Enhancing algal dominance and harvestability from wastewater treatment high rate algal pond

This paper investigates the influence of recycling harvested algal biomass on algal species dominance and harvestability in a pilot-scale HRAP over four months during a New Zealand summer. Two identical HRAPs were operated in parallel with and without harvested biomass recycling. Effluent from each HRAP flowed continually into two algal settling cones in series (3-h HRT each) to remove algal biomass. A portion of harvested algal biomass from the first algal settling cone was recirculated back to the HRAP on a daily basis. Daily algal biomass harvestability in the algal settling cones and Imhoff cone algal biomass settlability (after 10, 30, and 60 minutes and 24 hours) were measured five times a week. While similar colonial algal species were found in both HRAPs in summer, harvested algal biomass recycling promoted the dominance of Pediastrum sp. (>95% dominance). Increased dominance of the colonial algae greatly improved the efficiency of gravity algal harvest; >85% average biomass removal compared with ~70% average biomass removal in the control without algal biomass recycling. Furthermore, Imhoff cone algal settleability experiments in laboratory showed that recycling of harvested algal biomass could enable further reduction of the hydraulic retention time (and volume) of algal settling cone.