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dougweibel · 05-08-2021, 08:09

I have an intermittent, but frequent, problem with my blackwater system I cannot figure out. Sometimes I cannot dump the tank for no apparent reason!

My system is configured with a gravity dump tank. The hose from the bottom of the tank runs to a two-port Marelon sea-cock/thru-hull. When the handle is set to one side the valve directs waste from the tank hose out the thru-hull. When the handle is centered the valve is closed. And when the handle is to the other side waste is directed from the tank hose to the hose for pump-out through the deck fitting.

Sometimes when I move the valve from the closed position to the dump position the tank just does not dump though there does not appear to be any apparent clog or restriction.

I have verified that the vent line to the tank is clear and passing air. When the tank is failing to dump applying either air pressure or vacuum to the vent line to the tank has no effect.

I have verified that the hose from the tank to the seacock is clear. If the tank is failing to dump I can fill the pump-out hose with water from deck and then switch the seacock to the pump-out position, then watch the water in that hose flow in to the tank, at least down to the level of the tank, so there appears to be no restriction in the hose from the seacock to the tank. Also we have never had a problem pumping out.

On occasions when the tank has failed to dump I have managed to get it to dump by getting in the water and jamming my wash-down hose and a rag into the thru-hull, giving a blast of of pressurized water for several seconds, and repeating a few times if necessary till effluent starts to come out. When it does start flowing I never see anything indicating a blockage coming out, just effluent. This really sucks for the obvious reason but also because we are now sailing the Maine coast and the water is cold!

I've had this happen four times now and haven't figured it out. Ideas???

Bill O · 05-08-2021, 08:36

Due to the "fix" you mentioned, I'm going to take a guess that you have a blockage and it isn't something organic. It is possible you have petrified pieces of waste material, but thinking someone has put other things in your system that isn't dissolving/breaking down.
Would try the treatment to breakdown years of sludge. Can't remember the name of the product we used off hand, but worked well.

dougweibel · 05-08-2021, 08:48

Thanks Bill,

The baffling part is that if there is a blockage it seems to be in those few inches between the tank drain hose port on the ball valve to the thru-hull itself. This last time it happened, yesterday, I tried everything I could think of before getting in the water. Note that I filled up the pump-out hose with water from the deck port and this water flowed down the hose when I switched the ball valve to the pump-out position, meaning that it also flowed back up the hose from the ball valve to the tank. That happened quickly like there was no restriction whatsoever. I have taken a dowel and swirled it around inside the thru-hull from the outside and everything feels smooth as far as I can insert it.

Given that the blockage seems like it might be in the ball valve itself I don't see how I can effectively attack it with a treatment product.

**JoeRobertJr** · 05-08-2021, 09:09

Just tossing out a couple thoughts...
1. Boroscopes for smart phones are pretty cheep. I wonder if that could help you see what is going on.
2. Imagine a blockage in the tank at the discharge port that acts as a valve, like a ball of crud. That would allow the fresh water you add to the pumpout hose to enter the tank, but not let out the tank contents. Since the issue is intermittent, this chunk of crud may be moving around.

Nord Sal · 05-08-2021, 09:16

Given that it drains just fine in the pump-out position and that the line from the thru hull to the valve appears clear, and that your vent is clear, I'd check the valve. Since the valve appears to work between the 'off' and 'pump-out' positions it likely isn't a loose shaft spinning, I'd suspect an internal blockage in the 'dump' valve port. It maybe something that you can break free temporarily with the water jet which then settles back in place. I'd also suspect some foreign inorganic matter got into your system (piece of plastic bag or?).

Dsanduril · 05-08-2021, 09:31

Ahh, black water problems, isn’t boating fun?

If you can get the tank to drain, rinse well and drain some more. Close the seacock (or set to pumpout position). Remove the tank hose at the seacock (hopefully you rinsed and drained so any heel in the hose isn’t too nasty). Open the seacock to allow seawater to backflow into the boat (depending on configuration you may be able to catch in a bucket/pan, or may have to let it run to bilge - again, rinsing beforehand to reduce the yuck factor). Does water flow well? Full bore? Did you see any obstruction come out and into the boat?

Sounds a little scary but it really isn’t. Have a plug handy in case there’s a problem closing the valve, or keep the hose close by so you can jam it back on. Access may be fun, but that’s boating....

dougweibel · 05-08-2021, 10:29

JoeRobert - I'm amazed but you actually can get an IP68 borescope with 5 meter cable that hooks to a cell phone for $30! Great suggestion. I may go that direction if I don't have a breakthrough soon.

