Flushing Holding Tank at Sea

We have a 430 aft cabin trawler with two ~44 gallon tanks and the ability to discharge the holding tanks out side of the USCG limit at sea.

We have never even tried to use this feature but we're wondering if this option exists for others and what your experience has been.

I'm concerned that the waste line from the holding tank is below the waterline and that if the pump fails it would flood the holding tank.

Looking for best practices here...

Thanks, - Jim L

Jim, we do it regularly and our through hull is just below the waterline.

Go have a look at your set up. You should have a macerator pump that pumps the sewage overboard via a hose that is in a n (inverted U bend) shape taking the sewage up above the waterline and then down to the through hull.

If your pump fails the n bend will prevent any water intrusion.

Additionally you have a ball valve at the through hull that you can close.

BTW, while within the three mile zone that valve must be closed and secured by a lock or by taking the handle off. It will be checked during a CG inspection.

Assuming you have a washdown pump.... First pump out or dump the tank. Then...

1.open the deck pumpout cap and use the washdown pump to put about 6-8" of water into the tank (putting water into a tank via the deck fitting sends it into the tank at the bottom to stir up sludge and hold it in suspension so it can be flushed out).

2.Open the discharge thru-hull.

3. Turn on the macerator pump while you keep the water running into the tank.

4.When you see only clean water near the discharge thru-hull (it'll prob'ly be cloudy when you start), turn off the water and macerator pump and close the thru-hull.

--Peggie

HeadMistress said:

Assuming you have a washdown pump.... First pump out or dump the tank. Then...

1.open the deck pumpout cap and use the washdown pump to put about 6-8" of water into the tank (putting water into a tank via the deck fitting sends it into the tank at the bottom to stir up sludge and hold it in suspension so it can be flushed out).

2.Open the discharge thru-hull.

3. Turn on the macerator pump while you keep the water running into the tank.

4.When you see only clean water near the discharge thru-hull (it'll prob'ly be cloudy when you start), turn off the water and macerator pump and close the thru-hull.

--Peggie

Click to expand...

You may want to read his post again.

He should be able to repeat the operation for the other one. There should be a vented loop--or at least a loop--in each tank discharge line. What did I miss?



--Peggie

It’s done all the time and I’ve never heard of anyone having a problem. It would be very hard for sea water to flow backwards through the pump.
Even if the holding tank filled with water, it isn’t a disaster.
Just follow Peggie’s instructions.

HopCar said:

Even if the holding tank filled with water, it isn’t a disaster.

Click to expand...

That is exactly my wife's definition of a disaster.

(I follow the same routine at a pump out station. Pump out, refill, pump out again. Don't think the operator is too happy with the extra volume but solids settle.)

Hi Jim. Our 430 has two 26 gallon tanks with a diaphragm pump on top for overboard discharge. When underway, open the seacock for one of the tanks, turn the pump on at the electrical panel, and watch the amp gauge. When it is pumping fluid it will move slowly. When the tank is empty the gauge will jump quickly. Turn off the breaker and close the seacock. Repeat with the second tank. Never run the pump with the seacock closed and never leave the seacock open when not being used. If you have a different setup then disregard all of the above.

Thanks everyone. I know the forward tank has the loop above the waterline, I'll check the aft tank. Thanks for the information.

Jim

HeadMistress said:

He should be able to repeat the operation for the other one. There should be a vented loop--or at least a loop--in each tank discharge line. What did I miss?

--Peggie

Click to expand...

This was his question.

jimL said:

I'm concerned that the waste line from the holding tank is below the waterline and that if the pump fails it would flood the holding tank.

Click to expand...

99% of holding tanks are partially below the waterline. Worst case scenario, water will only fill the tank to the boat's waterline even if there's no loop in the line. It would have to seep through the macerator pump, which is possible, but would take a long time and could only happen if the thru-hull is left open when not actually in use to empty the tank.



