Question:

I am a treatment plant engineer in Connecticut, and I am interested in the role tank insulation plays on septic system effectiveness. Has anyone done a study, or would do a study of septic systems in a cold climate to determine if there are bacterial action benefits to keeping septic tanks warmer, or if the only benefit is keeping them from freezing?

Answer:

This is already November and the winter winds will soon be whistling. So your question is very timely. Freezing of the onsite sewage system can be a problem in cold temperature conditions, particularly if there is little or no snow cover.

In Minnesota and many other states with freezing winter temperatures and a minimum of snow cover, the major purpose of insulating a septic tank would be to prevent a freeze-up of the system. In some cases, additional heat is added to the tank, particularly if the system is used only on weekends, like a cabin in the northwoods.

I am not a bacteriologist, but I have learned from my colleagues that an optimum temperature for anaerobic bacteria is the temperature of the human body. This temperature is rarely achieved in a septic tank, but activity by the anaerobic bacteria in a septic tank would certainly increase as the temperature increases.

Typical septic tank temperatures in the summer in Minnesota might be 50 to 60 degrees. Septic tank temperatures have become even cooler with the cold-water clothes washing that has become common.

ADD INSULATING LAYER

Heat is lost from the septic tank into the surrounding soil. An insulating layer of at least four inches of expanded polystyrene on the top and down along the sides of the septic tank would certainly decrease the heat loss and keep the tank temperature higher. The increased anaerobic bacterial activity will increase the rate of breakdown of the sewage solids. (The polystyrene must be of the type that can be buried in the soil.)

I believe it is a matter of record that septic tanks in the southern part of the United States have a slower buildup of solids than tanks in the north. This is likely due to a higher temperature inside the septic tanks and the increased bacterial activity in the warmer tanks.

You are correct; there is a benefit to the anaerobic bacterial action if a tank is insulated and stays warmer. However, other factors involved in servicing and maintaining an onsite sewage treatment system make this of little consequence.

Today, there is a distinct trend for local sanitary codes to require pumping septic tanks every three years. There is also a trend to increase the required size of the septic tank for a home. With these factors in the picture, there is really little need to worry about increased bacterial activity. The septic tank will normally be serviced and the solids removed long before there is any danger of the tank filling with solids.

But there is a need to keep the onsite system from freezing under severe weather conditions. To prevent freezing becomes the major reason for the insulation. The layer of expanded polystyrene explained above would certainly be beneficial to prevent the septic tank from freezing.

Today many onsite systems have a pumping tank to deliver the effluent to a sewage treatment mound. The effluent from the septic tank is stored in the bottom of the tank until it is time for the pump to run. The effluent in the pump tank will be colder than the septic tank and is more susceptible to freezing. A layer of at least four inches of expanded polystyrene should be placed over the top and down along the sides of the pumping tank. Tank heaters are also available for use in pumping tanks and septic tanks.

BLANKET OF SNOW

While we’re on the subject of insulation, there are other parts of the onsite sewage treatment system to consider. Weather conditions seem to be changing. In Minnesota and Wisconsin, we used to get a foot or so of snow in December, and the insulating blanket of snow often stayed and was added to during the winter. Now there may be little if any snow in December, or we get the January “thaw” so there is no snow cover. When the temperatures drop to well below zero in January, the frost moves down in the soil. Or if we have a snow cover, snowmobilers might run over it and pack the snow down, reducing its effectiveness as an insulator.

It may be necessary to place an insulating cover of leaves or hay or straw over the drainfield portion of the system if there is no snow cover. Contractors should advise their customers of the need for an insulating cover over the drainfield and a fence or other warning to keep traffic from compacting the snow.

Another potential freezing point is the sewer line from the house to the septic tank. In these colder northern climates, it’s wise to use 4-inch sewer pipe that is enclosed in a larger pipe and insulated. Be careful that the house sewer is laid on a constant grade so all the liquid drains out.

Today systems are generally shallower, which is a good practice from a treatment standpoint. However, the shallow systems need to have protection from freezing conditions.

Onsite sewage treatment contractors should warn customers about the potential problems relating to freezing. This is particularly true for new systems, which may not have established a grass cover before winter sets in.