The severe rise in global surface temperature and its associated damages to the ecosystem are primarily attributed to anthropogenic carbon dioxide emissions. The present CO2 levels in the air today and the continuing emissions necessitate even the most ambitious development scenarios to rely on net negative CO2 emissions to limit the global temperature rise to 1.5 °C until the year 2100. Direct Air Capture (DAC) refers to the direct extraction of carbon dioxide from the ambient air and addresses the need for net negative emissions. As the technology is still in its infancy, understanding and modeling the occurring processes is crucial.
This thesis addresses the challenges in modeling adsorption and desorption processes in solid-sorbent Direct Air Capture, focusing on the complexities introduced by the porous structures of sorbent materials and the effects of humidity. Current models fail to incorporate the full spectrum of mass transfer processes, or do not consider the significant effects of humidity on the process. To bridge this gap, a detailed mass transfer and reaction kinetics model at the particle scale was developed, aiming to provide a more accurate framework for predicting the performance of amine-functionalized sorbents in DAC applications. The model was developed on a particle scale, and subsequently included in contactor models of a packed-bed and a monolith. The study considered all relevant transport mechanisms, finding that pore diffusion and reaction kinetics are critical in governing sorbent uptake dynamics. The contactor model effectively compares the performance of packed-bed and monolith reactors, highlighting a tradeoff between productivity and energy efficiency, with significant potentials for energy savings offered by the monolith.

Decentralized drinking water production continues to face challenges in achieving acceptable standards for human consumption. This is mainly due to difficulty in developing treatments that can operate effectively in low-resource settings, either due to complications associated with providing the system in a decentralized context or overall unsatisfactory performance of the technology adopted. Electrochemical systems are emerging technologies within decentralized settings which provide many advantages to being adapted in resource-limited areas. This study focuses on understanding the mechanisms behind the electrochemical
disinfection for the treatment of surface water. Laboratory experiments were conducted to evaluate the performance of an RuO2/IrO2 and a Magneli-phase reactor for the disinfection of a bacteria species, Escherichia Coli, and a virus species, ΦX174. The two reactors are characterised in terms of distinct anodic properties and their performance is assessed by promoting different electrochemical reactions for the generation of various disinfectant agents within the electrochemical cell, with specific focus on the formation of chlorine and Reactive
Oxygen Species (ROS). Chlorine-based disinfection proved to be the primary agent in the removal of both pathogens; a 4 log removal for E. Coli was achieved at an energy use of 0.41 kWh/m3 for RuO2/IrO2 and of 0.31 kWh/m3 forMagneli, and a 5 log removal for ΦX174 was achieved at 2.88 kWh/m3 for RuO2/IrO2 and for 1.18 kWh/m3 for Magneli.
To further assess the viability for the treatment of surface waters in decentralized settings, the RuO2/IrO2 was also tested in a field setting in Ghana making use of different water types to compare the utility in using an electrochemical reactor to produce chlorine for disinfection as compared to traditional disinfection methods.  Seawater, lagoon and river water were tested, achieving an energy use per gram of produced chlorine of 0.007, 0.046 and 0.066 kWh/gchlorine for a produced chlorine dose of 35 mg/L for seawater and 5 mg/L for the lagoon and river water. With these results, chlorine can be produced from river water at an equivalent dosage to traditional chlorine tablets, and at a cost that is 40 times lower.

In developing countries with increased industrialization, much research is carried out in order to recover the maximum ammonium from the environment. One of the methods to recover ammonium is Bipolar Membrane Electrodialysis (BPMED), a chemical-free technology that enables recovery and purification of the corresponding acid and base by the application of electrical energy. This thesis aims to evaluate five different BPMED cell configurations using the ammonium sulfate ((NH4)2SO4) salt. Experiments are based on evaluating operational parameters including flow rate, feed volume, base volume, and current density to achieve the maximum ammonium recovery using ammonium sulfate salt with efficient electrochemical energy consumption. Additionally, the optimum experiment was carried out with the best results of operational parameters to understand the overall impact on ammonium recovery and electrochemical energy consumption.

Green hydrogen produced by water electrolysis is one of the most promising technologies to realize the most effective way of utilizing intermittent renewable energy globally. Among the various electrolysis technologies, the emerging Anion-Exchange Membrane Water Electrolysis (AEMWE) shows promising potential for producing green hydrogen at a competitive price.

The low ion conductivity of the Anion Exchange Membrane (AEM) is found to be one of the major limitations of AEMWE. In this thesis, the AEM is aligned while casting using an alternating current electric field to improve the ion conductivity of AEMs. The stability of the electrically aligned membrane is tested using various characterisation techniques. The electrically aligned membranes showed a 300% increase in ion conductivity, with a slight decrease in mechanical stability.

