According to the 1967 edition of the Manual for Septic Tank Practice, the primary purpose of the septic tank was to “condition” the sewage and to protect the drainfield from clogging. Conditioning was understood to mean separating the solids from the liquid.

Septic tanks provide this protection by creating conditions where materials that are less dense than water will float (scum) and materials that are denser than water will sink (sludge). Today, we have a better appreciation for the role of a septic tank in domestic wastewater treatment. Beyond protecting the drainfield, retention of solids provides significant wastewater treatment: The material held by the tank cannot negatively affect the environment.

Further, septic tanks help to normalize the flow to the soil absorption area, and allow for some breakdown of organic compounds. The ability of septic tanks to hold solids depends on tank size, shape and placement.

Size matters

To maximize settling (or floating), we must minimize turbulence within the tank. The capacity (or volume) of a septic tank needs to be large enough to make the inflow from a toilet or kitchen sink seem insignificant. A toilet flush may be three to five gallons and have a significant solids content. When this small amount of liquid enters a 1,000-gallon septic tank, the velocity of the incoming liquid is reduced nearly to zero. By removing the entrance velocity, there is almost no turbulence in the tank, and the process of flotation or settling can begin.

A second way that size matters is that when a small volume of water enters the tank, the overall effect on the water level is very small. For example, if a typical 1,000-gallon tank is five feet wide and 10 feet long, the water surface area is 50 square feet. If a toilet flush creates an inflow of five gallons, the water surface will rise less than 3/16 inch.

The rate by which water leaves the septic tank depends on the depth of water above the bottom of the outlet invert. This very small rise in the water surface provides a very small driving force to move water through the outlet. During the toilet flush, five gallons of wastewater enters the tank in less than one minute. A rise of 3/16 inch means the discharge of the five gallons will be slow and will result in just a trickle of water from the outlet.

Local and state agencies typically prescribe the minimum septic tank volume based on the number of bedrooms or on the design daily volume. Often, these specifications are based on a minimum volume of twice the design daily flow. If the design flow is 450 gpd, then twice that volume is 900 gallons.

During design flow conditions, the hydraulic detention is two days – in other words, it would take two days for wastewater to move through the tank. The good news about this design approach is that most homeowners use less than their design flow, and wastewater may take five or six days to move through the tank.

This slower rate allows for even the smaller suspended solids to settle below the outlet baffle and become trapped in the tank. The two-day detention rule-of-thumb should be used as a minimum sizing standard for domestic and commercial applications. Depending on the solids content, even longer detention may be desirable.

Shape matters

Most regulatory agencies only specify septic tank volume. It is generally understood that a septic tank should be rectangular. A good rule of thumb is that the tank should be twice as long as it is wide. Length is measured from the inlet baffle to the outlet baffle. Liquid/solid separation improves as the distance between baffles increases.

The inlet invert should be two to three inches higher than the outlet invert. This distance allows for more scum material to rise onto the water surface. Inlet baffles absorb the inlet velocity energy and force the wastewater to flow downward.

Placement matters

A septic tank must be level and must be placed in a soil that will not settle beneath it. Any settling could allow the tank to tilt or crack. The effective volume is relative to the outlet invert, and if a tank is not level, the effective volume can be reduced, allowing solids to exit.

The septic tank is an essential wastewater treatment component and is often poorly appreciated. In order to function properly, it must be sized and placed according to good construction practices. Further, it must be recognized that as solids accumulate in the tank, the process of liquid/solid separation is hindered. The tank must be pumped when about 25 to 30 percent of its depth is occupied by solids.