The world is faced with problems related to the management of wastewater due to extensive industrialization, increasing population density and a highly urbanized society. The quality of wastewater effluents is responsible for the degradation of receiving water bodies, such as lakes, rivers, streams. In order to meet Goal 7 of the Millennium Development Goals “ensure environmental sustainability” and maximization of the health and environmental benefits associated with the use and discharge of wastewater, several legislations and guidelines have been developed, both at international and national levels. The two main processes for the removal of impurities from wastewater influents are chemical and biological. Because of the many drawbacks of chemical wastewater treatment, biological treatment is advocated in the last few decades. Biological phosphorus and nitrogen removal systems have been extensively investigated for municipal wastewater treatment over the past decades. Several questions have been raised on the role of the different microbial groups in the removal of nitrogen and phosphorus in activated sludge systems. In this paper, an attempt is made to give an overview of the population dynamics of the activated sludge. The role of the different microbial groups present in the activated sludge systems, with particular reference to bacteria and protozoa in the removal of phosphorus and nitrogen is also reviewed. This will enhance decisions that are science-based with respect to biological wastewater treatment.
Key words: Activated sludge, nitrogen, phosphorus, wastewater, bacteria, protozoa.
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