Ball Float Valve Non Return

[oakapple]

I have a ball valve fitted half way down an old irrigation water tank that has a flexible liner similar to a pond liner. The float arm of the ball valve has a verticle extension of about 600mm with the ball float attached to that, consequently the water level inside the tank is about 550mm above the ball valve itself!

I am worried that if the main water supply to the irrigation tank fails the water in the tank could syphon back into the mains and as there are strange nasty things floating around in the tank I do not want that to happen.

I do not want to move the ball valve above the water level so would it be possible to fit a non return valve of some sort on the supply side of the ball float valve and if so what is such a device called and where would I get one

Many thanks

 

[hi-spec plumbing heating]

water regulations state that you should have a double check valve installed in the mains pipe.

get them from any plumbers merchant.