This paper presents a novel technique for producing a CDI electrode through a straightforward process of converting activated carbon powder into a functioning stack for application in desalination. The foundation for this new technique is implementing the carbon layer via a novel carbon-on-mesh (CM) structure. This work elaborates on the CM formation process through flocculated carbon and filtration using the mesh as the filter medium. Alum-induced sweep flocculation, from a starting pH of 4.1 resulted in a good distribution of the PTFE binder to the carbon, as indicated by electron microscopy, producing a CM layer of 300 μm carbon thickness. Three electrode assemblies were handcrafted to demonstrate repeatability, comprising N = 20 cells. The stack’s performance was measured at 0.13, 0.13, and 0.11 mmol g− 1 adsorption capacity based on NaCl outlet concentration. The average salt adsorption rate (ASAR) performance was 0.74, 0.73 and 0.70 mg g− 1 min− 1 at 1.2 V (300 s adsorb and 300 s zero-volt desorb). These results align with other benchmark studies indicating that this technique is effective at recreating a carbon stack that has good electrosorption characteristics. This study demonstrates that a CDI stack can be made using common laboratory reagents and equipment thus reducing the barrier to entry for the technology.

Capacitive deionization is a promising technique for salt removal in brackish water applications. However, there are technical challenges to the longevity of operation related to the pH. In this study, we present a new technique for visualizing pH using a colorimetric dye injected into the spacer region between the anode and cathode. A purpose-built device in the form of a batch electrochemical microreactor was developed. In the process, many challenges were overcome, including the integration of a custom imaging system with an optical microscope that captures the electrosorption process within the narrow spacer region. A pH-sensitive dye was formulated with a rainbow hue range spanning pH 4 (red) through pH 7 (green) to pH 11 (violet) for the purpose of creating a visual pH map of the electrolyte phase. The initial solution prepared at 5000 mg/L NaCl was the source of dissolved ions for the batch. Dye was then added and adjusted to pH 7. Constant-voltage operation for 180 s consisted of observations of pH at the anode and cathode. The experiment was repeated at voltages between 0.9 and 1.8 V. Rapid lowering of the pH adjacent to the anode was observed from 20 s onward (indicated by a vivid orange color). Comparing the pH at the anode at 1.3 V (pH 5.8), 1.5 V (pH 4.7), and 1.8 V (pH 4.2) indicates that Faradaic reaction is the dominant mechanism, driven by increasing overpotential. This technique opens the opportunity to explore means of mitigating the adverse effects of pH by direct observation of the phenomena.

Much emphasis has been given to algal biomass growth in dairy farm wastewater. Most of the systems examined require productive land to be converted and/or freshwater use to dilute high concentrations of nutrients found in dairy effluent. A rotating algal biofilm (RABR) provides the capacity to grow algae without sacrificing productive land or freshwater. In theory, this system would overcome some of the economic and environmental challenges that other systems have. A combination of theoretical information, nutrient uptake formulas, and economic formulas were used to calculate the potential of biogas production from algae grown in an RABR with dairy effluents. The average nutrient uptake was 0.8 mgN/m2 per day and 0.1 mgP/m2 per day. The maximum methane production from the anaerobic digestion of algae was 112 m3/RABR·year. The minimum and maximum economic scenarios resulted in gross profits of NZD −2101 and −1922. After evaluating this system for the first time in the New Zealand dairy farming context, it was found that biogas production from an RABR is not a feasible option for New Zealand dairy farmers.

The anaerobic digestion of New Zealand low-input dairy farms has previously been evaluated for energy production, but farming systems have recently become more intensive with increased feed supplementation and feed replacement; therefore, we are studying how these changes affect the overall energy production for water heating. A combination of literature review, surveys, chemical analyses, biomethane potential analysis, and modeling were used for this study. On a case study farm with a solid separator, it was found that 558 MJ/day and 176–861 MJ/day could be produced with the solid and liquid portions of effluent, respectively. There is enough biogas to satisfy the dairy farm’s water heating requirements with a tankless water heater.

