Raised drain-fields are made of extra, above-the-ground layers of soils suitable for the treatment of effluent. In contrast to “as-it-is” native soil in traditional drain-fields, raised drain-fields are “engineered” for best effluent absorption and processing properties, to make up for limitations of the native land. They are used in the following cases:
a. Native soil is inadequate for the treatment of effluent – either too low percolation rate (clay, bedrock) or too high percolation rate (coarse sand, gravel, karstic rock, …).
Example of an unsuitable soil for the conventional drain-field (here mostly clay). Source: Minnesota Pollution Control Agency (USA)
b. High underground water table.
They add an extra absorption and filtration layer into the natural subsurface soil if the latter is too shallow to guarantee the efficient treatment of the effluent.
c. Small size of the available land
Suitable for the task “engineered” soil, has much better treatment capabilities than the typical native one. As a result, the active drain-field area can be reduced compared to the size of the traditional drain-field.
Raised drain fields use the same effluent disposal technologies as traditional ones. They make use of earlier discussed “pipe-in-stone” or chamber systems and can be deployed over beds (in most cases), or in trenches (less popular because require more space). The only difference is that they cannot be operated solely by forces of gravity because effluent must be lifted above the ground level. For this purpose, they need an extra lift station installed after the septic tank, consisting of a dosing tank, electrical pump, and control system.
Concept of a Raised Drain-Field Septic System. Source: “Septic System Design”, GroundStone, BC (Canada)
Another look at the mound-based Septic System. Source: “Types of Septic Systems”, EPA (USA)
Mound Drain-Fields Structure
Raised drain-fields can be built from a suitable local soil (it can be a lower-cost solution addressing shallow groundwater table). In general, however, it will be beneficial to use better than native soil materials. Selected types of sands for example, due to their excellent absorption and treatment characteristics are frequently used in municipal wastewater treatment systems. Residential, onsite, raised drain-fields using sands are commonly known as Mounds Drain-fields (or shortly – Mounds).
The area intended for the future septic mound must have at least a 1ft deep layer of suitable topsoil (in other words, it will be illegal to build it directly on the rock or clay). This topsoil must be arranged to form a sort of “foundation” supporting the operation of the mound. In practice, it means – the native soil must be cleaned from vegetation (including grass), tilled, and mixed with sand to create a soft “transition zone” from the future mound into the native soil. Technically speaking – we must make sure that the hydraulic loading rate of the transition zone equals the intended infiltration rate for the mound. Otherwise, the effluent, unable to infiltrate the native soil fast enough, will start to accumulate at the base of the mound.
After finishing the transition zone, the mound’s construction work can start:
Typically, the bottom layer of the mound (placed directly on mentioned above “foundation”) is built from medium-textured sand. It’s the most important part of the natural filter that will play a major role in the treatment of disposed effluent. That’s why special requirements for the sand: preferred range – from 0.02” to 0.08” (0.5mm to 2mm) with no more than a few percent contents of silts, clay, and fine sands (they will trap solids and help to build biomat). The “sand’s filter” must have a depth of at least 2 ft.
Work in progress – Mound-based “Pipe-in-stone” type effluent disposal system. Source: Minnesota Pollution Control Agency (USA)
On top of the sand, there should be a layer of gravel (for the “pipe-in-stone” distribution system) covered by a permeable geotextile to prevent infiltration of fine sediments into the absorption area. The upper layer (backfill) covering the drain-field consists of topsoil. Its depth must provide sufficient thermal insulation protecting the drain-field from freezing (minimum 6”). It must be also suitable for planting with grass to protect the mound from erosion.
Note that the naturally graded shape of the mound will help to divert rainwater away from the sensitive section of the drain-field. However, to work properly, the surrounding area must be able to absorb stormwater (if not, then the drainage system may have to be built around the mound’s base to evacuate it away).
How does it work?
Effluent seepage in the mound-based drain-field. Source: “Troubleshooting Mound Systems” by Sara Heger, (Onsite Installer).
Let’s analyze the percolation process in the mound-based drain-field. The effluent will infiltrate the sand and, on its way down, it will reach the native soil. Assuming “receptive” native soil, the effluent (possibly at a lower speed) will continue penetration, at the same time spreading horizontally over larger areas of the subsurface soil. The special “transition zone” will make this process seamless, effluent will continue the percolation process until reaching the underground water table.
Now, let’s imagine that there is no smooth transition zone at the ground-level, the native topsoil is compacted, or even worst, it’s poorly absorptive (clay). In such cases, effluent, on its way down, will hit a “permeability barrier” (discontinuity), accumulate at the mound’s base, and then start leaking all along its perimeter.
