European centre of excellence
for sustainable water technology

Science & Technology newsletter, September 2014

Published: 12 september 2014

Dear reader of the Wetsus S&T Newsletter, Edition September 2014,

First of all we would like to invite you to the annual Wetsus Congress, on October 6 and 7, 2014. Please find the program

The past period has been another exciting period for Wetsus, with many new scientific discoveries, some published and some still works-in-progress. And all the while we see from our present location our new Wetsus building getting more and more finished, and we look forward to starting our work early next year in such a beautiful, modern and inspiring architectural wonder.

But back to science! In this S&T Newsletter you can read about some of our scientific discoveries recently made. We have chosen to inform you of the results achieved by the team of Prof. GeertJan Witkamp (TU Delft), the team of Prof. Grietje Zeeman (WUR), and the team of Prof. Kitty Nijmeijer (Twente), in all three cases based on research performed in the Wetsus Laboratories. In addition, Prof. David Waite (Sydney, Australia), who chaired the IAP-conference that Wetsus organized this summer, will give his frank opinion about the field of water science and the important role Wetsus plays therein.

The three scientific papers discussed in this Newsletter are only three examples of the large output of Wetsus research published this year (more than 50 papers published until now, a record number for Wetsus). For a complete overview of all published papers, please go here.

Certainly more important than papers, are people. And the coming two months are very special in this respect. In the month of September, four Wetsus PhD students defend(ed) their thesis in Delft or Twente: Thijs van Leest, Yusuf Wibisono, Natalia Hoog Antonyuk, and Paula van den Brink. Then, on Friday, October 10, 2014, three PhD defenses of Wageningen University are scheduled to happen in Leeuwarden. A unique event, where (if all goes well) Wageningen University formally grants the PhD degree to three of their students, at a location outside university premises. In the history of scientific PhD defenses, this may never have happened before, and this will certainly be a most memorable event. For further information, see here.

Finally, I would like to bring to your attention to two important Wetsus-organized conferences. In just one month from now our annual Wetsus Congress will be held in De Harmonie, Leeuwarden, on Monday, October 6 and Tuesday, October 7, 2014. This year’s theme is “From Knowledge to Business”. Keynote speakers include:
- Prof.Dr. Rob Hamer, Director R&D of Unilever Vlaardingen, “Sustainable use of water, the need for new technologies and new habits”
- Prof.Dr. Alexander Friedrich, University Medical Centre Groningen, “Antimicrobial resistance in a connected world: a case for intersectoral infection prevention”
- Prof.Dr. Claus Hélix-Nielsen, Technical University of Denmark, “Biomimetic membranes: from concepts to applications”

The full program for Monday, October 6 and Tuesday, October 7, 2014, is available here. Registration to this important two-day event with 600+ attendants is still possible.

During June 21-25, 2015, Wetsus will host the 7th European SummerSchool on Electrochemical Engineering, an advanced-level summer school for PhD students and young industry professionals to be inspired by the science of Electrochemical Engineering. Only a limited number of individuals can participate in this prestigious and unforgettable event. For further information, please visit the conference website or send an email to .


Wishing you much joy in reading this Summer 2014 edition of the Wetsus S&T Newsletter!

Maarten Biesheuvel
Editor, Wetsus S&T Newsletter.

This edition:

1. Interview: Professor T. David Waite, Chair of the International Council of IAP
2. Study urges new regulations to allow reuse of black (toilet) water sludge in agriculture
3. “Morning star” crystals form in drinking water with addition of sulfate
4. Renewable energy through reverse electrodialysis requires combination of antifouling strategies

Interview: Professor T. David Waite, Chair of the International Council of IAP, impressed by research, industry support, and spirit at Wetsus

This past May, Wetsus was proud to organize and host the eighth biennial Interfaces Against Pollution (IAP) Conference, an international conference involving 250 attendees, 60 oral presentations, and 60 poster presentations. This year’s IAP Conference was titled “Interfaces in Water and Environmental Science.” The conference provides an international meeting platform for scientists working on colloid and surface science and/or heterogeneous catalysis in relation to the natural environment and environmental protection.

