Pump out question

[cosmo]

Random question:

I followed Peggie’s advice and added a second air hose from holding tank so the NoFlex can breathe and do its magic.

As we were pumping out today, it occurred to me: when pumping out, would it be beneficial to temporarily cork the air vents so that when most of the effluent is removed-but not all, the pump out starts sucking air from a vent and not all waste? When done, remove cork and off we go.

 

[Flatswing]

If you plug the vents, you risk collapsing the tank due to the vacuum. I would have thought the other way around. Maximum venting allows maximum effluent suctioning since the pump out hose is usually 1/1/2”” vs 3/4” vents. Of course once the tank outflow tube is not fully immersed, you will suck some air and the pump out hose starts jumping- but unavoidable.

 

[cosmo]

I hear ya. Without being too graphic, I always wonder if it’s still pulling from the tank or back flowing in the hose despite seeing movement in the sight glass. That’s where my thinking started. If it’s still trying to vacuum but it’s pulling more air than now than waste, would closing one of the two vents help really empty the tank?

Thanks for your reply.

 

[Flatswing]

Short answer: I don’t believe so. What you are observing represents the loss of “effluent” seal at the bottom of the tank dip tube. The air enters as the fluid level reaches that critical point. Subtracting more air from the vents doesn’t help. A brief pause in vacuum might allow more effluent to reach the dip tube in a baffled tank, but usually at that point you have to add more water to dilute what’s left, pump till the water seal is broken again, rinse and repeat. If it’s mid cruising season, I just accept the tank won’t empty 100%. End of season, I use the multiple dilutions method (try to go to pump out dock at a non busy time).

 

[HeadMistress]

Don't even THINK of trying that!

Tank Vents101
All tanks--water, fuel and waste--on a boat have vents. They all have two main functions: to provide an escape for air displaced by incoming contents and provide a source of air to replace contents as they're pulled out. When air displaced by incoming waste cannot escape out the vent--iow a blocked vent, the tank becomes pressurized, creating increasing back pressure that prevents the toilet from flushing and can result in a cracked tank if you keep trying.

Without a source of air to replace contents as they're being sucked out of the tank, the pumpout will pull a vacuum that'll prevent it from pulling out more than a gallon or two. A particularly strong pumpout can even implode a tank. If you're seeing anything moving through the pumpout sight glass, it IS coming from the tank.

A blocked vent on a fuel or water tank will prevent filling the tank and also prevent the fuel or water pump from pulling any out. Fortunately neither fuel nor water pumps are powerful enough to do more than prevent any water from coming out the faucets or prevent your engine(s) from running.

--Peggie

 

[cosmo]

Thank you. We are on that current method. I’m always looking for a better-more efficient way to do things.

 

[cosmo]

Thanks Peggie. We’ll stay on course.

 

[Comodave]

Don’t overthink it and take the chance of collapsing your tank, ugh. You probably will never be able to get absolutely all the sewage out. That is just how it works.

 

[dhays]

As others have indicated, you won't be able to entirely evacuate the holding tank with the vacuum hose. It depends on how close to the bottom of the tank the tube runs.

I do two things to make what is left at the bottom of the tank as innocuous as possible. Both I think we're Peggie's recommendations.
1) While emptying the tank, I (or a crew member) will direct a spray of fresh water from the dock hose at the holding tank vent. This sucks a lot of water into the vent back flushing the vent hose. I only have one vent. This helps keep the vent clear and add some water to the tank.
2) After I have emptied the tank, I then add fresh water from the hose to the tank via the pumpout opening. The idea is to dilute and disturb any sludge that may be sitting at the bottom of the tank. Then empty the tank again. I will repeat this 2 or 3 times and you will find that the effluent you see in the sight glass is just about clear.

It means that the remaining water in the tank that will slosh around while under way is now relatively clear.

 

[Bacchus]

Thank you. We are on that current method. I’m always looking for a better-more efficient way to do things.

On a previous boat with the black tank that was wide and not very tall I added a trap at the outlet. The tank was supported on a slightly raised platform and I lost nearly 2" of capacity when it started sucking air. Adding the trap at the outlet provided the seal so pump drew waste nearly to the tank bottom before the trap was exposed to air from the nearly empty tank... it added capacity and worked very well... not possible for all arrangements. If no clearance below the tank discharge I would consider adding shims under the opposite end of the tank so the discharge remained submerged and emptied more of the contents.
Air entering a black tank is your friend not an enemy.

 

[HeadMistress]

One more thought about tank vents: The vent thru-hulls boat builders use on holding tank vents are actually designed to keep sea water out of fuel and water tanks and make managing the tank aerobically impossible. Replacing those "vent" thru-hulls with open bulkhead fittings--the kind used in bilge pumps drains--corrects this and often makes adding a second vent unnecessary. In fact an aerator would accomplish a lot more without the need for additional vent line and another thru-hull. The open vent thru-hull also helps to prevent vent line blockages by allowing you to put a hose nozzle against it to backflush the vent line regularly (NOT during pumpout!) to flush anything trying to set up housekeeping in it into the tank. The improved air exchange also eliminates the need for a vent line filter--which btw, would rule out backflushing the vent line 'cuz filters are "toast" immediately if they get wet. An open bulkhead fitting is an inexpensive cure for 90% of holding odor issues.

--Peggie

 

[dhays]

The open vent thru-hull also helps to prevent vent line blockages by allowing you to put a hose nozzle against it to backflush the vent line regularly (NOT during pumpout!) to flush anything trying to set up housekeeping in it into the tank.
--Peggie

Only guessing here, but is the reason not to do it during pumpout is because the vent line has a smaller diameter than the pumpout hose and if the vent is filled with water it may enter the tank slower than the effluent is extracted creating a potentially high vacuum in the tank? At least that is why I spray the hose at the vent, rather than put the hose over the vent. I want to suck a spracy of fresh water into the vent but not block the airflow. However, my ignorance is boundless.

