Mercater pty ltd banner images


Treatment Plant Technology

Activated Sludge using IDEA Process Description

An activated sludge (heterogenous microbial culture consisting of bacteria, protozoa, rotifers and fungi) process is capable of converting most organic waste to a more stable inorganic form or to a cellular mass. Much of the colloidal and soluble organic matter in the influent is metabolised by a diverse group of micro-organisms to carbon dioxide and water. At the same time a sizeable fraction is converted to a cellular mass that can be separated by gravity sedimentation and removed from the process (To thickening tanks). Sewage (influent) enters the tank periodically at random. A baffle or distribution header prevents short-circuiting of influent to the decanter.

Stage 1 Aeration

Waste water is aerated in the presence of microbial suspension. Aeration feeds the oxygen demand of the organic mass as well as mixing the mixed liquor so as cell contact occurs. Aeration is activated in single (Sequence) or duel (Cascade) reactors tanks. Depending on the size of the plant and the stages the plant is being brought on line. Non aeration periods cause the activated sludge to seek oxygen from substrate compounds (Anoxic) which transforms nitrogen based compounds to release nitrogen biologically. Some other nutrients are removed biologically (Phosphorus) but to a lesser extent. (If cascading tanks are used Return Activated Sludge (RAS) is transferred back to the head of the reactors.-Delete all this).

Stage 2 Settle

Aeration is off and the cellular mass in the mixed liquor is allowed to settle by gravitational means after reaching a fixed top water level (TWL) in the tank. The coagulation of the biomass in the mixed liquor greatly assists, resulting in clear liquor below the surface level but on top of the tank settled sludge.

Stage 3 Decant

After a period of settling the Mercater motorised decanter system valve is opened causing treated effluent to decant from the tank by gravity. Once Bottom Water Level (BWL) is reached the decanter valve is closed and the flow stopped. The treated effluent is normally decanted to a contact storage tank.

Stage 4 Disinfectant

Disinfection occurs by the introduction of Sodium Hypochlorite in the contact tank during a decant phase. The mixing and then contact time for up to 30 minutes ensures disinfection to the classification required. Further disinfection processes can happen depending on the required finish of tertiary level treatment. UV systems are used for greater disinfection levels if the clarity of the treated effluent can be maintained. This will usually occur by using fine sand filters.


Suspended solids can be as low as 10mg/L. Biological Oxygen Demand (BOD) can be as low as 5mg/L. Total nitrogen can be as low as 6mg/L all with out additional filtration or enhancing chemicals providing plants are operated within the design parameters. Plant operation is very simple and all training is provided by Mercater. Energy usage is minimised by the control philosophy and live examples of energy usage can be provided on existing operating plants.

Mercater logo

All Rights Reserved - Mercater Pty Ltd - ABN 35 104 794 989

Wastewater Treatment Engineers