A review of potential methods for zooplankton control in wastewater treatment High Rate Algal Ponds and algal production raceways

High Rate Algal Ponds (HRAPs) can provide economical and efficient near tertiary-level wastewater (WW) treatment, with the nutrients recovered as algal biomass. HRAP performance can be negatively affected by the establishment of zooplankton grazers that can consume much of the algal biomass within a few days. Zooplankton management is therefore essential for maintaining WW treatment performance and algal productivity. This paper reviews zooplankton ecology in WW systems and eutrophic environments, and potential methods for zooplankton control in HRAPs. Promising options for zooplankton control include physical methods such as filtration, hydrodynamic cavitation, shear, bead mills; chemical methods such as increase of HRAP night-time CO2 concentration, promotion of the lethal un-ionized ammonia toxicity, use of biocides, and the chitinase inhibitor chitosan; and biocontrol using competitor and predatory organisms. CO2 and phototactic induced migration are proposed to concentrate zooplankton in specific areas to reduce the amount of pond water requiring treatment. Based on this review, we suggest that it may be most beneficial to maintain zooplankton grazer populations at low levels as part of a stable community, rather than to totally eradicate them. This will prevent the ecological imbalance of total control that could result in the establishment of other zooplankton species that are less easy to control.