How to stop Back-flow into overflow ?

[Mattyk_]

My plumber did not install an overflow when my water softener was fitted, something I found out when it failed and flooded the floor !
I have installed a 12mm overflow hose into a compression T joint, however I'm getting back flow, from both the washing machine, and probably the water-softener drain too. I have attached a diagram, What's the best way to stop this ? or an alternate way way to plumb the overflow into waste water ?

 

[Timmy D]

Mechanical trap between Softner and washing machine.

You’ve likely got a partial blockage causing the black flow. Need to unblock it

 

[Mattyk_]

I think the (low volume, partial) backflow is caused by (a) the low height differences between the s bends and (b) the low drop of the drain pipe. Neither of which are easy to change. I will also check for blockage,

What sort of mechanical trap ?

Do I have any other options ?

 

[Timmy D]

I think the (low volume, partial) backflow is caused by (a) the low height differences between the s bends and (b) the low drop of the drain pipe. Neither of which are easy to change. I will also check for blockage,

What sort of mechanical trap ?

Do I have any other options ?

If the washing machine backflow is capable of rising 200mm up the stand pipe to the softener, I highly suspect a partial blockage.

A trap like this.