Nutrient removal from wastewater reduces the environmental impact of its discharge and provides opportunity for water reclamation. Algae can accomplish simultaneous nitrogen and phosphorus removal while also adding value to the wastewater treatment process through resource recovery. The application of algae to wastewater treatment has been limited by a low rate of nutrient removal and difficulty in recovering the algal biomass. Immobilising the algal cells can aid in overcoming both these issues and so improve the feasibility of algal wastewater treatment. Trends for nutrient removal by algal systems over different wastewater characteristics and physical conditions are reviewed. The impact that the selection of algal species and immobilisation has on simultaneous nutrient removal as well as the interdependence of nitrogen and phosphorus are established. Understanding these behaviours will allow the performance of algal wastewater treatment systems to be predicted, assist in their optimisation, and help to identify directions for future research.

BACKGROUND: Immobilized algae are a promising tool to treat wastewater within a short time (<12 h) and simplify biomass harvesting compared with suspended algae. The potential of alginate-entrapped Chlorella vulgaris to bioremediate secondary (with and without nutrient supplementation), primary and two lagoon municipal wastewaters was investigated. The capability of the system to adapt to these wastewaters was analysed by determining biotic and abiotic nutrient removal, and further evaluated by comparison with suspended cells.RESULTS: The algal nitrogen (N) content (4.6-7.8 wt%) was closely related to the wastewater ammonium concentration (R 2 = 0.97). The algal cells did not adapt N uptake as effectively to wastewater nitrate concentration because both abiotic N and phosphorus (P) removal increased. The algal P content (1.2-3.2 wt%) varied in response to the wastewater P and was inversely related to the initial cellular P content. The algae thus adapted nutrient uptake to the wastewater N:P level and ratio when ammonium predominated. Biomass production (35-73 mg L -1 d -1 ) increased with dissolved organic and inorganic carbon with little impact from other wastewater characteristics. Immobilization did not affect N and P uptake per cell compared with suspended algae. CONCLUSION: Decoupling biotic and abiotic removal showed adaptation to the N:P of the wastewater and luxury P uptake had significant impact during treatment of different wastewater matrices, these traits were not affected by immobilization. Immobilization enables increased N and P removal rates compared with suspended algal systems as nutrient uptake per cell is not affected and higher concentrations of algal biomass within the reactor are facilitated.To meet the desired feed nutrient levels 184 mg L −1 NaNO 3 was added to SE1 + NO3, 115 mg L −1 of NH 4 Cl was added to SE1 + NH4, 268 mg L −1 of NaNO 3 to SE2 + NO3, and 156 mg L −1 of NH 4 Cl to SE2 + NH4.Immobilized algae adapt nutrient removal to different wastewaters www.soci.org

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Municipal wastewater reverse osmosis concentrate (ROC) poses health and environmental risks on its disposal as it contains nutrients and harmful organic compounds at elevated concentrations. This study compared a freshwater microalga Chlorella vulgaris and a marine microalga Nannochloropsis salina in suspended and alginate-immobilised cultures for batch and semicontinuous treatment of the ROC. The immobilised algae gave comparable nutrient removal rates to the suspended cells, demonstrating immobilisation had no apparent negative impact on the photosynthetic activity of microalgae. Semi-continuous algal treatment illustrated that the microalgae could remove significant amounts of nutrients (> 50% and > 80% for TN and TP, respectively), predominantly through algal uptake (> 90%), within a short period (48 h) and generate 335-360 mg DCW L-1 d-1 of algal biomass. The treatment also removed a significant amount of organic matter (12.7-13.3 mg DOC L-1 d-1), primarily (> 65%) through the biotic pathway.

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