Cost-effective bioregeneration of nitrate-laden ion exchange brine through deliberate bicarbonate incorporation

•CaCl2-induced partial sulfate removal with little residual calcium.•Salinity and alkalinity stabilization for stable brine biotreatment.•Hematite-enriched high-density activated sludge for brine biotreatment.•Sulfide-synchronized nitrate and nitrite reduction for carbon-limited denitrification.•Use of biogenic alkalinity and CaCl2 to induce CaCO3 flocculation in post treatment.

Bioregeneration of nitrate-laden ion exchange brine is desired to minimize its environmental impacts, but faces common challenges, i.e., enriching sufficient salt-tolerant denitrifying bacteria and stabilizing brine salinity and alkalinity for stable brine biotreatment and economically removing undesired organics derived in biotreatment. Incorporation of 0.25 M bicarbonate in 0.5 M chloride brine little affected resin regeneration but created a benign alkaline condition to favor bio-based brine regeneration. The first-quarter sulfate-mainly enriched spent brine (SB) was acidified with carbon source acetic acid for using CaCl2 at an efficiency >80% to remove sulfate. Residual Ca2+ was limited below 2 mM by re-mixing the first-quarter and remained SB to favor denitrification. Under CO2–H−CO3CO2–HCO3− system buffered pH condition (8.3–8.8), nitrate was removed at 0.90 gN/L/d by hematite-enriched well-settled activated sludge (SVI 8.5 ml/g) and the biogenic alkalinity was retained as bicarbonate. The biogenic alkalinity met the need of alkalinity in removing residual Ca2+ after sulfate removal and in CaCl2-induced CaCO3 flocculation to remove 63% of soluble organic carbon (SOC) in biotreated brine. Carbon-limited denitrification was also operated after activated sludge acclimation with sulfide to cut SOC formation during denitrification. Overall, this bicarbonate-incorporation approach, stabilizing the brine salinity and alkalinity for stable denitrification and economical removal of undesired SOC, suits long-term cost-effective brine bioregeneration.

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