Emma Thompson Brewster

Modelling recovery of ammonium from urine by electro-concentration in a 3-chamber cell

Electro-concentration enables treatment and nutrient recovery from source-separated urine, and is a potential technology for on-site treatment using a 3 compartment configuration that has anode, cathode and middle concentrate compartments. There is a particular focus on driving concentration towards the precipitation threshold in the concentrate compartment to generate solid ammonium salts, including ammonium bicarbonate. To evaluate controlling mechanisms and the feasibility of achieving high concentrations, a dynamic mechanistic model was developed and validated using experiments with synthetic urine. It was identified that high concentrations are prevented by increased back diffusion (diffusion from the middle chamber to the anolyte and catholyte) due to large concentration gradients, and the preferential migration of protons or hydroxide ions due to a loss of buffering capacity in the anolyte and catholyte (due to pH extremes). Model-based sensitivity analysis also identified that electrolyte ion concentrations (including buffer capacity) were the main controlling mechanisms, rather than membrane or electrolyte current transfer capacity. To attain high concentrations, operation should be done using a) a high current density (however there is a maximum efficient current density); b) feed at short hydraulic retention time to ensure sufficient buffer capacity; and c) a feed high in ammonium and carbonate, not diluted, and not contaminated with other salts, such as pure ureolysed urine. Taking into account electron supply and bio-anodic buffer limitations, model testing shows at least double the aqueous concentrations observed in the experiments may be achieved by optimising simple process and operational parameters such as flow rate, current density and feed solution composition. Removal of total ammonium nitrogen (TAN) and total carbonate carbon (TCC) was between 43-57% and 39-53%, respectively. Balancing the sometimes conflicting process goals of high concentrations and removal percentage will need to be considered in further application. Future experimental work should be directed towards developing electrodes capable of higher current densities. In addition it would be desirable to use ion exchange membranes with higher resistance to water fluxes and which limit back diffusion. Future modelling work should describe osmotic and electro-osmotic water fluxes as a function of the concentration gradient across the membranes and ionic fluxes, respectively. More generalised wastewater physico-chemistry speciation models should identify best methods where relatively simple Davies activity corrections do not apply.

Nutrient recovery from wastewater through pilot scale electrodialysis

Nutrient recovery performance utilising an electrodialysis (ED) process was quantified in a 30-cell pair pilot reactor with a 7.2 m2 effective membrane area, utilising domestic anaerobic digester supernatant, which had been passed through a centrifuge as a feed source (centrate). A concentrated product (NH4-N 7100 ± 300 mg/L and K 2490 ± 40 mg/L) could be achieved by concentrating nutrient ions from the centrate wastewater dilute feed stream to the product stream using the ED process. The average total current efficiency for all major cations over the experimental period was 76 ± 2% (NH4-N transport 40%, K transport 14%). The electrode power consumption was 4.9 ± 1.5 kWh/kgN, averaged across the three replicate trials. This value is lower than competing technologies for NH4-N removal and production, and far lower than previous ED lab trials, demonstrating the importance of pilot testing. No significant variation in starting flux densities and cell resistance voltage for subsequent replicate treatments indicated effective cleaning procedures and operational sustainability at treatment durations of several days. This study demonstrates that ED is an economically promising technology for the recovery of nutrients from wastewater.

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

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− 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− and HCO3−) are key towards the scaling-up of this technology.