Leaching Beds (Drain-fields)
The Drain-field is a crucial element of the onsite Septic System. It’s not just an area used for the disposal of partially purified wastewater (effluent). It is the final stage of the Wastewater Treatment System, the last one between the still “dirty” outflow from the Septic Tank and the protected, hopefully pristine-clear, groundwater aquifers. In other words, the main task of the drain-field is not just to absorb released effluent to prevent its accumulation on the surface), but to fully decontaminate it on its way down (percolation) before it has a chance to reach the underground water table.
Most popular, “pipe-in-gravel” type, gravity-based Effluent Disposal System. Source: “Septic Systems – Septic Tank/Leach Fields”, Chamberlain Septic and Sewer, (NY, USA)
The effectiveness of a water purification filter depends on its ability to trap contained in the water solids and dissolved contaminants. Most of us are familiar with filters used at home for the purification of drinking water. Based on active carbon, they are designed to trap dissolved minerals (mostly calcium & magnesium), as well as chlorine, traditionally used by the Municipal Water Treatment Plant for water’s disinfection. To be up to the task, they have engineered structure (density, the porousness of the carbon, etc…), suitable physical dimensions (length of the filtration path), and volume (guaranteeing reasonable lifespan). As we all know – these are passive, limited “capacity” filters that after processing the specified amount of water will lose their effectiveness and will have to be replaced.
Drain-field’s soil acts as a massive, natural (waste)water filter. In contrast to mentioned above carbon filters that are carefully engineered for specific tasks (trapping minerals and chlorine), the drain field’s soil is made by the “hands of nature”, and to make it worse – as the “treatment plant”, it also faces a much wider spectrum of contaminants, toxins and pathogens that must be broken-down, decomposed, removed, neutralized, destroyed… (whatever applies).
Effluent leaving septic tanks carries mostly invisible to naked eye contaminants in form of complex chemical compounds. Now, it is all up to Mother Nature (soil) to deal with them before the effluent discharges to underground water, closing this way the perpetual cycle. Source: UABDivulga (Barcelona Research and Innovation).
Soil is a critical part of the endless recirculation cycle of organic matter. Source: Loop For Your Soil (King County Wastewater Division – USA)
In short words, the natural drain-field:
a. Is “as-it-is”
The local soil can be composed of everything – starting from highly permeable sand through favorable soil with good-enough texture to impermeable clay. And, if you have bad luck, note, that “re-engineering” the local soil to improve its effectiveness of effluent processing is not an easy task ($).
b. Once used, it cannot be easily replaced by a new one
It is a costly process; in practice, it may be easier to relocate the drain-field to another location (provided, you have one suitable for that purpose within the borders of your property).
c. Must purify heavily polluted water (effluent)
Effluent, released from the septic tank contains suspended bits of organic matter, chemical compounds (often toxic), harmful pathogens, microbes… (you name it). What makes it worse – its level of contamination largely varies depending on seasonal temperatures and household “activities” (changes in the volume of generated sewage, the concentration of solids and disinfectants, toxicity….).
Sources of household-generated wastewater. Source: FAQ – “What are greywater sources”, (jalsevak.in)
However, despite so poorly controlled working conditions, we expect the drain-field to have:
d. Long lifespan
Under normal circumstances, the well-designed and maintained onsite Septic System (tanks, pipes, supporting hardware, drain-field…) should last for at least 30 years.
This may sound discouraging, fortunately, there is also good news:
e. Drain-field is a “multifunctional”, active (living) filter.
Mother Nature is very “resourceful”, so in most cases, the soil can act as a multifunctional filter providing physical, biological, and chemical treatments (by correspondingly – trapping, decomposing, and oxidizing contaminants).
In oxygen and nutrient-rich layers of top-soil, microorganisms, fungi, worms, plants, etc… are continuously regenerating soil’s “filtering” properties. Thanks to them, if not overloaded by the volume of effluent as well as the level of solids, contaminants, and toxins, the drain-field can keep its filtering abilities almost forever. In an endless “Life-death” cycle, dead microorganisms, worms, plants … are replaced by new ones keeping the natural filter active (unless we, humans, destroy this fragile environment by our “careless” actions!).
