Higher Life Forms and Their Role in Wastewater Treatment

The general awareness regarding microbes has greatly increased over the past few decades. However, little is known about their distant cousins, ‘Protozoans and Metazoans’, more popularly known as ‘higher life forms’. Both these categories of organisms are of great value to industrial effluent treatment. Microorganisms constitute 95% of population in an ideal activated sludge ,of which 4 % of this microbial population consist of higher life forms. Although these higher life forms contribute very little to the removal of organic nutrients, their presence however greatly enhances the clarity of the water and provides an indication of effluent condition (Eg. Toxicity,organic load, dissolved oxygen etc.)

Protozoa are single-celled microorganisms that come in a large variety of sizes and shapes. Their main function in the treatment process is to remove non-flocculent bacteria and very small floc that do not settle. Metazoa are multi-cellular microorganisms that feed on bacteria, algae and protozoa. They can have very simple to highly complicated physical structures.

Like all other livingbeings, the existence and survival of higher life forms depends onthe following parameters:

Temperature: Most higher life forms can survive and reproduce in the temperature range at which activated sludge processes are carried out. They grow best in ambient temperatures (15°C – 25°C)

pH- Higher life forms are more sensitive to pH than bacteria. They can luxuriantly grow at optimum pH range of 7.2-7.4 and have a tolerance range of 6.0 – 8.0.

Dissolved oxygenLike bacteria, higher life forms require oxygen to survive. Below par DO severely limits both the kind and number of protozoans.

Nutrition Most municipal wastewater treatment plants and effluent treatment plants, unless toxic, contain sufficient nutrients to support most higher life forms associated with wastewater.

One of the important attributes of the higher life forms is that they provide indication of effluent conditions and sludge age.The higher life forms indicative of stable effluent conditions and healthy activated sludge are as follows:

  • Amoeba

  • Flagellates

  • Free-swimming ciliates

  • Crawling ciliates

  • Stalked ciliates

  • Rotifers


Amoebae can only multiply when the nutrient level in the aeration basin is quite high or if there is very little competition for the food. Therefore, they dominate early in the effluent treatment process.

What does the presence of Amoeba indicate?

  • A sudden increase in influent BOD. This would make extra food available that will allow them to compete.

  • The presence of large amounts of particulate matter. Amoeba favor particulates.

  • Lack of oxygen. Amoebae move very slowly and require less oxygen then other protozoa.


Most flagellates absorb nutrients just like bacteria so they compete with bacteria for dissolved nutrients. Flagellates peak in number while the soluble nutrient concentration is high and the number of bacteria is still quite low. Soon after amoebae begins to disappear and while there is still high concentrations of soluble food.

What does the presence of flagellates indicate?

  • The wastewater still contains a large amount of soluble organic nutrients.

  • It is also indicative of some type of upset condition. Common triggers include – (1) high hydraulic loads, (2) increased organic loadings, (3) problems with dissolved oxygen concentrations.


Free swimming ciliates: They are usually covered with “cilia” which are hair-like projection that they used for locomotion and for sweeping food into their mouths contribute very little to the removal of organic material from the wastewater.They feed on bacteria not on dissolved organics.

Crawling ciliates: They have cilia on the under side of the body. The cilia are twisted together to form “tufts” or legs that are used for crawling along the floc. Crawling ciliates graze on floc particles and feed on the free bacteria on the edges of the floc

Stalked ciliates only have cilia surrounding the oral groove or mouth and are used to create a current that will bring food into the mouth. Stalked ciliates appear in mature sludge.

Types of Ciliates


Presence of free swimming ciliates is usually an indication of good treatment. They dominate after the formation of floc and when most of the organic nutrients have been removed. Appear after flaggelates starts to disappear

Presence of crawling ciliates indicates reduced population of disperse bacteria and increased flocs as crawling ciliates out compete free-swimming ciliates because they can find food within the floc unlike free-swimming ciliates .

Presence of colonial stalked ciliates indicate sludge ageing.As the sludge ages and minimal food becomes available, the colonial stalked ciliates begin to outcompete the single stalked ciliates for dominance.


Rotifers are the metazoans commonly found in waste water and plays a principle role in the activated sludge treatment .They feed on suspended particles and bacteria. They do an excellent job of polishing off and removing any remaining material in the water. They also secrete a sticky substance that helps the floc to remain firm and clumped together. Rotifers have shown to be helpful in reducing bulking by preying on the filamentous bacteria “Microthrix” which is associated with bulking in wastewater systems. Some wastewater plants use rotifers to identify whether they need to increase sludge wasting.

Also Read – Are Any Microbes Hardy Enough To Survive In Outer Space?

What do the presence of rotifers indicate?

Rotifers thrive in conditions with plenty of oxygen and low toxicity and are an indicator species for low Biochemical Oxygen Demand(BOD) and and stable wastewater systems

Though higher life forms formonly about 4% of microbial population, their contribution to the waste water treatment is significant. As the environment in the aeration basin changes, dominance of type of higher life forms changes too. The microorganism best suited for the environment will emerge until the environment changes again. Indications provided by the dominance of protozoan species dominance should not be relied on solely to troubleshoot wastewater treatment conditions, but this information is surely helpful in assessing the conditions of the activated sludge process.

Also Read – What Is Biological Wastewater Treatment And How Effective It Is?

Post by Priyanka Khaire