Operational optimization of batchwise nutrient removal from wastewater by carrageenan immobilized Chlorella vulgaris

Cyclic batchwise operations using carrageenan immobilized Chlorella vulgaris beads were employed to remove nutrients (N and P) from primary settled wastewater. The NH₄⁺-N removal efficiency at the end of each 3-day treatment cycle under Continuous-Contact mode operation dropped from 95.6% in the first cycle to 65.4% in the third cycle. If 3-day delay period was allowed at the end of each treatment cycle as in Contact-Delay mode operation, the NH₄⁺-N removal efficiency was maintained at over 90% even at the end of the third cycle. With a rehardening process at the end of each treatment cycle as in the Contact-Rehardening mode operation, carrageenan algal beads maintained over 95% NH₄⁺-N removal for all 4 cycles tested. Reduction of NH₄⁺-N was through algal uptake and bead adsorption. Over 90% PO₄^3--P reduction was recorded in all cycles in all the three operation modes through algal uptake and some other unknown mechanisms. Though rehardening process could strengthen and delay the problem, massive leakage became unavoidable once the gel beads were saturated with cells as a result of growth and proliferation within the gel beads.