Simultaneous removal of nitrate and chromate in groundwater by a spiral fiber based biofilm reactor

•Spiral fiber was utilized as biofilm carrier to get better adhesion of microorganisms.•Possible mechanism for nitrate removal was analyzed under Cr(VI) stress.•Excellent removal rates of nitrate and chromate were reached at low C/N ratio (0.9).•The biofilm reactor achieved high removal rate (nitrate ≥99%, Cr(VI) ≥95%).•The pH had impact on three procedure: denitrification, Cr(VI) reduction and DNRA. A spiral fiber based biofilm reactor was developed to remove nitrate and chromate simultaneously. The denitrification and Cr(VI) removal efficiency was evaluated with synthetic groundwater (NO3−-N=50mg/L) under different Cr(VI) concentrations (0–1.0mg/L), carbon nitrogen ratios (C/N) (0.8–1.2), hydraulic retention times (HRT) (2–16h) and initial pHs (4–10). Nitrate and Cr(VI) were completely removed without nitrite accumulation when the Cr(VI) concentration was lower than 0.4mg/L. As Cr(VI) up to 1.0mg/L, the system was obviously inhibited, but it recovered rapidly within 6days due to the strong adaption and domestication of microorganisms in the biofilm reactor. The results demonstrated that high removal efficiency of nitrate (≥99%) and Cr(VI) (≥95%) were achieved at lower C/N=0.9, HRT=8h, initial pH=7, and Cr(VI)=1.0mg/L. The technology proposed in present study can be alternative for simultaneous removal of co-contaminants in groundwater.