Membrane bioreactors for metal recovery from wastewater: A review

The need to remove or recover metal ions from industrial wastewaters is both financially and environmentally driven, financially in terms of cost savings through metal reuse or sale, and environmentally as heavy metal toxicity can affect organisms throughout the food chain, including humans. Current removal strategies are mainly based on physico-chemical techniques such as filtration, chemical precipitation and solvent extraction. All these "conventional" technologies have raised issues of efficacy when faced with low metal concentrations, high start-up or operating costs and low metal selectivity. Conversely, metal removal using biological and membrane processes is becoming more widely accepted as new evidence is gathered highlighting their lower cost, ease of operation, selectivity and efficacy. Precipitation of metal ions using biogenic hydrogen sulphide, produced by sulphate-reducing bacteria, is not a new technique, and is used by a small number of industrial installations worldwide. While this process has disadvantages such as the hazardous nature of the gas, the advantages inherent in utilising this source of sulphide are greatly enhanced when used in combination with membrane bioreactor technology. Initial studies have shown that the sulphate-reducing bacterial bioreactor coupled with a membrane can remove up to 90% of the metal ions present in an aqueous solution.