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Old 09-15-2015, 10:51 AM   #1
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Solving holding tank smelling issue

Ever since I bought my boat 6 years ago, my holding tank has been smelling. Mostly from the vent, not so much inside the boat, thank god. Nevertheless, it was always embarrassing to know my dock neighbors would get a whiff every time I would flush. No matter how much blue stuff I would drop in the toilet to try to hide the smell, it just wouldn't get rid of it. It was fine for a few days after a pump out but would come back as soon as there was a bit of stuff in the tank.

So this year I decided to tackle the issue and fix it once and for all. I found this post about a vent filter and thought it was the solution to my problem.

Chip & Debbie Willis on s\v Elegant'sea: SEALAND. SaniGardô Holding Tank Vent Filter re-built and working, fine!

I figured I could skip the step where I buy an expensive first filter and just build one from cheap parts at Home Depot. But I soon realized that it wouldn't really get rid of the problem, only mask it. After a bit of research on holding tanks I understood a few important things. It's all about air flow and the good versus bad bacteria that live in your tank. In short, a good air flow will allow for the good kind of bacteria that produce non smelly gases. The other thing I learned is that most holding tanks are built with a vent that is way too small to allow for a good air flow. The original vents are designed to allow for air to be displaced when filling or emptying. That's it.

So I thought about it long and hard and came up with a solution. First I installed a new bigger vent (5/8) instead of the 3/8 that was in place. Already a nice improvement. But since I had all the parts to build a filter I had an idea. Why not build an "active vent". Something that would circulate air. So on top of the parts I already had for the filter, I also bought a small 12v computer fan that I fitted inside the casing. The size of the hose for that additional vent is 3/4.

The fan is always running. The air displacement is not big of course but since it's constant, it's enough to bring the fresh flow of oxygen needed for the good bacteria to live. This totally solved the odor that was coming out of my vent. All and all the cost was about $25 and it took about an hour to build and install.

And while I was at it I also decided to solve another problem I had. It was always a guessing game for me to know when it was time to go for a pump out. Didn't have a gauge and didn't feel like buying an expensive electric one. So again I got a few cheap plumbing parts from Home Depot and build a "peep hole" about 1/3 from the top of the tank (poop hole might be a more appropriate name. lol). When I see the level of liquid through the glass, I know it's time. I can even unscrew the glass to clean it. Cost: less than $10.
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Old 09-15-2015, 11:09 AM   #2
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Nice work! Do you by chance have a Vacuflush head?
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Old 09-15-2015, 11:20 AM   #3
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Your problem is solved, that is the important thing.

However, I suspect that the reason it no longer smells is not that you have eliminated anaerobic bacteria (and the smell they produce) by creating an aerobic environment. (Without agitation, oxygen provided by the fresh air will not make it below the top couple of inches of crap in the holding tank. Below that level, the environment will be anaerobic and the nasty bacteria will continue to produce nasty smells that will migrate to the surface, just as without the aeration.)

I suspect that the reason it works is that the smelly air is being continuously diluted by fresh air (filtered) and blown out. A sure test would be to remove the filter material, put your noise right up to the vent and take a whiff.

But, in the end, it doesn't matter why it works, the fact is your solution works. Congrats.
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Old 09-15-2015, 01:42 PM   #4
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Well that's the thing, I never added the filter part of the project (activated coal). Only the fan to create a continuous flow of air. And agitation is provided by the fact that I am on a boat. Trust me, I did put my nose right on the thru hull of that vent. No odor whatsoever.
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Old 09-15-2015, 01:43 PM   #5
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Nice work! Do you by chance have a Vacuflush head?

