(431i) Harvesting Microalgae Cultivated on Domestic and Industrial Wastewaters Using Polymeric Flocculants

Increasing demand for freshwater consumption due to population explosion and industrialization has globally put stress on the existing limited freshwater resources. In 2019, global freshwater consumption was estimated at 4,600 billion m³ [1]. A large portion of this water ends up wastewater from various domestic and industrial sources. These wastewaters are mostly rich in nitrogenous and phosphorus-based compounds alongside dissolved organic carbon content, which increases eutrophication and contamination of ecosystems when discarded in freshwater systems untreated. Therefore, wastewater generated daily in vast amounts on a global scale presents challenges in terms of efficient treatment prior to disposal and recycling.

Microalgae cultivation and harvesting have gained momentum in the past few years for numerous emerging applications of biomass in the generation of high-value natural products in pharmaceuticals and nutritional supplements. Biomass is also increasingly implemented for water remediation applications as well as in biofuel production as a green energy source alternative. However, the large-scale biomass production, processing, and development costs challenge the economic feasibility of this process. Further, vast quantities of freshwater are generally required for cultivating microalgae for large-scale biomass production. Recent studies have also illuminated the potential of microalgae thriving in wastewater to in turn be used for wastewater remediation. Therefore, the integration of microalgae cultivation in wastewater for harvesting biomass targeted toward green energy production and wastewater treatment has attractive potential benefits. Further, the pollutants encountered in wastewater in terms of human consumption can in turn act as nutrients for increasing the productivity of microalgae alongside high-value metabolites such as carbohydrates, proteins, lipids, and pigments. Hence, wastewater from various sources can offer low-cost growth media for sustainably cultivating microalgae on a large-scale, while simultaneously reducing the cost and energy consumption of wastewater treatment. The treated water can be recycled for industrial or agricultural applications.

Microalgae harvesting at an industrial scale remains a techno-economic bottleneck for large-scale microalgae production chiefly due to the small size of microalgae cells and dilute microalgae concentrations in cultures. Consequently, large volumes of water must be separated to harvest microalgae from their cultures, rendering the process both cost and energy intensive. The harvesting stage in production alone can account for up to 30% of the total cost of biomass production. Therefore, harvesting microalgae from large quantities of water using energy-efficient methods is crucial for large-scale biomass production and to sustain a circular economy. A wide range of harvesting techniques has been applied commercially, among which, coagulation-flocculation techniques prove to be the most convenient and cost-effective. The application of commercial polyelectrolyte flocculants for bulk microalgae harvesting is considered one of the most economically-viable methods due to the low flocculant costs and low flocculant doses needed for efficient harvesting.

This work reports coagulation-flocculation tests performed for microalgae cultures cultivated in raw domestic wastewater and industrial process water obtained from the oil and gas industry in Qatar. Two types of high-molecular weight polyelectrolyte flocculants, namely a cationic and an anionic polyacrylamide were tested for their flocculation performance in the microalgae cultures. The electrokinetic and flocculation behavior of the cultures were tested by a range of experimental analyses including jar tests, residual turbidity, optical tests, zeta potential determination, and particle size distribution. In these studies, it was generally observed that the polyelectrolyte charge type significantly impacted the electrokinetics, adsorption, and flocculation behaviors of the stable microalgae cultures.

Cationic polyelectrolytes effectively destabilized algae and algae-bentonite cultures in small doses, in contrast to anionic polyelectrolytes. The underlying mechanisms involved are mainly adsorption, charge neutralization, and bridging interactions. Cationic polyelectrolytes displayed high adsorption affinity towards the negatively charged microalgae cells. For instance, in the cultures cultivated in raw domestic wastewater, cationic PAMs generated a maximum turbidity reduction of about 99% at a small dose of 3 mg/L. On the other hand, anionic PAMs could merely bring about a 16% reduction in turbidity at a flocculant dose of 40 mg/L, which is 13 times higher than the cationic PAM dose, thereby proving ineffective in destabilizing the cultures on its own. Anionic polyelectrolyte demonstrated low adsorption affinity for the negative surface of microalgae cells owing to steric hindrance from the negative functional groups on the polymer chains, resulting in its ineffectiveness as a flocculant in microalgae cultures.

Similarly, a complete surface charge neutralization was obtained for cultures flocculated with cationic PAMs at a low flocculant dose of 10 mg/L, enabling maximum flocculation. However, the excess addition of cationic polyelectrolyte beyond the optimum dose eventually caused a surface charge reversal on the microalgae cells, resuspending the flocculated cells in the culture. In terms of floc size analyses, anionic PAMs generally produced larger flocs of loose structure that remain suspended in the culture, while cationic PAMs generated dense and rapidly settling compact flocs. Therefore, the high molecular weight cationic PAM was established as the best flocculant among the examined polyelectrolytes for harvesting microalgae from cultures cultivated in domestic wastewater. Further, the addition of bentonite clay as a flocculation aid greatly was also preliminarily tested for one of the cultures cultivated in raw domestic wastewater. The presence of bentonite clay enhanced the flocculation behavior of cationic polyelectrolytes while also enhancing the floc structures and sizes. This is mainly attributed to the high sorption properties exhibited by the lamellar structure of the clay particles.

The impact of the growth phase of microalgae on the flocculation behavior of microalgae cultures cultivated in the industrial process water was also examined. Cultures in exponential and stationary phases were considered in this study. A clear distinction was observed in the flocculation analyses for the two cultures when treated with a cationic polyelectrolyte. While both flocculated cultures presented a sharp reduction in residual turbidity close to zero at low flocculant doses in the range of 6-8 mg/L, a dramatic surface charge reversal was obtained for stationary phase cultures on increasing the flocculant dose beyond the optimum range, unlike the exponential phase. Further, stationary phase cultures generated larger flocs than the exponential phase cultures at optimum flocculant doses. This may be due to the higher concentration of microalgae cells in the stationary phase in contrast to the exponential phase that rapidly aggregates, form large flocs and settle at the bottom. Further, the concentration of biopolymers secreted by microalgae during the growth phases reaches a maximum during the stationary phase. Their biopolymers are generally known to encourage flocculation. Overall, these studies demonstrated the successful utilization of wastewater generated from domestic and industrial sources as low-cost media for microalgae cultivation. These studies also establish the high harvesting efficiencies of commercially available, cost-effective, non-toxic, and versatile polyelectrolytes for efficiently harvesting microalgae from their stable cultures.

Reference

[1] A. Boretti, L. Rosa, Reassessing the projections of the World Water Development Report, Npj Clean Water. 2 (2019). doi:10.1038/s41545-019-0039-9.