Dr. T. David Waite, Professor of Civil Engineering at the University of New South Wales, Australia, is the Chair of the IAP Conference series. In this interview, he discusses the highlights of the conference this year at Wetsus, and the exciting future of the IAP series.

Foto artikel David Waite

Q. What was your impression of Wetsus during this year’s IAP Conference in May?

DW: I have been extremely impressed by Wetsus since my first interaction with the organization about two years prior to holding the 2014 IAP conference in Leeuwarden. The mix of high-quality research, focus on underpinning fundamental and applied research, and strong industry support and involvement is exemplary.

Q: In your opinion, what were some of the overall highlights at the water-themed May IAP Conference?

DW: Particular highlights for me were 1) the quality (depth, breadth) of the plenary lectures, 2) the strength of the theme sessions, especially that organized around the topic of Capacitive Deionization, and 3) the wonderful atmosphere that was created as a result of the excellent venue, high-quality organization and the generosity and spirit of the organizing team.

Q. Can you explain more about the main science and technology fields that play a major role in the IAP Conference Series?

DW: The scope of the conference ranges from fundamentals to applications. Conference topics have typically included:
- Surface and colloid chemistry in mainly soil and aquatic environments.
- Nature, role and fate of environmental particles.
- Role of interfaces in transport processes.
- Environmental catalysis and pollution.
- Remediation applications in soil, water and gas purification.
- Benign production processes based on interface science.

The choice of particular topics and plenary and keynote lectures is left to the local organizing committee for any particular conference. As such, each conference takes on a “flavor” representative of the organization hosting the conference.

Q. What are the future directions of the IAP Conference series? Or along these lines, what are the most important areas of future research in interfaces against pollution, especially involving water science?

DW: I have been surprised at the success of the IAP Conference series, both with regard to the interest by researchers in attending and by the very positive feedback received after each meeting. I believe this is due principally to the quality of the talks presented and the opportunity that has been provided at each of the meetings for productive exchange of ideas between participants. As such, it is important that these two features of IAP be maintained in future, starting with the next IAP conference in Lleida, Spain, in 2016.

As for topics, emphases and new directions, these will be influenced by both the local organizing team and overall trends in interfacial research and development. From my perspective, I expect to see increasing attention to “soft particles” and biointerfaces, treatment technologies based around new materials and electrodes, and increasing use of synchrotron-based techniques for characterization of interfaces and interfacial processes.


Study urges new regulations to allow reuse of black (toilet) water sludge in agriculture

Black water sludge (or treated wastewater from toilets) may not be as toxic for agricultural use as previously thought. While current Dutch regulations prohibit black water sludge from being reused in agriculture due to concerns about its high concentrations of heavy metals, a new study has found that black water sludge has a significantly lower heavy metal content than cow manure and phosphate fertilizers—both of which are currently used in agriculture. The findings suggest that new EU regulations should allow black water collected from vacuum toilets to replace part of the manure and phosphate fertilizers used in agriculture, which would reduce the heavy metal content in the soil/food cycle.

Lead author of the study is Taina Tervahauta, a PhD student at Wageningen University, working together with Sonia Rani and Lucía Hernández Leal, both researchers at Wetsus, along with Grietje Zeeman, Professor at Wageningen University, and Cees Buisman, Director of Wetsus and Professor at Wageningen University. The study is published in the Journal of Hazardous Materials.

