• Building a Holding Tank Filter

    Sniff, Sniff...If you spend quite a bit of time on your boat you may see folks with their noses in the air after you flush your toilet if your holding tank vent line does not have a filter. The holding tank has to allow air to escape as it fills with effluent and the process reverses as the tank is pumped out.

    Newer boats normally come equipped with a filter already in place but if your boat is older, sniff, sniff! You have two options, buy a new filter kit or build your own. The kits are pretty expensive but all that is required is to cut your vent line, install the filter then you are done. However, if you would like to save some money I will show you how to build one that costs around $30.

    Attachment 1482

    Here is a list of items required to build your own filter (depending on the plumbing stock of your local store you may be able to find a single fitting that accomplishes the duty of the various reducers).

    - aquarium charcoal (Wal-Mart)
    - Scotch Brite Pads (Wal-Mart)

    The remainder of these items were purchased from Lowes.
    - pvc cement
    - 2" pvc pipe (8 inches)
    - two 5/8" barb to 1/2" MIP adaptors (Watts # A-493)
    - 2" pvc threaded female adaptor (23286)
    - 2" pvc threaded male adaptor (331504)
    - two 2" x 1/2" bushings (51013)
    - 2" coupling (23902)
    - two J-hook pipe hangars (298079)

    Begin by gluing the coupling to the end of the pvc pipe, then glue the 2" x 1/2" bushing into the end of the coupling.

    Attachment 1483
    The picture below shows the parts glued in place.

    Attachment 1484

    Next thread one of the 5/8" barb to 1/2" MIP adaptors into the end of the bushing.

    Attachment 1485

    Now mark and cut the Scotch Brite into tow circular pieces that will serve as filters, one for each end of the assembly.

    Attachment 1486

    In this picture you can see the Scotch Brite discs prior to inserting into the pvc pipe. Go ahead and insert one of the disc into the pipe and ensure it is fully seated against the bushing at the end of the pipe.

    Attachment 1487

    Now glue the pvc threaded female adaptor onto the other end of the pvc pipe. The threaded female and male adaptors allow the filter to be opened so the charcoal can be replaced in the future (the filter will screw together).

    Attachment 1488

    Now glue the bushing into the threaded male adaptor, this will form the cap for the filter.

    Attachment 1489

    Insert the remaining Scotch Brite disc into the end cap.

    Attachment 1490

    Thread the remaining 5/8" barb to 1/2" MIP adaptor into the cap's bushing. The filter is now complete and ready for filling.

    Attachment 1491

    Fill the filter assembly with charcoal, then screw the cap onto the filter and it is ready for installation.

    Attachment 1492

    The full assembly mounted and connected in-line with the holding tank's vent.

    Attachment 1493
    JAT, rpark1957 and waynetegacay like this.


    Comments 40 Comments
    1. Link's Avatar
      Link -
      A bareboating school captain told me to add white vinegar to the holding tank to keep offensive odors at bay.
      This may be a good thing to try along with the vent filter.
    1. TimG's Avatar
      TimG -
      I have heard that as well but never tried it.
    1. greenghost39's Avatar
      greenghost39 -
      Thankyou Tim
      I've had to convert my system from raw sea pump out
      To a holding tank but I hadn't thought about the vent or filter
      Lol mine would have been one of those boats
      Giving off bad smells
      Or bubbling back out of the bowl
      Thankyou again I'll be reading this to detail
    1. greenghost39's Avatar
      greenghost39 -
      Lol m8s
      Keep up
      all these cocktails in the tank you might create a new
      Species of life form that eats our waste
      That would make it easier as long as it stays in the tank
      And didn't crawl out while we were reading our fishing magazine
      Jokes aside
      Great tips I'm definitely going to try them all
    1. JimMc's Avatar
      JimMc -
      I built mine a few years ago. It's very similar to Tim's. I used rubber couplers instead of gluing the PVC fittings. Brass barbed fittings corroded and I switched the barbed fittings to nylon. I used aquarium filter material rather than the Scotchbrite, both work to keep the charcoal in the filter.

      I made the mount out of Starboard. The blue straps are bungee cord. The lower side of the mount has holes the bungee goes through so it can't fall off when loose.