Negative. Macerator type.
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Old 09-15-2015, 01:58 PM   #6
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Trust me, I did put my nose right on the thru hull of that vent. No odor whatsoever.
That suggests a third explanation for the fact that you have successfully eliminated any trace of a foul odor: Nose blindness (an actual phenomenon). By exposing your nose to an extreme level of offensive smell, you have desensitized yourself to the levels you now encounter.
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Old 09-15-2015, 04:17 PM   #7
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Your problem is solved, that is the important thing. However, I suspect that the reason it no longer smells is not that you have eliminated anaerobic bacteria (and the smell they produce) by creating an aerobic environment. (Without agitation, oxygen provided by the fresh air will not make it below the top couple of inches of crap in the holding tank. Below that level, the environment will be anaerobic and the nasty bacteria will continue to produce nasty smells that will migrate to the surface, just as without the aeration.)
Actually passive--or active--ventilation above the surface in a tank that's no deeper than about 20" is very effective at keeping contents aerobic...it's deeper than that the contents at the bottom turn anaerobic.

Even when waste--all organic matter--breaks down aerobically, it generates CO2, which is heavier than air and can "suffocate" the contents, causing them to turn anaerobic. That's what aeration or ventilation has to run continously 24/7/365--to eliminate the CO2, thus PREVENTING odor from occurring.

Tank products that work WITH nature instead of against it help a lot! A bio-active (live aerobic bacteria) product digests the anaerobic bacteria that produce the foul smelling gasses (hydrogen sulfide and sulfer dioxide) and also methane, which is odorless. One of my favorite tank products is Odorlos...the active ingredient is nitrates, which promote oxygen release from organic matter--iow, from the waste in the tank--helping to maintain the aerobic environment in the tank necessary to prevent odor from occurring. Chemicals--the "blue stuff"--are non-selective in the bacteria they kill and also exhaust themselves fairly quickly, which is why they suppress odor for only a short time and also promote the production of anaerobic bacteria.

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I suspect that the reason it works is that the smelly air is being continuously diluted by fresh air (filtered) and blown out.
Good guess, and very close: it's working because the CO2 isn't being allowed to form a "blanket" on the surface inside the tank.

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But, in the end, it doesn't matter why it works, the fact is your solution works. Congrats.
Well...actually knowing WHY something works is necessary to keep it working or fix it if it ceases to work. It's also essential for anyone else who has a similar problem to understand how to cure it.
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Old 09-15-2015, 04:30 PM   #8
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Originally Posted by Fotoman View Post
Ever since I bought my boat 6 years ago, my holding tank has been smelling. Mostly from the vent, not so much inside the boat, thank god. Nevertheless, it was always embarrassing to know my dock neighbors would get a whiff every time I would flush. No matter how much blue stuff I would drop in the toilet to try to hide the smell, it just wouldn't get rid of it. It was fine for a few days after a pump out but would come back as soon as there was a bit of stuff in the tank.

I suspect you hadn't read Peggie's book first, yes?



And "blue stuff" often seems to be part of the problem...


Good to read you solved it!

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Old 09-15-2015, 04:39 PM   #9
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Good guess, and very close: it's working because the CO2 isn't being allowed to form a "blanket" on the surface inside the tank.
I know you are an expert in this area, but based on my previous research and amateur experimentation in my efforts to build an anaerobic chamber necessary to the creation of a suitable environment in which anaerobic bacteria would consume nitrates, I found that a relatively stable pool of nutrient rich water (one in which the depth is at least 50% of length (or width, whichever is greater) would go anaerobic within the top few inches, even with good ventilation above that. In the context of a relatively stable holding tank, I can't imagine the mechanism by oxygen consumed by aerobic bacteria in the bottom 80 percent of the tank could be replaced, even if all CO2 is removed. Once oxygen depleted, in my experience, only anaerobic bacteria can survive (and without competition of aerobic bacteria, they will thrive and produce a terrible smell).

