The Septic Tank
The septic tank collects household sewage. It is a watertight container located underground that is constructed of concrete or polyethylene. A sludge layer is forms at the bottom of the tank as heavier solids settle and a scum layer forms at the top as fats, oils and grease separate and float to the water surface. The septic tank provides an anaerobic (no oxygen) environment that promotes the growth of bacteria that breakdown human waste. The effluent between the top and bottom of the tank should have less 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 ( biological 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.
Gravity
Gravity dispersal is the older and cheaper of the two methods of dispersal. Gravity dispersal is based on the notion of progressive 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 (see picture X).
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 disbursed through one inch to inch and a quarter PVC pipe that has drilled holes spaced every two and a half to three feet. Effluent is pressurized from the pump or siphon through the holes, or orifices, in the pipe into the field. 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.
- Effluent standards for Type 1 systems are:
- Fewer than 400 mg/L Total Suspended Solids (TSS), and;
- Ffewer than 400 mg/L five day Biochemical Oxygen Demand (BOD5).
Type 2 Effluent: Septic tank followed by a package treatment plant.
Type 3 Effluent: Septic tank and a treatment plant with disinfection
- Effluent standards for Type 3 systems are:
- Fewer than 10 mg/L TSS and;
- Fewer than 10 mg/L BOD5 and’
- Median fecalcoliform density of less than 400 Colony Forming Units per 100 ml.
Types of fields
There are many types of disposal fields. The decision of matching the type of field with the property is based upon the findings of a site assessment. A site assessment looks at the design flow of the house, the soil and site constraints on the property to determine what type of field is appropriate.
Types of fields can range from rock and pipe to chambers for gravity systems and pressurized chambers to sand mounds for pressurized systems.
