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Septic system design alternatives: this article discusses the selection, design, installation, inspection, repair, and maintenance of Evaporation-Transpiration & Evapo-Transpiration Absorption as Alternative Septic Systems, also called Evapotranspiration Septic Systems.
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We include information from both state (New York) and federal (US EPA Wastewater Treatment Manual) sources as well as the advanced Tafgard™ evaporation/transpiration wastewater treatment system design by Taisei Kogyo Co., Ltd., Japan.
Evaporation - transpiration septic or effluent disposal systems may be selected for wastewater treatment and disposal at difficult sites. We also provide lists of product sources or specialists in this field. (Listings can be added here at no fee.) Citation of this article by reference to this website and brief quotation for the sole purpose of review are permitted. Use of this information at other websites, in books or pamphlets for sale is reserved to the author. Technical review by industry experts has been performed and is ongoing - reviewers welcomed and are listed at REFERENCES.
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(e) Evaporation-Transpiration (ET) and Evapo-Transpiration Absorption (ETA) Septic Systems Design Criteria
Evaporation-Transpiration (ET) Septic Systems, and Evapo-Transpiration Absorption Septic Systems (ETA) dispose of septic effluent from the septic tank by providing a surface area intended to allow the effluent to evaporate. ET systems depend entirely on evaporation while ETA systems make use of both evaporation and (limited) soil absorption of septic effluent.
The ETA gravel bed system shown in the center of our septic evaporation/transpiration system (left) was observed in the Hudson Valley, New York state and appears to be using evaporation to help dispose of septic effluent from a private onsite septic system.
If you click to enlarge the photo you 'll see a local storm drain basin at the far end of the graveled area. The storm drain catchbasin receives runoff from the paved street and nearby storm drains. We worried that the added water draining absolutely adjacent to the septic effluent evaporation bed will reduce the effectiveness of that system, particularly when in the foreground-left we see that the area includes intermittent rocky shale close to and even above the ground surface. An inspection of older residential properties down-slope from this relatively new neighborhood confirmed that once dry grassy lawns were a sodden, smelly mess, and we suspected that older properties were now receiving sewage effluent and storm drain runoff from the neighborhood shown here.
The following is from: New York Appendix 75-A.9 Alternative Septic System Designs. Information from the US EPA Wastewater Manual follows below.
[Regulation and System Design Criteria for Raised Septic Systems, Septic Mound Systems, Intermittent Sand Filter Bed Systems, Evaporation-Transpiration Septic Systems, Evaporation-Transpiration Absorption Septic Systems, and Other Alternative Septic Systems]
(1) Evaporation/Transpiration/Absorption Septic Systems - General
ET systems rely on the upward movement of moisture through the soil, surface vegetation and into the air rather than absorption into the soil. ETA systems also use the absorptive capabilities of the soil and are less dependent on evaporation and transpiration.
(2) Site Requirements for Evaporation, Transpiration-Absorption Type Septic Systems
(i) All systems previously discussed, except intermittent sand filters, have been determined to be unacceptable for the planned building site.
(ii) An expansion area equal to or greater than 50% of the required basal area shall be available on the site.
(iii) All minimum vertical and horizontal separation distances can be maintained as described in Table 2 from both the edges of the basal area and the designated expansion area.
(iv) An environmental assessment determines that the development of the site with this system is consistent with the overall development of the area and will cause no adverse environmental impacts.
(3) Design Criteria for Evaporation, Transpiration-Absorption Type Septic Systems
(i) The designer must consider all of the items listed below and be able to document from reliable sources (i.e., National Weather Service, Soil Conservation Service) the parameters used and show that the net outflow from the system exceeds the inflow without the exposure of sewage or partially treated sewage on the surface of the ground:
Total rainfall and snowfall.
The percentage of the rainfall and snowfall that will infiltrate into the soil and the percentage that can be expected to runoff the system.
The annual land evaporation rate of the area.
The vertical rise of water than can be expected in the soil due to capillary action.
The amount of transpiration expected from the surface vegetation.
The permeability of the underlying soil and the impact the system will have on the groundwater level.
(ii) The design must provide for a trench depth that is not greater than 30 inches below the surface.
(iii) Pressure distribution of effluent throughout the system is required.
