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Crystalline residue on top of waste tank


T and H

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Hi all. We have a waste tank under the bed (waste is pumped through a macerator from the toilet). The tank has been a bit smelly since we got the boat in the summer (more of chemicals than of waste, but sometimes smells of waste too).

We have taken up the bed panel to find grotty (!) crystalline residue on top of the tank. We did this about three months ago and cleared off all the residue. We have just taken up the panel again to find the crystalline stuff is back!

Has anyone encountered it before? Is it possible that the tank has tiny holes in and is somehow pushing crystals out of the tank? Does anyone know what it is? Some of them are white, some grey/green/black and some brown  

Have searched the forum and internet in general but really stumped! If anyone has any ideas we would be so grateful!!

Thanks,

Helena

 

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Weird! The smell might be the clue. A steel tank should not smell but of course the pipes and fittings might. The crystals have to be coming from somewhere and therefore I would guess it is small holes. I would suggest 2 things:

  • find the most accessible growth and clean it off as best you can, and wash with water to remove anything that might be in a hole (if there is a hole). Then inspect with a good magnifying glass. You may be able to see if a hole is present

  • Try and record what the smell is like when the tank is full (ish) or empty (ish). If the smell is consistently better when it is empty then that may point to the tank at fault rather than the pipes and fittings.

See if you can find someone with a thickness tester and see what you get around the areas of concern.

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From the pictures i assume these are on the top surface of the tank. Do you run the tank until its absoloutely FULL. It looks like its leaking from small corrosion holes on the top surface. Time for a new tank methinks or fit a cassette toilet as I did.

Edited by Loddon
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32 minutes ago, tree monkey said:

Complete shot in the dark but how about clean the debris off, smear a thin film of washing up liquid over where the corrosion was, flush the loo and look for bubbles?

Would that help identify if there are holes?

Depends on how big the breather pipe is, assuming there is one

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So helpful. Thanks everyone. 

Yes the residue is on the top. But no we have never run the tank totally full (that’s what’s weird about how stuff could force its way up through holes anyway). 

Good idea to to look for holes - magnifying glass is a great idea and will try washing up liquid. Breather pipe is about 3” though.

Initially we did think pipes would be more likely re the smell, and we did reseal round the pipes. Have considered replacing them but the residue stuff is pointing towards tank rather than pipes. Probably both!!! 

Yeah might need new new tank or cassette :-( time to save up. 

The previous owners used to self pump-out but I’m not sure whether this would make any difference. They also said that they had once had a problem with smell which turned out to be the gauge for how full it was so they disabled it and sealed it off (we pump out quite regularly so I’m still sure that we haven’t run it full).

Thanks for the advice!

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Tank must be porous, urine does create chrystaline deposits in drainage systems. If you can't afford to replace the tank I wonder if you cleaned the deposits off, and then paint the affected areas with bitumen might get you out of trouble temporarily? 

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So helpful. Thanks everyone! Maybe they did use yeast! We definitely haven’t. Yeah definitely not droppings, although I am going through  all the sci-fi options of what the monster in the tank could be..

Tank is definitely in in poor condition. We haven’t got hold of a magnifying glass but do think they are small corrosion holes (not sure how it gets corroded?). 

Bitumen is a great suggestion as buying a new tank is not really on the cards right now (Dear family. We didn’t get any of you a Christmas present as we needed a new poo tank).

Thanks again!

 

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1 hour ago, T and H said:

Tank is definitely in in poor condition. We haven’t got hold of a magnifying glass but do think they are small corrosion holes (not sure how it gets corroded?). 

If the tank is in poor condition, I would pump out asap and invest in a cheap porta loo. It would not be good if the sides of the tank that you cannot see are worse than the sides you can. No amount of bitumen would hold the contents inside. The clean up doesn't bare thinking about.

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2 hours ago, T and H said:

So helpful. Thanks everyone! Maybe they did use yeast! We definitely haven’t. Yeah definitely not droppings, although I am going through  all the sci-fi options of what the monster in the tank could be..

Tank is definitely in in poor condition. We haven’t got hold of a magnifying glass but do think they are small corrosion holes (not sure how it gets corroded?). 

Bitumen is a great suggestion as buying a new tank is not really on the cards right now (Dear family. We didn’t get any of you a Christmas present as we needed a new poo tank).

Thanks again!

 

I would be tempted to put a layer of 2 pack epoxy over the whole of the top and anything of the sides you can see. Maybe 1-2mm thick on the top. It will stop the internal smell getting out better than bitumen and it wont smell like bitumen will.

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Looks like a stainless steel tank, most grades of ss need oxygen to resist corrosion, if it's exposed to  oxygen free (anaerobic) conditions it will corrode, highly likely in a holding tank. Leesan stopped using ss tanks for that reason. 

