Newsletter Science & Technology

Dear all,

In just a few days, starting Monday October 1, 2012, the annual Wetsus Congress “Societal challenges: Call for innovative water technology” will be organized again in De Harmonie in Leeuwarden. Over 600 participants have already registered and we expect two full and exciting days covering all aspects of water technology, business, policy and science. For the complete program, please visit the following link. On-site registration will be possible. We hope to meet many of you next Monday and Tuesday !

www.wetsus.nl/business-educationevents/annual-wetsus-congress

As a runner-up to the Congress, it is our pleasure to introduce to you the first Wetsus Science&Technology Newsletter, showcasing a small selection of currently ongoing Wetsus research that has been published recently in peer-reviewed scientific journals. Science journalist Lisa Zyga helped us by interviewing the authors and “translating” the hard scientific content into a broader context story that we are sure you will find enjoyable to read.

In this first newsletter, we present scientific results from Delft University of Technology, from Wroclaw University of Technology, Poland, and from the FOM-institute AMOLF, Amsterdam, describing results ranging from two-dimensional modeling of biofouling in RO membrane modules, water desalination with wires, to the unexpected structure of water in the floating water bridge.

As you may understand, space is limited and this Newsletter can just cover a small cross-section of all the Wetsus research. However, in order for you to stay in touch and keep you updated on research progress within Wetsus, we are pleased to inform you that from now on Wetsus scientific papers are listed on an open-access webpage with a link to the article at the journal’s homepage. To view the list, please go to www.wetsus.nl/research/wetsus-scientific-publications.

The Wetsus Science&Technology Newsletter will be published three times per year, with the next issue published before Christmas this year. Any comments and suggestions you may have on the format and content of the Newsletter, we are pleased to receive via STnewsletter@wetsus.nl.

We wish you much joy in reading the S&T Newsletter, and hope to see you next week in Leeuwarden !

Kind Regards,

The Wetsus Science & Technology Newsletter Team,

Maarten Biesheuvel, Hester Henstra, Lisa Zyga & Cees Buisman.STnewsletter@wetsus.nl

Newsletter Science & Technology

Simulations of biofouling in RO membranes

Although reverse osmosis (RO) is one of the most widely used water desalination technologies today, the build-up of microorganisms on the RO membranes causes biofouling, which increases operational costs and reduces the amount and quality of water produced. To combat this problem, a team of researchers from Delft University of Technology has developed a mathematical model to investigate how hydrodynamic conditions and nutrient presence may affect biofouling.

In their study, performed within the Wetsus theme “Biofouling”, the researchers explored potential strategies that could result in biofilm removal to restore process efficiency. In addition, their simulation results can theoretically explain some recent experimental results and indicate new parameters that need to be measured for a better understanding of biofouling.

The researchers, Ph.D. student Andrea Radu, Dr. Hans Vrouwenvelder, Professor Mark van Loosdrecht, and Dr. Cristian Picioreanu, all at Delft University of Technology, have published their study on the biofouling simulations in RO systems in a recent issue of the Chemical Engineering Journal.

As the researchers explain, biofilms may develop in RO membrane systems from the start of operation, but they only begin to impact system performance when the biomass reaches a certain threshold volume and forms at certain specific locations. By keeping biofilms below this threshold amount and away from target locations, their effect on membrane performance can be minimized.

As the researchers showed in previous research, there are several possible ways through which a developing biofilm contributes to permeate flux decline, salt passage increase, and feed channel pressure drop increase – all processes that negatively affect the RO system’s overall performance.

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Illustration of biofilm development over time in the feed channel. Arrows indicate velocity vectors. At 72 h, biofilm colonies spread over the membrane surface. At 144-224 h, small biofilm streamers form on the spacers in the middle of the channel, then break off. Biofilm colonies on the membrane expand. At 224-232 h, a sloughing event occurs in a relatively short period, in which some biomass is removed from the channel. At 192-408 h, a quasi-steady state is achieved for biomass in the channel. Image credit: Radu et al. ©2012 Elsevier B.V.

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In the current study, the researchers used a two-dimensional mathematical model to analyze strategies that may reduce the impact of biofilms.

