When soils on a site have a limiting zone closer to the soil surface than 48 inches, Pennsylvania, via Chapter 73 requires that a filtration or advanced treatment unit (FATU) be placed in the treatment chain between the septic tank and the absorption area. These FATUs come in various shapes and sizes, but all provide what the sewage treatment industry would call secondary or biological treatment. Some of these units pass the septic tank effluent down through a prescribed layer of filtering material. Some of these units cause the effluent to flow over sheets of geotextile, which serves as a zone of biological contact that exposes the wastewater to the microbes that treat the wastewater, similar to a "trickling filter". All of these units are designed to expose the wastewater to aerobic bacteria, which reduce the septic tank effluent organic content (BOD5) of between 150 and 300 mg/L to less than 10. These treatment units, in theory, apply aerobic treatment to the wastewater before it reaches the absorption area so the soil, which has limited treatment capacity because of its shallow depth, has sufficient capacity to complete the treatment process.
Types of Filtration or Advanced Treatment Units
There are several FATUs currently approved for use in Pennsylvania. These FATUs can be divided into two general categories:
- those that pass the wastewater through an aerobic layer of media
- those that cause the wastewater to flow over a media where the aerobic bacteria are attached and they treat the flowing wastewater as it comes into contact with the media.
Both of these types of filters reduce the organic load (BOD) in the wastewater by bringing the waste into contact with the feeding aerobic bacteria. These units are also effective in reducing the total suspended solids (TSS) of the wastewater to less than 10 mg/L. The FATUs in each of these categories are listed below and described later in this article.
Those that cause the wastewater to flow through a filter media include:
- Peat Filters
- Coconut (Coco) Fiber Filters
Those that cause the wastewater to flow over a media include:
- AdvanTex Units
- Geotextile Sand Filters
A third treatment unit that yields about the same quality of effluent and employs the same biological treatment is the Aerobic Treatment Unit (ATU). The ATUs are often used as a substitute for a septic tank followed by a secondary treatment filter. Article F-174 explains the use and operation of ATUs.
Peat or Coco Filter
A peat filter is a tank of peat that receives effluent from the septic tank. The uniformly applied wastewater flows through the filter media. The filter can be accessed through two manholes so the performance of the filter can be monitored and maintained. The peat used in these filters must be changed every 8 to 10 years and a service contract is required. Figure 1 (above) shows a cross-section through a peat filter.
Coconut Fiber Filters
A coco fiber-filled filter is similar to the peat filter, except the filter media is chopped coconut fiber, often called coir in the erosion-control industry. The influent is dosed under pressure to the coconut fiber-filled filter where it passed downward through the coconut chips where aerobic bacteria biodegrade the organics in the wastewater. The coconut chips must be changed every 15 to 20 years and the coconut fiber filled filters require a maintenance contract. Figure 2 (above)shows a typical coconut fiber-filled filter.
AdvanTex Filtration Unit
AdvanTex is a commercially available unit that provides storage, filtration and aerobic biological treatment for septic tank effluent. The septic tank effluent flows into the bottom of the tank and then is pumped and sprayed over the top of vertical sheets of geotextile hung in the tank. As the water drains/flows downward over the geotextile sheets, aerobic bacteria biodegrade the organics and filter the wastewater, thus reducing BOD and TSS concentrations. After the wastewater has been recycled through the filter unit several times, it is discharged to the absorption area. These units also provide some nitrogen removal. Figure 3 (above) shows the earlier AdvanTex unit (left), which is usually placed above the septic tank. The newer model (right) is a self-contained unit that captures, sprays the effluent over the top of the hanging filters and directs the treated wastewater to the distribution area. A pressure distribution pump can be placed in the far-right chamber where effluent can be pumped to the absorption area.
Geotextile Sand Filter (GSF)
The geotextile sand filter (GFS) is an engineered wastewater disposal unit that applies pressure distributed wastewater from a septic tank to the top of a cuspated plastic core encased in multiple sheets of a geotextiles. As the water flows downward over the plastic and geotextile units, aerobic bacteria biodegrade the organics in the wastewater and filter the solids. The cuspated plastic core units rest on 12 inches of sand, through which the waste also flows before entering the native soil. Figure 4 (above) shows the full installation of a GSF unit (left) and one of the cuspated plastic core units (right).
Wastewater typically leaves a septic tank with a BOD concentration of about 150 to 300 mg/L. If the septic tank effluent is then passed through a filtration or advanced treatment unit described herein, the BOD concentration should be reduced to less than 10 mg/L. The total suspended solids (TSS) are also reduced to less than 10 mg/l. Filtration or Advanced Treatment units greatly improve the quality of wastewater distributed to the soil in the absorption area.
For additional assistance contact:
your local Sewage Enforcement Officer or County Extension Agent
Pennsylvania Association of Sewage Enforcement Officers (PASEO)
4902 Carlisle pike #268
Mechanicsburg, PA 17050
Pennsylvania Septage Management Association (PSMA)
Bethlehem, PA 18016