Assessment of potential zooplankton control treatments for wastewater treatment High Rate Algal Ponds

•Chronic CO2 injection controlled the density of microcrustaceans and rotifers.•Biocontrol using Moina tenuicornis and Heterocypris incongruens reduced or eradicated the populations of rotifer.•Mechanical filtration could control and even eradicate M. tenuicornis.•Mechanical hydrodynamic stress killed all microcrustaceans, and reduced the density of larger rotifers.•Phototaxis-induced migration promoted higher densities of M. tenuicornis in the upper layer of the water.

Cladocerans and rotifers rapidly consume beneficial microalgae and reduce the performance of High Rate Algal Ponds (HRAPs) for wastewater treatment and algal production. Potential zooplankton control treatments for HRAPs have been proposed and tested at a laboratory scale including CO2 asphyxiation, biological control using competitor species, filtration, and mechanical disruption using hydrodynamic shear stress. This paper aims to validate these treatments using outdoor mesocosms with physicochemical and operational conditions similar to those of full scale HRAPs. A continuous CO2 concentration of ~ 100 mg/L maintained low pond water zooplankton densities, while a continuous concentration of ~ 180 mg/L killed all microcrustaceans and rotifers present. As biocontrol agents, the cladoceran Moina tenuicornis at ~ 2000 individuals/L reduced average rotifer densities by 90% while the ostracod Heterocypris incongruens at ~ 1000 individuals/L removed all rotifers. Mechanical filtration using 300 μm and 500 μm filters eradicated M. tenuicornis after one and four filtration periods, respectively. Mechanical hydrodynamic stress killed up to 100% of microcrustaceans, and ~ 50% of larger rotifers. Furthermore, phototaxis-induced migration promoted higher densities of M. tenuicornis in the upper layer of the water column in an 8 m3 HRAP during periods of low solar radiation, suggesting that mechanical treatments should be performed at night and to the upper layer of the pond water. Overall, CO2 asphyxiation appeared to be the most reliable, versatile, and effective zooplankton control treatment.