Factors that Affect the Development of Activated Sludge in Wastewater Treatment

The presence of bacteria is vital for the wastewater treatment process. In an ideal wastewater treatment plant system, bacteria (about 95%) along with a small portion of fungi and protozoa should form a biological floc that is known as an activated sludge. This is essential for the degradation of organic substances in wastewater.

The activated sludge is developed in a multi-chamber aeration tank where aerobic conditions are optimized to keep the activated sludge suspended. These microorganisms digest organic matter and remove nutrients from wastewater to produce a high-quality effluent.

Bacteria are classified into three types: aerobic bacteria which require oxygen to survive, facultative bacteria which prefer oxygen but can live for some time without it, and anaerobic bacteria which die in the presence of oxygen.

The most important microorganisms in the activated sludge system are the aerobic bacteria. They digest the biodegradable material in wastewater as well as consume proteins, carbohydrates, fats and many other compounds.

Aerobic bacteria digest organic particles in a two-step process. Adsorption occurs when the bacteria secretes certain enzymes to dissolve organic food particles that are too large to go through the cell membrane into tiny units. Once the food is small enough to pass through the cell wall, absorption takes place.

Current treatment plant designs have done away with adsorption and only allow dissolved nutrients to enter the aeration basin. This helps the bacteria to work more efficiently by removing the need to break down nutrients before they can be absorbed in the cell walls.

Wastewater treatment plant operators must maintain a delicate balance to foster the most suitable environment for floc-forming bacteria. Although there will be factors that cannot be controlled such as the weather and types of nutrients present in the wastewater sludge, here are several factors that can be influenced ensure bacteria can grow and thrive:

Oxygen Utilization

Optimal bacteria digest food at a high rate thereby also utilizing a great amount of oxygen. This is measured in mg O2/hr/gm of mixed liquor suspended solids (MLSS). In general, a higher Uptake Rate is associated with a higher food to microorganism (F:M) ratio and younger sludge age. A lower Uptake Rate is associated with a lower F:M ratio and older sludge age.

Sludge Age

As bacteria start to develop in the tank, they grow singularly in small clumps and chains. They are very active with flagella (slender threadlike structure that enables microscopic organisms to swim). They do not have a well-developed slime layer and do not settle well. As the sludge ages, the flagella disappear and the bacteria accumulate more slime. Bacteria then starts to clump together forming a floc that is big enough to settle in the tank.

Dissolved Oxygen

Aerobic bacteria require at least 0.1 – 0.3 mg/L of oxygen to live. At least 2 mg/L of oxygen must be sustained in the bulk fluid for the bacteria in the center of the floc to get 0.1- 0.3 mg/L of oxygen. Otherwise the bacteria in the middle will die and the floc will start to disintegrate.


Mixing is essential to allow the bacteria, oxygen and nutrients to be contact with each other. In the absence of adequate mixing, the bacteria that has lost its flagella will not clump together and proper treatment will take place.


Bacteria require enzymes to digest nutrients. Enzymes are compounds produced by living organisms to allow for biochemical reactions to occur. These enzymes are very pH sensitive. Their optimal pH is between 7.0 and 7.5. Rapid pH changes could have a negative effect in the production of enzymes.


Biochemical reactions are slower in colder temperatures, so the system will require more organisms to do the work. Inversely, warmer temperatures require fewer bacteria to process wastewater sludge.


Bacteria cannot survive without basic nutrients such as carbon, nitrogen, and phosphorus as well as trace amounts of sodium, potassium, magnesium and iron. These are commonly found in normal domestic sewage. Industrial wastes lack sufficient nutrients and must be supplemented.

Dispersed Growth

Dispersed growth is a population of suspended, growing, non-flocculated (fragmented) bacteria, algae or fungi. In an ideal system, dispersed growth should be avoided so that bacteria can develop a “slime” layer and clump together to form floc. Dispersed growth can result from excessive shearing and /or surfactants that break up the floc.
Keeping the system working well is not always easy as water is not homogeneous and there are no absolutes. Therefore, it is imperative for the operator to observe and record what works well for his own treatment system. Once the operator is familiar with the control parameters that are best suited for his treatment plant, he will have a baseline upon which to gauge its performance and make appropriate adjustments when necessary.

If you are a municipality in Ontario and in need of a biosolids management solution, please feel free to contact us at 1 (877) 479-1388.


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