Impact of Temperature on the Onsite Septic System

The operation of an onsite Septic System can be affected by several exterior factors. Some of them like family’s lifestyle or drain-field’s soil we already discussed in a series of articles dedicated to onsite septic systems. The important, although often underestimated factor is the ambient temperature (in a wider sense – the climate zone).

Winter has its natural beauty, but Septic Systems may see it differently…. Source: “Do I Need to Winterize my Septic System?”, Space Coast Plumbing Inc. (Melbourne, Australia).

For most readers, it will be obvious that temperatures below the freezing point (32 oF or 0 oC), can stop the operation of the system. Of all elements of the Septic System, the most susceptible to freezing is the Effluent Disposal System, more precisely – the network of distribution pipes and the drain-field soil (especially, shallow layers making an absorption zone). Frozen wastewater will block the gravity-driven outflow of effluent and lead to spills from tanks and/or back up through the line pipe into the house’s plumbing system.

Much less understood is the impact of temperatures on the wastewater treatment process. This process largely relies on biological activities of invisible to naked eye anaerobic and aerobic bacteria. Like it’s the case of all living organisms, their activity and survival are possible only over a limited temperature range.

Onsite Septic System with the detailed structure of the effluent treatment zone. Source: “Color Infrared Survey for Identification of Failing Onsite Treatment Systems”, by William E. Roper

At temperatures below 35°F (2°C) the biological activity stops. It means, that crucial elements of the wastewater treatment process like the decomposition of organic matter and breaking-down chemical compounds will come to an end. The septic tank will serve only as a physical separator of solids and oils, while the drain-field (soil) – a zone of absorption and physical filtering. As the result, the septic system will be releasing untreated, contaminated wastewater into nature.

Impact of temperature on the Aerobic process (TSS – Total Suspended Solids, Bacterial Floc- masses of bacteria associated with fungal filaments). Source: “Performance of wastewater treatment during variable temperature” by Alisawi H.A.O., Applied Water Science (2020)

High temperatures have similarly destructive effects. Rising temperature decreases the amount of oxygen in the water, leading to the deterioration of aerobic activity (mainly in the soil, where the aerobic process is dominant). And, at temperatures exceeding 113 oF (45 oC) bacteria cease to function and eventually die. But there is more than that- high temperatures have also a negative impact on the process of separation of solids and oils in the septic tank. They affect the formation of scum (oils and grease easily solidify at low temperatures, so cooler is better) as well as increase the quantity of suspended solids in the effluent that will eventually find their way into the drain-field.

All in all – the optimal temperature range for residential wastewater treatment is between 68oF -to 104oF (20oC to 40 oC).

Note, that biological treatment in onsite residential wastewater is mostly carried out by Mesophilic bacteria. Industrial Wastewater Treatment Plants may operate at higher temperatures; however, they use thermophilic cultures of bacteria, not naturally present in residential sewage.

Four types of microbial communities: mesophiles, thermophiles, psychrophiles, and hyperthermophiles. Of those, mesophiles are naturally present in residential wastewater.  Source: “Performance of wastewater treatment during variable temperature” by Alisawi H.A.O., Applied Water Science (2020)


Residential, onsite Septic System consists of 4 major elements:

  1. Line pipe connecting the house with the exterior tank.
  2. Septic Tank(s)
  3. Effluent Distribution System
  4. Drain-field.


Residential, onsite Septic System. Source: “Septic (Onsite Sewage) Systems”, Allen County, Department of Health (IN, USA)

Operation of all of them can be affected by extreme temperatures, however, their impact is always lessened by the following favorable factors:

a. Statistically, about 1/3 of water used by an individual household is hot. Showers, bath and kitchen sinks, dishwasher, washing machine release large amounts of warm and hot water. This factor alone guarantees that even in winter, the average temperature of wastewater in the septic tank is always higher than the ambient one.


Kitchen and bath faucets are major sources of hot water directly ending in the drain. Source: “Four Ways to Get Instant Hot Water at the Faucet”, BillyGo Plumbing, Heating and Cooling (Dallas, USA).


b. The decomposition of organic matter also contributes to the rise of wastewater temperature. It’s a minor factor in septic tanks where dominates the anaerobic process, but it’s noticeable in upper layers of drain-field soil where dominates the aerobic process.


c. Surrounding soil represents an enormous heat accumulator. Due to the transfer of heat from deeper layers of Earth, at most inhabitable geographical locations (except for Arctic permafrost-zones), the soil 2-to 3ft deep below the surface, marks the border of the “no-frost” zone.


d. Snow (especially fresh and fluffy) is an excellent insulator protecting the soil from persisting low ambient temperatures as well as from the impact of gusts of “arctic-cold” winds. In fact, 1ft of fresh snow may be equivalent to 6 inches of R-18 fiberglass insulation (as the snow becomes more and more compacted, it gradually loses its insulating properties). Unfortunately, we do not have much influence on weather patterns, snowless winters will leave bare soil unprotected, depriving the septic system of the benefits of natural insulation.