Research into desalination of groundwaters for agricultural application in Australia has previously focused on the viability of large scale reverse osmosis plants. Capacitive Deionization (CDI) is an alternative method of desalination that has been demonstrated to be effective for desalting moderately saline (brackish) waters in pilot studies and can be powered using photovoltaics. Using a farm scale economic model coupled to a CDI performance model, potentially viable agricultural applications for CDI have been identified with variation in crop type and CDI configuration enabling optimisation. Scenarios for grapes, oranges, almonds, apples and tomatoes were modelled with maximum internal rates of return (IRR) and annualised profits (AUD$/ha/yr) determined. Groundwater bore salinity thresholds above which the regime is not feasible were found to be 4.2 dS/m for grapes, 5.5 dS/m for oranges, 4.4 dS/m for almonds, 14 dS/m for apples and 8.5 dS/m for tomatoes for a 60 ha crop with an investment period of 10 years. Costing of CDI desalinated water was scenario dependent with a large portion falling below AUD$1/kL. CDI desalination was found to be economically feasible for a range of scenarios and should be explored further as an option to assist with global water and food security concerns.

Capacitive de-ionization (CDI) systems are well-known for their low energy consumption making them suitable for applications powered by renewable energy. In this study, CDI technology is, for the first time, integrated with a suitably-scaled, stand-alone, renewable power system comprising photovoltaic panels and battery storage. Guidelines for designing and sizing such power systems are proposed including determining electrode charging current, PV panels and battery capacity. A 1 kW pilot plant was designed, constructed and operated to verify the proposed guidelines. Using the pilot plant, the total energy consumption of the system has been evaluated with different electrode charging currents and influent flow rates and the relationship between these parameters analyzed. This analysis has enabled the development of practical design guidelines for bulk water treatment with MCDI electrodes. The results of this study show that use of photovoltaic-powered MCDI water treatment, particularly when combined with energy recovery, is competitive against more mature water-treatment technologies for particular applications and at particular locations.

Membrane capacitive deionization (MCDI) is a promising technology for the removal of charged species from aqueous feed streams.

An innovative flow electrode comprising redox-active quinones to enhance the effectiveness of water desalination using flow-electrode capacitive deionization (FCDI) is described in this study. The results show that, in addition to carbon particle contact, the presence of the aqueous hydroquinone (H2Q)/benzoquinone (Q) couple in a flowing suspension of carbon particles enhances charge transfer significantly as a result of reversible redox reactions of H2Q/Q. Ion migration through the micropores of the flow electrodes was facilitated in particular with the desalination rate significantly enhanced. The cycling behavior of the quinoid mediators in the anode flow electrode demonstrated a relatively high stability at the low pH induced, suggesting that the mediator would be suitable for long-term operation.

Capacitive deionization (CDI) is attracting increasing attention as an emerging technology for the facile removal of ionic species from water. In this work, the feasibility of fluoride removal from low-salinity groundwaters by single-pass constant-voltage CDI was investigated and a model developed to describe the dynamic fluoride electrosorption behavior. Effects of operating parameters including charging voltage and pump flow rate as well as impact of fluoride and chloride feed concentrations on the effluent fluoride concentration and equilibrium fluoride adsorption capacity were studied and the obtained data used to validate the model. Using the validated model, the effects of various design parameters, including arrangement of multiple CDI cells, on fluoride removal were assessed. Single-pass constant-voltage CDI was found to be effective in removing fluoride from low-salinity groundwaters but, as expected, removal efficiency was compromised in waters of high salinity. The relatively simple electrosorption model developed here provided a satisfactory description of both fluoride removal and current evolution and would appear to be a useful tool for prediction of CDI performance over a range of operating conditions, cell arrangements and feed water compositions though scope for model improvement exists.

