Introduction
The United Nations Educational, Scientific and Cultural Organization (UNSECO) declared in 2009 that in the future it will be even more difficult to ensure a sufficient supply of clean water for everyone. Thus, it is important to explore and use new technologies to meet the increasing demand for clean water. One technology that offers possible solutions to overcome water shortage is membrane technology. Presently, this technology is successfully used for different types of water and industrial processes, due to its versatility. Furthermore the use of membranes can guarantee clean and hygienic water, which is a considerable improvement in comparison to the conventional water purification methods.
However, high investment and operating costs presently limit the application and circulation of this beneficial technology. Deficits of conventional membrane technologies are considerable due to manufacturing costs and high energy consumption during the application.
Membrane substitutes made of fine fibre nonwoven composites (Picture 1) could contribute to overcoming these problems. The target of research currently taking place at the Institute für Textiltechnik (ITA) of the RWTH Aachen University is to create an innovative leap within filter technology, by designing a composite structure made of melt electro spun and spun bond nonwovens. Furthermore, the research aims towards the development of a user-oriented, less risky and more environmental friendly filter techniques. One aspect is therefore a dialogue with society about the acceptance of new developments.
Global Megatrends Drive Innovations
The water sector currently faces major global challenges, according to UNESCO. These include an increase in world population, migration and globalisation, all of which are leading to a future where over 900 million people have no access to clean water [www12ZDF]. Beyond these circumstances, a changing consumer behaviour, technological progress and agricultural and industrial developments are having a huge impact on the global water budget.
All these developments are accompanied with an increasing demand for water and an increased pollution of the waste water [WWAP12]. Thus, the extraction of groundwater has worldwide more than tripled in the last 50 years. Factors of influence for the quality of drinking water in Europe are:
- Growing economy and rising urbanisation
- Increasing consumption of water
- Use of groundwater tripled over the last 50 years
- Pharmaceutical products in waste water
- On an average every German citizen received 10 packages of medicine in 2010
This trend points to the need to meet the increasing demands for clean water through appropriate technologies. Appropriate water quality is required for the population and for irrigation in agriculture. Also, industrial processes require a sustainable supply independent of a time and location with sufficient water of a defined quality. [WWAP12]
Membrane Technology for Clean Water
In waste water management, the use of sedimentation tanks is common. In these tanks the water rests for a period of time until the dirt particles have sunk to the ground. Afterwards the cleaner water is removed from the surface and routed into rivers or lakes for final filtration. Unfortunately this technology can only remove particles and dirt from the water, but a hygienic cleaning, removing bacteria and medicine, is not available.
A technology exists which can filtrate dirt as well as medicine or bacteria from the water by using membranes for filtration [Jud06]. However, the big disadvantages of this technology are the high investment costs of €70-150 per m² [Bre11, Pin06] membrane and the even higher cost of operation. About 50% of the energy of the whole sewage plant is needed for cleaning the membranes [Pin11]. A technology which can combine the advantage of the membrane technology with lower costs would be a big innovation for the waste water treatment.
Electro spun nonwovens for microfiltration
To combine a filtration performance similar to the membrane technology with lower costs, ITA is working on a nonwoven assembly out of spun bond and submicron fibres. The assembly (Picture 1) has two layers of nonwoven which are connected by a new bonding technology. The filtration layer consists of submicron fibres which improve the efficiency of filtration products without increasing the pressure drop [Gop06]. Furthermore the use of fibres instead of membranes has the advantage, that the filtration layer can be manufactured to the filtration task by controlling the pore size distribution of the layer via the fibre diameter [Hsi11].
The stabilisation layer created using a spun bond nonwoven makes up the second part of the assembly. This layer is needed for the stabilisation of the filtration layer during the filtration and cleaning process. The bonding between these two layers is adjusted to the requirements of the submicron fibres and the application in filtration systems. Because of bonding points in the submicron range, the fibre structure is preserved and the bonding does not have any influence on the pressure drop.
For the production of submicron fibres ITA used a multi nozzle melt electro spinning technology. Researchers could demonstrate that a spin pack with 64 nozzles runs with a stable jet formation. They are currently considering a further increase the number of nozzles for an even higher throughput that can match the properties of an extruder. Fibre diameters are currently in the range of 600nm. A further decrease of fibre diameter and a controlling of the fibre assembly for a specific pore size distribution are in progress.