Line Pipe

Typical residential houses have basements, so line pipes carrying wastewater to septic tanks are buried at least 4ft below the ground level. At that depth (except for arctic zones), the soil can stay unfrozen throughout the whole winter.

The time and amount of work necessary for the construction of the basement contradict the most important characteristics of container houses – remote location and short installation time. That is why, container-based houses are mostly installed on easy to make (undemanding) foundations like pavement blocks, piles, strips… For this reason, line pipes are typically installed at around 1ft below the surface, to allow for the construction of a traditional, gravity-based septic system.


For reasons like the need for natural aeration, the required minimum depth of absorption and treatment zones (3ft), unsuitable deep layers of soil, high water table, etc … conventional drain-fields are not buried deeper than 2ft below the surface. That leaves just about 1 -to- 1.5ft for the required difference of levels between the tank’s inlet and outlet as well as the slope of distribution pipes.  Otherwise, the pump station will be necessary to guarantee the flow and disposal of the wastewater.

By the nature of physics, the shallow line pipe will be exposed to freezing temperatures. As mentioned earlier, the large amount of household-generated wastewater is hot. Released almost instantaneously, it will rather safely reach the septic tank. However, all longer-lasting or permanent leaks will contribute to the build-up of ice in the line pipe and ultimately, its blockage!

Septic Tank

Due to mentioned earlier favorable factors (first of all – the use of hot water), effluent stored in the septic tank will be some 10-to-20 oF (6 -to-11oC) warmer than the temperature of the surrounding soil. While it will almost always prevent the tank from freezing, the bacterial activity may significantly drop, and so the efficiency of the pre-treatment process.

While we can’t change the weather, we can take few reasonable steps to minimize the impact of temperature on the operation of the septic tank:

a. Slow bacterial activity should be compensated by proportionally longer wastewater retention time. In other words, the capacity of the septic tank located in a cold climate zone must be larger than that required for an equivalent household in a warmer climate zone. Extended retention time will give bacteria more time to do the “job” as well as will help solids to settle at the bottom of the tank.

Note, that the minimum capacity of the septic tank at a given geographical location will be reflected in local regulations!

b. Walls of fiberglass (plastic) tanks may create better insulation barrier than thick walls of concrete tanks. It is especially true in wet (water-saturated) soils because concrete, due to its inherent porousness, is easily infiltrated by the water.

If possible, tanks located in cold climate zone should be protected by an extra layer of insulation (especially, the upper part of the structure facing the surface).

Spray foam-insulated septic tank. Source: “How Temperature Impacts Onsite Wastewater Treatment” by Sara Heger (Onsite Installer).


a. Water (rain and/or melting snow) infiltrating the soil in the proximity of the septic tank creates thermal bridges to the surface. Especially, frozen water accumulated around the tank will substantially decrease the surrounding temperature as well as limit the transfer of heat from deeper layers of soil.

It is always suggested to install the septic tank on a thick, compacted gravel bed, as well as to fill the space on all its sides with gravel. As high-percolation material, the gravel (in general – stones) will prevent the accumulation of water in the proximity of tanks. For the same reason, the tank should be located away from downspouts and any other areas where rainwater may be naturally collecting.

It will be helpful to install a water-impermeable fabric above the tank to divert rainwater away from it.


b. If possible, the tank’s risers (and lids) should be isolated to minimize the outward transfer of heat from the tank. Monitoring pipes (if any) must be capped and isolated.


Distribution Systems

In gravity-based distribution systems, the supply pipe, D-Box, and perforated distribution pipes are fed by effluent based on “demand”. The demand is naturally created by the inflow of wastewater from the house’s plumbing system (“what flows in, must flow out”).  In most cases, these are small volumes of wastewater created by such events as washing hands, numerous kitchen sink’s activities, brushing teeth, etc. Equally small volumes of outflowing effluent will be rather “seeping” down the distribution system. Carrying a little amount of heat, the effluent will be quickly dropping its internal temperature and eventually freeze before reaching the absorption zone in the drain-field.

Also, any imperfection in the network of feeding and distribution pipes (local depression, slumps, bows, etc, caused by activities on the surface of the drain-field), will lead to accumulation of effluent, the formation of ice, and ultimately blockage!  Unfortunately, with little (or rather no control) we have over gravity-based systems, not much can be done to improve this situation.

In contrast, pressure-dosed systems fully control the distribution process. The outflow of effluent (volume, pressure, and timing) is strictly controlled and can be programmed to accommodate the daily weather patterns (expectedly cold nights and warmer days). Larger doses of effluent move fast through pipes (pressure-driven as opposed to natural “seeping”), and once the pump shuts-off, remaining in pipes effluent is discharged back into the dosing tank.

