Help plumbing waste holding tank.

JoeAyers

Cadet
Joined
Aug 27, 2010
Messages
8
For the last 5 months I've been building a houseboat and am finally to the point where I need to get the blackwater holding tank plumbed. I will be moored 100% of the time with city water hookup, approximately 1 gallon of water per flush, and regular pumpouts (like after every weekend (4-6 people for 3-4 days at a time). The holding tank is 250 gallons and can be drilled any way I want. I would prefer not to use harsh chemicals instead using something like Raritan or Odorlos. my initial plan was to have a 1.5" vent that came out of the top of the tank and up approximately 10' and then turning 90 to the outside.

Will this be enough oxygen for a natural product? Should I add a second vent? If so I would either need to leave the vent about 6" above the holding tank, place it right next to the 1st vent, or have it 90 over then 90 up to run next to the 1st vent.

If you could design the perfect set up what would it be. My next week to work on the boat is March 30-April 10 and the black water tank is #1 on the list.

Thanks for any help.

Joe
 

smokeonthewater

Fleet Admiral
Joined
Dec 3, 2009
Messages
9,838
Re: Help plumbing waste holding tank.

the vent doesn't do much of anything to provide oxygen.... It simply allows pressure to equalize inside the tank while routing smelly fumes overboard... If you are pumping out that often then the choice of treatment isn't nearly as important. It gets much more important if you are going a few weeks.
 

Hank496

Seaman
Joined
Jun 13, 2010
Messages
59
Re: Help plumbing waste holding tank.

250 gallons sounds like a lot. Our 30' express cruiser's holding tank is only 28 gallons. Also as waste is mostly water, a full 250 gallon tank will add about 2,000 pounds to the boat, maybe a smaller tank is in order. Holding tanks are not supposed to be septic systems, so you really don't want to be promoting any kind of reaction in the tank. We regularly bleach our water supply and use detergent in the holding tank after pump outs, things you would never do to a septic system.

There should be two vents, one vent for positive pressure should be vented (ours uses a charcoal element), the other vent should be for negative pressure. The purpose of the filtered vent is that when the head is flushed the increase in pressure in the tank has some place to go. Going through the charcoal filter gets rid of the sewer smell. This filter should be above the effluent level because once it gets wet it goes bad. The negative pressure vent is to prevent the holding tank from collapsing when being pumped out.

You might look at the various marine sanitation systems provided by Sealand Dometic. There is a lot of info there that you can help you with the design of yours. You might also look into your state's local regulations on this as well. They may require USCG approved MSD hardware
 

smokeonthewater

Fleet Admiral
Joined
Dec 3, 2009
Messages
9,838
Re: Help plumbing waste holding tank.

one vent can handle both positive and negative pressure
 

Fireman431

Rear Admiral
Joined
Sep 17, 2007
Messages
4,292
Re: Help plumbing waste holding tank.

One vent will be sufficient. The vent in ours is only 3/4" hose. !.5" would be more than enough...possibly overkill.

If you are pumping your head out weekly, don't worry about adding deodizers or chemicals to the tank. This kills off the natural bacterial digestion inside the tank. The only thing I would recommend is to put a filter on the vent and a screen on the outside of the vent pipe so critters don't turn it into a condo.

Also, with a tank that large, you'll want to make sure it's centered in the boat. If not, when it gets to about half full, it's gonna list the boat.
 

JoeAyers

Cadet
Joined
Aug 27, 2010
Messages
8
Re: Help plumbing waste holding tank.

Thanks for the feedback. I guess I should have clarified. While technically it is a boat I am building, it is more of a condo on a giant barge. Two 40'x4'x2' pontoons run the length. The Holding tank sits between the two near the back of the boat. There are no concerns of listing or weight problems with a tank this large. Also, until I retire (probably another 25 years) I will never be there long enough to fill it before it gets pumped out.

Thanks again for the replies. I found some good information at this link incase anyone else is interested.

http://home.online.no/~lha-h/forum/phall.pdf
 
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