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-carboncontaining
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.