Poorly designed “Transition Zone” as well as the too high hydraulic load can lead to effluent seepage along its base. Source: “Sand Mound Not Taking or Holding Water” by Carl Weaver, SepticTankCare.
Side leaks represent another potential danger. In this case, however, seepage does not have much to do with the poor-quality transition zone, but rather with an inadequate mound-to-drain-field size ratio. The percolation of soil by the effluent is mostly driven by forces of gravity. In practice, however, the “path-of-least-resistance” is determined by many other factors (for example capillary forces, local variations of soil’s texture…). As a result, on its way down, effluent also spreads horizontally within expanding “cone-type” shape. To prevent side seepage, the mound’s geometry is regulated (typically maximum allowed slope is 6%).
Local regulations may impose the installation of monitoring wells in the raised drain-fields to periodically evaluate their “health”. Typically, they will consist of vertical, 4” diameter PVC pipes with ½” holes at the lower-end extending to the bottom of the layer of gravel and (another one) to the bottom of the mound. The latter allows for verification if effluent does not accumulate at the ground level (surface of the native soil).
Mound monitoring wells. Source: “Steps in Constructing a Mound (Bed-Type) Septic System” by Don D. Jones, Joseph E. Yahner and Edward R. Miller; Cooperative Extension Service, Purdue University (IN, USA)
The Pump Station (also known as Lift Station) consists of an extra (usually underground) dosing tank and a submerged pump that can move effluent “uphill”. The most popular control system turns the pump on when the effluent reaches a pre-determined high level and turns it off when it drops below a pre-determined low level. In other words, it acts the same way as a familiar sump-pump in the residential basement (just to make it clear, pumping wastewater is more demanding than pumping water, so sump-pumps cannot be used in septic systems). The control system must also signal failures. Alarms (buzz and flashing light) must be activated when the effluent in the dosing tank reaches a pre-determined alarm level. The alarm system should have an independent power circuit (or battery backup) to be able to activate alarms caused by the failure of a dedicated to the pump circuit fuse/breaker because such events are not easily detectable).
The concept of the Pump Station. Source: “How septic systems works”, Public Health – Seattle & King County (USA)
Commercial Dosing Tank w/controller. Source: “Mound Septic Systems”, Meade Septic Design Inc. (IN, USA)
The dosing tank itself serves as a safety buffer able to collect some volume of effluent during power outages. Unfortunately, longer periods of inactivity caused by the pump’s failure will temporarily put the whole household septic system out of use!
The need for the continuous availability of electrical energy is a “weak spot” of all active septic systems.
For more about tanks’ structure, see: Types of Septic Tanks
The fact that the pump creates pressurized (forced) flow conditions makes a big difference compared to gravity-based systems where typically, the effluent slowly runs through pipes. Pressure guarantees, that effluent quickly fills the entire length of distribution pipes, and so, it can be uniformly released across the whole area of the drain-field. In contrast, in gravity-based systems, effluent is mostly released into the soil in the first section of the distribution pipe.
Pressure-based distribution allows for smaller-diameter distribution pipes (typically 1 ½” instead of 4”) and smaller holes (1/4” instead of ½”) without the risk of clogging.
More sophisticated control systems can further exploit the advantages of active, pressurized distribution systems. For example, a time-based control system may activate the pump only for a specific time to “dose” the volume of effluent released by individual sections of the drain field. By using actively controlled Diverting Valve(s) (sort of D-Box in pressure-distribution systems), not only the dose (volume) of released effluent can be controlled, but also active and rest intervals for each individual section of the drain-field. While nature never “sleeps”, soil and all living microorganisms making this Natural Wastewater Treatment Plant will certainly welcome the opportunity to rest and recover their strength! Keep in mind that aerobic bacteria need oxygen, and in naturally aerated drain-fields, it takes time for oxygen to penetrate deeper layers of soil as well as for biogases to escape. Giving the soil time to regenerate makes a big difference!
In fact, it is proven, that small, frequently released doses of effluent lead to a much higher level of its purification compared to the situation when the drain-field is literally “flooded” with a large volume of effluent, typical in systems with “level-activated” pump. Also, the time-dosed distribution system can practically eliminate the ponding effect at the base of the mound!
Pumps allow for the installation of more efficient, pressure-dosed disposal systems as well as the selection of active & resting disposal sections. Due to higher performance, such drain-fields have reduced size (compared to what will be necessary for traditional subsurface systems). On the negative side – all raised drain-fields need access to electrical energy and are maintenance demanding!
