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OWDP Home > Online Resources > Conventional Drainfields

• Process Description
• Design Criteria
  • Configuration
    • Trenches
    • Beds
    • Seepage pits
  • Sizing
  • Effluent Distribution
    • Parallel
    • Serial
    • Continual

Please note:

1. This module may take up to 113 seconds to load.
2. This module conventional drainfields for land application of pretreated onsite wastewater. A more in-depth discussion on soils and site evaluation can be found in module "Soil & Site Evaluation and Characterization."
3. This information was adapted from the following sources by Kitt Farrell-Poe:
   1. Hoover, M.T., T.A. Disy, M.A. Pfeiffer, N. Dudley, R.B. Mayer, and B. Buffington. 1996. North Carolina Subsurface Wastewater System Operators Training School Manual. Soil Science Department, College of Agriculture and Life Sciences, North Carolina State University, Raleigh, NC and North Carolina Department of Environment, Health, and Natural Resources, Raleigh, NC.
   2. National Association of Wastewater Transporters, Inc. Introduction to Proper Onsite Sewage Treatment. St. Paul, MN.

The conventional drainfield consists of perforated pipes surrounded by media such as gravel or sand, covered with a fairly shallow layer of topsoil, and planted to grass. Effluent passes through the pipe and is stored by the gravel until it can be absorbed into the soil. The microbes, decomposing the waste, form a biological mat at the surface of the soil and the bottom of the drainfield. Microbes in the soil and mat feed on the waste, nutrients, and pathogens remaining in the wastewater.

Design criteria for conventional drainfields include: configuration, sizing, effluent distribution, and minimum vertical separation.

Configuration

Drainfields can be configured as trenches, beds, or seepage pits. The trenches are longer than they are wide, beds are wider than trenches, and seepage pits are deeper than they are wide. However, whatever configuration, they should be relatively shallow to maximize treatment of the waste.

Trenches
The trench is the most common of the natural soil treatment systems. Trenches are narrower than they are wide, typically no wider than three feet, and laid out along the contours of the soil. The method of distributing the pretreated wastewater can be either pressure or gravity. There are a number of different arrangements by which the trenches can be connected with each other and with the septic tank: parallel, serial, and continual. In Arizona, the designer also has the choice of using either a shallow or deep trench.

A typical trench is constructed by placing a one-foot-thick layer of washed gravel or stone in the bottom and placing the distribution pipe on top of the gravel. Then a layer of straw, untreated building paper, or a synthetic geotextile fabric is placed on top of the gravel aggregate. The soil material dug out of the trenches is then used to finish filling the trenches and cover the system. A typical drainfield consists of two to five trenches that are three feet wide by two to ten feet deep which are on nine-foot centers (however, in Arizona, by code the center-to-center distance is 10 feet). Trenches are only effective if the sidewalls do not interfere with each other in transferring oxygen. If the center-to-center distance is reduced too much, the trenches would behave like a bed. [Note: source of figure is National Association of Wastewater Transporters, Inc.]

Beds
A bed system is a wide area (wider than five feet) prepared to accept septic tank effluent, created below the surface of the soil, and built the same way as a trench system. They treat the effluent effectively as long as they are located in appropriate soils. Separation from groundwater is an important design factor.

Beds are more prone to failure than trenches. One reason is from reduced oxygen transfer. The sidewalls are sometimes too far apart to provide sufficient oxygen for the entire excavation and the biomat will become thicker. The thicker the biomat, the more slowly the water will leave the system. Another reason is the reduced sidewall surface area available for biological growth. Therefore, sizing of beds is critical, and designers should size beds with greater surface area than trenches receiving the same flow.

Alternatively, a bed system could use pressure distribution to apply the effluent to the soil. This would allow for better transfer of oxygen and would not require an increase in size over a trench system.

Seepage pits
An alternative to the common drainfield is the seepage or disposal pit. In this type of soil absorption unit, septic tank effluent flows to a vertically-oriented pre-cast pipe with sidewall holes, the whole of which is surrounded by gravel. The effluent seeps through the holes to the surrounding soil. Because of the vertical orientation, this alternative is often used when there is not enough surface area to accommodate a standard disposal trench.

In Arizona, seepage pits are regulated to be a concrete-covered circular pit with an excavated diameter of 4 to 6 feet, unless an alternative design is accepted by ADEQ. The seepage pit may either be gravel-filled or hollow-lined. A gravel filled pit must be entirely filled with aggregate with a breather conductor pipe which is a perforated pipe at least 4 inches in diameter placed vertically in the backfill. The breather pipe needs to extend from the bottom of the pit to within 12 inches below ground level. A hollow-lined pit is constructed of either a concrete liner or some other approved material. The lining must be placed on a firm foundation and excavation voids behind the liner must be filled with at least 9 inches of aggregate.

