Investigation of flocculation and rheological properties of microalgae suspensions cultivated in industrial process wastewater

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Microalgae harvesting at an industrial scale remains a techno-economic bottleneck for large-scale microalgae production that can account for up to 30% of the total cost ofbiomass production. A wide range of harvesting techniques have been applied commercially, among which, coagulation-flocculation techniques prove to be both convenient and cost-effective as pre-treatment techniques used in conjunction with other methods. In this study, process wastewater acquired from onshore natural gas facilities in Qatar was utilized to cultivate freshwater microalgae specie Scenedesmus sp. for simultaneous pollution abatement and biomass harvesting applications. Scenedesmus sp. was cultivated in two growth phases (fast and slow growth phases) to examine the influence of the microalgae growth phase on the flocculation process and rheological behavior. Effective flocculation was achieved using a commercial high charge density and high molecular weight cationic polyacrylamide-based flocculant. Flocculation efficiencies exceeded 97% for both fast and slow growth phase suspensions at an optimum flocculant dose of 10 mg/L. Optimum conditions were well corroborated by residual turbidity and zeta potential measurements. For fast growth phase suspensions, the optimum PAM dose coincided with a minimum residual turbidity of 2.35 NTU and a zeta potential of −0.52 mV. Similarly, the optimum PAM dose was fixed based on a minimum residual turbidity of 2.30 NTU and a zeta potential of +0.63 mV for slow growth phase suspensions. Large, easily settleable and compact flocs were obtained with average D50 values of 70.7 μm and 142.0 μm recorded for fast and slow growth phase suspensions, respectively, at optimum PAM dose. Moreover, based on rheological characterization studies, the flocculated suspensions demonstrated an overall non-Newtonian pseudoplastic (shear thinning) fluid behavior. Introduction of cationic PAM significantly improved the viscosity and yield stress of the flocculated suspensions around the optimum PAM dose range. At the optimum PAM dose, the initial viscosities were recorded as 10.37 and 39.19 mPa.s with yield stresses of 12.84 and 16.59 N/m2 for fast and slow growth phase suspensions, respectively. The flocculated biomass further displayed a viscoelastic solid (gel) behavior, characterized by high gel strength and resistance to shearing around optimum flocculant doses. At the optimum PAM dose, the storage moduli were recorded as 886.2 and 1068.6 mPa for fast and slow growth phase suspensions, respectively.