OPTIMIZATION OF IMMOBILIZED ALGAL CULTIVATION IN AQUACULTURE WASTEWATER

Based on the experiments of the present work, the optimal immobilized conditions for algal cultivation depend on the purpose of the cultivation. The optimal initial algal concentration, bead concentration, and bead size for nutrient removal and algal growth are different. If the purpose of the cultivation is to treat aquaculture wastewater and achieve high nutrient removal,

the optimal initial algal concentration, beads to culture medium ratio, and bead size seem to be 5.1  10⁵ cells/bead, 1:10.7, and 3.4 mm. If the purpose of the cultivation is to produce algal biomass for the production of high-value algae-based compounds and achieve high algal growth, the optimal initial algal concentration, beads to culture medium ratio, and bead size seem to be 1.2  10⁵ or 2.3  10⁵ cells/bead, 1:5.4, and 4.7 mm. If both high nutrient removal and algal growth are desired, the parameters could be adjusted somewhere between the optimal values for nutrient removal and algal growth.

7. CONCLUSIONS

Microalga S. quadricauda was able to successfully grow and remove nutrients in the course of the cultivation in aquaculture wastewater, which implies that the algal species holds potential to be utilized in the aquaculture wastewater treatment. Immobilization was observed to improve the nutrient removal from aquaculture wastewater. However, assimilation by algae was not the only nutrient removal mechanism in the bead treatment, since nutrients were also removed via bacterial assimilation, adsorption to alginate, phosphorus precipitation, and ammonia volatilization. Algal growth on the other hand was not enhanced in immobilized cultivation, and the leakage of cells from the beads to the culture medium was common in the treatments with immobilized algae. Neither nutrient removal nor algal growth was observed to benefit from the addition of Mg into the beads. Of the initial algal concentrations, beads to culture medium ratios, and bead sizes that were tested in the experiments, 5.1  10⁵ cells/bead, 1:10.7, and 3.4 mm seem to be optimal for nutrient removal from aquaculture wastewater in 4.5-day cultivation and 1.2  10⁵ or 2.3  10⁵ cells/bead, 1:5.4, and 4.7 mm for algal growth in 6-day cultivation.

However, the results obtained in the experiments are only directional estimates for various reasons. First of all, in many of the experiments, especially the ones investigating the nutrient removal, the differences between the treatments were small, and the statistical significance between the treatments could not be determined due to low sample size. In addition, some of the conducted measurements (ammonia concentration and cell counts) were not performed precisely according to the instructions, which reduces the accuracy of the results. The experiments investigating bead concentration and bead size had also issues in their experimental set-up and consequently the results of these experiments are influenced not only by the differences in the investigated parameter but also by the differences in the initial algal concentration.

Overall, immobilization seems to be an interesting technique to ease algal cultivation and harvesting. However, more research is still needed especially on large scale before immobilized algae can be applied for aquaculture wastewater treatment. In the future research it is highly important to carefully plan the experiments in advance to assure that solely the investigated parameter is causing differences in the results, and that the sample size is large enough to allow the statistical analysis of the results.

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