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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.

Electro-concentration of nutrients from waste streams is a promising technology to enable resource recovery, but has several operational concerns. One key concern is the formation of inorganic scale on the concentrate side of cation exchange membranes when recovering nutrients from wastewaters containing calcium, magnesium, phosphorous and carbonate, commonly present in anaerobic digester rejection water. Electrodialytic nutrient recovery was trialed on anaerobic digester rejection water in a laboratory scale electro-concentration unit without treatment (A), following struvite recovery (B), and following struvite recovery as well as concentrate controlled at pH 5 for scaling control (C). Treatment A resulted in large amount of scale, while treatment B significantly reduced the amount of scale formation with reduction in magnesium phosphates, and treatment C reduced the amount of scale further by limiting the formation of calcium carbonates. Treatment C resulted in an 87 ± 7% by weight reduction in scale compared to treatment A. A mechanistic model for the inorganic processes was validated using a previously published general precipitation model based on saturation index. The model attributed the reduction in struvite scale to the removal of phosphate during the struvite pre-treatment, and the reduction in calcium carbonate scale to pH control resulting in the stripping of carbonate as carbon dioxide gas. This indicates that multiple strategies may be required to control precipitation, and that mechanistic models can assist in developing a combined approach.

Source-separated urine is a promising stream for nutrient capture using electrochemical technologies. It contains the majority of macronutrients present in municipal wastewater in a concentrated, high ionic conductivity liquid and in N:P:K ratios suitable for agricultural application. The purpose of this study was to recover nutrients from urine, and particularly nitrogen as a solid without any chemical addition. Simulated source-separated urine was concentrated using a threecompartment electrochemical system, applying a range of current densities and feed compositions. Electro-concentration into a liquid concentrate reached maximum recovery of 72:61:79 % for N:P:K, respectively, from a synthetic feed simulating ureolysed and digested urine, with a specific electrical energy consumption of 47 MJ/kg N and current efficiency of 67 % for ammonium. Cooling the concentrate to-18°C resulted in solid ammonium bicarbonate crystal formation in samples with high ammonium bicarbonate ionic product and high relative ammonium bicarbonate ionic strength. Precipitation started to occur when ammonium bicarbonate ionic product was higher than 2.25 M 2 and ammonium bicarbonate accounted for more than 62 % of the total ionic strength of the feed. The maximum observed nitrogen recovery into solid ammonium bicarbonate reached 17 % using a current density of 100 A m-2. Based on these results, electro-concentration is a promising technology for urine nutrient capture. However, capture as solid ammonium bicarbonate is feasible only if higher recovery efficiencies are achieved by removing competing ions.

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Microbial electrochemical processes have potential to remediate acid mine drainage (AMD) wastewaters which are highly acidic and rich in sulfate and heavy metals, without the need for extensive chemical dosing. In this manuscript, a novel hybrid microbial/electrochemical remediation process which uses a 3-reactor system -a precipitation vessel, an electrochemical reactor and a microbial electrochemical reactor with a sulfate-reducing biocathode -was modelled. To evaluate the long-term operability of this system, a dynamic model for the fluxes of 140 different ionic species was developed and calibrated using laboratory-scale experimental data. The model identified that when the reactors are operating in the desired state, the coulombic efficiency of sulfate removal from AMD is high (91%). Modelling also identified that a periodic electrolyte purge is required to prevent the build-up of Cl À ions in the microbial electrochemical reactor. The model furthermore studied the fate of sulfate and carbon in the system.For sulfate, it was found that only 29% can be converted into elemental sulfur, with the rest complexating with metals in the precipitation vessel. Finally, the model shows that the flux of inorganic carbon under the current operational strategy is insufficient to maintain the autotrophic sulfate-reducing biomass. The modelling approach demonstrates that a change in system operational strategies plus close monitoring of overlooked ionic species (such as Cl À and HCO 3 À ) are key towards the scaling-up of this technology.

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