Note 1: The pump itself does not imply the pressure-distribution system. One can use the pump just to lift the effluent to a higher elevation from where it will be distributed by using traditional D-Box and forces of gravity. However, this will be the waste of existing resources (pump) and lost opportunity to reduce the size and increase the lifespan of the drain-field!
Note 2: Due to poor performance, in some countries, Gravity-based Distribution Systems are considered as outdated and may be banned from use in new Septic Systems.
Chamber-based Mound Drain-Fields
In conventional, gravity-based chamber-type drain-fields, effluent is freely flowing over the exposed native soil (in other words, there are no perforated distribution pipes). However, as mentioned above, by using the same approach in raised drain-fields, we will lose opportunities offered by pressure-distribution systems. That is why, in chamber-based raised drain-fields (including mounds), the effluent is almost always distributed by suitable pipes all along the length of the chamber’s tunnel. This approach offers all possible advantages of pressure-distribution systems combined with the abundance of air (oxygen) characterizing chamber-based systems.
Similarly, as in conventional systems, gravel is not required, instead, chambers are directly deployed on the top of a sand filter.
Summarizing: Thanks to engineered-designs and pressure distribution systems, raised drain-fields have much better efficiency of wastewater treatment and a longer lifespan than conventional, gravity-based drain fields. While they have great potential for reduced-size disposal area, in practice this “potential” is always negatively affected by the required shape of the mound (much wider at its base). In the first approximation, the poorer the native soil, the deeper the layer of the sand filter, the higher the mound and the wider its base.
Raised Drain-Fields: Downsides
In many cases, the mound-type drain-field is the only possible solution for an onsite septic system, so we must accept it with all its drawbacks. And these are:
a. Increased cost
The dosing tank, pump w/control system, and all ground-works necessary to raise the mound increase the cost compared to the traditional, gravity-based septic system.
Note, that the use of heavy machinery is often limited by regulations, to prevent compacting of the soil (it may disqualify the land for the intended use).
b. Higher probability of malfunction
In the majority of cases, raised drain-fields are built when the native soil does not meet percolation test requirements (either too low or too high). In both cases the design and quality of the “transition zone” are crucial. It’s a sort of an “interface” between an engineered world (mounds) and the reality of nature (clay, sand, gravel…). If not properly designed and built, raised drain-fields can leak around their bases or let not fully treated effluent discharge into underground water.
c. Air Valves
Unlike in traditional, subsurface disposal systems, the effluent held by supply pipes in raised drain-fields cannot discharge into the field once the pump shuts off. In cold climate zones, the effluent can freeze, putting the whole septic system out of use. To avoid this scenario, the supply lines must be discharged back to the dosing tank. This process, however, creates the siphoning effect that may suck soil to the distribution pipes via perforation holes (you do NOT want to see it happening). That is why all distribution systems in raised drain-fields must have air valves installed in the highest points of the piping.
There are not that many choices when it comes to the mound’s shape or type of vegetation you can plant over it. The point is – mounds will rarely make your landscape more attractive.
Despite your efforts, the mound-type drain-field rarely adds elegance to the surrounding landscape. Source: “How to Locate Your Raised Mound” by Carl Weaver, (Septic Tank Care)
Mound-based drain-field, you probably wouldn’t like to see in your yard. Source: “Mound Septic Systems”, Meade Septic Design Inc. IN (USA); Note that this mound drain-field was NOT designed by Meade, it was shown to prove that it can be done better!
Drip-Distribution is a refined version of “time-based” pressure distribution systems. Effluent carried by ½” diameter tubes is released into the soil in micro-doses via installed in tubing walls emitters. Thanks to the tightly controlled discharge of the effluent (by drops), the distribution tubing can be installed just 6 -to- 8 inches below the surface without the danger of creating health hazards.
Shallow distribution drain-fields have many advantages compared to traditional ones.
a. Good natural aeration
It creates an oxygen-rich environment crucial for the aerobic decomposition process as well as facilitates evacuation of gaseous by-products (CO2).
b. Rich in microorganisms, worms, fungi, and all other creatures
By feeding on contaminants and solids released with effluent, they are a vital part of the natural food chain.
c. Roots’ zone of the vegetation,
Plants, by their developed roots system, absorb many chemical components preventing this way their accumulation in the soil and/or contamination of groundwater. Note, that organic and ammonia nitrogen, and to a lesser extend phosphates (not mentioning a long list of other dissolved in the effluent minerals) serve as nutrients for plants. Additionally, by absorbing water (later released into the air by Evapotranspiration), vegetation decreases the hydraulic load of the soil (which can be a significant factor in warm climate zones).
