All Conventional Effluent Disposal Systems consist of Distribution Systems and Absorption Systems.
The Distribution Systems receive partially treated effluent from the Septic Tank(s), then convey it and distribute (spread) over the Absorption System. They may function based on forces of gravity or by the use of sewage pumps. Gravity-based systems are the least expensive and so the most popular. However, due to the inherently uneven distribution of effluent, they negatively affect the performance of the whole septic system.
The pump-driven distribution systems come in few variations, of which the most popular are pressure-dosed and drip-distribution systems. By controlling pressure, timing, and doses, they offer excellent uniformity of effluent’s disposal, and greatly contribute to the overall high performance of the Septic System.
The Absorption System (drain-field) receives distributed over its area effluent, absorbs it, and hopefully finish the treatment process by fully decomposing organic matter, breaking chemical compounds, and killing or neutralizing pathogens and microbes… The most popular versions of Absorption Systems are absorption beds, absorption trenches, and chambers. They can be built as Subsurface Absorption Systems (the most popular) or as Raised Absorption Systems (Mounds, Sand-Filters…).
Shortly: All gravity-based distribution systems bring a lot of randomness into the operation of the Absorption Fields. The pressure-based systems offer flexibility, predictability, and consistency!
Concept of the Effluent Disposal System. Source: “System Maintenance” – A&B Septic Service (NY, USA)
In practice, it is difficult to cut a clean line between Distribution and Absorption systems as they are interconnected. We start with the presentation of the most popular Effluent Disposal System combining gravitational distribution and “pipe-in-gravel” absorption fields (in both – trench and bed versions).
Conventional Effluent Disposal Systems
The effluent flows from the Septic Tank to the network of perforated PVC pipes. To guarantee good performance and longevity of the drain-filed, the system must be able to evenly distribute the effluent across the network of branching-out pipes. Otherwise, overloaded sections of the drain-field may get clogged, which will prematurely put them out of service.
In most conventional Septic Systems, the downstream flow of the effluent is governed by forces of gravity. In such a case, an important role plays the geometry of the distribution system – especially at the crucial splitting point where the single pipe carrying the effluent branches-out into few parallel sections covering the drain-field. This task is accomplished by the Distribution Box.
Distribution Box (D-Box)
The Distribution Box is used to evenly split the flowing effluent into the network of perforated drain-field pipes. While at first, it may look that the design and installation of the DB is an easy task, the reality is quite different.
Distribution Box. Source: “Distribution Box Replacement and Repair”, Northboro Septic Service Inc (MA, USA)
To guarantee the even distribution of effluent, the following conditions are necessary:
a. D-Box must be securely leveled
The distribution box cannot be just buried in the soil. Ideally, it should be placed on a level (onsite poured) concrete slab. However, to fully eliminate the impact of movements of soil, the slab (or D-Box directly) must be placed on a layer of compacted gravel (at least 1 ft). As being highly permeable, it will prevent the accumulation of water under the box, so at temperatures below the freezing point, the ground would not swell and warp, de-leveling this way the Distribution Box.
For the same reasons, the D-Box must be in the “no-traffic” zone!
b. All outlets must be located at the same level
In gravitation-based systems, the flow of effluent follows the path of “lowest level”. That is why in modules with several outflow paths (outlets), it is crucial to keep the best possible symmetry between them.
c. Inlet must be located about 2” higher than outlets
Modern, resin-based Active Even Flow Distribution Box for Gravity Septic Systems (Septi Surge – model 3A). The visible Tub Liner helps to “equalize” the outflow of effluent. It also acts as a “trap” preventing (or minimizing) the carry-over of solids (if any). Source: SeptiSurge.
d. Effluent flow across the D-Box must be laminar,
If the slope of the supply pipe is too steep, the effluent flow will be turbulent what may have an impact on the uniformity of the effluent distribution between outlets. To minimize this effect, the baffle acting as a “speed leveler” may be necessary at the inlet side.
