| OWDP Home > Online Resources > Principles of Subsurface Treatment | ||||
If you have not already visited the University of Rhode Island Virtual Tour of a conventional septic system, I encourage you to do so. Also, please read Chapter 7 (pp. 133-151) of Burks and Minnis.
Please note:
Many reasons exist for the use of onsite wastewater treatment. Large regional sewage treatment plants are not economical for many rural areas. Also, some mechanical treatment plants may not meet increasingly stringent water quality requirements for wastewater discharge into streams, rivers, and lakes. Thus, development often depends upon the proper use of septic systems.
Septic systems are designed to hold, treat, and disperse household wastewater. The liquid portion of what goes into the system will leave the system and may eventually reach groundwater or surface water. Household wastewater contains bacteria, viruses, household chemicals, and excess nutrients such as nitrates, all of which can cause health problems. Therefore, household wastewater must have adequate treatment to prevent water contamination. Figure 1 shows a schematic diagram of onsite wastewater system and its relationship to groundwater quality.
Different types of septic systems have been developed to suit
a wide variety of needs. The simplest and most commonly used is the conventional
septic system, which consists of a septic tank, an optional distribution box,
a drainfield, and the soil beneath the drainfield. Figure 2 depicts a
two-chamber septic tank with no distribution box conventional system, Figure
3 depicts a conventional system with a distribution box,
and Figure 4 shows a close-up of a distribution box.
Septic tank
Wastewater from toilets, sinks, showers, washing machines, and other drains
flows from the household sewer drain into the underground septic tank. The septic
tank is a watertight concrete box approximately nine feet long and five feet
tall. It is typically designed with a 1,000-gallon liquid capacity, but in most
states (including Arizona) the size of the tank is legally determined by the
number of bedrooms in the home. In the septic tank, waste components separate
-- the heavy solids settle on the bottom, forming a sludge layer, while the
grease and fatty solids float to the top, forming a scum layer. Bacteria in
the tank partially decompose the solids. Solids will build up in the tank and
must be removed periodically by a qualified professional contractor.
The relatively clear layer of wastewater that forms in the middle of the septic tank is called effluent. A buried pipe carries the effluent from the septic tank outlet to the soil absorption system where most of the treatment process occurs.
Soil
absorption system
The soil absorption system (also known as the drainfield, leach field, or disposal
field) receives the septic tank effluent and allows the effluent to seep into
the surrounding soil where germs and chemicals are removed through filtration,
adsorption, and other processes before reaching groundwater or surface water.
A typical soil absorption system consists of an excavation, gravel, distribution
pipe, geotextile fabric, and topsoil. The trench bottom and sidewalls provide
the soil surface area to support biological growth which is the major treatment
process of the system. The gravel supports the trench sidewalls, preventing
them from collapsing onto the distribution pipe. The geotextile fabric protects
the trench from fine particles entering the gravel which can seal the pores,
and the topsoil protects the trench surface and geotextile fabric.
Excavations are classified according to their width and number of distribution pipes. Narrower excavations (1 to 3 -5 feet wide) that contain one distribution pipe are called trenches (see Figure 5 for a typical cross section of a trench). Excavations wider than 5 foot wide that contain multiple distribution pipes are called beds or "seepage beds." Figure 6 shows a schematic of a gravel-filled seepage bed.
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 transfering oxygen. If the center-to-center distance is reduced too much, the trenches would behave like a bed.
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. Another reason is the reduced sidewall surface area available for biological growth. Depending on the jurisdiction, trench designs are based on the bottom surface area only (this design rule applies to Arizona); sidewall surface area is not factored into the design which in essence can provide a large 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!
Effluent moves through the pipes and seeps into the gravel below to the surrounding soil for final treatment. Soil particles filter out small suspended solids and organic matter, while soil bacteria break down harmful microorganisms and other organic components. Viruses adhere to clay particles in the soil and eventually die (see The Fundamental Microbiology of Sewage). The now treated effluent continues its downward flow through the soil layers.
Flow diversion devices
In many systems, flow diversion devices are used. The primary purpose of flow
diversion devices is to control the flow of the effluent into the drainfield.
A flow divider tee (Figure 7), dual
alternating systems with a diversion valve (Figure 8), and a tipping
D-box (Figure 9) are common flow diversion devices. The flow divider tee
and the dual alternating system control the flow into particular trenches or,
in the case of the alternating system, into separate drainfields. The bullets
below summarize the advantages and disadvantages of the dual alternating system.
Advantages of dual-alternating systems:
Disadvantages of dual-alternating systems:
A tipping D-box may be used on a slope to deliver equal amounts of effluent to each trench (see Figure 10 for a tipping D-box in action). The tipping D-box works by accumulating about 1.5 gallons of septic tank effluent in a pan which tips when full. When the D-box tips, it empties the effluent into a small chamber that distributes effluent to pipes going to each trench. This approach may be particularly useful on sloping sites.
