What's the blocked (external) drain for?

[aca99sdb]

I've been living in the house (1930 end of terrace) for a year now and have called someone out twice (7 months ago and recently) to unblock my drain as it was blocked and created a horrible smell. I thought I'd take a look myself after the last guy left and noticed that there is a second pipe that looks completely blocked. The main pipe at the bottom is where the blockage was cleared and the last guy said if it blocks again poke it with a pole and it should free it up but I have a couple of questions:

1) With the lower pipe, if this is unblocked, should there be any visible standing water in the pipe? I see some standing water after hosing the it all down. It seems to drain but standing water remains.
2) Any idea what the higher pipe is for? I assumed it was an overflow pipe but not exactly sure why neither of the 2 plumbers noticed it (or ignored it). It looks to me to be full of cr@p so should be cleared out?

Cheers in advance.

 

[bernardgreen]

Possibly it is an interceptor trap, the upper pipe being access for rodding the pipe beyond the trap; The trap is likely to need clearing out with a pressure jetting.

Interceptor traps - MG Drainage

Best to use a tradesman used to working on drainage rather than a plumber.

 

[boringoldcodger]

My guess is that it's a Buchan Trap (I believe there are other names / manufacturers). The idea is that normally all the water passes through the U bend, so yes there should always be standing water. There should be a removable plug in the upper pipe, if the U bend blocks and water builds up then you remove the plug, everything drains away, you unblock the U bend and then replace the plug.
These days this type of trap is considered unnecessary, they're a hangover from Victorian times when they felt it necessary to stop sewer gases going "backwards".

Bernard beat me to it.

 

[aca99sdb]

Excellent thanks. Looks like this is the one. I'll clean out the top to see how much rubbish is in there but it's good to know that standing water is expected!

 

[HERTS P&D]

Looks like you need to clean up your diet, that trap is full of grease and fat.
You may find a plug in the 'top' hole, that is the rodding access.

Andy