That is also consistent with my experience on my fresh-water flushed boat. When anchored, even for days on end, the tank doesn't smell because there is enough movement to agitate the contents, and enough fresh oxygen (from frequent flushing) to keep the aerobic bacteria alive and the anaerobic in check. But when the boat is tied to the dock for a week, there is no agitation and little flushing. It will get stinky within a week.
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Old 09-15-2015, 04:49 PM   #10
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I don't care unless the odor escapes the tank. Between WalMart and Lowes; ~ $12
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Old 09-15-2015, 05:05 PM   #11
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I don't care unless the odor escapes the tank. Between WalMart and Lowes; ~ $12
What is that?
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Old 09-15-2015, 05:12 PM   #12
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Sorry. After so much discussion, I thought an inline activated charcoal filter would be easy to recognize. That's a 9oz bottle of activated charcoal from WM aquarium supplies, a knee-high stocking for containing the charcoal in the canister made from the plumbing fittings.
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Old 09-15-2015, 05:13 PM   #13
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Ah, nice job! Thanks.
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Old 09-15-2015, 05:50 PM   #14
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I know you are an expert in this area, but based on my previous research and amateur experimentation in my efforts to build an anaerobic chamber necessary to the creation of a suitable environment in which anaerobic bacteria would consume nitrates, I found that a relatively stable pool of nutrient rich water (one in which the depth is at least 50% of length (or width, whichever is greater) would go anaerobic within the top few inches, even with good ventilation above that. In the context of a relatively stable holding tank, I can't imagine the mechanism by oxygen consumed by aerobic bacteria in the bottom 80 percent of the tank could be replaced, even if all CO2 is removed. Once oxygen depleted, in my experience, only anaerobic bacteria can survive (and without competition of aerobic bacteria, they will thrive and produce a terrible smell).

That is also consistent with my experience on my fresh-water flushed boat. When anchored, even for days on end, the tank doesn't smell because there is enough movement to agitate the contents, and enough fresh oxygen (from frequent flushing) to keep the aerobic bacteria alive and the anaerobic in check. But when the boat is tied to the dock for a week, there is no agitation and little flushing. It will get stinky within a week.
Ye gods...I thought I wrote the world's longest sentences till I read yours!

a). oxygen doesn't consume bacteria, it promotes aerobic bacterial activity.

b). CO2 isn't removed from the tank contents...aerobic breakdown generates CO2 which, without ventilation, forms a blanket on the surface that causes the tank contents to turn anaerobic. Once that happens, anaerobic bacteria create the stinky gasses.

You may not agree with this, but 25 years of experience with considerable input from sewage treatment bio-chemists has proven that passive ventilation above the surface coupled with tank products that work WITH nature instead of against it DOES prevent odor in most tanks...the operative word being "most." It does require that the tank be no deeper than 20", and a vent line that's relatively short (no longer than 5-6') and straight, with a maximum of a 45 degree rise and ideally at least 1" ID diameter...none of which is possible if the tank is buried in a deep bilge....in which case, aerating the contents is likely to be the only solution to odor. Vent filters should only a be a last resort when aeration isn't possible either because filters actually help to create the problem they're sold to solve by impeding the free exchange of air...all they can do is trap the odors, they can 't prevent them from occurring.

It's also important to note that the holding tank is rarely if ever the source of odor INSIDE a boat, because unless a tank is leaking odor from inside the tank has only one place to go--out the vent. It's possible to have both, but odor inside the boat and odor out a tank vent are two separate problems requiring different solutions.
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Old 09-15-2015, 06:00 PM   #15
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Hi All.

If a 12 volt fan adding air to the tank surface helps then would the addition of an aquarium type pump blowing air into the bottom of the black tank adding air also help.

It seems to me that addition of air and the resultant displacment of stale air must be a help.

Just wondering.

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Old 09-15-2015, 06:04 PM   #16
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Hi All.

If a 12 volt fan adding air to the tank surface helps then would the addition of an aquarium type pump blowing air into the bottom of the black tank adding air also help.

It seems to me that addition of air and the resultant displacment of stale air must be a help.

Just wondering.

Regards.

David.
Congratulations, you just reinvented the Groco Sweet Tank System.
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Old 09-15-2015, 06:25 PM   #17
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I once went down to Clearwater, FL to help a friend deliver his new boat back to TN. He and his wife had been down for awhile getting some stuff added. So I was in the bow cabin with the forward head. It just didn't seem to be flushing correctly. I told the owner, and he called the importer. The guy came down to the dock, and said he found the problem. Dirt daubers had built a nest in the vent line. So, he stooped down with a pocket knife to clean it out. The tank had built up pressure, and when the vent cleared it let go with a whoosh. The next thing I knew the broker was prostrate on the dock. He had taken a blast in the face. I learned a lesson from watching him.
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Old 09-15-2015, 06:59 PM   #18
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Y
a). oxygen doesn't consume bacteria, it promotes aerobic bacterial activity.

b). CO2 isn't removed from the tank contents...aerobic breakdown generates CO2 which, without ventilation, forms a blanket on the surface that causes the tank contents to turn anaerobic. Once that happens, anaerobic bacteria create the stinky gasses.

