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The activated sludge process probably the most important wastewater treatment technology used for both municipal and industrial wastewaters. The process utilizes microorganisms to decompose organic matter in wastewater. The activated sludge process is frequently subjected to various inhibitory pollutants, such as toxic organics, which have inhibitory effect on the biochemical reactions and decrease the efficiency of the wastewater treatment plants. Phenols are aromatic compounds, which is released to the environment by several industrial activities, as refineries. They are particular toxic to both humans and other organisms and thus they have attracted a lot of attention and concerns. Activated sludge process, among others have been studied as a potential process for phenols removal. However, in the same time they exhibit inhibitory effects on activated sludge systems.

This study presents the experimental results on the inhibition effects phenol and chlorophenols namely 2-chlorophenol (2-CP) and 2,4,6-trichlorophenols (2,4,6-TCP) at a concentration of 80-160 ppm on the activity of urban recycle activated sludge. The inhibition the biological processes was examined in in continuous flow aerobic reactors equipped with porous membrane in order to retain the suspended solids, while allowing treated water to pass through. A synthetic influent wastewater was used. The continuous flow system was monitored by analysing the reactor mixture for total suspended solids (TSS), dissolved oxygen (DO) and pH and the treated effluent for total carbon (TC), total nitrogen (TN), phenols, NH₄⁺, NO₃^- and NO₂^-. In control experiments (i.e. without phenols) the continuous flow aerobic reactors reached high levels of TC and TN removal of about 90%. The high removal of TN indicates that the reactors work under mixed aerobic-anoxic conditions. The results demonstrated that while carbon oxidation is not significantly affected by the addition of phenols in the influent wastewater nitrification and denitrification steps show considerable inhibition. Also, chlorophenols showed higher inhibition levels than phenol.

It was found that the removal of TC and TN has exhibited slight inhibition for both chlorophenols, however 2,4,6-trichlorophenols showed higher inhibition than 2-chlorophenols. The significant inhibition about 20 % in ammonia removal, nitrification rate and cell growth was observed for both 2-chlorophenols and 2,4,6- trichlorophenols.