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.

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.

Removal of ammonium from simulated ammonium salt solutions was done using bipolar membrane electrodialysis (BPMED), without the use of chemicals. The effect of spacer thickness and open area on the overall energy consumption of BPMED to transport ammonium from the diluate was assessed in batch experiments. The electrochemical energy consumption decreased from 28 MJ/Kg-NH4+ to 16 MJ/Kg-NH4+ when the spacer thickness decreased from 750 μm to 140 μm.
The removal efficiencies of ammonium from the diluate increased from 77% to 85% when the spacer thickness decreased from 750 μm to 140 μm. These results show that with the increasing spacer thickness, the open area and porosity also increase, which accounts for higher resistance on the membrane stack. However, besides open area and porosity, it is the thickness of the spacer that plays a major role in higher energy consumption. This study demonstrated the energy-efficient application of BPMED for the removal of ammonium from simulated ammonium salt solutions.1

Driven by the increasing demand for waste reduction and green energy production, an integrated system which combines an anaerobic membrane bioreactor (AnMBR) and a solid oxide fuel cell (SOFC) was proposed in this research project for blackwater treatment and energy production. The potentials of using an AnMBR for wastewater treatment and biogas production, and the feasibilities of producing energy from biogas with a SOFC have been investigated by many researchers. Although, combining the two equipment might raise new challenges and opportunities. The AnMBR pH has direct impacts on the biogas composition, which would subsequently affect the SOFC operational strategy. Therefore, this research project focused on the influence of the AnMBR pH on the SOFC operational strategy, which would provide insights for connecting AnMBR and SOFC. The AnMBR pH was controlled around 8 initially, and then reduced to 7. The composition of the biogas produced under each pH condition was analyzed before the biogas was conditioned for the SOFC operation. Biochar adsorption and CO2 addition were applied for biogas conditioning. pH 8 was favorable for biochar adsorption, whereas pH 7 was favorable for CO2 addition. The aim of biochar adsorption was to ensure that the H2S concentration remaining in the biogas after adsorption was less than 0.5 ppm, so that sulfur poisoning could be avoided at the anode of SOFC. A biochar column (BC) was attached to the AnMBR for the adsorption of sulfur compounds in the biogas. The BC was packed with biochar made of cow manure. The adsorption capacity of the biochar was measured to determine the amount of biochar required in the BC. After biochar adsorption, the ratio between CH4 and CO2 was balanced by adding CO2 to the biogas, to reduce the risk of carbon deposition at the anode of SOFC. The exhaust gas discharged by the SOFC could also be recycled as an alternative to CO2 addition. The performance of the SOFC system using the conditioned biogas as the fuel was assessed based on electric power output and fuel utilization efficiency. Based on the results of biogas production, conditioning, and utilization, the influence of the AnMBR pH on the SOFC operational strategy was analyzed. Furthermore, the potentials and the limitations of connecting AnMBR and SOFC were discussed.

Aromatic compounds have always been of concern regarding their toxicity to living organisms, including microorganisms. With more anthropogenic activities (e.g. coal gasification), the need for feasible treatment of industrial effluents is highly prioritised. With the anaerobic degradation process being a competitive solution, these compounds’ toxic impact on the biomass is still of concern. These implications influence the stability of the degradation process; thus, there was a search for mechanisms to make the anaerobic degradation process more resilient. One potential mechanism is enhancing syntrophic collaboration between different species and its corresponding electron transfer. Syntrophic collaboration in an anaerobic environment can be conducted using intermediates (e.g. hydrogen) or direct electron transfer. Direct interspecies electron transfer (DIET) is reported to be more energy efficient and more thermodynamically favoured over other mechanisms that include mediators (hydrogen/ formic acid). Conductive and semi-conductive materials have been investigated to simulate this direct interspecies electron transfer mechanism (DIET), with various materials being researched, such as iron oxides, zero-valent metals, and even carbon-based materials.
This study investigated the impact of magnetite addition (as a DIET-stimulator) on p-cresol degradation, methane production and sludge characteristics, with a further interest in membrane fouling mitigation. This investigation was conducted with continuous flow reactors and batch reactors. The continuous configuration was based on an anaerobic membrane bioreactor (AnMBR) fed with a synthetic-coal gasification-like solution of phenol and p-cresol to investigate mainly the conversion rate of p-cresol and monitor the influence on the methane production, sludge characteristics, and membrane fouling. At the same time, batch experiments were conducted to investigate the acetoclastic methanogenic pathway and p-cresol degradation as a sole carbon source. The continuous experiment lasted for 143 days but was divided into two separate phases with two different magnetite dosages, starting with 40 mmol/L in phase (I), then replacing the sludge with acclimatised one (from the control) with the addition of the second dosage (20 mmol/L) in phase (II).
A Magnetite dosage of 40 mmol/L showed signs of biomass-suppressed conversion capacity compared to the control, by which the reactor conversion rate deteriorated by reaching 212 mgCOD/ gVSS/d (under a feed of 900 mgPh/L & 900 mgPcr/L). Phase (I) showed no significant differences in the methane production rate between control and magnetite reactors. On the other hand, the batch experiments fed with 1 gCOD/L acetate showed that the magnetite reactor had a lower acetoclastic methane production rate than the control. It was suggested that the 40 mmol/L magnetite dosage was suppressing the acetoclastic methanogens, which was further contributing to the lower conversion capacity observed in the AnMBR by the end of the phase. With the same methane being produced in control and magnetite reactors, it was also possible that either hydrogenotrophic methanogenesis or the DIET pathways were enhanced; however the absence of intermediates (e.g. VFAs) and the similarity of the COD balance supported the possibility of the latter one. During phase (II), the conversion rate of both reactors (control and magnetite) reached 74 mgPcr/ gVSS/d, approaching the highest conversion rates reported in the literature. While the acetoclastic methanogens showed no significant difference in the batch experiment, the magnetite-AnMBR’s methane production rate was 10%-28% higher. Furthermore, the methane yield with magnetite supplementation showed an average enhancement of 15%. In addition, the batch experiment also showed that this magnetite dosage reduced the p-cresol conversion rate by 87% compared to the control.
Both magnetite dosages (20 mmol/L & 40 mmol/L) showed a reduction in the protein and carbohydrate content of the soluble microbial products (SMP) and the extracellular polymeric substances (EPS). Magnetite had adversely impacted the loosely-bounded EPS (regarding protein and carbohydrates), whereas it was shown to be significant compared to the control. The EPS-LB showed an inverse relation with the particle size distribution (PSD), verifying that the higher increase in the particle size could be correlated with the EPS-LB reduction by the magnetite. On the other hand, the fouling rate of the membranes showed an insignificant difference between both reactors. This was suggested to be related to the incomplete formation of a mature cake layer under the influence of low operational flux. However, with the reduction of the SMP/EPS, it was suggested that the formed cake layer would be more porous and permeable. This would mean that the cake-fouling and its corresponding resistance would be expected to be lower. As the cake layer acts as a protective barrier for the membrane, its reduction would lead to a higher risk of irreversible pore-blocking by fine particles from the magnetite and the sludge.

