Ammonium and nitrite oxidation rates in the reactors with different salinities

We collected biomedia from different RAS biofilters. To assess the salinity tolerance, the ammonium (NH4-N) removal capacity of the biomedia from Havland and Erko biofilters was evaluated at different salinities by performing short-term batch experiments. The selected test salinities were 0, 15, and 31 ppt to impose a significant shift relative to the original salinity for all biomedia groups. The biomedia from Havland and Erko were collected on September 26 and October 14, 2022, respectively, and were shipped on ice overnight to NTNU's lab. Upon arrival, to avoid starvation and biofilm detachment, the biomedia were incubated overnight at room temperature (∼21 °C) in ammonium medium at a concentration of around 7.5 mg/L with a salinity identical to their biofilter origin (35 and 15 ppt for the Havland and Erko biomedia, respectively). The next day, the biomedia were incubated for 24 h in the ammonium medium at the relevant test salinity (0, 15, or 31 ppt) to physiologically adapt the biofilms to a new salinity. The next day, the ammonium removal capacity of the biomedia was determined in batch reactors of 1 L filled with 350 mL biomedia and 600 mL ammonium medium at the corresponding test salinity, 0, 15, or 31 ppt. To ensure that both biomedia types had an equal surface area, the number of biomedia pieces was adjusted: 200 pieces for Erko and 80 pieces for Havland, corresponding to a total surface area of 0.257 m2. A magnetic stirrer was used to ensure proper mixing and aeration of the reactors. To determine NH4-N and nitrite (NO2-N) concentrations, 2 mL samples were collected and filtered through a 0.2 μm PES syringe filter (Sarstedt, DE) at 0, 15, 30, 60, 90, 120, 150, and 180 min after the batch experiments started. The samples were kept on ice during the experiment and then stored at −20 °C until analyzed using the Hach-Lange cuvette kit (LCK305 and LCK339) according to the manufacturer's instructions on the DR3900 spectrophotometer (HACH®, DE).