Activated Sludge System Process

Activated sludge is a process for treating sewage and industrial waste- waters using air and a biological floc composed of bacteria and protozoans.


In a sewage (or industrial waste-water) treatment plant, the activated sludge process is a biological process that can be used for one or several of the following purposes:

  • Oxidizing carbonaceous matter: biological matter.
  • Oxidizing nitrogenous matter: mainly ammonium and nitrogen in biological materials
  • Removing phosphate
  • Driving off entrained gases carbon dioxide, ammonia, nitrogen, etc.
  • Generating a biological floc that is easy to settle
  • Generating liquor that is low in dissolved or suspended material.

The Process

The process involves air or oxygen being introduced into a mixture of screened, and primary treated sewage or industrial waste-water (waste- water) combined with organisms to develop a biological floc which reduces the organic content of the sewage.

This material, which in healthy sludge is a brown floc, is largely composed of saprotrophic bacteria but also has an important protozoan flora mainly composed of amoebae, spirotrichs, peritrichs including vorticellids and a range of other filter feeding species.

Other important constituents include motile and sedentary Rotifers. In poorly managed activated sludge, a range of mucilaginous filamentous bacteria can develop including Sphaerotilus natans which produces a sludge that is difficult to settle and can result in the sludge blanket decanting over the weirs in the settlement tank to severely contaminate the final effluent quality.

This material is often described as sewage fungus but true fungal communities are relatively uncommon.

The combination of waste-water and biological mass is commonly known as mixed liquor. In all activated sludge plants, once the waste- water has received sufficient treatment, excess mixed liquor is discharged into settling tanks and the treated supernatant is run off to undergo further treatment before discharge.

Part of the settled material, the sludge, is returned to the head of the aeration system to re-seed the new waste-water entering the tank. This fraction of the floc is called return activated sludge (R.A.S.).

Excess sludge is called surplusactivated sludge (S.A.S.) or waste activated sludge (W.A.S). W.A.S is removed from the treatment process to keep the ratio of biomass to food supplied in the waste-water in balance, and is further treated by digestion, either under anaerobic or aerobic conditions prior to disposal.

Many sewage treatment plants use axial flow pumps to transfer nitrified mixed liquor from the aeration zone to the anoxic zone for denitrification. These pumps are often referred to as internal mixed liquor recycle pumps(IMLR pumps).

The raw sewage, the RAS, and the nitrified mixed liquor are mixed by submersible mixers in the anoxic zones in order to achieve denitrification.

Activated sludge is also the name given to the active biological material produced by activated sludge plants.

Activated Sludge Control

The general method of doing this is by monitoring sludge blanket level, SVI (Sludge Volume Index), MCRT (Mean Cell Residence Time), F/M (Food to Microorganism), as well as the biota of the activated sludge and the major nutrients DO (Dissolved oxygen), nitrogen, phosphate, BOD (Biological oxygen demand), and COD (Chemical oxygen demand). In the reactor/aerator + clarifier system:

The sludge blanket is measured from the bottom of the clarifier to the level of settled solids in the clarifier’s water column; this, in large plants, can be done up to three times a day.

The SVI is the volume of settled sludge in milliliters occupied by 1 gram of dry sludge solids after 30 minutes of settling in a 1000 milliliter graduated cylinder (Burton and Stensel, 2003).

Read Also: Industrial Wastewater Treatment Procedure

The MCRT is the total mass (lbs) of mixed liquor suspended solids in the aerator and clarifier divided by the mass flow rate (lbs/day) of mixed liquor suspended solids leaving as WAS and final effluent ( Burton and Stensel, 2003).

The F/M is the ratio of food fed to the micro-organisms each day to the mass of micro-organisms held under aeration. Specifically, it is the amount of BOD fed to the aerator (lbs/day) divided by the amount (lbs) of MLVSS (Mixed Liquor Volatile Suspended Solids) under aeration.

Note: Some references use MLSS (Mixed Liquor Suspended Solids) for expedience, but MLVSS is considered more accurate for the measure of microorganisms. Again, due to expedience, COD is generally used, in lieu of BOD, as BOD takes five days for results.


The general arrangement of an activated sludge process for removing carbonaceous pollution includes the following items:

Aeration tank where air (or oxygen) is injected in the mixed liquor.

