Sometimes this causes a maintenance issue. Materials that float (such as toilet paper) can become stuck in the inlet baffle. The success of the inlet baffle depends on the occasional high flow (such as laundry discharge) to flush floating materials through the baffle. Also, the tank producer and/or tank installer needs to provide a riser above the inlet baffle for easy access from the soil surface.
All septic tanks should have two compartments. Typically, the first and second compartment volumes are two-thirds and one-third of the total tank volume, respectively. A baffle in the compartment wall serves to block the scum and sludge layers from flowing into the second compartment. The compartment wall must also be designed to allow air to move across the tank.
The outlet baffle is the last chance to capture solids before liquid leaves the tank. This baffle will only allow clarified water (septic tank effluent) to leave the tank. Some tank manufacturers and/or tank installers include effluent screens as part of the outlet baffle.
Effluent screens are a recommended practice, serving to prevent excess solids from entering the soil absorption area. However, whenever an effluent screen is used, there must be a riser to the soil surface located above the to allow maintenance. Effluent screens must be cleaned.
Septic tanks do need to be watertight and structurally sound. The structural component is obvious—the tank must be able to withstand the various loads that will be placed on the side-walls and top.
The issue of watertightness is not so obvious. Certainly, tanks need to be tight to prevent wastewater from leaking straight out into the soil. However, it is just as important for the tank to be tight to prevent groundwater from seeping into the tank.
Water that enters the tank must be treated by the soil absorption area. If an excessive volume of groundwater enters the tank, then the detention time within the tank is reduced and the soil absorption area may become hydraulically overloaded.
Good tank design must incorporate a watertight seal between any joints in the tank body, must incorporate watertight connections for the house sewer pipe and the pipe to the soil absorption area, and must incorporate watertight seals around the riser connections.
John R. Buchanan is an associate professor at the Center for Decentralized Wastewater Management, University of Tennessee. E-mail firstname.lastname@example.org.