Soil absorption systems use the natural soil as part of the onsite wastewater system providing both treatment and ultimate dispersal of the water into the ground. Various configurations exist for soil absorption systems using either gravity or pressure distribution to disperse the water throughout the system.
Trench systems consist of shallow, level excavations, usually 1-5 feet deep and 1-3 feed wide. The excavated area is usually filled with six inches or more of a porous medium, such as gravel. Next, a distribution network is laid out over the media. A single line of perforated distribution pipe is laid in each trench. Building paper, straw, or some other semi-permeable barrier is placed on top of the network before the system is covered with soil. The wastewater trickles through the distribution network, through the media, and into the soil beneath the trenches. Treatment of the wastewater occurs in both the media and the natural soil.
A bed system consists of an excavated area, much wider than for a trench system but usually around the same depth. The excavated bed is filled with gravel with the distribution network laid out over the gravel. Building paper, straw, or some other semi-permeable barrier is placed on top of the network before the system is covered with soil. . The wastewater trickles through the distribution network, through the media, and into the soil beneath the bed. Treatment of the wastewater occurs in both the gravel and the soil sections beneath the bed system.
Alternating drainfields utilize the trench system dispersal option. The division of the soil absorption system into one or more active areas allows alternate use of each individual section over extended periods of time. Usually, alternating fields consist of two fields, each sized anywhere from 50-100% of the total soil absorption area required to effectively treat the wastewater. Flow to the fields are manually switched once or twice a year, often through a pull gate mechanism as shown in the graphic below. This practice extends the overall life of the soil absorption field by allowing the part of the system not receiving the pretreated effluent to “rest.” A resting period allows the infiltrative surface to rejuvenate naturally through biodegradation of the clogging mat.
Alternating Drainfields - Use of two drainfields interlaced offers extended
life of an onsite system because while one system receives water, the other ‘rests,’
– does not receive water.
For soil absorption systems on slopes, serial systems are common. A serial distribution system uses a series of trenches in succession. Often gravity-fed but can be pumped from trench to trench, the pretreated effluent moves from one trench to the other maximizing the infiltrative capacity of a trench before moving to the next trench.
Gravity- or pressure-fed, this soil absorption system is designed for shallow and slowly permeable soils where may have a travel path more horizontal than vertical. A single, shallow trench is dug following the natural contours of the site. Sand is placed on the trench bottom, followed by gravel and the perforated pipe. Building paper, straw, or some other semi-permeable barrier is placed on top of the gravel prior to covering the trench with soil. Contour systems can also be designed to use chamber systems.
Soil Absorption Systems - Trenches, beds, contour trenches, and low- pressure pipe (LPP) systems are highlighted.
Alternative Dispersal Options - Various alternatives to the conventional soil absorption/dispersal system to overcome site limitations for onsite wastewater treatment.
Chamber and Gravelless Systems
Chamber or gravelless systems use material other than gravel or rock in the trench or bed excavation to provide an infiltrative surface onto which the effluent is distributed. These systems provide some capacity to store effluent until it can be absorbed into the natural soil. Because these systems do not use gravel or rock, they may also inhibit sand and silt infiltration into soil pores. The risk of soil compaction is reduced because there is no need to use heavy equipment to haul and place gravel.
Gravelless and Chamber Systems: Alternative Drainfield Designs – Discussion on how gravelless and chamber systems differ from conventional soil absorption systems.
Alternatives to Gravel Drainfields - Conventional drainfield (i.e., soil absorption system) consists of a series of trenches or a bed lined with crushed stone or gravel and perforated pipes. Alternative aggregate media options exist that may provide better treatment and/or be cost-effective.
Drainfield Rehabilitation – Steps to assist in determining if your soil absorption system needs repaired and corrective measures are provided.
Pressure/Low Pressure Pipe System
Pressure and low pressure pipe (LPP) systems are shallow, dosed soil absorption systems. The effluent from the pretreatment unit flows by gravity to the pump chamber. When a predetermined level within the pump chamber is reached (usually marked by a float control), the pump forces the effluent through the distribution lines under pressure. This allows effluent to be evenly dosed at intervals with dosing frequencies determined by site and soil conditions.
In a pressure assist system, the wastewater flows by gravity from the pretreatment unit to the pump chamber where it is pumped uphill and dispersed by gravity through the distribution system. Other than the addition of a pump chamber, this type of system operates identically to a conventional trench or bed system.
Drip irrigation systems apply treated effluent to soil slowly and uniformly through a network of flexible, narrow plastic, polyethylene, or polyvinylchloride (PVC) tubing placed at shallow depths of usually 6-12 inches in the plant root zone. The effluent is pumped through the tubing under pressure but drips slowly from a series of evenly spaced openings in the tubing called emitters. Wastewater must be pretreated (septic tank or aerobic unit) and filtered prior to subsurface disposal via drip irrigation. This is because small solids can clog the tubing and emitters. One advantage of a drip irrigation system is minimal site disturbance because the flexible tubing can be placed around trees and shrubs.
Spray irrigation systems apply treated effluent aboveground to reclaim wastewater. The area to be irrigated must be vegetated and landscaped to minimize runoff and erosion. The wastewater must be treated to a high enough level to protect public health and reduce odors. For this reason, the wastewater must be disinfected (ozone, UV, or chlorine). After treatment, filtration, and disinfection, a pump equipped with timers sends the wastewater under pressure through the mains and lines of the spray distribution system at preset times and rates.
Spray and Drip Irrigation for Wastewater Reuse, Disposal - presents a brief overview of two types of wastewater irrigation systems - drip and spray systems.
Mound or At-Grade System
A mound system is a soil absorption system that is elevated above the natural soil surface using a suitable fill material, such as quality sand media. Wastewater is first pretreated by a septic tank or aerobic unit and then fed by gravity to a pump chamber where the effluent is dosed to the mound system. The purpose of the design is to overcome site restrictions, such as slowly permeable soils, shallow permeable soils over creviced or porous bedrock, and permeable soils with high water tables. In an at-grade system, the ground surface is the bottom of the trench. Construction consists of scarifying the ground surface to expose the existing soil and eliminating vegetation prior to adding gravel to the
Mounds: A Septic System Alternative - Defines a mound system and details advantages to their use.
Evapotranspiration (ET) systems are ideal in locations where the annual evaporation rate exceeds the annual precipitation rate. The effluent flows from the pretreatment unit to the sand bed underlaid with an impermeable liner. Capillary action in the fine sand causes the effluent to rise to the surface and escape through evaporation while at the same time, vegetation transports the wastewater from the root zone to the leaves where it is transpired as a relatively clean condensate. The design allows for complete wastewater evaporation and plant uptake with no discharge. Evapotranspiration-absorption (ETA) systems are similar to ET systems. However these systems are unlined and designed for use where soils are fairly impenetrable, but not entirely so. The ETA system disposes of wastewater in the same evaporation/transpiration manner but also allows effluent to trickle through underlining natural soils.
Technical Overview - Evapotranspiration Systems - design guidelines to follow for ET and ETA systems.
Evapotranspiration Systems - General overview of ET and ETA systems based on use, soil conditions, and climate