4 Ways to Balance Food to Microorganism Ratio (F:M) in Wastewater Treatment

Food to Microbe, Wastewater Articles

4 Ways to Balance Food to Microorganism Ratio (F:M) in Wastewater Treatment


Wastewater treatment requires a constant balance between the organic carbon coming into the plant and the quantity of microbiology available to consume the influent material. This relationship is known as the food to microorganism ratio (F:M).

The “M” portion of the ratio is also referred to as mass which is short for biomass. Regardless of how “M” is defined, this ratio is an important parameter that can be measured, tracked and controlled to help maintain effective treatment in mechanical wastewater facilities.

F:M imbalances cause plant discharge high in solids or nutrient content. There are several common factors that can cause deviations from a plant’s optimal ratio. These include, but are not limited to, the following:

Low Flow (Low F:M)

Wastewater plants may experience seasonal periods with low flows. These facilities are often in resort towns where flows are dependent on the influx of tourists and seasonal residents. When influent underloading occurs (due to low flows), the biomass in wastewater plants can starve causing reduced viability and and concentration.    

Extreme Infiltration and Inflow

High infiltration and inflow (I&I) primarily occurs in wastewater collection systems with cracks and surface openings that storm water can enter through. Added rain or snow melt dilutes wastewater biological oxygen demand (BOD) concentrations (aka organic carbon loads). Reducing the concentration of food in wastewater can starve and diminish the microbiology in the treatment process.

In some extreme I&I events, the biomass may be washed out because of high diluted wastewater flow through the treatment system.

High BOD Slugs (High F:M)

Wastewater plants often have very little control over the nature of influent entering the facility. High BOD slugs in wastewater can overwhelm the existing biomass which reduces treatment efficiency. Additionally, these loads can be toxic to nitrifying bacteria and other essential species in the treatment biomass. 

Once an imbalance in F:M is detected (see the calculating F:M heading below), there are at least four ways wastewater operators can adjust this ratio to maintain proper treatment in their systems. Under each item, actions to improve [a] low and [b] high F:M are discussed.  

The List

Depending on the wastewater plant design, operational flexibility, and budget some facilities might choose to do nothing and just wait for F:M to improve naturally. 

1. No Action

[a] Sometimes when the F:M is low, a plant can wait for the influent wastewater composition to change. For instance, a wastewater system in a resort town might take no action because they know flows and BOD concentrations will increase over the weekend or during tourist season. There is a chance the residual microbiology will recover after receiving a consistent food supply again.  

[b] Similarly, when F:M is high, a wastewater plant may choose to wait out a period of treatment with no action. High BOD slugs may occur only occasionally. After BOD levels decrease to normal, the biomass may or may not recover on its own.  


This option is the least expensive and operationally challenging. Also, there is little risk of making the treatment efficiency worse by adjusting the system dynamics.  


This methods risks producing poorly treated effluent. The likelihood of violating a permit limit can increases if the microbiology does not recover from high or low organic loading. Additionally, the biomass in a system can die without a food source, so even if organic loading increases, the microbiology in the plant can be too dispersed to treat wastewater. Running a treatment facility with an unbalanced F:M can lead to excessive filamentous bacterial growth and poor settling.  

2. Sludge Wasting

Activated sludge wasting is the process of removing biomass from mechanical treatment systems. Operators have control over how much activated sludge is wasted and how much is returned to the treatment basins to keep the biological process going.

[a] Systems with lower than optimal F:M can increase wasting to improve treatment efficiency. This decreases the “M” part of the relationship increasing the overall value.

[b]Wastewater plants that have a F:M higher than optimal can decrease wasting. Adjusting wasting in this way helps increase “M” which decreases F:M.    


Wasting and returning activated sludge is relatively simple. No added products are needed because the solution comes from within the system.  


Changes in F:M from optimal can cause activated sludge to have low abundance of residual microbiology. The biomass in the activated sludge can be mostly dead or shifted towards an undesirable community for treatment. In these instances, more returned activated sludge will not effectively improve treatment. 

Additional biological products may be required. Wasting more activated sludge can add to the cost of solids handling and disposal. 

3. Adjust Operational Factors

There are a few operational adjustments that can be made to help increase or decrease F:M. These include changing the treatment capacity and aeration practices in a wastewater treatment facility.   

