The predominant binding groups for metals on the surface of bacteria are likely to be carboxyl and amino groups in proteins [147]. The solubility of metals is enhanced when the pH drops below 5 hence prolonged exposure of granules to low pH may influence the extent of metal retention within the sludge granules and cause catalytic or inhibitory impacts depending on the concentrations of the metals rendered labile. But thus far the effects of interactions between metals and the granule walls on metal bioavailability and transport have not been clearly understood.