Sequential in situ hydrotalcite precipitation and biological denitrification for the treatment of high-nitrate industrial effluent

A sequential process using hydrotalcite precipitation and biological denitrification was evaluated for the treatment of a magnesium nitrate (Mg(NO3)2)-rich effluent (17,000 mg NO3−-N/L, 13,100 mg Mg/L) generated from an industrial nickel-mining process. The hydrotalcite precipitation removed 41% of the nitrate (7000 mg NO3−-N/L) as an interlayer anion with an approximate formula of Mg5Al2(OH)14(NO3)2·6H2O. The resultant solute chemistry was a Na–NO3–Cl type with low trace element concentrations. The partially treated effluent was continuously fed (hydraulic retention time of 24 h) into a biological fluidised bed reactor (FBR) with sodium acetate as a carbon source for 33 days (1:1 v/v dilution). The FBR enabled >70% nitrate removal and a maximal NOx (nitrate + nitrite) removal rate of 97 mg NOx-N/L h under alkaline conditions (pH 9.3). Overall, this sequential process reduced the nitrate concentration of the industrial effluent by >90% and thus represents an efficient method to treat Mg(NO3)2-rich effluents on an industrial scale.