Black Holding Tank slow to drain...

[mgirardo]

Our Black Holding Tank is slow to drain. Can this be caused by a bad Air Admittance Valve or a clogged vent cap or both? The tank dumps fine at first, but it eventually backs up and then very slowly drains. The grey tank does not do this. It flows just fine.

Thanks in advanced,

-Michael

[Bobbo]

Thanks for letting us know the final answer.

Not everyone does.

[mgirardo]

Thanks again for all the replies, suggestion and advice. I never did get a chance to test the vent, but it turned out to be the septic. When I disconnected the sewer hose, there was water about an inch from the top of the sewer connection.

-Michael

[mgirardo]

Thanks for that explanation Aron. I have found that not opening the black tank valve fully keeps the system from backing up. We are heading out in a few days and will let the campground owners know of the issue when we leave. Every site has its own septic, so I'll let them dig into it after we leave.

-Michael

[Aron]

Sewage systems generally are designed to be gravity flow (partially full pipes, alowing liquids to flow down while allowing gases to flow up using the same pipe), so if your line is full (potentially under pressure), that could be an un-fun day for you if the hose or connection ever leaks.

Sounds to me like the issue is with your sewage line, and nothing to do with your RV, since the flow is backing up in the hose (and presumably in the sewage pipe). Most likely there's a partial blockage somewhere in the sewer pipe, and when there's enough buildup of non-liquid materials at this choke point, it blocks it nearly completely (kind of like an ice dam effect). I suppose that if the line feeds to a septic drain field, its also possible that that system is just full and/or clogged, and can't keep up with demand. Or, its also possible that the system was designed poorly and there's no way for the sewer gas to escape, which could block new fluids from flowing down the pipe very quickly--you could tell if you see big bubbles working their way up the pipe against the flow.

In any case, I don't think that I'd try to keep diagnosing the problem using the actual black tank flow, so you don't have to deal with any leaks of that full pipe. I'm not really sure what I would do instead, though. Perhaps grind up some leaves and mix them in some water to simulate the solids? sounds like an issue the campground owners are going to have to take care of.

[2gypsies1]

If the sewer pipe is a few inches above the ground and your sewer hose is laying on the ground below it, try a support under your hose so that the sewage drains at an angle. You said you can lift the hose and it's heavy with wastewater. That, to me, is because your hose is below the sewer pipe. Give it a try. It's a cheap fix.

[mgirardo]

2gypsies wrote:
If the liquid rushes out at first and then slows, could it be that it's just empty?

We have a 5" clear straight extension on the sewer outlet, at the tank. When the hose slows, the clear extension is full. I can see waste moving through the extension slowly. The sewer hose is full of waste. I can lift it and feel that it is full the entire length of the hose.

Old-Biscuit wrote:
Unless a bathroom sink or shower has been plumbed to black tank no 'AAV' on black tank. Typically grey only.

Tanks that start out draining OK then slow way down are due to tank vent being blocked OR the cut-out disc was left in tank and gets sucked down to drain out let blocking it.

Thanks Old-Biscuit. I wasn't sure about the AAV, we have one in the bathroom, so I wasn't sure if it was for the toilet or the sink. I haven't seen another in the camper. Thanks for verifying.

My original thought was the vent, but I wasn't sure. I did inspect the vent exhaust and it looked fine. Thanks for the suggestion of the garden hose. I'll give that a try in the morning.


Our site is a seasonal site, no one else has used it since 2014. The previous site occupants occupied it for 5 years. They were an older couple (in the 80s) and only stayed a few months of the 5 month season. If they had clogged it, I think we would have had a problem last year.

Since our camper is over 7 years old, I think we would have found the "cut disk" sooner. The waste flows just fine out of the tank until the hose fills up. There is no leaking at the ground connection. The pipe is a few inches above the ground and it's a tight seal.

Thanks all.

-Michael

[Sam_Spade]

mgirardo wrote:

Now that I've written it all down, it makes me believe maybe it is the septic. Perhaps it is having trouble keeping up when dumping a full black tank.

Assuming that your sewer opening isn't ABOVE the tank outlet, then you might be right. It is possible that a previous user clogged it up and IT needs to be roto-busted.

Note: is it a common arrangement to have black and grey water discharged through different hoses ??

[old_guy]

when they cut the openings for the tanks, sometimes the plastic plug is not retrieved and it falls into the tank. that plug will get caught in the discharge tube and slow down the drains. Not a fun thing to try to get out.

[time2roll]

If the sewer hose is backed up then you have a sewer problem.
I would think this would leak at the ground fitting... or is the ground connection attached and sealed?

[Old-Biscuit]

Waste tank draining should only slow down when getting close to empty.

Sink/shower drains have 'air admittance valves' after the 'P' trap to prevent 'p' trap from getting sucked dry when draining tank
Unless a bathroom sink or shower has been plumbed to black tank no 'AAV' on black tank. Typically grey only.

Tanks that start out draining OK then slow way down are due to tank vent being blocked OR the cut-out disc was left in tank and gets sucked down to drain out let blocking it.

Garden hose full stream down vent line from roof will verify vent clear it water flows out of tank drain valve.

Floating cut-out disc...harder to get rid of

[Bobbo]

Mine did that. Until I was dumping one time and a plastic circle came out.

When they cut the hole for the toilet and/or the drain, the plastic fell into the tank and they left it there. It was partially obstructing the drain when nearly empty.

Finally, after about 2 years, it hit just right and came out. Luckily, I was at home using a Flojet macerator with a clear section above it. It trapped in the clear section, so I KNEW what it was. I never had a slow drain again.

[2gypsies1]

If the liquid rushes out at first and then slows, could it be that it's just empty? If your gauge is still reading full then that's a typical problem with the gauges, not your tank.

When it slows down, close the valve and try adding bukets of water in the toilet. Does it accept more water? If so, then it was probably empty after the big gush of water.

Or...is this something new to your RV or did this always happen?

[MarkTwain]

You might want to check with other RVers in the park and ask if they are having the same problem, if not, it is probably just the sewer drain at your site.

[mgirardo]

Thanks for all the replies, I should have been more clear. The problem isn't the tank. The waste drains quickly at first, but then as the sewer hose fills up, it slows down considerably. It will eventually drain after a minute or two. Once the hose is empty, I can turn the black tank rinser on and it flows without a problem.

I guess it could be a problem with the septic, each site has its own, but I don't have a problem draining the grey tank. When the black tank rinser is going, I can leave it on for 10+ minutes and it never backs up.

It seems like when the sewer hose gets full, the waste just moves through it much slower. The grey tank is only a 2" pipe, so the liquid doesn't move through the hose as quick and can't build up.

I did have my wife flush the toilet and keep it open for about a minute and it did not help the flow, but it was already backed up at that point. Today, I flushed the tank when it was only about 1/4 full and it drained no problem. I ran the black tank rinser for about 10 minutes with the valve open and it drained fine. I closed the valve and let it fill up about 1/4 full again and opened the valve, no back up.

Now that I've written it all down, it makes me believe maybe it is the septic. Perhaps it is having trouble keeping up when dumping a full black tank.

-Michael