Is it necessary to protect the septic tank and pipes from freezing?

Sometimes. Here we describe when, why, and how frost protection is needed or useful for sewer piping, septic tanks, effluent distribution pipes, and drainfields or soakaway beds.

To what depths are septic tanks or cesspools o

Septic Tank & Piping Frost Protection

Even in freezing climates, the septic tank serving an occupied home is unlikely to freeze, partly because of latent heat the bottom of the septic tank receives from the earth and partly because of the heat generated by the bacterial action going on in the septic tank, and mostly from warm wastewater entering from the building served by the septic system.

However in very cold climates, in winter weather with little or no snow cover, and where the septic system is receiving little or no active use - that is warm wastewater from the building is not flowing into the tank, frost protection is appropriate.

Watch out: DO NOT add antifreeze to a septic system. You'll kill the bacteria and destroy the drainfield.

[Click to enlarge any image]

Question: What keeps my septic tank from freezing?

2018/12/27 septic tank freeze said:

What keeps a septic tank from freezing,

We had to open ours yesterday and the soil was is now loose above.

It has been in use for 40 years and needs to be cleaned and will be as as soon as the weather in iowa permits.

we covered the dirt with leaves and a tarp. thanks, Bob

This Q&A were posted originally at SEPTIC SYSTEM DESIGN BASICS

Reply: warm wastewater, latent heat, bacterial action help keep the septic tank from freezing

Septic

Thanks for a great question and one that worries lots of people.

Why doesn't the septic tank freeze, and do we need to protect it from freezing?

An in-use septic tank, even when not below the frost line, has some warmth from incoming wastewater from the property (this is probably the main source of warmth), and additional heat generated by bacterial action and oxidation of waste in the septic tank.

A healthy septic tank and drainfield use both aerobic (using oxygen) and anaerobic (doesn't need oxygen) bacteria to break down waste.

Most of the bacterial action is taking place in the drainfield not the septic tank, so that plus the receipt of warm water in the drainfield can explain why if your drainfield trenches are not very deep you may see snow melting over the trench lines.

In addition, even when not in use and thus not receiving warm wastewater for a time, the septic tank has protection against freezing as well from its large thermal mass.

The anaerobic bacteria group in the septic tank also happen to like and work harder in a warmer environment. So they love it when you take a hot shower.

We don't try to bury the septic tank below the frost line.

Why?

At SEPTIC TANK DESIGN DEPTH we explain that septic engineers prefer to keep the drainfield and tank as high as practicable in the soil in order to avoid having to dig even deeper for the leach lines or soakaway bed piping and trenches - keeping those well above the high water table to avoid contaminating the environment with sewage effluent.

This design also helps assure that there is some oxygen available to soil bacteria - necessary in turn to support the aerobic bacteria family in their work breaking down septic pathogens and waste.

You can imagine that if we're to avoid the added complexity and cost of an effluent pump-up (to the drainfield) system in a conventional septic, we don't want to put the tank deeper than necessary - else effluent flowing to the drainfield by gravity would put the fields below available soil oxygen.

The consistency of heat production (or reservoir) inside working septic tanks has led some researchers to investigate the possibility of heat recovery from septic tanks - not something I'd recommend as we risk interfering with healthy septic tank operation.

Watch out: in extremely-cold climates OR in a very cold climate when a septic tanks and system are to be unused for a prolonged time, protection against septic tank freezing may be needed.

Watch out also: to the extent that climate change leaves some areas cold but without snow cover, the risk of septic system components freezing is increased.

• Leaks or drips in the house
  
  can form ice in the above-frost-line waste line between the building and the septic tank, causing a blockage, risking a sewage backup or burst sewer piping.
  
  Be sure there are no leaks, drips, running toilets in your home.
  
  Watch out: leaving water running or trickling in very cold weather, thinking you're avoiding frozen water supply pipes risks a frozen, burst drainline or septic component.
  