“The two-dimensional model proposed in this work emphasizes the complexities of flow pattern and solute distribution encountered in membrane processes and their potential effect on biofilm formation within the RO systems,” Radu said. “The outcome of our studies could give an indication regarding potential aspects that need to be considered and improved in the RO module design and operation for reduced biofouling.”

The simulations revealed that biofilm accumulation is more likely to occur in regions of low liquid shear stress and low flow velocity. By investigating various nutrient concentrations in the feed water and nutrient accumulation near the membrane surface, the researchers also found that a decrease in the amount of nutrients in the feed water seems only to help delay biofilm formation and its effects on process performance.

The numerical results agree with previous experiments that have found that biofilms tend to accumulate in certain zones and enhance the salt concentration polarization, which contributes to performance degradation.

In an attempt to find solutions to the biofilm problem, the researchers investigated the effects of different membrane cleaning strategies in their simulations. These strategies included increasing the flow velocity at different times in order to slough, or remove, part of the accumulated biofilm. For example, one strategy involves switching to operation at a higher flow velocity after significant biofilm accumulation. Another strategy is to use alternating periods of low and high velocity, promoting episodic sloughing.

Since increasing the flow requires additional energy input and comes at a cost of lower water recovery, these strategies do not necessarily provide a more economically efficient way of operation, the researchers noted. Identifying optimal operating conditions will involve a more detailed analysis of these costs.

“Biofouling is a complex issue, involving physical, chemical and biological factors,” Radu said. “Developing approaches to limit its consequences in RO plants is still a challenge. The mathematical model is a useful tool for guiding future experimental research needs by indicating specific situations in which biofilms are expected to impact process performance. Additionally, it can help clarify common misconceptions and provide a strong basis for the assessment of several theories.”

The researchers highlight the necessity of developing three-dimensional models to allow for better comparison with experimental results, although developing these models will not be a trivial task. Future modeling and experimental approaches may ultimately lead to an improved RO design with better biofilm tolerance, producing better quality water at a lower cost.

More information:A.I. Radu, J.S. Vrouwenvelder, M.C.M. van Loosdrecht, and C. Picioreanu. “Effect of flow velocity, substrate concentration and hydraulic cleaning on biofouling of reverse osmosis feed channels.” Chem. Eng. J. 188, 2012, 30. http://dx.doi.org/10.1016/j.cej.2012.01.133

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Water desalination simplified with pair of wires

In response to the increasing global demand for fresh water, researchers around the world have been developing more efficient techniques to desalinate salt water. In a recent study, a team of scientists led by Slawomir Porada, a PhD student at Wroclaw University of Technology, Poland, has shown how brackish water can be transformed into potable fresh water using just a pair of wires and a small voltage that if necessary can be generated by a small solar cell. The biggest advantage of the wire method is that it minimizes the mixing between the treated and untreated water, resulting in a highly efficient process.

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An illustration of the desalination process shows the treated water and brine in separate streams. The close-up of the wires (top circle) shows salt ions being attracted to and adsorbed by the wire electrodes. The close-up of an individual wire electrode (bottom circle) shows ions being stored in the porous electrode.

(a) Seven pairs of graphite rods/wires are dipped into brackish water. (b) An electrical voltage difference is applied between the anode and cathode wires via copper strips, causing the electrodes to adsorb salt ions. (c) Scanning electron microscopy image of the membrane-electrode assembly.

In the paper published in The Journal of Physical Chemistry Letters, Porada and colleagues, performing their research at Wetsus, designed a device that consists of two thin graphite rods or wires, which are inexpensive and highly conductive. The researchers coated both wires with a porous carbon electrode layer that allows ions to be stored inside the nanopores. Then the wires were clamped a short distance apart in a plastic holder, with each wire squeezed against a copper strip.

To activate the wires as electrodes, the researchers dipped seven pairs of wires into a container of brackish water and ran electrical wires from the copper strips to an external power source. Upon applying a small voltage difference (1-2 volts) between the two graphite wires of each wire pair, one wire became the cathode and adsorbed the sodium cations, while the other wire became the anode and adsorbed the chlorine anions from the salty water.

To remove the ions from the nanopores in the wires, the researchers manually lifted the wires out of the once-treated solution and dipped them into another container that became brine. When the voltage was removed, the wires released the stored ions into the brine, increasing its salinity.

