A mechanistic model for electrochemical nutrient recovery systems

Abstract

Electrochemical membrane technologies such as electrodialysis have been identified as key technologies to enable nutrient recovery from wastewater. However, current electrochemical models are focused on simpler solutions than wastewater and omit key outputs such as pH, or total cell potential. A combined physico-chemical and electrochemical model was developed which includes the mechanisms of competitive transport of ions, implicit inclusion of H(+) and OH(-), pH (including ionic activity and ion pairing), different factors contributing to total cell potential and a novel method for ion exchange membrane transport. The model outputs compare well with measurements from experiments and simulate secondary effects such as electrode reactions and current leakage. Results found that membrane, rather than boundary layer or bulk resistance was the major contributor to potential drop, and that apparent boundary layers were relatively thick (3 ± 1 mm). Non-ideal solution effects such as ion-pairing and ionic activity had a major impact, particularly on multi-valent Ca(2+) ions, which enhances the capability of electrodialysis to recover monovalent nutrient ions such as K(+) and NH4(+). Decreased resistivity of ion exchange membranes to specific ions (for example, in this case nitrate) could also be detected. The methods here are validated using a comparatively simple synthetic solution of five ionic components, but are able to be easily scaled for a more complex solution, and are also compatible with additional mechanisms such as precipitation, fouling, and scaling.

Resource Recovery from Wastewater by Biological Technologies: Opportunities, Challenges, and Prospects

“…For instance as entioned above phosphorous can be recovered (Cusick and Logan, 2012) and also nitrogen removal and recovery either via electrodialysis (Thompson Brewster et al, 2016) or as ammonium bicarbonate from source-separated urine has been investigated (Tice and Kim, 2014). In addition, the BES also offers the possibility to recover metals fom wastewater, recently reviewed (Wang and Ren, 2014).…”

A modelling approach to assess the long-term stability of a novel microbial/electrochemical system for the treatment of acid mine drainage

“…Other species transported into A2 are Cl − , NO 3 − and NaSO 4 − , while some species were found to transport in the opposite direction (A2 to A1), including H + , Na + and H 2 S (aq) . The rates of migration of each species are proportional to the activity, charge and Fickian diffusion coefficient as described in Thompson Brewster et al 7 Sulfate provides 96% of the anions (in [mol]) in the feed, resulting in a high coulombic efficiency for its migration, but there will always be co-transport by other anions and undesired back-diffusion of species (for example, Na + and H + ) where the concentration gradient across the membrane results in diffusion in the direction opposite to migration.…”

“…Based on the ionic flux, pH and speciation methods developed in Thompson Brewster et al , 7 the electrochemical (R2) and microbial electrochemical (R1) reactors were modelled using a Nernst–Planck equation, accounting for the speciation of different ionic species 17 and using a current proportioning method to describe membrane transport. Diffusion and migratory fluxes occurring across the anion exchange membrane (AEM) and cation exchange membrane (CEM) were represented by the Nernst–Planck equation and taking into account the components sodium, potassium, ammonium, chloride, acetate, calcium, magnesium, carbonate, sulfate, phosphate, aluminium, iron (2+), iron (3+), sulfide and nitrate, as well as acid–base pairing and ionic pairing.…”

“…No diffusion boundary layers (DBLs) were taken into account in the modelling as at the large chamber widths utilised, these were unlikely to be a limiting mechanism. 7 …”

“…Recently, modelling methods for electrochemical systems using wastewater (anaerobic digester liquor and source-separated urine 7,9 ) as electrolyte have been developed. However, AMD contains lower concentrations of organics and nutrients, and higher concentrations of heavy metals and sulfate compared to these other streams, and has not been modelled in electrochemical systems before.…”