Holding tank vent smell and aeration

Comodave said:

Time to get a new boat…

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But-But I dont smell nuttin. And since I stopped smoking the ashtray does not get full.

I was just wondering if the other recommendation of a vent hose on each side does same job.
Or a single hose fed to the bottom.
Alternatives to feed air and eliminate chemicals.

In our current boat we have a 50 gallon tank. It had the standard 5/8” vent hose with a filter. I left that in place and added 2 new 1.5” vents, one to each side of the boat. We haven’t had any smells. I do occasionally add some sodium perconate to the tank but that is more of just a why not than a necessity. With the really large vents to each side we get airflow through the tank no matter which way the wind blows.

The vent filter negates the value of a larger size vent line because it impedes the exchange of air with the gasses in the tank necessary to PREVENT odor, eliminating the need for a filter to block odor that it allows to occur.

--Peggie

I didn’t put a filter on the new larger 1.5” vents. The filter s on the small original vent hose and I can’t possibly reach it so I left it in place. But it doesn’t have anything to do with the new large vents.

It sucks when things still don’t improve... It could be that the issue is more with the tank’s overall chemistry, like you’re suspecting. If the tank is too acidic, that could be why the smell persists even with aeration..,. The sodium percarbonate should help raise the pH and add oxygen, but you might want to monitor the pH closely to ensure it’s staying in the right range for aerobic bacteria to do their job effectively.

Based on my results this year (mild smell most of the time, worse as the tank gets more fuel, but goes away for a couple of days after dosing the tank with Camco TST Max), I'm thinking my issue is that I'm just not pumping enough air into the tank. That, or I'm not getting enough movement in the tank and some part of the tank is still short on oxygen. Whatever is going on is certainly not the tank being anaerobic though, the smell is nothing like the stench from an anaerobic tank.

Over the winter I'll see what I can come up with for either more venting or an improved aeration system and hopefully I can get the tank down to basically no noticeable smell.

I had someone apparently get sick over a couple of trips during Christmas time and they barfed in the head (to be discrete I suppose). It caused a terrible smell that took some serious flushing to get back to normal. I think it just really created some smelly bugs that even aerating didn't help. It was something evil....

Just doing a little brainstorming....
It's impossible to provide too much oxygen to a holding tank..odor can only be generated in an environment that isn't getting enough. So.....

1. Putting the holes in bottom of the tube is sending the air to the bottom of the tank instead of distributing it throughout the contents. If you rotate the hose to move 'em to one side of the tube, the air can go up to where it's needed.
2. Check the volume of air your pump is generating against the specs for the Groco aerator pump (liters/minute). If yours is significantly lower that could be at least one of the reasons why it's not working as it should.
3. Are your holes too small to allow enough air to go out of them?

--Peggie

HeadMistress said:

Just doing a little brainstorming....
It's impossible to provide too much oxygen to a holding tank..odor can only be generated in an environment that isn't getting enough. So.....

1. Putting the holes in bottom of the tube is sending the air to the bottom of the tank instead of distributing it throughout the contents. If you rotate the hose to move 'em to one side of the tube, the air can go up to where it's needed.
2. Check the volume of air your pump is generating against the specs for the Groco aerator pump (liters/minute). If yours is significantly lower that could be at least one of the reasons why it's not working as it should.
3. Are your holes too small to allow enough air to go out of them?

--Peggie

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I've confirmed good upward bubbling action from the aerator tube, there's noticable bubbling action at the surface in the tank. I did add more holes at one point but there was no noticeable difference. Bubbling action is also audible in the engine room.

I'm thinking that it's a fairly large tank so I may just need to build a second aerator tube near the other end of the tank so that more of the tank is getting air introduced and gets agitated from the bubbles.

The air pump I'm using is listed at 5.6 liters per minute. Based on our fill rate and the pumps recommended for an aerobic septic it should be plenty. But thinking about it, holding tank contents on a boat are far more concentrated. And our heads are raw water flush, so that further increases the oxygen demand of the contents.

At this point I'm inclined to either add another air pump for a second bubbler or find a larger air pump to use along with a second bubbler tube. As you said, too much air isn't a problem, so overkill is better than not enough.

According to Marine Metal Products, their Power Bubbles aerator delivers 5.6L/Min which they say is enough for up to a 35 gallon live well tank. My holding tank is 50 gallons and it seems to work great. I doubt that the bubble distribution makes any difference.

It is surprising that you are still having smells after installing the aerator. Our current boat just has the 2 1.5” vents that I added and we don’t have anymore smell issues. Strange.

It surprised me too. But it's a bigger tank than many (70 gallons, a bit over 4 feet long) so I'm thinking that it might just need more air or I may have some area of the tank not getting enough air (even though the aerator and vent are at opposite ends). The some areas being anaerobic thought is supported by a few times with the tank 1/4 - 1/3 full where it smelled fine at the vent before leaving the dock, but after being out in somewhat rough seas where the tank contents got sloshed around a bit, the vent was a bit smelly.

