Impact of Natural Cationic Polymers on Charge and Clarification of Microalgae Suspensions

Microalgae-based systems for wastewater treatment, biofuel production, and nutrient recovery require effective solid/liquid separation processes that frequently include coagulation and flocculation steps where discrete particle (i.e., microalgae) suspensions are destabilized to initiate particle agglomeration, followed by conglomeration into larger flocs, respectively. The choice of coagulant affects process economics and sustainability. This study investigated the use of natural cationic polymers (chitosan and cationic starches with varying degrees of substitution) and the synthetic cationic polymer (polydiallyldimethylammonium chloride [pDADMAC]) to flocculate Chlorella protothecoides and Nannochloropsis salina. Algae surface charge and flocculation efficiency were examined in response to variable pH, coagulant dosage, cell concentration, and salinity. Overall effectiveness of coagulation, flocculation, and sedimentation is referred to as flocculation efficiency and is determined by measuring the percentage decrease in optical density of the microalgae suspensions. Cationic starch with a degree of substitution (DS) of 0.5 and chitosan neutralized the charge on C. protothecoides at lower dosages compared with pDADMAC. C. protothecoides flocculation efficiencies >95% were achieved with dense cultures (1 g/L algae dry weight) at cationic starch (DS 0.5) and chitosan dosages of 0.02 g/g algae dry weight. Zeta potential measurements indicated that complete charge neutralization was not necessary for maximum flocculation. Of the polymers tested, zeta potential analysis indicated that only chitosan was sensitive to pH. Maximum flocculation efficiency of N. salina was achieved through chitosan precipitation and subsequent sweep floc at pH 8.0. However, cationic starch (DS 0.5) and pDADMAC optimum doses were lower than that required for N. salina flocculation with chitosan.