Marjolein Vanoppen

Properties governing the transport of trace organic contaminants through ion-exchange membranes

Ion exchange membranes could provide a solution to the selective separation of organic and inorganic components in industrial wastewater. The phenomena governing the transport of organics through the IEM however, are not yet fully understood. Therefore, the transport of trace organic contaminants (TOrCs) as a model for a wide variety of organic compounds was studied under different conditions. It was found that in the absence of salt and external potential, the chemical equilibrium is the main driver for TOrC-transport, resulting in the transport of mainly charged TOrCs. When salt is present, the transport of TOrCs is hampered in favor of the NaCl transport, which shows a preferential interaction with the membranes due to its small size, high mobility and concentration. It is hypothesized that electrostatic interactions and electron donor/acceptor interactions are the main drivers for TOrC transport and that transport is mainly diffusion driven. This was confirmed in the experiments with different current densities, where the external potential seemed to have only a minor influence on the transport of TOrCs. It is only when the salt becomes nearly completely depleted that the TOrCs are transported as charge carriers. This shows that it is very difficult to get preferential transport of organic compounds due to the diffusive nature of their transport.

Increasing RO efficiency by chemical-free ion-exchange and Donnan dialysis: Principles and practical implications.

Ion-exchange (IEX) and Donnan dialysis (DD) are techniques which can selectively remove cations, limiting scaling in reverse osmosis (RO). If the RO concentrate could be recycled for regeneration of these pre-treatment techniques, RO recovery could be largely increased without the need for chemical addition or additional technologies. In this study, two different RO feed streams (treated industrial waste water and simple tap water) were tested in the envisioned IEX-RO and DD-RO hybrids including RO concentrate recycling. The efficiency of multivalent cation removal depends mainly on the ratio of monovalent to multivalent cations in the feed stream, influencing the ion-exchange efficiency in both IEX and DD. Since the mono-to-multivalent ratio was very high in the waste water, the RO recovery could potentially be increased to 92%. For the tap water, these high RO recoveries could only be reached by adding additional NaCl, because of the low initial monovalent to multivalent ratio in the feed. In both cases, the IEX-RO hybrid proved to be most cost-efficient, due to the high current cost of the membranes used in DD. The membrane cost would have to decrease from ±300 €/m² to 10-30 €/m² - comparable to current reverse osmosis membranes - to achieve a comparable cost. In conclusion, the recycling of RO concentrate to regenerate ion exchange pre-treatment techniques for RO is an interesting option to increase RO recovery without addition of chemicals, but only at high monovalent/multivalent cation-ratios in the feed stream.

Salinity gradient power and desalination

The use of alternative water resources will become ever more important in the future, increasing the need for water desalination. Conventional desalination technologies, that is, multistage flash distillation, multieffect distillation, electrodialysis, and reverse osmosis, are well established but require high energy consumption. The coupling of these processes with salinity gradient power (SGP) offers the double benefit of decreasing the energy demand by the production of salinity gradient energy and by decreasing the salt concentration of the feed prior to desalination. Opportunities and challenges to combine SGP and desalination will be discussed in this chapter.

Refinery and concentration of nutrients from urine with electrodialysis enabled by upstream precipitation and nitrification

Human urine is a valuable resource for nutrient recovery, given its high levels of nitrogen, phosphorus and potassium, but the compositional complexity of urine presents a challenge for an energy-efficient concentration and refinery of nutrients. In this study, a pilot installation combining precipitation, nitrification and electrodialysis (ED), designed for one person equivalent (1.2 Lurine d-1), was continuously operated for ∼7 months. First, NaOH addition yielded calcium and magnesium precipitation, preventing scaling in ED. Second, a moving bed biofilm reactor oxidized organics, preventing downstream biofouling, and yielded complete nitrification on diluted urine (20-40%, i.e. dilution factors 5 and 2.5) at an average loading rate of 215 mg N L-1 d-1. Batch tests demonstrated the halotolerance of the nitrifying community, with nitrification rates not affected up to an electrical conductivity of 40 mS cm-1 and gradually decreasing, yet ongoing, activity up to 96 mS cm-1 at 18% of the maximum rate. Next-generation 16S rRNA gene amplicon sequencing revealed that switching from a synthetic influent to real urine induced a profound shift in microbial community and that the AOB community was dominated by halophilic species closely related to Nitrosomonas aestuarii and Nitrosomonas marina. Third, nitrate, phosphate and potassium in the filtered (0.1 μm) bioreactor effluent were concentrated by factors 4.3, 2.6 and 4.6, respectively, with ED. Doubling the urine concentration from 20% to 40% further increased the ED recovery efficiency by ∼10%. Batch experiments at pH 6, 7 and 8 indicated a more efficient phosphate transport to the concentrate at pH 7. The newly proposed three-stage strategy opens up opportunities for energy- and chemical-efficient nutrient recovery from urine. Precipitation and nitrification enabled the long-term continuous operation of ED on fresh urine requiring minimal maintenance, which has, to the best of our knowledge, never been achieved before.

Assisted reverse electrodialysis : principles, mechanisms, and potential

Although seawater reverse osmosis (RO) is nearing its thermodynamic minimum energy limit, it is still an energy-intensive process, requiring 2–3 kWh/m³ at a recovery of 50%. Pre-desalination of the seawater by reverse electrodialysis (RED), using an impaired water source, can further decrease this energy demand by producing energy and reducing the seawater concentration. However, RED is hampered by the initial high resistance of the fresh water source, resulting in a high required membrane area (i.e., high investment costs). In this paper, a new process is presented that can overcome this initial resistance and decrease the RED investment cost without the need for additional infrastructure: assisted RED (ARED). In ARED, a small potential difference is applied in the direction of the natural salinity gradient, increasing the ionic transport rate and rapidly decreasing the initial diluate resistance. This decreasing resistance is shown to outweigh any negative effects caused by, for example, concentration polarization, resulting in a process that is more efficient than theoretically expected. As this effect is mainly important at low diluate concentrations (up to 0.1 M), ARED is proposed as a first step in an economic and energy efficient (A)RED-RO hybrid process.Seawater desalination: assisting reverse osmosisCoupling reverse osmosis with assisted reverse electrodialysis can reduce the cost of seawater desalination. While reverse osmosis currently accounts for more than 60% of our worldwide seawater desalination capacity, this crucial process operates at a high energy demand. A reverse electrodialysis (RED) pre-treatment of seawater, diluted with a waste water stream, reduces the energy demand by producing energy and by reducing the concentration of the seawater subjected to reverse osmosis. Nonetheless, low transport rates in RED require high-membrane surface areas, making the costs impractical. A team led by Marjolein Vanoppen at Gent University in Belgium design an assisted RED pre-treatment process, where a small potential difference is applied in the direction of the salinity gradient, increasing the ionic transport rate and decreasing the required membrane surface area, thus offering a more economically viable alternative.