Concrete D-Box w/baffle wall. Source: Jefferson Concrete Corp (NY, USA)
e. D-Box must be accessible for maintenance,
Typically, the D-Box should be located at least 1 ft below the ground level (in gravity-based septic systems, it must be located deeper than the outlet of the last tank). Underground location protects the D-Box from freezing, however, it makes access for periodic inspection and cleaning more difficult.
Note, that due to its underground location, the D-Box will need some protection (riser) preventing the soil and debris to fall inside when it is serviced.
As it can be seen from the pictures, Distribution Boxes can be made from concrete (either precast or onsite poured), plastic, or fiberglass.
Sometimes, D-Boxes are equipped with (or replaced by) Diverter Valve, which is used to alternate the flow of effluent to the selected section of the drain-field. The main idea behind this is to let the remaining sections of the drain-field to “regenerate” their abilities to absorb and process the effluent.
The benefits of using Diverting Valve are indisputable. Any tilt of the D-Box or partial clogging of an individual outlet (what may happen with time) will affect the split-ratio of effluent between drain-field sections leading to a hydraulic overload of some and cut-off of others. While the automatic control of Diverting Valve will certainly make the operation of the system much easier, with typical lengths of alternating cycles of about 1 week, the system can be operated manually. Little pain, but big gain – it has been proved that the Diverting Valve can substantially extend the lifespan of the drain-field!
Left: Bull Run Valve™ (2-Sections Diverting Valve), Right: practical, field-installed system with a riser-tube and valve-handle mechanism. Source: American Manufacturing Company
Connecting the 2nd valve to one of the main valve’s outlets will allow splitting the flow of effluent between 3 individual drain-field sections. Connecting extra valves at each outlet of the main valve will allow to individually operate each of 4 separate drain-field sections.
Note, that there are semi-automatic versions of Diverting Valves switched by effluent’s flow, however, to make it work, the effluent must apply a specified pressure on the valve, which is only possible in a system using pumps. Electrical versions of Diverting Valves are much more practical, however, due to needed electrical energy, they hardly fit into the concept of passive (gravitational) systems.
Below, we will focus only on solutions suitable for onsite residential septic systems, leaving behind Cesspools (now mostly illegal), Leach-pits (mainly used for greywater) as well as larger-scale multi-user systems using Lagoons or Wetlands.
Drain-fields (also known as Leach-Beds or Soaking Beds) belong to the class of conventional, horizontal dispersal systems. These relatively simple and certainly long-lasting (compared to vertical dispersal systems like cesspits) systems, come with one major inconvenience. Each drain-field takes a large area of your lot (which is already a problem), and subsequently, turns it into a zone of “limited use”.
Due to health-hazards, the effluent cannot be dispersed on the surface of the soil. That is why all drain-fields use subsurface zones for disposal of effluent. In practical implementation, trenches are dug along the drain-field, and then (assuming a favorable native soil) – they are filled with layers of gravel surrounding perforated PVC pipes. Such drain-fields are often known as “Pipe-in-Gravel” or “Pipe-in Stone”.
Concept of the traditional “Pipe-in-gravel” effluent distribution system. Source: “How does a Conventional “Leach Line” Work”; (JT’s Septic, AZ (USA)
“Pipe-in-Gravel”-type trench field. Source: “Effluent flux prediction in variably saturated soil zones within a septic tank—soil absorption trench” by Cara Beal and all; Research Gate
Gravel is the most common material deployed around the pipes. Itself hardly permeable, it creates a high percolation zone preventing the accumulation of effluent in the proximity of pipes. Additionally, with countless airgaps between stones, the gravel-zone acts as an “oxygen storage” feeding aerobic bacteria. This way, it minimizes the probability of clogging and forming adjacent to pipe biomat. Note, that, for that purpose, also other materials, like Styrofoam peanuts, peat moss, etc… can be used.
