Onsite Sewage Basics
Septic Tank
The septic tank collects household sewage. It is a watertight container located underground that is constructed from concrete and polyethylene. A sludge layer forms at the bottom of the tank as heavier solids settle and a scum layer forms at the top as fats, oils and grease separates and floats to the water surface. The septic tank provides an anaerobic (no oxygen) environment that promotes the growth of bacteria that break down human waste. The effluent between the top and bottom of the tank should have fewer solids. An outlet tee allows this cleaner effluent to exit the tank and leave the floating solids and sunken sludge behind. Septic tanks are typically sized to handle 3-days of design flow. Treatment efficiency of suspended solids and BOD (biochemical oxygen demand) is typically 30 – 40%.
Dispersal of Effluent
Effluent can be dispersed to the absorption or septic field in one of two ways – by gravity or under pressure. The type of distribution is dependent on soil type and unsaturated vertical separation in the soil.
Gravity
Gravity dispersal is the older and cheaper of the two methods of dispersal. However, gravity dispersal has the drawback of localized hydraulic and mass solids overloading. This overloading leads to progressive failure of the localized area and then ultimately total system failure. As the first three or four feet of each line is soaked with effluent, over time, a bio-mat builds up limiting the soil’s ability to accept effluent. As a result, the effluent moves further down the line to where the soil can take in effluent. At some point, all the soil in the lines has a bio-mat built up that limits the absorption of effluent and the system fails.
Gravity lines used to be constructed of clay tile but are now constructed of three- or four-inch perforated PVC pipe. The PVC pipe can either be laid on a bed of washed drain rock or underneath infiltrator chambers. Effluent is evenly distributed to the perforated pipe through a distribution box (d-box) which is typically two to three feet below surface.
Pressure
Pressure distribution uses either a pump or siphon to distribute effluent. Pressurized systems are more expensive than gravity systems, but the distribution of effluent is more uniform over the field. Effluent is dispersed through 1” or 1 ¼” diameter PVC pipe. The orifices or holes are typically spaced 2.5 feet to 3 feet apart. The very small orifices are typically only 3/16” diameter of less. Effluent is pressurized by the pump or siphon and squirts out the orifices all over the field at once for a short time (approximately 1 minute). Small frequent doses work better than a few large doses per day. This method gives better dispersal over the field, which allows the system to have a longer lifespan than a gravity system.
Types of Systems Based on Effluent Quality
Type 1 Effluent – Treatment with a septic tank. Typical effluent quality for Type 1 systems:
- Fewer than 80 mg/L Total Suspended Solids (TSS)
- Fewer than 300 mg/L five day Biochemical Oxygen Demand (BOD5)
Type 2 Effluent – Septic tank followed by a package treatment plant. Effluent standards for Type 2 systems:
- Fewer than 45 mg/L TSS
- Fewer than 45 mg/L BOD5
Type 3 Effluent – Septic tank and a more advanced treatment plant followed by disinfection. Effluent standards for Type 3 systems:
- Fewer than 10 mg/L TSS
- Fewer than 10 mg/L BOD5
- Median fecal coliform density of less than 400 Colony Forming Units per 100 ml.