Design of Septic Tanks

All onsite systems designs are judged in comparison with the basic septic tank/soil absorption system. So prevalent is this view that the septic tank/soil absorption system is commonly referred to as a “conventional” system, and most other designs are referred to as “alternative” systems.

Process Description

Design Criteria

Operation & Maintenance


Process Description

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. 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: (1) clarification and (2) 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 Criteria

Design and operational considerations for septic tanks include: (1) configuration, (2) materials, (3) structural integrity, (4) water-tightness (5) size, (6) appurtenances and (7) operation and maintenance. 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:

        TSS concentration of 75 mg/l (30 day average)

        BOD of 150 mg/l. (30 day average)

        Total Nitrogen (as N) of 53 mg/l (5 month average)

        95 percent of the time the LOG10 of the Fecal Coliform Colony forming units per 100 ml is less than or equal to 8.


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.


Typically, septic tanks are made of concrete, polyethylene or fiberglass. Steel and redwood have been used in the past but is no longer accepted by most regulatory agencies. 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.

Structural integrity

The long-term performance of the septic tank will depend on its structural integrity. Or 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.


Watertight tanks are a necessity for the protection of the environment and for the operation of the system. Each tank should be tested for water-tightness 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.


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.

Figure 1. Septic tank.

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.

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.


Influent baffles restrict and redirect the flow of the influent to help prevent short-circuiting. Baffles control the flow of the settleable and floatable materials. Effluent baffles prevent floatables, scum, or suspended solids from flowing into the leach field. Baffles come in many sizes and styles; the simplest is just a bend and extension in the inlet or outlet pipe. Baffles can also be concrete or fiberglass partitions attached to the ceiling and/or floor of the tank.

Compartmentalization can enhance the operation of the septic tank. A two-compartment tank helps to eliminate the possibility of short-circuiting wastewater through the system.

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. A filter restricts and limits passage of suspended solids out of the tank. 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 clogged it can easily be removed, hosed off and reinstalled.

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.


Operation & Maintenance

The system is designed to provide treatment and disposal for normal domestic sewage. No non-biodegradable material should be introduced into the wastewater treatment and disposal system. Plastic and paper (except toilet paper) are examples of non-biodegradable materials that should not be placed down the drain. Normal amounts of dirt and small non-biodegradable debris (buttons, dental floss, etc.) from washing will inevitably get into the system. These solids will be retained in the septic tank until it is pumped during its normal maintenance. Oils and grease should not be placed down the drain in excess quantities. Normal washing of greasy dishes is not considered excessive. Routinely draining fat from a frying pan, deep fryer, or roasting pan down the drain would be considered excessive. A garbage disposal may be used on the system but its use should be restricted. A garbage disposal should not be used for the bulk disposal of food preparation waste.

Because septic tanks are buried and are out of sight, many homeowners forget that septic systems require periodic maintenance. Failure to pump-out the septic tank is possibly the greatest single cause of septic system failure. After several years of use, a buildup of bottom sludge and floating scum will reduce the effective capacity of the system. As mentioned earlier this means the wastewater passes through the tank too fast, and solids may eventually plug the pipes in the leach field.

Figure 2. Poorly maintained septic tank.

To avoid leach field failures inspect the tank at regular intervals and pump when necessary. Due to many variables it is recommended that the tank be inspected every year and base pump-outs on these annual inspections. As the years pass, you should be able to see a pattern of sludge and scum accumulation. Always keep records of inspections and dates when the tank has been pumped.

The tank should be pumped by a licensed pumper with a vacuum tank truck. The pumper will use a hose and vacuum everything out of the tank (both solids and liquid). Waste pumped from the tank is called septage. It is approximately 5% solids and 95% water. The septage waste must be taken to a licensed disposal site because of the potential health problems with contamination.

The effluent filter on the tank outlet that may require cleaning more than once a year. This is accomplished by simply removing the filter and hosing it off back into the septic tank and re-installing the filter.