The effects of changes in cellular characteristics and dissolved organic matter (DOM) on dissolved air flotation (DAF) harvesting of Chlorella zofingiensis at the different growth phases were studied. Harvesting efficiency increased with Al3+ dosage and reached more than 90%, regardless of growth phases. In the absence of DOM, the ratio of Al3+ dosage to surface functional group concentration determined the harvesting efficiency. DOM in the culture medium competed with algal cell surface functional groups for Al3+, and more Al3+ was required for cultures with DOM than for DOM-free cultures to achieve the same harvesting efficiency. As the culture aged, the increase of Al3+ dosage due to increased DOM was less than the decrease of Al3+ dosage associated with reduced cell surface functional groups, resulting in overall reduced demand for Al3+. The interdependency of Al3+ dosage and harvesting efficiency on concentrations of cell surface functional groups and DOM was successfully modeled. (C) 2012 Elsevier Ltd. All rights reserved.