A sulfate reducing bioreactor controlled by an electrochemical system enables near-zero chemical treatment of metallurgical wastewater

Abstract

Metallurgical wastewaters are characterized by a low pH (<4), high concentrations of sulfate (15 gSO42− L−1), and metal(loid)s. Current treatment requires the consumption of chemicals such as alkali and high levels of waste sludge generation. In this study, we have shown that combining water electrolysis and sulfate reducing bioreactors enables the in-situ generation of base and H2, eliminating the need for base and electron donor addition, resulting in the near-zero treatment of metallurgical wastewater. By extracting cations from the effluent of the system to the bioreactor, the bioreactor pH could be maintained by the in-situ production of alkali. The current for pH control varied between 112–753 mol electrons per m³ wastewater or 5–48 A m−2 electrode area. High concentrations of sulfate in the influent and addition of CO2 increased the current required to maintain a steady bioreactor pH. On the other hand, a high sulfate reduction rate and increased influent pH lowered the current required for pH control. Moreover, the current efficiency varied from 14% to 91% and increased with higher pH and cation (Na+, NH4+, K+, Mg2+, Ca2+) concentrations in the middle compartment of the electrochemical cell. The salinity was lowered from 70–120 mS cm−1 in the influent to 5–20 mS cm−1 in the system effluent. The energy consumption of the electrochemical pH control varied between 10 and 100 kWh m−3 and was affected by the conductivity of the wastewater. Industrial wastewater was treated successfully with an average energy consumption of 39 ± 7 kWh m−3, removing sulfate from 15 g SO42− L−1 to 0.5 ± 0.5 g SO42− L−1 at a reduction rate of 20 ± 1 gSO42− L−1 d−1..Metal(loid)s such as As, Cd, Cu, Pb, Te, Tl, Ni and Zn were removed to levels of 1–50 µg L−1.