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Old 02-12-2014, 11:38   #1
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How to get rid of tank scum?!

Hey all,
We've discovered tank scum (or mold as we've been told) in our tank. It looks like black slimy kelp. Added bleach to the tanks to try to get rid of it but it just turned into milky white slimy kelp. It clogs up the fresh water pump filter and may also be the cause of pump problems. Does anyone have advice on how to get rid of the scum?
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Old 02-12-2014, 11:45   #2
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Re: How to get rid of tank scum?!

I should add each of our four tanks in 50+ gallons and only used about 8 oz of bleach per tank. Also our broker/friend said we could use Water Shock from west marine. Just wNted to get your thoughts.

Thanks
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Old 02-12-2014, 11:47   #3
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Re: How to get rid of tank scum?!

If it's just in the tank(s), I would disconnect them from your system and then give them a good scouring/flushing, assuming there is an access port. Drain them into a bucket until they are clear.

If you used 8 oz. per 50 gallon tank, then likely the water is disinfected, but the scum you see is the product of the (formerly) living bacteria. The issue is getting it out of the tank.

Going forward, you should treat your tanks once a year, with chlorine, 1/3 cup per 100 gallons. Ideally, you should then test for residual chlorine (buy a pool test kit at HD which will test down to .5 ppm, which is adequate) ...if there is still residual chlorine, then disinfecting is complete. If not, add more chlorine incrementally until you get residual chlorine in the test.

A bigger question might be what shape the rest of your system is in. After the tanks are clean and you've treated the water while filling them, run the water through the entire system, let it "soak" for at least 30 minutes, and then test what's coming out of your plumbing. If there is no residual chlorine, then there is more disinfecting required. Continue to cycle water through the plumbing runs until you get residual chlorine in your test.

I would shy away from Water Shock from WM for the simple fact that you don't know what's in it and how effective it is at disinfecting. Bleach (chlorine) is the de facto preferred method, tried and true, along with UV and ozone treatment.
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Old 02-12-2014, 19:39   #4
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Re: How to get rid of tank scum?!

You need to get rid of the biofilm. In the brewing industry, unfortunately, the only way to effectively get rid of both gram positive bacteria and gram negative bacteria and molds is by physically scrub the walls. This is one of the reason large fermentors have manholes. So you can climb in and scrub the walls to get rid of slime and scale. In between we used clean-in-place high velocity nozzles.

By just using chlorine you could be making things worse because the top of the biofilm dies and forms long chain polymers and saccharides that form a protective layer on the bacteria and molds inside below that layer, allowing them to reproduce at will, making the spread worse.

The only way around this is to physically clean the insides and then prevent it from coming back by using a sanitizer like bleach.
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Old 02-12-2014, 20:36   #5
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Re: How to get rid of tank scum?!

I have two 75 gal water tanks and two 40 gal fuel, port and star. They have been setting for a half a decade. Each has an inspection port at one end. I can see into the first baffled chamber, but no farther. i'm thinking of cutting some more holes in each chamber so as I can get a pressure washer wand down in there. Looking to get a way of physically cleaning them out and then going down the bleach road.
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Old 17-12-2014, 01:08   #6
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Re: How to get rid of tank scum?!

If your able to add access ports to the tank. Scrub with hot soapy water, it shouldn't be to hard to add 8"or 10" ports.
Just a hole and some bedding compound, and screws.


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Old 17-12-2014, 04:03   #7
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Re: How to get rid of tank scum?!

Bleach will destroy aluminum tanks over time, I prefer hydrogen peroxide

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Old 17-12-2014, 09:32   #8
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Re: How to get rid of tank scum?!

If your water tanks have no cleanouts, this is the solution.
Seabuilt - Access Plate Systems
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Old 17-12-2014, 09:45   #9
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Re: How to get rid of tank scum?!

You need to physically scrub the walls of your tanks. No chemical will magically clean this from the tank walls.

Once you've done this, disinfect them, rinse and use normally. If you're using treated city water in your tanks, you should have no problems in the future. If not, consider adding a small amount of unscented bleach every time you add water. And use a drinking water safe hose that's used for nothing else to fill the tanks. Don't leave it on the dock pedestal where other boaters can use it to rinse their holding tanks.
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Old 17-12-2014, 09:59   #10
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Re: How to get rid of tank scum?!

8 oz in 50 gallons is a lot. But you are going to have to manually clean those tanks out I think. Is the pumpout/drain from the bottom or the top?
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