CaCO3 supplementation of low-carbon wastewaters for the cultivation of Microalgae : a study with Desmodesmus multivariabilis

Abstract

Microalgal biomass cultivated in wastewater has the potential for refining to energy products such as biodiesel and biohydrogen with the additional benefit of also treating the wastewater. As many species of microalgae can employ both Heterotrophic and Autotrophic modes of carbon synthesis, low-carbon waters benefit from the addition of inorganic carbon to the water. Capital and operational costs are a deterrent to using CO2 and other alternatives may be more attractive. NaHCO3 is a popular alternative but as a result of its solubility and alkalinity quickly raises the pH of the water which inhibits algal growth due the presence of free ammonia at high pH values. In this experiment, the growth of a South African strain of Desmodesmus multivariabilis was studied in the presence of solid calcium carbonate. Calcium carbonate was chosen for its low solubility, which would potentially allow for its dissolution to be driven by the inorganic carbon uptake in the media. The performance was compared to that of aerated wastewater and wastewater with solid CaSO4 as a non-carbon-containing substitute. It was found that there were significant differences in the growth and metabolism of all three experiments. Growth in the presence of solid calcium carbonate and calcium sulphate showed a preference for attached growth in the vicinity of solids, while suspended growth was preferred when just air was supplied. Furthermore, the experiment with air showed the highest growth rate, nitrogen uptake and a biomass yield that was more than an order of magnitude higher than with CaCO3. The experiment with CaSO4 showed low yields and growth rates, possibility indicating and inhibitory effect of the CaSO4. In the presence of CaCO3, a very high yield of extracellular organic metabolites was observed. The presence of these metabolites, as well as the stability of the pH and low growth, is a possible indication that the organism was controlling the pH as a defence mechanism. Despite not being a favourable substrate for growing D. multivariabilis, the high yield of extracellular metabolites may have a commercial potential, and the nature and use these metabolites deserve further investigation.