Holding tank filter

[HeadMistress]

It's not necessary for the bubbler (air pump) to be in the tank. Do a little research on live well aerators to find one that uses an external pump. However, it is necessary for a perforated hose to come down from the top and then bend to run across the tank to distribute the air.

A lot of boat owners have installed DIY aerators..,some of them are sure to be able to offer you the help you need.

Back flushing the vent line is a good idea...but NOT DURING pumpout! Forcing water into the vent while the pumpout is trying to pull in the air needed to replace tank contents as they're pulled out interferes with the pumpout's ability to pull out tank contents.

--Peggie

 

[HeadMistress]

Greetings,
Mr. w. I think simply immersing an air tube without the bubbler/disperser would work with less danger of clogging. The end goal is to aerate the liquid (supply oxygen) and encourage aerobic bacteria to grow.

Where would any air come from without a "bubbler" (air pump)?

--Peggie

 

[Bacchus]

Question for Peggie or anyone else with experience…
Situation: I’ve got a 5/8” vent hose leading to a 50 gal holding tank. I replaced the almost blocked thru hull vent with a mushroom vent. I regularly back flush the vent when pumping out. It definitely works better. Access to our tank is limited. We have been using only Noflex for 8-9 years. 2 Vaccuflush toilets. All hoses new. Still stinks from the vent when flushing and we are over about 1/2 full. Doesn’t stink anywhere else.

Question: I would like to install an aerator/fish bubbler. Could I just put that bubbler air into the top of the tank and forgo trying to get it down into the liquid? In other words, the air would just get introduced into the tank just like having another vent. It would be a much simpler install and avoid possible plugging issues.

I have wondered the same... if more / larger vents work with only O2 exposed to the surface what's the difference if is pumped in?
I suspect there are variables that affect how well it works...

Surface area vs volume of liquid... I'll guess this is why half-full tank = no smell but larger volume / same Surface begins anaerobic action & smell. A large area shallow tank will have larger ratio of area to depth. Bubbles rising are likely the best... large Surface area AND some mixing / turbulence.

Does the tank get agitated often via rougher sea state or does it sit at a calm dock?

I dip tube from / near top but extending to near bottom likely just as effective as a tube at/ near tank bottom. Mine is horizontal near bottom as I used an abandoned overboard discharge port. I drill small (~1\8") holes and have never had a plugging problem. I think the 24/7 operation with positive pressure keeps most everything from plugging.

I can't think of any reason that a distribution / diffuser pipe would be an absolute requirement? If that were the case larger vents would not be adequate.
A diffuser certainly more effective than just a dip tube neat bottom, but even that will supply a stream of larger bubbles that provide some agitation and large surface area for oxygenation of liquid when you consider total surface area of a stream of bubbles vs just the air/liquid surface area... plus the bubbles positively/continuously refresh the blanket above the liquid vs hoping fresh air exchanges thru a single vent.

 

[RT Firefly]

Greetings,
Ms. HM. Was replying to Mr. w's post (#26) where he was intending to "...install an aerator/fish bubbler." THAT was the air source! I suggested immersing said air tube without a diffuser/bubbler INTO the effluent.

 

[dhays]

I have wondered the same... if more / larger vents work with only O2 exposed to the surface what's the difference if is pumped in?
I suspect there are variables that affect how well it works...

Surface area vs volume of liquid... I'll guess this is why half-full tank = no smell but larger volume / same Surface begins anaerobic action & smell. A large area shallow tank will have larger ratio of area to depth. Bubbles rising are likely the best... large Surface area AND some mixing / turbulence.

Does the tank get agitated often via rougher sea state or does it sit at a calm dock?

I dip tube from / near top but extending to near bottom likely just as effective as a tube at/ near tank bottom. Mine is horizontal near bottom as I used an abandoned overboard discharge port. I drill small (~1\8") holes and have never had a plugging problem. I think the 24/7 operation with positive pressure keeps most everything from plugging.

I can't think of any reason that a distribution / diffuser pipe would be an absolute requirement? If that were the case larger vents would not be adequate.
A diffuser certainly more effective than just a dip tube neat bottom, but even that will supply a stream of larger bubbles that provide some agitation and large surface area for oxygenation of liquid when you consider total surface area of a stream of bubbles vs just the air/liquid surface area... plus the bubbles positively/continuously refresh the blanket above the liquid vs hoping fresh air exchanges thru a single vent.

I agree completely with your logic. I did run a horizontal pipe across the bottom with hole drilled into it. I doubt it was necessary.

 

[HeadMistress]

I did run a horizontal pipe across the bottom with hole drilled into it. I doubt it was necessary.

It was. Aeration isn't just introducing more air into the tank above the surface...aerating oxygenates the contents...and to do that requires tubing that distributes the air provided by a bubbler or external pump throughout the contents.