Note: Biomat (Biological Mat) is a layer of soil with a very high concentration of living and dead anaerobic bacteria, partially decomposed organic solids, and by-products of the biodegradation process. Due to its clayish, sticky texture, it has poor absorption characteristics, creating infiltration barriers. Biomat mostly grows in zones where higher percolation soil changes to lower percolation one. The classic example is the transition zone between the gravel and the receiving native soil.
Conventional drain-field with individual tranches and distribution pipes connected to the DB. Source: “The Building Science of Septic Drain Fields”; Trent Hills Septic Service, (Ontario, Canada)
Bed-type drain-field in a large single trench with interconnected distribution pipes. Source: Author – Nonztp, Wikipedia,
Perforated PVC pipes are concluding components of the effluent distribution system. They mark the point where the household generated wastewater is released back to nature, sort of “Point of no return”, where we hand over the full control over effluent to Mother Nature. No wonder that this “Interface” between man-made system and Nature is highly regulated by Environment Protection Agencies, Health, and Local Authorities.
As it could be expected – relevant regulations governing the design and execution field works will largely vary between states and countries. There seems to be a common agreement that distribution pipes should have a diameter of 4 inches and ½ inch drain-holes. For almost everything else there are too many discrepancies so we can provide just the general guidelines:
- The drainage holes should be distributed in an “8-6-4” o’clock pattern with “6” at the bottom of the installed pipe. Note the difference, compared to traditional water drainage pipes with holes all over its perimeter. However, they are designed to collect and evacuate rainwater from the surrounding soil, while the effluent distribution pipes must do just the opposite – dispose of the inflowing liquid into the surrounding soil.
- The length of each distribution pipe (perforated section) should be no longer than 66 feet, however, for best performance, it is suggested to not exceed 40 ft run. It’s because in gravity-based systems if the pipe is too long, it’s far-end section will rarely (if ever) receive any effluent.
- The slope (gradient) of distribution pipes must be well-controlled and not exceed values imposed by local regulations. Unfortunately, there is no common understanding as to what the best geometry is. Some call for level-configuration, but you will also find suggestions to keep a 1:200 slope (about 1/16 of inch/feet) or even a 1:100 slope (about 1/8 of inch/feet).
To shed some light – the too small slope of the distribution pipe (or level), may prevent the propagation of effluent down the pipe. As a result, effluent will be discharged along its initial run or in “locally-warped” sections. In either case, it may lead to hydraulic overloading of some sections of the drain-field, leaving others inactive. On the other side, with a too steep slope, most of the effluent will reach the ending section of the pipe, causing the same result as above. Unfortunately, it’s the general problem of gravity-based distribution systems.
The bottom line is: More evenly effluent is spread over the area of the drain-field, longer is its lifespan, and higher the level of purification of the effluent.
- The width of trenches, the horizontal distance between the walls of two parallel trenches (or between distribution pipes in a bed-type drain-field) must comply with local requirements. While relevant regulations will vary depending on the type of soil, climate zone, etc…, just as an idea – the typical numbers may be (correspondingly) 3-to-5ft, 10ft, and 6ft.
- Local legislations may call for the installation of Monitoring and Cleanout standpipes. The monitoring standpipe is detached from the system of distribution pipes. Perforated in its subsurface section, it must reach the bottom of the trench. In contrast, the solid (not perforated) cleanout standpipes are connected to the network of distribution pipes. These vertical, capped pipes, extending above the ground may be necessary for fragile (arctic) and marginal zones.
Drain-field with Monitoring and Cleanout standpipes. Source: “Design Specifications for Sewage Disposal Systems”, based on Septic Systems in the Yukon (Canada)
Typically, the drain-field is covered by a layer of native soil and grass (no trees or shrubs are allowed because their roots may destroy the integrity of the system). It creates the “safety” buffer between subsurface effluent’s disposal zone and the exposure to humans and pets – landscape. For these reasons, its depth is regulated by law – usually at least 1ft of soil as a safety barrier, but no more than 2ft to keep the treatment zone within the reach of natural aeration. In gravity-based systems, the depth of the top layer of soil may be determined by the level of D-Box outlets and the required slope of distribution pipes. Also, in cold climate zones, a deeper layer of topsoil may be necessary to isolate pipes as well as absorption zone from freezing temperatures (if deeper than 2 ft, forced aeration may be required).
