Another Road Warrior Observation: F:M Issues in Wastewater Treatment

During my travels to wastewater facilities, I have repeatedly observed one operational issue that can significantly reduce treatment efficiency: an insufficient food supply for the microorganisms responsible for biological wastewater treatment.

Whether a wastewater treatment plant (WWTP) uses an activated sludge process, lagoon system, aerated treatment, anaerobic treatment, or an advanced water-reuse process, successful biological treatment depends on maintaining a healthy and active microbial population.

One of the most important factors is maintaining the proper food-to-microorganism ratio (F:M).

What Is the Food-to-Microorganism Ratio?

The food-to-microorganism ratio, commonly called the F:M ratio, describes the relationship between the amount of biodegradable organic material entering a biological treatment process and the amount of microbial biomass available to consume it.

When the balance between food and microorganisms changes significantly, treatment performance can suffer.

An unfavorable F:M ratio may contribute to:

Poor sludge settling
Excessive foaming
Unstable biomass
Reduced nitrification
Reduced denitrification
Increased odors
Higher effluent BOD
Higher total suspended solids (TSS)
Difficulty meeting ammonia or nitrate limits

These problems can make it more difficult for a wastewater facility to meet its National Pollutant Discharge Elimination System (NPDES) permit requirements.

Low Wastewater Flows Can Reduce the Available Food Supply

One common cause of F:M problems is a significant reduction in influent flow and organic loading.

During periods when communities experience fewer visitors or residents, wastewater plants may receive considerably less flow than expected.

Lower influent volumes can also mean lower concentrations or total loads of:

Biochemical oxygen demand (BOD)
Chemical oxygen demand (COD)
Total suspended solids (TSS)
Other biodegradable organic material

These materials represent much of the food available to microorganisms in a biological wastewater treatment system.

When the available food supply falls, microbial activity can decline and portions of the biomass may die off or become less active.

Case Example: Reduced Flow at an Arizona Wastewater Facility

During a visit to a municipality in Arizona, I observed a wastewater facility that normally received approximately 1.2 million gallons per day (MGD) during the winter season.

During the period discussed in this article, flows had fallen to approximately 850,000 gallons per day.

At the time, the municipality believed reduced seasonal population associated with the COVID-19 pandemic was contributing to the lower wastewater flows.

The reduced loading coincided with several operational challenges, including:

Excessive foaming
Poor settling
Nitrification problems
Denitrification problems
Difficulty maintaining permit compliance

This situation demonstrated how dramatically changing influent conditions can affect the balance between wastewater loading and microbial populations.

Household Chemicals Can Also Affect Wastewater Biology

Another concern during this period was the increased use of disinfectants, antibacterial products, and sanitary wipes.

When certain antimicrobial chemicals enter wastewater systems at elevated concentrations, they may negatively affect portions of the microbial community.

Sensitive populations, including microorganisms involved in nitrification and denitrification, can be especially vulnerable to toxic or inhibitory compounds.

Non-flushable wipes can create additional collection-system and treatment problems because they do not readily break down like toilet paper.

How to Correct an F:M Imbalance

There is no single solution for every F:M problem.

Operators should first determine whether the imbalance is caused by:

Too little organic carbon
Too much biomass
Insufficient microbial populations
Toxicity
Hydraulic changes
Poor oxygen conditions
Nutrient limitations
Other process conditions

Two strategies commonly considered are biostimulation and bioaugmentation.

Biostimulation: Increasing the Available Food Supply

Biostimulation involves supplying nutrients or carbon sources that support microorganisms already present in the wastewater treatment process.

When a treatment system has insufficient readily biodegradable carbon, supplemental carbon may help support microbial activity.

Carbon sources used in wastewater treatment may include:

BioLynceus® Candy Carbon®
Methanol
Acetic acid
Commercial supplemental carbon products
Suitable industrial or food-processing byproducts

The appropriate carbon source depends on the treatment objective, wastewater chemistry, microbial population, safety considerations, and operating costs.

Operators should evaluate supplemental carbon carefully rather than simply adding any available organic material.

Certain food products or animal feeds can introduce fats, proteins, solids, and other materials that may create additional treatment challenges.

Bioaugmentation: Increasing Microbial Populations

When an F:M imbalance is related to insufficient or damaged microbial populations, bioaugmentation may be considered.

Bioaugmentation involves introducing additional microorganisms into the treatment process to supplement the existing biomass.

Commercial microbial products are one option, but operators should evaluate factors such as:

Microbial viability
Species diversity
Product stability
Treatment objective
Application method
Storage requirements
Manufacturer quality control

The goal is to introduce microorganisms capable of supporting the biological functions required by the treatment plant.

Using Imported Activated Sludge for Bioaugmentation

Another approach is to introduce activated sludge from a nearby wastewater treatment facility.

Imported activated sludge can provide a substantial microbial population, but the process also has limitations.

Potential concerns include:

Transportation costs
Community concerns about hauling wastewater solids
Differences between treatment processes
Unknown microbial composition
Introduction of undesirable organisms
Potential introduction of excessive filamentous bacteria

Imported sludge that performs well at one wastewater plant may not necessarily be ideally suited to another facility.

For example, excessive populations of certain filamentous bacteria can contribute to foaming or poor settling when favorable conditions, including high fats, oils, and grease (FOG), are present.

Monitor the Biology Before Choosing a Solution

Maintaining an effective F:M ratio requires more than simply adding bacteria or carbon.

Operators should evaluate the entire biological treatment process, including:

Influent BOD and COD loading
Mixed liquor suspended solids
Sludge age
Dissolved oxygen
Temperature
pH
Ammonia and nitrate concentrations
Settling characteristics
Organic carbon availability
Toxic or inhibitory discharges

Understanding why the microbial population is struggling is essential before selecting a treatment strategy.

Keeping Wastewater Microorganisms Fed and Active

A wastewater treatment plant depends on microorganisms to perform much of the biological treatment.

When influent flows, organic loading, or microbial populations change, the food-to-microorganism balance can shift quickly.

Biostimulation may help when microorganisms need additional carbon or nutrients, while bioaugmentation may help when the system needs additional microbial populations.

The most effective approach depends on the specific wastewater system and the cause of the imbalance.

For more information about F:M ratios, wastewater bioaugmentation, or biostimulation, contact:

Rick Allen
970-586-3391
rick@biolynceus.net

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