Free ammonia resistance of nitrite‐oxidizing bacteria developed in aerobic granular sludge cultivated in continuous upflow airlift reactors performing partial nitritation

Abstract

Free ammonia (FA) inhibition has been taken advantage as a strategy to suppress the growth of nitrite‐oxidizing bacteria (NOB) in aerobic granules stabilized in a continuous upflow airlift reactor to achieve partial nitritation. However, after nearly 18 months of continuous exposure of aerobic granules to FA in the reactor, the FA inhibition of NOB was proven ineffective, and the partial nitritation gradually shifted to partial nitrification even though the long‐term granule structural stability remained excellent under the continuous‐flow mode. The extent of NOB resistance to FA inhibition was quantified based on the kinetic response of NOB to various FA concentrations in the form of an uncompetitive inhibition coefficient. It was confirmed that the NOB immobilized in larger granules under longer term exposure to FA tend to become more resistant to FA. Thereby, it was concluded that NOB can develop strong resistance to FA after continuous exposure, and thus, FA inhibition is not a reliable strategy to achieve partial nitritation in mainstream wastewater treatment. Practitioner points Nitrifying aerobic granules can remain structurally stable in continuous‐flow bioreactors. NOB developed free ammonia resistance after 6‐month continuous exposure. Larger aerobic granules tended to develop stronger free ammonia resistance. Free ammonia inhibition is not a reliable strategy for mainstream anammox.

Long-Term Stability of Nitrifying Granules in a Membrane Bioreactor without Hydraulic Selection Pressure

“…For this reason, it was reported that aerobic granules could not remain structurally stable for more than 80 days within MBRs operated without hydraulic selection pressure because the submerged membrane module retained both granules and bioflocs within the same reactor without selection [3][4][5]. However, our recent study demonstrated that nitrifying granules have remained stable within a completely mixed airlift reactor for 340 days under minimum hydraulic selection pressure in terms of the surface overflow rate (SOR) as low as 0.04 m h −1 [6]. Technically, such a minimum hydraulic selection pressure should be insufficient to keep the long-term stability of nitrifying granules because it is even lower than the SOR commonly used in full-scale wastewater treatment clarifiers that only retain activated sludge [7].…”

“…The airlift reactor (ALR) used in our previous long-term nitrifying granule stability study was used in this study (Figure 1A) [6]. The only change made was by replacing the previous overflow design with a membrane inserted for completely retaining all bioparticles inside the ALR (Figure 1B).…”

“…The air was introduced to the bottom of the ALR at an aeration rate of 0.3 L min −1 using an air pump. The substrate containing 50-60 mg L −1 ammonium-nitrogen (NH 4 + -N) along with other nutrients as described in a recipe of the previous study [6] was continuously pumped into the ALR at a flow rate of 0.3 L h −1 , giving a hydraulic retention time of 5.2 h. The seed granules were the anammox granules collected from a DEMON ® reactor operated by Hampton Roads Sanitation District, Virginia Beach, VA. As reported previously, the ALR was operated for 340 days without any granule stability issue [6]. After that, a membrane module (ZENON Environmental, ZeeWeed, Burlington, ON, Canada) was inserted into the ALR to turn it into a GMBR to totally eliminate the hydraulic selection pressure by retaining all biomass in the GMBR.…”

“…To understand the substrate utilization profiles in the granules, the mathematical model developed and calibrated in previous studies was adopted in this work [6,10]. Briefly, the model was set up by taking both the Monod equation and Fick's law into consideration.…”

The water industry toolbox

“…Biological treatment is also included in this issue. An et al (2021) study a significant problem appearing in reactors performing partial nitritation for subsequent anammox treatment, where nitrite‐oxidizing bacteria (NOB) develop free ammonia (FA) resistance. Using FA is a frequently used strategy to suppress NOB growth, but the authors demonstrate that typical operational conditions in aerobic granular sludge in airlift reactors lead to the development of large granules where NOB tend to become more resistant to FA.…”