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A method of accelerating the removal of ammonium and phosphate by the unicellular microalga Scenedesmus bicellularis is presented for municipal tertiary wastewater treatment using immobilized cells to obtain a high quality of effluents. Microalgal cells grown in defined medium were harvested by centrifugation and stored at 4°C in the dark for 8 months before immobilization. The concentrated cell suspension was then immobilized in alginate films supported on polypropylene screens. Immobilized cells were incubated in a water-saturated air stream enriched with CO2 at 750, 1,000, or 1,500 ppm for 3 h periods followed by 2 h periods without enrichment. The quantitative effects of these three CO2 enrichments on nutrient uptake from secondary municipal wastewater effluent were compared to a control laboratory air at 320 ppm under the same conditions of illumination, photoperiod, and humidity. The exposure cycle of 48-h nutrient deprivation in air with CO2 enrichment followed by 2 h of nutrient uptake from wastewater was repeated three times with a residual NH4N content dropping to 0% after 105 min for the 1,500 ppm CO2 treatment and to 34% of the initial level after 120 min for the control treatment. Complete PO4P removal required more than 2 h. The chlorophyll a contents obtained with 1,000 and 1,500 ppm CO2 enrichments were comparable. This study establishes that intermittent CO2 enrichment during nutrient deprivation of immobilized microalgal cells in a water-saturated air stream may accelerate tertiary wastewater treatment. |
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