Simultaneous Removal of Soluble Metal Species and Nitrate from Acidic and Saline Industrial Wastewater in a Pilot Scale Biofilm Reactor

Abstract Α pilot scale packed-bed biofilm reactor was set up and monitored for the treatment of wastewater originating from the hydrometallurgical recovery of metals from printed circuit boards (PCBs). The wastewater is characterized by: (a) low pH, (b) residual soluble metal species and (c) elevated concentrations of nitrate and chloride originating from the use of nitric and hydrochloric acid as leaching agents. Such wastewater could be treated in a bioreactor capable for the simultaneous removal of metals and nitrates, through complete denitrification, in presence of elevated chloride concentrations. However, the possible inhibitory effects of metals as well as the metals bioprecipitation should be investigated experimentally. Biological denitrification was studied under extreme conditions in the bioreactor inoculated with Halomonas denitrificans: at (a) pH 3-8; (b) metal content (Cu, Ni, Zn and Fe) at 50 mg/L and 100 mg/L, respectively (c) nitrate concentration 750-5,750 mg/L NO3- and (d) chloride concentration 5%-10% as NaCl. According to the results, denitrification proceeds rapidly through the formation of nitrite as intermediate which is sequentially reduced completely to nitrogen. The presence of metals does not affect the denitrification process. Iron, zinc, copper and nickel are sequestered from the wastewater via bioprecipitation. Both goals, namely metals removal and complete reduction of nitrate in presence of elevated concentrations of chloride, were successfully achieved by the treatment scheme. The proposed simple, robust and low-cost biological treatment unit is advantageous compared to the conventional wastewater treatment, based on metal precipitation via chemical neutralization, where the problem of nitrate removal remains unresolved.