  See DRAIN FREEZE PROTECTION
  
  And more details at PIPE FREEZE PROTECTION - home
• Insulate the septic tank top and risers:
  
  In very cold climiates (Alaska, Manitoba Canada, Siberia for example), you may need at least a foot of insulating material over the septic tank and extending 1-2 meters past the tank top in all directions.
  
  Temporary insulation can be achieved with mulch, straw, even leaves or hay, but don't bother to put a sprinkling of hay or straw or just an inch - that amount of insluating material is just too insignificant to make any difference.
  
  Particularly for little-used septic systems in very cold climates you should consider permanent septic tank insulation.
  
  Don't use fiberglass or any other insulation that absorbs water - it'll simply turn into a block oif ice. Where I've installed permanent septic system freeze protection we used 2" thick solid closed-cell foam insulating blocks covered with backfill.
  
  For septic tanks accessed by risers that reach a tank that is itself two feet or less below ground you may need to add extra insulation.
  
  For deeper septic tanks accessed by a septic tank riser, don't forget to either install insulated riser covers or otherwise protect the riser cover from freezing or you will be unable to have the tank serviced in winter should that be needed.
• Don't drive over the septic field
  
  ever with heavy equipment or you risk compacting the soil and breaking pipes - damaging the fields.
  
  But in winter when there's snow cover, even driving a light snowmobile or quad or ATV over the fields should also be avoided because compacted snow loses its insulating value. Leave the snow intact over the drainfield.
  
  More details are at DRIVING OVER SEPTIC SYSTEMS
• Check the drainfield or soakaway bed:
  
  if you see standing water or wet soggy areas the field is flooded and is already in failure. And it's also at risk of freezing solid.
  
  On the other hand, if a septic system field is working properly and disposes of the effluent it receives, then during a period of inactivity the pipes and trenches themselves should be empty.
• New septic fields with bare soil
  
  and lacking grass or leaf cover are at extra risk of freezing and may need temporary cover.
• Supplemental electric (or other) heat may be needed
  
  for some septic designs such as shallow pumping stations.
  
  Electric heat in the form of sewer pipe heating cables, circulating heaters, immersion heaters and internal pipe heaters, and even warm air blowers have been used to avoid freezing septic equipment.
  
  Temporary heat sources for abnormal conditions can also be used to protect septic tanks from freezing, such as using a combination of circulating steam or hot water along with insulating blankets, portable heaters, stock tank heaters (used to keep livestock drinking water from freezing).

Septic Freeze Protection Suppliers, Research, Advice

Above: septic mound system under deep snow cover, Two Harbors MN. This mound septic is also illustrated at MOUND SEPTIC SYSTEMS

• Aero-Stream, "How to Protect Your Septic Systemn During Freezing Temperatures" [web article] Aerostream, Aero-Stream®, LLC W300N7706 Christine Lane Hartland, WI 53029
  USA, Tel: (877) 254-7093 retrieved 2018/12/27, original source: https://www.aero-stream.com/protect-septic-system-freezing-temperatures/
  
  Excerpt: Septic systems that are used frequently are less susceptible to freezing because of the constant flowing water.
  
  [Septic] Systems that are not in constant use, or that lay dormant for long periods of time, are more likely to have issues in colder climates.
• Hanegan, T., and K. DenBraven. "The feasibility of using a septic tank as a heat source for geothermal heat pumps." (1999): 207-209.
  Abstract:
  
  A geothermal heat pump (GHP) system with three ground coils was installed in a residence in northern Idaho with a portion of the ground heat exchanger wrapped around the residential septic tank. The septic coil provided a significant portion of the heating for the residence over the heating season.
  
  There was no evidence of the septic tank freezing up or failing to properly function. Utilizing a septic tank as a heat source for GHP systems is feasible design option if the septic tank is used on a full-time basis. However, the tank should be surrounded on all sides by a large amount of soil and/or insulated from the ground surface to ensure that ground temperatures near the tank remain warm during the winter.
• Hickey, John LS, and David L. Duncan. PERFORMANCE OF SINGLE FAMILY SEPTIC TANK SYSTEMS IN ALASKA [PDF] Journal (Water Pollution Control Federation) (1966): 1298-1309. Retrieved 2021/03/30 original source: https://www.jstor.org/stable/25035610?seq=1#metadata_info_tab_contents
  
  Abstract
  
  Residential septic tank-seepage pit systems designed to FHA standards performed satisfactorily in the Anchorage and Fairbanks, Alaska, areas. Fifty percent of 127 systems observed in Anchorage functioned 10 years without malfunction; in Fairbanks 50 percent of 46 systems observed functioned 6.5 yr without malfunction.
  
