Septic holding tank from 4" pvc

Discussion in 'Boatbuilding' started by mariobrothers88, Feb 17, 2022.

  1. mariobrothers88
    Joined: Sep 2020
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    Location: San Diego, CA

    mariobrothers88 Senior Member

    Hi guys for my holding tank for the toilet I'm planning to use a simple hand flush rv toilet like the Thetford aqua-magic rv toilet and plumb it into 8 foot length 4"pvc. This will connect to a deck plate to allow pump outs and to a vent as well. I want the system to be as simple as possible. I got the idea from the sailing uma video where they used a 2 foot length of 4" pvc as the "holding tank". According to my calculations this should hold 8 flushes which would be all I would need. What do you guys think?

  2. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    That pipe will hold 5.2 gallons. Do you think you can flush with less than half a gallon of water?
  3. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    The pumpout tube needs to be dimensionally large enough to dip to the bottom of the pipe. And if the waste is not macerated; it will need to be like a 2" id tube.

    you need to subtract the pumpout tube from the volume calc and once you do so; not gonna be much holding power

    unless you are just flyin out the tube end, but I'd probably want to macerate wastes; so the pipe is okay, but the toilet not so good; you can valve
    To the sea, but I'd valve macerated wastes or it'll probably be trouble
  4. Barry
    Joined: Mar 2002
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    Barry Senior Member

    Dometic 711 - All-in-One Toilet System

    manual style

    Note the link above the video of the 711

    You may find the the Dometic 711 with the holding tank right under the toilet would be the best way to go. Amazon might help with a lower cost than a marine supplier. I suspect that there may be other manufacturers
    This system removes the hassle of finding space to house another holding tank. I would be hesitant to purchase a standard RV toilet as the ball closing valve at the bottom of the bowl can tend to leak vapors while the Marine
    closing valves seem more robust.

    We have had a Port a Potty, a couple of vacuum flush units , a macerator pump into a holding tank with a diaphragm overboard pump, and 2 of a type similar to the 711. These are almost foolproof.
    With the 711 style, you will need a macerator overboard pump, Y valve, anti siphon hose, and pump out fittings, hoses etc. I am assuming that you have a pressured water system as the bowl still has to be filled. Alternatively, the manual pump style toilet may be an option. Ie hand pump water from outside the hull, manually pump the waste into a holding tank, then pump overboard or pump out the tank.
    We have had the best success with a Jabsco macerator pump BUT we always carried a spare in case in case of a pump failure.
    DogCavalry likes this.
  5. Will Gilmore
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    Will Gilmore Senior Member

  6. wet feet
    Joined: Nov 2004
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    wet feet Senior Member

    This won't be what you might have hoped for but just do the job to the highest possible standard at the outset.No shortcuts or clever workrounds will save you from eventually having to deal with a pretty unpleasant problem and its likely to be at a time you haven't chosen.Can you lift the tank out or get a suitable pump to empty it prior to removing it?Taking the lid off isn't something to look forward to and if you don't have a lid and have to lift out a flimsy length of pipe full of nastiness-it gets worse.If you can't get a stainless tank made to fit then seriously consider putting together a GRP substitute from flat sheet,make sure there are pipe inlets in the top and put a macerator upstream of the tank.You might have a pump out fitting in the deck,but will it allow every last drop to be removed?Anything left will reduce the effective cruising time between visits to the pump out station so make sure the pipe inside the tank reaches a long way down.Doing it right will mean you can fit and forget,a half baked system will almost certainly spoil at least one weekend afloat and then require a few more to take it apart and do it right so you might as well go for the best solution first of all.This has been typed by one who has been onboard a boat when the lid came off a holding tank and wishes he hadn't had the experience......
  7. fallguy
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    fallguy Senior Member

    I can sum up the article/book rather easily.

    The smelly waste bacteria are anaerobic and live where oxygen doesn't. The best waste systems are in a high oxygen environment. This means lotsa ventilation. My 35 gallon waste tank uses two one inch vents and the vent intakes can be arranged so a clamshell is open forward on one side and faces aft on the other forcing air to pass across the tank which will minimize anaerobic bacteria and increase aerobic bacteria. This is the essence of her book. I'm still not sure about using the clamshells..the vents are both a full inch..

    Venting a 4" round pipe well might be tough.

    Not sure the rules in Mexico, but I'd want macerated wastes and a Y valve.

  8. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Wouldn't you rather be able to dump wastes out to sea Ron?

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