A service provider I have worked with for a number of years asked whether lower-profile (shallower) septic tanks provided adequate sludge and scum storage while functioning the same from a liquid-retention perspective. As with most questions I receive, the answer is it depends. If the tank has the proper volume and the inlet and outlet baffles are the proper lengths and functioning, the answer is yes. I also suggested he visit that location and keep track of scum and sludge depths to determine a maintenance schedule, rather than simply following the typical 2-3 interval.

The question prompts a review of how to determine tank volumes and proper depth of baffles to provide time to settle solids delivered to the tank and if the effluent is taken from the tank clear zone to be delivered for final treatment and dispersal.

The volume or capacity of any tank is dependent on liquid depth and surface area determined by the inside length and width of the tank. Liquid depth in a septic tank is determined from the bottom of the invert at the outlet baffle to the bottom of the tank. To provide proper settling, liquid depth cannot be less than 3 feet and no more than 6.5 feet.

These numbers are from research in the 1960s and early ‘70s by an agricultural research service engineer who spent almost his entire career looking at how water moved through tanks of different sizes. Based on the question, if the low-profile tank has a liquid depth of 36 inches or more and has the proper total volume, it will function as intended.

SETTLING AND STORAGE

Providing storage of the floating scum requires additional tank height above the liquid level at the outlet equal to 20% of the liquid depth. If the liquid depth is 36 inches; 0.20 x 36 inches = 7.2 inches. The total tank depth of the low-profile tank needs to be at least 36 inches plus 8 inches for a total of 44 inches inside. If the tank is made of concrete, additional depth and width are added for the thickness of the bottom, sidewalls and lid.

All state and local codes I have seen call for minimum septic tank volumes to provide for settling and storage. One of the most common volumes required for a three-bedroom residence with an estimated daily sewage flow of 450 gallons/day is 1,000 gallons. 1,000 gallons ÷ 450 gallons/day = 2.2 days of storage time which meets the general requirement of 2 days of liquid storage to allow for settling. 

To determine liquid capacity in the septic tank, measure the inside length, inside width and the height of the outlet from the floor. Common dimensions for a 1,000-gallon tank are 54 inches of depth, 4 feet wide and 8 feet long. Just to check this, the calculations would be 4 feet x 8 feet gives an area of 32 square feet. Thus, the tank has 32 cubic feet of liquid volume for each foot of liquid depth. Since each cubic foot of water has approximately 7.5 gallons, the tank has (7.5 x 32) 240 gallons per foot of depth. With 4.5 feet of depth (54 inches) the total tank capacity is 1,080 gallons (4.5 x 240).

For the lower-profile tank to have the required 1,000-gallon volume, a change in either the length or width (or both) of the tank is required. A 4-foot-by-10-foot tank would have 40 cubic feet per foot and volume of 300 gallons per foot resulting a total volume of 900 gallons; 100 gallons short of the required volume. It either needs to be longer still or wider to meet the minimum requirement. 

PLAN CAREFULLY 

Lower-profile tanks have their uses; the service provider who asked the question lives near mountains where soils can be very shallow over bedrock. Installing tanks into bedrock is hard and very expensive, so having options in terms of tank depths is important. At the same time, it is important to realize for the tank to do the job, you can only go so far in terms of making them shallow. 

In addition, we are much less certain about how well the shallower tanks will capture and store both sludge and scum while still providing the quiet clear zone to draw effluent from for final treatment and dispersal. Taking a little more time at installation to evaluate actual liquid capacity and then some follow up to evaluate how the tank is operating can avoid problems in the future. And obviously, this is where having effluent screens is helpful to prevent damage to the drainfield from solids.