• 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. ben2go's Avatar
      ben2go -
      On the end that doesn't open,you could drill the appropriate size hole,thread in the 5/8 barb,and glue the end cap on.The barb will self thread into the soft PVC.This may help reduce some cost.I love cheap and free solutions.
    1. TimG's Avatar
      TimG -
      That is definitely an option. Some hardware stores have the proper fittings that you can use and avoid 2 of the pieces as well.
    1. Cap'n Ray's Avatar
      Cap'n Ray -
      I'm going to do this when it's time to replace the 'store bought' filter that is on the Carver now... Them buggers are pricey!!!
    1. ben2go's Avatar
      ben2go -
      I think I am going to build two units kind of like this.One for my waste vent and one for my fresh water vent.Maybe I can keep all the nasties in the filter and out of my tanks.Read this all the way through.You'll get an idea of what can get in your tank and make everyone sick. The Marine Installer's Rant: Primordial soup
    1. TimG's Avatar
      TimG -
      An interesting read for sure and it makes you think...
    1. Cap'n Ray's Avatar
      Cap'n Ray -
      Makes me think alright... Think "ewwww!". I just put a gallon of bleach into about 150gal of fresh water. Gonna let that sit a day or two!! The tanks are super inaccessible on the Carver, but I'm a bottled water kinda boater anyway. Sure do want to be able to use the sinks and head, and possibly the shower though!
    1. ben2go's Avatar
      ben2go -
      Quote Originally Posted by Cap'n Ray View Post
      Makes me think alright... Think "ewwww!". I just put a gallon of bleach into about 150gal of fresh water. Gonna let that sit a day or two!! The tanks are super inaccessible on the Carver, but I'm a bottled water kinda boater anyway. Sure do want to be able to use the sinks and head, and possibly the shower though!
      Arggg same here.Bottles fur drinking.Tank water fur flushin and washin.

      Sorry.Middle of the night and all that's own is PC, Dead Man's Chest.
    1. TimG's Avatar
      TimG -
      Ray you may want to run some white distilled vinegar through the system as well. The vinegar will knock a lot of the various deposits loose, of course be ready to clean the filter many times.
    1. Cap'n Ray's Avatar
      Cap'n Ray -
      Thanks Tim, ya saved me some hassle again!! I was planning the vinegar (dads system for cleaning fresh water tanks in rv systems when I was young- bleach first, then vinegar) as it helps kill the bleach odor and taste, didn't even occur to me that it'd loosen all the mineral deposits and such, good heads up- I'm inclined to put a strainer or filter right before the water pump- probably do that now rather than later!! Ahh the projects list is STILL growing lol!!
    1. RIce's Avatar
      RIce -
      How much charcoal(carbon) did you use? Total fill or partial? Tks
    1. TimG's Avatar
      TimG -
      Rice I did a total fill of the unit, and up to this point I have noticed a decreased amount of smell when the had is flushed but not as much as I would have thought, but definitely better.
    1. RIce's Avatar
      RIce -
      Thanks, I'll build one and test it out.
      RIce in Dallas
    1. TimG's Avatar
      TimG -
      good deal let us know how it works for you!
    1. Tim Robertson's Avatar
      Tim Robertson -
      This is pretty cool. Trouble is the filters actually keep air out of the holding tank. Introducing oxygen into the holding tank is the best way to cut down on the smell. Common yeast and air will cut down on the smell. Although like everyone else, I use a retail product in mine. Septic systems used to have a small compressor that injected air into the tank to cut down on the smell.

      Of course there are two types of "smells". The first is the "long-term" smell. You know like when your tanks been sitting for a period. The above method is very effective in that situation. The other smell is the "short-term" smell and that comes from a whole lotta usage in a short time like a weekend. The above method doesn't do a thing in that situation because the enzymes don't have time to do their dirty work. I think the filters would help in that situation. Now how do we get both?

      Now, just a rant. Why don't manufacturers vent holding tanks out the top of the boat away from where people are walking/swimming/sitting?
    1. TimG's Avatar
      TimG -
      You should design an injection system that will automatically inject yeast into the tank, but then again I guess you could just flush it down the toilet.....hmmmmm Besides your boat smells worse than mine anyway.
    1. aoben's Avatar
      aoben -
      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. ben2go's Avatar
      ben2go -
      If you have a liquid tight pump out connection then up sizing the vent filter will help.Most places and boats I deal with they just slide a hose down the pump out and air comes in around the hose as the tank is emptying out.
    1. TimG's Avatar
      TimG -
      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.
    1. Roger's Avatar
      Roger -
      Can you easily disconnect the vent hose at the tank when you next pump out?
      If the tank doesn't collapse as usual you will know there is a blockage in the hose or filter.
    1. Cap'n Ray's Avatar
      Cap'n Ray -
      If its drawing that hard on the system, just open the flush valve while your pumping out. RV trick! And, a good spray wand through the flush valve to dislodge solids and sludge isn't a bad idea either. If your tank is collapsing, they're not getting everything out of it.
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