In the treatment of industrial wastewater, the biological removal of aniline & nitrogen often occurs simultaneously within the hyper-saline water matrix. This study focuses on pioneering the research
on combined aniline-nitrogen removal in a hyper-saline environment. The experimental approach assessed the cause of nitrite accumulation phenomena in this specific case study as well as the influence
of oxygen concentration and aniline as a carbon source on nitrification. The hypothesis of increasing aniline biodegradability by making it chemically react with nitrite was experimentally tested. In addition, data analysis of the full scale of this study was performed in order to create analytical models to represent its biological processes. It was found that the correlation between oxygen concentrations and nitrification was non-linear and differs from what is observed in the literature due to the significant influence of different boundary conditions. The unavailability of carbon sources during the nitratation process was the cause of nitrite accumulation, which crucially affects nitrification. The chemical reaction between aniline and nitrite (most likely polymerization) resulted in a compound that did not limit nitrification and had more biodegradable potential. Furthermore, an aerobic model was developed but the promising results were most likely caused by the mathematical optimization, which was done in order to bypass the lack of data. Nevertheless, respirometry was used to build an empirical model to detect nitrite accumulation in the effluent of the full scale, which showed promising results. This study can potentially pave the way for the application of Anammox technology in the combined treatment of aniline & nitrogen, which is discussed in the hypothetical design implementations.

Currently, organic micropollutants (OMPs) are continuously and uncontrollably released into the water environment worldwide, as the reason for their special properties, OMPs removal has been a global challenge. This study focuses on acetaminophen degradation by photo-electrolysis (PEC) activities, which is one of the promising advanced oxidation processes (AOP) technologies. First, we report the fabrication methods of the BiVO4/BiOI heterojunction on FTO glass, then characterised the prepared photoanodes with XPS, XRD, SEM, EDS, UV-vis and IPCE. The results demonstrated the BiVO4/BiOI p-n heterojunction had been successfully electrodeposited on the FTO glass. Further, the LSV and EIS analysis in this study showed the BiVO4/BiOI photoanode had less photocurrent density than BiVO4 when carried out in the solution of acetaminophen. Even if the heterojunction did not improve the photocurrent, it significantly enhance the acetaminophen removal efficiency in the PEC degradation process. BiVO4/BiOI photoanode achieved 99% degradation efficiency in 3 hours and obtained 0.019 mi n−1 of the reaction rate constant. Overall, these results indicate that BiVO4/BiOI heterojunction has a great application potential for the degradation of OMPs in the wastewater treatment plants secondary effluent.

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

A reverse osmosis (RO) membrane followed by a packed tower aerator and rapid sand filter are used by an agricultural company in Emmen to treat brackish groundwater to supply water for growing crops and cleaning purposes. The iron removal by the treatment plant is sufficient most of the time throughout the year. However, the farmer periodically (once per year) reported yellowish treated water, indicating insufficient iron removal. Nevertheless, it is still not clear what is the cause of the insufficiency of the iron removal. This thesis aimed to investigate the iron removal in the water treatment plant in Emmen and propose a solution to improve the iron removal. The iron removal of the treatment plant was investigated through a set of batch iron oxidation, flocculation, and filtration experiments. The pH of the water was the main parameter that influenced the oxidation of Fe(II). It was found that within the retention time that was available in the treatment plant’s tower aerator and the rapid sand filter (20 minutes), the Fe(II) was not fully oxidized and flocculated. At pH of the permeate in the range of 6 – 7, only <11.5% of the initial Fe(II) was oxidized in 20 minutes. Increasing the pH to 8 accelerated the oxidation of Fe(II), and the Fe(II) was completely oxidized within 30 minutes.Although removal of iron that is dominated by floc filtration was not achieved, the treatment plant also removed the iron through adsorption on iron hydroxide deposit in the packed tower aerator and sand particle. However, the Fe(II) adsorption capacity of adsorbents was low at low pH. The regeneration of the adsorption capacity was achieved through oxidation of the adsorbed Fe(II) which is also influenced by pH. COMSOL model showed that without regeneration, the adsorption capacity of new sand and iron oxide-coated sand was exhausted after 24 hours and 230 hours, respectively.When the concentration of CO2 in the permeate is in equilibrium with air, the pH of the permeate should be in the range of 7.9 – 8. However, the maximum pH of the permeate was 7 after aeration because the contact time of the tower aerator was not sufficient to completely strip the CO2. The tower aerator was also clogged by iron deposits that reduces the airflow and causes short-circuiting that decrease the CO2 stripping efficiency over time.Installation of a bubble column reactor was proposed to improve the CO2 stripping and increase the pH of the permeate. The Phreeqc model showed that the CO2 could be stripped until approx. 1 mg/L within 4 minutes, and the pH also increased to approx. 7.9. Moreover, the bubble column reactor will provide additional retention time for Fe(II) oxidation, and approx. 20 – 30% of the initial Fe(II) concentration can be oxidized within 4 minutes. The bubble column was considered preferable compared to the packed tower aerator because it was not susceptible to clogging by iron deposits and requires lower maintenance.