      I later reoriented it to vertical for better air flow and to prevent it from retaining moisture.
    1. Peter Markham's Avatar
      Peter Markham -
      Thanks Tim I am going to make one for our holding tank.
    1. rpark1957's Avatar
      rpark1957 -
      Building mine tonight!
    1. ben2go's Avatar
      ben2go -
      Pics aren't showing up in this post.
    1. rpark1957's Avatar
      rpark1957 -
      Need to log in first.
    1. ben2go's Avatar
      ben2go -
      Quote Originally Posted by rpark1957 View Post
      Need to log in first.
      Ummmm yeah,I made that post after trying to view pics.I have a feeling I was logged in.
    1. TimG's Avatar
      TimG -
      Ben are you still having the issues?
    1. ben2go's Avatar
      ben2go -
      Quote Originally Posted by TimG View Post
      Ben are you still having the issues?
      I thought there were pics in this post.If they are,I'm not seeing them.
    1. TimG's Avatar
      TimG -
      ahhhhhh I see, if you go to this post in the forum you won't see any, it is actually the comments section for an article on the first page, when you post a comment there, it actually feeds into the forum.
    1. ben2go's Avatar
      ben2go -
      Quote Originally Posted by TimG View Post
      ahhhhhh I see, if you go to this post in the forum you won't see any, it is actually the comments section for an article on the first page, when you post a comment there, it actually feeds into the forum.
      yeah,that's what got me.
    1. TimG's Avatar
      TimG -
      did me too the first time I saw it...haha
    1. packrat's Avatar
      packrat -
      Quote Originally Posted by TimG View Post
      aoben it does sound like you may need to get a bigger vent, but before you do that check and make sure your current vent is not partially clogged. Many of the vent lines have a wire mesh filter where they enter the tank to prevent anything from getting in the line other than liquid, however some don't so you may be partially clogged, especially if the holding tank has ever been filled to the limit.

      I built my holding tank out of fiberglass and all lines are 11/2" especially the vent. if your sucking out 11/2 you need 11/2 going in or if a weak tank it will collapse. new boats are going to 2 or 21/2" thru hulls.
    1. Seasons's Avatar
      Seasons -
      I had a clogged vent line from the previous owner. like concrete. Took a while to figure out why I would get leaks around seals. I'll try this vent filter, the commercial ones are stupid expensive for old school charcoal filtration. Thanks for posting this.
    1. TropicalHorizon's Avatar
      TropicalHorizon -
      I know this is a bit late, but I'm new to the forum. The easy way to get around this is to open up all of your heads so that air can get through the system and prevent the tank collapse. I also flush a bowl full of water down too while pumping out so that the lines get flushed out too. Just a thought... I know this works with both manual and vacuflush heads, but you do have to turn off the vacuflush pump. Not sure about other electric heads.

      Quote Originally Posted by aoben View Post
      This is way better than spending a $100 on a new/replacement filter.
      My problem is not so much the filter, its the way the system is pumped out. Whenever the Marina(s) pump out the system the tank, being plastic starts to collapse.
      Has anyone thought about the actual vent size and is there an easy solution? Seems like the only viable option is to increase the size of the vent hose.
    1. Cap'n Ray's Avatar
      Cap'n Ray -
      As it turns out, one school of thought is to prevent the anaerobic (bad) bacteria (which cause odor) rather than trying to filter it out. Large, free-breathing vents (2 or more) will allow aerobic (good) bacteria to flourish, eliminate anaerobic bacteria, and stop foul odor in its tracks. I've used this method with RV tanks in the past with exceptional results. 1 tablespoon of Rid-x instead of deodorizer or harmful chemicals, and two good sized vents (~1-1/2") allowed me to go two full weeks in summer (Arizona) approaching a full tank and never any odor. I plan to use a similar method on the Chris craft, but I plan to incorporate a small solar powered fan to circulate air. As long as you don't have any unventilated spaces (like sagging plumbing), you'll never have any odor. It's the anaerobic bacteria that produce all the stinky gasses- eliminate them, encourage the aerobic bacteria and you'll save a fortune on filters and chemicals.
    1. ASD's Avatar
      ASD -
      Great post. Thanks for sharing
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