Grease traps
Wastewaters containing high levels of fats, oils, and greases, such as those
from restaurants and diners, require a grease trap to remove grease entrained
in the wastewater (see Figure 11 for side view of a typical two-compartment
grease tank). The grease trap is actually the first
compartment of a two-compartment septic tank where the grease is cooled and
provided time to float to the surface. The grease is prevented from leaving
the first chamber by baffles and from the second chamber by effluent discharge
pipes extending below the presumed depth of oil and grease. The grease must
be pumped out periodically by a qualified professional contractor.
Sometimes, food service facilities use emulsifiers, cleaning products that keep oil and grease in suspension, to keep their indoor plumbing from clogging. Designers of such systems need to be aware that the use of emulsifiers may allow the oil and grease to short-circuit grease traps.
Effluent filters
Effluent filters are placed in the septic tank outlet pipe to filter out suspended
solids from the septic tank effluent before they are distributed to the
absorption field. In some jurisdictions (including many counties in Arizona),
effluent filters are required for new installations.
In a conventional septic system, gravity controls flow. That is, the sewage flows downhill from the house plumbing to the septic tank. The septic tank effluent then flows down to the D-box and to the pipe in the trenches. These pipes are called laterals. The pipe contains many large holes. However, because these holes are so large and the flow rate out of the tank is so slow, most of the septic tank effluent exits out of the first few holes in the pipe.
When the sewage effluent comes out of the first few holes nearest the D-box, it trickles down through the gravel underneath these holes and soaks into the soil (see Figure 12 to see a schematic of the formation of a clogging mat beneath a drainfield over time). After a month or so, a biomat begins to form where the gravel lies on the soil under the first part of the trench. The biomat allows some effluent to flow through it, but causes part of the effluent to flow laterally along the trench bottom to where a biomat has not yet developed. Over time (probably two to eight years), this biomat slowly forms along the entire length of the the trench. A more in-depth discussion is found further on in this module under Biomats (clogging mats).
Advantages of gravity distribution systems (mainly the result of less mechanization, such as pumps and other flow controls):
Disadvantages of gravity distribution systems:
Soil as a Treatment and Dispersal Medium for Wastewater
Soils consist of solid particles such as grains of sand and clay, as well as pores and openings between the solid particles. These pores can be filled with air, water, or a mixture of the two. Most of the actual wastewater treatment in a septic system occurs in the soil beneath the drainfield. As effluent enters and flows through the small pores in the soil, many of the bacteria that can cause diseases are filtered out. Some of the smaller germs, such as viruses, are adsorbed by the soil until they are destroyed. The soil can also retain certain chemicals, such as phosphorus and some forms of nitrogen. However, the nitrate form of nitrogen will move down through the soil. Thus, the water that continues on to the groundwater or surface water supplies should be relatively free of sewage contaminants except for nitrate which moves with water.
Biomats
(clogging mats)
During the life of the septic system, a biological mat or biomat (also known
as a "clogging mat") forms on the soil beneath the drainfield. This
mat is actually a mixture of solids from the effluent, biomass (old cells) from
microorganisms, and excretions from the microorganisms. Although the biomat
is usually 3/16" (5 mm) to 1-3/8" (30 mm) thick, the thickness increases
and decreases with changes in wastewater quality and quantity and with the growth
and decay of microbes.
The solids from the effluent begin the process by blocking soil pores. Soil microorganisms, in turn, thrive in this environment because the sewage contains all the nutrients they need to grow and multiply, producing more cells and more excretions. Another process that occurs under these conditions is that chemical dispersions alter the soil structure, causing some of the soil pores to close up. Insoluble compounds, such as iron sulfide, may precipitate and become a part of the mat. Biomats are highly effective in removing organic material and pathogens and detain viruses from the septic tank effluent. Biomats also have low permeabilities (about 0.5 ft/day) which essentially controls the wastewater infiltration. Figure 12 shows the formation of a biomat beneath the drainfield over time after 1 day, 1 month, 1 year, and 3 to 8 years of operation.
Advantages of biomats:
Disadvantages of biomats:
Creeping failure is a condition where the clogging mat becomes so intensely developed that no water can flow through it. A normal clogging mat allows slow movement of water through the small soil pores and has the benefit of dramatically enhancing wastewater treatment. However, problems occur if homeowners don't regularly pump the solids out of their septic tanks. In this case, these excess solids continually flow out to the drainfield and clog all the pores in the soil so that no water can flow through it. This intensely developed clogging mat creeps down the trench over time. It eventually can lead to the surfacing of untreated septic-tank effluent to the ground's surface and a malfunction of the system. All of this points to the need for regular maintenance, including removing solids from the tank.
While we don't yet have enough research-based information and practical experience to effectively manage the biomat, we do understand many of the factors that influence the development of it. Some of the factors which influence intensity of development of the biomat are:
Soil
suitability
Remember that not all soils are suitable for subsurface systems. Soils must
have the capability to both absorb and purify the sewage effluent if a septic
system is to work properly. When the soil is aerobic, meaning "with
air," the pathogens in the septic-tank effluent are removed as the effluent
trickles slowly down through the soil.
