there are many different ways to auto-top off a system. i would do a search for auto top off to get an idea.
i use a kinda tripple redundant system that works on house pressure for top-off. i have a water purifier attached to a
float switch that will turn on a solenoid to let water into the system. there is also a
float valve on the sump just in case the solenoid/float switch were to quite working. this float valve will mechanically cut off the incoming water into the sump. there is also an emergency water shut off valve on the water purifier to turn off all water into the area if the sump were to overflow.
G~
