Private

Sewage-Treatment
Systems

CEU 282
April 2020

Continuing Education from the

American Society of Plumbing Engineers
ASPE.ORG/ReadLearnEarn
 READ, LEARN, EARN: Private Sewage-Treatment Systems

Note: In determining your answers to the CE questions, use only the material presented in the corresponding continuing education article. Using information from other
materials may result in a wrong answer.

Private
Sewage-Treatment
Systems
Reprint from Plumbing Technology, Chapter 11. All rights reserved.

A private sewage-treatment system is used when no municipal sewer is available. It usually serves a limited number of people. The private
sewage-treatment system includes a septic tank and a leaching area. The septic tank, which is the principal piece of equipment in this system,
is a holding tank equipped with baffles. These baffles guide the slow-moving liquid to deposit the solids. The deposited matter is biodegraded
with the help of bacteria. A simple schematic of a septic-tank leaching-area arrangement system is shown in Figure 11-1.

Figure 11-1 Septic-Tank Leaching-Area Arrangement

Septic tanks may be constructed of the following materials:

• Poured concrete.
• Precast concrete.
• Cement blocks.
• Steel lined with cement.
• Coated steel.

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 READ, LEARN, EARN: Private Sewage-Treatment Systems

LEACHING AREA
Downstream from a septic tank is a distribution box followed by a leaching area, usually a tile field. To determine the site for the tile field,
percolation tests are required. These tests determine how quickly liquid (poured into a small test hole) will seep into the ground after satura-
tion. To install a leaching area in a particular location, tests must show that water percolates into water-saturated ground in a predetermined
amount of time. The ground under the leaching area acts as a natural filter. When the liquid percolates through the ground and reaches the
water table below, it is considered clean water.
If percolation does not occur within the prescribed time limit and the spot selected for the field cannot be changed, it is still possible to
use it as a leaching area. However, in this case, a man-made filter, usually a sand bed, must be installed under the tiles. This type of installation
increases the total cost, but it will make the installation feasible.
The calculation for the percolation test is based on the following formula:

C = t + 6.24
29
where: t = Time required for the water level to fall 1 in. after saturation (min)
C = Percolation coefficient (sq ft/gal)
6.24 and 29 = Practical coefficients

For the area to be acceptable as a tile field, the result of this equation should be 0.285 after 2 min and 1.25 after 30 min.
The tile field is not made up of ceramic tiles. Instead, the tiles are a special type of perforated pipes. These pipes are laid with perforations
face down on the ground and are installed with a small distance (1 to 2 in.) between each length of pipe (see Figure 11-2).

Figure 11-2 Tile Arrangement

The following must be considered when installing the leaching area:

• Locate field a minimum of 100 ft away from any ground or underground water supply.
• Locate field a minimum of 10 ft away from any structure.
• Area should have a slight downstream slope.
• Tile field width is recommended not to exceed 60 ft (100 ft maximum in special circumstances).
• Tiles shall be sloped 2 to 4 in./100 ft, and the field slope shall not exceed 6 in./100 ft.
• Clean, graded gravel; broken bricks; or washed rocks should be installed around the tiles.
• Entire field must be installed below the frost line or be covered with biodegradable material (hay) for protection against freezing.
• Tiles must be installed with a space between them.
• Distribution box (located downstream of the septic tank and ahead of the tile field) must have all outlets at the same elevation.
Nonmetallic, perforated tile pipes are 12 ft long and have a standard diameter of 4 in. Tiles may be clay bell, and spigot sewer pipes may
each be 2 to 3 ft long.
A dosing tank must be installed downstream of the septic tank and ahead of the distribution box in case the tile field is not large enough
to accommodate the inflow. The dosing tank must have a capacity of 70% of the tile field. This tank helps store the overflow before slowly
distributing it to the tile field.
A tile field is not the only method available for use in a leaching area. Another method involves allowing the liquid waste to seep directly
into the ground along some specially prepared and covered trenches with dividers (see Figure 11-3). This system is less expensive to install,
but the seepage into the ground is less uniform.

