Mysterious black residue on submerged stainless

[sbush]

One of our clients has a large commerical pool with a perimeter gutter system constructed of Type 304L stainless steel. After about five weeks of operation, they discovered a black residue, similar to carbon black but not grainy, only on one end of the pool, and only on the face of the stainless steel gutter where it is submerged in the pool water. The top of the gutter, above water, is clean and the Marcite (marblized, white sand and portland cement mixture) topcoat is clean. The residue will wipe off the surface and leaves no permanent stain; however, it will reappear the following day. We have collected a sample on a terry cloth swab, but we don't know what to analyze it for. The substance has no obvious odor. On the white terry cloth swab, the bulk of the sample appears to be similar to black ink stain, with light gray tint at the edges with some faint green tinting in a few spots. The operators have not seen any foreign matter floating in the pool or in the air above it. And, as I said, it appears only on one end of the pool (one end completely, plus the corners and a few feet on the adjacent sides).

Does anyone have any ideas or similar experiences?

S. Bush

http://www.water-eg.com

 

[Rich2001]

This might be Black algae. What is the pH? it might be slightly high.

 

[EdStainless]

Does this end get more/less sun? I am thinking like Rich, alge, or fungus is my first guess.
If you block sections of the gutter how uniform is the flow? If this is a bio-slime I would expect that this end is getting less flow.
What is pH and Cl?

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Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[sbush]

The pH is 7.5 and the free chlorine residual is 2.5 ppm. I don't have any test results on chlorides (Cl). Is this necessary?

The residue is not slippery to the touch and has no organic odor (or odor of any kind).

The area of deposition is on the "return side" of the gutter where the turbulence is greatest due to pumps discharging water back into the pool after filtration.

The filtration media is silica sand. There is no anthracite, carbon, or "black" filter media of any kind in use.

The pool is indoors and there is no direct sunlight available. The artificial indoor light is uniformly distributed on all sides of the pool.

Thanks again, for any help / suggestions / comments.

S. Bush

http://www.water-eg.com

 

[metengr]

sbush;
I have an above ground vinyl pool that likes to sometimes develop black algae or green algae growth during a certain time of the swim season. Black algae is normally hard to remove from surfaces, because it requires a stiff brush or stainless steel brush to remove after treatment - depress pH to 7 and shock the pool water. I have never seen this on metal surfaces, near the skimmer, only on the sides or bottom of my pool.

Just to be sure, I would have the water analyzed at a pool/spa supply shop (this is typically done for free) for free chlorine, alkalinity, pH and especially hardness ( dissolved minerals) that could be causing the residue (if anything you could eliminate the possibility). If the water quality is acceptable, I would drop the level of the pool to dry and clean the gutter surfaces thoroughly with some type of biocide treatment. If this is indeed black algae, the critters develop a barrier that cannot be easily penetrated with chemical treatment in water. Therefore, I would suggest dropping the level of the pool to dry the gutter and clean with a recommended polymer biocide.

 

[EdStainless]

It sound like the water treatment is right.
Take a piece of filter paper and wipe it.
Have it analysed for Fe and Mn.

Though I still suspect a biological cause.

What are you adding for biocide? Copper or silver? Better analyse for both of those also.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[sbush]

This pool is used by the Navy for training.

The treatment system consists of vacuum sand filtration, medium pressure ultraviolet light sterilization and calcium hypochlorite chemical disinfection.

What's puzzling is the fact that the residue only appears on the stainless steel surfaces and not the Marcite. The operators have suspended a stainless steel coupon in the water a few feet away from the gutter system, just to see what will happen. We are awaiting the results.

The pool was filled with municipal treated water meeting EPA drinking water standards and not containing high levels of iron, manganese, or organic matter. However, we will analyze our sample for iron/manganese/TOC.

Is there anything of an electrolytic nature, perhaps with the composition of the soil on one end of the pool, that could set up a reaction like this?

Thank you all for responding.

S. Bush

http://www.water-eg.com

 

[SMF1964]

Is this gutter continuously immersed in the pool water or is it intermittent? The bulk pool chemistry is 2.5 ppm residual chlorine, but what about the water in the gutter itself? If it is low or non-existent, it could be that you've got some sort of biological fouling going on that is using the stainless as a food source?

 

[sbush]

This is a rim-flow gutter system. Essentially, 75% of the gutter is submerged at all times. The manufacturer of the gutter system has details of the design on their web site:

http://www.paddockpool.com.

The gutter system is fabricated with a series of integral, conduits for carrying water away from the pool to filtration and treatment; and for returning water to the pool from the pumps. There is constant movement of water in the gutter system, although this is not visible from the pool or deck side. The pool is designed to "turnover" the entire hold-up volume at least once every 8 hours.



S. Bush

http://www.water-eg.com