Influence of light regime on the performance of an immobilised microalgae reactor for wastewater nutrient removal

Microalgae immobilised within a resin shaped into beads have demonstrated the ability to remediate nutrients from wastewater effluents within hydraulic retention times as low as 3 h. Methods to further optimise performance consider parameters relating to the bead with the impact of external conditions seldom investigated. Light is an essential parameter for microalgal growth with its effect on suspended cultures well documented. This work explores the influence of light on nutrient remediation by immobilised microalgae in order to recommend an optimal lighting solution for an immobilised microalgae technology based on Scenedesmus obliquus encapsulated within calcium-alginate beads. White light (400–700 nm) at a photon flux density (PFD) of 200 μmol∙m−2∙s−1 was determined optimal when illuminating a packed bed configuration. When considering phosphate, these conditions supported a remediation rate of 10.7 (± 0.01) mgP∙h−1∙106 beads−1 in comparison to 10.2 (± 0.01) and 10.1 (± 0.01) mgP∙h−1∙106 beads−1 for the blue (465 nm) and red (660 nm) spectra respectively. Although similar performance was demonstrated, light transmission trials determined white light to penetrate to greater bed depths resulting in a larger photoactive zone. A PFD of 200 μmol∙m−2∙s−1 was regarded as optimal when considering performance, attenuation depth and effective use of total supplied light. In addition, photoperiods trials determined lighting periods <12 h extended the overall treatment time.