It seems like an aerobic septic system typically recommends 80 liters / min for 500 gal/day. Our typical input to the tank is more like 5 - 8 gal/day when we're on the boat. Assuming I size for a worst case of 10 gal/day input, that should only need 1.6 liters / min of air to keep things happy. But the question becomes one of oxygen demand per gallon of tank contents which is likely much higher in a tank containing only black water (and more concentrated than what home toilets put out) plus additional oxygen demand from seawater flush.

As far as the aerator assembly itself, I haven't had any issues with clogging, but it's possible that the 20 or so 1/16" holes in the horizontal tube just aren't enough and are restricting airflow. I might need to look at adding some more holes or enlarging them slightly to see if that makes any difference.

I also wonder about air bubbling into the bottom of the tank versus just pumping some of the air through the top of the tank. I'd expect the agitation from bubbling to be helpful, but I don't know for sure.

We have a Dometic vent line filter installed and use the Dometic "juice" in the tank and we don't notice any odor when flushing - And our vent is just below our salon window. It sounds like others have not had as good of luck with filters?

rslifkin said:

I also wonder about air bubbling into the bottom of the tank versus just pumping some of the air through the top of the tank. I'd expect the agitation from bubbling to be helpful, but I don't know for sure.

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I assumed that you were introducing the air at the bottom of the tank. Is that not the case? I used some PVP pipe that went down from the top of the tank, then ran horizontally for a short length just an inch or 2 above the bottom. The holes are in the the horizontal section.

I don't think in my case that was necessary, but in you case with a larger tank, it might be helpful. I agree with others that you situation has stumped me. If there was a way to reasonably add a second aerator it might be worth a shot.

The aerators themselves can be found inexpensively. The hassle is the installation.

dhays said:

I assumed that you were introducing the air at the bottom of the tank. Is that not the case? I used some PVP pipe that went down from the top of the tank, then ran horizontally for a short length just an inch or 2 above the bottom. The holes are in the the horizontal section.

I don't think in my case that was necessary, but in you case with a larger tank, it might be helpful. I agree with others that you situation has stumped me. If there was a way to reasonably add a second aerator it might be worth a shot.

The aerators themselves can be found inexpensively. The hassle is the installation.

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Yes, I'm introducing air at the bottom. I built a tube that extends down to about 1" above the bottom of the tank and then extends 18" horizontally with air holes in the horizontal portion.

With a black water tank 30" x 50" x 12" high, any suggestions on what size air pump should I get? In a 120V version, they seem to be available in 100 - 300 gallons per hour, with some larger ones in the 600, 1000, 1200 GPH range. I understand that one cannot add to much O2, however the larger the pump, the more the continuous amp load when at anchor.

I bought a small aquarium air pump (20-40g size) and split the output into a large black tank and a large gray water tank, each well over 100 gallons in capacity. It runs 24/7 with pretty minimal power usage and minimal sound, and has had the desired results of eliminating odors. This is with a straight dip-tube style input tube with no horizontal extension.

I tried a larger air pump (for 60-100g) and the gurgling noise and pump noise was bothersome at night. The smaller pump does the job with almost imperceptible sound.

My conclusion is that the volume of air doesn’t have to be much if it is run continuously. The pumps are cheap so its not a big deal if you don’t get the size right on the first try. Make your best guess and adjust if necessary. Just give it a few days before passing judgement.

Stabi said:

With a black water tank 30" x 50" x 12" high, any suggestions on what size air pump should I get? In a 120V version, they seem to be available in 100 - 300 gallons per hour, with some larger ones in the 600, 1000, 1200 GPH range. I understand that one cannot add to much O2, however the larger the pump, the more the continuous amp load when at anchor.

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This is what I used. I bought mine was Cabellas but now I'd likely just order it from Amazon. It will run continuously and draws very little current. It is also quiet particularly if you don't mount it firmly against the hull. This has proven more than adequate for my 50 gallon holding tank. Based on your dimensions, Yours is around 77 gal? The pump above delivers 5.6 Standard Liters/min which would be roughly 90 gal/hr I think.

I have often considered putting the pump on a timer just for the heck of it. My experience has been that it would work on my boat even at a 25% run time.

Agree It doesn't take much air run 24/7. Start small vs large. I can hear faint Bubbling at nite when all us quiet but pump is very quiet.
Here is the pump I used. Still operating after 11 seasons.
Less than 1/2 amp at 12V. Little need to go large or 120V IMO.

From Specs:
"Super quiet 12V DC low amperage air pump system is designed to fully aerate two livewells up to 18 gallons, or one large well. Designed for use with flow through systems. Kit includes pump, 10' air line tubing, two large air stones, two anti-siphon valves, 6' of wire and copper battery clips.

12 V DCw, very low current +/- .46 amps
Twin bellows and double outlets produce aeration of 1 or 2 live wells up to 35 gallons
High volume twin bellows pumps 5.6 STL/min
Weighted glass bead airstones provide greater dissolved oxygen
Produces 99.5 % saturation of dissolved oxygen
Perfect for crowded livewells
Excellent emergency aquarium pump up to 100 gallons"

guy with a boat said:

This is with a straight dip-tube style input tube with no horizontal extension.

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This is good news because a straight dip-tube would be much easier to install. Because the footprint of my tank is not tall, do you think this might be detrimental?

Bacchus said:

Little need to go large or 120V IMO.

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The reason for 120V is that 120 V is easier to wire to the tank location