Basic Microbiology of Activated Sludge | Activated Sludge Process | Wastewater Treatment Plant

 Basic Microbiology of Activated Sludge

Organisms Involved

Activated sludge is a very complex ecosystem. Many different types of organisms are present and many interactions occur among the organisms. Bacteria, fungi, protozoa, and rotifers are among these creatures. At ambient temperatures in a mixed, aerated environment, the organisms must be able to grow on the "food" (substrate) accessible in wastewater. They must also have the ability to form flocs and settle out in secondary sedimentation.

There are four types of organisms: floc formers, saprophytes, predators, and nuisance organisms. The floc forming organisms are mostly bacteria. They form natural polymers which cause individual bacteria to clump together and become large enough to form flocs which can then settle out by gravity forces. The saprophytes are mostly bacteria. They degrade the organic matter (BOD). Some saprophytes are also floc formers. Protozoa and, to a lesser extent, rotifers are the primary predators. Their prey are the dispersed bacteria (those not in flocs). Note that individual bacteria do not settle well. This is why flocs are so important for the activated sludge process to work. Because distributed bacteria cause effluents to seem hazy, predators are vital in creating clean effluents. Finally, the nuisance organisms are filamentous fungi and filamentous bacteria. These are the organisms that cause difficulties with bulking. While most bacteria grow as spheres, rods or spirals, filamentous organisms grow in the form of very long rods, many times longer than wide. Thus the filamentous organisms do not form good settling flocs or flocs which compact very well.

Growth and Respiration

Most biological organisms use organic compounds in their growth and respiration processes. Organic compounds are complex materials consisting of a carbon skelton - carbon atoms linked together - to which other atoms are attached - usually hydrogen, often oxygen and sometimes others such as nitrogen, sulfur and phosphorus.

When organisms use organic compounds as food for growth the carbon atoms of the compound are used as a foundation on which to build new cellular organic material. The following equation can be used to represent this process:

Food Organics --- Organisms ---》 Cellular Organics (1)

This process requires energy because the organisms are building a new material, cellular organics. The energy is obtained by degrading, or burning, other food organics to carbon dioxide and water as follows:

Food Organics --- Organisms ---》 C02 + HOH + Energy (2)

The energy released is used in the equation's growth process (1). The burning process is referred to as respiration.

Organisms also require other materials, or nutrients, in their growth and respiration processes. A source of nitrogen is required in the growth process because this element is incorporated into the new cellular organic material. A source of oxygen is required in the respiration process to burn the organic material, the carbon, to carbon dioxide. Other nutrients which are also required include phosphorus and trace elements such as magnesium, cobalt, iron and manganese.

Because the energy released by respiration is used in the growth process, these two cellular functions are not independent, but are, in fact, closely integrated and controlled within the cell. The cell can thus be considered as a factory - taking in nutrients and discharging products.

The growth and respiration processes discussed above are carried out by most living organisms. The result is a biodegradation of food organics to waste products - CO2, H2O, residuals at a lower energy level - and to new cells which can then perpetuate the process.