20" is the maximum tank depth at which tank vents alone can provide enough air exchange to maintain a tank aerobically and there are several factors that influence that: vent line length and diameter, straight it is (I have a photo in my files of a vent line that actually makes a full 360!), degree of rise above horizontal (45 is the max because fresh air doesn't fall and even aerobic gasses don't rise any closer to vertical than that) and a vent thru-hull that actually allows air flow. Below that depth contents turn anaerobic of necessity.
And as for filters: oxygen PREVENTS odor from occurring...filters trap odor that's been allowed to occur due to insufficient oxygenation.

Fwiw, I don't make any of this up...it's all basic physics 101.

-Peggie

 

[Comodave]

Peggie, that makes sense since my tank is about 15” tall and the 2 1.5” vents seem to work fine in it.

 

[dhays]

I did run a horizontal pipe across the bottom with hole drilled into it. I doubt it was necessary.

It was. Aeration isn't just introducing more air into the tank above the surface...aerating oxygenates the contents...and to do that requires tubing that distributes the air provided by a bubbler or external pump throughout the contents.

20" is the maximum tank depth at which tank vents alone can provide enough air exchange to maintain a tank aerobically and there are several factors that influence that: vent line length and diameter, straight it is (I have a photo in my files of a vent line that actually makes a full 360!), degree of rise above horizontal (45 is the max because fresh air doesn't fall and even aerobic gasses don't rise any closer to vertical than that) and a vent thru-hull that actually allows air flow. Below that depth contents turn anaerobic of necessity.
And as for filters: oxygen PREVENTS odor from occurring...filters trap odor that's been allowed to occur due to insufficient oxygenation.

Fwiw, I don't make any of this up...it's all basic physics 101.

I believe you.

I had forgotten about the effect of the vent line configuration. If I recall, that information is included in your book. Time for me to re-read it again.

But you do reinforce @Bacchus as far as tank configuration. The more surface area between air and contents the more O2 is mixed in. My tank is reasonably shallow, but the vent line is definitely not ideal.

The aerator eliminated odors for me. I was just fortunate that I thought the horizontal pipe along the bottom with holes in it was a good idea at the time.

 

[Bacchus]

I did run a horizontal pipe across the bottom with hole drilled into it. I doubt it was necessary.

It was. Aeration isn't just introducing more air into the tank above the surface...aerating oxygenates the contents...and to do that requires tubing that distributes the air provided by a bubbler or external pump throughout the contents.

20" is the maximum tank depth at which tank vents alone can provide enough air exchange to maintain a tank aerobically and there are several factors that influence that: vent line length and diameter, straight it is (I have a photo in my files of a vent line that actually makes a full 360!), degree of rise above horizontal (45 is the max because fresh air doesn't fall and even aerobic gasses don't rise any closer to vertical than that) and a vent thru-hull that actually allows air flow. Below that depth contents turn anaerobic of necessity.
And as for filters: oxygen PREVENTS odor from occurring...filters trap odor that's been allowed to occur due to insufficient oxygenation.

Fwiw, I don't make any of this up...it's all basic physics 101.

-Peggie

I think we are saying the same thing...
Under certain conditions (shallow tank / liquid levels) vents can be adequate to provide aerobic conditions. My only point is it makes little difference how that air layer above the liquid gets there... static vent where natural flow is present or a pump that "injects" air above the liquid. Natural venting works so should "injected" air.
If air injected above the liquid doesn't provide aerobic conditions (due to depth? Etc?) Venting alone will probably fail.

A dip tube supplying a single stream of bubbles is likely marginally better that injecting above the liquid and providing a distribution manifold is the ultimate and not much more difficult than the simple dip tube... why not include a maifold?

For very large tanks a second manifold may be necessary / beneficial. The bubbler I used and reference in my write up actually has 2 output tube connections that could easily be used to supply 2 manifolds at opposite ends of a tank.

 

[HeadMistress]

My only point is it makes little difference how that air layer above the liquid gets there... static vent where natural flow is present or a pump that "injects" air above the liquid.

Nope...neither vents nor aeration create a layer of air and would cause more problems than they solve if they did.Think of the tank as a stuffy room...opening a window introduces a way for the room to "breathe" (exchange the stuffy air with fresh air). That's how tank vents work. When the vent(s) alone can't provide enough air exchange to oxygenate the contents, it may be necessary to use an air pump to pull in air and push it into the tank along with some simple plumbing inside the tank to distribute it.
It's as simple as that. Manifolding it would over-complicate it.

--Peggie

 

[Bacchus]

My only point is it makes little difference how that air layer above the liquid gets there... static vent where natural flow is present or a pump that "injects" air above the liquid.

Nope...neither vents nor aeration create a layer of air and would cause more problems than they solve if they did.Think of the tank as a stuffy room...opening a window introduces a way for the room to "breathe" (exchange the stuffy air with fresh air). That's how tank vents work. When the vent(s) alone can't provide enough air exchange to oxygenate the contents, it may be necessary to use an air pump to pull in air and push it into the tank along with some simple plumbing inside the tank to distribute it.
It's as simple as that. Manifolding it would over-complicate it.

