U.S. Inspect Home Inspection:

Advisory Report

February, 2004

Septic Systems Overview
Part Three of Three: Absorption and Treatment Systems

With so many types of septic systems out there it is easy to become confused when distinguishing between them. In this series we're going through several of the more common types of septic systems.

In this last installment we'll cover the different types of absorption and treatment systems. There are many and they all work in different ways, but in the end they all get the job done. Chamber systems, mound systems and seepage pits are more common than the rest. The others you may only see once or twice during your tenure as a Relocation Professional, but are good to know just in case.

Leach Chamber Absorption Systems

Leaching chambers are prefabricated plastic forms with open bottoms through which the effluent is absorbed into the soil. They are easier to install and more flexible than traditional leaching fields, but not always cheaper. During installation, leaching chambers can be installed around existing features of the land and if necessary, chamber systems can later be enlarged with relative ease.

This type of system can be installed in any location that a normal leaching field can. Leaching chamber systems are promoted as requiring less land to perform the same function as a traditional leaching field, however this statement is not accepted by all in the industry. ‘Infiltrator’ is a commonly seen brand of leaching chamber system and so, leaching chamber systems are occasionally referred to as ‘infiltrator’ systems.

Mound Absorption Systems

A mound system protrudes from the ground; and is generally used when the soil percolation rate is not suitable for a traditional leaching field or when the water table in the area is too high. Because the mound is elevated, any septic system with this type of absorption scheme requires a holding tank and pump. The pump not only serves to move the effluent to the lines contained within the mound but it also pressurizes the lines, allowing for greater absorption in a smaller area.

The mound itself is simply placed on top of the ground and is layered, typically with sand on the bottom, an area of gravel in the middle of the mound, and then a topsoil and grass on top. The pipes are run through the gravel aggregate so that the water disperses outwards to the sand, then the sand disperses the water further to maximize effluent contact with the underlying ground. As with any system that requires mechanical components like a pump, maintenance and eventual replacement of the parts are an issue.

Seepage Pit Absorption Systems (also known as Drywells)

A seepage pit, or drywell, is a deep, relatively wide hole in the ground that allows effluent to leach into the soil through the sides and bottom. The pit is lined with gravel and contains a chamber of sorts that is constructed of stone, masonry, or metal in which the effluent is dispensed. The chamber is sometimes partially filled with gravel to promote more even dispersion of the effluent. Although you may see these systems fairly often, they are no longer installed due to regulations requiring absorption areas to be 4 feet above the local water table.

Low-Pressure Pipe Absorption Systems

With a low-pressure pipe system, the lines and trenches are smaller and do not have to be buried as deep as would be required in a traditional leaching field. It utilizes a holding tank with a pump to transport the water into the lines and apply pressure, effectively forcing the water into the soil. The use of pressure prevents the lines from clogging as easily as they might in a traditional system and promotes equal distribution of effluent throughout the absorption area. A low-pressure pipe system requires less land than a traditional system, but can be more costly as additional equipment is necessary. Also to consider, is the maintenance of the pump and the inevitability that the pump will break down.

Evapotranspiration Absorption Systems

Evapotranspiration systems can only be used effectively in warm, dry climates. There are three different types that you might come across. Standard evapotranspiration is the most common, followed by evapotranspiration/absorption and last, mechanical evapotranspiration.

Standard evapotranspiration systems and evapotranspiration/absorption systems are fundamentally the same. A wide hole is dug into the ground, lined, and the pipes are run along the bottom in a bed of gravel. The remaining space in the bed is filled with carefully selected sand that draws the water up from the bottom and exposes it to the air. Carefully selected plants cover the top, utilizing the plant roots to draw water through the roots to the leaves, where the water is transpired. The only real difference between these two units is the bed liner that is used. A standard evapotranspiration system is lined with an impermeable liner, preventing effluent from entering the soil around the bed. On the other hand, an evapotranspiration/absorption system uses a permeable liner that allows water to seep into the surrounding soil.

Mechanical evapotranspiration systems on the other hand use one of two methods. An exposed rotating disk or an exposed, rotating cylinder covered with cloth. Both are continually dampened, but with the rotating cylinder there is the additional element of compressed air that is forced through it, turning the water into vapor. Mechanical evapotranspiration systems are rare, as the technology has not yet been perfected to the liking of the industry.

