Holding Tank problems

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Lemain, I suggest you actually read the posts, as it seems you do not understand what it is actually about!

The gravity system is simple and very effective, with no extra pipework and pumps; its called the KISS system-I'm sure even you can work out what that means. The system works 95% of the time

[/ QUOTE ]I totally agree; gravity rules. KISS is the guiding principle in the cruising world and more so than ever in the holding tank world. Anybody who disagrees might not have learned what they urgently do need to learn from a clogged holding tank pipe: sh** always flows downwards.
 
Maybe a Lavac does break things up better? I don't know. Getting solids through a hole is all about pressure (head). There isn't a big head available with a gravity tank. A Henderson can produce easily 5m head, probably far more.
 
Not sure about the advisability of pressurising the tank too much, but by closing off the vent the gravity tank can be pressurised using the toilet pump.

The joker valve on a standard toilet is very efficient at breaking up solids but it can't be expected to do much for baby wipes and other more durable materials. I have never had a blockage of any sort in any toilets, due to taking the normal precautions.

Also, on stainless tanks - there is no inherent reason why stainless steel should not be used for a holding tank. It's just a matter of correct specification, of the sheet steel and the weld filler metal. The majority of these tanks will have been made with 'general purpose' in mind, just not good enough for sewage. I'm fairly sure that the tanks and pipework for the sewage system on offshore platforms I have worked on were all stainless, although I cannot quote the grades.
 
I was wondering about that but I advise extreme caution. With my design, there can be a small airlock from time to time between the Jabsco and the tank. This results in a small pressure building up and, on occasions, it relieves itself back through the joker valve and into to the bowl rather than forcing the sewage into the tank (the tank has a large breather, this is just an airlock). When the contents of the pipe are unpleasant, the experience is, likewise /forums/images/graemlins/wink.gif So I think that trying to pressurise the tank (with a Jabsco, a Lavac wouldn't work?) would be a bad idea.

As for stainless and sewage, I wonder.... sewage rapidly becomes deficient in O2 which is a big problem for stainless. Even with a decent breather, does the sewage always get enough O2 through any floaters or crust to aerate the mass below? I don't know. All I do know is that I have never heard of a satisfied ss tank user. Tek Tanks might know --- they owe this forum a favour for all our plugs, maybe they could explain it to us.
 
<<< does the sewage always get enough O2 through any floaters or crust to aerate the mass below? >>

The smell is the test! I'm sure all holding tanks smell, but it's the presence or absence of H2S that indicates whether oxygen levels are sufficient. If oxygen level is high enough then aerobic bacteria thrive. If they are able to use all the available oxygen then anaerobic bacteria take over. Corrosion is accelerated under these conditions. Every flush replenishes oxygen as the seawater is quite highly aerated. A decently placed vent should admit more air.
 
We have a simple vetus plastic holding tank which vents out onto the deck! but no smell as its well vented. I have linked the anti-syphon pipe into this vent as well as it stopped the smell from that pipe as well. The out pipe is in the same place and same run as the original fitting, but with the holding tank fitted where the anti syphon vent would have been (a very big anti-syphon loop!). As I said KISS works for me, never clogs when we are at sea or at anchor, which 6 months of the year at least. No extra pumps or X over valves, easy to maintain, etc, etc, etc.

For the gravity side of life then its location, location, location; ours has enough room to access the inspection cover and get an arm in if needed, but also has a drop to the seacock of just over a metre. Unfortunately I am stuck with a 90 degree bend at the outlet due to lack of room!
 
Rather than a screwdriver I would use a hose and back blow from the outlet. I keep a hose for this purpose, I have ring of self-amalgamating tape wound round it a couple of inches from the end and this helps to provide a seal (cloth sometimes needed too).

If your problem is barnacles this won't solve it long term. If you use your tank a lot then limescale will be building up on the walls. This eventually flakes off in sheets and is a very effective outlet blocker.

For this reason we descale our system every year or so with muriatic (aka hydrochloric) acid. Just shut the stopcock and leave it to stew for half a day. I am not sure how this gets on with stainless steel (and toilet components need to be considered) and of course there may also be environmental considerations in your location, but it helps to stop the scale build and the barnacles.
 
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All I do know is that I have never heard of a satisfied ss tank user.

[/ QUOTE ]Here is one! My boat and tanks are 12 years old and they have had a very heavy use. No problems, touch wood......

But maybe that is the issue; its not about ss or plastic, the old adage 'boats and sailors rot in port' applies. Anything not used on a boat develop problems sooner or later, due to low usage. My theory, but I stick by it.
 
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The idea of a gravity discharge for raw sewage sounds daft to me. Stainless tanks will fail, and fail badly, in time.

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I've got both - gravity draining and stainless tanks - which work fine. As far as I know Oyster still do both as standard so the idea that they are fatally flawed may need a little more justification!

I have two blockages in 13 years (6 as liveaboard) one fixed (easily) from under the boat and the other, with slightly more work, by attaching the dinghy pump to the vent. Important features of the installation are the avoidance of sharp turns in the pipework and that the gravity drain has a larger cross section than the hose from the bowl to the tank.
 
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stainless tanks are a no-no and we hear nothing but problems from owners of gravity tanks. Such are the facts.

[/ QUOTE ]Interesting. I have heard of significantly fewer problems with gravity systems (including my own) than pumped ones. My observation; all production boats I have seen are fitted with ss tanks. So these builders are all wrong and Lemain is right? Hummmm...

Secondly, if you do not accept that the KISS principle and gravity is the preferred and best proven option; you must accept that you are going against a long tradition in seafaring.
 
Calgon tablets available in any supermarket will solve your limescale problems.

Disolve with warm water in the bowl, close the seacock pump through so its in the lift pipe and the outlet pipe,leave overnight. We do this every 10 days or so, works for us.

Aqua40, or diluted hydrocholic I know has been used by some people as well.
 
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