How septic tanks work.
Sewage flows to the septic tank through the house sewer. This pipe must have the proper slope; not too steep so that the liquids run away from the solids and not too flat so that the solids settle out in the sewer pipe. A grade of from one to two inches in eight feet is used. This is a slope of one to two percent. A one percent slope is a one-foot drop in a hundred feet of pipe.