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This information was adapted from the following sources by Kitt Farrell-Poe:
Septic tanks are called "septic" because they have no dissolved oxygen, and therefore only anaerobic bacteria. This means that you will not get any aerobic treatment in the system, instead you will get anaerobic treatment. Anaerobic treatment is good, but is not as efficient at breaking down the waste. On the other hand, septic tanks are low-cost and reliable.
A septic tank is a buried, watertight container used to clarify and partially treat wastewater. The septic tank has been in use in one form or another for over 100 years (for more in-depth discussion refer to A Brief History of Wastewater Management). The septic tank was originally designed to serve as a settling basin to separate scum and grit from the liquid. The effluent from the tank then was sent to a sewer or the soil for disposal. The clarification function of the tank was known, but the biological processes that partially digested the sewage were discovered by accident. Scientists found that the organic solids in the wastewater decomposed if they stayed in the tank long enough. Therefore, the septic tank is designed to accomplish two tasks: clarification and treatment.
Clarification is a function of the detention time and the water extraction method. Solids settle out of the water based on size and specific gravity. Smaller lighter particles take longer to settle than heavier particles. Clarification also includes the removal of fats, oils and greases, which float to the surface along with soapsuds and “scum.” The variables of a septic tank are: size, shape, number of chambers, number and style of baffles, and gas venting provisions.
Treatment consists of biological treatment by anaerobic digestion. Anaerobic treatment partially decomposes the organic matter into simpler compounds that can be treated further in the septic tank or discharged into the soil for aerobic treatment.
Design considerations for septic tanks include: configuration, materials, structural integrity, watertightness, size, and appurtenances. A well-designed tank can remove 60 to 90 percent of the total suspended solids (TSS) and 30 to 80 percent of the biochemical oxygen demand (BOD) resulting in:
Efficient clarification takes time to complete because fats, oils, greases, and suspended solids travel slowly in water and may require hours to either float to the top or settle to the bottom. The shape of the tank must be designed to maximize the detention time of the wastewater. Surface area is more critical for settleability than depth, so a shallow, wide tank is preferable to a deep, narrow tank if both have the same volume capacity. Shallow tanks are also easier to transport and install and poses less of a safety risk because the content level of the tank is not much deeper than the height of an average person.
An improperly configured tank will allow wastewater to “short-circuit” through the tank to the outlet. Short-circuiting can allow solids to migrate to the absorption field if the wastewater is not given sufficient time for the solids to settle out. A septic tank of 1000-gallon capacity is approximately 9 feet long and 5 feet tall. In Arizona, a septic tank of 1000-gallon capacity needs to be at least 8 feet long; tanks with greater capacity need to have their length be between two to three times their width.
In Arizona, two compartment tanks are required unless two separate structures are placed in series (in which case, the series is considered a unit and must meet the same criteria as a single tank). The first compartment, or inlet compartment, should be 67% to 75% of the total required capacity of the tank.
Concrete mid-seam tank
Most septic tanks are cast as two parts and joined with a seam. A typical
concrete mid-seam tank
has a thick, tar-like mastic material placed in the seam where the sections
join together. This tank has also been termed a two-piece tank.
Many precautions are taken to make this seam watertight, but they often
fail, allowing groundwater to seep into the tank and wastewater to seep
out.
Concrete top-seam tank 
The concrete top-seam tank is an improvement over the mid-seam tank
since the seam ends up closer to the ground surface and further from
any groundwater. While waters can still infiltrate into this tank, the
problem does not seem to be nearly as severe as with mid-seam tanks.
This design has frequently been used for pump tanks. Also, since the
seam is above the wastewater level in the tank, exfiltration of the
sewage through the seam and out of the tank is not a problem.
Materials
Typically, septic tanks are made of concrete, polyethylene, or fiberglass. Steel and redwood have been used in the past but are no longer accepted
by most regulatory agencies. (In Arizona,
steel tanks are permissible as long as the tank has a minimum wall thickness
of No. 12 U.S. gauge steel and is protected from corrosion, internally and
externally, by a bituminous coating.) Long term “creep,” resulting
in deformation has been a problem with polyethylene tanks.
