What are the control factors and calculations?

The proper performance of an activated sludge plant is enhanced with knowledge of five biological and physical elements that determine the efficiency of the process. These factors include:

  1. Organic and hydraulic loading on the aeration tank
  2. Dissolved oxygen (DO) in the aeration tank
  3. Biosolids wasting rate
  4. Return activated sludge rate
  5. Solids settling and compaction characteristics

Organic and hydraulic loading on the aeration tank

Organic loading refers to the number of pounds per day of biochemical oxygen demand (BOD) that enter the activated sludge process. Hydraulic loading refers to the million gallons per day (mgd) flows to a wastewater treatment process.

Quantity of microorganisms

Mixed liquor volatile suspended solids (MLVSS) is the quantity of microorganisms in an activated sludge system defined as the organic or volatile suspended solids in the mixed liquor of an aeration tank. This volatile part is utilized as a measure or indication of the microorganisms present.

The amount of microorganisms available for treatment is calculated as follows:

MLVSS (lbs.) = (Aeration, mg) x (MLVSS, mg/L) x (Conv., 8.34)

Dissolved oxygen in the aeration tank

DO is defined as the molecular (atmospheric) oxygen dissolved in water or wastewater.

The purpose of maintaining DO in the aeration tank is to ensure the biology in the aeration process has a prescribed DO range to remain alive and remove organic matter most effectively.

Two reasons to maintain the DO level between 1.0 milligrams per liter (mg/L) and 2.0 mg/L in the aeration tank are:

  1. Lower DO increases oxygen transfer rate (which is proportional to the difference between actual and saturation dissolved oxygen of 9 to 10 mg/L, temperature-dependent). The energy for oxygen transfer is reduced with the larger difference between saturation and operating, so DO of 1.0 mg/L is better than DO of 2.0 mg/L.
  2. DO is an indication of sufficient mixing to prevent dead zones in the aeration tank, so DO of 2.0 mg/L is better than DO of 1.0 mg/L.

The net result is this range is used in practice as an effective compromise.

Food-to-microorganism ratio

The food-to-microorganism (F:M) ratio is specified as the amount of food provided to bacteria in an aeration tank.

The F:M ratio is one of the primary controls used in activated sludge plants. This allows the operator to maintain equilibrium between the quantity of food available with the quantity of microorganisms in the tanks. The food available to the microorganisms is represented by the wastewater BOD.

While the best treatment may not occur at the same F:M ratio in different plants, the spectrum of conventional activated sludge plants is often 0.20 to 0.50. Activated sludge plants that work in the extended aeration mode typically work with an F:M ratio in the 0.045 to 0.20 range.

Since the operator usually has no control over the number of pounds of BOD entering the wastewater treatment plant, the F:M ratio is adjusted by modifying the number of pounds of MLVSS in the secondary system.

If more biomass is needed (raising MLVSS) the amount of biomass wasted must be scaled down, and if less biomass is needed (lowering MLVSS) the wasting rate must be escalated until the necessary pounds of biomass are met.

Two matters that must be remembered regarding making operational changes are:

  1. Biological systems react slowly to these types of control changes; Therefore, give the system time to adapt to a change before making another alteration.
  2. Admit that consistency is often the key to successful operation; Therefore, consider applying a moving average to calculate the pounds of BOD and make readjustments only when necessary.

The F:M ratio is calculated as follows:

F:M Ratio = BOD5 (lbs./day)

MLVSS (lbs.)

Cell residence time

Cell residence time (CRT) or sludge age (SA) represents the duration of time a particle of suspended solids has been retained in the activated sludge process.

The CRT or SA is calculated as below:

CRT/SA, (Days)  =   Total MLVSS (lbs.)

Total MLVSS wasted (lbs./day)

Biosolids wasting rate

The biosolids wasting rate is known as waste activated sludge (WAS), which is the excess growth of microorganisms that must be removed from the process to maintain the biological system stability. It is accomplished by pumping a portion of the return sludge to the solids handling facility.

Biosolids wasting of secondary sludge is one of the most important controls available to the operator.

Determining a wasting rate first involves deciding on a target CRT or SA. For guidance, the CRT for a conventional activated sludge plant may be at least seven days, or for an extended air plant the CRT may be established no less than 19 days.

When determining the target CRT, solve for the number of pounds of biosolids wasting rate known as waste activated sludge (WAS) as follows:

WAS (lbs.)  =  MLVSS in aeration (lbs.)

CRT (day)

Return activated sludge rate

Return activated sludge (RAS) refers to the settled activated sludge accumulated in the secondary clarifier before it is returned to the aeration basin to mix with incoming primary treated effluent.

An approach using the poundage formula to calculate the RAS flow rate is established when the pounds of solids drawn from the secondary clarifier (RAS) are equal to the pounds of material entering the secondary clarifier (MLSS) as follows:

Rq  =     (Q) x (MLSS)


In this formula, Rq is the calculated RAS (expressed in mgd), Q is the influent flow rate (in mgd), MLSS is the mixed liquor suspended solids concentration in milligrams per liter and RAS SS is the RAS suspended solids concentration in milligrams per liter.

Solids settling and compaction characteristics

Solids settling by the settleometer test determines the settling characteristics of mixed liquor. Solids compaction characteristics can be assessed by using the sludge volume index (SVI) and the sludge density index (SDI).

SVI index:

The SVI is defined as the volume in milliliters occupied by one gram of activated sludge that has settled in 30 minutes.

The SVI calculation relates sludge volume in milliliters to MLSS concentration in grams per liter as follows:

SVI  =         (mls settled)

(MLSS, mg/L) / (1,000)

SDI index

The SVI is defined as the grams of activated sludge that occupy a volume of 100 ml after 30 minutes of gravity settling.

The SDI calculation is:

SDI  =            (MLSS, mg/L) / (1,000)

(mls settled in 30 min.) / (100)


This article discussed control factors and calculations to help assess, troubleshoot and potentially improve wastewater treatment quality.


Known in the industry as “Wastewater Dan,” Dan Theobald, proprietor of Environmental Services, is a professional wastewater and safety consultant/trainer. He has more than 24 years of hands-on industry experience operating many variants of wastewater treatment processing units and is anxious to share his knowledge with others.