Gravity holding tank does not drain - SailNet Community
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post #1 of 15 Old 08-16-2011 Thread Starter
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Gravity holding tank does not drain

I'm new to this but would certainly appreciate any thoughts and/or advice. I purchased a 2001 Beneteau 331 last year, and the Jabsco manual head was plumbed so that a Y valve allowed you to either pump directly overboard, or into the holding tank. The holding tank could only be emptied by pumping it out. I believe this was/is the standard arrangement in the US.
Since I live and cruise in Canada, where pump-outs are less common and discharge is allowed in open water, I replumbed the head so it pumped into the holding tank, and put the Y valve on the holding tank outlet, so the tank could either be pumped out, or drained overboard. I also installed a holding tank gauge, along with new hoses and clamps, etc. Outlet hose is standard 1 ˝ inch.
The system worked great in tests - put water in the holding tank, put the Y valve in the drain overboard position, open the through-hull, and whoosh - the tank drained completely. The bottom of the holding tank is a couple of feet above the waterline. However, while on our first longer trip I noticed that the tank was not draining when both valves were open (glad I installed the gauge). This led to some rather unpleasant work with a long stick to unclog the drain. The same thing happened a couple of days later, and this time I could not get it to drain at all. I was able to empty it at a pump-out station when we returned home.
The holding tank vent is clear and the tank discharge hose is relatively short, although it does have a right-angle bend as it leaves the tank. I believe that solids are collecting at the bottom of the tank, and gravity isn't enough to push everything through the outlet hose. It seems as if there are two possible solutions:

1) Install a manual or electric pump to empty the holding tank
2) Install a macerating head

The first option involves providng additional suction to empty the tank, instead of just gravity. However, the best location for the pump would be at the bottom of the tank, and there's virtually no room there at all. To allow the tank to be emptied at a pump out station, the pump would also have to be downstream of the Y valve, which would again put it further from the tank and where any blockage would likely be.

The other option would involve installing an electric macerating head, which would hopefully eliminate the problem by grinding everything up before it goes into the holding tank, allowing it to drain by gravity as intended. It would also simplify 'head handling' for guests etc. I understand Jabsco makes electric heads which have the same dimensions, connections etc. as their manual heads.

I'm leaning toward the latter idea, but welcome your thoughts and advice. I guess the main question on my mind is if the tank will drain without problems if it contains only macerated waste. Can anyone vouch for this arrangement? Thanks for your posts! (I long to go on another trip without carrying a ‘stir stick’…)
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post #2 of 15 Old 08-16-2011
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My thoughts are to just add a macerator pump, between the y and the seacock. I would think that the pressure pump would, or should be able to clear any solids from the tank. It would be a bit cheaper than a macerating head and probably easy enough to install, though I know you did say space was limited.
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post #3 of 15 Old 08-16-2011
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I'm not certain that these things will help, but here's my thoughts:

In my camper I use a half cup of laundry detergent, and a half cup of water softener (calgon) in my black tank. This combination contains no formaldehyde, and will not damage any of the components in the system. In combination with traveling down the road, it keeps everything broken up and in suspension, and as a side bonus it lubricates the valves as long as they are kept wet.

For the above reason, I would suggest emptying after you've been sloshing it around a bit.

When the tank gets to the halfway point, let it slosh, then top it with water and empty it immediately.

Don't empty the tank when there is very little in it, or when it has been stationary for a period of time. Flow is your friend when carrying out waste.

One other thing that's kind of related -- People talk about the bacteria in black and grey tanks, but these are supposed to be holding tanks only and don't require bacteria for operation.

I hope some of this helped.
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post #4 of 15 Old 08-16-2011
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My Caliber has a gravity emptying holding tank and it goes straight to the seacock at which point it turns 90° and through the seacock. My head is an electric jabsco macerating head and I have only ever had one clog when dumping overboard. Eventually the waves off shore freed the clog.

I believe that chopping into the tank will be more effective and less prone to problems. Trying to pull clogged material(paper and solids) though the hose to the macerator may be difficult and even make the clog worse.

Tim R.
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1997 Caliber 40LRC

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post #5 of 15 Old 08-16-2011
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We have a t-connector on the holding tank on our Caliber 33. One side of the T goes to the deck pumpout fitting, the other side of the T is plumbed to manual Whale hand pump then to another T thats plumbed to the overboard thru-hull.

We can flush toilet to holding tank or directly overboard with Y-valve. We can get pumped out at a pumpout station or pumpout manually with Whale hand pump overboard when offshore.

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post #6 of 15 Old 08-16-2011
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Scale can accumulate at the bottom if the holding tank and block the discharge hole.

I had to run a snake up the hose after removing it from the discharge thru hull and break the dam. We flushed the tank several times with fresh water while at a pump out before we used the snake.

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post #7 of 15 Old 08-16-2011
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The ideal way to install a gravity drain holding tank is with the deck pumpout fitting directly above the seacock so you can clear a clog easily with a rod from on deck.
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post #8 of 15 Old 08-16-2011
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I added a macerator pump to drain my holding tank beyond the 3 mile limit. I added a Y-valve in the overboard discharge hose from the head. In one position the Y-valve allows the head to directly discharge overboard. In the other position, the macerator output goes directly into the overboard discharge hose.

BTW, you need to be careful not to run a typical (e.g., Jabsco) macerator pump dry.
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post #9 of 15 Old 08-17-2011
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Quote:
Originally Posted by PaulfromNWOnt View Post
........these are supposed to be holding tanks only and don't require bacteria for operation.
If you empty it and rinse it out almost daily, that may be true. However, we would have to motor over to the fuel dock for a pump out before we left each Monday, and that just isn't practical. It may not even be open when we leave. With three to four days away baking at 85 degs in the cabin, the aerobic bacteria in the holding tank better be healthy. By strictly avoiding any chemicals or soaps in the tank and using sufficient flush water, we have no problems.


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post #10 of 15 Old 08-17-2011
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My previous and current boats have a manual diaphragm pump in the holding tank discharge line for pumping it overboard when permitted. Works great.
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