How do I unblock an interceptor chamber?

Question

I have a house built in the UK in 1906, which has an Interceptor Chamber. Having identified it as such, I need to figure out how to unblock it. Water is currently backing up all the way to ground level, very very slowly draining away.

I believe there is a flap to another pipe under the surface of the water, slightly below ground level which I should be opening and trying to unblock, but I'm not really clear what I should be doing. Reading online seems to indicate that it's the U-bend/trap that's blocked so I need to clear that. Mainly, I don't want to do anything that could just cause a blockage somewhere else.

What I remember seeing when it wasn't filled up with water was a 6 inch (diameter) pipe that went down as far as I could see (presumably until it hit the near horizontal waste pipe). Within that, taking up about half of the diameter of the pipe is a flat cap (also called a tea-pot lid I believe). Not really knowing what I was doing I tried plunging down the pipe as best I could with a normal plunger, but don't think it did much as it wouldn't fit properly because the tea pot lid took up much of the room. What should I try next?

tl;dr: See title.

Answer 1

You are lucky there is a vertical pipe. The teapot lid is often at the bottom of the chamber, so it needs to be pumped out for access. As you suspect, this should enable cleaning out the main house trap. The pipe with the flap that is under water is to access the pipe beyond the trap. Either the pipe beyond or the trap can be the cause of the blockage. Dealing with the pipe beyond likely requires professional service with root cutting equipment.

So lets deal with the trap and hope that fixes it. You need to feed a plumber's snake down the pipe and through the trap to break up the blockage. I've also had luck simply jetting out the trap with a common garden hose with maximum water flow. How successful you are depends on the nature of the blockage. It may also require professional service with the proper equipment.

If you do require assistance, enquire about the cost of removing the main trap. It is typically no longer required by building codes and would eliminate a common trouble spot. Sometimes it's an easy job and other times it means digging up your garden.