You may not agree with this, but 25 years of experience with considerable input from sewage treatment bio-chemists has proven that passive ventilation above the surface coupled with tank products that work WITH nature instead of against it DOES prevent odor in most tanks...

It's also important to note that the holding tank is rarely if ever the source of odor INSIDE a boat, because unless a tank is leaking odor from inside the tank has only one place to go--out the vent. It's possible to have both, but odor inside the boat and odor out a tank vent are two separate problems requiring different solutions.
Not trying to argue with you, only to learn from your experience. It could give me an insight that will restart my quest for the perfect anaerobic chamber.

For clarity, I did not intend to suggest that oxygen consumes any bacteria. Just the opposite -- aerobic bacteria need oxygen to survive and without enough oxygen either die or get out-competed by anaerobic bacteria (which, by definition, do not require oxygen in their metabolic processes). This is an important distinction, because in my experience it isn't the CO2 that is a problem, it is the lack of oxygen. In other words, removing the CO2 isn't enough to keep aerobic bacteria alive if the oxygen they consume is not replaced. Once they die, there will be only anaerobic bacteria left, and that is what produces the smell.

And, as noted above, I don't doubt that ventilation works. I am just wondering about the mechanism that makes it important. I agree with your point that it is important to understand the mechanism, so I wonder how you determined that the bottom of the tank is aerobic. I suspect it is not, and suggest that if it were, there would be no need for a 20" depth limitation. A more persuasive explanation, to my mind, is that the ventilation continuously dilutes and displaces the offending gases such that no odor is detectable.

Also, I believe you are right that odor within the boat must have a different cause. Fortunately, I do not have that problem.
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Old 09-15-2015, 09:20 PM   #19
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[QUOTE=MYTraveler;369951 I agree with your point that it is important to understand the mechanism, so I wonder how you determined that the bottom of the tank is aerobic. [/QUOTE]

Boats rock...and when they do, that agitates waste at the bottom of a tank. Flushing a toilet into a tank forces air displaced by the incoming flush out the vent. If the the entire tank is aerobic, there's no odor out the vent when either one happens. If it's not, there will be.

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I suspect it is not, and suggest that if it were, there would be no need for a 20" depth limitation.
The 20" depth limitation was arrived at in the field. We began tracking the impact that sizes and shapes of tanks had on odor. The average depth at which odor out the vent from adequately ventilated tanks began to stink when the boat rocked or the toilet was flushed was 20". Is is possible to have a shallower tank that stinks...a deeper one that doesn't? Of course...20" is a rule of thumb that works.

Quote:
A more persuasive explanation, to my mind, is that the ventilation continuously dilutes and displaces the offending gases such that no odor is detectable.
One more time: when organic matter breaks down aerobically, there are no "offending gasses" to be dilutes or displaced...only CO2, which is odorless. A compost pile is a prime example of this: when regularly tossed to aerate, it doesn't stink...when it's not, it does.

Quote:
Also, I believe you are right that odor within the boat must have a different cause.
Thanks so much!
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Old 09-17-2015, 12:15 PM   #20
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Ours is a slightly different situation. We have electric pump Jabscos and 40 gal. holding tank that is pumped and rinsed twice each week. (My wife insists). About a month ago -- and really for the first time -- the aft toilet began stinking to high heaven at every flush. Twice poured in and worked through system a gal. of West's cleaner/deodorant (the blue stuff) and let it sit in the hoses overnight. Very small result and none after about two flushes. Replaced hose from toilet to tank. Surprised to get no benefit at all. Still emits odor big time with every flush. Could something weird be going on in the bowels of the toilet itself or what?

Thanks for any suggestions.
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