In contrast to conventional effluent distribution systems, drip-distribution systems allow for the selection of timing and size of doses to match characteristics of the drain-field. By doing so, we can optimize the whole distribution process to such extent, that these systems can be installed even in poor-quality soils (clay, shallow water table….). In fact, in most countries, regulations allow for the installation of drip-distribution systems over just a 1 ft deep layer of receiving soil (compared to 3ft required in conventional absorption fields). What greatly helps, is that the effluent treatment process in drip-distribution systems is carried out not only by the receiving soil but also by uptake by vegetation and evapotranspiration. As a result, the volume of percolating effluent is significantly reduced compared to that released by tubing and it also has a much higher level of purity.
Drip-distribution systems operate under high pressure (15 to 70 pounds per square inch (psi)). Effluent release, however, is controlled by integrated into the wall, pressure-compensating emitters (opening in the range of 30-to-45 mils), so it exits tubing at almost 0 psi (drops).
Tubing for drip distribution of effluent significantly differs from that used in drip-irrigation systems (water). It must have interior bactericide coating (preventing the growth of bacteria), exterior skin must comply with color-coding assigned for wastewater tubing (a clear sign of “do-not touch”), emitters should be protected by root growth inhibitor (tubing will be installed in roots zone of cover vegetation) and must guarantee a nominal discharge of about 1 gallon/h (to mention a few from the specifications’ list).
Bioline®Dripline – advanced, self-cleaning, pressure compensating dripline, specifically designed for wastewater. Left – a cross-section of an emitter, right – color-coded tube (pink), Bottom – emitter (exterior view). Source: Netafim USA (Wastewater Division).
In contrast to all previously discussed effluent distribution systems (gravity, low pressure, pressure-dosed…), the drip-distribution system requires a recirculation return line to the dosing tank. It’s part of the self-cleaning process flushing eventual solids from tubing back to the tank. In colder climate zones, it also allows for fast evacuation of the effluent back to the tank, preventing freezing.
Example of the effluent drip distribution system. Source: “Subsurface Drip Disposal System”, Norweco (Ohio, USA).
Summarizing: Drip-distribution allows for a reduction of the drain-field size, minimizes the risk of biomat formation, eliminates the ponding effect (accumulation & surfacing of effluent), and significantly extends the drain-field’s lifespan. And, as important as mentioned above technical aspects, drip-distribution systems can be also deployed around trees, and do not disturb local landscapes (like mounds do).
Disadvantages of drip-distribution systems:
a. Drip-distribution systems require larger dosing tanks. They must be able to accommodate daily peak flows of household-generated wastewater. Note, that wastewater may suddenly flow into the dosing tank in large volume, while it is distributed on a 24/7—basis in strictly controlled doses that cannot be changed “on-demand”.
b. Due to the shallow subsurface distribution process, received effluent must have a much lower level of suspended solids than the typical Type-1 Septic System can deliver. That’s why in many countries, regulations require that the drip-distribution system is fed by the Type-2 Septic Systems (Aerobic Treatment Unit (ATU)).
c. The system uses small-diameter tubing (1/2”) with small pressure-reducing emitters (holes). To prevent their clogging, the dosing tank must be equipped with a good-quality filter (disk, screen, or sand filter). Of these – due to low maintenance requirements, the most popular are disc filters (they can be cleaned automatically, under the control of the system).
d. Drip-distribution systems are the most complex of all effluent distribution systems. It’s a combination of plumbing hardware (pump, drip emitter tubing, emitters, effluent and air valves, filters, fittings, flow meter, return lines…) as well as electronics & software for automatic control of the process.
e. Cost – However, despite the increased cost (initial and installation), in many cases, it may be the only feasible solution (poor quality soil, small lot…). And let’s face it – the traditional gravity-based systems are part of the past, drip-distribution (an in general pressure-distribution) systems are the future!
Sand Filters represent reduced versions of Mound-based Absorption Systems. In the same way as mounds, they use selected types of sand as the effluent’s treatment medium. They also need a “transition zone” at the base, to prevent the ponding effect. However, unlike mounds, their contours are clearly determined by concrete walls, so they have significantly reduced size and are not prone to side seepage.
