Activated Sludge Plant


Activated Sludge Plant in wastewater treatment

Category / type: suspended biomass, activated sludge, extended aeration, constant flow rate.
Application: civil (or assimilable) wastewater secondary treatment.
Population Equivalents (PE) served: from 2 up to 50 (Packaged and/or site assembled) and more than 50 tailor-made.
Normative references and guarantees: EN 12566-3, d.o.p. According to Regulation (UE) n.305/2011.
Material: concrete, polyethylene, polypropylene, glass-reinforced plastic (GRP).

Activated sludge is composed by flocs (improperly called flocks) produced by active bacteria cells suspended and mixed in wastewater; bacteria remove biodegradable organic substance and nutrients from civil black and grey domestic or equivalent wastewater.
Inflow requires a mechanical pre-treatment (screens or fine screens) and a primary treatment like settling basins or septic tank, depending on the design setting.
Anyhow grey water pre-treatment (grease trap) is very important to restrain FOG (fats oil and grease).
Oxygen must be supplied to bacteria for biological reactions. There are several system to ensure oxygen supply and the necessary mixing into the reaction chamber: such as blowing air (or pure oxygen) into the tank through submerged membranes or through surface aerators and mixers.
Microorganisms oxidise organic carbon through cell metabolism and synthesis, producing carbon dioxide (CO2) and water.
Flocs (agglomerates of sludge particles) are suspended in reactor tank and dragged with the outgoing treated wastewater; the sludge is then removed in the secondary sedimentation tank by gravity. Most of flocs are recirculated to reactor to increase sludge concentration and to enhance the system efficiency. Effluent, already purified, can be discharged into a receiving water body (if any disinfection downstream is applied) or subjected to further treatment according to the standard purification required (tertiary treatment).
Output flow can be also processed for water reuse (see wastewater recovery).
The system requires power supply and maintenance by qualified labor.
In order to avoid personnel work on site and to optimize functioning of plants an automatisation system should be supplied such as PLC (programmable logic controller) with GSM module.
It’s possible to automate sludge recirculation and to store excess sludge into an accumulation and clarification vessel (thickening tank). In this arrangement, although it requires an additional part, a higher system complexity (not very remarkable) allows to reduce frequency of purging with a considerable saving in terms of operating costs (to be considered for the entire plant working life).


How It Works

Activated sludge process is generally part of a complex purification system. It’s classified as a secondary treatment and it usually needs a primary treatment (screening or sedimentation). Biological reactors can be divided into more oxidation lines in order to manage inlet variable flow (e.g. camping, hotels, etc.) or to guarantee a continuously treatment during maintenance activity.
The reactor tank is supplied with aeration devices (air diffusers) and mixing devices, useful to keep sludge in suspension (sometimes both devices are arranged in a unique system).
A secondary clarifier is present downstream to separate the biomass from the effluent and to collect sludge; the clarifier, also called secondary sedimentation tank, is often equipped with a recirculating system. Sludge must be collected into aeration chamber in order to increase MLSS concentration and sludge retention time. The excess sludge periodically must be removed to thickening tank. Supernatant created during thickening process must be collected in aeration tank.
Tertiary treatment, such as filtration or disinfection processes, may be added downstream. Biological processes that occur into activated sludge plants are effective in removing soluble, colloidal and particulate materials. Treatment is cheap and produces relatively small amounts of surplus activate sludge (SAS).
Biological reactor can be designed for nitrification and denitrification processes and also for phosphorus removal.
The main parameter for civil wastewater plant (or equivalent) projects is the population equivalent (PE); normally it refers to 60 g BOD5 per day and a flow rate from 150 to 250 litre per day.
Population equivalent (PE or sometimes used p.e.) is a conversion value which aims at evaluating non-domestic pollution in reference to domestic pollution fixed by EEC directive (Council Directive 91/271/EEC concerning Urban Waste Water Treatment) at 60 g/day related to BOD5.
European legislation defines Population total (PT) as a sum of population and population equivalent (PE).
A proper wastewater characterization for incoming organic loads a better estimation of nutrients are possible considering the possibility of dosing additives for biological reactions.
Basin sizing must be based on an accurate wastewater estimation (composition, organic pollutants and daily flow rate).
Sizing is normally based on population equivalent and standard organic load (BOD5); it is very important to understand exactly what is the main project parameter for each product and plant. For example, in European countries there are different standard value for inhabitants water consumption.
An excess sludge treatment is required to reduce water content and for a better storage.
Sludge recirculation influences sludge retention time (SRT) that may be greater than hydraulics retention time (HRT or residence time) and increase removal efficiency.
Their sizing should be more accurate then the one performed primary treatment, since the biological process requires more constant water supply (therefore organic loads).
Maintenance costs and purges frequency are very important parameters for assessment of the investment for waste water plants.
Small flow variability is accepted in biological line; it is, anyhow, better to divide the flow in several lines, especially for medium and large plants.
An equalization tank is often added upstream in small and medium installations in order to obtain a constant flow and to protect the plant from peaks pollutants.


