Anammox Sludge Preservation Strategies for Short-Term Reactivation

Anaerobic ammonium oxidation (Anammox) is an efficient biological route to nitrogen removal under anoxic conditions. The high sensitivity to environmental conditions, slow growth, and low sludge production for reactor start-up are some drawbacks to the full-scale application of this technology. Thus, preservation and successful reactivation of Anammox sludge could be one of the solutions. Anammox granule preservation (low temperature (4 °C) and freezing (− 4 °C) with KNO3) standouts as a practice for fast reactor start-up and improvement of its performance. However, the integrity of granules and bacteria activity must also be maintained during the preservation period to provide a fast reactor start-up. In the present study, the preservation was evaluated at 4 and − 4 °C as sustainable techniques for granular Anammox sludge storage. Subsequently, the reactivation of both Anammox sludge in up-flow bed reactors and the integrity of the freezing granules (preserved under − 4 °C) were analyzed by extracellular polymeric substances (EPS). Anammox can be successfully stored at 4 °C, for up to 3 months. It is also possible to recover the sludge in a short time (21 days), with a nitrogen removal efficiency higher than 80%. Under freezing conditions after 11 months (− 4 °C), the sludge can be recovered after 45 days, reaching around 80% nitrogen removal efficiency even with the granule’s low integrity. Based on these results, refrigeration and freezing can be used as cost-effective and feasible techniques for Anammox sludge preservation and reactivation. This is important for biomass transport and reactor start-up, accelerating its practical use for wastewater treatment plants.