Simultaneous removal of dissolved nitrogen species and hexavalent chromium by indigenous Achromobacter sp. CSND-B12 from tannery activated sludge

Tannery wastewater may contain nitrogen compounds and chromium simultaneously, creating challenges for treatment systems designed to address individual contaminants.This study aimed to isolate and characterize bacteria from activated sludge at a tannery wastewater treatment plant that can simultaneously remove dissolved nitrogen species and hexavalent chromium [Cr(VI)] under aerobic conditions.We recovered twelve isolates from the sludge through four enrichment strategies and screened them for growth under 100 mg L⁻¹ Cr(VI) exposure in nutrient-rich medium and three nutrient-limited media containing ammonium (MM-ACr), nitrate (MM-NCr), and combined ammonium and nitrate (MM-ANCr).All isolates grew, although growth and functional performance varied among the media tested.Based on screening results from the three nutrient-limited media, CSND-B12 consistently showed high biomass accumulation and apparent removal of dissolved nitrogen species and Cr(VI) across the tested media, with significant differences among isolates after 72 h of incubation (P < 0.05).CSND-B12 achieved an OD₆₀₀ of 0.543, with 66.80% apparent ammonium removal and 50.42%Cr(VI) removal in MM-ACr; an OD₆₀₀ of 0.475, with 96.10% apparent nitrate removal and 47.76% Cr(VI) removal in MM-NCr; and an OD₆₀₀ of 0.552, with 54.69% apparent removal of the combined ammonium and nitrate pool and 21.52% Cr(VI) removal in MM-ANCr.16S rRNA gene sequencing showed that CSND-B12 had 99.93% sequence identity to several closely related Achromobacter reference sequences, while phylogenetic analysis placed the isolate within the Achromobacter xylosoxidans clade.This isolate maintained measurable decreases in dissolved nitrogen species alongside Cr(VI) removal across all three nitrogen regimes.These findings identify CSND-B12 as a strain warranting further evaluation of its bioaugmentation potential in contained treatment systems receiving wastewater containing co-occurring nitrogen and chromium.