  Most malfunctions were due to failure of the secondary absorption systems.
  
  A substantial number of cases of early failure were observed in both areas caused by defects in design or construction. Temperature measurements made on 20 septic tanks and seepage pits showed that septic tank contents remained above 48° F (8.9° C) during the winter and that freezing in the secondary systems was not a problem. Adequate treatment was obtained at this temperature.
• Hotline Internal Water & Sewer Pipe Heaters, Hotline Inc., PO Box 1999, Granby CO 80446 USA Tel: 877-881-2980, Website: https://hotlineinternalpipeheater.com/
• Immersion heaters for septic tanks - more than 200 companies provide immersion heaters of various models. Contact your plumbing or heating supplier.
• Kadlec, R., Axler, R., McCarthy, B. and Henneck, J. (2000). (submitted). Subsurface Treatment Wetlands in the Cold Climate of Minnesota. Constructed Wetlands for Wastewater Treatment in Cold Climates. Advances in Ecological Sciences (in press).
• Minnesota MPCA DON'T LET YOUR SEPTIC SYSTEM FREEZE [PDF] (2018) Minnesota Pollution Control Agency, retrieved 2018/12/27 original source https://www.pca.state.mn.us/featured/dont-let-your-septic-system-freeze
• Oregon DEQ, SEPTIC SYSTEM DOs and DON'Ts [PDF] Oregon Department of Environmental Quality, Onsite Program, retrieved 2018/12/27, original source: https://www.oregon.gov/deq/FilterDocs/SepticDoDont.pdf
• Reed, Jane, Barbara J. McCarthy, Jerald Henneck, Richard P. Axler, Jeff Crosby, and Heidi Bauman.
  
  FREEZE SURVEY SUMMARY REPORT: ONSITE WASTEWATER TREATMENT SYSTEMS [PDF] (2001). - University of Minnesota, Duluth, retrieved 2021/03/30 original source: https://conservancy.umn.edu/handle/11299/200924
  
  Abstract
  This report summarizes the results of an informal survey mailed to pumpers and contractors of onsite wastewater treatment systems in northern Minnesota in the spring of 2000. The survey was developed jointly by the Sewage Technical Committee (TC) of the Iron Range Resources and Rehabilitation Board and Northern Lights Tourism Alliance (IRRRB/NLTA). The purpose of the survey was to determine, in general, the types of freezing problems associated with onsite systems during the winter of 1999-2000.
  
  Excerpts:
  About 20% of the systems (75 out of 369) with freezing problems were new construction; 3 7% of the systems (137 out of369) ranged between 1-5 years old; 25% of the systems (92 out of369) were between 5-10 years old; and 18% of the systems ( 65 out of 369) were greater than l O years old.
  
  The results suggest that systems less than one-year old had a higher freezing rate as compared to systems used for more than one year.
  
  In the industry, it is generally recognized that new installations, especially those installed late in the construction season, have a greater tendency to freeze during their first winter of operation as compared to systems used by homeowners for several years.
  
  Cold septic tanks may contribute to freeze problems on some new construction, especially when wastewater flows from the home are low or sporadic.
  
  The cold, disturbed soils, and less than adequate grass cover over a newer system, likely contributes to the problem. Both grass and snow cover help to insulate the ground and the onsite system components. When disturbed soil is placed onto a newly installed system, and the re-establishment of a grass cover is not complete, the frost depth can be quite extensive, especially in years with minimal snow cover.
  ...
  Protection from the cold is an obvious solution to the problem and a normal year of snow cover in northern Minnesota typically provides much of the needed insulation. But in winters with no snow cover until late December or early January, it may be wise to provide additional insulation for system components, especially those installed at shallow depths.
  