Both polyethylene and fiberglass tanks can easily be moved by a labor
crew, whereas concrete tanks are typically moved about by a truck equipped
with a crane and boom. Fiberglass tanks are often used in areas inaccessible
to concrete tank delivery trucks. Both
polyethylene and fiberglass tanks are more prone to “floating” than
concrete tanks. Regardless of the
material of construction, the tank must be watertight and structurally sound.
Concrete tanks can be precast or poured/cast-in-place. Some states allow curved blocks on a concrete floor (blocks plastered on the inside) or rectangular blocks on a concrete floor (blocks must have mortared joints and two coats of plaster on the inside).
In Arizona, cast-in-place concrete tanks need to be protected from corrosion by coating the tank with a bituminous coating, constructing the tank using a concrete mix that incorporates 15% to 18% fly ash, or other ADEQ-approved means. The coating needs to extend at least 4 inches below the wastewater line and cover all of the internal area above that point.
Structural
integrity
The long-term performance of the septic tank will depend on its structural
integrity. For concrete septic tanks, structural integrity is dependent on
the method of construction, the placement of the reinforcing steel, and the
composition of the concrete mix. For maximum structural integrity, the walls
and bottom of the tank should be poured monolithically. Where the walls and the bottom are poured monolithically,
the top should be cast in place with the reinforcing steel from the walls
extending into the top slab. In some
cases, a water seal is placed between the wall and the top.
By Arizona rules, all septic tank covers need to be able to support an earth load of 300 pounds per square foot. If the top of the tank is more than two feet below finished grade, then the septic tank and cover need to support an additional load of 150 pounds per square foot for each additional foot of cover.
Watertightness
Watertight tanks are a necessity for the protection of the environment and
for the operation of the system. Each
tank should be tested for watertightness and structural integrity by filling
the tank with water before and after installation.
Hydrostatic testing is conducted at the factory by filling the tank
with water and letting it stand for 24 hours.
If no water loss is observed after 24 hours, the tank is acceptable. Because some water absorption may occur with concrete tanks, the
tank should be refilled and allowed to stand for an additional 24 hours. If the water loss after the second 24-hour
period is greater than 1 gallon, the tank should be rejected (ASTM C1227 (Precast
Concrete Septic Tanks)). It is important
that the above procedure be repeated once the tank is installed and required
by law in Arizona (see R18-9-A314(E)(4)).
Size
A septic tank also accomplishes treatment through the biological
activity of anaerobic or facultative bacteria.
This type of biodegradation may take many hours to fully work, so treatment
efficiency is linked to detention time. Over
the years a number of empirical relationships have been developed to estimate
the required detention time. The recommended
detention time ranges from 36 to 48 hours, but the absolute minimum is 24
hours before being discharged to the soil absorption system. Tank size and
household water usage determine the detention time of the tank.
As mentioned above, efficient clarification takes time to complete
because fats, oils, greases, and suspended solids travel slowly in water and
may require hours to either float to the top or settle to the bottom.
A septic tank 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 and/or number of fixtures in the home. The septic tank will serve as a receptacle for all the settleable and floatable materials until the tank is pumped. For this reason, the tank design must include provisions for adequate storage. The storage capacity is based on the intended use of the tank and the anticipated pumping interval. A tank that is too full of solids will have a shortened detention time and will not function properly and will allow unwanted substances to pass through to the soil absorption system.
In Arizona, refer to Table 1 to determine the size of a septic tank for a single residence. For other than a single residence, the recommended design liquid capacity of a septic tank is 2.1 times the design flow into the tank as determined from ADEQ Table 1. Unit Daily Design Flows. It is also permissible to place septic tanks in series to meet the septic tank design liquid capacity requirements. The table is developed with to maintain a liquid level in the tank of at least 42 inches.
|
No. of Bedrooms
|
No. of Occupants
|
No. of Baths
|
Maximum Fixture Count
|
Minimum Septic Tank Size
(gallons) |
|
2
|
4
|
1-2
|
18
|
1000
|
|
3
|
6
|
1-2
|
18
|
1000
|
|
4
|
8
|
2-3
|
25
|
1250
|
|
5
|
10
|
2-4
|
32
|
1500
|
|
6
|
12
|
3-5
|
39
|
2000
|
|
7
|
14
|
3-5
|
42
|
2000
|
Baffles
Baffles are critical to the success of the septic tank, and the septic
tank will not function without them. Influent baffles restrict and redirect
the flow of the incoming wastewater to prevent short-circuiting. By
doing so, baffles control the flow of the settleable and floatable materials.
