Read the pulse from a liter counter ( can be from softner flow sensor or other sensor). Pre-program the number of pulse, than at the pulse reach emit a pulse contact (for dosing pump).
Power supply: 230/9-15Vac/dc 2VA. (Request 0,5w you can use the power supply of the softner).
0,5W with relays ON, for most of the time few mW.