Black water - why in the world would this happen?

[family affair]

My 270 holding tank issues still aren't over. Last year we couldn't pump out the holding tank and ended up sucking out the float switch. After clearing that out of the line, all the the ick came out.
Fast forward to the present: only 2 people used the head this year - just water. Both opened the raw water intake, but not the discharge since the macerator has been toast. Electric flush worked as normal. Sea cocks closed.
Today the cabin was a little funky. I opened the door to the head and water in the bowl was very high. I flushed it and someone top side said there was discolored water on the deck. The water was pushing out around the pump out fitting! I opened the cap and water was at the very top. I then went to the pump out station and only pumped out maybe 2-3 gallons. We dumped water into he bowl, flushed, and pulled that water out. How could that possibly be? I looked at the schematic and it doesn't seem possible. I think my tank is 10 gallons. Any ideas?

 

[seasick]

I have been investigating a similar issue on another boat. Randomly, I will find the toilet almost full of something. Currently the boat is on the hard.
It seems that the issue is related to rain, specifically heavy rains. I measured the level of the top of the holding tank and projected that back to the head and it seems to line up with the water level in the bowl. So water if backfilling the bowl. There are a few things going on here and probably also in your case.
First of all the bowl shouldn't allow the liquids to back fill the bowl. You probably have an old joker valve ( sometimes referred to as a duckbill valve. It is a pretty inexpensive part but can be messy to replace. They only last so long and should be changed regularly as part of routine maintenance ( I am guilty as charged)
The gunk is just the stuff in the holding tank and the gunk that accumulates over time in the waste tubing between the MSD and the holding tank.
The last question is "Where is the water coming from?"
In my case I suspect that the pumpout deck fitting is leaky or the vent clam shell is allowing water in during the heavy rains we have had recently.
Do you have a selector valve for pumpout and overboard discharge? If so do you know what position is was left in. Mine I think was in Pumpout and if the fitting leaked, that could explain the water filling the tank.

So, take a look at the deck/gunnel fitting for leaks and/or bad o-rings and also see what position the selector valve is in. One way to tell is to pour water into the deck fitting and see if the fill overflows and the water doesn't drain into the tank.
Let me know what you find and I will do the same when I get back to trouble shooting ( and Yes, my discharge macerator pump is not working either

 

[family affair]

I have been investigating a similar issue on another boat. Randomly, I will find the toilet almost full of something. Currently the boat is on the hard.
It seems that the issue is related to rain, specifically heavy rains. I measured the level of the top of the holding tank and projected that back to the head and it seems to line up with the water level in the bowl. So water if backfilling the bowl. There are a few things going on here and probably also in your case.
First of all the bowl shouldn't allow the liquids to back fill the bowl. You probably have an old joker valve ( sometimes referred to as a duckbill valve. It is a pretty inexpensive part but can be messy to replace. They only last so long and should be changed regularly as part of routine maintenance ( I am guilty as charged)
The gunk is just the stuff in the holding tank and the gunk that accumulates over time in the waste tubing between the MSD and the holding tank.
The last question is "Where is the water coming from?"
In my case I suspect that the pumpout deck fitting is leaky or the vent clam shell is allowing water in during the heavy rains we have had recently.
Do you have a selector valve for pumpout and overboard discharge? If so do you know what position is was left in. Mine I think was in Pumpout and if the fitting leaked, that could explain the water filling the tank.

So, take a look at the deck/gunnel fitting for leaks and/or bad o-rings and also see what position the selector valve is in. One way to tell is to pour water into the deck fitting and see if the fill overflows and the water doesn't drain into the tank.
Let me know what you find and I will do the same when I get back to trouble shooting ( and Yes, my discharge macerator pump is not working either

I appreciate the advice, but the boat is kept indoors. Still the other issue is why would I only be able to extract 2-3 gallons in a 10 gallon tank? The 270 does not have a selector valve.
I replaced the flush system last year. I can't remember if the joker valve was new or not. I'd need to look at what is included with the replacement unit.

 

[seasick]

OK. That changes things.
How about pressure in the tank due to the warmer temps? Perhaps the vent is blocked and the air in the tank is forcing the water back towards the head. That would also explain why there was only 2 or the gallons in the tank. It wasn't full to start with.
Grabbing at straws here

I will check the venting when I get a chance

 

[seasick]

I decided to search for info on the issue and found this video. It mentions the vent and also issues with mineral buildup. It didn't mention issues with the tank heating up in hot weather. The back flow they are referring to is when you flush the toilet and after it drains, it starts to refill.