Foto artikel Taina T

“Currently the wastewater sludge reuse regulations are solely based on the quality of sludge without consideration for the origin of wastewater,” Tervahauta said. “Due to this, sewage sludge and black water sludge are categorized under the same regulation although sewage sludge has significantly higher heavy metal content compared to black water sludge. Also, heavy metals in black water are primarily human originated (feces and urine), while heavy metals in sewage mostly originate from industrial effluents and surface run-offs. Since heavy metals in feces and urine are primarily from dietary sources, land application of black water sludge could reduce the heavy metal content in the soil/food cycle by excluding external heavy metal sources. Cow manure and phosphate fertilizers, widely applied on land, also involve external heavy metal sources, and by partly replacing them with black water sludge, many of the heavy metals in the soil/food cycle can be further reduced.”

By collecting black water from three different locations in the Netherlands from vacuum toilets, as well as collecting and analyzing data from previous studies conducted in 11 European countries during the past 30 years, the researchers made several key findings.

First, the new measurements of the heavy metal content of black water show that black water has significantly lower concentrations of nearly all of the eight heavy metals measured (As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn) compared to cow manure, phosphate fertilizer, and sewage water (of which black water is just one component that has been separated). The data from these three sources was taken from previous studies.

The new study also reveals that the majority of the heavy metals in black water comes from human feces and urine. The heavy metal content in feces and urine, in turn, originates primarily from dietary sources. Since the majority of the foods we eat are agricultural products, it means that the heavy metals came from agricultural land in the first place, so returning them to the land would essentially be completing the soil/food cycle, without the addition of external heavy metal inputs.

More specifically, the analysis of the data on average heavy metal loadings of black water reveals that feces contributes to the majority of Pb, Cd, Cu, Zn, Cr, Ni, and Hg levels (52%, 84%, 63%, 80%, 72%, 75%, and 69%, respectively). The one exception is As, which is due mostly to flush (tap) water (53%) and urine (47%). The contribution of toilet paper to most of the heavy metal content, including Pb, Cu, Zn, Cd, and Ni, is less than 10%. Therefore, the heavy metal concentrations are characterized primarily by human-originated content (feces and urine) rather than infrastructure-originated content (tap water).

When comparing black water with sewage water, the researchers emphasize that the two types of waste water have very different characteristics. Sewage water’s significantly higher heavy metal content is due to its different origins than black water. For example, the Hg and Pb content of sewage water is about 50-200 times higher than in black water. The difference is due in part to industrial effluents and surface run-offs that contribute to the higher heavy metal concentration in sewage. Source-separated, vacuum-collected black water excludes these external heavy metal inputs.

Despite their different characteristics, black water and sewage water are currently regulated under the same guidelines. Both types of waste water are currently prohibited from being reused in agriculture. Currently, most black water and sewage water is incinerated, which not only mineralizes the organic carbon into CO2 and destroys the plant nutrients nitrogen and phosphorous, but also requires additional energy input.

With the EU “Sewage Sludge” Directive 86/278/EEC currently under revision, the results of this study should prove very useful in improving both the safety and effectiveness of reusing waste water in agriculture.

In particular, the results highlight the benefits of replacing at least part of the manure and phosphate fertilizers with black water due to its lower heavy metal content. In a 2012 study, Zeeman calculated that black water and kitchen refuse has sufficient phosphate to replace 25% of the global artificial phosphate fertilizer need. To minimize the risk related to pathogens, black water sludge should be pasteurized at 70 °C before being applied to agricultural soil. In the same 2012 study, Zeeman also showed that hyper-thermophilic treatment can be applied on black water with high energy efficiency.

As manure provides a significant stream of nutrients, the researchers also suggest that heavy metal content could be further reduced by regulation of the heavy metals in livestock feed. Improving soil quality is an important area of future research.

“Lack of organic matter in soil is one of the most crucial factors declining soil fertility,” Tervahauta said. “Importing of organic matter from virgin organic carbon pools, such as peat bogs, to other regions causes imbalance in the global carbon cycle. Future research is needed to improve the returning of organic matter from local waste streams into agricultural land. This could include drawing of global organic carbon balances and supporting environmental legislation towards closing organic carbon cycles locally.”

