Rust-like material in black tank

[chaps2018]

I changed my black valve last week and found a bunch of rust in the tank. There was also particles of rust that were blocking the valve to causing it to leak.

The only metal that I am aware in the black tank circuit are the sensors which I suspect are stainless. I have had my share of construction debris so this could be a possibility.

It seemed the rust material adhered to the inside of the tank as opposed to being just a bunch of loose particles.

I’m pretty diligent about the fill and dump practice. I am the only owner, no kids.

This makes me wonder if it’s the chemicals that I’ve been using for the past 4 yrs.

Initially I used happy camper but then stopped after a few months. For the most part I’ve been using the TST orange liquid, Thetford valve lube and occasionally dometic toilet bowl cleaner/tank chem pouches.

Anyone who has found this rust material in the black tank use these chems?

PS there was an old thread on this topic but never came to a conclusion.

PSS I don’t let the chems sit in the tank while we are not using the rig.

[TandW]

Struvite and or vivianite precipitate onto the walls of sewage containments. Since the abs tanks are not rigid this usually only turns into the flakes you are likely seeing.

[chaps2018]

Ok, so it appears my tank has kidney stones? That is funny but with your explanation it makes sense.

I do wonder if it is related to my chemical selection since it is not necessarily a common problem.

[astrnmrtom]

Just shooting from the hip here, but I wonder about iron precipitating out of the water via either the pH of the black tank water, or another chemical reaction. The rust may not be from anything rusting in the tank. Do you camp in area with high iron in the water?

If a tank treatment works by altering the ph of the water, this may have something to do with it.

Also found this: https://www.filterwaterdirect.com/in...-iron-bacteria

You mentioned construction debris. While I wouldn't suspect metal shavings to be likely in a plastic tank, I have found a surprising variety of things left in places from construction. An example might be a drill bit that came loose and fell into the tank or a nut or bolt. Still I can't imagine something that size generating that much rust although it might be "food" for the above mentioned bacteria.

One final thought is; are you sure it was rust, not a rust colored material?

Interesting mystery. Inquiring minds want to know.

[chaps2018]

Update--starting to see the same thing in my gray tank. I don't think it is rust but rather scale as noted above. I notice it as the final bits of gray water are exiting the pipe--the water looks like gritty hot chocolate (no smell tho). I have been to a couple of high-iron water content locations for several weeks (6 weeks at Elkhart Campground in late 2021 and in Indiana/Illinois again in 2022) and expect that may be the cause. This would also explain why it is not a widely seen problem (ie, not the toilet chemicals causing it but rather localized water sources).