Soil properties change across landscapes, but soils may also vary greatly on adjacent sites. This drastic variation in soils helps explain why one septic system can be failing while the septic systems at neighboring homes work fine. Systems installed in soils that can't absorb the septic-tank effluent usually malfunction, leaking nearly raw, untreated sewage to the soil surface or roadside ditches. Systems in soils that can absorb the effluent but that do not have aerobic conditions usually contaminate the groundwater because no treatment occurs.
Correcting a malfunctioning system and all of its associated problems is costly. To help avoid this problem, many states require a comprehensive soil and site evaluation before a permit is issued to construct a septic system. A more in-depth discussion of soil and site characteristics is found in the Soil & Site Evaluation and Characterization module. However, deep, well-drained soils with loamy textures are usually the best sites for conventional septic systems.
Several changes may be made to a site to make it suitable for a conventional septic system. The structure and components of a conventional septic system may also be changed to enable onsite treatment in a particular area. Site and system modifications include:
Drainage to
lower the water table
Tile drains are installed to intercept groundwater and lower the existing water
table (see Figure 13, tile drains to lower the water
table). This technology may make onsite wastewater treatment possible in
areas with wet soils due to high water tables. It increases the thickness of
unsaturated aerobic soil conditions if the drainage system and its outlet are
maintained.
Drainage
to intercept laterally flowing water
On some sites, the only suitable area for a drainfield may be near the bottom
of a slope. In these cases, a drain may be installed to intercept water on the
slope (see Figure 14, drainage to interecept laterally-flowing
water). The outlet for this drain should be below the system (see Figure
15 drainage outlet). Please note that the drainage
system is an important part of this septic system.
Placement on a steep
slope
Where the site is a steep slope, distribution may be done through a series of
pipes stepping down the slope. Figure 16 shows the pipe components of a serial
distribution system using a stepdown and a clearer picture of the stepdown
structure and how two trenches are connected by the stepdown. Figure 17 illustrates
another approach to serial distribution, showing a closeup of one of the drop
boxes in a serial distribution system. The drop boxes can be in the center
or at either end of the trench. This serial distribution approach may be desirable
because of its low cost, only slightly more difficult installation than a typical
conventional system, the little maintenance that's needed, and its ability to
promote biomat development. On the other hand, this system may overload the
front end of trenches, may increase the likelihood of creeping failure, and
may not use the entire soil treatment zone.
Placement
in saprolite materials
Many soils in the southeastern and western United States are underlain at a
shallow depth by a material called saprolite. The word "saprolite"
literally means "rotten rock." Saprolite material looks like rock
and retains the banding, layering, and fracture pattern of rock, but it is highly
weathered and can be easily dug up like soil. Often when onsite systems are
placed in saprolite, special designs are used. For instance, the trenches may
be placed much deeper than normal (4 to 6 feet deep) or a sand-lined trench
system design (see Figure 18, sand-lined trench system)
may be used if the soil will not let water flow through it to the saprolite.
Shallow placement
systems
The shallow placement system is used when there is a limiting condition in the
soil, such as rock, low water table, or a restrictive layer. If the limiting
condition is not too close to the ground surface, then the infiltrative surface
(the bottom of the trench) can be installed shallower than normal to maintain
an aerobic soil treatment zone between the trench bottom and the limiting condition.
The rest of the system is installed just as with the conventional system.
Large
diameter pipe systems
In some areas of the country where there are no local sources of clean gravel,
it can become very costly to transport gravel long distances. If this is the
case, one option is to install a large diameter pipe system. This system is
like the conventional system except the trenches are narrower and do not contain
gravel. Instead, large diameter (8 to 10 inches inside diameter) corrugated
polyethylene tubing is placed in the trenches and covered with excavated soil
material. A fabric sleeve covers the tubing. This approach has also been used
on very steep slopes (up to 60 percent) where power equipment can't operate
and where the soils are loamy enough to allow the trenches to be excavated by
hand.
Panel block systems
Another trench option without gravel is a panel block system. Here, prefabricated
panels about 8 inches wide and 16 to 24 inches tall and made of porous concrete
are placed on sand in the trench. The parts of the trench alongside the panels
are also filled with sand to the top of the panel and then the entire system
is covered with soil. The septic tank effluent flows through openings in the
middle of the panels and seeps out through the porous concrete.
Pump to distribution
box
Using a pump to control flow to the distribution box may be helpful on sites
where the drainfield may not be at an ideal site. Advantages include the ability
to pump the septic tank effluent up to a drainfield that is located at a higher
elevation than the house and the opportunity to have dosing and resting cycles.
This modification, however, is more costly and requires more maintenance than
the conventional system. Also, poor distribution to the trenches and overloading
the front end of the trench may be problems.
Pressure manifold
Distribution can be controlled even better by using a pump to a pressure manifold.
The pressure manifold system can be seen in Figure
19. Advantages of this modification are that it allows dosing and resting cycles
and enables the septic tank effluent to be equally divided among the trenches
so that no trench is overloaded with effluent while other trenches go unused.
Disadvantages include greater cost and more maintenance, as well as overloading
of the front end of trenches.