Model-based evaluation of algal-bacterial systems for sewage treatment

Abstract

Many papers on wastewater treatment by algal-bacterial systems have been published. Current reactor models cannot model the simultaneous removal of organic matter, ammonium and phosphate from an algal-bacterial reactor. Because a powerful theoretical analysis platform is not available, how the consortia function in bioreactors is still not clear. In this study, the activated sludge model NO.3 was extended with a modified algal biokinetics for modeling wastewater treatment by algal-bacterial consortia. The applied model was calibrated and verified by experiments. The model shows that phosphate accumulating organisms (PAOs) and nitrifying bacteria consumed over 80% of phosphate and 60% of ammonium under different aeration conditions, respectively, whereas algae played a minor role in removing nutrients. In addition, the model shows that maintaining alkalinity at 1 mol/m3 (100 mg/L CaCO3) can significantly promote nutrient removal by increasing activities of PAOs and nitrifying bacteria, despite that algae consume less nutrients. Light intensity decreased quickly in suspended algal-bacterial consortia, which was accurately modelled. Compared with algae, bacteria played a more significant role in reducing light intensity. A defined light utilization efficiency decreased by 50% when doubling biomass concentration, showing that concentrated biomass should not be applied at large-scale when artificial illumination is applied. The model predicts that efficient wastewater treatment by algal-bacterial consortia at a biomass concentration of approximately 1 g/L can be achieved in eight hours under non-aerated conditions. This study proved that biological wastewater treatment by algal-bacterial consortia can be modeled, while heterotrophic bacteria, nitrifying bacteria, PAOs and algae simultaneously function in the consortia. The model can be a useful tool to study and optimize algal-bacterial reactors.