Evapotranspiration and Evapotranspiration/Infiltration Systems for Septic Effluent Disposal
Onsite Wastewater Treatment Systems Technology Fact Sheet 6 - EPA 625/R-00/008
Description of Evapotranspiration and Evapotranspiration/Infiltration Systems for Septic Effluent Disposal
Onsite evapotranspiration wastewater treatment systems are designed to disperse effluent exclusively by evapotranspiration. Evapotranspiration (ET) is defined as the combined effect of water removal from a medium by direct evaporation and by plant transpiration. The evapotranspiration/infiltration (ETI) process is a subsurface system designed to dispose of effluent by both evapotranspiration and infiltration into the soil. Both of these systems are preceded by primary pretreatment units (e.g., septic tank) to remove settleable and floatable solids.
The influent to the ET or ETI units enters through a series of distribution pipes to a porous bed. In ET systems, a liner is placed below the bed to prevent water loss via infiltration unless the soil is impermeable. The surface of the sand bed is planted with water-tolerant plants.
Effluent is drawn up through fine media by capillary wicking and evaporated or transpired into the atmosphere. In ETI systems, effluent is allowed to percolate into the underlying soil.Modifications to ET and ETI systems include mechanical evaporating devices and a broad array of different designs and means of distribution, storage of excess influent, wicking, and containment or infiltration prevention. Some newer studies are using drip irrigation with distribution to forested areas with purported success.
Tafgard soil based wastewater treatment systems - developed in Japan by Taisei Kogyo Co., Ltd., this system uses a combination of a five-chamber waste treating septic tank designed by Taisei, effluent disposed-of by an evaporation-transpiration system through a Tafgard geotextile and aerated soil (effluent spread horizontally and upwards from distribution piping). The Taisei septic tank utilizes a chain of bacteria, protozoan, metazoans and various kinds of enzymes.
Illustration (above) of the Tafgard soil based evaporation transpiration system using Tafgard geotextile over effluent distribution piping is courtesy of Taisei Kogyo Co., Ltd.
Consequently, the developer asserts, sludge does not increase, thus it does not need to be desludged, minimizing the maintenance cost. The longer the food chain becomes, the more energy is consumed by respiration. We pose that tank cleaning will in fact probably be needed, but apparently at a much less frequent interval than other less sophisticated treatment systems. This design uses no pumps or electrical power, and is described as most suited for homes, mountain lodges and public toilets in remote areas, such as beaches, national parks, ski resorts and campgrounds where there is no electric power source, or where there is no place to discharge effluent.
Typical applications of Evapotranspiration and Evapotranspiration / Infiltration Systems for Septic Effluent Disposal
ET and ETI systems are best suited for arid (evaporation exceeds precipitation) climates. If ETI is selected, soil percolation is also an important consideration. Both systems are often selected when site characteristics dictate that conventional methods of effluent disposal are not appropriate (e.g., unprotected sole source aquifer, high water table or bedrock, tight soils, etc.).
Although these systems normally follow septic tanks, additional pretreatment may be employed to minimize clogging of the ET/ETI system piping and media. They are sometimes used as alternative systems during periods when normal disposal methods are inoperable, for example, spray or other surface irrigation. Also, these systems have been widely used for seasonal homes in areas where year-round application of ET/ETI is not practical and conventional methods are not feasible. Year-round ET systems (see figure 1) require large surface areas and are most feasible in the areas shown on figure 2. ETI systems can be employed to reduce the infiltrative burden on the site during the growing season. Such applications can also result in some reduction in nutrients, which are transferred to the overlying vegetation (USEPA, 1999).
Figure 1. Cross section of a generic evaportranspiration bed (adapted from NSFC)
Figure 2. Relative suitabilityof Evapotranspiration and Evapotranspiration/Infiltration Systems for Septic Effluent Disposal
Design assumptions for septic evaporation transpiration systems
The design evapotranspiration rate is site specific. Some areas are arid (precipitation < evaporation) but lack the solar radiation or wind velocities necessary to efficiently evaporate wastewater throughout the year. Therefore, simple use of well-known evaporation estimates like Pentman, Blaney-Criddle, and Jensen-Haise will not likely be satisfactory.
In fact, historically, the definition of workable ET rates for an area has been a trial and error process, which is further complicated by the system design and the plants used. The primary variables that have an impact on the potential ET rate are climate, cover soil, and vegetation.
The most important system variables, which control the movement of wastewater to the surface, are media and the depth to saturated (stored) water. Most published designs are suspect because they store the wastewater so deep that the wicking properties of the fill and the area (voids) through which water must rise restrict delivery of water to the surface where it is evaporated.
