The shocks became an important issue in operating wastewater treatment plant, because it may lead to biological unit damage in a short time. This study had been done to experience two separated ways in toxic shock happening prediction. Two glass basin (10L volume) used to conduct the study. The work was with completely mixed activated sludge. The studied ways of prediction were 1-the specific oxygen uptake rate (SOUR) of biomass in aeration tank , and 2-microscopic images monitoring. Two types of toxic material were used , the first was phenol (organic) and cyanide (inorganic inhibitors) with 40mg/l concentration in each of the basin separately. The results revealed that the two ways were useful in shock prediction. SOUR has raised with phenol shock in a short time , and has sharp dropped with cyanide shock (about 15 min.). the microscopic images revealed that filamentous growth happened in the bioflocs after short time of phenol shock. While a dispersed growth and pinpoint flocculation has appeared after short time of cyanide shock (leading to the biomass damage).

Key words:activated sludge, toxic shock, shocks early prediction