Modeling the low pH limit of Nitrosomonas eutropha in high-strength nitrogen wastewaters

Abstract

In wastewater treatment, the rate of ammonia oxidation decreases with pH and stops very often slightly below a pH of 6. Free ammonia (NH3) limitation, inhibition by nitrous acid (HNO2), limitation by inorganic carbon or direct effect of high proton concentrations have been proposed to cause the rate decrease with pH as well as the cessation of ammonia oxidation. In this study, we compare an exponential pH term common for food microbiology with conventionally applied rate laws based on Monod-type kinetics for NH3 limitation and non-competitive HNO2 inhibition as well as sigmoidal pH functions to model the low pH limit of ammonia oxidizing bacteria (AOB). For this purpose we conducted well controlled batch experiments which were then simulated with a computer model. The results showed that kinetics based on NH3 limitation and HNO2 inhibition can explain the rate decrease of ammonia oxidation between pH 7 and 6, but fail in predicting the pH limit of Nitrosomonas eutropha at pH 5.4 and rates close to that limit. This is where the exponential pH term becomes important: this term decreases the rate of ammonia oxidation to zero, as the pH limit approaches. Previously proposed sigmoidal pH functions that affect large pH regions, however, led to an overestimation of the pH effect and could therefore not be applied successfully. We show that the proposed exponential pH term can be explained quantitatively with thermodynamic principles: at low pH values, the energy available from the proton motive force is too small for the NADH production in Nitrosomonas eutropha and related AOB causing an energy limited state of the bacterial cell. Hence, energy limitation and not inhibition or limitation of enzymes is responsible for the cessation of the AOB activity at low pH values.

Growth of Nitrosococcus-Related Ammonia Oxidizing Bacteria Coincides with Extremely Low pH Values in Wastewater with High Ammonia Content

“… 5 However, rather than NH 3 limitation, a direct effect of the high proton concentration on the energy conservation is the likely reason for the low pH limit of 5.4 of AOB from the Nitrosomonas europaea lineage. 12 Correspondingly, Nitrosomonas OTU 3 disappeared in this study as soon as the pH dropped to values below 5.4 in all experiments ( Figure 1 ).…”

“… 41 Low intracellular pH values also hamper the energy generation in certain bacteria, for example, AOB affiliating with the Nitrosomonas europaea lineage ( Section 4.1 ). 12 pH homeostasis is therefore an essential requirement for the survival of bacteria at low pH values. 20 HNO 2 impedes pH homeostasis under acidic conditions as it diffuses passively across the cytoplasmic membrane and decreases the intracellular pH value.…”

“… 53 This study shows that nitrification of urine, manure, digester supernatant, or another wastewater with high ammonia content is prone to low pH values, if the ratio of alkalinity to total ammonia is less than 2. When nitrifiying such solutions, any decrease of the pH far below the typical limit of 5.4 12 should be prevented. Otherwise, acid-tolerant γ-proteobacterial AOB will grow in, which has two detrimental consequences: first, the loss of the microbial community, which is responsible for high-rate nitrification at neutral pH, and, second, the emission of hazardous volatile nitrogen compounds such as NO, N 2 O, NO 2 , and HNO 2 .…”

“…The main population of AOB in urine nitrification reactors at neutral pH values was found to be affiliated with the Nitrosomonas europaea lineage. 12 Their activity was shown to cease at pH values close to 5.4. 12 Hence, it has to be expected that a population shift from this acid-sensitive to other, acid-tolerant, AOB is responsible for low-pH nitrification in urine.…”

“… 12 Their activity was shown to cease at pH values close to 5.4. 12 Hence, it has to be expected that a population shift from this acid-sensitive to other, acid-tolerant, AOB is responsible for low-pH nitrification in urine. A complete population shift from Nitrosomonas europaea to Nitrosomonas oligotropha has been observed in a reactor operated with synthetic low-strength nitrogen wastewater as the pH dropped from above 6 to 4.5.…”