© 1st EWaS-MED International Conference, “Improving Efficiency of Water Systems in a Changing natural and financial Environment”, Thessaloniki - Greece, 11-13 April 2013. Fate and seasonal variation of microbial pathogens and Candida population degrading phenol in a sewage treatment plant S. Mahgoub*, H. Abdelbasit, H. Abdelfattah and S. Hamed Microbiology Department, Faculty of Agriculture, Zagazig University, Zagazig 44511, Egypt *Corresponding author: E-mail: mahgoubsamir@gmail.com, Tel/Fax: +20552287567 Abstract The fate and seasonal variation of several microbial pathogens (MPs), including Salmonella spp. (SS), Escherichia coli O157:H7 (EC), Listeria monocytogenes (LM), Staphylococcus aureus (SA), biomarker bacteria and Candida spp. (CS) were investigated in a municipal sewage treatment plant (MSTP) located in Zagazig city, Egypt employing an anaerobic/ anoxic/oxic (A/A/O) process to monitor their incidences in both influent and effluent throughout the seasons of 2011. Enhancing the activity of Candida populations and the bacterial biodegradation activities in the anaerobic-anoxic-oxic process is an axial pathway for the removal of phenol. In summer season, phenol degradation in MSTP was about 85% which was higher than that in winter season (60%). The chemical treatments routinely used in MSTP can effectively reduce 70% of microbial pathogens in wastewater in summer and more than 80 % in winter. The concentrations of microbial populations in the effluent were much higher in summer and spring than in winter and autumn, which was closely related to degradation of phenol. Therefore, this study may raise a particular concern regarding the removal of microbial pathogen and phenol from wastewater in summer seasons. Keywords: microbial pathogens; phenol; Candida; sewage treatment plant; A/A/O. 1. INTRODUCTION In most wastewater treatment systems any pathogen removal that occurs is a fortuitous by-product of the principal design objective (usually organic carbon removal). A biological wastewater treatment system contains many types of microorganisms, such as bacteria, protozoa, fungi, metazoan, viruses, and algae, while bacteria comprise approximately 95% of the total microbial population [1] and play a key role in the purification of water quality. Secondary treatment is one of the key components of a wastewater treatment plant. It involves the biological reduction of biochemical oxygen demand, suspended solids and toxicity of industrial wastewaters and the production of low nutrient, environmentally benign outgoing effluent. Effluent also has been shown to reduce the incidence of toxic chemicals and pathogenic bacteria [2; 3].These functions are carried out by the resident microbial community which is considered the foundation of the secondary treatment process. Reclaiming and reusing wastewater before thorough treatment to reduce the concentrations of waterborne pathogens e.g. helminthes, protozoa, fungi bacteria, and viruses poses a health risk and great concern [4; 5; 6; 7; 8; 9].Anaerobic/ anoxic/oxic