Noflex/sodium percarbonate bad for steel tanks?

[dasnider]

Hi all,

I've got a 20-year-old steel blackwater tank in my boat, which is so far beating the odds by not leaking - yet. In fact it seems remarkably solid, at least in the places I can get to.

I've recently switched from Eparcyl to Noflex Digestor, which is mostly sodium percarbonate. The improvement has been remarkable - no more odor! However after thinking about it I'm a bit concerned: sodium percarbonate works by introducing oxygen into the black water, which causes aerobic bacteria to flourish and reduces the anaerobic bacteria (which cause the smells). But oxygen also causes rust! Perhaps one of the reasons my tank has lasted so long is that there was very little oxygen in the black water.

By introducing more oxygen into the tank (in addition to Noflex I also added a fishtank bubbler) am I signing a death warrant for my venerable old steel tank??

I'd appreciate advice from anyone who has experience or expertise in this area... (Looking at you, Peggie! ;-)

- David

[rslifkin]

I wouldn't be surprised if more oxygen increases the corrosion rate in the tank. But Noflex is also somewhat basic if memory serves, so it should make the tank contents less acidic.

Personally, I'd plan to replace the tank before it fails and not worry about whether smell reduction tactics are slightly shortening its lifespan.

[dasnider]

Thanks rslifkin, good to know.

I'd love to replace the existing tank but unfortunately it's underneath the floor and an extensive set of built-in cabinets and a woodstove on a tile platform, all of which was nailed AND glued in place. The tank is the full width of the boat and it'd be a major project to get it out.

The builder says he's never had a tank fail yet, so I'm going to cross my fingers. Maybe in a year or two I'll be ready to bite the bullet and rip the saloon apart.

- Dave

[thinwater]

Originally Posted by rslifkin

I wouldn't be surprised if more oxygen increases the corrosion rate in the tank. But Noflex is also somewhat basic if memory serves, so it should make the tank contents less acidic.

Personally, I'd plan to replace the tank before it fails and not worry about whether smell reduction tactics are slightly shortening its lifespan.

He makes a good argument, or rather raises a good question. I doubt that has been tested, so people will guess.


Yes, percarbonate releases H2O2 and sodium carbonate (yes, I know they are ions in water). First, you can be sure the peroxide is consumed yearly instantly. The tank will remain anoxic (you could measure DO and redox potential if curious). Thus, the primary corrosion mechanism will remain sulfate reducing bacteria, which create acid. I'd be more worried about the pH staying above 7.5. So my guess is that the effect is minor. But I'm with RSLIFKIN; I would not add oxygen by any mechanism. Open steel waste tanks always fail at the waterline because the oxygen is highest in the splash zone. In fact, it can generally only rise high enough in the splash zone (the bacteria suck up all of the oxygen farther down). Your tank is semi-sealed, so the oxygen eve in the air space is probably 5-10 times lower than than our atmosphere.



I'll be honest and say that although I'm a chem E and do wastewater treatment, I've never investigated that. I am amazed that the tank is not leaking; uncoated steel usually has a safe life expectancy of only a dozen years or so at typical thicknesses. In the absence of oxygen, as you see, it can do much better.

[dasnider]

Thanks #thinwater, just the sort of info I'm looking for - and from an expert in the field! I'm going to stop the oxidation treatments and just live with a slightly smelly boat again. Rather do that than deal with a leaking tank!

BTW, the one place I did find visible corrosion was around the tank gauge, which was a nylon housing but held on with stainless steel screws. There was some corrosion spreading out from the gauge hole, which I was able to repair with a larger mounting plate. I'm guessing it started with the screws and then spread out as air was able to get in around the compromised screw holes. I sounded the few parts of the tank I was able to access and the metal sounded quite solid everywhere else.

The tank is made of 8mm steel, so that may be another reason it's lasted so long. At some point I'll bite the bullet and replace the whole wastewater system, incl. hoses, but hopefully this'll buy me some time.

- David

[peghall]

Oxygen isn't corrosive, in fact is the essential "ingredient" in preventing odor because it creates an aerobic environment in the tank. When organic matter breaks down AEROBICALLY it generates CO2, which is odorless...but generates sulfur dioxide and hydrogen sulfide--the stinky gasses--and methane, which is odorless but flammable, in ANAEROBIC conditions. It's the reason why running streams don't stink but stagnant ponds do. Ever notice that sewage treatment facilities have ponds with fountains in them? This is the reason. Aerating a holding tank is an effective odor prevention that works when the tank vent doesn't allow sufficient air exchange with the gasses in the tank to work.

Urine is the real culprit in the destruction of metal sewage holding tanks...it's so corrosive that it can eat through welds at seams or fittings (the first leak) in as little as 5 years and can turn the bottom of the tank into a colander in an average of about ten years. Wall thickness of metal tanks determine the rate at which urine eats through them.

--Peggie

[dasnider]

Thanks for chiming in, Peggie! I don't dispute that oxygen is beneficial for aerobic bacterial breakdown of black water, and I'm very happy with the results from a smell perspective.

But I think you're wrong about oxygen not causing corrosion: oxygen + H2O + iron = rust, which is a form of corrosion. So for me, with my metal tank, it's a tradeoff between odor vs corrosion, and I'd rather live with a slightly smelly boat than risk having to do an emergency tank replacement.

