NEUTRALISING THE pH OF ALKALINE BAUXITE RESIDUE USING A CONTINUOUS FLOW BIOREACTOR SYSTEM

Bauxite residue is a highly alkaline (pH 12) byproduct of alumina refining. The alkalinity of bauxite residue is a major environmental challenge and one of the key goals of remediation efforts is to neutralise pH. Recently, a microbially-driven method of pH neutralisation has been developed which has the potential for the treatment of both existing residue deposits and new residue streams. With this method, the fermentative metabolic pathways of the introduced microbial community produce organic acids and carbon dioxide, which can successfully lower the pH of bauxite residue below a target level of pH 8. To efficiently treat new residue streams, a continuous flow bioreactor system would be required, capable of neutralising large volumes of residue. It would require ongoing inputs of organic carbon to support fermentation and pH neutralisation before the neutralised residue could be discharged or treated by further remediation processes.

A laboratory scale bioreactor system using Western Australian bauxite residue from Alcoa of Australia Limited, glucose, and a mixed microbial inoculum was trialled to optimise glucose inputs, hydraulic retention time, and solids loading during microbially-driven pH neutralisation of bauxite residue in a continuous flow system. The continuous flow system successfully neutralised bauxite residue at an influent pH of 12, maintaining effluent pH <8 with hydraulic retention times of 16-17 hours in initial trials. Results from further work aimed at lowering the hydraulic retention time, increasing bauxite residue solids loading and reducing the glucose input will also be presented, as well as supporting data to identify the microbial species and fermentation pathways active within the bioreactor system.