  Although insulation adds cost to a system, it might be money well spent in those years when snow cover is minimal. Straw placed over the system at least 12 inches deep is typically effective at minimizing frost depths.
• Scherer Tom, PRECAUTIONS CAN PREVENT FROZEN SEPTIC SYSTEMS [PDF] (2010) North Dakota State University, NDSU Extension Service, ND Agricultural Experiment Station, retrieved 2018/12/27 original source https://www.ag.ndsu.edu/news/newsreleases/2010/sept-20-2010/precautions-can-prevent-frozen-septic-systems
• Septic Heater Co., 12141 Gates Ave., Northfield, Minn. 55057 (ph 888 417-3784 or 507 649-0831; Email: chris@septicheater.com; website: www.septicheater.com)
• Septic Heater Company, PO Box 891 Tioga, ND 58852 Website: http://www.septicheater.com/product Tel: 1-888-417-3784 Email: Chris Norgaard chris@septicheater.com
  Website excerpt:
  The Septic Heater prevents septic systems from freezing in the winter.
  
  It fits securely to the existing access pipe (4" SDR35) above a septic tank, a lift station, or a drop box in the drainfield and blows warm air into the system.
  
  The air is naturally vented out the existing soil stack (main vent) of the home. The Septic Heater is easy to install.
  
  Simply slide it onto your 4" access pipe. The heater is powered using an outdoor extension cord. Put it on in early winter, and remove it in the spring.
• Simons, A. P., and F. R. Magdoff. Disposal of Septic Tank Effluent in Mound and Sand Filter‐Trench Systems on a Clay Soil. Vol. 8, no. 4. American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America, 1979.
  Abstract:
  
  Small‐scale mound systems and conventional sand filter‐trench systems for disposal of septic tank effluent were established on a clay soil. During a wet year (1976), major differences in soil moisture patterns were observed between the two systems.
  
  Unsaturated flow of effluent occurred through the gravel/sand interface of the mound systems, although a slight pressure‐head buildup occurred at the bottom of the mound fill. Conventional sand filter‐trench systems were under water throughout most of the 1976 monitoring season.
  
  However, when the dry conditions of 1977 allowed the water table to drop below 1.2 m, unsaturated conditions occurred below the gravel/sand interface of both the mound and sand filter systems.
  
  Water tension patterns below the gravel/sand interface indicated development of the early stages of clogging (but not permanent ponding) in the mounds, while those within the sand filter‐trench system indicated the development of a restricting layer.
  
  Rejuvenation of the mound infiltrative surface occurred during the warm and dry 1977 spring and early summer.
  
  Temperatures below the gravel/sand interface dropped to about −3°C during the winter in the mound systems, but stayed just above freezing in the sand filters.
  
  A lack of snow accumulation on top of the mounds probably allowed the subzero temperatures.
• Wallace, S., G. Parkin, and C. Cross. COLD CLIMATE WETLANDS: DESIGN AND PERFORMANCE [PDF] Water Science and Technology 44, no. 11-12 (2001): 259-265.
  
  Abstract:
  Constructed wetlands are gaining widespread use as a simple, low cost means of wastewater treatment. Introduction of constructed wetlands technology into the northern United States has been limited by the ability of conventional wetland systems to operate without freezing during the winter.
  
  A design approach using subsurface-flow constructed wetlands covered with an insulating mulch layer has been demonstrated to prevent freezing. However, introduction of a mulch layer will affect oxygen transfer rates, pollutant removal performance, and plant establishment.
  
  These factors must be addressed for successful application of constructed wetlands technology in cold climates.

 

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

Illustration: depth of snow cover is a key factor in septic system freeze-ups (Reed 2001) cited below. [Click to enlarge any image]

We discuss this illustration and also report on a reader's experience with graywater disposal system freezing (or not-freezing)

at GREYWATER FREEZE PROTECTION

...

Continue reading  at GREYWATER FREEZE PROTECTION or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

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