By Jael Batty

Activated sludge is a mixture of microorganisms that come in contact with and digest biodegradable materials (food) from wastewater. Once most of the material is removed from the wastewater, microorganisms form floc and settle out as sludge. Some type of microorganism will always grow in the system. The organisms that will dominate will be the ones that are best suited to the environment.1

Microorganisms that are natural to the wastewater environment play a vital role in the wastewater treatment process. Beneficial bacteria, protozoa, metazoa, algae, and fungi feed on organic material in wastewater, breaking it down. Bacteria clump together, or floc, forming masses that settle and separate from wastewater liquids. This settled mass is called sludge. This week, we’re going to talk specifically about wastewater bacteria.

Aerobic, Anaerobic and Facultative Bacteria

Bacteria account for 95% of the microorganisms in wastewater. They are single-celled microorganisms that are classified based on their response to oxygen.

Aerobic bacteria use oxygen, which is added mechanically, to break down wastewater contaminants, converting it into energy. Bacteria use this energy to grow and reproduce. Anaerobic bacteria obtain oxygen from their food source. As anaerobic bacteria break down sludge, they produce methane gas. This methane gas can be used as an alternative energy source, called biogas, to fuel the wastewater treatment process. Anaerobic bacteria can also be used to lower the amount of phosphorus in the effluent. Although they prefer oxygen, facultative bacteria can switch between aerobic and anaerobic forms in response to their environment.1,2

Bacteria Consume Organic Material

Bacteria consume biodegradable organic materials, including proteins, carbohydrates, and fats through adsorption and absorption:

During adsorption, food particles that are too big to pass through the cell membrane and bacteria stick to each other. The bacteria secrete enzymes, which dissolve food particles into very small units. These small units of food can now pass through the bacteria’s cell wall. Absorption is the process by which smaller dissolved units of food pass into the cell membrane.1

Five Phases of Bacteria Growth

The mixture of microorganisms and wastewater in the aeration tank is called “mixed liquor.” Bacteria goes through 5 phases of growth in the mixed liquor.

  • During the lag phase, bacteria adapt to the environment, developing the required enzymes to digest nutrients. Bacteria break down nutrients with enzymes that only work in favorable conditions. Bacteria cannot survive if enzymes are not working properly.
  • Influent wastewater contains high levels of nutrients that bacteria use for growth and energy. Bacteria begin to grow and reproduce during the accelerated growth phase. Growing bacteria move about in search of nutrients, quickly multiplying. They do not settle to form floc.
  • During the declining growth phase, nutrient levels begin to drop and bacteria compete for nutrients. When food levels are low, bacteria slow down to conserve energy and do not grow or reproduce. Nutrients are used for energy and cell maintenance.
  • Bacteria levels remain constant during the stationary phase. They form a thick slime layer of waste products on the outside of the cell wall. This slime layer causes bacteria to clump together to form floc.
  • The number of bacteria reduces during the death phase.1

Food-to-Microorganism Ratio

The amount of food available to microorganisms is calculated by the food-to-microorganism (F/M) ratio. The F/M ratio is the measurement of incoming food divided by the microorganisms (in lbs.) in the system. This is determined by dividing results of the BOD/COD test by the mixed liquor volatile suspended solids (MLVSS).

If the mixed liquor is not held long enough for nutrient levels to drop, a high F/M ratio develops in which bacteria are dispersed and continue reproducing and moving about actively. This prevents floc from developing, resulting in settling problems and turbid effluent. Only when nutrients are limited are bacteria able to develop the slime layer and clump together to form floc.3

If the F/M ratio is too low, nutrient deficiencies can occur. An environment deficient of phosphorus, nitrogen, and/or sulfur interrupts the development of the cell wall, creating water resistance, which results in floc dispersion, sludge bulking, and foaming. It becomes difficult for nutrients to penetrate the cell wall. Slime bulking may occur in environments deficient in phosphorus or nitrogen or high in organic acids.1,3


The F/M ratio can be adjusted with bioremediation, which is a process of treating wastewater to encourage the growth of existing microorganisms. With biostimulation, supplemental nutrients, vitamins, minerals, organic acids, and pH buffers are introduced into wastewater to create a hospitable environment that stimulates naturally occurring microorganisms. Bioaugmentation, the addition of microorganisms, may be used to restart activated sludge systems or to aid in the breakdown of a targeted pollutant.

Bioremediation increases the bio-oxidation of wastewater and reduces grease, sludge, and odor levels in wastewater treatment plants, lagoons, and ponds. Bioremediation is used to help degrade heavy metals, petroleum compounds, and hazardous wastes. Biostimulant and bioaugmentation products help reduce wastewater toxicity, sludge handling costs, and power use.4


Bacteria break down organic material in wastewater and form the floc that settles and separates solids from liquids. Controlling F/M ratio–a vital component to creating a favorable environment for wastewater bacteria–can be accomplished through the use of bioremediation products.

The presence and activities of bacteria and other microorganisms indicates the condition of wastewater throughout the stages of treatment. Check in next week to learn about wastewater bioindicators.

Shop Bioremedial products now


  1. T Glymph (2005). Wastewater Microbiology: A Handbook for Operators,
  2. Kentucky Department for Environmental Protection (2012). Types of Bacteria Used in Wastewater Treatment,
  3. R Fuller (2017). Food-to-Mass (F:M) Ratio, The Wastewater Blog,
  4.  LM Coelho, HC Rezende, LM Coelho, PAR de Sousa, DFO Melo, and NMM Coelho (2015). Bioremediation of Polluted Waters Using Microorganisms,

About the Author

Larry Cooper

Director, Sustainability & Knowledge Management, Huma, Inc. Lifelong learner, master gardener, rescuer of greyhounds, grandpa. Once served detention for placing ecology flag on top of his high school.

Related Posts