survey on biological growth in improved activated – sludge system by pac

in this research, the effects of the addition of powdered activated carbon (pac) into the aeration zone of an activated-sludge (as) system for treating tehran oil refinery effluent, was investigated during more than 12 months in a pact pilot-scale model. besides the evaluation of organics removal efficiency and determination of basic design factors kinetic coefficients, a series of experiments were conducted in order to study the variations in biological growth (especially bacteria and monocellular organisms) related to pac addition. after sampling from mixed liquor and microscopical studies, adequate samples were filtered, dewatered and fixed. the samples were then coated by 100 a films of (au) and were studied by scanning electron microscopy (sem). results showed significant improvement of bacteria and monocellular organisms in the presence of pac, which may be raised from providing more contact surface and better nutrient availability for microorganisms, decreasing the effects of toxic shock loads and providing better conditions for microorganisms against intensive mixing. electronic photomicrographs by sem confirmed good attachments of ciliate to pac particles, along with appropriate efficiency of bacteria diffusion into its meso and microspores. also, the comparison between the count number of protozoa in two pilots of pact and as, showed better conditions for growth of microorganisms, using pac.