As global warming proceeds with increasing consequences, new and improved solutions that can mitigate the increasing CO2 emissions are becoming ever more important. Renewable energy technologies are advancing and along with carbon capture technologies, they promise to lower global emissions and help combat climate change. Renewable energy sources can be used to form value-added chemicals from captured CO2 through a method known as CO2 electrolysis. A novel approach to CO2 electrolysis is bicarbonate electrolysis, which uses carbon capture solutions directly to make products. By integrating the capture and conversion process this way, effectively bypassing the energy intensive steps of CO2 recovery required for conventional gas fed operation, CO2 conversion to products can become even more sustainable.

The objective of this thesis is to explore ways to improve the selectivity of carbon monoxide (CO) in a bicarbonate electrolyser. In this experimental study, focus is also placed on the stability of the process, characterising the selectivity over time. Causes of selectivity decline are examined as well as methods of improvement. Furthermore, the effect of pH on CO selectivity and stability is given special consideration. Literature in the field of (bi)carbonate electrolysis was reviewed to gather understanding on the process, to clarify recent advances made and to find areas for improvement. Based on the findings from the literature review the experimental study was designed.

Experiments were conducted in a membrane electrode assembly (MEA) flow-cell in constant current fashion, applying a current of 100 mA/cm2. The membrane chosen was a bipolar membrane (BPM) as it offers the possibility of operating with distinct electrolyte environments, separating the 3M bicarbonate catholyte from the 1M potassium hydroxide anolyte. Gas diffusion electrodes (GDE) were prepared by spray-coating silver nanoparticles on the surface, using Nafion ionomer as binding material. An interdigitated catholyte flow plate was used which ensured the bicarbonate would pass through the GDE due to its discontinuous channels forcing the flow through.

By introducing a catalyst-membrane gap through inserting a hydrophilic porous spacer between the GDE and the BPM, CO selectivity was improved from 50% to 78% in peak production, recording 55%
averaged over 3 hour operation. This enhancement in selectivity can be explained by the defined pH gradient resulting from the gap, permitting a low pH at the BPM for protons to react with the bicarbonate, liberating i-CO2; and a higher pH at the catalyst for CO2 conversion to CO while suppressing the hydrogen evolution reaction (HER). An optimum gap was found to be 135 - 270 μm. These results compare with the previously highest reported CO selectivity values from the literature at ambient conditions and 100 mA/cm2. While improving the selectivity, the stability of CO was not improved by the catalyst-membrane gap. The pH was found to affect both the selectivity and stability of CO, with higher bicarbonate pH leading to reduced selectivity but improved stability. This behaviour is explained by the reduced i-CO2 liberation and increased carbonation reactions taking place at higher pH levels…

A transition towards renewable energy sources is necessary to globally achieve net-zero carbon emissions. To achieve reliable renewable energy production, storage of sustainable energy is key. The utilisation of hydrogen, whether for long-term or short-term storage, could provide a vital solution in achieving reliability for sustainable energy demand. However, producing green hydrogen (water electrolysis) is not cost-competitive with fossil fuels and other hydrogen production methods. An original alkaline electrolyser design has been created that can help with lowering the CAPEX and OPEX when optimized. In common alkaline electrolysers, electrodes are completely immersed in liquid. However, the electrode surfaces of the novel design apply wetted surfaces through the use of capillary forces to suck up electrolytes. Compared to immersed electrode surfaces, the wetted surface shows a reduction in (bubble) resistance leading to lower voltage losses. Additionally, the overpotential can be further reduced with, for example, the use of a PES membrane instead of a common Zirfon membrane, better contact with the anode by welding it to the current collector, and adding PTFE to the anode for better gas removal. Prior to testing the assembled cell design, the electrodes were tested individually to understand the mechanics of each electrode. Here, we report that the electrodes perform well individually in alkaline media.
However, our findings reveal that the assembled cell performs poorly, mostly because of significant overpotentials brought on by increasing resistance, along with other drawbacks including salt formation and exceeding the legal gas crossover limit. Some of these issues regarding performance are still poorly understood and therefore need further investigation.

