Waterproofing drinking-water tanks: approvals, curing and testing

01Watertight and safe to drink

A tank holds water under constant pressure from inside, and its outer face often sits against wet soil in underground tanks. Unlike a roof, its inner face touches what people drink: a lining that is watertight but leaves odour, taste or substances in the water is not acceptable.

02Approvals for drinking-water contact

Drinking-water products are tested and listed under recognised schemes, such as NSF/ANSI/CAN 61 for drinking-water system components, or the UK WRAS approval based on BS 6920 testing. An approval covers a specific product, often with conditions — cure time, maximum water temperature, sometimes tank size — and local authority requirements apply as well.

03Systems used inside tanks

• Cementitious coatingsRigid or flexible grades approved for drinking water; they bond to concrete and tolerate damp surfaces.
• Crystalline treatmentsCrystals that grow within the concrete; approved grades exist. They do not bridge moving cracks.
• Epoxy liningsA smooth, cleanable lining over a repaired, levelled substrate; the approval and the cure are critical.
• PolyureaFast-curing sprayed linings, where the product is approved for drinking water.

04Preparation comes first

1. Empty, clean and inspect the tank; record cracks, honeycombing and leaks.
2. Repair: honeycombing and spalls with repair mortar, leaking cracks injected, the wall–floor junction and pipe entries detailed.
3. Surface preparation to the lining’s needs: an open texture for cementitious systems; a sound, dry, profiled and levelled surface for epoxy.
4. Fillets at internal corners where the system calls for them.

05Curing, testing and commissioning

1. Cure for the time and conditions the TDS states, with ventilation in closed tanks.
2. Fill test: the tank is filled and the level and outer faces are watched for the period in the specification.
3. Cleaning, flushing and disinfection per the authority’s or the specification’s procedure before use.
4. Records: products and batch numbers, approvals, cure times and test results — the documents that support the warranty.

06Common mistakes

• Lining over active leaks or unrepaired cracks.
• Epoxy on damp concrete: blisters and debonding.
• Filling before the lining has cured: taste or odour and a soft lining.
• A non-approved product, or an approved one used outside its conditions.
• Untreated pipe entries and wall–floor junction.