Taina Tervahauta, Sonia Rani, Lucía Hernández Leal, Cees J.N. Buisman, and Grietje Zeeman. “Black water sludge reuse in agriculture: Are heavy metals a problem?” Journal of Hazardous Materials 274 (2014) 229-236.

Most of the heavy metals in black water come from human feces and urine, which in turn originate primarily from dietary sources, which themselves often come from agriculture. Using black water sludge as agricultural fertilizer would be returning these heavy metals to the land and completing the soil/food cycle.
Most of the heavy metals in black water come from human feces and urine, which in turn originate primarily from dietary sources, which themselves often come from agriculture. Using black water sludge as agricultural fertilizer would be returning these heavy metals to the land and completing the soil/food cycle.

“Morning star” crystals form in drinking water with addition of sulfate

A medieval weapon that looks like a spiked club, called a “morning star,” has inspired the name of a new hybrid crystal that forms in drinking water reverse osmosis (RO) concentrate when sulfate is present. Westus postdoc Martijn Wagterveld and coauthors have for the first time observed the morning star crystal, which is composed of two polymorphs of calcium carbonate (CaCO3): vaterite “clubs” with aragonite “spikes.” The study may provide insight into the growth of pearls and other biominerals, as well as the crystallization of biomimetic materials.

Wagterveld, along with Wetsus researchers Miao Yu, Henk Miedema, and Geert-Jan Witkamp (who is also a Professor of Environmental Biotechnology at TU Delft), published the study on the effects of sulfate in drinking water RO concentrate in a recent issue of the Journal of Crystal Growth. As Wagterveld explained, the research idea arose somewhat by surprise while the scientists were experimenting with ultrasound.

“We were looking at something completely different, namely how ultrasound influences the crystallization of calcium carbonate in the presence of magnesium and sulfate, compounds found in reverse osmosis concentrate,” he said. “This was to see if we could modify/prevent the crystallization of calcium carbonate with ultrasound. It did not give very interesting results, but we found this weird morning star crystal. Since it was not described in the literature yet, we started to focus on this crystal with our experiments.”

Foto 2 artikel Martijn

The researchers’ main finding is that increasing the sulfate concentration of drinking water RO concentrate decreases CaCO3 formation and changes the preferential CaCO3 polymorph from vaterite to aragonite. Aragonite spikes grow directly on the vaterite surface, resulting in the morning star configuration.

As a second finding, the scientists observed that the presence of a moderate magnesium concentration also produces morning star crystals with even larger spikes than without magnesium, even at relatively low sulfate concentrations. In the presence of both magnesium and sulfate, single crystals of aragonite also form on their own, apart from vaterite. These observations suggest that sulfate and magnesium have an additive effect.

Wagterveld explained that observing and identifying the tiny crystals presented difficulties that required cutting-edge technology to solve.

“The identification of the crystal was a big challenge,” he said. “We could not use ‘normal’ Raman spectroscopy to identify the polymorphs in the morning star crystal. All Raman spectroscopes are equipped with optical microscopes. With these microscopes it is impossible to discriminate the morning star crystals from other crystals, because the details of the morning star crystal are too small. However, we found a start-up company that was building an integrated scanning electron microscope with a Raman spectroscope. It took several months before their system was operational and we could finally confirm that the crystal really consisted of the polymorphs vaterite and aragonite.”

This study is not the first time that hybrid structures of vaterite and aragonite have been observed. In fact, their combination is also seen in freshwater pearls. In pearls, as in the morning stars, vaterite is usually located at the center and aragonite as the smooth outer layer. While scientists are aware of the role of magnesium ions in pearl formation, a role of sulfate has not been acknowledged, although the sulfate ion is present. The present results may lead to further investigation of a possible role of sulfate in pearl formation.

As for why the morning star crystals form in drinking water RO concentrate, the researchers propose that a combination of factors leads to the decrease in the CaCO3 precipitation rate and the aragonite polymorph preference.

