Holding Tank Design

S

Steven

Member
Joined
Jan 1, 2000
RO Number
1517
Messages
19
I am looking into replacing my 40 gallon black water holding tank and have noticed two different fitting configurations. On some, like the Sealand, the discharge hose is attached to a fitting on top of the tank which is then attached to a tube going to the bottom of the tank. To empty the tank, the contents are pumped up the tube and out. Another design has a fitting for the discharge hose on the lower part of one end of the tank and the contents are just pumped out the bottom of the tank.

My current setup is with the fitting on the lower end of the tank. I have room to install either style. I can see a few advantages to each design. Any thoughts on which is the better configuration?

Thanks.
 
If you are going to all that trouble and expense, I would convert to a Lectrasan or Purasan. All I know who have them say it is one of the best investments they ever made in their boat.
 
I'f you're replacing the tank, I'd go with the top pumpout fitting...less chance for leakage if the fitting/hose breaks.
 
i agree, a dip tube at the top is better as you dont' have to worry about leaks.

Lectrasans are nice but not allowed everywhere and you dont' want to pump out treated wasted when people are swimming around the boat, do you ? i woudn't have a boat without a holding tank.
 
With the pumpout at the top, there is another advantage besides less chance for leaks. When the pumpout fitting is at the bottom of the tank, fittings and associated hosing will stay full of effluent whenever there's anything in the tank - more chance for a clog to develop, and a MUCH BIGGER change for the hose to permeate and stink up the boat.

On the downside, you never actually get 100% of the contents out of the tank as the dip tube stops just short of the bottom of the tank, but who really cares? The new plastic tanks are just about impervious to permeation (hey, ya like them 50 cent words?) so there isn't usually a problem from this setup. It also costs a little more to have the tank built that way, but not enough more that it'll put you in the poorhouse.

Just make sure you have 4-5" clearance above the top of the tank to allow for the fittings and hose connections.
 
Pascal, your comment begs the question and Vic, while you are here, Just how "filthy" is the discharge from a Lectrasan or Purasan? Probably not as clean as revered bottled water, but cleaner than say the James River or the Chesapeake or Narragansett Bay's at Red Tide?
 
It's about the consistency of a watered-down "smoothie" from 7-11 - Coast Guard regs say "no visible floating solids." As far as color, it ranges between milky white and cafe au lait, depending on how long it was inside the treatment system, and how much it's been bleached. The actual "quality" of the effluent generally exceeds the Coast Guard requirements by quite a large factor - much better than the outflow from the average municipal sewage treatment plant.

Requirements for a Type I MSD: Type I. A flow-through discharge device that, under the test conditions described in 33 CFR 159.121, produces effluent having a fecal coliform bacteria count no greater than 1000/100 milliliters, and no visible floating solids. A Type I MSD is commonly a physical/chemical type (macerator/chlorinator).

Average output from a Lectra/San or Electro-scan is 20 or less fecal coliform per 100 milliliter sample - 50 times better than the law calls for.
 
Thanks for the feedback. Most of the tanks seem to come with 1/2 inch fittings for the vent line. Doesn't it need to be bigger than that? If so, do you have to adapt a bigger hose down to the 1/2 inch fitting?
 
1/2" fitting for a holding tank vent line is WAY TOO SMALL. It should be minimum 3/4" and preferably larger. For a water storage tank, a 1/2" vent line is fine, but not for a holding tank - you'll never get anywhere near enough air into the tank to break down and liquify the solids. All you'll get is stink out the vent line every time the toilet is flushed.
 
You must log in or register to reply here.
Back
Top