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Old 17-09-2020, 10:56   #1
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Holding Tank Pumpout Not Fully Evacuating

I have recently acquired a boat with a 40 gallon holding tank. When doing a pumpout (liveaboard marina) it leaves about 2 inches in the bottom of the tank. This results in the worst and smelliest of the waste settling to the bottom and never being pumped out. Is this typical? Is there any modification I can do to get it to pump out the tank fully?
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Old 17-09-2020, 11:08   #2
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Re: Holding Tank Pumpout Not Fully Evacuating

The hole that the pump out hose is attached to - is it all the way at the bottom of the tank? If not it will obviously leave stuff below it.

Another thing you can do is make a hole in the top of the tank, pass a PVC tube down to the most dependent part of the tank and connect the pump out hose to it - Peggie suggested this to me, I am sure shell chime in too.
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Old 17-09-2020, 11:11   #3
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Re: Holding Tank Pumpout Not Fully Evacuating

Id bet the pump out fitting is connected to the top of the tank with a dip tube. If this tube is too close to the bottom its easy to clog, but 2 seems like too much.
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Old 17-09-2020, 11:28   #4
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Re: Holding Tank Pumpout Not Fully Evacuating

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Id bet the pump out fitting is connected to the top of the tank with a dip tube. If this tube is too close to the bottom its easy to clog, but 2 seems like too much.
I don't think anything is clogged. I flushed out the tank real well and flushed out the pump out tube, the tank is clean now and it's pretty much just clear water in the tank. Water seems to flow fine and pump outs work quickly without issue other than it never gets it all. Even with just water in the tank the pump out stops at the same 2" level.

I am new to boat plumbing however I was thinking the pickup tube might be too short, but I don't know what is too long either. I haven't pulled that apart yet but wanted to gather some info before I did. If I can extend that tube how close to the bottom of the tank would be proper? Is it likely just a PVC pipe with fitting on the top which I could easily replace or extend with Home Depot parts?
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Old 17-09-2020, 11:32   #5
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Re: Holding Tank Pumpout Not Fully Evacuating

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Id bet the pump out fitting is connected to the top of the tank with a dip tube. If this tube is too close to the bottom its easy to clog, but 2 seems like too much.
Yes the pump out hose connects to a fitting on the top of the tank, with a tube going down into the tank. But I don't think it's clogged, maybe just too short but I'm not sure if that pickup tube comes with the tank at a predetermined length for the tank or is that likely just a poor install?
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Old 17-09-2020, 11:39   #6
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Re: Holding Tank Pumpout Not Fully Evacuating

This is common. The Pumpout works on a vacuum. It is impossible to get everything. Similar to a bilge pump. Once the vaccuum is broken and air starts getting sucked into the system, the pumpout can't pump anymore. what is left in the sanitation hose from the the tank to the pump out deck plate, drains back into the holding tank.

It is what it is.
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Old 17-09-2020, 11:41   #7
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Re: Holding Tank Pumpout Not Fully Evacuating

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Originally Posted by Stasis View Post
Yes the pump out hose connects to a fitting on the top of the tank, with a tube going down into the tank. But I don't think it's clogged, maybe just too short but I'm not sure if that pickup tube comes with the tank at a predetermined length for the tank or is that likely just a poor install?
Poor install or design. My money says the dip tube stops 2 from the bottom. How close you can get it to the bottom is a crap shoot (pun intended). To high, and you get the problem you have now. To close and it can clog. Im sure theres a sweet spot, but I dont know what it is.
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Old 17-09-2020, 11:44   #8
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Re: Holding Tank Pumpout Not Fully Evacuating

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This is common. The Pumpout works on a vacuum. It is impossible to get everything. Similar to a bilge pump. Once the vaccuum is broken and air starts getting sucked into the system, the pumpout can't pump anymore. what is left in the sanitation hose from the the tank to the pump out deck plate, drains back into the holding tank.

It is what it is.
Hmm, seems like a poor design. I realize some will back flow after the pump out stops but it seems like a bit much. I suppose I could add water while it is pumping out to help dilute it down a bit, but the engineer side of me thinks it could be improved. Maybe a tube which goes almost to the bottom of the tank but have a side facing opening?
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Old 17-09-2020, 11:47   #9
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Re: Holding Tank Pumpout Not Fully Evacuating

As everyone said, you never get it all.

Most people try to flush with fresh water and pump again. As many times as the pump-out attendant will allow you. Most are OK with one or two flushes. After that they start getting annoyed. Especially if they pay by volume to dispose of it.
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Old 17-09-2020, 12:21   #10
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Re: Holding Tank Pumpout Not Fully Evacuating

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Hmm, seems like a poor design. I realize some will back flow after the pump out stops but it seems like a bit much. I suppose I could add water while it is pumping out to help dilute it down a bit, but the engineer side of me thinks it could be improved. Maybe a tube which goes almost to the bottom of the tank but have a side facing opening?
RE: "add a little water" It's not an issue with solids, the entire contents are (or should be liquid).

