To control activated sludge, you need to “have a handle” either on aeration, sludge-wasting or return-sludge flow.

Aeration-rate adjustment is probably the simplest way to go. It involves, however, more than measuring once each day aeration-basin dissolved oxygen (DO) concentration. For example, consider factors that influence dissolved-oxygen levels in the aeration basin and adjust those parameters for balance.

Aeration balance simply means looking at the basin’s dissolved-oxygen profile and adjusting the air flow rates, in a diffused air system. In a mechanically aerated system, consider the number and location of mechanical aerators. By balancing aeration rates, dissolved-oxygen levels can be maintained throughout aeration treatment.

A properly maintained diffuser can also improve aeration efficiency.

In controlling aeration consider that the aerated oxygen required is based on the direct relationship between the influent bio-chemical oxygen demand (BOD) concentration and the aeration basin dissolved oxygen.  As BOD concentration entering the aeration basin rises, the amount of oxygen required to maintain dissolved oxygen levels rises also.

Another factor is how much aeration is required to maintain a given level of dissolved oxygen that is directly proportional to the amount of bacteria in the aeration basin.


Sludge wasting is also important to activated sludge process control.  Sludge wasting rates affect the following:

•                  Growth rate of the bacteria

•                  Oxygen consumption

•                  Mixed liquor settle-ability

•                  Occurrence of foaming/frothing

•                  The possibility of nitrifying

•                  Nutrient quantities needed

•                  Quality of the final effluent

Wasting removes solids buildup in the activated sludge system, which is due to the solids amounts in the aeration tank influent being greater than the solids amounts in the secondary clarifier effluent. If sludge is not wasted, the secondary clarifier eventually fills up with solids.

Methods of determining the wasting rate include maintaining constant mixed-liquor solids-level control in the aeration tank, as well as a constant food-to-microorganism balance by controlling F/M Ratio or by using Sludge Age (SA) or Mean Cell Residence Time (MCRT) in the overall activated sludge system to control wasting.

In determining the appropriate sludge wasting target value, consider sludge age, foam, mixed-liquor color & concentration, settle-ability, solids rising and how much sludge is in secondary clarifier.

Finally, to optimize based on sludge-wasting, identify the best sludge age, consider seasonal influences on sludge-wasting values and make small sludge-wasting flow-rate adjustments as needed.


Return sludge is the liquid removed from the bottom of the secondary clarifier and pumped back to the aeration basin. Return sludge is a mix of water and solids settled out in the secondary clarifier. The solids include live bacteria. Therefore, if there is failure to remove sludge from the clarifier bottom, the level of settled sludge will get deeper and deeper. Within a certain time it will begin spilling over the clarifier effluent weirs, and flow into unwanted areas.

Changes in return-sludge flow-rate affect return-sludge and mixed-liquor concentration in the aeration tank. To control return-sludge flow, monitor and evaluate performance resulting from return-sludge flow-rate changes.

Correct return-sludge flow rate is important. The consequences of incorrect flow should be avoided. These include shut-off of the return-sludge flow rate, causing the secondary clarifier to fill up with solids and a decrease in bacteria, dissolved oxygen and fresh food in the aeration tank. Another alternative to avoid is setting return-sludge flow to maximum, causing settled solids in the secondary clarifier to decrease and returned sludge, containing more water, to enter the aeration tank, as well as decreased aeration-tank bacteria.

In determining the correct return-sludge flow-rate, monitor settled-sludge depth along with flocculated sludge solid-particle size and sludge-wasting flow-rate changes.


Mass Balance refers to the amount of solids entering the treatment unit and the amount of solids leaving the treatment unit. While using the mass-balance approach, consider both the level of settled solids in the secondary clarifier and the amount of solids leaving the secondary clarifier.

The “sludge-quality” approach to return-sludge flow control relies on loading, process balance, and sludge-quality characteristics to reveal the clarifier-sludge flow rate that will best satisfy the net requirement of all these interacting variables. The calculated demand satisfies the coordinated requirements imposed by changing mixed-liquor sludge concentrations and quality, sludge-solids distribution between the aeration tanks and the final clarifiers, and the wastewater flow rates. This approach may yield results that entail either settling too fast, settling too slow or normal settling conditions.

In summary, for return-sludge control, always keep return-sludge flowing, make small adjustments, and repeat adjustments after adequate time passes to evaluate prior adjustment.

Overall process controls for activated sludge has been described above. However, if you have specific issues controlling activated sludge or other wastewater queries, please submit a question.