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Old 28-10-2014, 08:38   #16
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Re: Holding Tank Vent Sizing

Has anyone tried the Groco sweettank? It seems like it would be the best way to assure plenty of aerobic action. I'd think it would make up for a smaller than optimal vent, although they do recommend dual (port & starboard) vents. It does add an energy cost and a bit of complexity, but for holding tanks that are difficult to vent well, I think it might just work.

My holding tank is difficult to vent well due to it being deep in the bilge. The vent hose goes pretty much straight up about 5-6 feet. I can increase the hose size to 1" without too much trouble, but that's about it.
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Old 28-10-2014, 08:40   #17
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Re: Holding Tank Vent Sizing

Mine are 1 1/2 inch mushroom vents mounted port and starboard of my main hull (I have a 40 foot trimaran). I don't have an issue with heeling, however, even if I were in storm conditions, bashing into head seas, filling the holding tank with seawater, there would be no problem as the tank is located above the waterline. In the open sea I normally just leave the holding tank drain valve open, creating, in effect and actuality, a direct discharge toilet. When I get into port, I close the drain valve, dump a shot of K.O. (Peggie Hall's concoction of aerobic bacteria soup) in the toilet and pump it into the holding tank, and I'm good to go. Because the vent lines are oriented into the prevailing wind at anchor, weathercocking makes one direct the air into the tank and the other exhausts the gases, which, by the way, don't smell as foul as dead anaerobic bacteria with formaldehyde and "perfume". If I am in a remote location, I don't need to store my holding tank contents. My waste isn't a toxic mix of formaldehyde, so it's more like the sea lions and other beasties I share this environment with.

By the way, there is no need for a macerator pump (hooray!) since the toilet pump, a LAVAC with conventional bilge pumping, completely disintegrates the waste as it enters the holding tank. One less item to have to repair on the boat, and a particularly odious one at that.
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Old 28-10-2014, 08:56   #18
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Re: Holding Tank Vent Sizing

I haven't done this yet on anyone's boat, but it just occurred to me as a possible way to install an above-water holding tank in conventional boats: many anchor lockers are large spaces in which the chain and rode fill only the bottom. The bottoms of the locker are also often above the water line to allow drainage. One could build a holding tank that would occupy the otherwise unused space at the top of the locker, and have a system similar to mine. The Groco Sweet tank is an interesting compromise, but complicated and energy intensive. The toilet doesn't care where the tank is located as the pump is lifting water and waste anyway.
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Old 28-10-2014, 09:32   #19
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Re: Holding Tank Vent Sizing

The toilet cares if the tank is too far away.
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Old 28-10-2014, 09:43   #20
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Re: Holding Tank Vent Sizing

The toilet pump only cares how high it has to lift. Gravity takes care of the trip to the tank. Just don't make the journey a long slog to the high spot of the passage. Plumbing has been around for a couple thousand years and folks are pretty understanding of the process. As one can see from my pics, the toilet dumps immediately into the top of the holding tank. But, then, I built the boat so I could choose where to locate the head.
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Old 28-10-2014, 09:49   #21
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Re: Holding Tank Vent Sizing

Quote:
Originally Posted by Roy M View Post
The toilet pump only cares how high it has to lift...
Wrong.

If the tank is far from the toilet, you have two choices:
Pump and pump and pump to clear the line of solid waste
or
Leave solid waste sitting in the line.

To clear a long run means that, each time the toilet is used, a larger volume of the tank is taken up.
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Old 28-10-2014, 12:57   #22
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Re: Holding Tank Vent Sizing

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Originally Posted by Saltyhog View Post
Has anyone tried the Groco sweettank? It seems like it would be the best way to assure plenty of aerobic action. I'd think it would make up for a smaller than optimal vent, although they do recommend dual (port & starboard) vents. It does add an energy cost and a bit of complexity, but for holding tanks that are difficult to vent well, I think it might just work.
That one would be an easier to trial than larger vents. If you dropped in a live well bubbler through the inspection port then taped everything up....

http://www.amazon.com/Marine-Metal-P...+bait+air+pump
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Old 28-10-2014, 19:47   #23
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Re: Holding Tank Vent Sizing

The Groco Sweetank system is a great solution in larger tanks--40-60 gallons, but at $200 +, it's a bit pricy for a tank under 30 gallons. Unfortunately a bait well aerator won't work in a holding tank...it's necessary to run piping all the way across the bottom of the tank to provide air to the whole tank. A baitwell pump only provides a single column air, which doesn't work....and just running a piece of hose with holes in it across the bottom will only get you a piece of hose with clogged holes in very short time. There's a whole lot more engineering in a waste tank aerator than is readily apparent...which explains why nearly 10 years after Groco first introduced the Sweetank system, no one else has put a competing version on the market.
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