This thesis constitutes a part of the "IMPRESS" project which is aiming at the production of bio-plastics by utilizing hemicellulosic hydrolysates. The hydrolysate stream, after following the steps of acid hydrolysis and neutralization, consists of monosaccharides (C5/C6 sugars), salt and sugar degradation products. The C5/C6 sugars need to be concentrated by a factor of 10 to be purified afterwards in downstream processes. Evaporation has been the most dominant technology in the sugar treatment sector, however the current study aimed to investigate tubular nanofiltration (NF) and vacuum membrane distillation (VMD) units for concentrating the C5/C6 sugars, as more sustainable alternatives. Synthetic solutions with C5/C6 sugars and salt were used to assess the performance of the NF and MD membranes in terms of sugar and salt rejection before the experiments with the real hydrolysate solution. Among the different NF and MD membranes, the one that met the C5/C6 sugar rejection requirements (>90 %) during the synthetic solution experiments was tested for concentrating the C5/C6 sugars. The most suitable technology for concentrating the C5/C6 sugars was also assessed for its energy consumption. Additionally, single filtration tests were conducted to characterize the NF membranes, while surface tension and contact angle measurements were performed to predict the membrane wetting phenomena during the VMD operation. Based on the C5/C6 sugar rejection values (range of 22-81 %) obtained with ceramic and polymeric NF membranes, it was deduced that the tubular NF membranes were not capable of meeting the C5/C6 sugar concentration goals. The polymeric membranes performed better than the ceramic ones in terms of C5/C6 sugar rejection, with a maximum sugar rejection of 81 %. However, the polymeric membranes were found to be more prone to fouling, showing a maximum decrease of 34 % in the water permeability after 4 hours of operation with the hydrolysate solution. Additionally, the characterization of the NF membranes based on the Donnan Steric Pore Model (DSPM) revealed membrane pore sizes larger than those provided by the manufacturer. On the contrary, the 0.2 μm MD membrane showed >99 % of C5/C6 sugar rejection and was used for the sugar concentration tests. The maximum C5/C6 sugar concentration factor (CF) achieved with the VMD was 8 with a negligible sugar loss (<1 %) in the permeate, indicating that concentrating the C5/C6 sugars by a factor of 10 is feasible with this technology. Surface tension and contact angle measurements disclosed inconsistencies in the hydrophobicity of the MD membrane, which can be linked to the membrane wetting phenomena occurred during the VMD operation. The fouling of the MD membrane (50 % of flux decrease after 39 hours of operation) was found to be reversible with complete flux recovery after chemical cleaning and drying of the membrane. The energy assessment of the VMD technology showed that the main energy consumer was the cooler, contributing to 96 % of the total consumed energy. Based on the cooler's efficiency, the energy consumption of the VMD unit was calculated to be in the range of 207-736 KWh/m³ of distillate. When compared with multi-effect evaporators with up to three effects, the VMD was found to be more energy efficient. Investigating less energy-intensive alternatives for concentrating the C5/C6 sugars, experiments with electrodialysis (ED) showed 90 % removal of both acid and salt from the raw and neutralized hydrolysate water, respectively, making it feasible for reverse osmosis (RO) membranes to be further tested for concentrating the C5/C6 sugars. Therefore, a treatment scheme of ED and tubular RO membranes is proposed for further research on concentrating the C5/C6 sugars. Overall, a pre-treatment step with the most permeable ceramic NF membrane is suggested, as evinced by the high color removal achieved (elimination of big foulants) and the high permeation of the C5/C6 sugars and salt.

The abundant presence of nitrogen in natural waters can be a threat to both humans and environment. Therefore, municipal and industrial wastewater streams need to be treated before their disposal in the environment. Currently used biological and physical-chemical treatment methods have drawbacks such as high greenhouse gas emissions, high energy use and high use of chemicals. This research used bipolar membrane electrodialysis (BPMED) as a technology for the removal of ammonium sulfate from industrial wastewaters with extreme characteristics. The industrial stripper/scrubber system removing nitrogen from industrial waters creates an ammonium sulfate rich water stream with a pH of 2, a temperature of 70 degrees Celsius and concentrations up to 250 g/L. The study investigated the influence of a low pH, high concentration and high temperature on the removal of ammonium and sulfate from wastewater and in situ generation of sulfuric acid and production of ammonium hydroxide. Key performance parameters to measure the influence of these extreme conditions were the efficiency of nitrogen removal in the form of ammonium current efficiency and ammonium removal efficiency}) and sulfate removal removal efficiency. Other key performance parameters of interest were the consumed energy for this removal in the form of electrochemical energy consumption, the purity of the generated acid and base and the final ammonium concentration in the base and the sulfate concentration in the acid. Model wastewater with a pH between 2 and 10, a concentration between 50 - 250 g/L ammonium sulfate and with a temperature between 20 degrees Celsius and 40 degrees Celsius was treated with BPMED. The influence of pH, concentration and temperature was researched independently in experiments of 180 minutes, after which the removal efficiency and the quality of the produced acid and base were assessed.

