These growths are known as biofilms or 'microbial slime' and are most noticeable in areas of the house with a moist environment, such as bathrooms and kitchens. Bacteria and fungi present in the air, water and household dust can attach to damp surfaces and multiply to form a visible black slime or stain in various colours. If not removed as soon as they form, biofilms can grow into certain materials, such as tile grout and shower curtains, permanently staining them.

Biofilms can form on any surface that comes into contact with water, such as:

  • the inside of cold water taps
  • the base of tap fittings
  • shower heads and shower curtains
  • tiles in the bathroom/shower area
  • drains and plugholes
  • toilet cisterns and toilet bowls
  • kitchen sink draining surfaces
  • washing machines

Although biofilms can appear very unpleasant they are generally harmless and do not pose a risk to health.

Unfortunately, there is nothing we can do to the drinking water supply to prevent the growth of biofilms within customer’s properties.

The best way to prevent growth is to improve ventilation, to allow these damp areas in kitchens and bathrooms to dry rapidly, and the only way to keep on top of them is regular cleaning of all affected surfaces.

It is also important to reduce the amount of food available for the bacteria and fungi. Food sources can include shampoo, soaps, propellants from hairspray and deodorants.

Black slime associated with tap fittings can be removed by cleaning the inside of the tap or around the base of the tap with a small brush dipped in a mild solution of bleach.

The tap should be thoroughly flushed immediately afterwards. If a tap has a plastic insert or any other detachable fitting attached to it, the fitting should be regularly removed, cleaned and disinfected.

All water distribution networks are susceptible to biofilm growth. Biofilms are kept to a minimum in the distribution network (prior to entering customer’s properties) via the following:

  • maintenance of adequate residual chlorine levels throughout the distribution system
  • comprehensive networks maintenance programme, including regular mains flushing and pipe replacement
  • service reservoir maintenance, including regular cleaning programme
  • adequate filtration and disinfection of raw water sources, prior to release from the water treatment works

The efficacy of the above is monitored through our comprehensive sampling regime at water treatment works, service reservoirs and customer’s taps. In addition to this, we also sample mains following renewals, repairs and shutdowns. All of these samples are analysed for the presence of coliforms, a key indicator to the presence of pathogenic microorganisms.