Operation and maintenance

Qualified personnel is not always required for maintenance operations, instead of SBR or MBR plants. Normal cleaning operation can be carried out by medium skilled personnel.
Mechanical equipment (mixers, aerators, pumps, etc.) must be subject to periodic inspections and maintenance.
It is necessary to ensure a regular oxygen supply in the oxidation compartment and the dissolved oxygen rate must be regularly checked by oxygen meter. Otherwise sludge characteristics, inflow and outflow, must be checked by specific instruments like DO meter, TSS probe, pH meters, thermometer, Imhoff cone, etc.
Inlet water is checked to evaluate treatment performance and to control abnormal intakes of pollutants that could completely stop biological process or allow development of an useless bacterial population (for example, filamentous bacteria).
A very serious problem concerning activated sludge process is the phenomenon known as bulking.
Bulking is a condition that can occur in secondary sedimentation chamber depending of particularly bacteria grown that, due to a bigger specific surface, have difficult to settle and aim to float on surface escaping to outlet.
Basically with this condition around, it will be very difficult in order to get a good separation of sludge and water and this will lead to carry over of solids to the discharge side and clog up the final polishing filter.
Nutrient and chemicals are added to control bacteria process and improve its efficiency.
These operations can be realised by the use of a partly or wholly automatized system equipped with remote control.
This solution is suitable for constant flow rate. Long inactivity periods can lead to the arrest of biological reactions and, thus, require a new start up procedure.


Regulatory framework

The European standard EN 12566-3 “specifies requirements, test methods, the marking and factory production control for packaged and/or site assembled domestic wastewater treatment plants (including guest houses and businesses) used for populations up to 50 inhabitants…”
The manufacturer shall state the nominal organic daily load, expressed in kg of BOD5 or BOD7 per day and the nominal hydraulic daily flow, expressed in cubic meters per day.
Plant’s Nominal Designation is the proper daily organic load value (QN) (3.7).
The small wastewater treatment plant must demonstrate compliance with the performance of efficiency… and to operational data declared by the manufacturer: the declaration must be expressed in percentage.
It is also to be considered that the structural resistance and the hydraulic efficiency must be tested only on a representative model of the product line. Representative model for structural resistance test must be the bigger, supposedly as the one of series with the worst characteristic, while the one for hydraulic efficiency test must be the smaller, presuming the one with the worst efficiency.
The standard describes statics and hydraulics methods to determinate resistance and hydraulic efficiency.
Informations to be included in the test report and in the CE mark are also detailed in ZA.3 annex.
For small domestic wastewater treatment plants up to 50 PT (therefore for activated sludge plants), a Declaration of Performance (DoP), is required, as provided by the EU Regulation. 305/2011.


Features and benefits

An activated sludge process is appropriate for a wastewater treatment plant with constant pollution and flow rate typically coming for medium and large communities.
It needs qualified personnel, care in installation connection and technical equipments (spare parts, monitoring equipment, etc.).
Activated sludge process is appropriate in almost all climatic conditions, although treatment efficiency is very responsive to temperature. Low temperatures require higher volume to get the same quality result.ASP
Activated sludge plants, with proper pre-treatments, are used to treat wastewater coming from medium and large residential areas.

Advantages (benefits)

  • resistant to organic and hydraulic shock;
  • good BOD and pathogens reduction;
  • limited nutrient reduction;
  • good effluent quality;
  • medium skilled personnel needed;
  • reduced area compared to natural systems;
  • no wastewater homogenization (equalization recommended);
  • system integration upgrade (tertiary, irrigation reuse, etc.);
  • automatic and remote controls functions;
  • reduced and stable excess sludge.


  • higher initial investment compared to primary treatments;
  • power supply needed;
  • only constant flow rate (not long inactivity periods admitted);
  • system start up needed;
  • spare parts and planned maintenance;
  • accurate sizing;
  • excess sludge disposal.


Products in category Activated Sludge Plant