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By repeating this cycle eight times, the researchers measured that the salt concentration of the original brackish water, 20 mM, is reduced to about 7 mM. Potable water is considered to have a salinity of less than roughly 15 mM.

The new technique’s biggest advantage lies in the fact that mixing between the treated water and brine can be minimized since the two types of water are split in separate containers. Only about 0.26 mL of brine per electrode is transferred between containers between cycles, which is much less than the mixing that occurs in other techniques where the treated water and brine are in the same container, limiting the efficiency.

The new technique can also be made very inexpensive, since the device consists merely of carbon rods or wires to conduct the electrons, onto which the activated carbon slurry is simply painted to create the porous carbon electrode. The wire pairs can be used repeatedly without degradation, giving the device a long lifetime.

“Because of its simplicity and low cost, this method might out-compete state-of-the-art technologies for certain applications, and may also have advantages over capacitive deionization (CDI or cap-DI), a novel water desalination method which has recently become commercially available,” Bert Hamelers of Wetsus said. “Also, the voltage required is low, just 1.2 V for instance, and DC (direct current), perfectly compatible with solar panels. Thus it can be used at off-grid or remote locations.”

Although methods such as distillation and reverse osmosis are still superior for desalinating sea water (500 mM salinity and higher), the new technique is more suitable for brackish groundwater of 100 mM salinity or lower. Consumer applications may include removing so-called “hardness ions” from drinking water, while industrial uses may include treating waste water so that it can be reused. As a result, industries would no longer need to take in new fresh water nor dump waste water at high financial penalty.

The researchers also found that the method’s efficiency could be improved by adding a membrane coating to the electrodes. For instance, a cationic membrane on the cathode wire has a high selectivity toward sodium cations while blocking the desorption of chlorine anions from within the electrode region. As a result, cationic (and, on the anode wire, anionic) membranes could enable the electrodes to adsorb and remove more ions than before.

The researchers predict that these improvements could increase the desalination factor from 3 to 4 after eight cycles, with 80% of the water being recovered (i.e., 20% of the original water becomes brine).

The researchers also want to use the technique to treat large volumes of water, which they say could be done by automating the process and using many wire pairs in parallel.

“This research continues by testing better membranes and by packing the wires more closely,” Porada said. “We also want to test ‘real’ ground/surface waters, not only artificial simple salt mixtures as tested now.”

More information:S. Porada, B.B. Sales, H.V.M. Hamelers, and P.M. Biesheuvel. “Water Desalination with Wires.” J. Phys. Chem. Lett. 3, 2012, 1613.http://dx.doi.org/10.1021/jz3005514

Newsletter Science & Technology

Scientists may have discovered new kind of water in water bridge phenomenon

In the water bridge phenomenon, a few kilovolts applied between two beakers of water causes the water to climb out of the beakers and form a freely hanging string in the air that connects the two beakers. A team of researchers at the FOM Institute for Atomic and Molecular Physics – AMOLF in Amsterdam in collaboration with Wetsus has now demonstrated that the water in the water bridge may differ from ordinary bulk water on a molecular scale. A parallel study by Wetsus researchers in collaboration with NASA Ames, the US space agency research centre located in Mountain View, California, have found evidence to suggest that the water inside our bodies may be more similar to bridge water than bulk water.

The water bridge phenomenon is not new. In 1893, the British engineer Sir William Armstrong discovered that, by placing a cotton thread between two wine glasses filled with chemically pure water, and applying a high voltage, the water would rise to the edges of the glasses and flow along the cotton thread. Eventually the cotton thread was pulled into one of the glasses, leaving a water bridge between the two glasses for a few seconds.

The water bridge was widely forgotten until 2007, when Wetsus researcher Elmar C. Fuchs and his coauthors rediscovered the phenomenon and ignited new research interest.

One of the first video demonstrations of the floating water bridge, recorded by Elmar C. Fuchs and Jakob Woisetschläger in 2007.

In the past five years, various research teams have investigated many of the water bridge’s macroscopic properties, such as its mass, density, and temperature gradients. Studies have shown that the bridge’s stability (it can last for several minutes) can be explained in the framework of electrohydrodynamic theory due to the balance between polarization forces, capillary forces, and gravity. But whether the water bridge differs from ordinary bulk water on the molecular scale as well as the macroscopic scale is still an open question.