Unfortunately, without any physical barrier in between, the structure of topsoil over gravel is not sustainable. With rainwater or melting snow, topsoil will migrate towards the gravel zone, filling (clogging) free space between stones. Gravel mixed with soil will be continually losing its high permeability, starting instead trapping solids and contaminants. The natural process of aeration will be drastically reduced, which will kill aerobic bacteria and leave the effluent treatment at the mercy of anaerobic ones. This, in turn, will lead to the build-up of biomass creating an almost impermeable zone surrounding distribution pipes. As a result – the distributed effluent would not be able to infiltrate lower layers of the native soil, effectively, stopping the operation of the whole Septic System! To prevent this from happening, tops and sides of trenches (or whole bed) must be covered by a physical barrier (sort of coffee filter), preventing migration of topsoil, sediments, silt, and any other fine particles down into the gravel drainage zone.
The suitable Soil Filter Fabrics must be:
The biodegradation process crucial for the treatment of effluent by the drain-field soil largely depends on aerobic bacteria. To keep them active and alive, the “treatment-zone” of the drain-field must be continuously aerated, while generated by bacteria biogases (CO2) evacuated. Usually, these two processes happen naturally, but to keep it this way, the Soil Filter Fabrics must be breathable.
Poor-quality soil barrier will considerably limit the lifespan of the drain-field. That is why, it is strongly suggested to use professional-grade, heavy-duty, woven, geotextile fabrics specifically designed for septic drain-fields. Note, that most weed-barriers used for landscaping, will sooner rather than later rot, and decompose. On the other side, more durable (but also not-breathable) tarp-type plastic sheets will suffocate aerobic bacteria due to lack of oxygen and high concentration of accumulated carbon dioxide.
The replacement of the Soil Filter Fabrics is a costly business!
Example of propylene-based, woven, geotextile fabrics.
Vertical cross-section of the “Pipe-in-stone” drain-field (all measurements in millimeters). Source: JD Pipes (UK)
Note, that a non-woven geotextile (as shown in the above picture) has lower strength compared to a woven one, however, it also has lower water permeability what may be beneficial in wet climate zones, as it will reduce the saturation of soil by rainwater.
At the low level of contamination, characterizing already pre-treated effluent, the aerobic treatment is the solution of choice. The decomposition process is faster, it produces “cleaner” effluent, and lower content of biogases compared to what can be achieved by the anaerobic treatment.
The crucial requirement for the aerobic process is good aeration (presence of oxygen and evacuation of biogases). Unfortunately, in conventional “pipe-in-gravel” systems, the aeration of subsurface soil is left to forces (or rather caprices) of nature. The truth is, that only topsoil is rich in oxygen, and microorganisms, but it only marginally takes part in the overall effluent’s treatment process. Deeper we go, lover the level of oxygen and an anaerobic treatment process gradually takes over.
The lack of efficient aeration in conventional drain-fields was addressed by Chamber technology (also known as Vault). In these effluent disposal systems, “pipe-in-stone” trenches (or beds) are replaced by series of arched plastic chambers (vaults). Like in traditional systems, supply pipes from the D-Box deliver effluent into the distribution area, but here the similarity ends. Instead of entering perforated pipes, effluent is released into the “tunnel” created by connected in series arched chambers. The bottom part of the tunnel is open, so the effluent comes into direct contact with the native soil.
Depending on the quality of soil, a layer of coarse sand may be placed on the native soil before the installation of chambers. It will also help to level the base and by that, improve the performance of the whole septic system.
What makes the difference?
