Biofilms, Mould, Slimes and Staining

Water is a food product. Areas that are left damp within a house for prolonged periods provide an ideal environment for the growth of many different, generally harmless, common organisms such as bacteria, fungi (including airborne mould) and algae. Biofilms contain a mixture of microscopic organisms that have colonised, grown on a surface, and sometimes secreted a slimy, protective layer around them.

Consumers typically notice biofilms/mould as: pink, orange or black staining in showers/on grout; on shower curtains or around taps; black fungal growth/slimes on washing machine seals; black mould at the back of washing powder dispenser drawers; slimy stains above the waterline in toilet cisterns, in toilet bowls, on rubber inserts within taps, on tap washers, in drains/plugholes, in refrigerator drip trays or in dehumidifiers. A jelly-like substance can form within taps if they are not regularly internally cleaned with a bottle brush combined with dilute chlorine solution. It is important to always ensure that water fittings comply with the Water Supply (Water Fittings) Regulations 1999, to reduce the risk of biofilm growth.

The presence of these growths is a natural phenomenon and generally does not indicate a deficiency in the drinking water supply or a risk to health. However, these growths can look unsightly. Soaps, aerosols or other sprays can provide an additional food source and encourage the growth of organisms.

If consumers wish to control biofilm growth, it is important to keep areas as dry as possible and clean them regularly with suitable cleaning solutions, following manufacturers’ instructions to avoid damage to surfaces and to comply with health and safety precautions. If staining growths are not routinely, adequately cleaned, they can become a permanent feature. Consumers often must replace shower grout or flexible sealant (around baths/showers) to improve its appearance, following mould damage.

In areas prone to regular wetting or condensation, it is helpful to dry and clean surfaces quickly, increase heating and always provide adequate ventilation by opening windows/using extractor fans.

Occasionally, other stains can develop in consumers’ properties. Dripping taps can cause deposits to gradually build up on sinks/baths. Consumers sometimes notice green/blue stains from copper (often used for domestic pipework/hot water tanks); brown stains from iron (often historically used for private or public water pipes); dark brown or black stains from deposits of natural manganese or white/brown stains from water hardness (calcium carbonate). These can appear if the pressure or flow has changed in the local distribution area or within a property. They can generally be cleaned away with appropriate cleaning products.