The excessive nitrogen in waterbodies, often caused by the discharge of ammonia-rich wastewaters, leads to eutrophication, disrupting aquatic ecosystems. Wastewater treatment plants play an important role in the removal and recovery of nitrogen from wastewater streams, thereby preventing pollution. Air stripping in combination with acid scrubbing has emerged as a promising technology not only for removing ammonia from wastewater, but also for recovering it as a valuable fertilizer, ensuring circularity. However, factors such as continuous consumption of chemicals and the hazardous use of strong acids need to be addressed. To reduce chemical consumption, a combination of air stripping and organic acid scrubbing with bipolar membrane electrodialysis (BPMED) has been proposed.
This study focused on optimizing BPMED for the recovery of ammonia and citric acid from ammonium citrate scrubber effluents. The impact of current density, membrane configuration, feed solution characteristics (pH and initial N concentration), and temperature on recovery efficiency, current efficiency, and energy consumption of a BPMED system was evaluated. The limiting current density (LCD), a key factor in the normal operating range of the system, was determined using the Cowan and Brown method, yielding a critical value of 1.01 A/m2.
Comparative experiments conducted on three BPMED configurations, including 3-chamber BPMED (BPCA), 2-chamber base BPMED (BPC), and 2-chamber acid BPMED (BPA), revealed the superior performance of the BPC in terms of current efficiency, energy consumption, and running time. The optimal operating time of BPC was determined to be 120 minutes, achieving a recovery efficiency of 55.9%, a current efficiency of 44.2%, and an energy consumption of 8.4 kWh/kg-N.
Moreover, regression models were established using Box-Behnken design (BBD) from response surface methodology (RSM) to optimize operating conditions (pH, initial N concentration, and temperature), maximize recovery efficiency, current efficiency, and minimize energy consumption. Verified by analysis of variance, normal probability plot, and residual analysis, the model showed high accuracy and significance.
Univariate analysis elucidated that pH and initial N concentration were found to be important variables, while temperature was not. Increasing pH (3–7) enhanced recovery and current efficiency while decreasing energy consumption. Higher initial N concentrations (2–10 g/L) improved current efficiency, decreased energy consumption, but reduced recovery efficiency, emphasizing the need for a careful balance. Temperature variations (20–40°C) had no significant impact on BPMED. Critical factors limiting ammonia recovery efficiency, current efficiency, and energy consumption were identified, including solution conductivity, H+ ion leakage, water migration, and NH3 diffusion.
Furthermore, the study revealed non-significant interactions between these variables through 3D response surface plots and 2D contour plots. Adjusting operational variables proved feasible for optimizing performance indicators. The optimized conditions (pH 6.05, initial N concentration 6.67 g/L, temperature 30°C) were experimentally verified, and the predicted values were in good agreement with the actual values, confirming the reliability of the optimization model. Specifically, the recovery efficiency was 52.9%, the current efficiency was 45.4%, and the energy consumption was 7.0 kWh/kg-N.
Energy evaluation of the BPMED system in BPC configuration under optimal conditions showed significant energy efficiency. Based on the comparison with the available literature, the integration of BPMED with air stripping and organic acid scrubbing could improve energy efficiencies and lower chemical consumption while offering a closed-loop system.
Future research should explore principles to inhibit ion leakage and water migration, analyze the combined effects of various operating variables using RSM, and validate the potential for lower energy consumption in full-scale BPMED. Developing continuous BPMED processes is crucial for full-scale application, and integrating BPMED with other processes such as air stripping and acid scrubbing may enhance ammonia recovery and production efficiency.
The insights gleaned from this study provided a solid foundation for enhancing ammonia recovery processes from ammonium citrate scrubber wastewater, thereby promoting sustainable and resource-saving industrial practices.

Alkaline water electrolysis emerges as a promising technology for green hydrogen production, playing a significant role in global decarbonization. Nickel-based electrodes are widely used in alkaline water electrolysis due to their excellent catalytic properties for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). However, nickel electrodes often experience a decrease in activity over time. Attempts of existing literature to investigate nickel deactivation employ conditions that differ from industry standards. Therefore, a more profound understanding of these phenomena under industry-relevant conditions is crucial for averting specific degradation pathways in future electrode design. This thesis investigates the deactivation phenomena and the timescales of untreated nickel electrodes during the OER and HER at 323 K and 30 wt% KOH, employing a current density of 1 A cm-2. The electrolyte concentration and current density match industry standards, and the temperature aligns more with industrial practices than literature. The OER overpotential increases under these conditions in 2.5 hours by 0.19 V and still increases after this period. The HER overpotential increases by 0.65 V and stabilizes after 20 minutes. Electrochemical and surface analytical tests suggest that both reactions primarily deactivate due to a redox reaction.

Producing green hydrogen can be done using alkaline water electrolysis. Recycling water might help to fulfil the large water demand of hydrogen production facilities. In fertiliser production, steam condensate could be a recyclable water source for hydrogen production. This condensate is relatively clean as its main contaminant is a small concentration of ammonia. This report researches the effect of adding ammonia to the electrolyte of an alkaline water electrolysis cell.

When steam condensate is used instead of ultra-pure water, it is important to find out what happens to the added ammonia and if it affects the production rate and the production efficiency, or if it degrades the equipment. The experiments in this report used three different ammonia concentrations and the electrolyte was made using potassium hydroxide. The electrolysis cell used nickel mesh electrodes and a Selemion™ anion exchange membrane. Each of the experiments were carried out at 2.15 Volts for a duration of 50 minutes.

It was found that adding 1 mmol/L ammonia to the electrolyte decreased the current density of the cell. Doubling the ammonia concentration led to an even larger decrease in current density of up to 19%. A significant effect on the Faraday efficiency was not measured. During the experiments, the ammonia was partially stripped from the electrolyte due to its high alkalinity. Another part was oxidised to produce nitrogen gas and nitrate. Some of the ammonia had not reacted after the 50-minute experiment and could be measured in the spent electrolyte. After all the experiments were carried out, the cell was disassembled. The cathode showed significant signs of degradation. However, the many starts and stops between experiments could be the primary reason for this degradation. The ammonia could have accelerated the degradation, but this was not proven.