--Peggie

One thing I liked about my vacu-flush system was the robust pump. I installed a “Y” in front of it and used it all the time to empty the holding tank.

It was a very reliable system. Are VF’s still popular these days or are they not so?

makobuilders said:

One thing I liked about my vacu-flush system was the robust pump. I installed a “Y” in front of it and used it all the time to empty the holding tank.

It was a very reliable system. Are VF’s still popular these days or are they not so?

Click to expand...

Did you not have a separate macerator pump between the holding tank and the through hull for overboard discharge?

No I solely relied on the VF pump. My understanding is the the soil gets macerated during its travels from the head to the holding tank

makobuilders said:

No I solely relied on the VF pump. My understanding is the the soil gets macerated during its travels from the head to the holding tank

Click to expand...

Not quite. There is no macerator in a Vacu Flush system. The sewage does get broken down somewhat by the force of the vacumn as it forces it through the system into the holding tank.

If pumped out through the deck then it never does go through a macerator. If going overboard via a through hull it should be moved via a macerator sitting between the holding tank and through hull.

So it is important in a VF system to use Marine/RV paper if putting into the system at all.

jimL said:

Thanks everyone. I know the forward tank has the loop above the waterline, I'll check the aft tank. Thanks for the information.

Jim

Click to expand...

If you have a diaphragm pump for overboard discharge then you don't need a loop above the waterline. There is a duckbill valve that will keep water from going back up the hose.

On our MS 400 our anti siphon loop is only for when you're going directly from head to overboard. The macerator pump sucks out directly from holding tank to the through hull. Pumping your tank out probably does not go through anti siphon loop.

Not quite. There is no macerator in a Vacu Flush system. The sewage does get broken down somewhat by the force of the vacumn as it forces it through the system into the holding tank.

True. There's no macerator in manual toilets either...but it doesn't matter because solid waste is 75% water anyway, plus it gets broken up going through manual toilets and VFs, so it dissolves very quickly in the tank...and so does quick-dissolve TP. All of which makes maceration, whether in the toilet or the overboard discharge pump totally, unnecessary except in the those rare occasions when the tank might be dumped with an hour or less after solids and TP land in the tank.


At the risk of being accused of "selling" it, y'all really do NEED my book!


--Peggie

Take a good look at the pump location. Some are tied to the tank at the tank bottom. They could back flow.

Some like mine, a Sealand diaphragm unit, are mounted atop the tank which may make the pump above the water line. With this the seawater will not backfill the tank even if the pump goofs up.

One way to find out for SURE whether the pump and/or tank is above or below the sea level is connect a clear vinyl hose to the nearest seacock. Install a small valve on the other end of the hose and close it. Drag the vinyl hose over to the tank and raise it above the tank and pump as far as you can. Open the ball valve. The seawater will rise to the same level as the sea water outside. Now you know whether or not you need worry.

I have to pump out at sea fairly often because there are no pump out stations where we go. I treat the tank with Noflex to help neutralize it. I usually close the thru Hull valve, but sometimes leave it open if I will be using it again with the next couple days when we are on the move. As previously noted, there is the loop, and these pumps are positive displacement which act like a check valve. So I don’t think not closing the thru Hull is an issue. Long term, yes, I do make sure the valve is closed.

We have both a salt water and a fresh water supply to our heads. Also for the wash down pump. We only use the salt water supply if we haven’t had time or there’s some other reason we been unable to make RO water.
If you flush your holding tank (and when you flush your head) try to use only fresh water. That cuts down odor to a major degree also is better for the head components.

Take a good look at the pump location. Some are tied to the tank at the tank bottom. They could back flow.

Below waterline tank discharge thru-hulls should always be kept closed except when actually dumping the tank. But even if left open, unless the duckbills in a SeaLand diaphragm pump aren't so old and worn that the slits have become holes, at worst sea water will only seep very slowly through it. Sea water will also have a hard time doing more than seeping through a macerator.