Oceanic CO2 capture technology can be used as a negative emission technology, or pre-treatment to reduce inorganic fouling (i.e., scaling) potential when further processing seawater. In this work CO2 from (synthetic) seawater was captured electrochemically via bipolar membrane electrodialysis. Although previous studies showed promising results regarding energy consumption (kJ/mole CaCO3), inorganic fouling is a drawback, which is why an inorganic fouling control and removal study was done. The effect of applied current density and flowrate on inorganic fouling build-up and dissolved organic carbon removal was investigated for 2 cell pair configurations. For inorganic fouling removal 5 methods were
investigated. This research was a proof of concept.

Brine streams from industry is a burden to the environment if not disposed properly. “Zero liquid discharge” (ZLD) is an ideal way of converting saline streams from waste to resource through multiple membrane-based or thermal-based treatment technologies. The utilization of the recovered resource minimizes both costs and environmental impacts. Among the brine treatment technologies, the Eutectic Freeze Crystallization (EFC) is a promising thermal-based technology to obtain water and salt in high purity. EFC technology has higher energy benefit and simpler equipment than conventional evaporation technology. This technology is feasible for continuously recovering pure salt and ice in different stages, and separating them spontaneously. In this study, batch EFC experiment was conducted to determine the solubility curve of Na2SO4 under the different concentrations of NaCl. The effect of NaCl on the solubility of Na2SO4 at the low-temperature interval was investigated. It was found that with the increasing concentration of NaCl, the common ion effect between Na2SO4 and NaCl reduced the solubility of Na2SO4 and depressed the eutectic point (EP) of Na2SO4-H2O system. The decreasing EP showed a linear trend. A thermodynamic model in OLI Studio was used to evaluate the solubility of Na2SO4 as well. The modelling result was compared with the experimental data. The experimental result about the EP of Na2SO4-H2O system has a better agreement to the theoretical value than the eutectic concentration of Na2SO4. Besides, the effect of NaCl was compared with KCl in the OLI Studio, the salting- in effect of KCl promoted the solubility of Na2SO4. A scenario study was carried out to explore a proper process to treat reverse osmosis (RO) concentrate from a demineralised-water-producing (DWP) plant that was rich in Na+, SO42- and Cl-. The thermodynamic model was used to simulate the continuous EFC process and investigate how the different pre-treatment processes affect the recovery of Na2SO4. Four scenarios with different pre-treatment technologies were put forward based on the existing processing facilities: Scenario 1: RO – NF – EFC, Scenario 2: RO – TOC – NF – EFC, Scenario 3: RO – TOC – EFC, Scenario 4: RO – TOC – NF – RO – EFC. All the scenarios could reach more than 92% recovery of H2O. Among the four scenarios, Scenario 4 accomplished the highest recovery of Na2SO4 (98.4%) by -3℃. But the scaling tendency and poor permeate quality of the second RO unit made spontaneously. The gap between ice yield and Na2SO4·10H2O (Mirabilite) yield was significant in all the scenarios. It was suggested only to recover H2O rather than both H2O and Na2SO4, which can be achieved by means of Scenario 1 or 3. In Scenario 2, the close nucleation temperature of ice and mirabilite brought problem for separation work. To conclude, EFC is a newly emerging technology that can achieve a sequential removal of minerals from brine and is sustainable. There are barriers to overcome in impurity studies and those problems deserve attention. Further developments should be stimulated on recovering H2O and salt with an advance retrieving method, for example the design of multi-stage separation process, and the synergistic effect of different ion species and the combined effect of the organic compounds.

Large nitrogen emissions and depositions in countries like The Netherlands have had a negative impact on human health and natural ecosystems. Nitrogen ends up in water bodies where it causes eutrophication, which can lead to a decrease in biodiversity. Nowadays, it is removed from water at wastewater treatment plants (WWTPs) via denitrification – nitrification or the anammox process. In both of these processes the end product is N2 gas, which is again emitted into the atmosphere. In recent years, however, nitrogen in the form of ammonia (NH3) has received increased attention as a valuable resource and can be found in different types of wastewaters. To produce energy from ammonia, solid oxide fuel cells (SOFCs) are used, to which ammonia should be fed in its gaseous form. For the production of ammonia from wastewater, a low carbon-to-nitrogen ratio is desired. The overall goal of the research project is to identify nitrogen-rich wastewaters from the industry and apply a series of treatment steps for the production of ammonia. Protein-rich wastewaters are especially suitable for the production of ammonia. Effluents fitting these criteria have been identified, so far, in several industrial settings, namely the slaughterhouse, food and dairy industry. Proteins can be converted to ammonia through anaerobic digestion, while producing biogas in the form of carbon dioxide (CO2) and methane (CH4), which can also be used for energy purposes. Next to proteins, carbohydrates and volatile fatty acids (VFAs) also make part of protein-rich wastewater. However, not a lot is known about the co-digestion of proteins and these simple carbon sources. Some aspects have been investigated, but thorough research on the whole degradation is needed to fully understand the process. The master thesis research presented in this report focused on the anaerobic degradability of proteins in the presence of sugars and VFAs. Three proteins (bovine serum albumin [BSA], casein and gelatin) were selected from the identified industries and assessed based on degradation efficiency and kinetic rates, under mesophilic batch test conditions. Furthermore, the conversion of protein to ammonia was assessed and parameters to define a biological ammonia potential (BAP) are defined. An increase in the degradation coefficient after the co-digestion with sugar was observed for gelatin (1.1 to 1.6 d-1) and BSA (0.57 to 0.68 d-1). Pure proteins were degraded efficiently with 71 - 96% compared to 76 - 97% of the co-digested batches. A combination of conversion efficiencies and the newly introduced biological ammonia potential gave a good indication on the conversion of protein to NH+4 . Acidification with VFAs resulted in process instabilities with respect to the methane production rate, which was due to the applied ratio of propionate to acetate. To prevent this in future trials, amino acid analysis to predict the production of VFAs from protein was evaluated with a maximum deviation of 14% of measured to theoretical values. This study demonstrated the efficiency of co-digestion to degrade protein-rich substrates. Further research is required to unveil the impact of high ammonia concentrations and explore the microbiological aspect of protein fermentation. Finally, considerations for a test to assess the biological ammonia potential are given.

