Waste pipes - purposeful or bodge?

[JonJC]

Please take a look at this set-up (new house to me, but about 40 years old and bit of a mess with a lot of bad DIY - not sure if this is bad DIY or not). It's a waste pipe from one double kitchen sink and one washing machine to the right of the picture. On the left it goes outside to low-level drain on the other side of the wall. Does the elbow joint hump in the corner have a purpose or is it a bodge because someone had the elbow joints but didn't have a whole piece of pipe to connect without the elbow joints? Will it cause problems if I take out the elbow-joint hump and replace with a single length of straight pipe to the corner? At the moment, when I run the tap into either of the sinks, it backs up until water starts coming up into the other sink - could the hump be the problem? Before anyone asks, the waste pipes themselves are now completely free of blockage all the way from sinks and washer to the outside drain.

Any thoughts welcome.

I worked as a plumber's mate for about a month 40 years ago which means I'm willing to have a go but in reality, I know next to nothing!

Kind regards,

Jon (newbie here)

 

[johnny2007]

They forgot to fit a trap to the washing machine stack and thought that that hump would work the same
No, seriously, possibly the sink was in that corner at one point and some bodgers didn't bother to repipe properly.
Cut the waste from the left of that hump, and make all new.
Install a proper washing machine stack with trap.
Sink waste should connect below the washing machine trap, not above.
Personally I would change the main run to 50mm.

 

[johnny2007]

Red line I would do 50mm and blue lines washing machine stack with trap and sink (with trap of course)

 

[Madrab]

Not needed, take it out and put a trap on the standpipe for the washing machine.

 

[JonJC]

Thanks, Johnny2007 and Madrab.
That's really helpful and not a major job - he says, hopefully.