Two-step startup improves pollutant removal in membrane-aerated biofilm reactors treating high-strength nitrogenous wastewater

The goal of this study is to develop a two-step startup strategy for establishing a layered biofilm in membrane-aerated biofilm reactors (MABRs) treating strong nitrogenous wastewater. We hypothesized that using a two-step startup strategy based on the deliberate deposition of nitrifiers on the membrane in the first step and of heterotrophs away from the membrane in the second step would improve the removal rates of ammonia and COD compared to a one-step startup procedure. The results demonstrate that MABRs with a two-step startup had more than double the specific removal rates of ammonia and COD compared with the reactors using a one-step startup. Direct use of biofilm inoculum exposed to high ammonium concentration can shorten the startup duration. It was found that the abundance of the amoA gene in biofilms from MABRs with nitrifying biofilm inoculum was two orders of magnitude higher than that with nitrifying sludge inoculum after 63 days of operation. The sequences for introducing COD in the second step of the startup either stepwise or gradually had less pronounced effects on the rates of substrate removal. The provided startup strategy is beneficial for practical operation of MABRs for treating high-strength nitrogenous wastewater.