BACKGROUND: Waste mussel shells present a significant environmental problem in the area of Northern Greece, since they are produced in vast quantities and there is no legislation for their appropriate disposal or use. In the present study thewaste shells were used for the removal of dyes and heavy metals from aqueous solutions while powdered mussel shells were added into activated sludge processes for the removal of hexavalent chromium. RESULTS: Powdered mussels shells were used in standard adsorption experiments for the removal of methyl blue and methyl red aswell as for the removal of Cr (VI), Cd and Cu. Also theywere added into laboratory scale activated sludge reactors treating synthetic wastewater with hexavalent chromium, in order to investigate the potential attribution to the treatment efficiency. CONCLUSION: Adsorption experiments indicated almost 100% color removal. High removal efficiencies were observed for the metals, especially in the case of chromium and cadmium reached concentrations even below 1mgL-1. The addition of powdered mussel shells in the activated sludge processes enhanced the removal of chromium and phosphorus, while enabled the formation of heavier activated sludge flocs and thus enhanced the settling properties of the activated sludge. (C) 2017 Society of Chemical Industry