Overall, the benefits of using steam condensate do not seem to outweigh the drawbacks. The ammonia caused a significant decrease in current density. On the long term, ammonia might cause electrode poisoning which would further lower the current density. The experiments also found nitrate in the spent electrolyte. Having additional pollutants in both the spent electrolyte and the produced gasses might introduce additional disadvantages to using an ammonia containing electrolyte. If steam condensate is used as a water source, a treatment step is advised. Aeration could be used to strip the ammonia from the electrolyte. To get a more accurate insight into the effects of ammonia on alkaline water electrolysis, additional research is necessary. In future research, the effects of ammonia should be measured more accurately, and the long-term effects should be researched.

As part of the efforts to mitigate climate change, the electrochemical reduction of CO2 into valuable chemicals and fuels has been identified as a pivotal technology due to its potential for CO2 utilization and ability to store excess electricity as chemical energy. While significant progress has been made in optimizing various aspects of the electrochemical CO2 reduction system, an unexplored area pertains to the improvement of the anodic reaction. The conventional anodic reaction, namely the oxygen evolution reaction (OER), is constrained by kinetic and thermodynamic unfavorability, reliance on precious metal catalysts, and the need for costly downstream gas separation.
To address these limitations, a novel approach has gained traction within the research community: the paired electrolysis of CO2 reduction reaction (CO2RR) and glycerol oxidation reaction (GOR). How- ever, these studies mostly employ expensive platinum group metal (PGM) catalysts in flow cells, failing to address cost dependency or scalability for future industrial applications due to inefficient energy use in this electrolyzer configuration. Therefore, this study intends to understand and optimize the paired electrolysis of CO2RR with GOR in zero-gap electrolyzers while comparing the performances of Pt (a relatively rare PGM catalyst) and Ni (an abundant non-PGM catalyst). The effects of applied cell po- tential, glycerol concentration, active surface area, and ion exchange membrane type on GOR product selectivity and the system’s energy demand are evaluated through potential and current controlled ex- periments. Gas chromatography (GC) and proton nuclear magnetic resonance spectroscopy (H-NMR) are used for product analysis.
This thesis demonstrates the viability of paired electrolysis in zero-gap electrolyzers, yielding major products like formate and lactate alongside minor byproducts such as acetate, glycerate, and dihydrox- yacetone. The results reveal Ni’s superior performance over Pt at current densities below 200 mA/cm2 in zero-gap electrolyzers. The negative influence of increasing applied potentials on faradaic efficien- cies (FEs) is presented, particularly in Ni, likely due to side reactions like OER or formate oxidation. The study also illustrates that increased glycerol concentrations decrease FEs and system activities due to heightened viscosity-related diffusivity issues. Moreover, the tests conducted using the Ni anode in zero-gap electrolyzers utilizing bipolar membranes (BPM) show a minor reduction in product selectivity likely caused by the increased amounts of OH– ions near the anode coming from the water dissociation reaction (WDR).
The study also uncovers that the anticipated significant reduction in the cell’s energy demand with the replacement of OER with GOR is not observed in zero-gap electrolyzers. No conclusive improve- ments are observed for either catalyst when anion exchange membranes (AEM) are employed, and only marginal improvements in the cell’s energy demand are achieved when bipolar membranes are used with Ni. Although this behavior is speculated to be a consequence of the absence of a flowing electrolyte near the anode, further investigations are needed to identify the cause of this unexpected lack of improvement in the energy demand.

Lignocellulose is an abundant source of feedstock and consists of hemicellulose, cellulose, and lignin. Pre-treatment is done to fragment lignin and hydrolyse the hemicellulose and cellulose fractions into simple sugars which is then converted into bioplastics. The pre-treatment generates a mix of sugar solutions, lignin breakdown products and inorganic ions. However, the inorganic ions act an inhibitor for fermentation and needs to be removed. Electrodialysis is investigated as an alternative process to ion-exchange to desalt the sugar solutions for downstream processes. While the sugars are neutral compounds, the other organics are negatively charged and can cause fouling on the positively charged anion-exchange membrane.

The process waters are C5C6 and C5 sugar streams and the main ions present are sodium and sulphate. A desalting target of 60% is set for these two ions and the experiments are done using a lab-scale electrodialysis set-up. Tests are run to check the desalting rate, effect of filtration, powdered activated carbon (PAC) and acidification, persistence of fouling and the effect of cleaning-in-place. The increase in average resistance and run time is used as an indicator of membrane fouling.

