Waterproofing a potable water tank has to satisfy two separate requirements at once: keep water in (or, for underground reservoirs, keep groundwater out) and not contaminate the water it holds. That second requirement rules out a large share of otherwise perfectly good waterproofing products — many standard membranes and coatings aren't certified as safe for contact with drinking water, so 'food-grade' certification is a genuine specification requirement, not a marketing label.

Polymer-modified cementitious coatings are the most common choice for concrete tanks, largely because they bond permanently to the concrete substrate and are available in food-grade formulations. Some are elastic and crack-bridging, useful for tanks that flex slightly under the weight of water or ground movement; others are rigid and rely on the crystalline or dense-matrix effect to resist water pressure. Epoxy coatings are also used, valued for strong adhesion and abrasion resistance, particularly where the tank sees regular cleaning or mechanical wear.

Cracks and joints need attention before any coating goes on — a food-grade crack filler or putty seals visible defects first, since coating over an active crack just delays the leak rather than stopping it. For new tank construction, incorporating a waterproofing admixture directly into the concrete mix gives an extra layer of protection that doesn't rely solely on a surface coating remaining intact for the tank's entire service life.

For a tank already in service and showing leaks, hydrophilic polyurethane injection grout is a practical repair option — it can be injected from outside the tank without draining it, sealing cracks and joints in foundation walls and floors from the exterior. That matters practically: draining, cleaning, and prepping a tank surface for a full internal re-coat is expensive and puts the water supply out of action, so a targeted external repair is often the more realistic first option.