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 READ, LEARN, EARN: Private Sewage-Treatment Systems

Figure 11-3 Septic Tank and Leaching Area

MAINTENANCE
A well-maintained septic-tank drainage system has an expected operating life of 20 to 30 years. Certain maintenance procedures and
observations help to ensure good system operation, for example:
• Septic tank must be pumped (emptied) by specialists at the rate recommended by the system manufacturer or the installing contractor.
• Leaching area must be kept free of bushes and trees.
• No structure (e.g., decks) must be built above the leaching area.
• Effluent sent down the drain should not contain:
A. Cooking oil and/or fats.
B. Food scraps.
C. Plastic material or tin foil.
D. Non-biodegradable objects such as cigarette butts or tampons.
E. Concentrated or aggressive chemicals (they may kill the needed bacteria growing in the tank).
• Use non-colored toilet paper.
• Keep the conveying tank supply pipes conventionally clean (rod through once a year).
• Conserve water, as this will help to prevent the septic tank and the system from overloading.

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 READ, LEARN, EARN: Private Sewage-Treatment Systems

ASPE Read, Learn, Earn Continuing Education

You may submit your answers to the following questions online at aspe.org/ReadLearnEarn. If you score 90 percent or higher on the test, you will be notified
that you have earned 0.1 CEU, which can be applied toward CPD or CPDT recertification or numerous regulatory-agency CE programs. (Please note that it is your
responsibility to determine the acceptance policy of a particular agency.) CEU information will be kept on file at the ASPE office for three years.

Expiration date: Continuing education credit will be given for this examination through April 30, 2021.

Thank you to J. Francisco DeHoyos, CPD, of the Central Texas and Houston Chapters for authoring this month’s quiz.

CE Questions — “Private Sewage-Treatment Systems” (CEU 282)

1. Which statement about a private sewage-treatment system is false? 7. Effluent sent down the drain should contain:
a. It is used when municipal sewer is not available. a. Food scraps.
b. It usually serves a limited number of people. b. Sanitary drainage
c. The system includes a septic tank and a leaching area. c. Non-biodegradable objects.
d. The septic tank and a leaching area must be vented d. Concentrated chemicals.

8. A septic tank must be pumped (emptied) by a specialist when:

2. Nonmetallic, perforated tile pipes are 12 ft. long and have a standard a. The septic tank is full.
diameter of: b. The tile system is backing up.
a. 2 inches c. Only once every two years.
b. 4 inches d. At a rate recommended by the system manufacturer.
c. 6 inches
d. 3 inches 9. In the percolation calculation formula, the percolation coefficient is
expressed in:
3. Septic tanks may be constructed from the following materials except for: a. Cubic feet / Hour
a. Steel lined with cement. b. Cubic Feet / Minutes
b. Cement blocks. c. Acres / Gallons
c. Fiber Glass. d. Square feet / Gallons
d. Coated steel.
10. In case the tile field is not large enough, what should be installed to
4. What is located between septic tank and a tile field? accommodate the inflow?
a. Distribution box. a. A settling basin.
b. A back water valve. b. A lift pump.
c. A leaching area. c. A dosing tank.
d. An effluent filter. d. A retention system.

5. When installing the leaching area, which of the following should be 11. Which can be used in leaching areas to protect them from freezing?
avoided? a. Cement cover on top.
a. Area with a slight downstream slope. b. Hay.
b. Tiles must be installed without space between them. c. Pressurized the tile system.
c. Tiles sloped 2 to 4 in./100 ft. d. Adding liquid antifreeze to the septic tank.
d. A field slope of 6 in./100 ft.
12. The ground under the leaching area acts as a natural filter. When the
6. A well-maintained septic-tank drainage system has an expected liquid percolates through the ground and reaches the water table below,
operating life of: it is considered:
a. 30 to 50 years. a. Gray water.
b. 5 to 10 years. b. Recycled water.
c. 20 to 30 years. c. Clean Water.
d. 50 to 60 years. d. Reclaimed water.

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