Non-Faradaic (ion electrosorption) and Faradaic (oxidation-reduction) effects in a batch-mode capacitive deionization (CDI) system were investigated, with results showing that both effects were enhanced with an increase in charging voltage (0.5-1.5 V). Significant concentrations of hydrogen peroxide (H2O2) were observed with the generation of H2O2 initiated by cathodic reduction of O-2 with subsequent consumption occurring as a result of cathodic reduction of H2O2. A kinetic model of the Faradaic processes was developed and found to satisfactorily describe the variation in the steady-state concentration of H2O2 generated over a range of CDI operating conditions. Significant pH fluctuations were observed at higher charging voltages. While the occurrence of Faradaic reactions may well contribute to pH fluctuations and deterioration of electrode stability and performance, the presence of H2O2 could provide the means of inducing disinfection or trace contaminant degradation provided H2O2 could be effectively activated to more powerful oxidants (by, for example, ultraviolet irradiation).

Charging capacitive deionization (CDI) at constant voltage (CV) produces an effluent stream in which ion concentrations vary with time. Compared to CV, charging CDI at constant current (CC) has several advantages, particularly a stable and adjustable effluent ion concentration. In this work, the feasibility of removing fluoride from brackish groundwaters by single-pass constant-current (SPCC) CDI in both zero-volt and reverse-current desorption modes was investigated and a model developed to describe the selective electrosorption of fluoride and chloride. It was found that chloride is preferentially removed from the bulk solution during charging. Both experimental and theoretical results are presented showing effects of operating parameters, including adsorption/desorption current, pump flow rate and fluoride/chloride feed concentrations, on the effluent fluoride concentration, average fluoride adsorption rate and water recovery. Effects of design parameters are also discussed using the validated model. Finally, we describe a possible CDI assembly in which, under appropriate conditions, fluoride water quality targets can be met. The model developed here adequately describes the experimental results obtained and shows how change in the selected system design and operating conditions may impact treated water quality.

Capacitive deionization (CDI) is an emerging water desalination technology in which pairs of porous electrodes are electrically charged to remove ionic species from water. In this work, the feasibility of fluoride and nitrate removal from brackish groundwaters by batch-mode CDI was investigated. Initially, the effects of flow rate, initial fluoride concentration, and initial coexisting NaCl concentration on fluoride removal were studied. The steady-state fluoride concentration declined as the initial fluoride concentration decreased while initial NaCl concentration remained constant. Due to the competitive electrosorption between fluoride and chloride for limited pore surface sites, a higher initial chloride concentration resulted in a higher equilibrium dissolved fluoride concentration. A simplified one-dimensional transport model for dual anions was developed and found to reliably describe the dynamic process of removal of both fluoride and chloride ions in CDI cells over a range of well-defined operating conditions. Based on the ability of the model to describe fluoride removal, it was extended to description of nitrate removal from brackish groundwaters and also found to perform well. Thus, the approach to description of ion removal, at least in batch studies, appears robust and should assist in optimization of design and operating conditions such that optimal removal of trace ionic species is achieved even when high background concentrations of salt are present.

Constant flux filtration is a common mode of operation for submerged membrane filters and membrane bioreactors. A model was developed to describe the pressure rise as a function of time or processed volume taking into account both the cake formation and cake consolidation stage. The approach described here is based on the Nelder Mead optimisation method to calculate best solution for key parameters. These include the consolidation time constants and compressibility parameter that can be used to describe the filtration over a wider range of conditions. The consolidation time constant calculated for flocculated yeast shows a power law relationship with flux over a range of conditions. Furthermore, the concept is extended to sheared systems. A shear-dependent model for constant flux filtration is presented over a narrow range of shear rates constrained by the minimum shear required to distribute cake evenly and the maximum shear permissible before cake erosion/lift is induced. Additional work is required to model the cake behaviour above the point at which cake lift occurs.