One factor is that the sulfate and magnesium ions interfere directly with the CaCO3 crystal growth, in which sulfate substitutes for carbonate and magnesium substitutes for calcium. These substitutions not only reduce the overall CaCO3 precipitation rate, but they are also more likely to affect the vaterite polymorph than the aragonite polymorph. As a result, aragonite becomes the preferred form.

Another factor that likely contributes to the observed changes is that an increase in sulfate concentration results in lower supersaturation ratios of the CaCO3 polymorphs. Since more of these molecules dissolve, less precipitate forms.

Although CaCO3 crystallization is not Wagterveld’s main research interest (that would be Wetsus’ CapMix project), he continues to be involved with crystallization research. He is currently supervising several PhD students, one of whom is working on crystallization.

R.M. Wagterveld, M. Yu, H. Miedema, and G.J. Witkamp. “Polymorphic change from vaterite to aragonite under influence of sulfate: The ‘morning star’ habit.” Journal of Crystal Growth, 387 29 (2014)

Scanning electron microscope images of calcium carbonate precipitate in drinking water RO concentrate in the presence of various sulfate concentrations. As sulfate concentration increases from A.1 to A.6, precipitation rate decreases and aragonite spikes grow on the vaterite surfaces. Sulfate concentrations: (A.1) 0 mM, (A.2) 3 mM, (A.3) 5 mM, (A.4) 10 mM, (A.5) 30 mM, (A.6) 60 mM.
Scanning electron microscope images of calcium carbonate precipitate in drinking water RO concentrate in the presence of various sulfate concentrations. As sulfate concentration increases from A.1 to A.6, precipitation rate decreases and aragonite spikes grow on the vaterite surfaces. Sulfate concentrations: (A.1) 0 mM, (A.2) 3 mM, (A.3) 5 mM, (A.4) 10 mM, (A.5) 30 mM, (A.6) 60 mM.

Renewable energy through reverse electrodialysis requires combination of antifouling strategies

By mixing seawater and river water, reverse electrodialysis (RED) can theoretically supply well over 2 TW of power, which is close to the present worldwide electricity demand. In reality, that number is severely limited by fouling of the ion exchange membranes due to clay, charged colloids, and organic substances. In a new paper published in Environmental Science & Technology, David Vermaas, a recent PhD graduate (cum laude) from the University of Twente, and coauthors have found that different antifouling strategies have different advantages, so that a combination of strategies will likely prove most effective for optimizing RED power density.

Foto 2 artikel David

Vermaas, who also worked at Wetsus, collaborated with Wetsus researcher Michel Saakes, University of Twente Professor Kitty Nijmeijer, as well as Damnearn Kunteng and Joost Veerman, both researchers at REDstack, which is the first spin-off company of Wetsus.

RED stacks consist of a series of alternating cation exchange membranes (CEMs) and anion exchange membranes (AEMs). When seawater and river water flows in the compartments between these membranes, the flows form a salinity gradient over the membranes, transporting ions in the seawater compartment toward the river water compartment. Each membrane creates an electric potential that can be used to drive an electric current.

The performance of RED stacks has been well investigated in models and under laboratory conditions. However, little research has been done on RED stacks under realistic conditions, using natural streams of seawater and river water.

What is known is that, under realistic conditions, the CEMs and AEMs suffer from fouling if no anti-fouling strategies are applied. This fouling decreases the power density by approximately 50% during the first day, followed by a slow further decrease in power density over the next several weeks.
Although general antifouling strategies exist, most of them are too expensive or environmentally undesirable for RED due to the large quantities of water that would be involved in future commercial RED power plants.

Two types of antifouling strategies that are cheap and environmentally friendly, and therefore promising candidates for RED, are feedwater switching (periodically switching the seawater and river water) and air sparging (periodically injecting pressurized air).