In “pipe-in-stones” systems, effluent is released into the thick layer of stones with gaps filled with air. Unfortunately, the gravel is not directly exposed to air, which considerably slows the process of natural aeration. The lack of oxygen and rising concentration of CO2 (due to decomposition of solids) will suffocate aerobic bacteria.
In chamber-based systems, effluent is released into tunnels filled with air. Large, open space under the vaults is readily infiltrated by air with the help of ventilation standpipes. This creates natural conditions for aerobic bacteria to thrive. No wonder, that chamber-based systems perform much better than conventional pipe-in-stones ones.
Concept of the Chamber-type wastewater distribution system. Source: “Types of Septic Systems”, EPA (USA)
Concept of the Chamber-type wastewater distribution system (here, for simplicity of graphical presentation, the D-Box is omitted). Source: New Brunswick Technical Guidelines for On-site Sewage Disposal Systems (NB Gov, Canada)
The series of Infiltrator Chambers is a good example of the practical implementation of this technology. They offer superior mechanical strength, quick-latching mechanism, lateral flexibility (allowing for up to 10 degrees left/right turns) to avoid potential “obstacles”…
The Quick4 Plus™ Standard chamber (size – 34″W x 53″L x 12″H). Source: Infiltrator Water Technologies, (CT, USA)
Side openings (visible along the bottom sections of the chamber’s structure) expand the leaching area to both sides of the tunnel, allowing the system to handle the instant inflow of a larger volume of effluent. However, thanks to their louver-design matching the angle of the surrounding soil, chambers are protected from infiltration of soil, so unlike “pipe-in-gravel” systems, they do not need any geotextile protection.
Note, that in some states/countries, side openings are mandatory, (and their dimensions are specified by local regulations).
The benefits of the chamber technology are:
a. Good natural aeration (often forced by extra standpipe vents).
An abundance of oxygen creates favorable living and “working” conditions for aerobic bacteria and so, a sort of guarantee for the efficient operation of the drain-field.
b. No need for gravel
c. Gravel itself, may not cost too much, but the transport may!
d. Strong physical structure
In contrast to “pipe-in-stone” drain fields, chamber-based ones can tolerate much heavier surface loads (traffic).
e. Elimination of clogging of perforated pipes
Perforations (holes) are prone to clogging by suspended in effluent solids (oils, grease…). In other words – it’s the gain without proverbial pain.
f. No need for Soil Filter Fabrics
Plastic chambers (vaults) create a strong physical barrier for any sediments, soil, silt that may migrate with rainwater. As a result – the active area under the plastic vaults making “Effluent Treatment Plant”, can stay undisturbed by external factors.
Note, that in wet climate zones, water-impermeable barriers (sort of tarp fabrics) are often deployed just below the layer of topsoil to divert rainwater away from the effluent disposal zone.
And the best is:
g. Reduced size of the drain-field.
Well, it’s not guaranteed, the last words always belong to the Local Authority. However, due to the higher efficiency of effluent treatment by chamber-based systems, usually 25% -to-50% reduction of the drain-field size can be expected compared to that needed by “pipe-in-gravel” systems.
Nothing is free, so all these advantages have their price – in this case, a higher cost of Chamber Distribution systems compared to Pipe-in-gravel ones.
Comparison of the Chamber (left) and Traditional “Pipe-in-Gravel” effluent distribution systems. Source: The Natural Home (AskingLot.com)
Typically, both described above – “Pipe-in-gravel” and Chamber effluent disposal systems are implemented as gravity-based subsurface systems. Both can be deployed in individual trenches or over common beds, have similar limits for the maximum length of the run and dept of the covering layer of soil. Being the most economical, they are also the most popular onsite residential Septic Systems.
Unfortunately, our Mother Nature is not always so favorable. Poor quality soil (clay or sand), shallow bedrock and/or water table, unfavorable local topography (steep slopes), too small lot…. and certainly, few other limitations that will be pointed out by a Licensed Specialist may give us no choice but to accept more complex and expensive solutions like Raised Drain-fields, Pressure-dosed, and Drip-distribution systems.
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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