Effluent baffles prevent floatables, scum, or suspended solids from
flowing into the drainfield.
Baffles come in many sizes and styles, the simplest just a bend and extension in a pipe. Baffles can also be concrete or fiberglass partitions attached to the ceiling of the tank. No significant difference has been found between using plastic and concrete baffles [Burks and Minnis]. More elaborate baffling systems can include pipes, screens, and weirs that direct and/or filter the flow.
Baffles often work better in conjunction with effluent filters that prevent suspended solids from leaving the tank. The advantage of effluent filters is that they filter the effluent of large suspended materials. The disadvantage of effluent filters is that they plug up. A monitoring device should be used with an effluent filter to avoid backup into the house.
Another type of baffle is a gas-deflecting baffle. Gaseous products of anaerobic digestion are generated in the solids at the bottom of the septic tank and large bubbles can belch up from the solids layer. When belching occurs, the gas disrupts the solid layer and can allow some solids to migrate to the outlet pipe and be carried to the drainfield. If this happens often, the soil absorption area can become clogged with solids. To prevent this occurrence, the outlet pipe can be fitted with a deflector (gas-deflecting baffle) to prevent the gases from traveling up the pipe. The deflector can be a 45° bend or a conical device manufactured to fit over the outlet pipe. The use of other baffling devices can also reduce the problems caused by belching gases.
Risers
Risers provide surface access to the septic observation and pump-out lids. Plastic
tank risers 
or manhole risers can be purchased and added to tanks even after the tank has
been installed and covered. These risers allow easy access for operation and
maintenance of septic tanks and pump tanks.
Often concrete risers 
are cast at the same time as the tank and installed during construction of the
system. These are most important for pump tanks. Risers need to extend well
above grade and be sealed to the tank with hydraulic cement so that surface
waters cannot infiltrate into the tank.
It is important that the discharge piping comes up into the riser to allow
easy equipment access 
to the quick disconnect threaded union, gate or ball valve, float tree, and
nylon pump rope.
Here are some websites to obtain manufacturer information and specifications:
Tank outlet boots
Tank outlet boots are rubber connectors that can be placed where pipes enter
and exit tanks to improve watertightness. The wide end of the boot goes in the
tank knockout; the pipe is slipped into the narrow end of the boot. Gaskets
help seal the pipe/boot and boot/tank outlet connections.
Effluent
filters
The effluent filter has been one of the most significant improvements in septic
tank design in decades. The most serious
problem with septic systems is the migration of solids, grease, or oil into
the leach field. The effluent filter
is an effective way to prevent this. The filter 
is 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. Solids in a filtered system’s effluent discharge are significantly
less than those produced in a non-screened system.
The effluent filter is relatively inexpensive and can be quickly installed
or retrofitted. If the filter becomes plugged it can easily
be removed, hosed off into the septic tank, and reinstalled 
.
In some jurisdictions (including many counties in Arizona), effluent filters are required for new installations. Because effluent filters plug up, a monitoring device should be used with an effluent filter to avoid backup into the house.
Here are some websites to obtain manufacturer information and specifications:
Access
openings
And each tank must have at least two access openings that are at least
20 inches in diameter. One of the access openings needs to be over the
inlet end of the tank, and the other access opening needs to be located
over the outlet end. If the first compartment is over 12 feet long,
then another access opening needs to be placed over the baffle wall.
All access openings are required to be within 6 inches below finished
grade.
The top of the tank has covered, removal manholes to allow for routine inspection and pumping. For easier location and access to the manholes, a riser should be constructed over each manhole, extending from the top of the tank to the ground surface. For new tanks the riser should be precast into the tank.
Grease
tanks 
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. 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.