In the current study, the researchers conducted RED fouling experiments at the Wetsalt demo site in Harlingen using freshwater from a nearby canal and seawater from a nearby harbor. By comparing the feedwater switching and air sparging strategies over the course of two months, the researchers found that both antifouling strategies significantly influence the degree and type of fouling in an RED stack.
In particular, feedwater switching has short-term advantages, specifically by creating disturbances in the feedwater flow and reversing the electric field, which significantly reduces colloidal fouling. However, feedwater switching is not sufficient to avoid fouling in the longer term, since fouling continues to slowly build up.

In contrast, air sparging does not have the same short-term advantages of feedwater switching, but its effective scouring of the channels removes fouling and so greatly minimizes the drop in water pressure that would otherwise occur. In short, air sparging removes colloidal fouling that would otherwise accumulate over longer periods of time.

Although neither antifouling strategy can prevent a decrease in power density, the two strategies can reduce that decrease. Without any antifouling strategies, the power density decreases in the first four hours of operation to 40% of the originally obtained power density, and slowly decreases further thereafter. With feedwater switching, the power density remains initially at 60% of the original power density, followed by a further decline. With air sparging, the power density decreases to 40%, but then remains relatively steady for the following weeks.

“The most useful work we did during my PhD is the step from laboratory use of RED to practical application of RED,” Vermaas said. “That involves fouling and anti-fouling strategies (this paper), but also the use of profiled membranes, which yield the necessarily improvement in power density and practical advantages in building stacks. Those two developments became the most important pillars of my thesis. And it's great to see that these results are now used for the larger scale application of RED in the pilot plant facility at Afsluitdijk.”
At the end of these tests, the researchers performed another experiment to investigate whether the RED stacks could be taken apart and cleaned in order to fully restore their performance. Using a brush, the researchers manually removed the visible fouling material from all the stacks. Then they stored the membranes of the RED stack without antifouling treatment in demineralized water, and the membranes from the other stacks in brine to test whether the brine treatment would remove some of the humic substances due to ion exchange.

They found that the brine turned brown, indicating that the effects of the noncolloidal foulants is at least partly reversible. However, when the researchers rebuilt the RED stacks and tested their performance with natural feedwater as before, they found that the cleaned stacks performed worse than the new stacks at the start of the first experiments.

These observations indicate that not all fouling effects are fully reversible. The researchers aren’t sure of the cause of the irreversible fouling effects, but they do emphasize the need for ion exchange membranes that are insensitive to these foulants. Specifically, monovalent selective AEMs and CEMs coated with a thin layer that has an opposite charge to that of the foulants could help address this problem.
Overall, the results show that a combination of short-term (feedwater switching) and long-term (air sparging) antifouling strategies, along with special membrane coatings, are urgently needed to maintain a high power density for RED energy generation. The knowledge from this research is applied in the pilot RED plant facility at the Afsluitdijk that is initiated by REDstack, Wetsus, and Fujifilm.

Vermaas briefly worked at REDstack after working at Wetsus, but recently he has embarked on two other endeavors.

“First, I've started a new company with another former Wetsus PhD student, Jan Post,” he said. “Our company is AquaBattery (, and our business is related to Blue Energy, but distinguishes in a way that we only use (probably closed) systems for energy storage. Closed systems do not aim for electricity production, but for electricity storage. The need for electricity storage is rapidly growing due to the increased use of, for example, solar panels and wind power, and such systems can buffer this energy.
“The second job I'm working on is quite different: I have a postdoc position at TU Delft on solar fuels, i.e., hydrocarbon fuel production from solar energy. I thought that would be challenging as well, and it shares the same aspects that I like (renewable energy, electrochemistry, engineering).”

David A. Vermaas, Damnearn Kunteng, Joost Veerman, Michel Saakes, and Kitty Nijmeijer. “Periodic Feedwater Reversal and Air Sparging As Antifouling Strategies in Reverse Electrodialysis.” Environmental Science & Technology. 2014, 48, 3065.

SEM photo of fouling
SEM photo of fouling
Fouling on an RED ion exchange membrane
Fouling on an RED ion exchange membrane
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