Resource recovery from waste streams is indispensable to shifting from a linear economy to a circular economy. Industrial concentrate is a waste stream that is actively researched to explore and expand the prospects of resource recovery. Strontium (Sr), an alkaline earth metal, while found regularly in concentrate streams has not been a focus for recovery. Through this study, the recovery of Sr as strontium carbonate (SrCO₃) via precipitation from concentrate streams has been investigated. The objective of this study is to characterize SrCO₃ precipitation in terms of factors that influence it. Specifically, the effect of multiple factors has been investigated on the amount of Sr precipitated (as % relative to the initial amount of Sr) as well as the purity of the corresponding SrCO₃ precipitate (%). Factors that could affect either the amount of Sr precipitated or the purity of the precipitated SrCO₃ were identified based on theoretical knowledge. Equilibrium-based simulations were performed using OLI studio to confirm their impact. The conditions associated with the identified influencing factors were investigated in concentrate from a reverse osmosis unit during drinking water production. A Box-Behnken Design was implemented to quantify the relationship between the identified factors and two responses (i) Amount of Sr precipitated and, (ii) Purity of SrCO₃ precipitated. The range of the factors in the design was established based on their ranges as observed in the concentrate. The value of pH and concentrations of strontium, inorganic carbon (C), calcium (Ca), barium (Ba) and inorganic sulfur (S) were hypothesized to have quantitative and qualitative effects on the SrCO₃ precipitated. Magnesium (Mg) was hypothesized to have a qualitative effect on the SrCO₃ precipitated. Direct SrCO₃ precipitation from concentrate was not practical due to considerably higher concentration of other ions. To facilitate SrCO₃ precipitation from concentrate, the concentration of the other ions was reduced by recovering them as precipitates. The recovery of these precipitates was investigated through variation of process conditions via OLI simulations. Theoretically, it is possible to recover 0.0014g of SrCO₃ (100% purity) per liter of concentrate along with 0.684g CaCO₃ (99.95% purity), 0.161g Mg(OH)₂ (96% purity), 0.0083g Ca(OH)₂ (99.99% purity) and  NaCl solution (4.65 M). Using Box-Behnken Design, the mathematical relationship between the amount of Sr precipitated (% relative to initial concentration) and purity of SrCO₃ precipitated (%) subject to the identified factors was established. The equations had a R² of 98.75% and 97.38% respectively. S was found to not have any significant impact on SrCO₃ precipitation while a positive interaction effect between Sr and Ca was also identified. %Sr precipitated=0.82098+0.01319(pH)-0.04004(Sr)+0.11581(C)-0.07343(Ca)-0.01314(pH*pH)-0.04299(Sr*Sr)-0.04073(C*C)-0.02041(Mg*Mg)+0.01818(pH*Ca)+0.05369(Sr*C)+0.07436(Sr*Ca)-0.05763(C*Ca)  SrCO₃ purity(%)=0.5155-0.20923(pH)+0.12593(Sr)-0.03178(C)-0.06115(Ca)-0.07187(Mg)+0.0617(pH*pH)-0.0388(Sr*Sr)-0.0295(C*C)+0.0456(Mg*Mg)-0.0485(pH*Sr)+0.0595(pH*C)+0.0405(pH*Ca)-0.0903(pH*Mg)+0.0481(Sr*C)+0.0614(Sr*Ca)-0.0453(C*Ca) The predictive R² of these equations are 93.45% and 86.32% respectively. This is representative of their prediction accuracy and hence their applicability to replace experiments and simulations. These equations, however, are built on outputs from OLI simulations. Therefore, their accuracy is subject to the predictive capability of OLI software. Experimental results based on the chosen Box-Behnken Design are needed to either replace the OLI simulations results entirely or account for the inaccuracy of OLI simulations.

