Holding tank vent smell and aeration

For a long time we've had an issue with holding tank vent smells when flushing the heads. No smells just sitting, but a horrendous smell from the vent when flushing. We've tried various combinations of Odorlos, Noflex, sodium percarbonate and KO with only a small improvement from any of them.

This past winter I changed out the vent line from 5/8" to 1" and added a straight mushroom thru hull. Still not good enough. So about 10 days ago I added an aerator to the tank using one of the Marine Metal Power Bubbles 12v air pumps. The aeration tube I built goes down to about an inch from the bottom and runs about 18" across the tank (diagonally forward and across starting from one corner at the end opposite from the vent). It's got a bunch of tiny holes in the bottom of the tube and is happily bubbling away along the whole 18" length of the lower tube.

I started out with a half full tank, and after a few days, the smell had tapered off to minimal. And it was no longer the stench of anaerobic waste breakdown, but a very sharp, sour smell (although very faint). We had the tank pumped a few days ago and flushed with water, figuring that might help get rid of the last bit of smell. Added about 8 ounces of KO to the 70 gallon tank and have used it a bit over the last few days.

At this point, the smell is still not gone. It's actually a little worse than it was before pumpout. There's a little bit of crud on the tank walls, but not a lot (I can still see the waste level through the sides of the tank). It's pushing enough air through the tank to have it bubbling pretty noticeably, and on cool mornings I'm actually getting condensation drips from the vent fitting due to the humid air being expelled. The smell this morning is a little better than when I left the boat yesterday (and the tank did get sloshed around a good bit yesterday), but it's still noticeable if you stand just downwind of the vent (or hold your hand to the vent and then smell it).

Any thoughts on what I might be missing, or what might be wrong with the chemistry of the tank where aeration can't kill the smell? Is it possible the tank is too acidic? I just threw a few ounces of sodium percarbonate into the few gallons of waste that are in there, so we'll see if that helps any. It should raise the pH in the tank a bit as well as adding extra oxygen.

I have put Groco Sweetank systems in 2 boats and after 2 hours the smell was gone completely and never came back for the 8 years I owned the first boat. Never used any chemicals at all in that boat. This boat had the regular 5/8” vent, useless for odor control, so I added 2 1.5” vents and no smell so far. I do add some sodium perconate periodically.

Comodave said:

I have put Groco Sweetank systems in 2 boats and after 2 hours the smell was gone completely and never came back for the 8 years I owned the first boat. Never used any chemicals at all in that boat. This boat had the regular 5/8” vent, useless for odor control, so I added 2 1.5” vents and no smell so far. I do add some sodium perconate periodically.

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It was based on your experience and others that I was expecting better results than I'm seeing so far. I figure I have to be missing something simple at this point.

Maybe your “stuff” is just too potent…

"It's actually a little worse than it was before pumpout."

This might be an indication that there might be a hardened build up in the bottom of your tank that got disturbed during flushing and pump out causing more smell.

The build up in mine was cement like. I broke it up into little pieces with a stick and vacuumed it out.

I also wonder if it isn't your line from the head to the holding tank that's the cause due to build up or permeation.

https://www.trawlerforum.com/forums/s31/holding-tank-cleaning-easy-way-46357.html

This is a procedure that OC Diver used to clean his holding tank. We used it and it made a noticeable difference in ours. Maybe try something like this

mahal said:

"It's actually a little worse than it was before pumpout."

This might be an indication that there might be a hardened build up in the bottom of your tank that got disturbed during flushing and pump out causing more smell.

The build up in mine was cement like. I broke it up into little pieces with a stick and vacuumed it out.

I also wonder if it isn't your line from the head to the holding tank that's the cause due to build up or permeation.

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The tank is only 4 years old and it's been flushed out regularly, so I know there's not a lot of buildup in there. There may be some, however. It's definitely not a permeation issue though, as the smell is coming out of the vent. No smell issue inside the boat anywhere.

I gave it a hefty dose of sodium percarbonate about an hour ago and the smell has now decreased noticeably. So I'm thinking either it needed a blast of extra oxygen (and foaming) to help break up some crud in the bottom of the tank, or the tank was possibly too acidic (percarbonate will raise the pH).

