Resources recovery from wastewater is pursued as it facilitates a circular economy, which a society runs with a minimal environmental impact. Eutectic freeze crystallisation (EFC) has been identified as a potential technology that can assist to achieve such goal. High quality of salt and ice can be retrieved with this treatment with a note of high energy efficiency. In this paper, based on an actual case of a Spanish silica production company, freeze and eutectic freeze crystallisation are found to be applicable to a complex RO concentrate, which involved Na, Mg, Ca, Sr, Ba, SiO2. Quality ice and mirabilite products were also produced, which the impurities attached can be further washed away easily. The influence of the impurities on the eutectic point of sodium sulphate solution is investigated. A conservative recovery rate of 42wt% and 55wt% are obtained respectively for mirabilite and ice. Heat transfer characteristics of the equipment are also conducted, the boundary layers in the working solution and in the coolant have a clear influence on the overall heat transfer. A heat transfer of 404W/K-m2 is obtained for the EFC cooling, which is found to be comparable to past study. Energy consumption for a complete EFC process on the RO concentrate is estimated to be 0.1437 kWh/kg-solution, which is only 22% of the energy consumed in heat crystallisation. Column crystalliser is suggested to be a possible method to resolve impurities accumulation issue. Torque was also identified to indicate nucleation and crystallisation during the process. A crystal size distribution model is developed and can be used to evaluate or predict the performance of an EFC system. During the thesis study, a successful demonstration and training was provided to the staff from Fundació CTM Centre Tecnològic, which is the respondent for the Spanish case and will later perform a EFC experiment in the treatment plant.

Water scarcity emphasizes the necessity of water recovery from saline sources, which are plentifully available. Nanofiltration (NF) is widely implemented in water reclamation treatment particularly in industrial field. However, the presence of silica hinders the possibility of obtaining a high recovery in a NF membrane system. In this work, the effect of silica concentration on the scaling of a NF membrane was investigated. Three types of feed water (demineralized water, water with 100 mg L-1 and 300 mg L-1 of SiO2) were fed to a single stage nanofiltration element. All cases were carried out at room temperature, pH values ranged 7 - 8 and a constant recovery of 11±1% for 24 hours continuously. The mass transfer coefficient (MTC) was used as method to determine the scaling of the membrane module. Results showed that no silica scaling occurred during the system runtime. Nonetheless, for all cases, an initial decrease in MTC could be detected. On the one hand, based on some literature reports, it is likely that a part of silica (mainly colloid) had precipitated on the membrane surface forming a more porous layer than the precipitation of polymerized monomeric silica. But to verify this argument, a further autopsy method is demanded. On the other hand, concentration polarization may also contribute to the initial decline in mass transfer coefficient. But the reason for the decrease for demi-water case remains unknown. In brief, Nanofiltration membrane can operate at a silica concentration up to 300 mg L-1 for 24 hours in lab-scale. The guideline (120 mg SiO2 L-1) suggested in the industrial standard (ASTM, D4933-08) was judged to be relatively conservative. For more practical purpose, further study is needed.

