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BACKGROUND: This work investigates whether the membrane bioreactor (MBR) process can effectively treat industrial wastewater containing high cyanide concentrations; cyanide may inhibit the biological processes, rendering such processes inadequate. In this work, the changes in process performance and microbial activity under gradually increased free cyanide (CN −) concentrations (1,3 and 10 mgCN − L −1) were investigated in two pilot-scale MBR configurations. The MBR systems consisted of two parallel operating lines: a single aerobic (MBR 1) and an aerobic/anoxic (MBR 2). RESULTS: Cyanide removal was not adversely affected by the increase in cyanide concentration since removals up to 90% were obtained. Despite the inhibition that occurred, ammonia and organic carbon removal efficiency did not decrease, even at the highest cyanide concentration of 10 mgCN − L −1 , and it remained high (>95%) in both systems. Nitrification (sAUR) was inhibited by 35%, 54%, and 64% in MBR 1 and by 16%, 25%, and 36% in MBR 2 , with the addition of 1, 3, and 10 mgCN − L −1 , respectively. The aerobic respiration (sOUR) was inhibited considerably less with an inhibition of 19%, 37%, and 45% in MBR 1 and 9%, 16%, and 21% in MBR 2. The biomass maintained under both aerobic and anoxic conditions (MBR 2) was more tolerant to cyanide compared to the biomass that was acclimated under only aerobic conditions (MBR 1). CONCLUSION: At high cyanide load conditions, the MBR systems showed satisfactory removal of both cyanide and conventional pollutants despite the inhibition observed, indicating that MBR systems can successfully respond to the treatment of industrial wastewater that is heavily contaminated with cyanide.

Α pilot scale packed-bed bio lm 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 denitri cation, in presence of elevated chloride concentrations. However, the possible inhibitory effects of metals as well as the metals bioprecipitation should be investigated experimentally. Biological denitri cation was studied under extreme conditions in the bioreactor inoculated with Halomonas denitri cans: 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 NO 3 and (d) chloride concentration 5%-10% as NaCl. According to the results, denitri cation proceeds rapidly through the formation of nitrite as intermediate which is sequentially reduced completely to nitrogen. The presence of metals does not affect the denitri cation 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.

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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.