3. Results and discussion
3.1. Microalgal growth and nutrients removal at 25 C
Only the results from the cultures at 25 C are presented here but similar trends were observed for the cultures developed under the other two conditions tested: B15–12, B15–18 (data not shown).
For the batch experiment at 25 C and a photoperiod of 12 h, the chlorophyll concentration profile followed the same trend as those of VSS and TSS (Fig. 1a). Additional results showed a linear correlation (R2 = 0.95, data not shown) between the chlorophyll concentration and the volatile solids (VSS) under our experimental conditions. This suggests that the chlorophyll concentration could be used as an indicator of microalgal growth or at least photosynthetic activity under our conditions.
The TSS/VSS ratio increased (Fig. 1a) over the last 80 h of the experiment. This could be due to an increase in the TSS brought about by mineral precipitation as the pH increased towards the end of the experiment. We propose phosphate as the likely candidate (see Section 3.4). The control experiments in which the pH was constant at around 8 exhibited no significant precipitation or increase in the VSS/TSS ratio.
The dissolved oxygen concentration had a tendency to increase during the experiment under the different conditions where there was algal growth, indicating a prevalence of photosynthetic activity over heterotrophic carbon-oxidation and nitrification.
Nitrification was observed when the cultures were grown under obscurity (Figs. 1 b and 4 c); the decrease in ammonium concentration was accompanied by nitrite formation that peaked at 70h and then decreased, presumably due to further nitrite assimilation by the microalgae. Comparing the nitrite concentration profile for the illuminated cultures at 25 C (Fig. 1b) and the cultures in obscurity (Fig. 4c), one can expect nitrite uptake by microalgae to occur after ammonium exhaustion under illuminated conditions. The nitrate concentration remained approximately constant and low (about 2 mgN L 1) over the entire experiment (data not shown). Its consumption due to denitrification was unlikely to occur under aerobic conditions and increasing dissolved oxygen concentration in the culture. On the other hand, data obtained for the cultures in obscurity (Fig. 4c) suggest no nitrification by conversion of nitrite to nitrate after ammonium exhaustion.