Heavy Metal Removal (Copper and Zinc) in Secondary Effluent from Wastewater Treatment Plants by Microalgae

Microalgae is used for the removal of heavy metals from a wastewater treatment plant discharge. Laboratory-scale experiments are described that characterize the heavy metal uptake of copper and zinc by three rnicroalgae strains: Chlorella vulgaris, Spirulina maxima, and a naturally growing algae sample found in the wastewater from a wastewater treatment plant (containing Synechocystis sp. (dominant) and Chlorella sp. (common) and a few cells of Scenedesmus sp.) Tests were conducted using untreated and autoclaved secondary effluent as a substrate. In the untreated secondary effluent trial, the microalgae removed up to 81.7% of the copper, reaching a lowest final concentration of 7.8 ppb after 10 days. Zinc was reduced by up to 94.1%, reaching 0.6 ppb after 10 days. The removal rates varied significantly with the microalgae strain. Higher heavy metal removal. efficiencies were obtained in the autoclaved secondary effluent than the untreated secondary effluent, suggesting microorganisms already present in secondary effluent contribute negatively and compete with microalgae for nutrients, hindering microalgae growth and uptake of heavy metals. Inoculated samples showed decreased heavy metal concentrations within 6 h of initial inoculation, suggesting microalgae do not require long periods of time to achieve biosorption of heavy metals.