What you will do if you have one water tank used for both fire and building facility , there is no space to have tank to solve this issue with NFPA ?

Having one tank is not a problem but having old small tank is a problem regarding size. NFPA has nothing to do for the number of tanks they need specific capacity for fire water which has to be used by Fire pumps only and can be achieved by having suction line of building water pump above the level of fire suction line. But in case of small tank i will prefer to have Over head water tank for building use can be normal plastic tanks or have separate GRP tank at any location but it depends on the building and its architecture\\structure

I knew that is very important to have separate tank for Fire and building facilities but my quotation if you have old building and you have one tank only used for fire and facilities , how you are going to manage this with alignment with NFPA ?

Put the pressure/level  switch into automatic mode. If the tank is50% empty the pump will automatically shut on to pump water supply.

Dear brother,

that all depends upon the size of tank. that how much water we can save in that is quite complete or less  that depends upon usage and further according to me much better is should be seprate tank for building and fire safety. you can make under ground water tank