NS - yes, I'd tend to agree. It just is such a small area, less than a linear foot, that I'm having trouble visualizing how it can happen. Seems like once I get it opened up it should flush whatever out.

Dsandduril - this is a great suggestion and one I wouldn't make to myself as I, like most I expect, hate to contemplate a 'yuck factor' job. In this case access is fairly easy and cleanup wouldn't be bad as it is all at the bottom of my generator locker, which is a somewhat small enclosed space with its own bilge pump.

peghall · 05-08-2021, 12:39

One of the few disadvantages to gravity drain tanks is, if the thru-hull remains closed for an extended time, sludge can build up in the discharge line. That buildup will start at the thru-hull and gradually fill up the hose...but that's not telling you anything you don't already know.

You've said you can always clear it/break it up by poking it from the outside of the thru-hull...which means that any sludge "ball" has to be not much bigger than a golf ball, but packed pretty tight just from settling.

So I'm gonna toss out a WAG that this only happens when it's been some time--maybe as little as just a couple of weeks--since you'd opened the thru-hull to dump, but that you've used the head a good bit during that time, always pumping out. Just long enough to create that golf ball size plug.

If I'm right, the solution MIGHT be to open the thru-hull to flush out the line immediately after each pumpout so it can't ever remained closed long enough to create the plug. You'd be doing it with clean water down the pumpout line, but you might want to be discreet to avoid offending any hard core enviro-zealots who might be aware of what you're doing.

Just an idea that may or may not be worth the time it took you to read it.

--Peggie

dougweibel · 05-08-2021, 14:21

Thanks Peggie,

What you've said is a thought that has occurred to me. My biggest misgiving about that being all that is going on is that over the last month the history had been we have been able to be offshore and dump about once a week. The first time the tank didn't dump till I freed it from in the water. The second and third times it dumped fine. This last time again it would not dump without jetting it with the wash down hose, and the valve was only closed for six days.

peghall · 05-08-2021, 14:41

How long have you had this gravity discharge tank? Did it work flawlessly for a while, then start having this problem? Or is it a new system that's had this problem from the start? If it's a new problem in a previously fine system, has anything changed in the overboard part of the plumbing or has gotten old enough to need replacing? Have you switched to a different tank product? If so, from what to what?

I don't have any answers...just hoping yours might trigger an inspiration.

--Peggie

dougweibel · 05-08-2021, 15:16

Thanks Peggie,

It is an existing system that developed the problem, I think. In the two years we have had the boat this is the first time that we are frequently filling, or partially filling, the tank and then dumping. In the past we were generally in areas where we pumped out, or in the Bahamas where the valve was left in the dump position most of the time. We never saw the problem before this season, but we didn't have cause to cycle the valve frequently in the past.

You ask an insightful question. We had a few different treatment products on the boat from the previous owner which we use when we leave the boat - usually for 1-3 months a couple times a year. It may be that this problem developed after we ran out of one product and started using another, West Marine Eliminodor. Also that product may have been sitting on the boat for several years. This thought too has crossed my mind, though I would guess that if the product generated some goo in the system it would have worked its way out by now. Maybe not.

Nothing else has changed except we replaced the discharge hose from the head to the blackwater tank. This definitely happened in the interval immediately before the problem arose. The other hoses, tank discharge, pump-out, and vent, have not been touched.

Nicholson58 · 05-08-2021, 15:24

Quote:

Originally Posted by Bill O

Due to the "fix" you mentioned, I'm going to take a guess that you have a blockage and it isn't something organic. It is possible you have petrified pieces of waste material, but thinking someone has put other things in your system that isn't dissolving/breaking down.
Would try the treatment to breakdown years of sludge. Can't remember the name of the product we used off hand, but worked well.

I agree. Inspection port? Yuck!

We allow no paper, only poo in the heads. Paper, condoms, feminine products, plastic or some other item. Perhaps a loose bit on the valve.

Bill O · 06-08-2021, 05:25

Doug,
Here's the product I mentioned in an earlier post at liquefies and helps remove sludge build up. A little expensive but worked well to help remove the petrified "stuff" in our tank.

peghall · 06-08-2021, 07:49

It could very well be that the WM product is your culprit. I just checked it out on the WM site and it's a chemical product that can only be shipped via ground and requires a signature because it's listed a "hazardous."

I'm very familiar with the product Bill recommended--Zaal No-Flex Digestor Noflex Digestor ...in fact it's the tank product I now recommend. It's also the top rated tank product because it not only prevents odor, but also dissolves sludge, something few if any other tank products do AND is enviro-friendly, requiring no special handling. So I'd definitely give it a try, being sure to follow directions. It's available from a number of sources, so you may want to shop price.

--Peggie