Effect of immobilized microalgal bead concentrations on wastewater nutrient removal

A unicellular green microalga, Chlorella vulgaris, entrapped in calcium alginate as algal beads were employed to remove nutrients (N and P) from simulated settled domestic wastewater. A significantly higher nutrient reduction was found in bioreactors containing algal beads (at concentrations ranging from 4 to 20 beads ml^-1 wastewater) than the blank alginate beads (without algae). A complete removal of NH₄/⁺-N and around 95% reduction of PO₄/^3--P was achieved within 24 h of treatment in bioreactors having the optimal algal bead concentration (12 beads ml^-1, equivalent to 1:3 algal beads:wastewater, v/v). The NH₄/⁺-N removal was significantly lower at low (around 4 beads ml^-1) and high (>15 beads ml^-1) algal bead concentrations. On the other hand, the effect of bead concentration on phosphate removal was less obvious, and bead concentrations ranging from 8 to 20 beads ml^-1 showed comparable percentages of phosphate reduction. Algal uptake and adsorption on alginate gels were found to be the major processes involved in the removal of N and phosphate in the present study. In addition, NH₄/⁺-N could be lost via ammonia volatilization while PO₄/^3--P was removed by chemical precipitation, as alkaline pH was recorded in the immobilized microalgal treatment system.