ABSTRACT
The operation of fixed activated sludge system for treatment of wastewater containing heavy metal compounds [chromium, lead and nickel], by using of fixed activated sludge [FAS] system is studied in this research. A Plexiglas tank consisted of three sections including a downward-flow aerated reactor, an upward-flow aerated reactor and a gravity sediment unit, fed with the synthetic wastewater. The results showed that COD removal efficiency in the FAS is about 96% and the acclimation time for microorganisms is short. While chromium, lead and nickel removal efficiency in the fixed activated sludge at concentration of 1 mg/lit. is 84%, 75% and 80%, respectively, by increasing concentration of them to 5 mg/lit, the removal percentage increased to 90%,84% and 87%, respectively. When concentration of chromium, lead and nickel increased to 10 mg/lit, the removal efficiency became 85%, 95% and 92%, respectively. Concentration of heavy metals [chromium, lead and nickel] at 50 mg/lit; caused the removal efficiency of 86%, 96% and 95.1%, respectively. By rising concentration of heavy metals up to 100 mg/lit, the removal ratio efficiency became 86%, 97.6% and 97%, respectively. At each stage, increasing of heavy metal concentration [chromium, lead and nickel] caused to decreasing of COD and MLSS removal efficiency at the beginning of period but COD and MLSS removal efficiency re-increased by micro-organisms acclimation to changes. However this increase is less than best previous stage condition of COD removal efficiency and showed a downward trend. Therefore, heavy metal compounds removal in the fixed activated sludge is not just a biological process, indeed some part of these compounds are removed by adsorption on sludge. Altough this part is negligible in comparison with biological removal of these compounds. Heavy metals which are found in the sewage of many industries like petrochemical industries, refinary plants, paper mills and chemical industries, disrupt the wastewater treatment plants performance by affecting microorganisms and synthetic reactions due to their toxicity. Consequently, the concentration of these compounds exceed the maxiJIlum contamination level [MCL] in the effluent of wastewater treatment plants and causes many health and environmental problems. Since the ability of some microorganisms for degrading of heavy metal compounds are proved, biological treatment is widely used for removing heavy metals from industrial wastewaters. Bioreactors with fixed activated sludge beds are one of suitable methods in this subject. In this research, the performance of activated sludge bioreactors with fixed beds is reviewed for removal of three heavy metal compounds i.e. chromium, lead and nickel in industrial wate waters. Fixed activated sludge system [FAS] is a composition of activated sludge system and trickling filter which media are used for forming biofilms in the aeration tank to enhance the treatrnnet performance. A synthetic solution is prepared to provide a wastewater with a COD of 550-600 mg/L