The second priority of the research is the bonding technology for the submicron fibres. The fixation of fine fibre layers onto nonwoven substrates with conventional bonding technologies is not feasible. Apparently, conventional methods (e.g. calandering, needle-punching) destroy the fine fibres. Therefore, we are investigating unconventional bonding technologies. One approach is the application of bonding points in the submicron and nano range using laser technology. The absorber is applied by electro spraying it onto the substrate, then a layer of submicron fibres is placed on top of them. Since most thermoplastic materials are transparent to the wavelength of the laser, the heat of the laser is induced precisely at the absorber points. Through the induced energy on the absorber point, the adjacent fibres melt and connect with each other. We could observe that this bonding technology has no influence on the pressure drop during filtration tests.
Assessment of the opportunities of the presented technology
The aim of the research is an alternative for membranes for waste water treatment, which combine lower costs with the same functionality. A reduction of the production cost is possible, because of the replacement of the membrane by nonwovens which eliminates the need for expensive and complicated to recycle solvents. The asymmetrical assembly of a single stabilisation layer and a single filtration layer makes it easier to clean the filter and can help to reduce operating costs. The challenge of the research is to combine these possible advantages of nonwovens with the same performance like membranes in the microfiltration of waste water.
Global Outlook
The development of membrane technology for the Urban Water Management in Germany continues. Low prices for nonwoven fabrics will help to reduce water and sewage costs for the population and offer the opportunity to secure water resources for the future.
The application of membrane processes in wastewater treatment will increase worldwide by 15% and in the treatment of drinking water by about 20%. This process can do its part to reduce the worldwide water problem and to help the majority of the world's population – thus, International Transferability is given.
Summary
In view of the increasing global demand for water, it is important to further exploit the potential of membrane technology. Therefore, nonwovens are promising for the use in liquid filtration. The Institute für Textiltechnik aims to create a membrane substitute from a nonwoven composite and thus to remedy the shortcomings of the membranes used today. Crucial factors for the success of the project are the reduction of energy consumption by 30% during the operation, reducing the investment costs by 20% for membranes and reducing the environmental impact during production by omitting of the use of solvents.
Part of an effective technology development process is the dialogue with the society about acceptance and acceptability of new developments. The improved water quality is more than socially relevant, for example, precarious topics such as hygiene and health are affected. The active participation of the public creates a sense of, and, participation.
References
[Bre11] Brepols, C: Operating large scale membrane bioreactors for municipal waste water treatment. London: IWA Publishing, 2011 ISBN: 9781843393054
[Gop06] Gopal, R., Kaur, S., Ma, Z., Chan, C., Ramakrishna, S., Matsuura, T. Electrospun nanofibrous filtration membrane. Journal of Membrane Science 281, 2006, S. 581-586
[Hsi11] Hsiao, B.S.; Chu, B.; Hongyang, Ma; Burger, C. Ultra-fine cellulose nanofibres: new nano-scale materials for water purification Journal of Materials Chemistry 21 (2011) No.21, S75077510
[Jud06] Judd, S. The MBR Book: Principles and applications of membrane bioreactors in water and wastewater treatment. Elsevier: 2006 ISBN-10: 1-85-617481-7
[Pin06] Pinnekamp, J.; Friedrich, H.; Membrantechnik für die Abwasserreinigung Aachen: FIW-Verlag, 2006
[Pin11] Pinnekamp, J., Veltmann, K., Palmowski, L. Prozessoptimierung von Membranbelebungsanlagen (ProM), (2011) Abschlussbericht zum gleichnamigen Forschungsvorhaben, gefördert vom Ministerium für Klimaschutz, Umwelt, Landwirtschaft, Natur- und Verbraucherschutz des Landes Nordrhein-Westfalen (bisher unveröffentlicht).
[www12ZDF] ZDF Article “Wasser wird knapper und kostbarer” http://www.zdf.de/ZDF/zdfportal/web/heute- Nachrichten/4672/6604554/44849a/Wasser-wird-knapper-undkostbarer.html, 27.06.2012.
[WWAP12] WWAP (World Water Assessment Programme) The United Nations World Water Development Report 4: Managing Water under Uncertainty and Risk (2012). Paris, UNESCO