Contents<br />1- Introduction <br />2- Types of organisms <br />3- The role of protozoa in wastewater treatment<br />4- Protozoa as indicators of activated-sludge Performance<br />5- Effect of toxicants on protozoa in sewage treatment processes<br />1\\ Introduction<br />Increasing environmental pollution and the continuous development of new chemicals and drugs has led to ever growing concern about the potential effects of these compounds directly or indirectly on human health. As concerns water pollution, protozoa seem to be an excellent tool to assess both toxicity and pollution Biological wastewater treatment processes rely on the natural self-purification ability of microbial communities. <br />Nevertheless, they differ from aquatic environments due to certain characteristics, such as a strong flow of organic load into the system, accelerated decomposition processes, short biomass turnover time, and the prevalence of heterotrophic organisms. Due to these characteristics, biological wastewater treatment processes can be regarded as man-made ecosystems subjected to extreme conditions.<br /><br />2\\ Types of organisms <br />Protozoa<br />Approximately 4% of the microorganisms in wastewater are protozoa, which are single-celled aerobic microorganisms. Protozoa improve effluent clarity by digesting suspended particles and bacteria. Protozoa are sensitive to temperature, pH, dissolved oxygen, and nutrient availability. The presence of certain protozoa are indicators of treatment system performance. Nutrient deficiencies or low dissolved oxygen content will limit the number and type of protozoa.<br /><br />3\\ The role of protozoa in wastewater treatment<br />The positive effects of protozoa on carbon mineralization by bacteria in activated sludge are well known . The excretion of mineral nutrients by protozoa results in an accelerated usage of the carbon source by the bacteria . Furthermore, protozoa excrete growth-stimulating compounds that can enhance bacterial activity . <br />Protozoa release inorganic and organic products into their surroundings. These products are mainly recycled nutrients, such as nitrogen, phosphorous, and organic carbon, but might also include stimulatory compounds that contribute to the dissolved organic carbon pool and affect the physiological state and growth of bacteria .<br /><br />4\\ Protozoa as indicators of activated-sludge Performance<br />Bacterivorous ciliates colonizing activated sludge can be subdivided into three functional groups on the basis of their behaviour: <br />(1)free-swimming ciliates, which swim in the liquor phase.<br />(2)crawling ciliates, which move on the surface of the sludge floc.<br />(3) sessile ciliates, which are firmly attached to the sludge floc.<br />The plant start-up phase is characterized by the presence of species typical of raw sewage: <br />The First free swimming bacterivorous ciliates and small heterotrophic flagellates. These microorganisms cannot be considered typical of these environments because they are not linked to the presence of sludge flocs. With the growth of sludge, they are replaced by other functional groups. <br />The second phase is characterized by the growth of ciliates typical of the activated-sludge habitat: crawling and attached ciliates. In this phase, a species-rich community can be observed, but its species structure changes with the progressive formation of activated sludge. <br />The third phase (the stabilization phase) is characterized by a ciliate community whose structure reflects the stable condition of the aeration tank environment, with a balance between the organic loading and the sludge that is produced, removed and recycled. <br /><br />5\\Effect of toxicants on protozoa in sewage treatment processes<br />In recent years, the presence of toxicants in both surface waters and sewage has become a common occurrence. Originating mainly in industrial sewage, they affect the performance of biological wastewater treatment processes, reducing or inhibiting the activity of microorganisms<br /><br /><br />جامعة المستقبل الجامعه الاولى في العراق