In the majority of cases, sand filters are installed above the ground, however, they can be also built as underground units or in any configuration “in-between”. Sand filters are best suited for pressure distribution systems (low pressure or pressure-dosed). The gravity-based distribution system is also possible, although not suggested, because it will significantly increase the size of the filter, nullifying one of the major advantages of sand filters – small size.
The bottomless sand filter is a space-saving solution on problematic soils. Source: “Alternative Septic Systems for Vermont Wetland Challenges”, AJFoss, NH (USA)
An interesting variant of sand filters is a Recirculating Sand Filter System. In contrast to conventional sand filters, it is closed at the bottom so the purified effluent can be collected, evacuated, and discharged into a suitable area (in Type-3 septic systems even directly into nature).
Concept of the Recirculating Sand Filter. Source: “Types of Septic Systems”, EPA (USA)
Landscaping over a septic drain field
Due to potential health-hazards, drain-fields belong to the class of “limited-use” zones. It does not mean, however, that drain-field should look like an abandoned “no-man’s-land”. Just the opposite – for many reasons, drain-fields should be planted, and the only question is, which plants are suitable (and which not) for growing over the drain-fields.
Aesthetic reasons are probably the first coming to mind. In fact, the drain-field makes part of your lot and so, it should nicely fit into the overall concept of the landscape. For sure, if left “as-it-is”, Mother Nature will take things into her own hands, letting the drain-field to overgrown by weeds.
Source: “Planting on your septic systems, Landscaping ideas for your drain field and tank”, GroundStone (Canada)
But as important as aesthetics, are also practical reasons. Vegetation protects the soil from erosion as well as is part of the “water and soil treatment plant”. Plants’ roots are helping to aerate the topsoil, absorb some nutrients (including those carried by effluent, (if by chance they ended-up in upper layers of the soil), remove moisture, etc… In this sense, any shallow-roots vegetation adds an extra “safety layer” between the trenches soaked with harmful effluent and ongoing “life” at the surface.
Guidelines for selecting drain-field plants:
a. No threat to the integrity of the drain-field
Aggressive, deep-roots plants (trees, shrubs…) cannot be planted over and near the edge of the drain-field. Expanding roots, will destroy Soil Filter Fabrics, clog and/or damage pipes, change their geometry, etc..
b. Supportive for the operation of the Drain-field.
Most likely, the spectrum and concentration of chemical compounds in the layer of soil above the drain-field will be quite different from those characterizing the local native soil. We like it or not – migrating microorganisms, worms, naturally occurring capillary actions will bring to the surface some of the contaminants released with effluent. Some plants may have low a tolerance to increased levels of salinity or alkalinity, to nitrogen, etc… (typical for residential wastewater). Planting them over the drain-field may just add the problems. In contrast, some other plants may thrive in such a nutrition-rich environment.
“Crawling” plants with an extensive number of leaves (class of “groundcover species”) should be also avoided. The thick natural umbrella over the surface significantly decreases the evaporation of moisture from the soil. It also prevents the aeration of the soil and discharge of biogases (both actions necessary for proper operation of the biodegradation process).
Despite all these restrictions, the list of suitable plants is long (for details, please consult nurseries). However, the truth is that probably the best solution, meeting most of mentioned above requirements is lawn turf!
It’s strongly suggested to consult specialists for selection of suitable plants. Source: “Septic Safe Plants & Landscaping”, Advanced Septic Services (FL, USA)
c. Low maintenance
From the aesthetic point of view, eye-pleasing flowers will have many supporters. You should keep in mind, however, that when working with plants over the drain-field, you must ALWAYS wear protecting gloves (anyhow, it is a good gardening practice). It is not just the probability, it’s the fact: Drain-field soil may contain much more pathogens and toxins than any other gardening soil. It does seem obvious then, that by choosing low-maintenance plants for drain-fields, you will minimize the danger of exposure to “contaminated” soil. Another, maybe not that obvious benefit – any drain-field is a fragile area. Minimizing traffic over it can only help to keep it in good working shape.
d. No veggies or fruits
Just for the completeness of the subject: Drain-fields should never be considered for planting veggies and fruit plants. In fact, in most countries, drain-fields (and areas along their perimeters) are banned from veggie and fruit gardening!
To find more information relevant to onsite Septic Systems, please select:
- Intro to Septic Systems
- Household-Generated Wastewater
- Septic Tanks
- Types of Septic Systems
- Greywater Disposal Systems
- Dry Toilets
- Intro to Effluent Disposal Systems
- Conventional Effluent Disposal Systems