Impacts of variable pH on stability and nutrient removal efficiency of aerobic granular sludge

The impact of pH variation on aerobic granular sludge stability and performance was investigated. A nine-day alkaline (pH ¼ 9) and acidic (pH ¼ 6) pH shocks were imposed on mature granules with simultaneous chemical oxygen demand (COD), nitrogen and phosphorus removal. The imposed alkaline pH shock (pH 9) reduced nitrogen and phosphorus removal efficiency from 88% and 98% to 66% and 50%, respectively, with no further recovery. However, acidic pH shock (pH 6) did not have major impacts on nutrient removal and the removal efficiencies recovered to their initial values after three days of operation under the new pH condition. Operating the reactors under alkaline pH induced granules breakage and resulted in an increased solids concentration in the effluent and in a significant decrease in the size of the bio-particles, while, acidic pH did not have significant impacts on granules stability. Changes in chemical structure and composition of extracellular polymeric substances (EPS) matrix were suggested as the main factors inducing granules instability under high pH.