Successful startup of the single-stage PN-A (partial nitrification-anammox) process by controlling the oxygen supply.

The single PN-A (partial nitrification-anammox) reactor offers a cost-effective solution for nitrogen removal. However, optimal control of the PN-A reactor is challenging due to the interactive mechanisms among the oxygen supply, bulk liquid DO (dissolved oxygen) concentration, and the balance of various functional bacterial species. In this study, a mathematical model was used to derive the optimal control variable for the maximum nitrogen removal, and an experimental PN-A reactor was operated to verify the model simulation results. The model simulation results indicate that the oxygen supply to the ammonium load ratio is the key factor to control the single-stage PN-A reactor for optimal TN removal. For optimal TN removal, the oxygen supply to the ammonium load ratio should be 1.9 mg O2/mg N. The DO concentration is not the key control parameter to get the maximum TN removal as the optimal TN removal could be achieved under a wide range of DO concentration. The model simulation results were verified in the experimental PN-A reactor under oxygen transfer rate ([Formula: see text]) at 52 day-1, HRT at 24 h, and ammonium load ratio of 0.55 kg N/(m3∙day).

Experimental investigation and mathematical modeling of the competition among the fast-growing "r-strategists" and the slow-growing "K-strategists" ammonium-oxidizing bacteria and nitrite-oxidizing bacteria in nitrification.

Based on their differences in the kinetic values, the nitrifiers could be classified into the fast-growing "r-strategists" and the slow-growing "K-strategists" bacteria. However, the difference in the kinetic values originated not only from the intrinsic differences among the nitrifier species, but also from other factors, i.e. sludge floc morphology, and the environment in which the nitrifiers were cultivated. It is not clear how these factors interact and affect the measured kinetic parameters and the competition among the "r-strategists" and the "K-strategists" bacteria. In this study, the kinetic parameters of nitrifiers cultivated in the SBR (sequencing batch reactor) under different substrate concentrations were monitored, together with the identification of nitrifier species and sludge floc morphology characterization. The results showed that the r-AOB and r-NOB ("r-strategists" ammonium-oxidizing bacteria and nitrite-oxidizing bacteria, i.e., Nitrosomonas and Nitrospira) were the dominant nitrifiers in the SBR reactor. A mathematical model describing the competition between r/K AOB and NOB showed that r-AOB and r-NOB could be enriched in the SBR. The experimental investigation supported the model simulation results. The model simulation also revealed that the different r/K AOB and NOB species could be enriched in different DO concentrations and SRT conditions, which could be manipulated to promote the growth of r-AOB and NO2- accumulation for the autotrophic nitrogen removal using ANAMMX.