Effects of Fe(II) on anammox-hydroxyapatite system after temperature reduction shock

The effects of Fe(II), as well as the low-temperature shock on the anammox-hydroxyapatite (Anammox-HAP) system, were investigated in the upflow anaerobic sludge bed (UASB) reactor. The nitrogen removal efficiency (NRE) decreased from 84.69 % to 57.85 % with temperature decreasing from 35 °C to 18 °C, and further declined to 46.24 % with Fe(II) concentration increasing from 1.56 mg/L to 5.00 mg/L. The secretion of extracellular polymer substances (EPS) increased by 23.99 % in response to the rising concentration of Fe(II). Meanwhile, the content of heme c and the activity of hydrazine oxidase (HZO) enhanced by 75.52 % and 30.26 %, respectively, while the electronic transport system activity (ETSA) decreased by 56.44 %. Despite the unfavorable influence of Fe(II) and low temperatures, the anammox-related genus Candidatus Kuenenia remained the dominant genus involved in the Anammox-HAP system, with the relative abundance increasing from 30.85 % to 38.54 %. PICRUSt2 was used to predict gene abundances, and genes related to nitrogen metabolism (nirS, nirK) and the iron metabolism (fur, feoB) showed a decreasing trend. In summary, the elevated Fe(II) concentrations at low temperatures potentially hindered the nitrite reduction process, possibly by reducing the abundance of functional enzymes or the capacity for electron transfer.