To prepare for future challenges, such as possible upcoming organic micro pollutants in the source water, including traces of medicines, pesticides, and industrial by-products, it is expected that the conventional treatment does not ensure a reliable quality of drinking water. As a result, during the past few years, Oasen started to research a new treatment concept based on 100% reverse osmosis (RO) membrane filtration to provide an excellent barrier for organic micro pollutants. However, the water produced by the RO membranes, called permeate, is corrosive, bitter in taste and does not comply with the drinking water regulation standards in the Netherlands. Therefore, a certain degree of remineralisation is crucial to solve these problems and improve the water quality. A commonly used remineralisation process is to filter the desalinated water through a calcite contactor, providing the appropriate amount of bicarbonate and calcium in the water. In order to properly design and operate the calcite filters as well as to predict the final water quality, it is essential to understand the processes that occur in the filter.
The aim of this study was to find the best kinetic calcite dissolution model in order to understand the calcite grains dissolution behavior inside the filter and subsequently to adequately design and operate the calcite filter. Therefore, extensive pilot research was conducted to investigate the effect of various parameters on calcite dissolution such as the calcite grain size, velocity and carbon dioxide concentration. On top of that, the dissolution was modelled based on a successful empirical expression given by Yamauchi et al. (1987). However, it was found that the effect of the flow rate on the diffusion boundary layer around the calcite grains has not been taken into account in the study carried out by Yamauchi et al. (1987). Therefore, the effect of velocity on the calcite dissolution coefficient was investigated at five different velocity ,i.e., 5, 10,15,20,30 m/h. From there, a function was developed to describe the correlation between flow rate and the dissolution rate coefficient. In order to calculate the equilibrium concentration, the chemical reactions were simulated using PhreeqPython (Phreeqc built in Python).
The main difference of this study compare to previous investigations was the low temperature of the water (12 oC vs 22-40 oC) and the smaller grain size of the calcite (0.5-1.2 mm vs 1-2 and 2-3mm) which was tested. Besides that a high range of CO2 dosing (1.45- 9.5 mmol/l) was tested. As expected from theory, the dissolution rates was strongly affected by the varied parameters. It is concluded that the smaller grain size of 0.5-1.2 mm reduced the required empty bed contact time (EBCT) to15 min where operating the filter with the larger grain size of 1-2mm needs a minimum EBCT of 25 min to reach calcite equilibrium. The CO2 dosing is recommended to be less than 3 mmol/l, since the CO2 efficiency will drop under the 60% at higher CO2 concentrations.

Eventually, the optimal design will be introduced for the remineralisation process at Oasen treatment plant De Hooge Boom located in Kamerik. For this purpose various operational scenarios were compared on capital and operational cost. The overall cost including, both Capital expenses (CAPEX) and Operational expenses OPEX, was estimated between € 0.048 and 0.064 per m3 for different scenario’s where 71% consists of investment cost. The total treatment cost of this design is 0.057 €/m3 and the investment cost was found to be € 1.351.000 which is 32% less than price estimated by previous study done by Oasen.

Worldwide fresh water scarcity is nowadays becoming a reality rather than a futuristic concern. Non-conventional sources of wastewater are being considered as a solution for this problem, since fresh water sources will not be sufficient to satisfy the world’s water demand in the following years. Extensive research is being done on different wastewater treatment technologies and methodologies, and lately, reverse osmosis (RO) membrane filtration has caught more attention as a solution for reclaiming and reusing wastewater. Nevertheless, the biggest problem RO membranes face is fouling, which increases energy consumption, and therefore, costs. Organic/particulate fouling affects the mass transfer of an RO membrane, but it can also contribute to biofouling, worsening the mass transfer and causing pressure drop on the RO system. For this study, a RO crossflow cell was used in order to investigate the effect of crossflow reversal on particulate fouling removal by testing two different feed spacers: (a) 1.25-mm high cavity spacer and (b) 0.71-mm high zigzag spacer. The experiments were carried out with an average crossflow velocity of 0.36 m/s, an average permeate flux of 20 L/m2 hr (LMH), an average feed electroconductivity (EC) of 14.90 mS/cm, and a feed temperature in the range of 21-25 C. Four runs were done, each with an average runtime of 1050 minutes. Two runs tested the cavity spacer, one with a crossflow reversal done at minute 806, and the other run with a crossflow reversal done twice every working day. The last two runs follow the same methodology but using the zigzag spacer. Each run used a new membrane and spacer. The mass transfer was graphed as a Mass Transfer Coefficient (MTC), and its development with time was analysed for identifying fouling, as well as particulate fouling when applying the crossflow reversal method. Crossflow reversal proved to be a reliable method for removing organic/particulate fouling. The cavity spacer caused a more severe organic/particulate fouling, but allowed it to be removed by crossflow reversal. The cavity spacer also proved to be less energy-consuming if applied to a full-scale RO facility. On the other hand, the zigzag showed to have a better effect on flow mixing and concentration polarization disruption inside the feed channel, thus keeping a higher mass transfer for a longer period of time. A more frequent crossflow reversal showed was not more efficient in removing organic/particulate fouling.