Water is the most dominant substance on the planet and has always been a synonym for life. On this planet, water has always been plentiful. Therefore, the conclusion of the United Nations in 2015 was even more shocking. The United Nations estimated that in 2030 a water gap would exist of 40%. The withdrawal of fresh water would be 40% larger than the amount that is replenished every year. We are living on borrowed time, and we need to close this gap. Since most water on the earth is saline, desalination could be a possible solution. Unfortunately, desalination is an energy-intensive process, and it will always be. There are different methods. However, energy consumption would always be rather large. Much of our energy is still created by fossil fuels in most areas in the world. Our increasing demand for fresh water can lead to a further large fossil footprint and therefore more climate change. Climate change negatively impacts our water supplies. A negative spiral could be the result.
The world is consuming a large amount of energy. Unfortunately, this is not done efficiently. It is estimated that 72% of worldwide energy consumption is lost after conversion. Waste heat is an unwanted product as a result of this inefficient conversion. It is estimated that 63% of this waste heat has a temperature of 100 degrees Celcius or lower. A temperature that is not even sufficient to boil an egg, but it is energy nonetheless. If this energy could be harvested, the water gap could be closed by desalination without increasing the carbon footprint. However, how to do this? A solution could be found in Japan and your dry cleaner. In the seventies, when the oil crisis was at its peak, Japan had a problem. It has a high need for air conditioning which has a high need for energy. A device was created that could use waste heat to create cooling with silica gel. The general public knows silica gel as moist eaters that are included in your dry cleaning. By placing silica gel in a closed environment, liquid water could be evaporated. Evaporation needs energy, and this could create cooling. In early 2000, some folks in Singapore realised that this technology could also be used to desalinate. Adsorption Desalination was born. Adsorption desalination (also known as AD) is a boiling point elevation and vapour pressure lowering desalination technique. Silica gel has a high affinity to adsorb water vapour. Since this is done in a closed environment, the pressure is lowered till boiling. When an energy source is placed inside the liquid, an equilibrium can be found without lowering the pressure further. Once the silica gel is saturated, it is heated till a certain temperature that it starts to desorb the vapour again. This vapour is collected and condensed into a clean product. The mass transport takes place by a pressure difference. Since the evaporation takes place in vacuum conditions, it is around room temperature. Therefore it can have a low sensitivity for erosion. Since it is an evaporation technique, it also has a low sensitivity for fouling. On paper, it can be a desalination technology with low operational costs compared to others. In reality, a couple of barriers have to be conquered. One of the barriers is the characteristics of the core element of this technology, the silica gel. The reason that silica gel can adsorb water vapour so well is due to two main elements. First, it is an incompletely dehydrated polymeric structure of colloidal silica acids. Therefore it can create hydrogen bonds, polar bonds and weak electron bonds with other molecules. Water is a bipolar molecule, and therefore silica gel can adsorb water exceptionally well. It can create hydron bonds efficiently, and due to the bipolar nature of water, multiplayer on top of each other can be formed. The second element is the high specific surface of silica gel. It has many pores, and therefore a specific surface equal to human lungs can be formed. In the end, the best qualities can adsorb until 40% of their body weight.
These two elements determine the maximum amount of vapour silica gel can adsorb. However, two parameters determine if silica gel can adsorb. These two parameters are the temperature and vapour pressure. If the temperature is high, the vapour pressure has also to be high to ensure adsorption and the other way around. The same account for the opposite effect. Therefore energy transfer is vital for smooth operation. Unfortunately, the thermal conductivity of silica gel is extremely low. Besides energy transfer to change the temperature of silica gel to go from adsorping to desorbing and reverse, much energy has to be transported during the adsorption and desorption phase. During adsorption, water vapour transforms from a gas phase to a semi-solid phase. In this phase, the energy level is lower. Since energy cannot be destroyed, this energy has to be removed. The same applies to the opposite. If the heat exchanger is not well designed, the adsorption and desorption phase are prolonged, and the daily water production diminished. Even if the whole set-up is well designed, there is still one element of silica gel that can also diminish the water production of the set-up: the degradation of silica gel. Multiple mechanisms can degrade silica gel, but the two most important are fragmentation and pore pollutions by metal ions. Fragmentation (the breaking up into smaller parts) occurs by the high thermal stress the silica gel endures and the pollutions by the feed. A part of the sorption ability is destroyed or temporarily inhibited. The use of acid water can remove the pollution, so maintenance on the silica gel is necessary. Replacing the silica gel by other sorbents like zeolite is possible. Unfortunately, it has a lower adsorption capacity which leads to a less compact device and is, therefore, more expensive. Another unfortunate fact is that zeolite desorbs at a higher temperature. When using zeolite, adsorption desalination can no longer use waste heat of low quality to desalinate. The last problem with using silica gel, but also other sorbents, are the instabilities with the sorption characteristics in vacuum conditions. The sorption characters depend as said before on the temperature and the vapour pressure. Inside vacuum conditions, a disturbance of the vapour pressure can negatively influence the sorption characteristics and can lead to desorption at the wrong moment. Since it is done in a closed environment, it can lead to condensation which lowers the vapour pressure further. A complete breakdown of performance can be the result. Disturbances can happen through air leakage, which is are a common problem in industries. There are some solutions, but in the end, the permanent one is a high-quality vacuum system and intensive maintenance. Even with these barriers, adsorption desalination is still an attractive technology due to its relationship with its own energy consumption pattern. The energy consumption is high but stable and barely sensitive to change in conditions like feed salinity and recovery. Its energy consumption stays around the 40 $kWh/m^3$ in most cases. Only a small part, around 1.38 $kWh/m^3$ or even less has to be mechanical energy. All others can come from waste heat. If the other operating costs are kept low, it is an exciting technology to use, especially for brine treatment since waste heat is an inexpensive energy source. Some researcher claim it is a free energy source, but that statement cannot be valid. To use waste heat, infrastructure is necessary to collect it and transport it. It is not the same as putting a plug in the wall. There is probably enough waste heat in the world to close the water gap. As an example, the Netherlands is the second biggest producers of waste heat in Europe and creates enough to produce probably more 850 $km^3$ potable water. Unfortunately, there are some uncertainties. Waste heat is only classified in three rough categories, low, medium or high. Low is everything below 100 degrees Celcius. It could be that most energy is trapped in waste heat with a temperature of 30 degrees. More validation is necessary since the future of adsorption desalination is intertwined with waste heat. Without waste heat, it cannot be an inexpensive desalination technique. Since it is still an experimental technology, not a lot of data is available to create a solid picture of the costs involved with adsorption desalination. Only two studies exist today, and both estimate that the cost for seawater desalination using adsorption desalination is between 50% and 70 % of the cost while using reverse osmosis. Only one study gives insight into the different sectors of cost. What is notable is that the other costs, replacement cost for parts and chemical cost for adsorption desalination are kept at zero. A strange conclusion since silica gel can degrade and replacement can be necessary, chemicals are necessary for the heating and cooling water AD uses. No conclusion can be made about the final costs of AD, but it the conclusion of these studies seems unlikely. For seawater desalination, the costs are probably in the same range as reverse osmosis. Furthermore, there is a strong case that AD can remove biological pollutants since it is an evaporation technique where temperatures are reached until 80 degrees Celcius. There are a few remarks and only solid empirical data can prove these claims. The original intent of this research was to research these claims. Unfortunately, the set-up was not functioning properly due to practical problems like air leakages. Even with this result, much knowledge was gained, and a few remarks and conclusions can be made. To conclude, adsorption desalination is a newly emerging technology with many interesting aspects. It can provide an alternative for standard desalination techniques while being sustainable. There are a few barriers to overcome, but it deserves attention. Further developments should be stimulated since the water should be close in an environmentally friendly matter. Adsorption desalination is a sustainable solution that should always be considered.

