Zooplankton community influence on seasonal performance and microalgal dominance in wastewater treatment High Rate Algal Ponds

High Rate Algal Ponds (HRAPs) with artificial addition of CO2 provide improved tertiary-level wastewater treatment over conventional HRAPs. One of the greatest challenges for performance and management of HRAPs with CO2 addition is the establishment of zooplankton grazers that can consume much of the algal biomass within a few days. High food availability and a near neutral pH provide optimal conditions for the establishment of zooplankton taxa and control strategies require an understanding of their population dynamics in HRAPs. Unfortunately, available literature lacks long-term assessment of such dynamics in HRAPs with CO2 addition. Here, we assess the environmental and biological parameters of two wastewater HRAPs with CO2 addition over a period of 14 months, in relation to HRAPs performance and operation. Eight species of zooplankton established in the HRAPs, with higher water temperatures and longer detention times promoting larger populations. Grazing pressure was associated with changes in: 1) the dominance of microalgal species; 2) large and rapid reductions of productivity; 3) reduced nutrient removal; 4) increased colony size, number of cells in colonies and formation of protective spines in microalgae; and 5) higher biomass settleability. Maintaining a dominance of colonial microalgae, operating with short retention times, and facilitating competition among zooplankton species showed greatest potential for reducing and controlling grazer populations.