The Hidden Health Risks of Black Spore Mould in Potable Water Tanks

Black spore mould in a potable water tank is not just an aesthetic problem. It is a contamination risk that can affect water quality, create health hazards for maintenance teams, and put compliance at risk. This article explains what causes it, what the risks are and the most effective long-term solution.

What is Black Spore Mould?

Black spore mould is a type of fungal growth that can develop on porous or degraded internal tank surfaces. In water storage tanks, it forms when moisture, organic material and a suitable surface combine to create the right conditions for microbial colonisation.

It is not the same as the black mould commonly found in bathrooms. In a potable water tank, the concern is specific black spore fungi that can release spores directly into stored water, affecting both water quality and the safety of anyone who works inside the tank.

It tends to develop in tanks where the original internal surface has begun to break down. Once the surface becomes porous, it gives mould the foothold it needs to grow and spread.

Important point about black spore mould in water tanks:

Black spore mould does not need a lot to take hold in a potable water tank. Minor surface degradation, inadequate cleaning schedules or a tank that has simply aged beyond its original protection are all enough to allow microbial contamination in water tanks to begin.

Why Are GRP Tanks Particularly Vulnerable to Black Spore Mould?

GRP tanks (Glass Reinforced Plastic tanks, also known as fibreglass tanks) are widely used for potable water storage across the UK. They are lightweight, durable, and cost-effective. But they do have a weakness.

Over time, the internal surface of a GRP water tank can degrade. This happens through age, chemical exposure, mechanical stress, or simply years of use without a protective coating in place. Once the surface starts to break down, it becomes porous.

A porous surface is exactly what black spore fungi need to colonise. The tiny air pockets and rough texture give mould the grip it needs to anchor and spread. Standard cleaning can remove visible growth on the surface, but it cannot reach deep into a degraded substrate.

GRP surface degradation creates microscopic pores where mould can anchor
Chemical attack over time strips the protective resin layer
Poor or infrequent maintenance accelerates surface breakdown
Older tanks that have never been relined are at the highest risk
Even tanks that look clean on the surface can harbour fungal growth in water tanks at a deeper level

Find out more about the risks specific to GRP tank lining on our GRP tank lining page, including how our ACOTHANE DW coating addresses microbial growth.

The Health Risks of Black Mould in Potable Water

Black mould in potable water tanks presents risks that go beyond a failed inspection. Even in small amounts, mould growth inside a water storage tank is dangerous for the people who use that water and the people who maintain the tank.

The key health concerns are:

Spore release into stored water. Mould spores in drinking water are a contamination risk that can affect the safety of the water supply
Taste and odour contamination. Even trace levels of mould can cause noticeable changes to the taste and smell of stored water, raising immediate concerns for occupants and users
Respiratory risks for maintenance personnel. Anyone entering or working close to a water tank with active mould growth is at risk of inhaling spores, which can cause respiratory irritation and in some cases more serious health effects.
Compromised water quality compliance. Facilities with mould present in their water storage systems risk failing regulatory inspections under the Water Supply (Water Quality) Regulations.

Compliance risk: Drinking water stored in tanks with active mould contamination does not meet the standards set out in UK water quality regulations. Failing to address this puts facilities managers, building owners and facilities operators at risk of enforcement action.

This is particularly important for high-occupancy buildings, healthcare facilities, hotels, schools and any site where potable water hygiene is a regulatory requirement. You can read more about our work in specific sectors on our NHS tank lining, hotel tank lining, and schools and universities tank lining pages.

Signs of Black Spore Mould in a Water Tank

Black spore mould is not always visible on a surface inspection. But there are warning signs that something is wrong inside a potable water storage tank.

Dark or discoloured patches on the internal walls or floor of the tank
Noticeable taste or odour changes in the water supplied from that tank
A tank that has not been relined and is more than 15 to 20 years old
Visible surface cracking, chalking, or delamination of the GRP substrate
Previous failed water tank inspection results related to microbial contamination
A tank that has had recurring mould growth despite regular cleaning

If a water tank keeps returning mould growth after cleaning, the cleaning process is not solving the underlying problem. The surface itself needs to be sealed. Read our article on how tank relining prevents contamination in water tanks for more detail on why surface protection is the long-term answer.

Regular water tank inspection and maintenance is the best way to catch problems early. COVAC offers nationwide water tank inspection and relining services. View our water tank inspection hatch options and our water tank refurbishment service for tanks that need more than just a reline.

The Solution: Acothane DW Tank Lining

Cleaning alone cannot fix a degraded GRP surface to prevent black spore mould. The only reliable long-term solution is to seal the internal surface completely with a high-performance, WRAS-approved tank lining.

COVAC applies Acothane DW, a fully approved WRAS-listed polyurethane coating, specifically designed for use inside potable water tanks. It creates a non-porous and chemically resistant internal surface that gives black spore mould nowhere to anchor or grow.

What Acothane DW Delivers:

Once applied, the Acothane DW coating completely transforms the internal environment of the tank. Here is what it prevents and provides:

Black spore mould cannot anchor or spread on the sealed surface
Surfaces remain smooth and hygienic, making routine cleaning far more effective
The non-porous barrier resists chemical attack and microbial colonisation
Potable water quality and safety are maintained to regulatory standards
A minimum 10-year guarantee is provided on all COVAC lining work
The coating is WRAS approved and safe for contact with drinking water

The coating system is the result of nearly 30 years of application experience across the UK. It is not an off-the-shelf product applied by general contractors. COVAC technicians are trained specifically in its application and surface preparation, which is critical to the performance of the finished lining.

Find out more about the full coating process on our COVAC coating system.

For tanks that need more than a reline before a coating can be applied, our tank refurbishment services handle structural defects before the protective coating goes on.

FAQS

Is black mould in a water tank dangerous?

Yes. Black mould in a water tank can release spores into stored water, cause taste and odour contamination and create respiratory risks for anyone entering or working near the tank. In a potable water context, it also represents a compliance risk under UK water quality regulations.

Can you clean black mould out of a water tank?

Cleaning black mould out of a water tank can remove visible surface mould temporarily. But if the underlying GRP surface has degraded, mould will return because the porous substrate continues to harbour spores. The only lasting solution is to seal the surface with a WRAS-approved tank lining such as Acothane DW, which creates a non-porous barrier that prevents regrowth.

What is Acothane DW and is it safe for potable water?

Yes. Acothane DW is a solvent-free polyurethane coating approved by WRAS (Water Regulations Advisory Scheme) for use in contact with drinking water. It is the coating COVAC has applied to potable water storage tanks for nearly 30 years. It is safe and resistant to microbial growth.

Why Choose COVAC

COVAC has been relining and refurbishing water tanks across the UK for over 25 years. We work with the NHS, the RAF, major hotel groups and facilities management companies.

Every tank job comes with a written guarantee and is carried out by trained, experienced technicians.

Over 25 years of water tank lining experience nationwide
WRAS-certified and DWI-listed coating system
Minimum 10-year guarantee on all lining work
ISO 9001 and ISO 14001 certified
Coverage across all UK regions: Southern England, London, Midlands, Northern England and Scotland

Read our articles on how tank repairs improve water quality and our complete guide to tank lining types, materials and regulations for further background.