This study evaluates the biological solid reduction in a conventional activated sludge system with an anoxic/anaerobic side stream reactor receiving 1/10 of return sludge mass. Influent iron concentrations and feeding modes were changed to explore the consistency between the influent iron concentration and yield values and to assess the impact of feeding pattern. The results indicated that sludge reduction occurs during alternately exposure of sludge to aerobic and anoxic/anaerobic conditions in a range of 38-87%. The sludge reduction values reached a maximum level with the higher iron concentrations. Thus, it is concluded that this configuration is more applicable for plants receiving high iron concentrations in the wastewaters. (C) 2015 Elsevier Ltd. All rights reserved.