Partial? holding tank system - is that a thing?

[xyachtdave]

I’m currently on a Dufour 325 flotilla in Greece and had a blocked holding tank yesterday.

As tempting as it was to call the engineer to sort it out I couldn’t bring myself to ask for help so decided to ‘man up’ and sort it out with the very basic items onboard.

I had a feeling it was emptying as I didn’t hear a rush of water when I opened the valve but put it down to not having much in it until the breather started to leak.

A basic set up, gravity dump valve in at the bottom, toilet waste in at the top, a breather and luckily a deck fitting which was fortunately above the tank.

We took the boat a few miles out and after a bit of banging about with the scrubbing brush through the deck fitting the blockage was cleared.

Had this not worked my next move was going to be trying a sail batten out the main bent up though the hull fitting :-/

 

[Daydream believer]

To help stop the holding tank from blocking I have taped up the tank breather. This means that when the heads pump is operated the black water tank is pressurised & it pushes any effluent out in to the sea.

 

[samfieldhouse]

To help stop the holding tank from blocking I have taped up the tank breather. This means that when the heads pump is operated the black water tank is pressurised & it pushes any effluent out in to the sea.

I did wonder if the breather actually interfered. It certainly didn’t help with pump out!

 

[Fr J Hackett]

I did wonder if the breather actually interfered. It certainly didn’t help with pump out!

It's pretty obvious that a breather won't help by trying to pump out under pressure the contents but a tank with the extraction pipe going to the bottom of the tank when a vacuum is used to suck out the contents the breather actually serves a useful purpose in allowing the pressure (negative) in the tank to equalise and stopping the tank imploding or collapsing.

 

[Daydream believer]

It's pretty obvious that a breather won't help by trying to pump out under pressure the contents

When you say "pump out under pressure" are you referring to the heads pump or the extraction pump on the dockside?

 

[Fr J Hackett]

When you say "pump out under pressure" are you referring to the heads pump or the extraction pump on the dockside?

I am referring to people that try to pump out a blocked holding tank by pressure rather than the dockside extraction that uses suction, negative pressure.

 

[Daydream believer]

Well if one is at sea, having the breather taped up is a sensible move. It pressurises the holding tank a small amount helping to push efluent out of the final pipe in to the sea. That - from my experience- is where the blockage occurs.
I have in the past pumped the heads up as hard as I can then gone to the bilge where the final pipe from blackwater tank to sea cock is. By using a small clamp I have squashed to pipe back and forth & have watched the effluent gradually work its way along the pipe . Then if I am lucky I have not had to disconnect it as there is a bang on the holding tank as the metal sides flex back in & the blockage clears.
Sometimes I have had to disconnect by the seacock & by pressurising the tank it has emptied the pipe into a container - or over me.
I was once on the dockside pumping station in ZeiriKzee . The pressurising trick did not work so my crew had the hose ready when I removed the holding tank filler cap. As I did the cap was forced out of my hand & c..p shot 9 inches in to the air. So it demonstrated how much pressure one could get just using the heads pump. As it turned out the station pump was inoperative so I still had to disconnect at the seacock & one of my crew drained it from there. He was as happy as a pig in s..t that day