Question:

My comments on septic tanks are as follows: In a two-compartment tank there is no way to create separate water levels unless flow between the compartments is severely restricted. No one is going to suggest this approach. Different water levels do not create attenuation (slower flow). More attenuation equals more residence time, and residence time equals water volume divided by flow rate. Septic tanks are sludge/ scum separation vessels. More residence time equals better separation. Residence time equals water volume divided by flow rate. For a water volume of 300 gallons and flow rate of 150 gpd the residence time is two days assuming perfect mixing (which is to be avoided) and perhaps 12 hours for stratified flow (which is desired).

Answer:

Thank you for your comments and thoughts about flow through septic tanks. I will explain my understanding of the hydraulics of flow through septic tanks with and without a compartment.

First of all, I agree that septic tanks are sludge and scum separation vessels. Solids that float become a part of the scum layer. Solids that sink become part of the sludge layer. If the solids are organic, as they should be, anaerobic bacteria act on the solids to reduce their volume. The purpose is to discharge effluent with very few fine organic solids. I disagree, however, with many of your comments about septic tank flow.

Many years ago, tests showed the bottom of the outlet tee should be located at 40 percent of the liquid depth of the tank. This distance was measured from the invert (bottom) of the outlet pipe. Another recommendation was the top of the outlet tee should extend above the bottom of the outlet pipe to a distance of 20 percent of the liquid depth of the tank. This dimension should provide adequate volume for floating scum storage.

Residence time in a septic tank is difficult to define. You stated a septic tank with 300 gallons of water volume and a flow rate of 150 gallons per day will have a residence time of two days. Your math may be correct, but the flow pattern is likely not correct. It is unlikely that when a gallon of sewage flows into the inlet of the 300-gallon septic tank a gallon that has been in the tank for two days, or even 12 hours, will flow out.

DON’T ASSUME

It is more likely that some of the sewage discharged into that septic tank from a toilet flush or other sewage action will flow out of the tank from that inflow. The dynamics of flow and mixing in a septic tank are such that it’s not likely to assume residence times as you have.

When sewage flows into a septic tank, the liquid level rises in the tank. With the rise in liquid level, effluent begins to flow out of the tank. When we had our own onsite sewage treatment system, I researched the time of sewage inflows and outflows of our septic tank.

Our septic tank had a volume of 1,000 gallons. The fastest wastewater discharge into the septic tank was from our automatic washer. About 20 gallons of water would flow into the septic tank over about a two-minute period.

The septic tank was four feet wide by eight feet long. This is a surface area of 32 square feet. The 20 gallons of wastewater was equivalent to 2.67 cubic feet of volume. (There are 7.5 gallons per cubic foot.)

The 20 gallons of wastewater would raise the liquid level in the tank by one inch. This would be the maximum depth of flow in the outlet pipe. The flow in the outlet pipe was fastest just after the wastewater flowed into the septic tank. As the liquid level dropped in the septic tank, the rate of outflow slowed. The 20 gallons that flowed into the tank in two minutes took more than 20 minutes to flow out of the tank.

In terms of hydraulics, this action is called “attenuation” or dampening of a flood wave. It is simply the process of slowing down a rapid inflow into any hydraulic vessel. This attenuation is exactly what takes place in the many dams along the Missouri River. Floodwaters flowing down the river raise the level of the water impounded above the dam. The rate of outflow is controlled by adjusting the outlet gates.

TANK SPECIFICATIONS

A slowing of the sewage flow also happens in a septic tank without a compartment, as explained above. However, a septic tank with a compartment experiences even more attenuation of the flow. The compartment wall must be liquid tight and have an outlet tee installed in the compartment wall. The outlet pipe of the second compartment must be two inches lower than the outlet pipe of the tee in the compartment. This is how a properly compartmented tank is constructed. The septic tank will have two liquid levels, and it is desirable to restrict the flow between compartments.

The design of a septic tank with a compartment requires that the first compartment contain at least half, but not more than two-thirds of the total volume of the tank. If our 1,000-gallon septic tank had been compartmented with 600 gallons in the first compartment, the liquid level in the first compartment would have raised 1.67 inches with the 20 gallons of wastewater. The outflow into the second compartment would begin as the level rose in the first compartment.

As the liquid level in the second compartment began to rise, the outflow from the tank would slowly begin. The final outflow from this septic tank will be much slower than a tank without a compartment. This slower flow will allow a clearer effluent to flow out of the tank.

I do not have any physical data on the total time of outflow from such a compartmented septic tank. But in my opinion, the total outflow time will be at least double the 20 minutes I observed in my septic tank with no compartment. A septic tank with a proper compartment has a slower outflow than a tank of the same volume without a compartment.