Black streaks in commercial spa

[ccpc raj]

I have a commercial spa that is inground gunite. It is starting to form black streaks around the wall of the spa. This is definately not Black Algea. My readings are as follows:

Chlorine- 3.0
pH-7.6
Alk-70
Calcium hardness- 150
CYA-0
TDS-500

I brushed with a steel brush and got some of the stains off, but some came back within 3 days. I have drained the spa and refilled it with fresh water and rebalanced the spa. This spa also has a heater and some above ground copper piping. I am at my wits end, please help if you can[/b][/b].

[chem geek]

Your numbers look good. You could have the water tested for copper in case the stain is from high levels of copper -- such stains can look green to black. What is interesting is that they are near the surface, but since this is a spa the aeration of the water makes the pH rise and perhaps it's rising more near the surface precipitating out copper in the water.

If you do find that there is copper in the water, then a metal sequestrant would be the solution, BUT given that you drained and refilled it would be strange to have that much copper so soon.

You can distinguish between a metal stain vs. an organics stain by taking a Trichlor puck/tab and rubbing it against the black area. If it fades, then it's organic. If it's organic, then it's more like a nasty tar-like scum line so perhaps an enzyme product might help break it down before it forms (scum balls may help as well), but that doesn't tell you the source -- do you have any trees near the spa? Perhaps it's some form of pitch.

I agree that with 3 ppm FC and no CYA in the water that it is unlikely to be algae. That's a very high level of chlorine, but I know you probably aren't allowed to use any CYA in the commercial spa water (too bad for people using the spa -- the higher disinfecting chlorine level will flake their skin and degrade their swimsuits much faster and will also create disinfection by-products such as nitrogen trichloride faster).

Richard