Once the vacuum starts sucking air, it won't get the rest. If the pickup tube is on the side or the top, the gap from the bottom to the top of the side mounted fitting, or the gap from the pickup tube to the bottom will never be able to be sucked.

Your best bet would be a bottom mounted fitting.
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Old 17-09-2020, 12:32   #11
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Re: Holding Tank Pumpout Not Fully Evacuating

This is common. You can try using oderloos to help with the smell after every pump out. I find that the crystals work better than the liquid form.

Think of thee holding tank from a systems view:

- See if your hoses need replacing - if they appear to have crystals on the outside that means they are way past due for replacement.
- Don't use bleach of germ killing chemicals in the tank
- Don't use salt water for flushing
- As often as viable or at least two or three times a year pump out and then fill the tank with a quart of liquid laundry detergent and then go for a sail. Pump out again.
- Make sure your vent hoses are clear, well attached, and have air flow.
- Always pump out as often as possible and don't let the solid sit to long. They eventually become hard as rock and you need something like muriatic acid to clean out the bottom.
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Old 17-09-2020, 12:40   #12
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Re: Holding Tank Pumpout Not Fully Evacuating

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This is common. You can try using oderloos to help with the smell after every pump out. I find that the crystals work better than the liquid form.

Think of thee holding tank from a systems view:

- See if your hoses need replacing - if they appear to have crystals on the outside that means they are way past due for replacement.
- Don't use bleach of germ killing chemicals in the tank
- Don't use salt water for flushing
- As often as viable or at least two or three times a year pump out and then fill the tank with a quart of liquid laundry detergent and then go for a sail. Pump out again.
- Make sure your vent hoses are clear, well attached, and have air flow.
- Always pump out as often as possible and don't let the solid sit to long. They eventually become hard as rock and you need something like muriatic acid to clean out the bottom.
Hmm, the boat does use salt water for flushing, but it doesn't seem too feasible to change that. I had tried the West Marine ExterminOder but it didn't make a big difference. Hoses are not factory so they must have been replaced at some point, I don't see any crystals on the outside. I did verify the vent hose is completely unobstructed.
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Old 17-09-2020, 13:22   #13
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Re: Holding Tank Pumpout Not Fully Evacuating

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Yes the pump out hose connects to a fitting on the top of the tank, with a tube going down into the tank. But I don't think it's clogged, maybe just too short but I'm not sure if that pickup tube comes with the tank at a predetermined length for the tank or is that likely just a poor install?


Never said its clogged, just that if the tube gets too close to the bottom chances of clogs increase.

A 1.5 ID tube 1.5 from the bottom wont be clogged by a lump that will pass the hose. A 1.5 tube 1/2 from the bottom can be clogged by debris that would otherwise pass through the tube.

Only real way to get more out is to put the tube on the bottom, or suck from the bottom with a bottom mounted fitting.....this is less ideal.
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Old 17-09-2020, 14:07   #14
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Re: Holding Tank Pumpout Not Fully Evacuating

Ideally the diptube should be just long enough to touch the bottom, cut at about a 25 degree angle. Any flatter can trap undissolved TP or recently deposited solid waste under it...a sharper angle leaves too much in the tank.


Is your diptube cemented into a thread-barb hose fitting? If so, and if no one made the mistake of using sealant on the threads, you should be able to remove the pumpout hose (warming it a little will make it easier to get off the fitting) to pull out the diptube to see what the bottom of the tube looks like and how long it is.

Measure the tank depth (a metal tape measure should fit through the tank fitting) and measure the tube. If it's long enough to reach the bottom, but cut flat across the end, cut it at the 25 degree angle, reinsert into the tank and you're done.

If it's too short, a new thread-barb hose fitting, a new piece of PVC pipe that has an OUTER diameter that lets it fit into the hose fitting (use PVC cement to secure it), cut to the right length and angle should solve the problem.


If your "handyman" skills aren't quite up to doing this yourself, you're sure to have dockmates who'll help you get it right and maybe even assist with the work if the tank has only clean water in it.



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Old 17-09-2020, 18:13   #15
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Re: Holding Tank Pumpout Not Fully Evacuating

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Hmm, the boat does use salt water for flushing, but it doesn't seem too feasible to change that. I had tried the West Marine ExterminOder but it didn't make a big difference. Hoses are not factory so they must have been replaced at some point, I don't see any crystals on the outside. I did verify the vent hose is completely unobstructed.
We just used to pour water from the sink in.
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