The industry standard is a polypropylene drain line that is rigid yet flexible.
Water softener drain line running.
According to most plumbing codes a water softener drain line must not be connected directly to the household waste system.
Many of the home improvement stores sell big box water softeners and installation kits to get them connected.
The main supply line to the rest of the house continues on to the softener.
In the garage there is a panel that has a shut off for the water coming into the house and this would be the best place to install the softener.
The outgoing line from the water heater then runs to interior fixtures and faucets that dispense hot water.
A softener drain line must not be connected directly to the waste system but must be emptied into a laundry tray floor drain or properly trapped special outlet preserving an air gap of at least two times the diameter of the drain line but in no case less than 1 1 2 above the top of the receptacle used.
The most common reason might be the impossibility to draw brine from the tank.
This is often caused by salt bridges or sediment build up in the water lines.
This is to prevent non potable water from flowing backwards possibly mixing and contaminating potable drinking water.
A cold line that runs to interior fixtures and faucets and a line that feeds the water heater.
Water softener install where to drain the water.
Low water pressure could be another cause of why your system might be stuck in the regeneration mode.
Once the water leaves the softener it should branch into two lines.