--Peggie

Semantics again... my bad if it's not a "layer"... I know not very scientific. Maybe better just saying "My only point is it makes little difference how that air above the liquid gets there..."

For your stuffy room analogy... if it's an interior room ( which is a better anology for a holding tank) there are no windows to open. Would it make any difference if the air exchange was simply via an open duct (vent pipe) or via a ventilating system that included a fan to propel the air?
My thinking is as long as the fresh air mixes with the air above the liquid and is available to provide O2 to the liquid does it make a difference how the air got there?

Other factors are much more important... is there any mixing, sloshing, stirring of the liquid to enhance the air/O2 exchange? (rock & roll from seas, etc)

 

[HeadMistress]

Actually there is a "window" in a holding tank: the vent, including its thru-hull. The diameter of the vent line(s) and other factors I mentioned above impacts it's ability to allow the tank to "breathe deeply" enough (passive ventilation) to supply enough air to the contents to oxygenate them, which is necessary to prevent odor from occurring. If it can't, aerating the contents using a pump to supply air to them with plumbing inside the tank to distribute it is the only solution
Installing a fan in the vent line would create an obstruction that only allows the tank to "inhale" or "exhale" depending on whether the direction the fan is aimed, but not both.
Rocking and rolling wouldn't help the tank do anything but "exhale" the stinky gasses.

--Peggie

 

[dhays]

Actually there is a "window" in a holding tank: the vent, including its thru-hull. The diameter of the vent line(s) and other factors I mentioned above impacts it's ability to allow the tank to "breathe deeply" enough (passive ventilation) to supply enough air to the contents to oxygenate them, which is necessary to prevent odor from occurring. If it can't, aerating the contents using a pump to supply air to them with plumbing inside the tank to distribute it is the only solution
Installing a fan in the vent line would create an obstruction that only allows the tank to "inhale" or "exhale" depending on whether the direction the fan is aimed, but not both.
Rocking and rolling wouldn't help the tank do anything but "exhale" the stinky gasses.

--Peggie

@HeadMistress you are one of the most patient and long suffering women on the internet. I appreciate it greatly.

My limited take-home understanding is that what mattes most is the amount of air exchanged in the tank. As you noted before, simply having a layer of air over the tank doesn't help if it isn't being exchanged adequately. A larger diameter vent hose or a non-restrictive vent thru-hull are only helpful if they sufficiently increase the exchange of the air in the tank. Air-water gas exchange is depended on the concentration of the gas in each. If the air isn't exchanged, then the O2 concentration decreases as the aerobic bacteria in the tank consume it, and through that air-water exchange consume the O2 in the air in the tank.

A bubbler helps in three ways. It is exchanging the air in the tank by pumping in a steady supply of fresh air and then the "stale" air is forced out the vent. This keeps the concentration of O2 in the air in the tank high and the concentration of CO2 low. Secondly, it increases the surface area for that air-water gas exchange as each small bubble provides a sphere of air-water contact that can allow that gas exchange to occur. Finally, the bubbles provide a small amount of agitation to the tank fluid contents which helps with trying to make the tank waste more homogenous.

Of course if my understanding is close to being correct, all it means is that I'll soon forget it and go back to my normal steady state of ignorance.

 

[Bacchus]

@HeadMistress

I have no interest in belaboring the discussion as I think we agree on almost all points and seem to be going back and forth on the semantics. Everything I have learned about the subject have been a result of your book and your many posts on the subject and the education has allowed me to solve "stinky tank syndrome" on 2 vessels.

The keys I think we agree completely on...

The vapor / air above the liquid in a holding tank needs 02 levels approximating the atmosphere (20%) and needs to be refreshed to avoid decreasing O2 and increasing CO2...
[I doubt it matters how 20% O2 levels are achieved so long as there is also a vent that allows gasses to escape during normal operation and air to enter when discharging liquid contents.]

Vent filters prevent the above and essentially guarantee the tank will smell, and possibly worse, if they become wet/ blocked!

Certain tank configurations may make even 20% O2 levels in the vapor ineffective. ( e.g. a tall deep tank with low liquid / vapor surface area). In those cases a Bubbling system with a submerged air diffuser may be used to ensure the liquid remains oxygenated. Any method of actively moving air into the tank does not replace or negate the need for a vent.

 

[HeadMistress]

Amen!! And I think we've about beaten this subject to death. As my final contribution to it, I know that every boat can have unique sanitation plumbing challenges that make following one-size--fits-all advice and/or instructions difficult if not impossible. So I'll be happy to help anyone who asks work out the best solution for your individual system....so feel free to send me a PM that includes your email address (email is SO much easier to use than PMs!)

--Peggie