Gravity fed holding tank woes

alexincornwall

alexincornwall

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Hi all,

If you're currently eating a meal then perhaps move on to another thread for now...

We're currently having a lovely time cruising the Vendee coast but have run into a nasty issue with our holding tank, which I can't quite get my head around. Suggestions very welcome, particularly those that don't involve me getting my hands too dirty.

Like most cruisers, we open the holding tank when offshore, and then close again in marinas. We've never had any issues with our gravity fed system but now something seems to be amiss. A few days ago I was getting a distinctive whiff of what I had diagnosed a full holding tank. This came as a surprise as it had been opened just a couple of days before. My diagnosis was unfortunately confirmed when, much to my horror, I observed the contents escaping through the tank vent whilst another family member flushed.

I assumed that something had blocked the tank at the outlet pipe (probably the kids using excessive tissue), so I condemned the heads until we could reach a pump out station, which we did the next day. That all went fine but I couldn't guarantee that the blockage would have been removed. So, with the pump in operation, I opened the holding tank seacock and much to my relief, could hear seawater being sucked into the tank via the pump out vacuum. That's it I thought, we have water coming up through the outlet, therefore the blockage must be removed and the contents will flow out.

Unfortunately, it would appear not. The holding tank has been slowly filling up again but when I open the seacock I still don't hear the swoosh of liquid leaving the boat. It would appear that there is still a blockage, but I can't understand where given the successful experiment at the pump out station.

I'm preparing myself for another pump out and then donning as much PPE as possible for an internal tank inspection, but hoping one of you will spare me with the answer to my problem please.
 
One possibility which would fit those facts is that you have a blockage inside the tank which floats away from the outlet when sea water comes in, but which blocks it again when trying to exit. Since you evidently have a tank inspection hatch, all you need is some Marigolds, a torch and a bit of resolve. Everything that's in there is your family's after all - less fun doing it when it's someone else's!

Good luck.

BTW FWIW mine has a macerator pump on the exit pipe rather than relying on gravity.
 
When I worked on flotilla the usual solution involved getting in the dinghy and using a long wire with the end bent into a hook shoved up the outlet pipe. If you do this after you have pumped out at least you don’t get the full load rushing out while you are in close proximity. Suggest the kids always put loo paper in the bin rather than down the loo in future!
 
I’ve had several blocked holding tanks over the years. Normally it’s a blockage (full or partial ) in the discharge pipe. Try blowing it back from the seacock outlet. I get in the dingy with the dinghy pump and Pump back up the pipe. You’ll hear a gurgling sound from the tank and with luck the blockage will clear. You’ll need to wrap tape around the pump dinghy pump pipe so it make a reasonable seal with the skin fitting.
Other options involve opening the tank inspection hatch and clearing the mess inside by hand- not to be attempted until all other options have failed!
 
On the only occasion that my tank filled completely, so much so that it flowed back into the toilet, the cause was flakes of scale that had fallen off the tank walls, blocking the outlet. In my case I have an inspection hatch directly over the outlet, through which I was able to rod from above. Once the scale was broken, which happened immediately, the contents flowed out. Since then I always clear scale off the walls at the end of season, quite easy with Greek mains water pressure.
 
alexincornwall said:
Yes, that’s a consideration. Maybe not blocked as such, but possibly full of water.
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I doubt whether the breather could block with anything that is sufficiently solid to stop it being sucked out when you open the tank outlet.

Is your top outlet and dip-pipe for the pump out directly above the holding tank outlet. If it is, that is the optimal design and allows you to insert a broomstick handle or cane or whatever from above and directly clear the outlet.

A broomstick handle works for me every time and it's a clean job that takes one minute. :)

Richard
 
alexincornwall said:
Yes, that’s a consideration. Maybe not blocked as such, but possibly full of water.
Click to expand...

Apparently insects can build a nest in them.

