Chitosan destabilizes the algae suspended in aqueous medium by changing the surface charge on the algal biocolloids. Zeta potential is a convenient measure for exploring the interaction between flocculants and algae cells (Henderson et al., 2008). The zeta potential of pure N. oleoabundans in media was found to be −16.9 ± 1.9 mV, consistent with reported values for algal biocolloids which are generally negative at most pH levels ( Ma and Liu, 2002). Chitosan is a cationic polyelectrolyte and is expected to coagulate negatively charged suspended particles found in naturally in turbid water ( Divakaran and Pillai, 2002). When the dose of chitosan was increased from 50 mg/L to 75 mg/L, zeta potential remained unchanged although the turbidity removal rate increased significantly ( Fig. 2). This shows that nonelectric factors contributed to flocculation, most likely bridging between cells facilitated by the chitosan polymer chain. This is consistent with the results of the pH-adjustment tests described previously. A further increase in chitosan level to 100 mg/L led to a decrease in the absolute value of zeta potential and therefore instability of the colloidal system—colloids with high absolute value of zeta potential are electrically stabilized while colloids with low values tend to coagulate or flocculate. The best turbidity removal rate was achieved at the 100 mg/L concentration of chitosan. The mechanism of coagulation of chitosan with N. oleoabundans therefore consists of polymer adsorption followed by neutralization of the colloidal charges. At higher doses of chitosan (>100 mg/L), an increase in zeta potential was found indicating restabilization of the colloids. This increase was correlated with a decrease in turbidity removal rate. This likely occurs because at high chitosan doses, the surfaces of individual algae particles are completely shielded by chitosan, making it impossible to bind multiple particles via bridging. Another contributing factor would be charge buildup due to excessive adsorption, causing repulsive interactions to exceed attractive interactions.