When I finally do manage to replace the tank (with a plastic one!) I'll definitely put in a better ventilation system and go back to using percarbonate.

- David

[dasnider]

@thinwater, you noted that "The primary corrosion mechanism will remain sulfate reducing bacteria, which create acid." Do you (or does anyone else) know which type of bacteria (aerobic or anaerobic) creates more acid and thus more corrosion? If it's anaerobic then this is a good case for oxygenating (encourages growth of aerobic bacteria, which suppress the anaerobic ones), whereas if aerobic bacteria create more acids then I should stop oxygenating, so that the anaerobic ones will flourish...

- David

[thinwater]

Originally Posted by dasnider

@thinwater, you noted that "The primary corrosion mechanism will remain sulfate reducing bacteria, which create acid." Do you (or does anyone else) know which type of bacteria (aerobic or anaerobic) creates more acid and thus more corrosion? If it's anaerobic then this is a good case for oxygenating (encourages growth of aerobic bacteria, which suppress the anaerobic ones), whereas if aerobic bacteria create more acids then I should stop oxygenating, so that the anaerobic ones will flourish...

- David

The bottom of the tank is always anaerobic. Not the bulk liquid, but the film next to the metal, because oxygen does not penetrate the stuck on sludge.



In my expereince, steel sewage tanks always perforate around the splash zone, where there is a lot of oxygen. This waste was mostly from portable toilets, at the plants I am thinking of, so the solids/urine ratio is probably not that different from boats.



One thing that puzzels me is using plain steel. I would not expect it to last more than 5 years at best, probably less. Are we sure it is not coated (epoxy) on the inside? That is normal practice. Then it is all about how good of a job they did.


If in doubt, use a nitrate-based additive instead. Odorlos for example. Camco makes some (read the ingredients).

[Lepke]

I knew a company that sold septic tanks that didn't need a drain field. It increased aerobic activity by using a bubbler at regular intervals. The final liquid coming from the tank was clear and bacteria free. Not needed in a tank you pump out.

On a steel tank adding something with sodium (salt) in the name doesn't sound like a good idea.
Who knows what the alloy of your steel tank is, but iron plates haven't been made since before WWII.

[dasnider]

Thanks for all the suggestions, @thinwater & @lepke. The tank is made of 8 mil steel, so that could explain why it's lasted so long. It's long and shallow - it's located under the floor between the ribs, and stretches the width of the boat, almost 4M. The toilets flush into the port side, and the pumpouts (and the single 1" vent) are on the starb side.

I suspect another reason for its longevity in addition to the thick steel is that it's not well-vented, so there's not much O2 getting in there to cause rust. I'm reluctant to change anything, given that it's lasted so long, but I would like to reduce the odor. What would a nitrate-based additive like Odorlos be likely to have on corrosion, either from rust or from acidity?

[pbmaise]

A steel tank with no leaks cannot transmit an odor into the vessel. So you may be treating the tank for a problem that resides elsewhere.

Have you replaced the blackwater hoses and lines to the heads?

[dasnider]

Thanks for the suggestion, @pbmaise, but I don't know whether the tank is leaking or not, and I haven't replaced the hoses yet for the same reason I haven't replaced the tank: they're inaccessible without extensive deconstruction/demolition. They're 20 years old so there's undoubtedly some odor coming through, but reducing the holding tank odor (e.g. by introducing more oxygen) would reduce any odors leaking through the hoses as well. However I'm reluctant to oxygenate it if that would exacerbate the corrosion; I'd rather live with the odors for a bit longer!

[thinwater]

Originally Posted by pbmaise

A steel tank with no leaks cannot transmit an odor into the vessel. So you may be treating the tank for a problem that resides elsewhere.

Have you replaced the blackwater hoses and lines to the heads?

[I'm assuming he meant 8 mm (314 mil), not 8mil.]


The other problem can be odor from the vent outlet, depending on location. I had a cat where the vent was up-wind of the hatches and every time the head was flushed ... weew. Move the vent or install a vent filter (which is a complication of its own--not the best solution for most, but it can work if done properly and the hoses are good).


Sail Delmarva: Holding Tank Odors vs. Carbon Vent Filter

[pbmaise]

Originally Posted by dasnider

Thanks for the suggestion, @pbmaise, but I don't know whether the tank is leaking or not, and I haven't replaced the hoses yet for the same reason I haven't replaced the tank: they're inaccessible without extensive deconstruction/demolition. They're 20 years old so there's undoubtedly some odor coming through, but reducing the holding tank odor (e.g. by introducing more oxygen) would reduce any odors leaking through the hoses as well. However I'm reluctant to oxygenate it if that would exacerbate the corrosion; I'd rather live with the odors for a bit longer!

My one off vessel also had plumbing that wasn't well thought out for future repairs. For example hot/cold lines ran to my forward head and then split into Tee with multiple hose clamps before going to shower and sink. These were located under the shower and impossible to maintain. This meant I had six potential leak paths and six hose clamps that couldn't be maintained in that one area alone.

I also couldn't get to hose clamps to the taps of the shower. When you factor in the number of heads and galley fittings that also couldn't be maintained I had at least 36 - decades old hose clamps I couldn't reach.

Kind of makes you worried if you don't turn off the water pumps when leaving the boat for the day. One might easily drain their entire fresh water tank into the vessel.