Strange. The first boat that I put the SweetTank in had a fiberglass 80 gallon tank with a 5/8” vent. Could not see into it at all. But almost exactly 2 hours after I turned on the air the smell was gone. And it was horrible, my wife wouldn’t use the head if anyone was on the dock because it smelled so bad when you flushed the head.

I'm surprised and can only guess...
Only other though I had was could it be some permeation of the vent line that is coming out with the Bubbling?
You might try a backflush of the vent hose?
Only other thought is keep at it with SPC and bubbling.

Bacchus said:

I'm surprised and can only guess...
Only other though I had was could it be some permeation of the vent line that is coming out with the Bubbling?
You might try a backflush of the vent hose?
Only other thought is keep at it with SPC and bubbling.

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If there's permeation in the vent hose, it's not enough to smell it with a sniff of the hose inside the boat. And the hose is only a few months old anyway. Plus I have backflushed it a couple of times this year.

I think the "keep at it" thought is probably the right one. Let it keep bubbling, dose it with SPC a few more times and just see what happens. It may just need more time and attention for everything to reach a happy, non-smelly state.

See post #4…

Did you PM Peggie? She might have something else to try.

with a bubbler you are adding a constant airflow. Without the tank percolates and vents when new stuff is added. Right there I can see less smell with a constant air flow out v. occasional.
That the bubblers work has been shown by several TF members. So what remains is built up crud on bottom and walls reactivating?

I'm hoping that the increase in smell noticed yesterday is just from sloshing stirring up some crud on the tank walls, etc. The thing that I find odd is that sharp, biting, sour smell that was still left when the rest of the smell settled down (although it was faint enough to mostly ignore it). It's very different than any smell I've had from the tank pre-bubbler. But maybe it just needs more time.

I haven't PM-ed Peggie yet, I figured I'd gather some ideas from the hive first.

No need to PM me...Are you only running the bubbler while aboard?


--Peggie

HeadMistress said:

Are you only running the bubbler while aboard?


--Peggie

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It's been running 24/7 since the install (about 10 days ago).

The aeration tube I built goes down to about an inch from the bottom and runs about 18" across the tank (diagonally forward and across starting from one corner at the end opposite from the vent). It's got a bunch of tiny holes in the bottom of the tube and is happily bubbling away along the whole 18" length of the lower tube.


Are there only holes in the bottom of the tube, or is the entire tube "perforated" every few inches? How big are the holes?


--Peggie

I've been thinking... The bubber has either made the condition aerobic or it hasn't...everything you've said indicates it has. Aerobic conditions prevent organic material from generating anything but CO2. So I don't think the source of your "sour" odor is organic, it has to be chemical or another inorganic "ingredient" in the tank, maybe a "witches brew" of tank products? What else besides K.O.--which is organic--have you used in the tank? Nor should there be any drips of condensation out the vent...I've never heard of excess humidity in a holding tank before. Is it sitting next to an engine or generator that could be heating it? I don't THAT would create the odor, though.

It's unlikely that buildup on the walls of the tank are the culprit...they're just the animal fats in waste. But depending on what else you've used or what someone may have flushed, the sludge in the bottom is a possibility.



--Peggie

HeadMistress said:

Nor should there be any drips of condensation out the vent...I've never heard of excess humidity in a holding tank before. Is it sitting next to an engine or generator that could be heating it? I don't THAT would create the odor, though.

--Peggie

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Peggie
I have to disagree... whilevKi have never noticed or looked for condensate in the vent fitting the Bubbling process... small size bubbles vs just air on the surface lends itself to higher dew points. With bubbles the surface area / volume ratio of air is large and has to enhance evaporation and increase the dew point.
If the air is saturated a small temp change results in condensation. Still should not cause any smell.

The holding tank is in the engine room, so it does get a little bit warm. We last ran the boat yesterday, IR gun says the tank is currently 80* on the lower sides. Hull bottom next to it shows 78*, which is about the temperature of the outside water. I've only had the condensation drips when it's cool outside in the morning and the outer part of the vent line is cooler than the tank (it was about 70* this morning). Because of the angle of the hull side where the vent fitting is mounted, the last few inches of vent hose are angled slightly down to the fitting. I don't think the condensation drips are really a problem, just a symptom of pushing humid air out of the tank through the vent when the outer end of the vent line is cool enough to condense some of the moisture. It only makes sense that the air in the tank will be pretty close to saturated humidity-wise.