Diagnosing Waste Back Flowing into Your Jabsco Marine Toilet | Xylem US

Here we show you what to check in your system to figure out what may be causing waste to back flow into your Jabsco marine toilet, including checking the joker valve, the vent, and the discharge line itself. Don't forget to wear gloves.

www.xylem.com

 

[Hookup1]

Still the other issue is why would I only be able to extract 2-3 gallons in a 10 gallon tank?

...but not the discharge since the macerator has been toast.

Is it possible there are a lot of solids still in the tank? I would do some testing of the holding tank. At the pump out station fill with water from hose and empty. Do it several times. Time how long it takes to pump tank out.

What are you going to do with the macerator? Probably a Grocco in tank pump. Several posts on this site that re-engineer a external pump. It's not difficult to remove the pump from the tank. It's just in a awkward spot. You have to be outside the head and most of the work is one-handed.

 

[family affair]

Great points to consider.
I wondered about the solids. They didn't have a hose at this pump out to backfill. Had one been available, I would have tried that.

I need to check the vent again. That one seems plausible and makes sense.
I need to check tank accessibility on the 270. Hopefully it isn't a PIA.

 

[Hookup1]

I need to check tank accessibility on the 270. Hopefully it isn't a PIA.

I have a 268 Islander. Same setup as yours. On your knees, twisting into the head compartment, stretched out over the top of the toilet and working with one hand - yes its a PITA.

Blow the vent back with a garden hose.

Use your washdown hose or fresh water hose if none at the marina.

 

[232Grady]

Great points to consider.
I wondered about the solids. They didn't have a hose at this pump out to backfill. Had one been available, I would have tried that.

I need to check the vent again. That one seems plausible and makes sense.
I need to check tank accessibility on the 270. Hopefully it isn't a PIA.

This is always a problem with Grady waste system, you would think they would address this. It was not a problem in the past, over the side. Now with family's and environmental issues it is! GW? I just purchased a 5th wheel, their is a big problem with the dreaded pyramid (different learning curve here) + holding take sprayers (flushing). Bottom line, a lot of research and "fixes" for the same problems (but ahead of boats). They (we/me) use "Thetford Tank Blaster Cleaner", they also use a lot of water (keep things "stirred up"). That's info for going forward.

The problem you both are experiencing seems to me IMO, is related to the vent. Use air and blow from out side to tank (if you cannot empty all the way using pump out-the tank is collapsing-vent is plugged?) No hose at pump out? Bucket, washdown hose, etc. Most people use personal "small bags" for toilet paper, bad thing happen inside holding tanks (with paper).

It's amazing how many GW owners are dealing with toilet/waste problem and reply "I just don't use it"! Maybe Kris Carroll & David Neese @ GW could address this problem and FIX it!

 

[family affair]

It baffles me the lack of robustness in marine systems, especially considering the price. Considering this system has been barely used, it is disappointing. Great designs consider the "what if's." Instead it is designed as "someone else's problem."

 

[family affair]

Well, the vent was blocked. I pulled the vent line off and paper came out. I didn't have much time to work on that part of the boat this weekend, so all I did was force water back through the line. The toilet bowl has not backed up since. So now the question is whether or not the tank is full of solids or was overfull a while ago and blocked the line. Good times ahead!

The tank is translucent appears to have the tank float threaded into the top. I couldn't break it free by hand and I'm not sure if it is sealed or glued in place. Has anyone removed one before? If so, how and did you end up breaking the threaded cap?

 

[232Grady]

Glad you found the "problem", it's never over when it involves the "Grady system ". If you must take out the gauge, I would call 800-867-1411 "Marine Sanitation & Supply-Seattle. Ask for advice, etc. They have been there, done that! I would precede that with tank blaster, see my previous post. Fill tank full and let sit a day, then go run "sloshing", empty. Fill tank "blaster" 3/4, go out sloshing.
From this point on use small container for paper OR SINGLE PLY PAPER (Amazon). NO West Marine 2 ply! Good Job!!!

"There are no plumbers at sea!"