A dosing pump is required in most modern septic systems. Older septic systems that did not have proper gravity fall needed dose pumps. Generally, they pump the liquid to a sand filter bed. Newer systems pump the liquid from the house to a mound or flood-dose trenches.
Dose pumps range anywhere from 4/10 HP all the way to 2 HP in some specialty systems. They measure water levels and pump water out when it reaches a certain level in the tank. Their purpose is to minimize the risk of flooding the septic tank or leach field.