In their recent study, published in Physical Chemistry Chemical Physics (PCCP), FOM researchers Lukasz Piatkowski, Hinco Schoenmaker and Huib Bakker collaborated with Wetsus scientists Elmar Fuchs and Adam Wexler to investigate this question.

Using a laser, the researchers excited the water bridge’s water molecules and then measured how quickly the molecules relaxed to their ground states. In this process, called vibrational energy relaxation, the molecules’ excess energy dissipates by being transferred to the lower-energy states in the same molecule or in surrounding molecules

The floating water bridge occurs when a high potential difference is applied between two beakers of water.

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until the entire system reaches a state of thermal equilibrium. The water’s molecular structure may affect how quickly energy is transferred, thereby changing the energy relaxation rate.

In their experiments, the scientists did observe a significantly faster vibrational energy relaxation time in bridge water (about 630 fs) than bulk water at 0°C (about 740 fs, which agrees with previous experiments). As shown in the figure, the bridge water relaxation time lies in between the relaxation times of water and ice at 0°C (384 fs). The 630 fs time has not been previously observed for bulk water at any temperature.

The scientists wondered if the difference may be due to the electric field or the field-induced higher temperature, but further tests ruled out both possibilities. In fact, a higher temperature should slow down the relaxation rate, not accelerate it, making the fast relaxation rate even more surprising.

The scientists observed a second difference between bridge water and bulk water, which is the time it takes for the system to reach equilibrium, the process that occurs immediately after vibrational relaxation. For the water bridge water, this time was 1,500 fs, which is much longer than the 250 fs it took for bulk water to reach equilibrium.

Although the effects are opposite (the vibrational relaxation is accelerated while the equilibration is delayed), the researchers think that both changes indicate a different structural arrangement of water molecules in the water bridge compared to bulk water.

“According to previous calculations, much higher field strengths are necessary to influence the microscopic structure of water,” Fuchs said. “These experiments on the floating water bridge make it clear that even

much weaker fields than previously thought are sufficient to significantly influence the microscopic behavior of water molecules.”

In the future, the scientists plan to investigate whether a partial orientation of the water dipole could explain both effects. They hypothesize that a partial orientation would reduce the degrees of freedom of the water molecules, allowing lower-energy modes to accept vibrational energy more quickly while slowing down the equilibration of the energy that occurs over many water molecules.

Although it may first seem that bridge water is a rare oddity, the Wetsus researchers discovered another surprise when they found that the water bridge is compatible with living systems. In collaboration with NASA researchers, the researchers published a study in Physical Biology in which they added bioluminescent bacteria to the beakers and monitored the bacteria’s movement through the bridge water, a seemingly hostile environment due to its high voltage. Unexpectedly, the bacteria that went through the bridge water actually showed more active behavior afterwards than the bacteria that remained in the beaker and a control group that was not exposed to any voltage.

Maybe the results should not be surprising, however, since in 2007 scientists measured that the electric potential in living cells is between 50 and 300 kV per cm, which is at least 10 times higher than the potential in the water bridge. The results suggest that, contrary to popular belief, water in living cells may not be ordinary bulk water, as Fuchs explains.

The vibrational lifetime of bridge water is in between those of water and ice at 0°C.

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“Is it possible that the water bridge water is more similar to water in living cells than ordinary water, and that this is the reason why the bacteria become more active if they are transported through the bridge?” Fuchs asked. “Could this mean that biochemical reactions can be recreated more accurately in bridge water than in bulk water? Further investigations are definitely required in order to answer these questions.”

More information:L. Piatkowski, A.D. Wexler, E.C. Fuchs, H. Schoenmaker, and H.J. Bakker. “Ultrafast vibrational energy relaxation of the water bridge.” Phys. Chem. Chem. Phys. 14, 2012, 6160.http://dx.doi.org/10.1039/C1CP22358E

A.H. Paulitsch-Fuchs, E.C. Fuchs, A.D. Wexler, F. Freund, L.J. Rothschild, A. Cherukupally and G.-J. W. Euverink. “Prokaryotic transport in electrohydrodynamic structures.” Phys. Biol. 9, 2012, 026006.http://dx.doi.org/10.1088/1478-3975/9/2/026006

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