Model-based Evaluation of Effects of Temperature on Nitrogen Removal in Low- and Moderate-temperature Type Anammox Reactors

Abstract

A mathematical model was developed for predicting effects of temperature on nitrogen removal in continuous-flow reactors via anaerobic ammonium oxidation (anammox). A cardinal temperature equation with inflection for the bacterial growth was incorporated into the anammox model of low-temperature type reactors, Low-R1 and Low-R2 inoculated with activated sludge respectively in Kumamoto and Hokkaido. The maximum specific growth rate (µopt) was calibrated to 0.052 d-1 at the optimal temperature (T opt) of 26℃ between the minimum temperature (T min) of 4℃ and the maximum temperature (T max) of 36℃ for anammox bacteria in Low-R1 (correlation with measured data, r = 0.851). The values of µopt, Topt, Tmin, and Tmax were calibrated to be 0.089 d-1 , 31℃, 0℃, and 36℃, respectively for anammox bacteria in Low-R2 (r = 0.995). A typical exponential temperature equation was incorporated into the anammox model of a moderate-temperature type reactor, Mod-R inoculated with activated sludge in Kumamoto. The maximum specific growth rate at 30℃ was calibrated to 0.055 d-1 with the temperature coefficient of 0.104℃-1 for anammox bacteria in Mod-R (r = 0.987). The mathematical model simulated treatment of ideal wastewater containing ammonium at 50 mg-N/L and nitrate at 60 mg-N/L at hydraulic retention time of 0.5d and sludge retention time (SRT) of 50-150d at 10-30℃. The simulations predicted stable nitrogen removals in Low-R1 and Low-R2 at low temperatures in comparison with Mod-R. Although Low-R1 at short SRTs showed lower nitrogen removals than Mod-R at high temperatures around 35℃, Low-R2 demonstrated robust T-N removals in the whole temperature range. In conclusion, the low-temperature type anammox reactors, especially Low-R2, would have a sufficient potential to develop a versatile nitrogen removal process without any heating system in mild climate conditions.