Protected by special additives and coatings, vinyl pool liners can withstand the extremes of sunshine, heat, cold and constant exposure to chemically treated water. However, even the highest quality vinyl liner is subject to staining if proper water balance is not maintained, or if debris is allowed to remain in the water.
The following technical information should help pool professionals avoid the black and pink stains that often plague vinyl liners — and deal with them when they arise.
Black staining that appears on vinyl pool liners can originate from a number of sources, and primarily falls into two categories: metal staining and black algae. Depending on the type of stain, different treatments are required to correct the problem.
Copper, iron and manganese may be introduced into the pool via source water. They can form oxides in chlorinated pool water, and can precipitate out of solution, resulting in stains on the pool liner. These stains are generally black, brown or gray. Copper can also dissolve from copper or brass fittings in the plumbing when pool water pH conditions of less than 7 occur. This metal may also be present in some algaecides, though most now use copper in a chelated or complex form that remains in solution.
The presence of metal staining can be confirmed by treating a small portion of the stained area with a pH reducer to dissolve the metals. If the stain can be removed by this treatment, the staining is a result of metal deposits, and the remainder of the stains can be treated in a similar manner. If not, the stain is likely due to an organic source such as black algae (see below). If the staining is due to metals, the pool water may need to be treated with a metal treatment — such as a sequestering or chelating agent — once the staining has been removed, in order to prevent a reoccurrence.
Black algae appear as a series of small black spots on the pool liner. They are very tenacious organisms with a chlorine-resistant coating. First, brush the algae spots using a nylon brush to open up the algae coating. Next, test the pH of the water and reduce it to the lower limit of the normal operating range (7.2) to improve the effectiveness of the chlorine. Then, superchlorinate the pool and add a dose of a quaternary (“quat”) type algaecide. Make sure to follow the recommended dosage from the manufacturer, as excessive usage may result in foaming. Continue to brush the algae stains to maximize the penetration of the chemicals. Vacuum the dead algae to the drain once they have been killed.
Twenty-four hours after superchlorination, add a dose of a polymer algaecide (“polyquat”) as per the manufacturer’s recommendations. Polyquats are more expensive than regular quaternary algaecides, but they’re also more effective in controlling these resistant types of algae.
Once the staining has been removed, resume normal chlorination and water balance. Remember, the best protection against algae growth is a constantly held free chlorine level in the range of 1-3 ppm, a total alkalinity between 80 to 120 ppm, a pH between 7.2 to 7.6, and a calcium hardness of 200 to 300 ppm.
Another type of gray/black colored stain can occur when dye-producing microorganisms colonize the back side of a vinyl liner. The microbial dye becomes visible on the pool side of the liner as it wicks through the liner’s material, creating an irregularly shaped blotch. The stains on the pool side can be temporarily diminished through superchlorination, but they’ll reappear, since the source of the stain originates from the back side of the liner. Installation of a polyethylene barrier between the vinyl liner and the walls and floor of the pool can provide a barrier to these types of organisms.
Pink blotches that appear on liners are also likely caused by bacterial dye. Because the dye is highly soluble in the plasticizers used in flexible PVC pool liners, it can easily migrate through the liner.
The portion of the dye that is exposed on the surface can be bleached by chlorine; however, new dye will continue to migrate to the surface. The bacteria can become established on either the water side or back side of the liner. Growth on the water side may occur if free chlorine levels are allowed to drift below 1.5 ppm at the same time that organic matter and bacteria have accumulated in the water.
Superchlorination at this stage will rid the pool water of the contamination. But if the dye has penetrated below the surface, staining tends to linger indefinitely.
Growth on the back side may not take place directly on the liner, but rather on some other material in contact with the liner such as soil, or on a backing material like foam, felts or taping. Even though an anti-microbial agent is incorporated into the vinyl formulation, the dye can migrate from unprotected components and stain areas well beyond the point of infestation. If a lot of pink dye is visible on any backing material, it will very likely be the source of the problem.
If the liner is replaced, all contaminated materials must be removed and the entire pool shell (floor and walls) must be disinfected with a liquid chlorine spray or other suitable disinfectant.
Special problems are presented by locations that have high water tables, which continually bring water loaded with micro-organisms to the back side of the liner. Using disinfectants at these sites may be ineffective, since they will be quickly washed away. A possible defense may be some type of barrier layer; either a plastic sheet, perhaps polyethylene between the pool shell and liner, or a barrier coating of some kind applied directly to the pool shell.
Following the diagnosis techniques here, it’s often possible to head off vinyl stains before they spread, and possibly even to remove them — if they’re caught in time.