An integral part of most sump pumps and especially sewage pump installations is a check valve installed in the discharge pipe of the pump. In most plumbing codes there is a requirement for a full flow check valve to be installed in any application where a sump or sewage pump is installed. This prevents the back flow from the sanitary or storm sewer siphoning back into your basin or sump pit after the pump shuts off. (See typical installation below)
The check valve is a fitting that has a flapper that acts as a one way trap door by allowing the water flowing out to open the flapper while the pump is running. When the pump stops the water in the discharge line forces the flapper or trap door closed.
There is a secondary benefit to having a check valve, it helps prevent the constant recycling and re-pumping of the same water left in the discharge pipe when the pump shuts off.
Below is a drawing of a typical sump or sewage pump installation. Note the need for a 3/16″ diameter bleed hole in the discharge pipe. We suggest that it be located below the “On” point of the pump being installed. The bleed hole is to prevent an air lock and in essence it would allow water to fill the pump base with water, rather than air.