Impact of hydraulic retention time, backflushing intervals, and C/N ratio on the SID-reactor denitrification performance in marine RAS

In recirculating aquaculture systems (RAS) the high water re-use in combination with insufficient treatment of the process water can lead to the accumulation of nitrate, among other metabolic end products. For the efficient removal of nitrate in a marine RAS, a Self-cleaning Inherent gas Denitrification Reactor (SID-Reactor) was investigated in this study. Within three consecutive experimental trials the effect of varying hydraulic retention time (HRT), backflushing intervals (BFI), and carbon to nitrogen (C/N) ratios on water quality parameters and denitrification performance (rate and efficiency) were monitored. Different HRTs of 2, 4, and 6 h and additionally BFIs of 10, 30, 60, and 90 min were evaluated. The tested C/N ratios, realized using methanol (MeOH) as a carbon source, were 1.1, 1.5, 1.9, 2.1, 2.3, 2.7, 3.1, and 3.5 (mg MeOH per mg NO3−-N). The experiments revealed that a HRT of 2 h resulted in the highest denitrification rate (497 g d−1 m3 biocarriers) but a lower denitrification efficiency of 64%. A HRT of 6 h had highest denitrification efficiency of 81% but a lower denitrification rate (253 g d−1 m3 biocarriers). Furthermore, it was evident that backflushing intervals every 10 min resulted in a decreased denitrification efficiency of 29%, while intervals every 90 min increased the maintenance effort. Overall, backflushing intervals every 30 and 60 min showed the best results. A C/N ratio of 2.3 seemed to be sufficient to ensure an optimal denitrification performance, incorporating all single tested water quality parameters. The results of this study allow an easy, efficient and safe application of a SID-Reactor with the purpose of nitrate removal in marine RAS.