Effects of abrupt salinity increase on nitrification processes in a freshwater moving bed biofilter

The nitrification process is a widely used biological approach responsible for ammonia and nitrite removal in recirculating aquaculture system (RAS) biofilters. Given this pivotal role, the influence of different water quality parameter on nitrification efficiency is important information for RAS operations. One influencing parameter is salinity, and salinity fluctuations in freshwater RAS biofilters are reported to affect the nitrifying bacteria. This study investigated the effects of abrupt increase in salinity in freshwater RAS on substrate-dependent (1'-order) as well as substrate independent (0'-order) nitrification rates. A 100% inhibition was found for surface specific removal (STR) of total ammonia nitrogen (TAN) and surface specific nitrite removal (SNR) when salinity was abruptly increased to 25 parts per thousand and above. A fast turnover (i.e. steep decline in [NH4-N+] and [NO2-N-]) were observed at lower salinities (<= 10 parts per thousand), while limited/no degradation of either ammonia or nitrite was seen at salinities above 25 parts per thousand. At low substrate loading (1'-order process), removal rate constants (k(1a)) of 0.22 and 0.23 m d(-1) were observed for ammonia and nitrite degradation, respectively, declining to 0.01 m d(-1) when adding marine RAS water increasing the salinity to 15 parts per thousand. Similar observations followed at high nutrient loadings (0'-order process) with STR and SNR of 0.10 and 0.12 g N m(-2) d(-1), respectively, declining to 0.01 g N m(-2) d(-1) at 15 parts per thousand. When salinities of 25 parts per thousand and 35 parts per thousand were applied, neither TAN nor nitrite degradation was seen. The results thus demonstrate a pronounced effect of salinity changes when freshwater RAS biofilters are subjected to fast/abrupt changes in salinity. RAS facility operators should be aware of such potential effects and take relevant precautions.