 

[HeadMistress]

Don't do ANYTHING involving the vent DURING pumpout! Spraying water at it may not completely block it, but it reduces the amount of air that the pumpout pump can pull in enough to prevent it from working efficiently and and the only thing it can do to anything IN the vent line (bits of waste from a tank overflow, insects, etc) is get it wet...a fine mist can't flush it out. And if you still have a "vent" thru-hull it can't even do that much, although it might clean out the thru-hull enough to prevent a total vent blockage.

I doubt if any of you realize that holding tanks require maintenance...they need to be flushed out 2-3 x/season--and especially last thing before winter or other extended layup--to remove any sludge It's easy to do: Put 4-6" of water into it VIA THE DECK PUMPOUT FITTING 'cuz that sends the water into it at the bottom to stir up any sludge and hold it in suspension so it can be pumped out. Pump that out...repeat, repeat, till you're pumping out clean water. If you have an overboard discharge pump, repeat one more time with it running to flush out the macerator pump and its plumbing. It would also be wise to completely rinse out the tank with detergent (Dawn is great for this) and water to clean the walls.

At the risk of appearing self-serving, I also suggest you consider picking up a copy of my book (see link in my signature below,, just click on the title). Its title (my publisher's idea) is a bit misleading, 'cuz although it does deal with every source of odor on a boat, their causes and cures. it's actually a complete "marine sanitation system 101" manual that includes everything a boat owner needs to know about US regs, equipment plumbing, operation and maintence--preferably preventive 'cuz prevention is always easier than cure and you can do it when convenient....the need to cure never happens when it is.
(end of shameless self promotion)

--Peggie

 

[Mac2]

Another alternative: My holding tank is several hundred gallons. It was taking close to two hours to pump out at the local pump out station. A professional pump out boat employee suggested I add air to the tank to speed up the process (not happy waiting for me!). I ran a small air line from my compressor to my holding tank and taped into a unused cap. I added a valve at the end. When pumping out, I only need to open the valve half a turn. I can hear the flow rate and it is a dramatic difference between a closed valve and a half turn. If I open the valve more, it makes no difference in flow rate. It takes about 30 minutes to empty the tank now. I have a one inch vent hose with an open end. Im guessing it is a 20 foot run. It was not possible to add another vent line , so this was a great solution for me.

 

[Robjbecht]

Don't even THINK of trying that!

Tank Vents101
All tanks--water, fuel and waste--on a boat have vents. They all have two main functions: to provide an escape for air displaced by incoming contents and provide a source of air to replace contents as they're pulled out. When air displaced by incoming waste cannot escape out the vent--iow a blocked vent, the tank becomes pressurized, creating increasing back pressure that prevents the toilet from flushing and can result in a cracked tank if you keep trying.

Without a source of air to replace contents as they're being sucked out of the tank, the pumpout will pull a vacuum that'll prevent it from pulling out more than a gallon or two. A particularly strong pumpout can even implode a tank. If you're seeing anything moving through the pumpout sight glass, it IS coming from the tank.

A blocked vent on a fuel or water tank will prevent filling the tank and also prevent the fuel or water pump from pulling any out. Fortunately neither fuel nor water pumps are powerful enough to do more than prevent any water from coming out the faucets or prevent your engine(s) from running.

--Peggie

Peggie -

I just recently pumped out. I noticed when I was done and disconnecting from the pumpout hose, there was a vacuum between the hose and the tank. The pumpout pump was off and connection closed. This made me think that this might be an indicator that my vent hose was blocked as there should not be a vacuum from the tank side. Am I thinking correctly? I am thinking about just replacing the vent and hose and call it a day.

Any advice appreciated. Thanks.

Rob

 

[HeadMistress]

I'm pretty sure you diagnosed the problem correctly: a blocked tank vent.
A blockage in the vent LINE is very rare.
All tank vents have two main functions: to provide an escape for air displaced by incoming contents and provide a source of air to replace contents as they're pulled out. When air displaced by incoming waste cannot escape out the vent the tank becomes pressurized, creating increasing back pressure that prevents the toilet from flushing.

Without a source of air to replace contents as they're being sucked out of the tank, the pump will pull a vacuum that'll prevent it from pulling out more than a gallon or two. A particularly strong pumpout can even implode a tank.

When air displaced by incoming waste cannot escape out the vent the tank becomes pressurized, creating increasing back pressure that prevents the toilet from flushing.

Do NOT use the toilet again or try to pump out or dump the tank until you've cleared the vent.

A blockage in the vent LINE is very rare, so just replacing the vent line may not solve the problem. But because he two most common locations for a vent blockage are the vent thru-hull and the other end of the vent line--that end of the hose and the vent fitting on the tank, replacing the "vent" thru-hull with an open bulkhead (aka "mushroom")fittingis a very good idea for two reasons: it allows air exchange with the gasses in the tank, creating the aerobic condition in the tank necessary to prevent odor from being created and it allows you to prevent future vent blockages by putting a hose nozzle against it to back flush the vent line regularly...anything trying to set up housekeeping in it will get a water slide ride into the tank.

If there's a filter in the vent line, remove it...once a filter gets wet--which can happen when a tank is overfilled or even due to heeling--the charcoal swells and blocks the vent. Filters also impede the air exchange necessary to prevent odor.

--Peggie