Effect of preservation temperature on the nitrification performance and morphological structure of bioflocs

This study aimed to determine the optimal storage temperature for bioflocs. A One-Way experimental design was employed, where bioflocs were stored at room temperature (25 °C), refrigerated (4 °C) and frozen (−20 °C) for periods of 7, 14, 21 and 28 d. The nitrification performance of bioflocs declined under all three conditions as storage time increased. After 28 d, the ammonia oxidation rate at 4 °C was (0.16±0.10) mg/(L·h), significantly higher than the rates at 25 °C (0.08±0.09) mg/(L·h) and −20 °C (0.01±0.09) mg/(L·h). Bioflocs stored at −20 °C had almost no ammonia oxidation activity by day 7, with a 99.52% reduction from the initial rate. The nitrite oxidation rate at 4 °C after 28 d was (0.19±0.03) mg/(L·h), which was significantly higher than that of bioflocs stored at 25 °C (0.10±0.05) mg/(L·h) and −20 °C (0.14±0.02) mg/(L·h). Microscopic observation revealed that of bioflocs stored at 25 °C became looser, with a decrease in volume average size from (245.92±21.51) μm to (148.50±9.90) μm by day 28. Conversely, bioflocs stored at −20 °C increased in size from (245.92±21.51) μm to (310.82±29.67) μm. No significant changes in structure or particle size were observed for bioflocs stored at 4 °C. In conclusion, a storage temperature of 4 °C is recommended for bioflocs. This study provides insights into appropriate storage conditions and duration, which can inform the storage and reuse of bioflocs in practical applications.