Application of a prototype-scale Twin-Layer photobioreactor for effective N and P removal from different process stages of municipal wastewater by immobilized microalgae.

In the view of limited phosphorous resources and tightened discharge regulations, the recovery of phosphate and nitrate from wastewater is of great interest. Here, the integration of microalgae into wastewater treatment processes is a promising approach. A prototype-scale Twin-Layer photobioreactor immobilizing the green alga Halochlorella rubescens on vertical sheet-like surfaces was constructed and operated using primary and secondary municipal wastewater. The process was not impaired by suspended solids, bacteria or loss of algal biomass by leaching. The average areal microalgal growth was 6.3 gm(-2) d(-1). After treatment, P and N concentrations in the effluents could efficiently be reduced by 70-99%, depending on element and type of wastewater. Mean effluent values of ⩽ 1.0mg L(-1)P and 1.3 mg L(-1)N met the legal discharge limits of the European Water Framework Directive and show a potential to comply with upcoming, more stringent legislation.