The raw C5C6 sugar solution could not reach the desalting target but once the solution is filtered, a desalting of 75.5% (Na+) and 66.5% (SO42-) is achieved. The average resistance also drops significantly. The fouling is removed by a CIP and is deemed to be ‘reversible’ in nature. On the other hand, the C5 sugar solution did not improve in performance after filtration. Although the desalting target could be achieved after filtration, the average resistance rose to 130 ohms. A CIP did not have any effect and increased run-time and drop in desalting efficiency was observed. The main reason for fouling in both cases is attributed to the presence of lignin degradation compounds. Lignin is more concentrated in the C5 sugar as compared to the C5C6 sugar solution due to its production process. In C5 sugar, the irreversible fouling pattern is hypothesized to be due to adsorption whereas in case of C5C6 sugar the reversible fouling pattern is cake fouling. PAC was dosed to remove the organic foulants in C5 sugar and improved the average resistance to 48 ohms and significantly reduced the run time. Desalting of 61.4% (Na+) and 87.2% (SO42-) was reached. Acidification protonated the feed and reduced the charged interactions, thereby both reducing run time and the average resistance. Desalting of 65.79% (Na+) and 68.02% (SO42-) was reached after acidification.

To conclude, the studied ED system could desalt the organic C5C6 and C5 sugar streams after a treatment step. Lignin degradation products were found to heavily foul the AEM membranes as its increased presence in C5 sugar stream led to a more severe fouling which could not be restored even with a chemical CIP. However, mitigation of the degradation products in the pre-treatment step could ensure that the solution was desalted by the ED unit. Thus, quantifying the composition of organic streams is imperative to predict the propensity of fouling in an ED unit.

Brine producing industries progressively become the centre of attentionas they carry greater environmental consequences. Despite the extensiveliterature that can be found on desalination, limited information is availableabout the practical comparison of the technologies with regards to the brineconcentration performance and associated energy consumption. This thesis studyaims to contribute to bridging this knowledge gap by comparing reverse osmosis(RO), electrodialysis (ED) and vacuum membrane distillation (VMD). RO wasstudied by simulations using WAVE software, while ED and VMD were studied byperforming lab-scale experiments. Feed NaCl concentrationsbetween 20 and 80 g/L were considered in this study. The achievedconcentration factor (CF) for RO was greatly influenced by the feedconcentration. It was found that applying higher netdriving pressures resulted in higher recoveries, but recoveries are limited by feed concentration. In the VMDstudy, the correlation between operating temperatures and permeate flux wasfound to be positive. A decrease in permeate flux wasobserved with increasing solute content, which is related to the vapourpressure lowering phenomena. It was found that the effect of operatingtemperature on permeate flux is greater than the effect of the feedconcentration. Condensation contributed to the highest energy consumption(81%), followed by the vacuum pump (18.9%). Efficient heating andcooling pumps result in a significant energy consumption decrease. In the ED study it was found that applying different current density (CD) did not influence the CF, so it ismore advantageous to apply lower CD to achieve the same desalination from anenergy saving point of view. Furthermore, the CF decreased with increasing feedconcentration and thistrend intensifies with larger volume ratios. The effectof absolute water transport became more significant at higher feedconcentrations and resulted in overall dilution of the concentrate stream. Thedistribution of mass transport was similar for different volume ratios, meaningthat higher CF can be achieved by larger volume ratios. Higher energy consumptionwas related to higher feed concentration and higher volume ratios resulted in higherenergy consumption due to smaller concentrate volume.RO, VMD and ED proved to be suitabletechnologies for concentrating different NaCl solutions. RO and ED showed verysimilar concentration performance and energy consumption. The concentration limit for RO was 118g/L, while for ED it was 140 g/L. RO was able to achieve higher CF with lowerenergy consumption at concentrations < 60 g/L, compared to ED. EDoutperforms RO at concentrations > 60 g/L. Theenergy consumption of ED and RO can be reduced by applying a multi-stageconfiguration. The VMD results showed that the energy consumptionto achieve a similar CF as RO and ED, increased by a factor of 61. VMD is moreadvantageous at higher feed concentrations and even more so with waste heat. VMD and RO are suitable for cases that require high qualitypermeate whereas ED is not. The technologies could also be applied togetherto minimize waste production, whilst prioritizing concentration.

This research aims to analyse the non-ohmic resistance of multiple ion-exchange membranes (IEM) in series, for a better understanding of membrane resistances. These membranes can be found in series in energy storage systems such as acid-base flow batteries (ABFB).
The effect of different current densities on the ohmic and non-ohmic resistance of bipolar membranes (BPM), under forward and reverse bias were studied using electrochemical impedance spectroscopy (EIS). Hereafter, an anion exchange membrane (AEM), cation exchange membrane (CEM) and BPM were examined individually and in series using EIS, to compose a simplified equivalent circuit for a whole ABFB triplet.
For the effect of different current densities on the BPM, an exponential decline of the non-ohmic resistance of the diffusion boundary layer (DBL) was found as a function of the current density. Under forward bias, the DBL resistance becomes significant. The resistance of the water dissociation reaction (WDR) was minimal under all current densities. Flow experiments validate a depletion and enrichment of the DBL during water association and dissociation, respectively.
To compose a simplified equivalent circuit for an AEM, BPM and CEM in series, it was found that the specific resistances of all membranes can be summed at their respective frequencies. This made it possible to construct a simplified equivalent circuit for a whole triplet containing an AEM, BPM and CEM.