Present ET system designs normally employ 20-mil polyethylene liners where the soil is too permeable and ground water contamination is likely. Most employ distribution systems placed in 12 inches of gravel (0.75 to 2.5 inches) at the bottom of the bed. Spacing of the distribution pipes is 4 to 12 feet, with lower values preferred for better distribution. Wicking is accomplished by a 2- to 2.5-foot layer of sand (0.1 millimeter) and a loamy soil-sand mix to raise the water to the surface or a thin layer of soil at the surface. Most have employed the formula:
Each of these factors is open to some degree of interpretation. Because these systems are large and expensive, there has been a tendency to minimize their design size and cost, resulting in significant failure rates. Typical ET estimates range from 0.01 to 2.0 centimeters per day.
The contribution of plants has remained a matter of controversy. ET bed sizing has varied from 3,000 to 10,000 square feet and higher. A water balance based on at least 10 years of data is calculated to provide sufficient storage for nonsurfacing operations or to estimate nonatmospheric volumes to be infiltrated.
The modern use of shallow trenches for SWIS is strongly related to the maximization of ET, and such systems could be classified as ETI systems. Further, the use of shallow serial distribution where topographic relief is available is a classic application of the ETI concept, that is, shallow trenches close to the surface, full of wastewater, with only a short wicking distance to the evaporative surface. Such a system fulfills all the described features of an ideal ETI system. Similarly, drip irrigation uses the shallowest of all SWIS burial requirements and, by nature, maximizes ET potential.
Performance of Evapotranspiration and Evapotranspiration/Infiltration Systems for Septic Effluent Disposal
There have been few studies of ET and ETI systems. Most ET system studies have been less than impressive. In most cases the fault has been related to poor design assumptions, for example, over-estimating the ET potential of shrubs and trees planted on the surface and of the overall potential of ET itself. Poor system design has been somewhat offset by leaking liners that give the appearance that the system is performing adequately. Inadequate wicking has been overcome by raising water levels. However, better ET assessment and more rational designs will improve performance at increased costs.
ETI systems have generally worked well, but no scientific studies have been performed to verify this observation. ETI systems do fail when the ET contribution is overestimated, but many times the placement of the wastewater higher in the soil profile offsets that error by increasing the infiltrative capacity of the site.
Management needs of Evapotranspiration and Evapotranspiration/Infiltration Systems for Septic Effluent Disposal
ET systems are very sensitive to variations in construction techniques. Poor construction can defeat their utility through poor liner installation, poor placement and choice of wicking media, compaction, and inadequate surface drainage mitigation.
Operation and maintenance requirements are minimal, often consisting of simply mowing the grass on the surface. Replanting cover crops to improve cold season performance has been suggested but offers little return. Shrubs or small trees planted on the ET system generally improve active (warm) season ET and hinder ET in the dormant (cold) season. Therefore, the O/M needs of the system should be limited to two to three short visits to observe and record the water height in the observation well.
These tasks require about 1 to 2 hours per year of unskilled labor. No energy is required. ET system salt buildup, if not diluted by precipitation, may require some media replacement after 5 to 10 years of operation depending on water supply characteristics. There are no known safety issues with these systems as long as they are fenced or otherwise isolated from children's play areas.
ETI systems are very similar to SWIS systems, and their management requirements are similar to those of ET systems. Because ETI systems infiltrate wastewater, they have ground water and surface water contamination concerns like those of other SWIS designs, and they may require monitoring of effluent impacts depending on the uses of ground water and performance standards to protect them.
Risk management issues of Evapotranspiration and Evapotranspiration/Infiltration Systems for Septic Effluent Disposal
Because ET systems are large, there may be some visual aesthetic problems. Odors are usually not a problem, but they can be on occasion. Flow peaks during low ET periods could result in overflows, thus leading to the usual restriction for year-round ET use in areas where ET does not exceed precipitation by more than 2 inches per month. These systems do not function when their surface freezes. They are typically unaffected by power outages since they are generally fed by gravity. Toxics also have no impact unless they are phytotoxic and would then kill the surface vegetation.
Typical Costs of Evapotranspiration and Evapotranspiration/Infiltration Systems for Septic Effluent Disposal
Because of their large size and specific media (and often liner) requirements, ET systems are generally expensive, reinforcing their use as a "last resort" alternative. Installed costs of $10,000 to $15,000 and higher are possible depending on climate and location. O/M costs are relatively low, on the order of $20 to $30 per year, but they could increase if the system fills and requires pumping. ETI systems have capital and O/M costs similar to a SWIS.
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Technical Reviewers & References
References for Evapotranspiration and Evapotranspiration/Infiltration Systems for Septic Effluent Disposal
Design Manuals for Septic Systems
Onsite Wastewater Disposal Books