Effect of nickel on the comparative performance of inverse fluidized bed and continuously stirred tank reactors for biogenic sulphur-driven autotrophic denitrification.

The comparative performance of an inverse fluidized bed reactor (IFBR) having high density polyethylene beads as carrier materials for biofilm formation and a continuous stirred tank reactor (CSTR), both maintaining autotrophic denitrification using biogenic sulphur (ADBIOS) in the absence and presence of nickel (Ni), was studied. The reactors were compared in terms of NO-N and NO-N removal and SO-S production throughout the study. A simulated wastewater with an inlet NO-N concentration of 225 mg/L and a decreasing concentration of biogenic sulphur (bio-S) from 1.5 to 0.375 g/L was used. Both reactors were operated at a hydraulic retention time (HRT) of 48 h for 140 days and at an HRT of 42 h for the following 68 days. A more efficient ADBIOS was observed in the CSTR than IFBR throughout the study due to a better mixing of the feed wastewater in the bulk liquid and a higher availability of bio-S to the suspended cells. The NO-N removal efficiency in the IFBR decreased by approximately 41% when the feed bio-S was reduced to 0.375 g/L, while it remained unaffected in the CSTR. Conversely, the presence of Ni did not significantly affect NO-N removal in both reactors even at a feed Ni concentration of 120 mg/L. The highest NO-N removal rates achieved were 86 and 108 mg NO-N/(L·day) in the IFBR and CSTR, respectively, in the presence of 120 mg/L of feed Ni at an HRT of 42 h. Batch studies conducted with acclimatized biomass showed that the continuous-flow operation mode in both reactors played a major role in helping the autotrophic denitrifiers to tolerate Ni toxicity.