Sometimes, effective flocculation is achieved simply by changing the pH of the algal broth. In one study, extensive flocculation was achieved at pH values between 11.8 and 12, without other added flocculants (McCausland et al., 1999). This method removed 95% of the algal biomass from suspension and the sludge obtained had good settling characteristics and mechanical resistance. In other instances, extreme pH values can cause adverse effects. In a study of EPA recovery from Skeletoma costatum, a pH of 10.2 sedimented 80% of the suspended cell mass but the cells lysed and released the intracellular contents (Blanchemain and Grizeau, 1999). Effect of two alkalis on cell removal by flocculation from suspensions of Chlorella is shown in Fig. 3 (Yahi et al., 1994).