Municipal and industrial wastewater contains a significant amount of dissolved nitrogen. This is the results of the organic protein degradation and of the large employment of nitrogen (N), usually in the form of ammonia (NH₃), in the industry. Currently, nitrogen is removed in wastewater treatment plant (WWTP) by means of biological treatments. Globally, the most applied treatment consists in the combination of nitrification and denitrification. This technique is characterized by a high energy demand (44.3 MJ per Kg_NH4⁺ removed)  which accounts for 70% of the total energy consumption in the WWTP. A less energetically intensive alternative treatment is the Anammox process. However, this also presents significant limitations, especially when dealing with extremely polluted stream and in the flexibility of operation.
An alternative to reduce the energy cost of nitrogen removal by valorizing the content of residual streams is explored by the N2kWh project. The novel concept underlying the N2kWh project aims to use NH₃ from waste streams as a fuel source for a solid oxide fuel cell (SOFC). In this way, the energy cost for biological N-removal processes is cut, and, ideally, energy can be even produced. WWTP digested reject water was recognized as potential N-source for energy recovery as a result of its relatively high total ammonia nitrogen (TAN) concentrations (up to 1.5 g − N ⋅ L^-1). A (selective) concentration step and pH regulation are needed in order to convert the TAN in the reject water, mostly present in form of ammonium bicarbonate (NH₄HCO₃), into NH₃ gas which can be stripped and directly fed in theSOFC. Previous studies proved electrodialysis (ED) to be the most energetically efficient technology for the removal and concentration of ammonium bicarbonate. However, due to the high alkalinity in the obtained concentrate, a large amount of chemicals is required for the pH regulation. An interesting opportunity to avoid this chemical addition is the employment of a bipolar membrane electrodialysis (BPMED). Through this unique technology, it is possible to simultaneously achieve the concentration of TAN, as well as the regulation of pH without chemicals.
The main objective of this work was to evaluate the potential of BPMED for the recovery of NH₃ in the boundaries of the N2kWh project. The energy performance of BPMED was compared with the alternative use of conventional ED plus sodium hydroxide for pH control. In order to obtain valuable information on the operation, three setups were employed: regular ED, BPMED and BPMED coupled with two membrane vacuum stripping devices (VMS). For this purpose, a synthetic solution obtained by dissolving 6.6 g ⋅ L^-1 NH₄HCO₃ salt in demiwater was used to simulate the digested reject water. Experiments were designed to characterize the BPMED operation and clarify which processes influence the performance.
Experimental laboratory results showed that, in terms of energy consumption for the NH₄⁺ removal, the BPMED used more energy compared to the ED. The removal of 90% of the initial NH₄⁺ is achieved by using an average of 13.2±0.1 MJ ⋅ Kg_NH4^-1. This value was 3.3 times higher than that achieved with regular ED. This discrepancy was explained by the extra-elements in the stack and the water dissociation process, responsible for the higher voltage measured during BPMED operation. The energy consumption was proven to be higher also as a consequence of the lower current efficiency for salt transport (73% compared to 95% of conventional ED). This was due to the more severe undesired diffusion processes, mainly gas diffusion and hydroxide leakage from the alkaline stream to the diluate, taking place within the BPMED stack. However, the energy consumption for ammonium removal in BPMED was still more than three times lower than the energy consumed by nitrification-denitrification and comparable to what used by anammox. 
The implementation of the membrane vacuum stripping modules, in series with the BPMED, only slightly increased the overall current efficiency (up to 3.4 % current efficiency gain). The benefit in current efficiency was less significant for higher concentrations and pH in the alkaline stream. Besides that, the tested technology tandem (BPMED+VMSs) was capable of stripping NH₃ and, consequently, reduce the gas diffusion over the stack. The energy consumed for the production of NH₃ and for the ED operation to reach a pre-defined pH and TAN concentration was estimated to be equal to 82.55 Wℎ per L of concentrate. This value was substantially higher than what consumed by BPMED (48.7 Wℎ) to achieve the same result in the alkaline stream.
To conclude, the studied BPMED system was demonstrated to be superior in terms of energy consumption for the recovery of NH₃ gas when compared to the combination of ED and chemical addition. BPMED showed also crucial advantages for the environment, design and safety of the treatment facility. The energy consumption for the removal of nitrogen with BPMED was proven to be lower than what used in the competitive biological technology. Finally, neither the technology nor the operation methods employed were specifically designed/optimized for this application. This made the use of BPMED for the N2kWh purpose even more interesting and promising.

Water as a coolant for industrial products or processes is widely used. Efficient operation of Open Recirculating Cooling Water Systems (ORCWS) relies on controlled cooling water conditioning programs that prevent corrosion, fouling, scaling and microbial growth. Water intensive industries feel greater pressure to decrease their water demand as water resources become increasingly stressed and discharge restrictions become more stringent. Soft alkaline cooling water (SACW) might be a good alternative for the conventional water and chemicals intensive ORCWS operation. However, this type of water requires complete reevaluation of the conditioning programs. Therefore, this research focused on the corrosion processes in SACW and a beginning was made in evaluating non-toxic and phosphorus-free corrosion inhibitors.