As far as tank products, the only things that have gone into the tank in the last couple of years are KO and sodium percarbonate. So there shouldn't be any chemical concerns unless we sucked up some contaminated flush water somehow (heads are seawater flush, boat is in fresh water). The only thing that gets used to clean the heads is CP.

As far as the aeration tube, the 18" long horizontal tube has 1/16" holes drilled on the bottom along its length, about 1 inch apart (I didn't want too many holes, figuring that a more forceful flow of air will better avoid clogging). The overall tube is basically an L shape. No holes in the vertical part, just the lower horizontal part. From what I can see by shining a flashlight through the tank walls, there's visible bubbling along that whole 18" length and the bubbling is audible as well. So there's definitely plenty of movement from the aeration and plenty of air going in. It doesn't stir up the whole tank though, as the tank is just over 4 feet long.

Perhaps I missed this in the messages posted so far, but you don't have a charcoal vent line filter in your vent line, do you? Following Peggy's advice we removed ours entirely and a persistent stank that annoyed us for years vanished almost instantly. I know vent line filters are standard on lots of boat sanitation systems but they badly inhibit airflow, even brand new.

kthoennes said:

Perhaps I missed this in the messages posted so far, but you don't have a charcoal vent line filter in your vent line, do you? Following Peggy's advice we removed ours entirely and a persistent stank that annoyed us for years vanished almost instantly. I know vent line filters are standard on lots of boat sanitation systems but they badly inhibit airflow, even brand new.

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Nope, no filter on the vent line. Just about 6 feet of 1" hose from the tank to the vent fitting.

I think the answer may be that it just needed more time. I've ignored it for the last couple of days and there has been pretty minimal addition to the tank. Just gave the vent a sniff and there's pretty much no perceptible smell.

I'll have to monitor and see if it stays that way or if the problem returns at some point.

Glad it is better.

Wondering whether the holes in the horizontal tube would work better if they were on the top of the tube instead of on the bottom where sludge can clog them...


--Peggie

HeadMistress said:

Wondering whether the holes in the horizontal tube would work better if they were on the top of the tube instead of on the bottom where sludge can clog them...


--Peggie

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It's possible that may be a long term concern, but for now it seems to be bubbling along the whole length of the tube nicely. It's only been in place for 2 weeks, so not enough time to clog. The tube also sits about an inch above the bottom of the tank, so I should hopefully never have that much sludge in there. I do plan to pull the tube and inspect at the end of the season once it's been in use for a few months so I can see how my design is holding up.

My thought with holes on the top was that anything that gets into the aeration tube will get stuck in there and could cause clogs (if I ever have to turn it off for service without the tank empty). With holes on the bottom, anything that gets in should get blown back out of the tube once it has airflow again.

That makes sense.

Peggie
I fretted a bit about holes when I first installed my bubbler...
How many, top or bottom, size, etc wondered about plugging as well as effectiveness.
After some time and thinking more about it I've come to the following conclusions...

It doesn't matter... don't over think it. ... if it required a dispersed Bubbling action or specific bubble size, density, etc simple venting (even adequate size & design vent) would not be effective as it doesn't provide any Bubbling, agitation, etc

If providing adequate venting is effective (which I believe from your experience and my last boat success with just more venting) I have to believe that an air pump providing fresh air to the tank space above the liquid would also be effective. If so Bubbling is not essential and the specific bubble details are unimportant as long as it provides adequate fresh air to the air space and surface of the holding tank.

Any comments or insights that counter the above conclusions?
As always, thanks for your knowledge & contributions here on TF.

If the bubbler is left on 24x7 it probably won’t get plugged up since the air should keep the stuff from coming into the tube, probably.

Holding tank filter

I had the odor problem several years ago. After some thought, I crated a vent filter from pvc tubing using charcoal filter material from an aquarium supply co. lasts quite a while. When the odor starts to build, I just change the charcoal. Bob