Water Tanks: Cleaning and Disinfection

For potable water tanks, the National Regulations in force, such as Order No. 114 of 04.02.2014, Art. 25, Par. 6, do not specify an exact cleaning or disinfection frequency, but these actions are required whenever necessary. Likewise, in international specialty litreature (ANSI/AWWA C625), no specific frequency is defined for cleaning and/or disinfection, although such operations are considered mandatory whenever needed.

For fire protection tanks, inspection and cleaning should be carried out at intervals not exceeding three years, or whenever conditions require it. These recommendations are detailed in the following standards: SR EN 12845, FMDS 2-81, and NFPA 25.

Examples of Cleaning and Disinfection:

1. Membrane Coated Water Tanks:

2. PAFSIN Water Tanks:

3. Concrete Water Tanks:

4. Tanks with Other Surfaces (Ceramic):

As shown in these images, the largest accumulation of crust-type deposits occurs in the zone where the water surface meets air. This level is naturally variable, but the gradient colour effect indicates concentration of salts, minerals and eventually biological growth in areas influenced by oxygen intake.

Cleaning and Disinfection Procedure: Cost Estimation

Cleaning calculations must consider the following aspects / consumptions:

• Solution consumption: 1 litre / 10 sqm – theoretical, for GRP, Ceramic or Vitrium surfaces, which are extremely smooth, chemically inert and in cases of annual maintenance.
• For tanks supplied from wells with dosed chlorine, consumption may reach 1 litre / 6 sqm; in some situations, two applications are required if the tank surface is very dry.
• Hose losses: approx. 0.1 litres / 1 meter.

Work Procedure

1. If the surface is dry, thoroughly wet the walls until the superficial layer of deposits begins to soften. Ideally, the tank should be emptied no more than 12 hours before the cleaning intervention.
   Note:
   • For tanks where deposits of a few millimetres can be removed by hand, a preliminary cleaning with pressurized water is recommended.
   • First, the tank must be cleaned of all deposits and fully vacuumed.
2. The solution is applied on the walls with a reaction time of approx. 20 to 30 minutes.
3. Rinse with low-pressure water jet, which may also be done using a standard garden hose.
4. Depending on the level of wall contamination, a second application may be required if deposits are crust-like.
5. Disinfection is performed with a reaction time of 20 to 30 minutes.
6. Theoretically, a final rinse is required if the water is microbiologically clean.

Concentrations

1. For chemical washing we use two components: the acid cocktail and the reaction activator.
   a) As a standard practice, one dose of activator is calculated per canister of 22 kg (20 litres) of the acid cocktail.
   b) For heavily loaded tanks – with brown-dark or black deposits – the chemicals are used at maximum concentration. In this case, the reaction activator is dissolved in 5 litres of water and then mixed together with one canister of the acid cocktail in a larger container (60–70 litres).
   c) For moderately loaded tanks – typically those cleaned annually or every two years – the acid cocktail may be diluted at a 1:1 ratio, meaning that for every litre of Rapid TW, one litre of water is added.
2. For disinfection: As a standard practice, concentrations of 3–5% are used, meaning that for 20 litres, 1.5 kg (1.25 litres, considering that 50% hydrogen peroxide has a density of 1.2 kg/l) is added..

Our technology is imported from France and is the same technology used by VEOLIA in its projects worldwide. The efficiency of biofilm removal is ensured by the complex formulation of the substances used, which does not require mechanical scrubbing of the surface:

• Provides maximum protection against Legionella and other organisms resistant to alternative treatments such as chlorine
• Microorganisms cannot develop resistance to these solutions
• Cleansing agents decompose into water and oxygen
• Do not cause unpleasant taste or odour
• Formulated to minimize and limit corrosive effects
• No neutralization required, reducing costs, labour, water consumption and energy use – residual water pH is continuously monitored
• The solutions are fully and rapidly biodegradable
• The disinfection performed by our company is far superior because it disinfects the entire tank surface by spraying, using a disinfectant with an active ingredient concentration of 5 percent Hydrogen Peroxide and Silver, unlike bulk chlorine disinfection which occurs at concentrations of maximum 0.5 percent (ten times lower)

Below is a comparative analysis of disinfectants commonly used for potable water network disinfection

Biocides Comparative Table	Ammonium Salts	Phenolics	Aldehydes	Chlorine	Hydrogen Peroxide	Stabilized Hydrogen Peroxide
Spectrum	Medium*	Wide	Wide	Medium	Wide	Wide
Required Reaction Time	Short	Long	Long	Short	Long	Short
Effectiveness	Medium	Poor	Good	Medium	Poor	Good
Toxicity	Moderate	Moderate	High	Moderate	Low	Low
Residues	Yes	Yes	Yes	Yes	No	No
Corrosive Effect	Low	Moderate	Low	High	Low	Low to Moderate***
Environmental Protection	Poor	Poor	Poor	Poor	Good	Good
Odor	None	Yes	Yes	Yes	None	None
Biodegradability	70% in 20 days	85% in 20 days	90% in 20 days		100%	100%
Biofilm Action	No	No	No	No	Yes**	Yes**

For the classic high-pressure washing method (fire hose / pressure washer 140–180 bar), the following aspects must be considered:

1. For newly built concrete tanks, the concrete formulation is more economical and a hydrophobic coating of 5 to 10 mm penetration is applied at the end (e.g., Sipex, Xypex). When such tanks are washed with high pressure, the hydrophobic coating is completely removed, leading to future infiltration and water loss.
2. Another disadvantage is that after each washing, the surface becomes increasingly rough, creating cavities that promote bacterial growth. Chlorine dosing in water becomes insufficient for effective disinfection.
3. Finally:
   • Disinfection is carried out by filling the entire tank volume with chlorinated water – the tank is filled at the end of the process and chlorine is dosed accordingly. When performed correctly, the concentration must be at least 10–20 ppm. After disinfection, this method requires additional costs for draining the tank and neutralising the chlorine in compliance with environmental regulations.
   • Note: The duration of such an operation is significantly longer, depriving the operator of the tank’s availability. Moreover, the quantity of potable water required for rinsing is several times higher. As a result, the washing costs and the overall efficiency of the activity generate increased direct and indirect expenses, including the need for rehabilitation of contact surfaces, with a direct impact on the tank’s service life and operational performance.

With highest regards,
Ing. Szokoly Viktor
General Manager Proex Top SRL