Removal of trace mono-valent inorganic pollutants in an ion exchange membrane bioreactor: analysis of transport rate in a denitrification process

The ion exchange membrane bioreactor (IEMB) uses a mono-anion permselective membrane as a barrier between a water stream, containing a target polluting anion, and a biocompartment, containing a suitable driving counter-ion for coupled counter-diffusion of the target pollutant and a microbial culture capable of its bioreduction to harmless product(s). A resistances-in-series trace counter-ion approximation model, based on the Fick formalism and the Donnan equilibrium principle is proposed for analysis of the transport rate of dilute inorganic anionic pollutants with relevance to drinking water treatment. Transport of co-ions (cations) is negligible due to their electrostatic repulsation (Donnan exclusion) from the positively charged membrane. The model is tested for a system, containing nitrate as the target pollutant and chloride as the driving bulk counter-ion. The effect of the most important process variables on the target pollutant flux are assessed using the model, making possible the comparison between different IEMB process options, which can be quantified and used for process design. The impact of the bioconversion reaction on the mass transfer resistance to the target pollutant transport is also discussed.