Sizing

The size of all the systems is based on the flow of wastewater and the soil. All soil has a set capacity for accepting the waste, which depends on the soil texture and structure, and also on the strength of the waste. In Arizona, the pretreated effluent quality must meet the following criteria:

• TSS = 75 mg/L, 30-day arithmetic mean;
• BOD₅ = 150 mg/L, 30-day arithmetic mean;
• total nitrogen (as nitrogen) = 53 mg/L, 5-month arithmetic mean; and
• total coliform level = 100,000,000 (log ₁₀ 8) cfu/100 mL, 95th percentile

The greater the waste strength, the larger the system must be. This is true for all system types, and although each type of system introduces water into the soil differently, sizing for the system you choose is critical. Some new devices are claimed to significantly reduce system size, but at some point the soil will not accept any more wastewater, causing failure.

The sizing of trench systems can be looked at in two ways: bottom area and sidewall area. Using the bottom area means that no credit is given for the sides of the system. This method provides the biggest safety factor. For example, a trench with six inches of sidewall surface on each side provides an extra square foot of absorption area available for each linear foot of trench length (0.5 ft high x 1 ft long x 2 sides = 1 sq ft). This amounts to 25 percent more infiltrative surface that is not accounted for in the design code. Using both the sidewall and the bottom for sizing allows less area to be used. You will note that effluent will move through the soil sidewalls as well as the bottom, no matter how the size of the system is calculated. [Note: source of figure at right is National Association of Wastewater Transporters, Inc.]

In Arizona, when computing the trench-bottom absorption, you may include a trench sidewall area between 12 and 36 inches below the distribution line [note: rule clarification #9]. Thus, up to 9 square feet of sidewall area per linear foot of trench (up to 3 ft² for each sidewall and 3 ft² for the trench bottom) may be used in the design without submitting an alternative design. A designer may propose an alternative design, in areas where seasonal saturation of surface soils does not occur, that includes up to a nominal 11 ft² of absorption area per linear foot of trench (up to 3 ft² for the trench bottom and a nominal of 4 ft² for each sidewall). For such an alternative design, especially for a shallow trench installation using the maximum sidewall distance, the designer must consider the elevations and slopes of the land surface, trench bottom, and distribution pipe if approval of the nominal 11 ft² is desired.

In Arizona, there are two methods for determining disposal field size: percolation test methods and soil characterization. Table 1 comes from the Arizona Administrative Code (R18-9-A312(D)(2)(a) and provides the maximum soil absorption rate used to calculate disposal field size based on percolation tests. Table 2 is used to determine maximum SARs for shallow and deep disposal fields using the soil evaluation method. If soil characterization and percolation test methods yield different SAR values, the most conservative value must be used unless approved by the Department of Environmental Quality.

Note: A disposal field described in R18-9-E302 is not allowed unless approved by the Department.

In Table 2, the questions are read in sequence starting with "A." The first "yes" answer determines the maximum SAR used to calculate disposal field size.

NOTE: A disposal field (gravity flow to shallow trench, deep trench, bed, chamber system, or seepage pit) is not allowed, unless approved by the Department and an applicable SAR is provided.

All water use, and thus the total flow of wastewater, must be accounted for when sizing the system. Users of the system (household residents) should know that any reductions in water use will benefit the system. It's a good idea to build some extra capacity into the system, because household water use can increase as well as decrease.

Shallow and Deep Trenches. The following table lists the Arizona design criteria for both shallow and deep trenches. Whichever trench is used, the trench bottoms must be level.

Notes:

1. For more than 24 inches, SDR 35 or equivalent strength pipe is required.
2. The distance between the bottom of the distribution pipe and the bottom of the trench bed.

Beds. The following table lists the Arizona design criteria for beds.

Effluent Distribution

Parallel distribution

The parallel distribution system directs wastewater flow into all trenches in the soil treatment simultaneously. Trenches are constructed to be of equal length and depth and in the same soil, so that treatment occurs at the same rate in each. In theory, this allows for equal use of all parts of the system. In practice, since the trenches are never identical to each other, the result is unequal flow.

Differences between the trenches are unavoidable: one trench may be dug in slightly more permeable soil or be slightly deeper or longer. Wastewater will enter all the trenches at the same time and at the same rate, but since it won't all be treated and leave the trenches at the same rate, the result can be backflow, as water leaves a full trench and moves to one that is emptying faster. The system can freeze in winter since the solid pipes between the trenches and the distribution box sometimes contain standing water.

