After a while I find the shower heads in my bathrooms become clogged by black flecks of what is obviously some kind of organic material. A similar material accumulates in my cold-water taps if they have not been used for some time, but in this case it is in the form of a black ribbon. I recall from visits made to water-treatment plants in my student days that the passage of water through a filter leads to the build-up of a zoogloea 鈥� a translucent jelly-like layer of organic matter. But if something similar to this process is taking place in the shower, why is the material black and exactly what is it?
鈥� I know this black jelly-like material well because it formed an integral part of my PhD studies, which were focused on the sort of water filters mentioned above. The material is a biofilm: a jelly made up of a layer of bacteria and their extracellular products. These commonly form at the interface between a solid and a flowing liquid.
The black coloration comes from manganese, which is common in groundwater. When water emerges from a spring, any manganese it contains will be in the form of soluble ions. Part of the treatment process is to oxygenate the water with an air cascade, and in some areas chlorine is added to the water to oxidise both iron and manganese. The insoluble manganese oxides created in this process are filtered out before water enters the distribution system.
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An alternative to this chemical oxidation is to allow a biofilm of manganese-oxidising bacteria such as species to form within a sand filter as the water passes through the filter. This film can build up into a sheath that, in the case of Leptothrix discophora, can be up to 20 times the diameter of the bacterial cells.
The ribbon-like nature of the slime shows that the biofilm has formed under conditions of fluid shear. In effect, the film is stretched out by the flow of water. These ribbons are known to biofilm researchers as 鈥渟treamers鈥�. Your questioner is seeing a biofilm of Leptothrix coloured black by manganese oxide. The water is safe to drink: manganese toxicity is not a problem in municipal water systems, but it can cause staining in laundry and plumbing. These biofilms are not at all dangerous, just messy.
鈥淭he ribbon-like nature of the slime shows that the biofilm has formed under conditions of fluid shear in which the film is stretched by the water flow鈥�
Chris Hope, Lecturer in oral biology, University of Liverpool, UK