How to Clear Blocked Holding Tank Vent ? - Cruisers & Sailing Forums

Clogged Holding Tank Vent Hose - Cruisers & Sailing Forums

"Last gasp at this potential smelly problem.
The tank is gravity operated effectively. To dump tank you simply open the large outlet valve and bombs away... to cause it to implode a high level of vacuum needs to be applied and the tank sealed for this to happen.
If the breather is blocked it simply stops working...only a partial discharge.
If the breather is too small in diameter it just takes longer to empty
As regards smells, remember when the tank is being dumped air is drawn in through the breather pipe, not out.. however if the tank is full it will most definately smell and this will come from the breather pipe..
Also most important the tank needs to be dumped as often as possible rather than leaving the bacteria do its trick..
I try to empty it each time before returing to the berth and also flush it thru regularly with soapy freah water and there ends the problem with tanks, breathers, smells etc..."

Holding tank breather diameter
 
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I has a similar effect on the outlet seacock of my forward head pump out.

The seacock had became partially block with barnacles such that the head would pump out but when a piece of loo roll wend down it completely blocked the seacock.

After a while the pressure would slowly seep out until the next flush.

I did something similar to RichardS using a prigi rod to displace the barnacles. I initially tried using my SCUBA tank at full pressure to no effect only a rigid rod would do.
 
My money would be on some debris in the tank acting as a valve, allowing stuff in and not out. This would almost certainly be unauthorised paper or wipes since ordinary loo paper seldom if ever gives trouble. At least, it hasn't in 20 years with us. The solution will depend on the construction of the tank. In some cases you might be lucky and suck the blockage out. Otherwise, it might allow you to use a flexible tool to clear it, though I don't think I could get at mine this way. If all fails, I would suggest a series of fills and pumps with strong antiseptic before disconnecting the tank to clear it.

If it were just a blocked vent, some content would drain. You could clear the tank fully by pumping air into it, in stages.
 
alexincornwall said:
Yes, that’s a consideration. Maybe not blocked as such, but possibly full of water.
Click to expand...

You said in post 1 that "I observed the contents escaping through the tank vent whilst another family member flushed." This would seem to rule out coopec's suggestion of a blocked vent.
 
I'm having similar problems with my gravity holding tank as well this year. Unfortunately the tank outlet has to go through a 90 degree bend on the seacock. The tank blocked on the bend or hull fitting when sitting in the mud. I managed to rod the pipework and it cleared. I have now put a valve in the tank vent pipe so when being used as a sea toilet the tank is part of the sealed system and hopefully blasts everything out as a lavac should. I going to renew the pipework this winter and see whether I can move the seacock to a better position, but as ever on a 1970s boat life is a compromise. The holding tank was only fitted for being anchored in swimming areas and so far has worked well. Whether the tank should be pressurised is another matter, but I haven't noticed it expand when pumping out - yet.
 
I had visions of this happening when I fitted a holding tank so had a flush inlet sited directly above the outlet so I could rod through from the top without having to take anything apart. Sounds like something is sitting around that won't go up the pump out or down the outlet. It may be that the outlet is one side of the tank baffles, if it has them, so whatever is causing the blockage isn't getting across to the pump out. It may need to be broken up so may need going via the inspection hatch.
 
I'm looking forward to my last century type toilet stuffing up because I'll get my angle grinder out (with a new cutting disc) and chop the whole lot out and replace it with a compost toilet.

But that's after I gethe yacht in the water.

And yes PVB I've been building it for 20 years!?. (PVB What about uploading some shots of the yacht you built?)?
 
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neilf39 said:
I had visions of this happening when I fitted a holding tank so had a flush inlet sited directly above the outlet so I could rod through from the top without having to take anything apart. Sounds like something is sitting around that won't go up the pump out or down the outlet. It may be that the outlet is one side of the tank baffles, if it has them, so whatever is causing the blockage isn't getting across to the pump out. It may need to be broken up so may need going via the inspection hatch.
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I fitted a high pressure jet running off the galley pump (2 bar) pointing directly at the tank outlet to break up any clods of tissue etc. Simple on/off cock, give it a blast before running the outlet macerator and once or twice during the run. It also gives the chance for a fresh water rinse after the tank is empty, which the pump benefits from rather than standing for days/weeks in residue.

BTW all that was possible because I had access to the inside of the tank through the afore-mentioned hatch.
 
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johnalison said:
Mine doesn't have an inspection hatch. I don't know how many do.
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The gravity tank in my old Hallberg-Rassy was stainless steel, with no inspection hatch. It worked well for the 19 years I owned the boat. The gravity tank in my current boat is HDPE and has a screw-in access hatch, which I'm pleased to say I haven't needed to unscrew yet!
 
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