Difficulties with existing approaches to the modelling of filtration of compressible materials under constant flux conditions are addressed in this paper and a new approach to modelling simultaneous consolidation and cake build-up based on dewatering theory is presented. Flocculated yeast is used as the test system with the predicted trans-membrane pressure rise as a function of time under constant flux conditions compared with experimental data. Good agreement is observed between model and experimental trends. The close correspondence between experimental and predicted results also suggests that it may be possible to predict trans-membrane pressure rise during constant flux filtration on the basis of material properties determined through simple constant pressure steady-state experiments.

Separation of liquid from concentrated slurry is a significant process in laboratory and engineering applications. In an earlier paper (X.M. Wang, S. Chang, P. Kovalsky, T.D. Waite, Multiphase flow models in quantifying constant pressure dead-end filtration and subsequent cake compression. 1. Dilute slurry filtration, J. Membr. Sci., in press), we investigated the applicability of the Smiles and Tiller multiphase flow models to description of the filtration behavior of dilute yeast slurries and here extend the analysis to consideration of concentrated slurries. Smiles’ and Tiller's models are employed to quantify dynamic filtration behavior while a numerical technique derived from Smiles’ method is utilized for cake compression stage characterization. The numerical method for Smiles’ model for concentrated slurry filtration is the same as that for dilute slurry, while an iteration step is added to the method for Tiller's model. The results obtained indicate that the Smiles and Tiller models are equivalent in quantifying filtration behavior and cake structure of concentrated slurry and reveal that the cake structure and the ensuing filtration behavior are affected by the initial slurry solid fraction. The cake formed from slurry with a higher initial solid fraction tends to have a lower average solid fraction and a lower average specific resistance than is the case for lower initial slurry solid fractions. The variance of superficial liquid velocity through the cake during concentrated slurry filtration is much more significant than is the case during dilute slurry filtration suggesting that caution should be exercised in employing the conventional filtration model in concentrated slurry filtration data analysis. During the compression stage, the compression rate is relatively constant at the beginning then drops sharply before adopting a relatively stable value at longer times. Concomitantly, the solid compressive pressure of the upper surface of the cake increases slowly at the beginning, quickly after that and then slowly again.

A numerical technique for quantifying the key material properties that describe how a flocculated suspension behaves under constant pressure filtration is presented with unrestricted compressive yield stress and permeability models used to describe these material properties. Using an iterative procedure, the optimal parameters for these models are calculated as are pressure and solid fraction distribution profiles. Input parameters to the numerical analysis are flux and final cake height data obtained from batch filtration experiments, which are driven to steady-state. This technique is validated against piston driven filtration (odeometer) and centrifuge experiments for zirconia, soil particles, and yeast assemblages. The compressive yield stress calculated from filtration experiments agrees well with values obtained by odeometer and centrifuge studies for all particle systems studied at both the low and high solid pressure regions. Similarly, the calculated permeability agrees well with the measured permeability. (c) 2007 American Institute of Chemical Engineers

Apart from the empirical conventional filtration model, a number of rigorous multiphase flow models are available for the description of the dead-end cake filtration of compressible slurries. In this study, Tiller's and Smiles’ models are compared with regard to their quantification of the dynamic filtration behavior of “dilute” flocculated yeast slurry during dead-end constant pressure filtration. Steady-state filtration is employed to obtain the compressive yield stress and specific resistance of the cake as functions of solid fraction. It is found that, by virtue of these cake properties, the governing equations of Smiles’ and Tiller's model can be numerically solved. The results show that Smiles’ and Tiller's models are equivalent in quantifying filterability and specific resistance, as well as solid fraction, superficial liquid velocity and solid pressure profiles. The compressible property of the cake is demonstrated by the dependence of either filterability or average specific resistance on the applied pressure. For dilute slurries, the applied pressure has a significant influence on solid fraction profile but has little influence on superficial liquid velocity profile with the maximum variance in superficial liquid velocity in the cake being determined by the solid fraction of the slurry. In the dead-end filtration of dilute slurry, the superficial liquid velocity through the cake is almost uniform and the specific resistance can be approximately obtained from correlation of filtration data by the conventional model.