Drain field

In the majority of cases, a drain field, protected by about 2ft of the topsoil overgrown by grass, should be able to absorb discharged effluent within the reasonable range of winter’s ambient temperatures. The “pipe-in-gravel” systems offer a few feet deep layer of gravel, ready to “instantly” absorb a large amount of effluent before it can freeze. Similarly, chamber-based systems offer large vaulted-space able to accommodate a significant volume of effluent. So, at most geographical locations (with exception of Arctic zones and/or prone to freezing, permanently wet areas), the capability of absorption is rarely altered by low temperatures.

Aerobic versus anaerobic process conditions. Source: “Aerobic Composting And Anaerobic Digestion” by Craig Coker, BioCycle.

In contrast, the effluent treatment process in lower layers of soil may be seriously affected. The decomposition of organic solids and neutralization of pathogens almost entirely depends on bacteria and the availability of time. For that to happen, contaminants must be first trapped by particles of soil, then subjected to chemical and electrical interactions, broken-down to simpler structures, digested, neutralized by naturally occurring in the soil “antibiotics” (fungi)…. It’s a complex process in which surface vegetation through its system of roots also plays an active role.

Basically, the biological treatment process of effluent follows the rules imposed by Mother Nature. The yearly cycle of all living organisms includes seasons of growth (spring and summer), maturity (autumn), and season of hibernation (winter). We like it or not, living in the soil bacteria, microorganisms, fungi, worms, as well as plants simply follow nature!

In contrast, we, human beings, protected from seasonal changes of weather by walls, roofs, heating & cooling systems, using electrical lights to change the night into day, we created our own, artificial environment that does not have to comply with the rules of nature. In this particular case, we generate wastewater 24/7 throughout the whole year, but Bacteria do not care much about that, they live their own lives. Bending Mother Nature and forcing Her to follow our rules has limits ☹.

Practical Notes:

  1. Due to the severe impact of low temperatures on the operation of residential, onsite septic systems, they are rather impractical, inefficient, and often banned from use in Arctic zones (especially in permafrost areas)! It does not mean that individual houses are deprived of these basic systems, it means, however, that Local Authorities provide tightly controlled regulations and allow only for construction of systems specially designed for such environmental conditions.


  1. Traditional Septic Systems rarely freeze if houses are permanently occupied. However, it may not be the case of seasonal houses, vacant for even only one or two weeks. The truth is that hot water poured down the drain is the biggest factor keeping the septic systems alive. Low usage or lack of it, will inevitably lead to the freeze of the tank.


  1. Continuous water leaks ending in the drain (toilet flush-system is the most frequent example) are the strongest contributors to the freeze of the septic system. Small quantities of water move along the pipes in smooth, thin-films layers. They are first to freeze within the pipe and (after accumulation) fully block it!


  1. Fresh snow acts as a free insulating blanket spread over the septic tank(s) and the drain-field. Do not remove it as it will expose the surface to ambient temperatures and allow frost to get deeper into the ground. Do not compact it, as it will significantly lose its insulating properties (it’s not that much snow itself, but rather zillions of air gaps in fluffy snow that create the insulation barrier).


  1. Vegetation overgrowing drain fields not only is part of the natural Wastewater Treatment Plant (as explained in previous chapters). It also creates a layer of thermal insulation and even if it’s only a small contribution, it all adds up. BTW- lawn turf, due to its uniformity, is certainly a better solution than any ornamental flowers.

winter protection

Spread a layer of straw or leaves over the system to provide insulation. Source: “Preparing Seasonal Septic Systems for Winter” by Sara Heger, Onsite Installer


6. Pipes are often blocked by frozen wastewater accumulated in a “low-point” (bowed) areas. Such deformations can happen due to driving vehicles over the sensitive areas, by sagging soil (typical for newly installed systems), by heavy lawnmower (tractor), playing children, etc…



Antifreeze is a mixture of distilled water with either ethylene-based glycol, or with ethanol or propylene-based glycols. The most popular, ethylene-based antifreeze has a lower freezing point and better heat-transmission properties compared to ethanol or propylene-based types of antifreeze. Unfortunately, it is highly toxic, so it can be used ONLY in closed systems, like automotive engine cooling systems. Due to serious health-hazards, it cannot be released in the raw form into nature!

The RV & Marine Antifreeze from Splash is designed for use with recreational vehicles, boats, spas, pools, and vacation homes.


In contrast, ethanol or propylene-based antifreeze is relatively safe (in small quantities they are used in many personal care products like cosmetics). They are commonly used for the protection of RVs blackwater tanks, for de-icing airplanes (which means they are released in large quantities into nature). The bottom line – in moderate quantities the ethanol or propylene-based antifreeze (known as RV Antifreeze) can be used for the protection of onsite septic systems. Given the very fragile septic tank’s environment, even this type of antifreeze should be used ONLY as a last resort solution! The main rule still holds – less we mess-up with nature, the better it is for all of us (and for nature).

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