Septic System Design

The purpose of a home’s subsurface sewage disposal system (septic system) is to dispose of the water generated by the occupants in such a manner that the soils on the property can disperse it without causing an adverse effect on ground water and in turn on public health and the environment. To accomplish this a septic system consists of the following elements:

1. A sewer line that connects the home’s plumbing to the septic tank;
2. A septic tank that allows for the settling of soils and provides the initial treatment of the septage. A properly functioning septic tank will reduce pollutant levels and produce an effluent of fairly uniform quality. This is accomplished by providing inlet and outlet baffles to reduce the velocity of liquid moving through the tank and to prevent solids from leaving the tank. Tanks installed since January 1991 now consist of two compartments in order to do a more effective job of attaining the above objective;
3. A distribution system that directs the flow of effluent from the septic tank to the leaching system in such a manner to insure full utilization of the system. Most systems are “gravity” systems, meaning the flow runs through piping and distribution boxes without the assistance of any mechanical device, such as a pump or siphon;
4. A leaching system, which disperses the sewage effluent into the surrounding natural soils. There are many types of leaching systems. The specific type utilized on a particular property is usually dependent on the soil conditions which exist on the site. Most residential installations utilize stone-filled leaching trenches, but galleries, pits and beds have historically been used.

For a leaching system to function properly it must:

1. Provide enough application area. The application area is the amount of surface area of soil within the leaching system where sewage effluent is applied (referred to as “wetted” area). The amount of application area needed for a given house depends on the characteristics of the soils on the property and the daily flows (in gallons) generated from the house.
2. Be surrounded by natural soil conditions which will be able to dissipate and disperse the discharge without becoming over saturated.
3. Provide enough capacity to store effluent during periods of unusually heavy use or when rainfall or subsurface flooding reduces the ability of the system to disperse the liquid. Note: Curtain drains or ground water interceptor drains are sometimes installed upgrade of the leaching system to minimize high ground water conditions.

It is important to realize that, once a system has been installed, only one of the above factors can be controlled by the homeowner. The homeowner can control how much water is actually being discharged to the system. Since each system has a set maximum capacity, it behooves the homeowner not to exceed that amount.

1. Plumbing fixtures may exhibit difficulty in releasing their contents (slow draining, bubbling, backups, etc.). This condition may be system-related but it could also indicate just a clog in the interior piping or sewer line. You should have the interior piping checked before proceeding with an investigation of the sewage disposal system.
2. Large volume discharges (such as, washing machines, dishwashers and bathtubs) cause either a backup, as noted above, or, an overflow of sewage above the septic tank or leaching field. If this conditon is usually at its worst during and/or directly following a heavy rain event, then the septic system is indeed suspect. If backup alone occurs independent of wet weather, you might first check for a partial blockage of the main drain that has occurred some distance from the house. In such cases a small discharge will simply be held by the main waste pipe, draining slowly past the blockage, while a large discharge will cause a backup.
3. Foul septic odors in storm drainage piping, catch basins, footing drain piping or curtain drain discharges may indicate that sewage from your property or an adjacent one is entering these ground water systems.