Preliminary Soil Assessment
Three (in many countries mandatory) steps should be undertaken, before starting the process of evaluation of the soil for the future drain-field. These are:
a. Certification of the Environmental Zone
It is the certification issued by the Local Authority, confirming that your site is Not located in the Groundwater Source Protection Zone. If that’s NOT the case, most likely you will have to put on the shelf your plan for the onsite Septic System. Note, however, that it is a highly unlikely situation, because in such a case, in the first place, you wouldn’t have the rights to build the house on your lot!
An example from Canada. Source: Urbanworkbench (Flickr.com)
b. Depth of the suitable soil
The suitable drain-field soil must have some minimum depth to guarantee sufficient purification of the effluent before it reaches limiting factors like impermeable bedrock, highly permeable sand, gravel, or groundwater table. The exact limit may vary depending on local regulations. In a first approximation, however, we can assume that at least 3ft (about 1m) distance from the surface to the limiting factor(s) will be required to qualify the land for construction of a traditional subsurface drain-field.
Of all mentioned limitations, the shallow water table is the most serious problem as it leads to direct pollution of potentially potable water used by nearby wells. The depth of the groundwater table is determined by the Trial Site Assessment Test (TSAT). It calls for drilling a deep test hole proving, that the groundwater table is below the minimum required level.
Any of the mentioned soil conditions may disqualify the projected area for use as a drain-field. Note, however, that installing a Type-2 or Type-3 septic tanks system and/or “re-engineering” the drain-field by adding artificial sand mounds, may improve the decontamination of effluent to such a level that the whole onsite Septic System will meet requirements imposed by EPA and Local Authority. Additional help in this direction may also offer more sophisticated pressure-distribution systems.
c. Location out of flooding zone
Sites located in flooding zones, cannot be used for drain-fields. Note, that such zones are identified by local administrations (and insurance companies ($$$)), so obtaining such a document is an easy task.
In flooding zone, onsite septic systems are banned. Source: “Is My Property in a Flood Zone? – The Easiest Way to Determine If You’re at Risk” by Margaret Heidenry (Realtor).
Armed with these three official documents you can go to the next step:
Evaluation of the soil
Drain-field soil plays a crucial role in the design of the whole onsite Septic System. Its ability to absorb the projected volume of effluent and filter its contaminations will have a direct impact on the necessary system of septic tanks as well as the effluent distribution system. In other words, the drain-field determines if a conventional (and lowest-cost) Type-1 Septic Tanks System will be good enough, or you will need more complex and costly Type-2 or Type 3 systems to meet local environmental requirements.
That’s why the first step of the onsite Septic System design is the evaluation of the soil in the area projected for the future Drain-field (leaching-bed).
1. Percolation Test (Perk Test)
The percolation test determines the water absorption rate (in other words, how quickly water can infiltrate the soil). The Percolation Rate is defined as:
Percolation Rate (PR) = (Volume of water)/ (infiltration time, per specified area] and is expressed in [gallons/(minute] per square feet.
In practice, several holes (typically 5 to 7 ft deep) must be drilled in the soil over the area intended for the drain-field. Holes must be cleaned from debris, presoaked, and then filled with water. The percolation rate is calculated based on the time necessary for the specified volume of water to percolate into the surrounding soil. The number of test holes, their diameter, depth, locations, testing period (up to 24 hours), and frankly several other details that must be respected to make the test valid) are regulated by Local Authorities based on the size of the drain-field, geology of the terrain, type of the soil, environmental requirements…
Field measurements of the percolation rate. Source: “Percolation Test- Soil Absorption Capacity”, The Constructor (Civil Engineering Home).
Given the standardized diameter of the test hole and the testing process, usually, the PR is expressed in Inches/minute. It represents the drop of water level in the standardized-diameter hole measured in inches/time.
