Pennsylvania has approved the use of At-Grade systems as a disposal method for on-lot wastewater. The at-grade system was developed and can be designed for sites with restrictive soil conditions, especially limiting zones at less than 48 inches. Thus, the at-grade systems may serve as an alternative to placing an elevated sand mound on some sites.
Components of the At-Grade System
The at-grade system has four major components: a treatment tank, a filtration or advanced treatment filter unit (if required), a dose tank, and an at-grade soil absorption area constructed on the soil surface (see Figure 1). Each of these components will be described below.
Figure. 1. Schematic of an At-Grade System.
The wastewater flowing from your home first enters a treatment tank. The treatment tank is most often a two-chamber septic tank. The septic tank encourages the heavier solids to settle to the bottom of the tank and the lighter scum to float to the surface. The remaining liquid is discharged to the next stage of the system. In place of a septic tank, a properly sized aerobic treatment tank may be used in which case a stream of air containing oxygen is forced through the wastewater encouraging the biological breakdown of the organics in the wastewater.
Filtration or Advanced Treatment Unit
The filtration or advanced treatment requirements are dependent on the depth to the site's limiting zone. In cases where the depth to the limiting zone is greater than 48 inches, no filtration or advanced treatment is required. In all cases when the depth to the limiting zone is less than 48 inches, the septic tank effluent must be conveyed to a filtration or advanced treatment unit where the wastewater is filtered to remove and biodegrade a large portion of the wastewater's organic components. The approved filtration or advanced treatment units for sites with limiting zones at depths less than 48 inches below the land surface are the Orenco AdvanTex Unit or one of the Peat or Coco Fiber Filters. Neither the at-grade or any other soil-based on-lot wastewater system can be installed on sites where there is a rock limiting zone closer to the soil surface than 16 inches, or where there is a seasonal high water table closer to the soil surface than 10 inches. As a point of interest, the Geotextile Sand Filter (GSF) systemis an approved alternative to the at-grade system for sites with limited zones at depths less than 48 inches.
Orenco AdvanTex Filter
AdvanTex is a commercially available unit that is placed after a septic tank. 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 the BOD and TSS concentrations. After the wastewater has been recycled through the filter unit several times, it is discharged to the next unit or the absorption area. These units also provide some nitrogen removal. Figure 2 shows the earlier AdvanTex unit, which is usually placed above the septic tank. The newer model (Figure 3) 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 dose chamber where it can be pumped to the absorption area.
Figure 3. AdvanTex Filtration Unit
Peat or Coco Fiber Filters
A peat or coco filter is a large tank, filled mostly with peat or coco (coir) fiber, into which the septic tank effluent is pumped. The wastewater percolates slowly down through a layer of peat, where the biological treatment occurs. The treated wastewater is collected at the bottom and piped to the dose tank and on to the absorption area.
Figure 4. Peat or Coco Filter
After the wastewater has been treated in the approved filtration or advanced treatment unit (assuming additional filtration or advanced treatment is required), it is piped to and collected in a small single-chamber tank known as a pump or dose tank. When a pre-set volume of wastewater has been collected, float switches in the dose tank excite the pump, which transfers a prescribed volume of wastewater to the at-grade system's soil absorption area. Generally the wastewater is dosed several times daily.
The absorption area required will differ depending on whether the limiting zone is less than or greater than 20 inches below the land surface.
Greater than 20 inches
If it is more than 20 inches to the limiting zone, the wastewater is dosed to a bed-type absorption area. The absorption area is sized according to the results of a standard Perc Test (see the Site Evaluation for On-lot Systems). There are two similar, but different approved pipe-aggregate-backfill configurations. The absorption area is a pair of pipes located in a bed of aggregate placed on the soil surface after the soil has been chiseled across the slope. In the first case, Option A (Figure 5), a 10-inch deep bed of aggregate is placed parallel to the soil surface with the two distribution pipes located off-center and at the up slope edge of the bed.
In Option B (Figure 6), the aggregate bed is placed on the soil surface with the top of aggregate level and has a minimum depth of aggregate of 10 inches. Here the two distribution pipes are centered in the aggregate.
Figure 5. At-Grade Soil Absorption Geometry, Option A.
Figure 6. At-Grade Soil Absorption Geometry, Option B.
Less than 20 inches
If the limiting zone is less than 20 inches below the land surface (Shallow At-Grade System), the wastewater is dosed to a trench-type absorption area. The trench is much like Options A or B described above, but only one pipe is used and the elevated area is smaller. The trenches may not overlap each other on a sloping area. The absorption area is sized based on the site assessment made by a professional soil scientist. The absorption area sizing procedure is outlined in Site Evaluation for On-lot Systems.
The at-grade on-lot wastewater disposal system was created as an alternative to requiring an elevated sand mound on some sites having a limiting zone between 20 and 48 inches. The shallow version of the at-grade system can also provide additional flexibility on sites where a limiting zone exists at less than 20 inches. The at-grade system is a very shallow mound with a maximum height of between 16 and 42 inches depending on the slope. The at-grade mound has a lower profile and is less intrusive than the elevated sand mound. The at-grade system also yields better quality effluent. The disadvantage of the at-grade system is that you will most likely be required to include a filtration or advanced treatment unit between the septic tank and the absorption area and regular maintenance is required.
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