Hi i have an Allen and Heath ZED 22 with no onboard effects. I would like to use an external Behringher unit but am having difficulty in connecting the processed signal back to the mixer. i would like to control the amount of effects individually per channel

Send audio from board via the "Aux1 Out" to the input of the effects unit. Now connect the output of the effects unit to "ST1" and control the amount of effects to each channel via each channels "Aux1" control. Adjust the Aux1 controls in the output stage (controls on the right side) for proper levels.

Older boards called this connection "effects send" for the output to the effect, and "effects return" for the effects out back to the board. The better choice was to use "effects send" into the effect but connect the effects output to an unused channel on the board. Using this scenario allowed EQ adjustments to be made before mixing it out to the amps and speakers.