Every water tank, no matter how well sealed, accumulates sediment, biofilm and — in warm climates — the conditions bacteria need. Cleaning and disinfection is therefore a scheduled maintenance activity, not a response to a complaint. Done properly it restores water quality and protects the tank; done badly it introduces contamination and can damage the internal surfaces.

This procedure follows the sequence used for potable water tanks, in line with the general principles of published water supply practice such as BS EN 806-5 and equivalent national guidance. Confirm the specific requirements with your local authority before carrying out work on a public supply. As a water tank manufacturer we supply cleaning and disinfection guidance with every potable tank, and the steps below summarise it.

How Often Should a Tank Be Cleaned?

Two separate activities should be distinguished:

• Inspection — internal and external visual check, typically every six months.
• Cleaning and disinfection — full drain, clean and chlorinate, typically every 12 months for potable water in normal conditions, more often in dusty environments, after any contamination event, following repairs, and before a new tank is commissioned.

Where the water is stored for long periods or the tank runs warm, shorten the interval. Where turnover is high and the water is treated and monitored, the period may be extended on the basis of sampling results.

Before You Start

• Plan the outage — notify building occupants and arrange alternative supply if needed.
• Isolate — close the inlet, and lock or tag the valves so nobody refills the tank mid-procedure.
• Drain safely — discharge through a controlled route. Do not dump a full tank onto a floor that cannot take it.
• Confined space — a tank interior is a confined space. Entry requires a risk assessment, atmospheric testing, a standby person and a rescue plan.
• Equipment — soft-bristled brushes, non-abrasive cleaning tools, approved disinfectant, clean water for rinsing, test kit, PPE, and lighting suitable for wet areas.

Never use abrasive pads, steel wool or aggressive solvents on a GRP or coated internal surface. They damage the gel coat or lining, and a damaged surface harbours bacteria permanently.

Step 1: Drain the Tank

Drain completely through the normal outlet, then remove the remaining water and sludge from the floor with a wet vacuum or a pump. Retain a sample of the water before draining if you need to establish the cause of a quality problem.

Step 2: Remove Sediment

Shovel or vacuum out the accumulated sediment. This material is the main food source for biofilm, so removing it matters more than scrubbing visible surfaces. Dispose of it as waste — not into the drainage system that serves the building.

Step 3: Clean the Internal Surfaces

Wash down walls, floor, roof underside, tie rods and the tank fittings with clean water and a soft brush, working from the top downwards. Pay attention to:

• Joint lines and sealant beads, where biofilm collects.
• The waterline — the band where the water level normally sits.
• Corners, tie rod connections and around every penetration.
• The underside of the roof, which is often overlooked and often the dirtiest surface.

Inspect as you work. Cracks, blisters, damaged sealant or corrosion found at this stage should be recorded and repaired before the tank is refilled.

Step 4: Chlorinate

Disinfect with a chlorine-based solution at the concentration specified by your local regulations and the tank manufacturer — typically in the range of 20 to 50 mg/l free chlorine for a contact period of several hours, depending on the guidance in force. Practical points:

• Mix the dose accurately. Under-dosing leaves contamination; over-dosing damages some coatings and lengthens the flush.
• Ensure all internal surfaces, including the roof underside and fittings, are contacted.
• Respect the required contact time — usually a matter of hours, not minutes.
• Use only approved disinfection chemicals. Never use domestic bleach with additives or perfumes.

Step 5: Flush

Drain the chlorinated water to a controlled discharge point, then flush the tank and the downstream pipework with clean water until residual chlorine falls to an acceptable level. Because chlorinated water can be harmful to aquatic life, confirm where the discharge is permitted to go before starting.

Step 6: Sample and Return to Service

Refill with potable water and take samples for bacteriological testing — commonly for total viable count, coliforms and, where relevant, Legionella. Do not return the tank to normal service until the results meet the required standard. Record the date, the dose used, the contact time and the sample results; this log is the evidence that the maintenance programme is being followed.

Common Mistakes

• Entering the tank without a confined space assessment.
• Using abrasive tools that scratch the internal gel coat.
• Failing to clean the roof underside.
• Refilling before the chlorinated water has been fully flushed out.
• Skipping sampling because the tank “looks clean”.

Design details make this work much easier. A tank with a proper manway, adequate internal access, smooth moulded surfaces and correctly positioned fittings can be cleaned thoroughly in a day. Our potable water tanks are specified with access and internal finish designed around maintenance rather than around manufacturing convenience.

Frequently Asked Questions

Can a tank be cleaned without draining it?

Diver cleaning is used for very large or difficult-to-isolate tanks, but it is a specialist activity with its own hygiene controls and does not achieve the same result as a full drain, clean and chlorination. For anything up to a normal commercial tank size, drain-down cleaning is preferred.

Does cleaning the tank require specialist contractors?

In many jurisdictions, yes, for potable water installations — the work is regulated and the contractor must be able to certify the disinfection. If you carry it out in house, ensure the competence, equipment, confined space procedures and sampling are all in place.

How do I know the tank needs cleaning?

Indicators include taste or odour complaints, discoloured water, visible sediment at outlets, rising bacteriological counts, or the simple fact that the scheduled interval has elapsed. Waiting for a complaint means the tank has already been out of spec for some time.

Does the material affect cleaning frequency?

A smooth, non-porous surface accumulates less biofilm and is easier to clean, which is one reason stainless steel and moulded SMC panels are preferred for potable water. Any GRP water tank supplier should be able to state the internal surface finish and the cleaning regime it supports.

Summary

Drain, remove sediment, clean without abrasives, chlorinate at the correct dose and contact time, flush, sample and record. Followed once a year with six-monthly inspections in between, that routine keeps a potable water tank compliant and the building’s water safe.

Need a tank that is designed to be maintained? Talk to our team about access arrangements, internal finish and the maintenance schedule for your application.