A lab-scale partial nitritation granular sludge air-lift reactor was operated in continuous mode treating low strength synthetic medium (influent ca. 50 mg-N-NH₄ ⁺/L). Granules were initially stratified with AOB in the external shell and NOB in the inner core at 20 °C. Once temperature was decreased progressively from 20 °C to 15 °C, nitrate production was initially observed during several weeks. However, by maintaining relatively high ammonium concentrations in the liquid (ca. 28 mg-N-NH₄ ⁺/L), effluent nitrate concentrations in the reactor decreased in time and process performance was recovered. Batch tests were performed in the reactor at different conditions. To understand the experimental results an existing one-dimensional biofilm model was used to simulate batch tests and theoretically assess the impact of stratification, dissolved oxygen (DO) and short-term effects of temperature on time course concentrations of ammonium, nitrite and nitrate. This theoretical assessment served to develop an experimental methodology for the evaluation of in-situ batch tests in the partial nitritation reactor. These batch tests proved to be a powerful tool to easily monitor the extent of stratification of nitrifier guilds in granular sludge and to determine the required bulk ammonium concentration to minimize nitrite oxidation. When nitrifier guilds were stratified in the granular sludge, a higher bulk ammonium concentration was required to efficiently repress NOB at lower temperature (ca. 19 versus 7 mg-N-NH₄ ⁺/L at 15 and 20 °C, respectively).

Kinetics of iron reduction, formation of vivianite and the microbial community in activated sludge from two sewage treatment plants (STPs) with low (STP Leeuwarden, applying enhanced biological phosphate removal, EBPR) and high (STP Cologne, applying chemical phosphate removal, CPR) iron dosing were studied in anaerobic batch experiments. The iron reduction rate in CPR sludge (2.99 mg-Fe g VS ⁻¹ h ⁻¹ ) was 3-times higher compared to EBPR sludge (1.02 mg-Fe g VS ⁻¹ h ⁻¹ ) which is probably caused by its 3-times higher iron content. Accordingly, first order rate constants in both sludges are comparable (0.06 ± 0.001 h ⁻¹ in EBPR vs 0.05 ± 0.007 h ⁻¹ in CPR sludge), thus potential rates in both sludges are comparable. The measured Fe(III) reduction rates suggest that all iron in STP Leeuwarden and STP Cologne can be turned over within 15 h and 44 h respectively. Mössbauer spectroscopy and X-ray diffraction (XRD) indicated vivianite formation within 24 h in both sludges. After 24 h, 53% and 34% of all iron were bound in vivianite in the EBPR and CPR sludge respectively. Next generation sequencing (NGS) showed that the microbial community in the CPR sludge comprised more genera with iron-oxidizing and iron-reducing bacteria. Iron reduction and vivianite formation commence once activated sludge is exposed to oxygen free conditions. Our study reveals that the biogeochemistry of iron in STPs is very dynamic. By understanding the interactions between iron and phosphate crucial processes in modern sewage treatment, such as chemical phosphate removal or phosphate recovery from sewage sludge, can be optimized.