[a] Turning off aeration basins and trains can help increase F:M. In doing so, operators can control the volume of wastewater treated. Wastewater is treated in a smaller area which concentrates influent BOD loads. By decreasing the volume of the treatment basins, the “M” side of the equation (see below) is decreased, resulting in a higher F:M value.

[b] Increasing the aeration in a treatment process can help decrease F:M. Turning up aeration adds dissolved oxygen to the treatment system. Most microorganisms that break down organic material for growth and function require oxygen to facilitate the process. In the presence of high BOD slugs, oxygen can be a limiting factor for the rate at which BOD is consumed and more biomass is created. By increasing aeration, the influent food is decomposed rapidly and the high F:M is quickly stabilized.


Turning off treatment basins and increasing aeration are both solutions that can be implemented without added commercial products. This can help keep costs down. Wastewater operators learn to adjust their systems to accommodate changes in organic loading will increase their chances of maintain consistent treatment efficiency.

Caution! Frequent changes to operational factors can be disastrous. Changes to wasting, and treatment volume should only be made after trends over several hours or days show that a change is required.


The limitations of adjusting treated wastewater volume and aeration are as follows; The above operational adjustments might not have a big enough impact on F:M to improve treatment efficiency. Operational costs can increase from running aerators on high settings for long periods of time. 

4. Add External Organic Carbon or Microbiology

A great way to balance F:M in wastewater treatment is to supplement with external organic carbon or biological products. These solutions can provide a fast and effective way to adjust F:M.  

[a]Adding an external source of organic carbon can help increase the F:M in a wastewater treatment facility. Organic carbon is an essential requirement to grow a strong treatment biomass. Increasing organic carbon loads in a wastewater plant boosts the “F” side of the ratio. This results in a higher F:M value. Many sources of organic carbon can be used to increase F:M. Some of these include, but are not limited to:

  • Methanol
  • Glycerin
  • Brewery waste
  • Dog food
  • Commercial organic carbon products.

Candy Carbon® is a BioLynceus® recommended source of external organic carbon for wastewater systems. This product is composed of humic substances derived from earth materials. The qualities which set Candy Carbon® apart from other external carbon supplements include: 

  • Lower dosing rate to achieve improved F:M and treatment efficiency
  • Treating with Candy Carbon® does not contribute to other treatment issues

[b]High F:M can be lowered by implementing additional treatment microbiology (bioaugmentation) in wastewater processes. The “M” side of the ratio is increased bringing the value down to a more desirable number. Depending on the source of microbiology used, this method can be highly effective for decreasing F:M but can also cause additional treatment issues.

In other words, product selection is most important when implementing bioaugmentation. The following bullets provide several common sources of microbiology, commonly called “bugs” for wastewater treatment:  

  • Activated sludge from another wastewater plant
  • Dried bugs
  • Live cultures

The Team at BioLynceus® recommends ProBiotic Scrubber® II as a source of “good clean bugs” for F:M issues. ProBiotic Scrubber® II contains diverse, live communities of wastewater treating bacteria cultured from natural soils and sediments. This source of microbiology stands apart from others because: 


Increasing the concentration of microbiology in a system by adding a clean bug product is highly effective for reducing high F:M. This solution will not cause additional treatment issues relating to the types of species added to the process. Having an organic carbon or biological product on hand gives operators a fast solution to tackle their F:M issues.   


Commercial external organic carbon products can be expensive. Methanol products can be dangerous to handle which can increase operational hazards and inconvenience.

Adding dog food to these systems is inexpensive but can cause issues with fats, oils, and grease accumulation and toxicity.
Furthermore, implementing activated sludge from another wastewater plant can seed the facility with unwanted filamentous bacteria and trash. 

Closing Thoughts

The relationship between incoming BOD and active microbiology in a system has a profound impact on the wastewater treatment efficiency. Balancing F:M in wastewater facilities is an active task which most often involves operational adjustments and may requiring adding commercial products.

The final section of this article provides the parameters and equations used to calculate F:M. By knowing the optimal F:M for a wastewater plant, operators can implement some of the solutions above to keep the ratio in range to properly treat wastewater. 

Calculating F:M 


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