The many variations of the humble water tank have evolved in different ways, according to the kind of water use that they are designed to complement. Despite the differing requirements for commercial, industrial, farming and domestic applications, the basic intention behind water storage remains the same: water tanks, by design, should protect the water from contamination or negative influences, such as bacteria, viruses, algae, fluctuating pH levels, and the accumulation of minerals or gas.
Increasingly, however, water companies and municipalities are being encouraged to focus their efforts on sustainability and the environment in their water distribution and management systems. In terms of tanks, both the type of materials used and the application of protective coatings have become a consideration of experts with a greener focus.
A basic guiding rule for decisions about tank materials is that all things should be considered in view of the site-specific circumstances of the tank. This means that, depending on the water use, tank materials could range from plastics (polyethylene, polypropylene), to fibreglass, concrete, stone and steel in various forms (welded, bolted, carbon or stainless).
With the environment in mind, steel is the sustainable choice. The construction of steel tanks produces less carbon dioxide (CO2) than other materials, and if you choose a bolted design, there’s an even greater reduction. Steel is also 93 per cent recyclable, so when a bolted steel tank reaches the end of its useful life, it’s more economical and requires less energy to remove and recycle it, compared to removing one of its concrete or field-welded counterparts.
A word on corrosion
Corrosion is a water tank’s biggest enemy, and, unfortunately, no material is immune to degradation in all environments. The corrosion control of different materials should be considered carefully at the design stage, and should be matched to the environment in which water tanks will be situated. That said, protective coatings are the most widely used technique for controlling both interior and exterior corrosion of water tanks. High-quality coatings will give durability and longevity to your tank, so they should be a top priority.
Protective coatings work by separating the surfaces that are vulnerable to corrosion from the corrosive elements of the surrounding environment, so chemical compounds and moisture are prevented from having contact with the substrate. By choosing the most effective coating for the particular application, and by selecting the right thickness of the coating according to the substrate, the life expectancy of bulk water storage structures can be significantly extended. Other economic benefits include fewer repairs and lower operating costs.
The newer high-solids epoxies are not only better for the environment, but they also offer a number of application and performance advantages, as well as being more user-friendly. The lower molecular weights of these epoxies create smaller, lower-viscosity molecules, resulting in increased crosslink density, which provides a greater resistance to chemicals and moisture, and improves adhesion. Epoxy coatings are flexible, not brittle, and because they can bend without cracking, they provide a durable coating. For a tank’s exterior, you can combine an epoxy coating with a polyurethane topcoat for better protection against the environment.
Protective coatings provide thorough protection for the external surfaces of tanks, but with the interiors of tanks usually creating a far more aggressive corrosive environment, it can be sensible to combine a protective coating with cathodic protection in steel tanks. These two factors working together make for an effective tank maintenance system, as their relationship is symbiotic. Cathodic protection works in conjunction with a coating system, to mitigate corrosion at pinholes, voids and holidays, while the coating electrically protects the bulk of the interior surface, which lessens the voltage required for the cathodic protection system to function. High-solids epoxies have been shown to work well with cathodic protection systems because their protective films contain a lower percentage of pinholes and voids.
A four-year study conducted by the United States Navy concluded that cathodic protection could double or even triple the time between re-coatings. Of course, this kind of performance has to be monitored and adjusted so as to produce the correct amount of electrical current for optimum protection. And there is a risk of overdoing it - too much current output can cause coating disbondment and blistering. But, if you can extend the working life of your water tank, the benefits will be felt not only in your pocket, but also in your sustainability outcomes.
Ultimately, though, the length of time a tank lasts is dependent on other factors, too: namely, water quality, maintenance, and the positioning of the tank. While water storage is an integral part of any water distribution system, the individual parts of the system need to be considered separately, as well as together with the whole, for the system to be wholly sustainable. Remember that as well as tanks, water sources and conduits need to be assessed regularly and treated accordingly to maintain a healthy water distribution system.