This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
In order to prevent any possibility of overflow, every house tank should be supplied with an overflow pipe of sufficient size to carry off easily the greatest quantity of water that may be discharged into it. The overflow from a house tank should never be connected directly with a sewer or soil pipe, even if provided with traps, for the water may seldom flow through this pipe, thus allowing the trap to become unsealed through evaporation. It is much better to let the end of the overflow pipe be open to the atmosphere or drop over some fixture which is in constant use.