sand.
Suggested by: (NGO Forum) at 20.03.2021

Anaerobic Treatment
The anaerobic baffled reactor is used as pretreatment, to remove solids from the waste The plant is located next to a creek line that has been widened. It is a topography composed of plains and plateaus. The site has yet to be flooded, however potential flooding could occur in large rainfall event.
Suggested by: (IFRC) at 20.03.2021