Reverse osmosis (RO) is one way of safely removing critical trace substances from wastewater or drinking water.1 However, when RO filtration is carried out, biofouling and organic fouling in the spiral wound elements represent a hurdle. Even after ultrafiltration (UF) has acted as a barrier, bacteria can settle in the permeate. This is not necessarily due to the integrity of the UF module but to the fact that the system is not hermetically sealed. Bacteria can thus enter the tank via the air and then often multiply exponentially there, since the organic substances present in the wastewater reach the UF permeate and serve as nutrients there.
Wastewater Treatment Research Project
The comparison and evaluation of treatment concepts for treated wastewater for use in the industrial and agricultural sectors is the aim of a project named MULTI-ReUse (www.water-multi-reuse.org) funded by Germany’s Federal Ministry of Education and Research (Bundesministerium für Bildung und Forschung [BMBF]). The investigations will be carried out across a network of nine participants in total, comprising research facilities, manufacturers, and users, at a pilot plant at the wastewater treatment plant in Nordenham, Lower Saxony. Conventionally treated wastewater is fed into the pilot plant and treated further. The aim is to produce service water that can be used in the industrial sector instead of drinking water. In flexible treatment chains, UF, RO, activated carbon filtration, and ultraviolet (UV) disinfection processes are combined to produce defined water qualities.
Reduction of Biofouling by Feed Spacer
The most practical way to reduce biofouling is to prevent the growth of bacteria in the spiral wound element by minimizing spaces with slow cross-flow velocity. For this purpose, a new feed spacer was tested in direct comparison with a standard spacer. The feed spacer is a crucial component of spiral wound elements, as it influences both the height and the flow of the feed channel.
Compared to the standard spacer, the new feed spacer has different thread filament heights (see Fig. 1). Thick and thin filaments alternate, thus the feed-channel height can be defined and, at the same time, sufficiently strong turbulences can be generated with thinner filaments in order to reduce the concentration polarization on the membrane surface. Simulations show that this leads to fewer areas with less overflow.
Pilot Process
The pilot unit in Nordenham consists of two separate lines in order to allow the comparison of different process conditions. Following the biological stage, the wastewater is pretreated by precipitation or flocculation followed by UF (see Fig. 2). Whereas the function of UF is to remove particles, the removal of salts and micropollutants is performed by RO.
Another objective was to implement a stable and energy-efficient process. Each RO line has a four-inch pressure pipe with three installed elements. The element type Lewabrane® B085 ultralow pressure (ULP) 4040 is installed in the pressure pipe RO1, and a special Lewabrane B085 ULP 4040 with alternating strand design (ASD) is installed in the pressure pipe RO2.
Investigations at Delft University of Technology confirm that ASD has an influence on biofouling (see Fig. 3). There, test cells were equipped with different spacers and flowed over. Bacteria were prepared in the tank, and their growth was promoted.2 The test showed that the pressure loss along the feed spacer increased continuously.
After 90 days, the standard spacer’s pressure loss due to biological blockage had increased by 54 percent, while the pressure loss of the standard spacer with ASD had only increased by 30 percent. Typically, a system is cleaned when the pressure increase has reached approximately 15 percent. As such, the standard spacer had to be cleaned after about 45 days, while the spacer with ASD needed cleaning after 65 days. This, of course, has an influence on chemical consumption and downtime.
In order to validate this result under real test conditions, two four-inch pressure pipes were equipped with RO elements in which a standard spacer or the ASD spacer were installed in parallel as part of the project.
Nordenham Results
During almost 200 days of testing at the Nordenham pilot, the elements were exposed to different temperatures and water qualities. During the period indicated, cleaning was carried out on days 83 and 147 on both lines.
At the end of the test phase, the elements were removed and opened to determine the type and strength of the fouling. Here it became clear that the fouling in the elements with the ASD spacer was significantly lower than with the standard spacer (see Fig. 4).
Summary and Outlook
RO is a safe technology used to remove pollutants from water. Energy and chemical requirements represent hurdles for a wide range of applications in waterworks. Although energy requirements have been significantly reduced in recent years through improved membranes, and the use of biocides and cleaning chemicals could be significantly reduced, there has not yet been a breakthrough. Although LANXESS is working on a further research project to improve the spacer by surface modification, it is likely that only an overall concept with different technologies, such as adsorber resins, will ultimately prevail in order to better control organic fouling on the membrane. IWW
About the Authors: Uli Doelchow began working with LANXESS as an application engineer after receiving his master’s degree in chemical engineering from the TU Berlin in 2015. He is also involved in R&D activities.
Jens Lipnizki has worked in the field of membrane filtration for 15 years and is currently the head of technical marketing for membranes with LANXESS. His main focus is the development of the LewaPlus® software and providing technical support for the international sales team. He has a master’s in chemistry from the University Dortmund (Germany) and London University (UK) and a PhD in membrane fouling from the Technical University of Denmark.
References
1. Lehmann, S., J. Ogier, J. Lipnizki. “Erfahrungen mit Umkehrosmose als vierte Reinigungsstufe”, wwt, 2018, 6, 10–13.
2. Araújo, P. A., J. C. Kruithof, M.C.M. Van Loosdrecht, J. S. Vrouwenvelder. “The potential of standard and modified feed spacers for biofouling control,” Journal of Membrane Science, 2012, 403–404, 58–70.
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