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Old 06-12-2011, 06:00   #1
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Is Gravity Discharge Possible from a Blackwater Holding Tank ?

I've searched the following topic, but couildn't pick up a relevant thread, so appologies if this has already been done to death recently.

I'm completely rebuilding the head compartment on my UFO34 and want to install a small blackwater holding tank under the side decks, as well as having direct overboard discharge, to cover all options.
I'm thinking a 25 litre holding tank, which is quite small, but probably sufficient for the limited use it will get. I'm a bit space constrained too.
It would reduce the complexity of the plumbing system if the holding tank gravity fed down to the through hull fitting - probably a drop of say 30 - 60 cms (1-2 feet). I will be using the standard 1 1/2" piping, valves and thru-hulls.
The toilet is a manual Jabsco, so the solids from the toilet WON'T be going through a macerator pump before reaching the holding tank.

My concerns are:
Will I end up getting blockages if I rely on gravity feed to discharge the holding tank into the sea?
Is there anything I can do (chemicals into the holding tank, etc) to reduce the possibility of blockages?
If I did instal a pump to assist discharge the holding tank into the sea, what sort of manual pump is recommended?

Just try not to visualise what we are talking about........!!

Thanks.

Ash
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Old 06-12-2011, 06:25   #2
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pirate Re: Is gravity discharge possible from a blackwater holding tank?

The only thing that causes blockages in my experience is toilet paper and feminine 'extra's'...
create a seperate waste disposal for those (bin whatever) and you'll be fine..
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Old 06-12-2011, 06:34   #3
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Re: Is gravity discharge possible from a blackwater holding tank?

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Will I end up getting blockages if I rely on gravity feed to discharge the holding tank into the sea?
Yes you will. By the way it's illegal to dump untreated, raw sewage in many areas not to add that it's gross.

Quote:
Is there anything I can do (chemicals into the holding tank, etc) to reduce the possibility of blockages?
Install a macerator pump on the outfall of the holding tank.
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Old 06-12-2011, 06:36   #4
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Re: Is gravity discharge possible from a blackwater holding tank?

Second that - install a macerator on the way in to the holding tank and it should gravity drop fine after that. Is a macerator not mandatory in WA?
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Old 06-12-2011, 06:39   #5
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pirate Re: Is gravity discharge possible from a blackwater holding tank?

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Yes you will. By the way it's illegal to dump untreated, raw sewage in many areas not to add that it's gross.
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Whats 'Gross' to you is 'Munchies' down the food chain...
I know... Life's a loada Crap....
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Old 06-12-2011, 06:55   #6
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Re: Is gravity discharge possible from a blackwater holding tank?

We have a similar system on a small trawler. It works fine, HOWEVER!, without a macerator you will need a large discharge thru hull. If the head were electrified and used a macerator there you could get by with a 1" thru hull on the tank discharge. Without any macerator you will need what may turn out to be an impractically large thruhull. We use a 2" gate valve. Do not forget you need a vent on the holding tank.
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Old 06-12-2011, 11:41   #7
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Re: Is Gravity Discharge Possible from a Blackwater Holding Tank ?

I had a gravity drained holding tank for 12 years. It clogged very easily, since TP and etc would settle on the bottom of the tank. The only way we got it to not clog was to throw TP into a separate waste basket.

I really recommend a pump.
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Old 06-12-2011, 12:27   #8
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Re: Is Gravity Discharge Possible from a Blackwater Holding Tank ?

Both of the Sunsail Charters I have done had a similar configuration. They skipped any y-valve arrangement and plumbed the head output directly to the tank. You gravity drained it when you left port or pumped it if you are staying.
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Old 06-12-2011, 14:21   #9
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Re: Is Gravity Discharge Possible from a Blackwater Holding Tank ?

If possible, put the pump-out directly above the discharge so if you do get a plug, you can rod it out from the pump-out
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Old 06-12-2011, 14:30   #10
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Re: Is Gravity Discharge Possible from a Blackwater Holding Tank ?

If you are going to have limited use save yourself a ton of money and headaches and go to composting head. You'll have a lot more storage space as well.
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Old 06-12-2011, 21:08   #11
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Re: Is Gravity Discharge Possible from a Blackwater Holding Tank ?

This should answer most, if not all, of your "is it possible" and "how to do it" questions. EDITORIAL written by Charles Kanter sometime in the 90s for one of the DIY mags or PS...can't remember which.

The best way to prevent clogs: Water...on a regular PREVENTIVE maintenance schedule. Close the thru-hull, put several inches of it (no need to fill the tank)--fresh or salt, doesn't matter--into the tank via the deck pumpout fitting...open the thru-hull again. Repeat at least once.

Use only quick dissolve TP--the cheap flimsy stuff that almost dissolves in your hand if it gets even damp--in limited amounts...and don't worry too much about solid waste. It's at least 75% water to begin with...gets broken up going through the pump and the plumbing, so dissolves in the tank in water very quickly. Be sure to add water to the bowl ahead of solids (use a cup from the sink!) so there IS some for solids to dissolve in.

Yes, you will need to use a tank product.

The entire tank has to be above the waterline to be able to drain via gravity. If any of it is below waterline, it'll fill to the waterline instead of draining.
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Old 07-12-2011, 16:48   #12
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Re: Is Gravity Discharge Possible from a Blackwater Holding Tank ?

Thanks Peggie, the Kanter article makes for interesting reading.

Thanks to everyone for their valuable input, you have come up with a few great ideas that I had not thought of.

I'm keen to give the gravity feed idea a go. With proper placement above the waterline, good maintenance, clear instructions for the crew on how and when to use it, plus placement of the tank such that potential blockages can be probed without major disassembly, I think I can make this work without a manual or macerator pump in the system.

I'll let you know in 6 months when the refit is complete.

Ash
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Old 07-12-2011, 17:00   #13
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Re: Is Gravity Discharge Possible from a Blackwater Holding Tank ?

There are products for leach lines and septic tanks that are solutions of specific bacteria that gobbles up waste in holding tanks. Work great. We also agree with the idea of having the deck pump out located directly over the tank discharge.
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Old 07-12-2011, 17:20   #14
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pirate Re: Is Gravity Discharge Possible from a Blackwater Holding Tank ?

Don't suppose they make holding tanks with U bottoms... pipe coming of the bottom of that would improve things no end...
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Old 07-12-2011, 18:12   #15
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Unicorn, we have been using a similar system for over a year now. No Y-valve. TP and everything is manually pumped to the holding tank and from there gravity takes it out to the remora, flathead and other bottom feeders. No macerator, no chemicals. The outlet from the bottom of the tank is 2", I think. Never been blocked between the holding tank and the ocean ..... yet! Touch wood.
Seems simple and easy. Good luck!
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