Why Water Tanks Become Contaminated
Every water storage tank accumulates contamination over time, regardless of installation quality. Sediment from the mains supply settles on the tank floor, creating a nutrient-rich base for bacteria. Biofilm — a slimy layer of microorganisms — colonises tank walls and provides a protected environment for pathogens including legionella.
External contamination is equally problematic. Tanks with damaged or missing lids admit insects, rodents, bird droppings, and airborne debris. In London's older buildings, many loft tanks were installed decades ago with inadequate covers that have since deteriorated. Even a small gap can introduce contamination affecting the entire downstream water supply.
Types of Water Storage Tanks
London buildings contain a wide variety of water storage tank types, each with different maintenance requirements and common failure modes. Understanding your tank type helps you anticipate potential issues.
| Tank Type | Common Location | Typical Issues | Lifespan |
|---|---|---|---|
| Galvanised steel | Lofts, roof spaces | Internal corrosion, rust contamination | 15–25 years |
| GRP (fibreglass) | Plant rooms, roofs | UV degradation, algae if exposed to light | 25–40 years |
| Plastic (polyethylene) | Lofts, plant rooms | Deformation if unsupported, lid degradation | 20–30 years |
| Sectional (bolted panels) | Plant rooms, basements | Joint seal failures, panel corrosion | 20–35 years |
| Concrete / rendered | Underground, basements | Cracking, coating failure, ingress | 30–50+ years |
The Professional Cleaning Process
Professional water tank cleaning follows a systematic process designed to remove all contamination and restore the tank to a safe condition. The engineer begins with a pre-clean inspection, documenting the tank's condition with photographs for your records.
The water supply is isolated and the tank drained. Interior surfaces are cleaned manually to remove all sediment, biofilm, and debris. For larger tanks, this may require confined-space entry with appropriate safety equipment and breathing apparatus. After cleaning, the tank is disinfected using a controlled chlorine solution , rinsed, and refilled. Post-clean water samples verify the water meets safety standards.
Tank Inspection Checklist
During every clean, our engineers inspect the tank against a comprehensive checklist. These are the key items assessed — any failures here require remedial action.
- •Lid condition — fitted securely, no gaps, screened overflow and vent
- •Internal surfaces — free from corrosion, scaling, or coating failure
- •Insulation — adequate to prevent freezing and temperature fluctuation
- •Pipework connections — no signs of leaks or deterioration
- •Ball valve operation — filling correctly, not stuck or overflowing
- •Support structure — tank base and frame structurally sound
- •Labelling — tank clearly marked with capacity and date of last clean
Cleaning Frequency Guide
The appropriate cleaning frequency depends on the building's risk profile. Your legionella risk assessment should specify the exact frequency for your property, but the table below gives general industry guidance.
| Building Type | Recommended Frequency | Notes |
|---|---|---|
| Residential blocks | Annually | More often if issues found |
| Offices and commercial | Annually | Consider 6-monthly for high-occupancy |
| Hotels and hospitality | 6 – 12 months | Based on occupancy patterns |
| Care homes and hospitals | 6 months | High-risk vulnerable occupants |
| Schools and nurseries | Annually | Clean during school holidays ideally |
| Industrial premises | Annually | More often if process water involved |
Warning Signs Your Tank Needs Urgent Cleaning
Don't wait for the scheduled clean if you notice any of these signs: discoloured water at outlets, unusual taste or smell from the cold supply, visible debris in taps, or a positive legionella test result. If your tank hasn't been cleaned in over two years, treat it as a priority regardless of visible signs.