Due to climate change, the sea temperature is rising. This temperature change has an effect on the phytoplankton population. Phytoplankton is responsible for more than 50 percent of the oxygen production on earth, and is therefore crucial for life on earth. In this report, the research question is: is the temperature increase of the water important for the development in space and time of the plankton population in a 2D model of a coastal area of an ocean?

To investigate this, first a 2D model for a coastal area of an ocean is formed. As a basis for this model the 0D model of Steele and Henderson is used to describe the predator-prey model for zooplankton and phytoplankton. Due to this model a repeating pattern in time arises, which is called a limit cycle. This predator-prey
model is expanded to a 2D model by applying convection (v) and diffusion (D) to it, representing the current and dispersion in the water. The chosen parameters and boundary conditions are inspired by the coastal area of the west coast of Portugal.

To evaluate the effect of the water temperature rising, first, the effect of diffusion and convection are studied by answering the following questions: is the convection important for the development in space and time of the plankton population in a 2D model of a coastal area of an ocean? And: is the diffusion important for the development in space and time of the plankton population in a 2D model of a coastal area of an ocean?

The diffusion is dominant in the direction perpendicular to the coast. Due to the diffusion the limit cycle of the predator-prey model is suppressed. Its range gets smaller until the plankton population is completely constant for D > 2.5 m^2/s. This critical value is included in the realistic value of D, which varies from 1 to 2000 m^2/s.

Due to convection, the limit cycle corresponding to the boundary condition occurs in the direction parallel to the coast, making every point in the domain steady state. The higher the velocity v corresponding to the convection is, the less oscillations in phytoplankton density there are in the domain, thus the larger the wavelength of the oscillation is. For v > 0.06 m/s the current is so fast that the predator-prey model has little time to develop before it reaches the right boundary, making the left boundary value more important. This critical value is included in the realistic value of v, which varies from 0.03 to 0.28 m/s.

The model experiences both diffusion and convection more or less equally when the Péclet number vH^2/DL≈1, resulting in D/v≈50 m. If this number is significantly larger than 50 m, diffusion dominates the solution. If the number is significantly smaller than 50 m, convection dominates the solution.

Now the main research question can be answered. By increasing the temperature of the water the growth rate of phytoplankton increases. Due to this increase, the timescale of the predator-prey (τpp ) decreases. The value of D and v for which the predator-prey model and diffusion and convection all influence the solution
equally depends on the timescale with the factor 1/τpp . A smaller time scale means that the predator-prey model contributes more to the model. However, this change is very small in comparison to the scale of the realistic values. In conclusion, in the formed 2D model the temperature increase of the water is not important for the development of the plankton population in space and time.

The results of this report are influenced by the assumptions and approximations made. In this study, several processes, both physical and biological, are described with constants and simplifications. Improving the models of these processes is left for further research.

Excess nitrogen disposal into the environment increased with the discovery of an artificial way of nitrogen gas conversion to ammonia, and its implementation to industrial scale. The excess wasted fixed nitrogen ended up in rivers, lakes, oceans which resulted in degrading air, water and soil due to exceedance of the amount of nitrogen that can be absorbed by soil and plants. Currently, the global wastewater treatment objective has been adapting sustainable solutions by shifting from contaminant removal to resource recovery. Nutrients that were once considered as waste are now being considered as resources. Bipolar membrane electrodialysis (BPM-ED) in combination with stripper/scrubber is one of the combined technologies that aim at contaminant removal and resource recovery.

This study looks into energy efficiency and treatment performance from ammonium sulphate and ammonium citrate scrubber effluents by BPM-ED technology. A three-compartment BPM-ED was used to investigate the use of salt mixture in the feed solution and factors affecting ammonium yield.

Salt mixture of ammonium sulphate and tri-ammonium citrate were used. It was found that use of a salt mixture of ammonium sulphate and tri-ammonium citrate decreased the energy consumption and increased the current efficiency for both of the salts. Use of a mixture of ammonium sulphate and tri-ammonium citrate resulted in better energy performance than either of the salts when they were used alone. Ammonium recovery performance was higher when tri-ammonium citrate concentration was higher in the initial feed solution. This was due to increase in buffer capacity and basic pH in feed solution due to OH- leakages from base compartment. Sulphate ions were favored over citrate ions in their transfer through anion exchange membranes due to higher mobility of sulphate ions. As a further investigation a BPM-ED stack with different combinations, perhaps a two-compartment stack, can be tested to get a deeper understanding to optimize the operation.

