1. What is an on-lot sewage system?
On-lot sewage systems are composed of two primary parts:
Septic tanks or an aerobic tank
Remove the settleable and floatable solids from the wastewater
An absorption area
Utilizes the ability of the soil to filter and treat the remaining effluent before it reaches the water table
Proper maintenance of the septic tanks or aerobic tank will prevent more costly problems with the absorption area

2. Sources of household sewage
Household sewage is a combination of wastewater from toilets, showers, tubs, sinks, washing machines, dishwashers, water softeners and garbage disposals

3. Types of on-lot sewage systems
The basic types of on-lot systems in Chester County are:
Cesspool
Septic tanks with seepage pit
Septic tanks or aerobic tank with seepage bed
Septic tanks or aerobic tank tank with standard trenches
Septic tanks or aerobic tank with elevated sand mound
Other technologies not listed here are used in special circumstances

4. Cesspools and Septic Tanks with Seepage Pits
Cesspools and septic tanks with seepage pits are no longer permitted for installation
Many older homes with systems installed prior to 1970 may have these systems in use.

5. Cesspools
Cesspools are constructed of open-jointed walls (concrete block or stone) and open bottoms
The solids in the wastewater settle to the bottom, while the effluent seeps through the jointed walls and bottom

6. Septic Tanks with Seepage Pits
The septic tank provides for the separation of solids from the liquid while storing the accumulated solids
The seepage pit allows the effluent from the septic tank to infiltrate into the soil
The seepage pit is constructed similarly to a cesspool

7. Septic Tank-Soil Absorption Systems
The septic tank is a watertight chamber constructed of concrete
One of the most important components of the septic tank is the baffle
The inlet baffle forces the wastewater from the building sewer line down into the tank
The outlet baffle prevents the scum layer from moving into the soil absorption area

8. Septic Tank-cont.
Septic tanks have an inspection port for checking the condition of the inlet baffle
Septic tanks also have a manhole access port for cleaning the tank
Older homes may need access risers placed on the tanks to bring the manhole opening closer to grade to allow for proper maintenance

10. In-Ground Soil Absorption Areas
The soil absorption area receives the liquid effluent from the septic tanks and distributes it over a specific area.
The effluent then filters through the soil under the pipes and is treated chemically and bacterially by the components of the soil.
The size of the soil absorption area is based on the size of the house and the percolation rate of the soil.
State law requires that there be at least four feet of usable soil for the effluent to percolate through before it reaches the water table.

11. Types of Conventional Soil Absorption Areas
The soil absorption area may be:
An inground seepage bed,
A set of trenches,
Or an elevated sand mound.
The type of system installed will depend on the slope of the property, the depth of usable soil, and the percolation rate.

12. Leaching chambers may be used in place of gravel.
Inground Seepage Bed
For seepage beds, the entire absorption area is excavated and lined with crushed stone.
The pipes (laterals) are then placed over the stone and the entire bed area serves as an absorption area for the effluent.
Leaching chambers may be used in place of gravel.

13. The effluent is then absorbed into the soil where it is renovated.
Inground Trenches
A standard trench absorption area consists of two or more excavated trenches in which perforated pipes or laterals distribute effluent into a layer of crushed stone under the pipes.
The effluent is then absorbed into the soil where it is renovated.

14. Gravity Distribution of Effluent
Equal distribution of the effluent is very important to the proper functioning of the system.
Most trenches and in-ground bed systems utilize gravity distribution to the absorption from the septic tanks,
In all trench systems a distribution box is used to split the flow equally to the individual laterals in the absorption area. It is important that this box remain level.
In most bed systems, a piping loop is used instead of a distribution box.

15. Pressure Dosed Systems
Some systems may require pressure distribution due to the permeability characteristics of the soil.
In this type of system, the effluent from the septic tanks flows by gravity into a dosing tank.
The dosing tank forces effluent through the distribution lines in a controlled dose so that all pipes discharge effluent at the same rate throughout the absorption area.
The pressure distribution system has two distinct advantages.
The entire length of the lateral is utilized and,
The soil absorption area is allowed to rest between doses.

16. Elevated Sand Mounds
The elevated sand mound system is used in areas with reduced permeability, shallow soils, and poor drainage characteristics.
Soils in these areas require the addition of sand above the ground to provide for adequate renovation of the sewage prior to reaching the water table.
The absorption area is constructed similarly to a standard seepage bed or trench, except that the system is required to use pressure distribution and a layer of sand must be placed between the crushed stone and the natural soil cover.
A soil berm is placed around the mound to protect it and to provide a suitable base for the establishment of a vegetative cover.
The mound is preceded by a dosing tank which must be equipped with an audible and visual alarm system to alert the homeowner when a pump failure occurs.

17. Elevated Sand Mound

18. Aerobic Treatment Tanks
An alternative to the septic tank is an aerobic treatment tank.
These systems utilize an air compressor or a churning propeller to maintain an aerobic environment for the growth of aerobic microorganisms.
These microorganisms break down the sewage like the anaerobic bacteria found in the septic tank.
Proper functioning of this type of treatment tank is highly dependant upon the diligent maintenance of the mechanical components of the aerobic system.