Modification of hydraulic retention time in open raceway ponds to prevent rotifers and cladocerans establishment

Abstract

Cost effective strategies to control zooplankton density in open pond algal cultures are not currently available, and those successfully tested at pilot-scale are generally difficult to implement at full-scale.Here we propose a method to calculate the hydraulic retention time (HRT) for open raceway pond algal cultures, below which undesirable zooplankton species would be prevented from establishing. We hypothesised that a HRT shorter than the generation time (GT) of zooplankton species should prevent their establishment by flushing the individuals out of the pond before they can reproduce. We reviewed published GT data of zooplankton species known to be detrimental to open raceway ponds performance and plotted GT against temperature over the range typical of algae pond operation (10 to 30 °C). We fitted trend lines to describe how the shortest, average, and longest GT of each zooplankton genera or species varies with temperature. We then validated the method against 29 months of operational data from two identical 8 m3 wastewater high rate algal ponds. Finally, we tested the method in a laboratory experiment performed under favourable growth conditions.Validation of the method against actual operation data showed that rotifers did not establish when HRT was shorter than their GT. However, cladoceran establishment was not predicted by the method likely due to their stratification in the water column depending on the intensity of the solar radiation and resulting avoidance of the pond outflow. The laboratory experiment confirmed that rotifers cultured under favourable growth conditions established at the shortest GT predicted by the method and HRTs shorter than the GT eradicated rotifer populations.We propose that operating ponds with HRTs that are up to ∼20 % shorter than the actual pond HRTs will control densities of zooplankton species evenly distributed in the water column to low levels.