Settling tank (usually referred to as “final clarifier” or “secondary settling tank”) to allow the biological flocs (the sludge blanket) to settle, thus separating the biological sludge from the clear treated water.

Treatment of nitrogenous matter or phosphate involves additional steps where the mixed liquor is left in anoxic condition (meaning that there is no residual dissolved oxygen).

Types of Plants

Activated Sludge System Process

There are a variety of types of activated sludge plants according to Burton and Stensel (2003). These include:

Package Plants

There are a wide range of other types of plants, often serving small communities or industrial plants that may use hybrid treatment processes often involving the use of aerobic sludge to treat the incoming sewage.

In such plants the primary settlement stage of treatment may be omitted. In these plants, a biotic floc is created which provides the required substrate.

Package plants are commonly variants of extended aeration, to promote the ‘fit & forget’ approach required for small communities without dedicated operational staff. There are various standards to assist with their design.

Activated sludge is another method of providing secondary treatment to waste-water, whereby a mixture of waste-water and biological sludge (microorganisms) is agitated and aerated. The biological solids are then allowed to settle out.

The name “activated sludge” comes from the biological mass formed when oxygen (in the form of air) is continuously injected into the waste- water. In this process, micro-organisms are thoroughly mixed with organics under conditions that stimulate their growth.

As the micro- organisms grow and are mixed by the agitation of the air, the individual micro-organisms clump (or flocculate) together to form a mass of microbes called activatedsludge. About eight cubic feet of air are required for every cubic foot of waste-water.

In the activated sludge process, waste-water flows continuously into an aeration tank where air is injected into the waste-water to mix the waste- water with the activated sludge, and also to provide the oxygen needed for the micro-organisms to break down the organic pollutants.

The mixture of waste-water and activated sludge is called mixed liquor. The mixed liquor flows to a secondaryclarifier (settling tank) where the activated sludge settles out. Some (usually twenty or thirty percent) of the settled sludge is returned to the aeration tank (and hence is called return sludge) to maintain a high population of microbes to break down the organics.

Since more activated sludge is produced than is needed for return sludge, the excess sludge is removed and disposed of.

Oxidation Ditch

In some areas, where more land is available, sewage is treated in large round or oval ditches with one or more horizontal aerators typically called brush or disc aerators which drive the mixed liquor around the ditch and provide aeration. These are oxidation ditches, often referred to by manufacturer’s trade names such as Pasveer, Orbal, or Carrousel.

They have the advantage that they are relatively easy to maintain and are resilient to shock loads that often occur in smaller communities (i.e. at breakfast time and in the evening).

Oxidation ditches are installed commonly as ‘fit & forget’ technology, with typical design parameters of a hydraulic retention time of 24 – 48 hours, and a sludge age of 12 – 20 days. This compares with nitrifying activated sludge plants having a retention time of 8 hours, and a sludge age of 8 – 12 days.

Deep Shaft

Where land is in short supply sewage may be treated by injection of oxygen into a pressured return sludge stream which is injected into the base of a deep columnar tank buried in the ground. Such shafts may be up to 100 metres deep and are filled with sewage liquor.

As the sewage rises the oxygen forced into solution by the pressure at the base of the shaft breaks out as molecular oxygen providing a highly efficient source of oxygen for the activated sludge biota.

Read Also: Industrial Waste-water Treatment Trickle Filters

The rising oxygen and injected return sludge provide the physical mechanism for mixing of the sewage and sludge. Mixed sludge and sewage is decanted at the surface and separated into supernatant and sludge components. The efficiency of deep shaft treatment can be high.

Surface aerators are commonly quoted as having an aeration efficiency of 0.5 – 1.5 kg O2/kWh, diffused aeration as 1.5 – 2.5 kg O2/KWh. Deep Shaft claims 5 – 8 kg O2/kWh.

However, the costs of construction are high. Deep Shaft has seen greatest uptake in Japan, because of the land area issues.

Deep Shaft was developed by ICI, as a spin-off from their Pruteen process. In the UK it is found at three sites: Tilbury, Anglian water, treating a waste-water with a high industrial contribution; Southport, United Utilities, because of land space issues; and Billingham, ICI, again treating industrial effluent, and built (after the Tilbury shafts) by ICI to help the agent sell more.

Deep Shaft is a patented, licensed, process. The licensee has changed several times and, currently, it is Aker Kvaerner Engineering Services.

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Benadine Nonye

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