Aeration-induced CO2 stripping, instead of high dissolved oxygen, have a negative impact on algae-bacteria symbiosis (ABS) system stability and wastewater treatment efficiency

Utilizing additional aeration to enhance the pollutants removal in the ABS system has been extensively studied. It is widely accepted that that higher aeration rate poses limitations in algae growth. However, this phenomenon is always attributed the phenomenon to the high dissolved oxygen (DO), and the CO2 blow-off that results in inorganic carbon (IC) deficiency is often ignored. In this research, different aeration rates were set under the same DO condition to prove that CO2 stripping, and not DO, is the main cause of algae growth limitation. It was apparent that ideal algae growth conditions, at an aeration intensity of 100 mL/min, was deteriorated due to CO2 stripping, leading to a large amount (15.62 mg/L) of unused NO2- and NO3-. Meanwhile, algae death caused by CO2 stripping damaged the phosphate assimilation process. Furthermore, insufficient IC resulted in competition between autotrophic bacteria and algae, inducing the drastic decline of nitrifying bacteria (whose growth cycle is especially long) numbers. For the relatively lower aeration rate (20 mL/min), the ABS system presented a more stable condition and higher pollutants removal efficiency than both the non-aerated group (0 mL/min) and the high-aeration group (50 mL/min and 100 mL/min). This finding demonstrates that CO2 stripping in the aeration process is the main cause of damage to the ABS system, which guides the ABS application.