Influence of water saliny changes on the operation of the RAS biological treatment system under low temperature conditions

In fish-breeding circulation systems the change-over of a biofilter from operation in fresh water conditions to sea water conditions (or vice versa) requires a smooth gradual transition. However, a one-time water change is feasible faster and technologically easier. This determined the relevance of the study. The goal was to determine the influence of one-time water salinity changes (sea to fresh and fresh to sea) on the ability of the activated sludge biocenosis of biofilters to maintain or restore its activity in a cold-water RAS. Six model biofilters were filled with sea water and six with fresh water. With a simultaneous decrease of water temperature from 21 to 9 °C, at the first stage a start-up period of biological treatment was carried out. At the second stage — in a stable operating regime, sea water was simultaneously changed to fresh water, and fresh water to sea water. Concentrations of ammonium, nitrites and nitrates were determined at the start of experiment, then on the next day after a bacterial culture and ammonia as a source of ammonium nitrogen adding, and then — 2 times a week. A steady decrease in the concentration of ammonium in water to 0.2 mg/l, nitrite — to 0.3 mg/l was considered as the indicator of the end of each stage. The duration of the start period of biological treatment in both fresh and sea water was approximately the same and amounted to 138 and 134 days, respectively. With the simultaneous change of sea water to fresh or fresh water to sea, the complete destruction of the biofilter activated sludge biocenosis didn’t happen, that indicated by the continuing increase of the nitrates concentration. However, the ability of the biofilter biocenosis to purify circulating water from ammonia nitrogen and nitrites naturally decreases with the following gradual performance restoration. The adaptation of biofilter biocenosis to work under conditions of rapid desalination was 1.5 times faster than the adaptation of biocenosis to rapid salination.