Wastewater nitrogen and phosphorus removal by free and immobilized microalgal systems

The employment of the unicellular green algae Chlorella was demonstrated as a model system in removing nutrients (N and P) from the primary settled wastewater in free and carrageenan immobilized states. The growth and nutrient removal performance was related to the type of organic-N substrates and affected by the bacterial interactions in the studies using synthetic wastewater. In wastewater, the nutrient removal was shown to be cell density dependent, correlated to the algal growth and photosynthetic status in terms of chlorophyll content. Physiological acclimation process was proved to be an effective mean in reducing the retention time for treatment process.; Carrageenan as an immobilizing matrix or the heating process associated with the gelling process had no adverse effect on the growth (k and {dollar}rmmusb{lcub}max{rcub}{dollar}), chlorophyll status, nutrient uptake and the nitrate reductase activity of C. vulgaris when compared with the free cells. Carrageenan gel beads were shown to be an NH{dollar}sb4sp+{dollar} shuttle in adsorbing the ion, temporarily storing it and releasing it when ambient conditions were depleted of the ion. Simultaneous N and P removal from primary settled wastewater was found to be promising. Over 95% N and almost complete P were removed for a retention time of 3 days. This was just half of the time required by the free cell systems operated under similar conditions. The carrageenan immobilized algal system persisted for three 3-day cycles in a continuous batchwise operation before cell leakage became serious. Nitrogen removal efficiency dropped from 95% in the first cycle to 65% in the third cycle. The incorporation of a Delay process between the consecutive cycles sustained the N removal efficiency at 90% though the leakage problem was not alleviated. The alternative incorporation of a Rehardening process extended the operation life to four cycles with a sustaining 95% N removal throughout the process. With this understanding and proper reactor design, the upscale of the immobilized algal system is promising. The maximal cell holding capacity of the gel became a practical limitation of the immobilized system with respect to leakage problem. Good growth of the algal cells in the interior of the bead was confirmed by tracing the temporal confocal images and SEMs. Confocal microscopy is a promising diagnostic technique in delineating the spatial distribution of the algal cells in the beads.