Factors affecting ammonium yield that were investigated were H+ leakages and ammonia diffusion. It was found that H+ leakages were same for ammonium sulphate and potassium sulphate feed solutions and H+ leakages were not affected by ammonia diffusion. Ammonia diffusion resulted in higher energy consumption and loss of ammonium recovery potential. Ammonia diffusion increased even with experimental time of 60 minutes. Ammonia diffusion rates also increased through experimental time of 60 minutes. Ammonium and potassium transfer rates from diluate compartment did not follow a specific trend within the first 30 minutes. Clear reasoning behind this was unknown and the results were not completely satisfactory. In current study, it was assumed that ion cross-over and back diffusion were identical for ammonium and potassium. Investigating ion cross-over and back diffusion of the ions in BPM-ED could bring a deeper understanding to the energy efficiency and treatment performance.

This study aims to explore hydrogen production via alkaline water electrolysis using municipal effluent. The experiments were conducted in a flow cell electrolyzer using carbon fibre and Ni-foam as the anode, with another Ni-foam electrode employed as the cathode for all experiments. Synthetically prepared effluent and real municipal effluent obtained from the Harnaschpolder water treatment plant effluent was used with potassium hydroxide (KOH) as the supporting electrolyte. The composition of the synthetic effluent was prepared with 23.0 ± 2.1 mg O2/L humic acid (Sigma Aldrich) as the primary organic pollutant. Experiments were conducted to investigate the effect of electrolyte concentration ranging from 0.01M -1M KOH and the effect of applied current density ranging from 25 A/m2, 50 A/m2, 100 A/m2 and 150 A/m2. Further experiments were conducted to assess the effect of humic acid concentration ranging from 23.0 ± 2.1 mg O2/L to 90.0 ± 0.7 mg O2/L. This was subsequently investigated under potentiostatic conditions at an applied cell voltage of 1.5V using the carbon fibre anode and under galvanostatic conditions for the Ni-foam anode. Regarding the Ni-foam anode, the current density was calculated based on the concentration using the relationship for limiting current density of humic acid oxidation. The investigation focused on assessing the performance of the electrolyzer in terms of volumetric hydrogen production rate and energy efficiency of hydrogen production. The performance data was also compared with conventional alkaline water electrolysis using Ni-foam as the anode and cathode in 1M KOH solution. In addition, the extent of humic acid oxidation was also assessed in terms of COD removal efficiency, TOC removal efficiency and changes in the spectral scans obtained via UV-Vis spectrophotometry.
The performance data revealed that the energy efficiency of hydrogen production was lower for both synthetic and municipal effluent compared to alkaline water electrolysis; this was evident for both carbon fibre and Ni-foam anode. Further, the energy efficiency was also higher when the Ni-foam anode was used compared to the carbon fibre anode. For the electrolysis of municipal effluent, a maximum energy efficiency of 75 ± 2.7% was obtained for the carbon fibre electrode, whereas for the Ni-foam anode, the maximum energy efficiency obtained was 83 ± 3.0% at an applied current density of 12.55 A/m2 with 1M KOH as the electrolyte. Despite this observation, the volumetric hydrogen production rates were not significantly affected because the rates converged closely to their theoretical values; this was also evidenced by the coulombic efficiency of hydrogen evolution reaction (HER), which exceeded 90% in all the experiments.
Concerning the extent of humic acid degradation, the Ni-foam anode was able to oxidize some of the humic acid molecules during electrolysis, and a maximum COD removal of 24.6 ± 8 % was observed after electrolysis of synthetic effluent at an applied current density of 100 A/m2 with 1M KOH as the supporting electrolyte. In contrast, the carbon fibre anode was not able to do so. Instead, the humic acid molecules tended to adsorb on the surface of the carbon fibre anode, evidenced by the increase in COD and TOC after electrolysis. Similar behaviour was also observed for the carbon fibre anode when the effect of humic acid was investigated under potentiostatic conditions, whereas under galvanostatic conditions for the Ni-foam anode, oxidation of humic acid was evident. The extent of oxidation was dependent on the duration of electrolysis, i.e., humic acid concentrations of 49.8 ± 0.98 mg O2/L and 90.0 ± 0.7 mg O2/L required a duration of 6 hrs and 8 hrs to achieve a COD removal efficiency of 43.5 ± 5.0 % and 60.4 ± 4.2 %, respectively. Additionally, the possibility to integrate an electrolyzer with an aerobic treatment plant to supply high purity oxygen was investigated. The comparison was done using BioWin simulations with a standard aerobic bioreactor for a flow of 10,000 m3/day under two conditions: 1) with atmospheric O2(baseline) 2) with high purity O2(95% purity). The investigation revealed that combining a 1 MW electrolyzer with the aerobic wastewater treatment plant would provide sufficient amount of oxygen for the proper functioning of the high purity O2 plant. In addition, the flow requirement to produce the required amount of oxygen was only 0.03% of the discharged effluent (i.e., only 3.1 m3/day of the discharged 9708 m3/day).
Further, the use of high purity oxygen in the aerobic wastewater treatment plant was accompanied by energy savings of 1.2 kWh (4320 kJ) due to reduced load on the air pumps

