Thick crust on blackwater tank: suggestions?

[dasnider]

Got a rather sticky problem with my head, so to speak, that I'm hoping I can get some suggestions on!

My boat has a 20-year-old steel holding tank that I'm trying to avoid replacing. (It's underneath the floor and under some extensive built-in cabinetry and a wood stove, all of which would have to be removed.)

When I replaced the sender unit recently I discovered there was a couple of inches of gooey sediment at the bottom of the tank, covered with a hard, inch-thick crust. The previous owner used to leave the boat for long periods and the sediment probably dried out each time, adding to the crust.

I was able to break some off and pull it out through the sender hole (thankless task!) and it's quite hard. However the tank is almost 4M long and I can only reach a small area. I'm reluctant to put in aggressive chemicals for fear of corroding the old tank. Here's what I'm planning to do; if anyone has experience with this then please chime in!

• Cut a new hatch on the starb side, by my pumpout flanges.
  
• Reach in (yuck!) and break up and remove as much of the crust as I can
  
• Go to a pumpout station (or hire a pumpout barge) and stick the pumpout hose into the hole.
  
• Run water into the port side of the tank, to flush sediment toward the pumpout hose.
  
• Hopefully this will also push the crust toward the hole, so I can continue breaking it up and removing it.
  
• Continue flushing, pumping and breaking up the crust until things look clean - or until I can't take it any more!

Let me know if any better suggestions. Or if anyone wants to help then I'll provide beer & pizza! ;-)

- David

[peghall]

First, welcome aboard cruisersforum, David!

I hate to break it to you, but that tank needs to be replaced with a plastic tank. When holding tanks became necessary in the early '80s lot of boat builders installed metal holding tanks, mostly aluminum ...steel was a major upgrade. However by the early '90s it had become obvious that metal was a bad choice because urine is so corrosive that it eats through any metal holding tank into a colander within an average of about 10 years. The first leak typically happens in about 5 years--a weld in a seam or a fitting, meanwhile urine was gradually turning the bottom of the tank into a colander. That "crust" is now the only thing that's preventing it from leaking. Hopefully it's empty and dry, allowing you to cut it up to remove it without making a major NASTY mess. So do NOT put any water into it...or if you have, don't use any more.

Ronco Plastics is your best source for a new tank. They make top quality thick walled rotomolded polyetheylene tanks for a very reasonable price and have more than 400 shapes and sizes, over 100 of which are non-rectangular. And they install fittings in the sizes and locations specified by the customer when they make the tank. There are retailers who sell Ronco tanks...Ronco sells direct for a much better price and they're great to work with.

Ronco Plastics Water and Waste Holding Tanks catalog

Ronco Plastics marine Tank drawings

You'll need new hoses too...Raritan SaniFlex is the top-rated sanitation hose...not only does it have a 10 yr warranty against odor permeation but it's also so flexible it can be bent almost as tight as a hairpin without kinking. Defender has it for the best price and they sell it by the foot (many retailers only sell 50' coils). Raritan Sani / Flex Sanitation Hose at Defender


I'll be glad to answer any questions...

--Peggie

[dasnider]

Thanks for the reply, Peggie - I was hoping you might chime in on this, as I've read your excellent recommendations on tank maintenance! Couple of additional points:

The tank is not dry - it's been in almost constant use since we bought the boat 5 years ago. The crust is sort of sitting on top of the sludge, and when I don't pump for a while it gets submerged under the black water.

The tank is not leaking so far, and the only visible corrosion was around the level gauge sending unit, which had stainless steel screws. I've replaced that with a nylon cover.

I'd love to replace the tank with a plastic one, but it ain't gonna happen for a while - unless, of course, it does start leaking! Getting it out would mean a major demolition and reconstruction of our saloon & master bath, including floor, cabinetry, woodstove, a bulkhead, sink, etc. The tank is made of 8 mil steel and the builder says he's never had one of the tanks start leaking yet, so I'm going to try to nurse it along for a few more years.

When we do finally have to replace it I'll definitely go with plastic, and will replace the hoses at the same time, per your recommendation. I'm in Amsterdam at the moment, so will have to find a local tank supplier - I'm sure there are plenty, but if you know of any good ones then pass them on!

Meanwhile I'm dosing the tank with Odourlos (can't find Noflex Digestor over here!) and have installed a fishtank bubbler to help aerate it. Not really any odor to speak of inside the boat, touch wood! I'm hoping that if I clean it out I can get another 5 years or so from it. I'm a little worried about cutting a hatch in it, though - the vibrations could cause a weld seam to fail or something - so maybe I should just leave it be. The sludge and crust take up a lot of room, though - I have to pump every few days, so would like to empty it out properly.

Thanks again for the info and suggestions, and let me know if you have any other ideas for extending the life on this thing!

- David

[peghall]

Only suggestion I have for extending the life of your steel tank is, don't even think of removing the crust or putting any holes in it.

Now that I know where you are....when you're ready to replace it, TekTanks in the UK TekTanks will be your best source for a new tank. They make excellent quality tanks and aren't that far from Amsterdam.

Vetus premium sanitation hose is excellent, rated very close to Raritan Sani-Flex, is as easy to use and a bit less expensive. Hoses are not where you want to save every penny you can if you only want to replace them every 8-10 years...cheaper hoses can permeate with odor in as little as a year or two.

I hope you'll keep me "in your loop" as your holding tank situation progresses (perhaps "evolves" is a better choice of words). Yours is a bit of a unique situation that I really want to follow and I'm always glad to help if/when needed. I'll send you a PM (private message) that includes my email address (so much easier than PMs!) and hope you'll use it to keep me updated.

Peggie

[dasnider]

Thanks for the referrals, Peggie; good to know about Vetus hose, which I can get at my local Vetus dealer, and I'll check out TekTanks when the time comes, hopefully not too soon!

Happy to let you know how it goes, esp. if I manage to find a good solution to this problem.

- David

[Sea Q]

Originally Posted by dasnider

Thanks for the reply, Peggie - I was hoping you might chime in on this, as I've read your excellent recommendations on tank maintenance! Couple of additional points:

The tank is not dry - it's been in almost constant use since we bought the boat 5 years ago. The crust is sort of sitting on top of the sludge, and when I don't pump for a while it gets submerged under the black water.

The tank is not leaking so far, and the only visible corrosion was around the level gauge sending unit, which had stainless steel screws. I've replaced that with a nylon cover.

I'd love to replace the tank with a plastic one, but it ain't gonna happen for a while - unless, of course, it does start leaking! Getting it out would mean a major demolition and reconstruction of our saloon & master bath, including floor, cabinetry, woodstove, a bulkhead, sink, etc. The tank is made of 8 mil steel and the builder says he's never had one of the tanks start leaking yet, so I'm going to try to nurse it along for a few more years.

When we do finally have to replace it I'll definitely go with plastic, and will replace the hoses at the same time, per your recommendation. I'm in Amsterdam at the moment, so will have to find a local tank supplier - I'm sure there are plenty, but if you know of any good ones then pass them on!

Meanwhile I'm dosing the tank with Odourlos (can't find Noflex Digestor over here!) and have installed a fishtank bubbler to help aerate it. Not really any odor to speak of inside the boat, touch wood! I'm hoping that if I clean it out I can get another 5 years or so from it. I'm a little worried about cutting a hatch in it, though - the vibrations could cause a weld seam to fail or something - so maybe I should just leave it be. The sludge and crust take up a lot of room, though - I have to pump every few days, so would like to empty it out properly.

Thanks again for the info and suggestions, and let me know if you have any other ideas for extending the life on this thing!

- David

Noflex where is over here DM me Its available in UK now