The ciliates are so named because of the cilia, small hairs that are distributed over the entire body. Ciliates are generally ovoid or pear-shaped and maintain their shape by means of a tough but flexible pellicle. Cilia protrude through the pellicle in a variety of patterns. The term ciliate comes from the Latin word “ciliate” which means eyelash.
Free-swimming ciliates range in size from 40-100 micrometers. Rapid, rhythmic cilia movement propels them through the liquid. Some are completely covered with cilia while others have cilia in rows or spirals around the cell. Euplotes, Colpidium, and Paramecium are common examples of free-swimming ciliates.
Some ciliates have specialized cilia that look and function like legs, allowing them to crawl around on floc particles and “flick” up the bacteria so that they can consume them. These are called crawling ciliates and tend to stay on the floc more than free in the bulk water. Aspidisca is one example of a crawling ciliate. The main purposes of their cilia are to propel the organisms and to gather food into their mouths (cytostome). They feed mostly on bacteria and other single cell organisms. They are sometimes identified by their smooth gliding or “swimming” motion through a sample or by the “crawling” movements around a piece of floc.
Ciliates are typical colonizers of biological sludge. Under optimum conditions, their numbers range from 1,000 to100,000 cells/mL. A sudden reduction in the number of individuals, or occurrence of encysted, inflated or dead ciliates, is an indication of shock loading with toxic substances or organic overloading. Therefore they are indicators of toxicity in wastewater systems such as ASB’s or Activated Sludge. Since they move fast and they prey on bacteria, ciliates helps to produce a low TSS and turbidity effluent. Some plants use the presence of ciliates to predict the quality of the plant’s effluent.