Application of the modern microbial techniques changed the paradigm about the microorganisms performing nitrification. Numerous investigations recognized representatives of the genus Nitrospira as a key and predominant nitrite-oxidizing bacteria in biological nutrient removal systems, especially under low dissolved oxygen and substrate conditions. The recent discovery of Nitrospira capable of performing complete ammonia oxidation (comammox) raised a fundamental question about the actual role of Nitrospira in both nitrification steps. This review summarizes the current knowledge about morphological, physiological and genetic characteristics of the canonical and comammox Nitrospira. Potential implications of comammox for the functional aspects of nitrogen removal have been highlighted. The complex meta-analysis of literature data was applied to identify specific individual variables and their combined interactions on the Nitrospira abundance. In addition to dissolved oxygen and influent nitrogen concentrations, temperature and pH may play an important role in enhancing or suppressing the Nitrospira activity.

Temperature is a key operational factor influencing the anammox process kinetics. In particular, at temperatures below 15 °C, the specific anammox activity (SAA) considerably decreases. This study aimed to describe the temperature dependence of the anammox process kinetics in the temperature range from 10 to 55 °C, including the specific characteristics of "cold anammox". The commonly used Arrhenius and extended and modified Ratkowsky equations were examined. The Ratkowsky equations yielded a strong correlation (coefficient of determination, R2 = 0.93-0.96) between the measured and predicted data over the analyzed temperature range (10-55 °C). However, these equations could not correctly reflect the anammox temperature dependence at temperatures below 15 °C (R2 = 0.36-0.48). Therefore, a new generalized temperature model was proposed. The generalized temperature equation (GTE) considered the division of the analyzed temperature range into three temperature ranges: 10-15 °C, 15-35 °C and 35-55 °C. The ranges correspond to "cold anammox", "(low) mesophilic anammox" and "thermophilic anammox". The applied approach yielded a strong correlation between the measured and predicted SAA (R2 = 0.97) over the temperature range from 10 to 55 °C and over the low-temperature range from 10 to 15 °C (R2 = 0.99). Overall, the GTE could enhance the predictions of the temperature dependence of the anammox process kinetics. The GTE can help examine anammox-based bioaugmentation systems operating at both high temperatures (sidestream reactors) and low temperatures (mainstream reactors).

The aim of this study was to determine a short-term and long-term effect of temperature on the anammox rate and determination of temperature coefficients in the Arrhenius and Ratkowsky equations. The short-term effects of temperature on the anammox granular biomass were investigated in batch tests at ten different temperatures in the range of 10–55 °C. The maximum overall nitrogen removal rate of 1.3 gN gVSS⁻¹·d⁻¹ was observed at 40 °C. The minimum rate, close to 0 gN gVSS⁻¹·d⁻¹, was observed for the limits of the analyzed temperature range (10 and 55 °C). The activity tests carried out at 55 °C showed an irreversible loss of the activity due to the observed biomass lysis. Subsequently to the batch tests, a sequencing batch reactor (SBR) was operated at different temperatures (from 30 to 11 °C) to determine the long-term effects of temperature. The system was successfully operated at 15 °C, but when temperature was decreased to 11 °C, nitrite started to accumulate and the system lost its stability. The temperature coefficient (θ) was 1.07 for the batch tests carried out in the temperature range of 10–40 °C. On the contrary, during the long-term SBR operation, substantially different θ had to be estimated for two temperature ranges, 1.07 (T = 15–30 °C) and 1.65 (T = 11–15 °C).

The aim of the study was to determine the pH effects on nitrogen removal in the anammox-enriched granular sludge. The experimental data were extracted from a 4 L completely-mixed batch reactor with the granular sludge at different initial pH values (6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, 10.5) and constant temperature T = 30 °C. Simulations were run in GPS-X 6.4 using a comprehensive mechanistic model Mantis2. Two kinetic parameters, the maximum specific growth rates of ammonia oxidizing bacteria (AOB) and anammox bacteria, were optimized at different pH scenarios. The inhibitory effects of the pH extremes on the anammox-enriched sludge were discussed in terms of the inhibition of free nitrous acid and free ammonia and metabolic mechanisms. Two different pH functions were used to examine the pH effects on the nitrogen removal kinetics. The pH optima for AOB and anammox bacteria were 7.4 and 7.6, respectively. The maximum specific growth rates of AOB and anammox bacteria at the pH optima were 0.81–0.85 d⁻¹ and 0.36–0.38 d⁻¹ (at T = 30 °C). The measured specific anammox activities (SAAs), predicted SAAs by Mantis2 and fitted SAAs by the Michaelis pH function at the pH optima were 0.895, 0.858 and 0.831 gN/(gVSS·d), respectively (VSS: volatile suspended solids).