• Sarukkalige, Ranjan
• Weerasinghe Mohottige, Tharanga
• Ginige, Maneesha P.

Abstract

Microbial-electrochemical reactors or bioelectrochemical systems (BES) are a promising environmental biotechnology. A key advantage of BES is its effective use of electrodes to stimulate and control microbial degradation of organics from a waste stream. Together with add-on features such as the use of ion selective membranes, the technology can be versatile for various environmental and industrial applications including wastewater treatment, resources recovery (nutrients, energy as fuel gases, metals/ compounds of economic value) from waste streams, and remediation of contaminated environments. BES is also being increasingly explored as an auxiliary technology for augmenting conventional waste treatment processes such as anaerobic digestion (AD). Nevertheless, practical application of BES is often hindered by two inherent properties of the waste stream: (1) low ionic strength causing high internal ohmic resistance; and (2) low pH buffering capacity causing drastic acidification hampering anodophilic activity of microbes. Although these constraints can be easily rectified by dosing chemicals, such approach is impractical, increases operating costs and may cause secondary pollution. Therefore, it seems rational to develop the BES technology for treating waste streams that are intrinsically saline and alkaline. Here, we explored the use of BES for the treatment of a highly saline and alkaline industrial waste stream associated with alumina refineries. Aluminium is typically extracted from bauxite ores in Bayer processes, where the ores are reacted with caustic liquor (Bayer liquor) under elevated temperature. Since the refining process is suppressed by the organics, particularly sodium oxalate (Na2C2O4) accumulated in the liquor, effective removal of these organics is crucial. Considering that the refinery liquor typically has a pH of >12 with also an high concentration of sodium (~25 g NaCl L-1), we therefore explored the use of a synthetic alumina refinery liquor as a BES feedstock to facilitate the destruction of organics and recovery of caustic soda for reuse. The concept was first tested by starting up a cation exchange membrane-equipped dual chamber BES using activated sludge as the microbial inoculum. The results suggested that a successful start-up of an alkali-halotorant anodic biofilm was achieved with acetate and formate as organic electron donors. However, oxalate was only poorly degraded by the anodic microbial community even after a prolonged period (>300 days) of operation, plausibly due to a lack of an oxalate-degrading microbial group in the established anodic biofilm as supported by the microbial community analysis. To overcome this problem, a new start-up strategy was devised using graphite granules with an active aerobic oxalate-degrading biofilm as the BES anode. The results showed that the aerobic biofilm readily (< 3 days) became anodophilic, facilitating current density of 363 A m-3 at a hydraulic retention time of 3 hours (~20 kg oxalate removed m-3 d-1). The biofilm also degraded acetate and oxalate (coulombic efficiency 80%) concurrently without apparent preference towards acetate. Further, caustic soda was generated at the cathode with (electrical) energy requirement lower than figures reported in other studies. Overall, this study suggests that BES is suitable for simultaneously removing organics and recovering caustic soda from alumina refinery liquor. Similar approach can be applied for treating other alkaline and saline waste streams.

Topics

• density
• impedance spectroscopy
• compound
• aluminium
• strength
• Sodium
• current density