Edited by nb Innisfree
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4 minutes ago, nb Innisfree said:

Looks like a stainless steel  tank, most grades of ss need oxygen to resist corrosion, if it's exposed to  oxygen free (anaerobic) conditions it will corrode, highly likely in a holding tank. Leesan stopped using ss tanks for that reason. 

But I thought you had to have abrasion too, the abrasion takes the protective oxide layer off the lack of oxygen prevents a new one from forming?

Stainless does appear to have a few nasty habits.

.............Dave

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Dr Bob - this is hilarious. We did think bitumen might be difficult so got some similar roof sealant stuff instead that was recommended for metal and easy to apply. Turns out.. Incredibly smelly and creates some fairly noxious fumes...we have evacuated!

Swings and roundabouts though - went out for a nice pub dinner and staying with friends who have a bath. 

...but 2 pack would have been a good idea!!

Thanks mad dog. We have managed to pick the tank up after pumping out and look at the bottom and all the sides. Everywhere is fine apart from the top (weird but true). 

That is interesting about corrosion. Makes sense. I wonder how the oxide could have been damaged. I guess anything could have gone in the tank at some point.

PS the cling film in the picture was from an experiment a couple of months ago when we suspected the pipes of causing the smell. Entirely ineffectual.

 

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1 hour ago, dmr said:

But I thought you had to have abrasion too, the abrasion takes the protective oxide layer off the lack of oxygen prevents a new one from forming?

Stainless does appear to have a few nasty habits.

.............Dave

Dunno but it does appear to affect the inside of waste tanks. Maybe water tanks too? Could be the wrong grade of ss used due to ignorance? 

Many questions, not enough answers. 

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15 minutes ago, nb Innisfree said:

Dunno but it does appear to affect the inside of waste tanks. Maybe water tanks too? Could be the wrong grade of ss used due to ignorance? 

Many questions, not enough answers. 

COR-TEN steel stops corroding and stabilises after a while, but in very wet tropical conditions it doesn't work, maybe certain ss grades behave in a similar way? 

Maybe regular aeration of an ss tank would be worthwhile? 

More questions? 

Edited by nb Innisfree
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15 hours ago, mad dog n' Englishman said:

If the tank is in poor condition, I would pump out asap and invest in a cheap porta loo. It would not be good if the sides of the tank that you cannot see are worse than the sides you can. No amount of bitumen would hold the contents inside. The clean up doesn't bare thinking about.

The only reason for suggesting bitumen is because it would appear that it is vapour rather than liquid escaping, and a cheap, temporary solution. In no way would it constitute a viable long term repair.

I wonder if the top has failed due to grinding debris? Was told by someone that the steel dust from grinding locks onto the stainless and acts similarly to an anode, rusting at an accelerated rate.  

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1 hour ago, BWM said:

The only reason for suggesting bitumen is because it would appear that it is vapour rather than liquid escaping, and a cheap, temporary solution. In no way would it constitute a viable long term repair.

I wonder if the top has failed due to grinding debris? Was told by someone that the steel dust from grinding locks onto the stainless and acts similarly to an anode, rusting at an accelerated rate.  

dunno about that but grinding debris can certainly make a right mess of car  windscreen :D

There is a dissimilar metals corrosion between mild steel and stainless but the potential difference is not too serious, but I believe that in the absence of oxygen the stainless won't form its protective layer so has a much greater potential.

I used to repair the chassis on my old VW van with stainless and with hindsight I think I was pretty lucky that it worked as well as it did.

...............Dave

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8 minutes ago, dmr said:

dunno about that but grinding debris can certainly make a right mess of car  windscreen :D

There is a dissimilar metals corrosion between mild steel and stainless but the potential difference is not too serious, but I believe that in the absence of oxygen the stainless won't form its protective layer so has a much greater potential.

I used to repair the chassis on my old VW van with stainless and with hindsight I think I was pretty lucky that it worked as well as it did.

...............Dave

When our boat was in for rebottoming I installed a stainless water tank and noticed that grinding waste had attached itself to the tank, you couldn't easily brush it off, not scientific but something I observed. I didn't realise that stainless formed a coating, is it similar to the oxidized surface found on aluminium? 

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1 hour ago, BWM said:

When our boat was in for rebottoming I installed a stainless water tank and noticed that grinding waste had attached itself to the tank, you couldn't easily brush it off, not scientific but something I observed. I didn't realise that stainless formed a coating, is it similar to the oxidized surface found on aluminium? 

I didn't either till recently, there is some good stuff on the www about it. It looks like its a bit more complicated than just an oxide, and that its the various additives (chromium etc) reacting with the atmosphere that are responsible. I suppose if was just an oxide of steel then it would be rust!

............Dave

 

Edited by dmr
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