However, that isn't a reason to eliminate the vented loop between the discharge pump and the thru-hull.



--Peggie

Peggy you’re the best. Thanks for the work you’ve done on this issue.
Can you say why you give this advice? We’ve been liveaboards commuting between the Caribbean and New England. Of course we close our thru hulls when coastal in the US. But have left them open on passage and in the Caribbean. Seem to get less clogs in the segment from holding tank to thru hull that way. We have two holding tanks. Ones below the water line and macerator pump dependent. The other is gravity fed.
Also any comments about using pvc pipe

Peggy, - thanks for your advice and I did buy you book several years ago. I highly recommend it for everyone.

Jim

The gravity drain tank is entirely above the waterline, so no problem leaving that thru-hull open all the time...in fact that's the better way to do it because it allows you to flush directly overboard via that tank. Keeping the thru-hull closed can allow the tank discharge line to become packed with sludge.

However, safety standards call for keeping all below-waterline thru-hulls closed except when in use, which can be a bit impractical. A vented loop between the macerator pump and thru-hull is called for, but mounting the discharge pump above the top of the tank can make the loop unnecessary.

As for pvc pipe, as long as it's a straight piece, "soft coupled" with about a foot of hose at each connection to anything rigid, and any long sections supported, there's no reason not to use it.

--Peggie

jimL said:

Peggy, - thanks for your advice and I did buy you book several years ago. I highly recommend it for everyone. Jim

Click to expand...

Thanks for the kind words!


--Peggie

Pumping your tank out probably does not go through anti siphon loop.

Holding tank discharge line does not through a vented loop to get to the deck pumpout fitting. It MAY go to a y-valve--one side off it going to the deck pumpout fitting, the other side going to the discharge pump (macerator or electric diaphragm) and then over a vented loop to the thru-hull.

I strongly recommend that you spend a little time tracing the plumbing lines on your boat to learn where they go and the components that may or may not be in them. This will make trouble-shooting problems a LOT easier!

--Peggie

menzies said:

Jim, we do it regularly and our through hull is just below the waterline.

Go have a look at your set up. You should have a macerator pump that pumps the sewage overboard via a hose that is in a n (inverted U bend) shape taking the sewage up above the waterline and then down to the through hull.

If your pump fails the n bend will prevent any water intrusion.

Additionally you have a ball valve at the through hull that you can close.

BTW, while within the three mile zone that valve must be closed and secured by a lock or by taking the handle off. It will be checked during a CG inspection.

Click to expand...

My interpretation of the coast guard regulations does not require holding tank valves to be locked with in the 3 mile limit. Y valves yes and type 1 or type 2 pass through treatment tanks need to be secured if in a no discharge zone. Type 3 holding tanks are not pass through devices nor y valves so they are not subject to the same requirement

https://www.dco.uscg.mil/Our-Organi...ineering-Division/Mechanical-Engineering/msd/

Your interpretation is incorrect...you've wandered into the weeds that apply to equipment mfrs (not uncommon...a least you didn't stumble onto Title 46!)

33 CFR 159.7(c) "Requirements for vessel operators" 33 CFR 159.7 is applicable part of the regulation and seacocks are "valves."

(c) When operating a vessel on a body of water where the discharge of untreated sewage is prohibited by the Environmental Protection Agency under 40 CFR 140.3, the operator must secure each Type III device in a manner which prevents discharge of sewage. Acceptable methods of securing the device include -
(1) Closing each valve leading to an overboard discharge and removing the handle;
(2) Padlocking each valve leading to an overboard discharge in the closed position; or
(3) Using a non-releasable wire-tie to hold each valve leading to an overboard discharge in the closed position.

--Peggie

Personally, I meet that rule by doing 2 things: bright orange ziptie holding the seacock closed and the macerator breaker is kept off so that 2 switches would be needed to power the macerator.