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Cost-saving waste water filtration

May 15, 2001

Nobody likes to find coffee grounds in their morning "brew", which is why coffee percolators are so popular. But the last drops emerge slowly from the meanwhile compacted coffee in the filter, causing the waiting sleepyhead to lose patience! The operators of sewage plants experience a similar situation to separate solids out of their brown "brew". But they have the technical means to accelerate the process. The waste water passes horizontally over the filter, so that the sludge deposited on the filter can only reach a certain depth – any further solids are swept away by the current. From time to time the filter has to be cleaned. This system has a drawback: The pressure causing the liquid to flow at the required rate is generated by pumps. These pumps are powered by electricity, and electricity costs money. And the more waste water there is, the higher those costs. But they can be reduced by between 80 and 90 percent through the use of rotating membrane filters – such as those developed by the Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB in Stuttgart.

"One of the reasons why membrane filters, despite their advantages, are still not being deployed in low-value-added operations such as sewage plants is the fact that they are expensive", admits Stefanie Wisst, an engineer at the IGB. "But the initial investment can be quickly recuperated through savings in running costs." This is how it works: The filter consists of a stack of membrane disks mounted on a rotating shaft. As the filter rotates in the waste water, it creates a flow effect, and the quantity of solid waste retained by the filter can be regulated efficiently by varying the speed of rotation. The filtrate flows into the disks and is drawn off through the hollow shaft running through the middle of the filter. The solid phase is deposited on the disks, but once it reaches a certain depth it is expelled by the centrifugal force of the rotation, and collected. Even if the waste water contains a high proportion of solids, the energy required is way below that needed by a conventional cross-flow filtration plant.

This new project is being sponsored by the German research ministry. According to Stefanie Wisst, its applications extend beyond municipal plants for the purification of waste water: "The process could also be of interest for filtration plants in the chemical and biotechnological industries, or in the metalworking industry where there is a need to process waste coolants and lubricants." Representatives of these and other industries will learn about the benefits of the process at the Envitec trade fair in Düsseldorf, May 14 to 17.


Fraunhofer-Gesellschaft