The research comprised the production of synthetic cooling water (SCW) and the evaluation of corrosion of mild steel in this type of water, with and without inhibitors. The selected corrosion inhibitors were NovoTraqua® without phosphorus (NT0) and sodium silicate (Si200). PHREEQC simulation were used to validate the SCW composition. A desk study was carried out to evaluate the applicability of corrosion indices. The average corrosion rate and inhibition efficiency were determined by mass loss tests of mild steel specimens with an immersion time of up to 4 weeks. The development of passivity was followed by open circuit potential (OCP) measurements. The pitting susceptibility was tested using cyclic potentiodynamic anodic polarization (CPP) measurements. 3D digital microscopy, SEM/EDS and XRD were used to evaluate the surface morphology and the composition of the top layer.

The addition of NT0 or Si200 to the SCW resulted in the inhibition of uniform corrosion but no complete protection against localized corrosion was realized. Si200 was the best performing inhibitor, also for inhibiting crevice corrosion. The immersion of mild steel in SCW resulted in natural passivation by the formation of a Fe2O3 oxide layer. The SCW did not have the ability to immediately repassivate the specimens after localized breakdown of the protecting passivating layer. The addition of NT0 or Si200 reduced the aggressiveness of the environments at the pits but did not accomplish complete repassivation.

Lab experiments with SCW can provide initial insights in the behavior of mild steel and can contribute to the decision making in more cost intensive research on alternative operation of ORCWS. Better understanding of the protection mechanisms would be obtained when more advanced techniques like Fourier Transform Infrared Spectroscopy (FTIR) and Electrochemical Impedance Spectroscopy (EIS) are used. Although cooling water corrosion inhibition by means of silica is a mature research topic, it is recommended to proceed with research on silica as a corrosion inhibitor in the specific conditions of SACW.

Brine that is produced during reverse osmosis (RO) could be further concentrated by another RO step. In the absence of bivalent cations, silica is a limiting factor because a high concentration of silica can cause irreversible fouling of the membranes. To prevent silica scale formation during the further concentration of brine with RO, silica should be removed in a separate process. Silica gel beads have a large specific surface area and can be used to precipitate silica. Previous research on the removal of silica using silica gel beads for solutions with a relatively high silica concentration showed that a removal of 30-60% is possible.
In this research the effect of the dosage of silica gel beads, pH, temperature and the presence of 1 or 10 mg/L of ferric and aluminum on the removal of silica using silica gel beads from a solution with a low silica concentration was investigated. Starting with a silica concentration of 180 mg SiO₂/L a removal up to 30% was obtained after 8 hours (pH 8.5, 21°C).
The removal of silica using silica gel beads was limited, because silica gel dissolved when the solution is unsaturated with regard to silica. The equilibrium concentration was approximately 102 mg SiO₂/L. The difference between the concentration of silica in solution and the equilibrium concentration appeared to be a driver of the removal. The increase in removal efficiency as a result of using a higher dose of silica gel seemed to be limited. Operating at pH 10 instead of 8.5 resulted in a lower removal efficiency. Lowering the pH to 7 or 5 did not result in a decrease of the removal efficiency. Changing the temperature to 10 or 35°C or adding 1 or 10mg/L of ferric or aluminum did not increase the removal efficiency either, but even decreased the efficiency.
The obtained removal efficiency might be too low for direct application of fixed bed column filtration with silica gel beads as interstage silica removal technique in a RO system that is used to concentrate brine. A combination of filtration with silica gel and a technology that can concentrate the brine without scaling to concentrations up to 500 mg SiO₂/L could be an interesting solution to make brine treatment more energy-efficient, because the need for an evaporation step is minimized.

This thesis provided an overview of the achievable ammonium concentrations, and the energy used to achieve these concentrations, by ion exchange, electrodialysis and reverse osmosis. The need for this overview arose during research for the N2kWh project, where one step in the process focused on increasing the ammonium concentration of (anaerobic digestion) reject water. The research presented some knowledge gaps. As available reports focused on the ammonium removal from the solution, the concentration in the residual stream was barely mentioned. Even when concentrations were reported, the conditions between the discussed technologies made it hard to make a comparison between the three technologies. Even less was reported on the energy that was needed to achieve the various concentrations.

To compare the three technologies, a 1.5 g NH4+/L solution was used as feed water for all three technologies. The technologies used different techniques to increase the ammonium concentration (using zeolites, electrical current over a membrane or high pressure through a membrane). This made comparing the technologies, very difficult. By using the same feed water for all experiments, the achieved concentrations and the energy that was used in the process could be used to compare the three technologies.

The highest ammonium concentration that was achieved during the experiments (7.1 g NH4+/L) was produced by Electrodialysis. Ion exchange achieved a concentration of 6.0 g NH4+/L, while a laboratory scale Reverse Osmosis achieved 4.5 g NH4+/L. The amount of energy used by the three technologies to produce 6 g NH4+/L was: ion exchange = 0.24 kWh, electrodialysis = 6 kWh, reverse osmosis = 4 kWh (per m3 of produced concentrated NH4+ solution). ROSA software was used to estimate energy use for Reverse Osmosis to achieve 6 g NH4+/L (as the laboratory scale Reverse Osmosis was unable to provide sufficient pressure to achieve this concentration). So ion exchange was the most energy efficient, but the maximum achievable ammonium concentration was the lowest of the three technologies. Electrodialysis was able to achieve the highest ammonium concentration but was less energy efficient and used more energy in the process.