The copper line is the water softener discharge line into the main septic tank system. It does show a trap but should there be an airgap here?
Absolutely!!! Personally, looking at the pic you posted, I would insist the entire system be disinfected prior to my using it. ANY backup will cause that line to become contaminated. Yech!!!
Cut a hole in the drain pipe , squeeze a copper pipe in that hole, then use glue solvent to plug that hole where copper meets ‘plastic’ pipe , and dont use any support straps or hangers to hold the copper pipe that has weight of water and a rubber hose on it. I say the air gap is least of the problem.
It looks like they threaded it to me. Regardless, the cross-contamination issue is the biggest one
Right-O! 
Thanks everyone! This is why I love this forum.