Effect of preservation techniques on the regeneration of gel entrapped nitrifying sludge

Gel entrapped nitrifiers may be used in wastewater treatment plants as the main N-removal system, or as an additive to existing biofilm or sludge systems. In any case, gel entrapped nitrifiers may be stored for safety measures and added in cases of a collapse of the activity in the plant. Different preservation techniques; freezing, drying and lyophilization, were evaluated as means to optimize the conservation of the nitrification activity immobilized in PVA-SbQ gel beads during storage. Glycerol, sucrose and trehalose were added in various amounts as cryoprotectants. After 2–3 months of storage, the nitrifiers were reactivated in 450 ml CSTR using a synthetic nitrification medium with excess ammonium. Beads frozen without any additives or in 15% glycerol maintained 60% and 40% of the original nitrification rate they had before conservation, respectively. After preservation by drying and lyophilization, 10% or less of the activity was recovered. However, even those beads reached 100% of the previous activity within 4–9 d of cultivation. In the case of drying, addition of 10 mM trehalose improved the initial activity significantly. When lyophilized, the presence of protectants up to 100 mM seemed to have a positive effect on the reactivation of the nitrifiers. The stress of the drying process apparently altered the structure of the gel beads so that they never fully regained the initial shape and volume, but without any significant loss in the obtainable maximum activity. It is concluded that for long time storage at a wastewater treatment plant, freezing without any additives should be the preferred preservation technique. For transportation, however, when weight and size are important economical factors, drying may be a better alternative.