The waste waters included in this study would typically require pretreatment (filtering
and sterilization) before they could be used in algal cultivation. Reject water and
liquid fractions from the composting process needed to be diluted to lower the high
concentration of nutrients. Sterilised and non-sterilized fish farm waters were used
without dilution. In laboratory scale experiments, E. gracilis and C. pyrenoidosa were
cultivated in each of the selected waste waters. Reject water and non-sterilized fish
farm water were not used for the cultivation of Selenastrum sp (Table 3).
Waste fractions originating from biowaste processing (compost leachate, process
water and press water) were found to be the most promising for algal biomass
production. The nitrogen and phosphorous concentration of the fish farm
waters were lower and also contained less organic carbon (BOD) than the other
waste fractions. This probably limited algal growth. Algal growth in the reject water
was also poor, probably due to the high concentration of ammonium (>100 mg
NH4 -N L-1), which was converted to toxic ammonia when the pH increased during
cultivation. The pH increase was a result of algal photosynthesis, that utilizes
dissolved carbon dioxide from the solution and thus raises the pH. The very low
concentration of magnesium in the reject water may also have hampered algal
growth (Park et al. 2010).
The composting leachate, in contrast, was not only rich in macro nutrients, but
also contained plenty of all micronutrients. Pre-composting water also contained
considerable amounts of dissolved zinc (0.5 mg L-1) and some nickel and chromium
(0.25 mg L-1). If the water is recycled within the cultivation process, the concentrations
of heavy metals would need to be monitored and controlled to ensure that the
concentrations would not exceed the toxic thresholds of the cultivated algae.