Constructed Wetland Absorption Systems

A constructed wetland absorption system utilizes the filtering qualities of plant roots and stone to mimic the otherwise naturally occurring process found in wetlands. The system is comprised of shallow trenches lined with either a permeable or impermeable liner. The trenches are filled with gravel, sand, or a mix thereof and carefully selected vegetation are planted within. The lines are run along the bottom of the trenches and the system is set up so that the effluent runs either through the gravel/sand or below the gravel/sand. Occasionally, other methods of effluent disposal may be required and to accommodate this, the design will include an outlet pipe.

While contained within the trenches, exposure to the air and the plant roots aerate the effluent, breaking it down further. The plant roots also form what is known as a rhizome network that additionally filters, or ‘polishes’ the effluent. The water is then disposed of via evaporation, transpiration, and into the surrounding soils if a permeable liner is used. Since constructed wetlands utilize the same methods of water disposal as an evapotranspiration system, they are also more practically used in hot, dry climates.

Sand Filter Treatment Systems

Sand filters are not final absorption systems, but treat the water further prior to release into an absorption system, with exception to a bottomless intermittent sand filter. There are two fundamentally different types of sand filter systems: recirculating and intermittent. In addition, there are three different types of intermittent sand filters.

Recirculating sand filters come in the form of a tank of sorts. This type of system requires a recirculating tank, where the initial discharge from the septic tank is held. The tank is filled with sand, on top of which the lines are run and discharge the water. As the effluent trickles through the sand it is filtered and aerated, further breaking it down and eliminating odors. When the water reaches the bottom, a portion of it is released to an absorption area and the remaining water is returned to the recirculating tank. Although very effective, recirculating filters require near constant maintenance and can be very expensive to maintain.

Intermittent sand filters are set up so water only passes through the filter once. Water is introduced intermittently, to prevent overloading of the filter. This type of sand filter is lined with an impermeable liner, has a small layer of gravel at the bottom, a large fill of sand in the middle, and another small layer of gravel on top. The line is run through the top layer of gravel and the effluent trickles through the sand. The gravel at the top aids in dispersing the water to maximize exposure to the sand. The difference between the types of intermittent sand filters lies in how the filtered water is disposed.

A gravity system allows the water to flow freely, via a pipe at the bottom of the filter, into the absorption area and must therefore be located above it. Gravity sand filters are usually constructed above ground because of this. In a pump system, the water is pumped into the absorption area, defeating the limitations of a gravity system. The third, a bottomless system, will retain the effluent. It will either: not have a liner at all, or it will have a permeable liner to allow the filtered effluent to absorb into the soil underneath.

Disinfection Treatment Systems

Disinfection systems are used as a secondary system, when the primary wastewater treatment does not provide the desired result. The effluent must still be disposed of via either one of the absorption systems described above, a traditional leaching field, a sprinkler system, into a stream or other nearby body of water (if local regulations allow it). The different methods of disinfection are basically a concentrated form of the common threads seen in primary wastewater treatment. You can disinfect with air (ozone), chemical (chlorine), or light (ultraviolet).

Because of the costs required to set up and maintain the ozone disinfection process, this type of system is predominantly used by local municipalities and used as part of the wastewater treatment process, not just to treat the effluent. The ozone is produced onsite via electrical discharges either in dry air or pure oxygen and introduced into the wastewater via a diffuser, which breaks the gas into fine bubbles. Ozone dissipates rather quickly in normal air and therefore has little to no impact on the environment. However, ozone is toxic to humans and if the system doesn’t allow the gas to dissipate properly, it can be hazardous.

Chlorine is the most commonly used method of disinfection because it is a proven and more cost-effective technology than other disinfection techniques. The chlorine can be introduced to the wastewater or effluent in the form of gas, liquid, or solid. In a residential system, chlorine is introduced in what is known as a ‘dosing’ chamber. After chlorine introduction, it must rest to allow the effects to take hold. Some local authorities require dechlorination of the treated water because residual chlorine can affect the plants and organisms in the soil surrounding the system.

Ultraviolet disinfection is arguably the fastest method of treating wastewater and effluent. In less than a minute, it can destroy nearly all bacteria with which it comes in contact. The ultraviolet light is introduced via mercury arc lamps that can be placed in the water or suspended above the water. Once the system is initially installed, it is relatively easy to maintain, as the lamps only require replacement every few years. The major maintenance cost is the electricity required to operate such a system.

Click here for the December 2003 article about traditional Septic Systems.

Click here for the January 2004 article about common "Alternate" systems.

Click here for more information more about Septic Systems.

Contributed by Justin Gore
Quality Assurance Supervisor
U.S. Inspect, LLC

Diagrams used under license from The Illustrated Home, © 1998 Carson Dunlop & Associated Ltd.