A comparative study of four systems for tertiary wastewater treatment by Scenedesmus bicellularis: New technology for immobilization

Immobilization appears to be one of the best techniques to separate physically micro-algal cells from their culture medium for the purpose of algal tertiary wastewater treatment. High operation costs and other drawbacks of large-scale physico-chemical methods of harvest led to a comparative study of biotreatment systems. Before treatment began, Scenedesmus bicellularis cells were conditioned (starved) under four different sets of conditions: 1) non-immobilized cells with air bubbling (NCA); 2) cells immobilized in alginate beads (CBW) and 3) cells immobilized on alginate screens (CSW), all conditioned in synthetic culture medium depleted in N and P; 4) cells immobilized on alginate screens but conditioned in air at 100% relative humidity (CSA). Starvation was started under a light:dark photoperiod of 16:8 h. Starved cells were then used to treat wastewater for a 2-h period. The performance of each system was evaluated by determination of residual NH₄-N and phosphate ions and by growth (dry weight, total chlorophyll, cell count, protein content). We then tested the capacity of microalgae immobilized on screens to eliminate N and P from a secondary municipal wastewater effluent and examined the influence of temperature and starvation. The quality of treated effluents was improved considerably with the system using CSA or CSW model. For CSA model, the protein content was 22.4 pg cell^-1 compared to 12.9, 9.5, 9.1 pg cell^-1 for NCA, CBW and CSW models, respectively. The CBW and CSW models were efficient for chlorophyll synthesis. The residual ammonium content in natural wastewater after 2 h of treatment with CSA model was 39% at 6±2 °C and reached 100% removal at 18±2 °C. With the first 2 h, the removal of orthophosphate was inferior (53%) at 6±2 °C, but 88 to 100% at 18±2 °C depending on starvation times. Long starvation times (72 or 96 h) caused damage to cells and uptake of nutrients was lower than with 54 h starvation. This work demonstrates that by using immobilization on screens, removal of nutrients from wastewater was higher than with conventional biological tertiary wastewater treatments (free cells or bead-shaped alginate particles).