Even when there is no backflow problem, there may be significant hydraulic head between the top of the system (the distribution box) and the trenches, with wastewater in lower trenches being forced up to the soil surface -- a surface failure. These systems must, therefore, be sited where the slope is not very great, often not steep enough to take advantage of gravity. To achieve even distribution of wastewater to the soil, a pump is required to deliver the wastewater to the trenches.

Parallel distribution systems fail entirely when a single trench fails, so the whole soil system is not used to its full potential. Unlike the continual system, in which a failure at one part of the system can also mean failure of most or all of the system, the parallel system offers no benefit in terms of improved effluent quality. Interestingly, the parallel distribution system is the most common in the United States, although it has the most potential problems.

Features of Parallel Distribution

• Common application
• Best for level sites

Dual alternating systems allow for the septic tank effluent to be dispersed in one of two or more trench systems. Once the first trench system fills up, it then gets diverted to the second trench system. Often times, people manually turn a valve on July 4th to let the trench system that was receiving effluent to recover while the second system receives the septic tank effluent.

Advantages of dual-alternating systems:

• relatively low cost,
• ease of installation, and
• allows periodic resting of infiltrative surface - increasing soil's absorptive ability .

Disadvantages of dual-alternating systems:

• front end of trench can overload,
• requires larger drainfield than conventional, and
• must switch diversion valve every 6 to 12 months.

Serial distribution
In the first design, serial distribution (sometimes called "drop-box distribution"), wastewater from the septic tank flows into the first trench until water has ponded and the trench is full. Then the water flows into the second trench until it, too, is full, then into the third and so on. The first trench will tend to be full all the time. When the water level in that trench drops, it will receive wastewater immediately. But aside from the order in which water reaches them, the trenches function independently, each receiving wastewater at the rate it is treated in that trench. If one is draining more slowly than the others, perhaps because it's located in less permeable soil, it will receive less wastewater. If one tends to drain quickly, perhaps because it receives more sunlight and more water is lost through evaporation, it will receive more wastewater. Since the trenches are not directly connected, there is no hydraulic head from trench to trench -- water does not move more quickly into or through the second or third trenches because they are downhill from the first one. [Note: source of figure is National Association of Wastewater Transporters, Inc.]

The serial distribution system allows for flexibility (see figures below). If higher soil treatment system capacity becomes necessary, or if one of the trenches fails, another trench can be dug and connected to the septic tank without any alterations to the existing trenches or their distribution lines. This system can also be constructed on steeper slopes than other types. Although use of some slopes may be impracticable because construction machinery cannot safely be operated on steep hills, the serial distribution system itself has no maximum slope limits. This is because of the absence of a hydraulic head between the trenches. This site flexibility may allow these systems to be constructed on the most suitable soils on a lot, or at an ideal distance from other developments, such as wells, driveways, or surface water bodies.

Onsite wastewater treatment system with drop boxes (serial distribution). (Source: National Association of Wastewater Transporters, Inc.)	Serial distribution construction plan. (Source: National Association of Wastewater Transporters, Inc.)

When serial distribution systems fail, they tend to fail in series; that is, the first trench fails after having been used to its full benefit. The second trench will become the first trench, and will also be used to its full benefit. Since no part of the soil treatment system depends on any other, the failure of any part can be remedied by the construction of a new trench and the abandonment of the failed one.

Features of Serial Distribution

• No slope maximum.
• No need for even lengths of trenches.
• Easy expansion — construct and connect new trenches as needed.
• Quick inspection — check only the ends of the trenches.
• No standing water in solid pipe between septic tank and drop boxes.

Continual distribution
In continual distribution (also known as "in-line distribution"), trenches are connected so that all wastewater passes through the first trench on its way to the second, which it passes through on its way to the third. This system probably treats the waste more thoroughly than the serial system, since all wastewater moves through the whole series of trenches. Water entering the second or third trench has already been treated by passing through the length of the previous trench. High levels of organic matter or suspended solids tend not to reach the last trench, so effluent leaving the system is likely to be much cleaner. The biomat at this end of the system will be a thinner layer, since there is little in the wastewater for the bacteria to consume, and the water will drain more quickly. The soil pores at the end of the last trench will probably never become plugged with inorganic solids or grease. [Note: source of figure is National Association of Wastewater Transporters, Inc.]

The first trench, however, is fairly likely to have problems with soil pores becoming clogged or with the buildup of an impermeable biomat layer. This trench must handle more than its "share" of suspended solids and organic matter. When any part of the system gets plugged or otherwise fails, the rest of the system is shut down without having been used to its full potential.

Features of Continual Distribution

• System filtering of the effluent
• No slope maximum
• No limits on trench length
• Easy expansion