The global concern of the increasing levels of CO2 is growing quickly in the recent years. Therefore, a lot of research is currently underway with respect to closing the carbon cycle. The electrochemical reduction of CO2 is a promising technology that could help utilize the CO2 as a feedstock to produce chemicals and fuels, while storing the excess energy generated from renewable energy sources in chemical bonds. Due to its simplicity and economic feasibility, the conversion of CO2 to CO has a high potential in the industrial market. Membrane Electrode Assembly (MEA) is an interesting electrochemical reactor configuration to produce CO on industrial scale due to the low ohmic losses and reduced risk of catalyst poisoning. Optimizing the catalyst and operating conditions are key steps towards the commercialization of the process. This research focuses on understanding the influence of different process parameters on the CO selectivity while analyzing the performance challenges. Multiple inlet flow rates of CO2 were tested at different current densities to evaluate its impact on the faradaic efficiency. The experiments were performed using KOH-exchange MEA cell with gas diffusion electrodes and Sustainion membrane. Since the product of interest is CO, Ag-based catalyst layer was sputtered on the gas diffusion electrode. The cathodic products were identified and quantified using gas chromatograph. The experimental results have shown that increasing current density resulted in lower CO selectivity, while the inlet flow rate did not have a significant effect. It was also shown that the cell could not achieve higher than 200mA/cm2 due to the accumulation of salts blocking the gas flow channel.
On top of that, a simple 2D model was developed in COMSOL Multiphysics to understand the mass transport and concentration distribution in the gas flow channel. The model was not able to simulate the complexities of the electrochemical process and represented an ideal plug flow reactor. It is understood that the incorporation of reaction kinetics and current distribution is necessary to replicate the real scenario.

Industrial and municipal wastewaters often contain a high concentration of NH4+
To prevent excessive discharge into the environment, these wastewaters need to be treated. Electrodialysis has been shown to be a suitable technique for the removal and recovery of NH4+ from both synthetic and real wastewaters such as digestate rejection water and industrial condensate. These wastewaters can contain varying concentrations of Mg2+, Ca2+ and CO32-. Treating these waters with ED for NH4+ recovery also causes recovery of these divalent ions. The recovery of these ions can potentially cause scaling. Scaling is undesired and needs to be prevented. The main objective of this research was to analyse what the applied current density, Mg2+/NH4+ ratio and total ion concentration had on the the selective transport between NH4+ and Mg2+.

Electrochemical reduction of the \ce{CO2} into the chemically valuable \ce{CO} at an industrial scale is a promising way to reverse the worrying rise of \ce{CO2} levels in the atmosphere. Profitable operation at an industrial scale requires high CO partial current densities, producing a CO-rich output, while keeping the energy consumption and the price of the electrolyser low. Conventional \ce{CO2} reduction (\ce{CO2}R) to CO electrolysers suffer from the competing hydrogen evolution (HEV) reaction and mass transport limitations. Therefore, they can produce a maximum CO partial current densities of 20 mA cm$^{-2}$. The current densities can be increased by working at higher pressures or using gas diffusion electrodes, but to date, both require high energy inputs. Therefore, we investigated the potential of using a flow-through electrolyser (FTE) to produce \ce{CO} partial current densities exceeding the 20 mA cm$^{-2}$, while keeping the potential attractive. Porous flow-through electrodes can overcome mass transport limitations by their large reactive surface area and their ability to decrease the diffusion boundary layer thickness. To obtain suitable porous electrodes, we electrodeposited Ag on microporous Ti substrates. We used an aqueous \ce{CO2}-saturated \ce{KHCO_3} solution, as a well-proven electrolyte for \ce{CO2}R to \ce{CO} on a Ag catalyst. Our FTE was capable of overcoming the mass transport limitations. In a one-off test, it produced a \ce{CO} partial current density of 100 mA cm$^{-2}$, at a Faradaic efficiency (FE) of 55 \%. We could reproduce \ce{CO} partial current densities of 60 mA cm$^{-2}$ at a FE of 25 \% in a 0.05 M \ce{KHCO_3} solution. We experienced a decrease in activity towards CO when using more concentrated electrolytes. However, high cell potentials of more than 5 V, were required to exceed \ce{CO} partial current densities of 20 mA cm$^{-2}$. The required high potentials can be ascribed to ohmic losses and low selectivities towards CO. We believe that the insufficient deposition of the Ag on the microporous Ti electrode was the main reason for the low selectivity. Implementing the improvements, that are mentioned in the report, could contribute to make the FTE a promising candidate to reduce \ce{CO2} to CO at an industrial scale effectively.