Holding Tank Mystery?

[Verities]

Sadly, Haul out time looms. Two days ago I ran over to the Marina to do the seasons last pumpout and rinse. As we typically employ a mobile pumpout boat, this was my first solo attempt to use the equipment. Pumped the tank and rinsed and pumped again. Felt all was good and back to the mooring I went.

Yesterday noticed an odor aboard and noticed that water was seeping from under the seal of the inspection port on the holding tank. Not good! Turns out the tank was completely full. Doesn't make any sense. Maybe there is/was an airlock keeping waste from being emptied??

Any ideas out there? Your input is greatly appreciated.

[jstevens]

are you sure the pump was running in the right direction (sucking from the deck fitting) rather than pumping into the tank?

John

[Verities]

There was only an on and off button. Followed the instructions from the dock master. That would make sense if they had it set incorrectly.

[Jd1]

Plugged vent maybe? That could also explain the seeping on the inspection seal as the vacum could have damaged the seal.

[jstevens]

Understand that the pump has only an on/off switch is not reversible. However, the must be mean to evacuate the pump-out machine. One way would be to have two ports for the hose - one for pump-out and one for evacuation. If the hose was left in the in the evacuation port then nothing would be pumped out and your rinsing the tank would fill it with water.
Just guessing on my part.

John

[Verities]

Both those ideas are very likely. I know that as a 32 yr old boat the vent hose is small, probably 1/4" ID, and likely clogged. It's been on the to do list. This winter for sure. It's the last of the plumbing that's original to the boat. Full of salt I'd guess.

I'll pay better attention to the machine next time over. Gotta be smarter than what your working with

Thanks for your help!

[peghall]

Tank vent line is blocked. 32 years ago they were the same size the industry uses now--5/8". Tank vents have two main purposes--

1. provide an escape for air in the tank displaced by incoming contents, and
2. provide a source of air to replace tank contents as they're pulled out

If no air can escape the tank, it will become pressurized as the toilet pumps more and more contents into it...this can have some unpleasant (albeit entertaining to uninvolved spectators) consequences...the least of which is a geyser when the deck pumpout fitting cap is removed, the worst being a burst tank...and several things in between.

If there's no source of air to replace tank contents, the pumpout or discharge pump will pull a vacuum that prevents more than a gallon or so from being pumped out. Worst case can even be an imploded tank if the pumpout is strong enough.

If you pumped out and your tank is still full, it's a 100-1 odds thana blocked tank vent is the reason. Unless you want to risk any of the unpleasant consequences I mentioned, do NOT use the toilet again or pump out or dump--or let the pumpout service near your boat--again untill you've done the following:

First open the deck fitting cap to relieve any pressure...I'd be sure to keep arms length UPwind of it when you do this. Backflush or scrape out the vent thru-hull (knock out any screen in it...screens cause more problems than they solve or prevent). Then remove the vent line from the tank and clean out that end of the hose and the vent fitting on on the tank.

NOW you should be able to pump out the tank. To keep the vent line open, back flush it through the vent thru hull by sticking a hose nozzle up against it (if your vent thru-hull fitting won't let you do this, replace it with one that does) every time you wash the boat and/or pump out. I wouldn't use the same hose that you use to fill your water tank(s).

[zboss]

You can also take off the inspection port. This would provide all the vent you need. I have done this when mine was blocked. Very unpleasant but it worked.