Effects of specific wavelengths of lights on performance of biofilm photobioreactor for treating septic tank effluent

Abstract

Biofilm photobioreactors (BPBRs) employing algal-bacterial symbiotic reactions were designed as an on-site post-treatment system to treat septic tank effluent containing high concentrations of organic and nutrient matters. In this study, three BPBRs equipped with blue (peak wavelength ∼ 460 nm), red (peak wavelength ∼ 635) and mixed blue and red (50:50) lights for growing algal biomass (Chlorella sp.) were continuously operated at hydraulic retention times (HRT), light intensity and light/dark period of 6 days, 124 μmol/(m2.s) and 12/12 h, respectively. The experimental results illustrated that the chemical oxygen demand (COD) removal efficiency of 85 % and the highest total nitrogen (TN) and total phosphorus (TP) removal efficiencies of 87 and 84 %, respectively, were found in the BPBR with mixed blue and red lights, corresponding to the data of biomass productivity. Mass balance analyses of the BPBR performance indicated that TN (9.2 mg N/(L.d) or 74 %) and TP (0.73 mg P/(L.d) or 73 %) were mainly removed via biomass assimilation. Overall, the results of this study suggested that the mixed blue and red lights could enhance the biomass productivity and increase the treatment efficiencies of the BPBR system, resulting in the treated effluent meeting the discharge standards.