On 25 July, 20:53, Michael D wrote:
Today I finally rodded my drains (not done for at least 5 1/2 years
and maybe decades). *There are a couple of things confusing me:

1. *In the inspection chamber before going into the street I found 2
pipes, one above the other. *Pls see the photo he *http://bit.ly/aYDCKm
(the lower hole has got white foam in). *Is the upper hole some kind
of overflow?

2. *When I lifted the cover off I couldn't see the lower hole. *It was
obscured by gunk and what I thought was a shard of broken pipe. *When
I cleaned things out I found the shard of pipe was actually the cap /
cover thing in the 2nd photo. *It was on its edge at an angle of about
45 degs to the opening of the pipe. *I've no idea what it is or
whether it's supposed to be there.

Any ideas?

Michael

It's an interceptor trap:

http://www.draindomain.com/interceptor%20traps.html