Comparative study on attached-growth photobioreactors under blue and red lights for treatment of septic tank effluent

Abstract

Attached-growth photobioreactors (AG-PBRs) employing low-cost attached-growth media were applied to treat septic tank effluent which contained abundant organic and nutrient matters as well as pathogenic microorganisms. This study investigated effects of blue and red LED lights on organic, nutrient and pathogenic removals, biomass productivity and compositions of microbial community in the AG-PBR system. The experimental results showed the blue AG-PBR to be more effective in removing chemical oxygen demand (COD), total nitrogen (TN) and ammonia nitrogen (NH3–N) and generating biomass productivity than those of the red AG-PBR (P < 0.05). Mass balance analysis indicated that the TN and total phosphorus (TP) were removed mainly by assimilation into the biomass. The TN removal rates via nitrification and denitrification processes in the blue AG-PBR were found to be higher than that of the red AG-PBR, corresponding to the observed results of bacterial biomass and abundances of nitrifying and denitrifying bacterial species in the treatment systems. The maximal areal algal biomass productivity of 47 gDW/(m2. d) in the blue AG-PBRs was found to be higher than those of other algal attached-growth systems. Although, the red and blue AG-PBR systems could effectively treat the septic tank effluent to meet the national and international discharge standards, based on treatment efficiencies and biomass productivity, the blue AG-PBR is recommended for treatment of septic tank effluent.