In many countries, on top of the detailed step-by-step PR test procedure, it is also required that the testing must be done during the wet season. The main reason is that the higher level of moisture in the soil and a possibly higher level of the water table will slow down effluent’s absorption. As a result, marginal drain-fields meeting relevant requirements in dry seasons may be not able to operate safely in wet seasons.
Note that there is an optimum range for the percolation rates, making the soil suitable for a drain-field. Too low PR corresponds to soils with a high content of clay or layers of bedrock, unable to absorb effluent. Too high PR rates correspond to sandy soil, gravel, or karstic rocks that will literally suck the water. Unfortunately, high PR rate signals lose soil’s texture, and so, its inability to trap, hold, and process contaminants.
2. Soil Profile Test
It’s a more detailed test usually followed by the PR test. It requires excavation of a larger pit exposing the vertical profile of the soil for its visual examination and eventual laboratory evaluation of samples. The location and depth of the pit are determined by the Licensed Specialist, who will then, proceed with the examination of the geomorphology of the soil, its texture, porosity, color, moisture, chemical composition (may need lab tests), and other factors that may be important if the soil only marginally meets relevant requirements.
Typical, soil profile with 5 layers (horizons). Source: Author, Wilsonbiggs (Wikipedia)
In some countries, the Soil Profile Test is mandatory as it reveals more information than the simple PR test. Note, that the performance of the drain-field largely depends on the depth of the “receiving soil” (soil representing the “treatment zone”), and that can be revealed only by examining the vertical profile of the soil.
Both, the Soil Profile Test, and the Percolation Test are quite-well standardized. However, in the field, you will face plenty of personal decisions made by the Licensed Specialist running the test. At first, it may be confusing. Keep in mind, however, that Licensed Specialists have much-needed experience and knowledge to quantify the impact of the dry or wet season on the test results as well as what will satisfy relevant local requirements!
Once the soil is tested, the Licensed Specialist should propose the optimal solution for the type of the Septic Tanks, drain-field (subsurface, raised, “pipe-in-stone”, chamber, gravitational, pressure-dosed, etc….), and calculate its size. The latter is based on an empirical formula considering the following factors:
- Volume of generated sewage
- Type of the Septic System
- Properties of the soil (based on PR and/or Soil Profile) tests
- Type of the proposed drain-field
- Type of the distribution system
- Climate zone
As an example, the estimated area for the drain-field can be calculated as follows:
S [sq. ft] = k x AVS/ EPR
S – “active” disposal area of the drain-field (sometimes called “contact area”). Note that the physical size (perimeter) of the drain-filed will be larger.
AVS – Average Volume of Sewage [gallons/day] = Number of persons x average interior water use per person per day
EPR – Effluent Percolation Rate [gallons/sq. ft per day]
K – “empirical coefficient” reflecting the selected type of Septic Tank System, Drain-field, and Distribution System as well as usually difficult to quantify factors like geological and topographical features, climate zone, local practices ….
To give you some idea, the table below shows Maximum Hydraulic Loads for given types of soil as a function of the performance of the Septic System. For those unfamiliar, the type of the Septic System determines the level of purification of the outflowing effluent. Type-1 (Anaerobic) provides the lowest level of treatment (dirtiest effluent), Type-2 (Type-1 followed by an Aerobic stage) provides a higher level of purification while Type-3 (Type-2 followed by a Disinfection stage) neutralizes pathogens, microbes….
For more details see: Types of Septic Systems
Maximum Hydraulic load for various soils as a function of the type of the Septic System. Source: Modified from the original version in “Septic System Design”, GroundStone, BC (Canada)
To continue, please select:
- Conventional Effluent Distribution Systems
- Unconventional Effluent Distribution Systems
To find more information relevant to the onsite Septic System, please select:
- Intro to Septic Systems
- Household-Generated Wastewater
- Septic Tanks
- Types of Septic Systems
- Greywater Disposal Systems
- Dry Toilets