by Brian Tomasik
First published: 25 Oct. 2017; last update: 12 Nov. 2017


This piece discusses unicellular and multicellular organisms that are brought into existence by sending waste to septic systems. There's an enormous variety of septic-system designs, and this piece only offers broad generalizations. While septic systems create some invertebrate suffering, I suspect that flushing organic matter down the drain into a septic system tends to create fewer new invertebrate animals than disposing of that waste in many other ways, such as composting or throwing the waste in your yard.

A rough calculation suggests that if septic drain fields stimulate lawn plant growth, the increase in invertebrate suffering due to this extra plant biomass might be nontrivial; further exploration of this point is required.


Decomposition of organic matter feeds small organisms. These organisms are sadly born without their consent into short lives that quickly end in potentially painful deaths.

I consider invertebrate animals like nematodes and rotifers to be more sentient per Joule of organic matter consumed than bacteria, since invertebrate animals have nervous systems, are capable of learning, and so on (Tomasik "Brain ..."). Therefore, if a given quantity of organic matter must be consumed, I probably prefer for bacteria to eat it than for animals to eat it, since this will presumably create less sentience-weighted total suffering. Therefore, I ceteris paribus favor septic-system designs that support fewer animals. I would speculate that more anaerobic septic systems support fewer animals, since most animals require oxygen to live, but I don't have specific data on that.


Septic systems are common in rural areas. Wikipedia ("Publicly ...") says that publicly owned treatment works in the USA serve "75 percent of the total population. The remainder is served by decentralized or private septic systems." Wikipedia ("Onsite ...") concurs, saying that onsite sewage facilities "account for approximately 25% of all domestic wastewater treatment in the United States."

Wikipedia ("Onsite ...") explains what while in nature, "animals such as mice, rats, flies, and parasites" participate in waste recycling, onsite sewage facilities "typically attempt to exclude them to prevent out of control population explosions and infestation, and prevent spread of vermin and disease."

Organisms in septic tanks

The first step in a septic system is typically a septic tank.

Steve's Plumbing (2014): "All septic tanks contain naturally occurring bacteria to aid in the decomposition of solid waste".

Wikipedia ("Septic tank") says: "Settling and anaerobic processes reduce solids and organics, but the treatment is only moderate. [...] The term 'septic' refers to the anaerobic bacterial environment that develops in the tank which decomposes or mineralizes the waste discharged into the tank."

Seman (n.d.), p. 12: "Septic tanks takes several days to reduce organic material through use of anaerobic bacteria".

Wikipedia ("Septic drain field") likewise says that anaerobic digestion occurs in septic tanks, which I take to imply that there are no invertebrate animals in septic tanks (other than those that may be incidentally washed down sink drains, toilets, etc.)?

Aerobic systems

However, Texas Cooperative Extension (2002) explains (8m52s) that some systems add aeration chambers for aerobic decomposition. Can these aeration chambers contain invertebrate animals because they have oxygen?? My impression is that most onsite sewage facilities don't have this aerobic step.

Wikipedia ("Aerobic ..."): "An aerobic treatment system [...] is a small scale sewage treatment system similar to a septic tank system, but which uses an aerobic process for digestion rather than just the anaerobic process used in septic systems."

Organisms in septic drain fields

Wikipedia ("Septic drain field") explains that septic drain fields "remove contaminants and impurities from the liquid that emerges after anaerobic digestion in a septic tank."

Are drain fields aerobic?

Geomatrix (2016) claims that "a traditional leach field [is] anaerobic". However, other sources dispute this somewhat.

Wikipedia ("Septic drain field") says:

Just as the septic tank is sized to support a community of anaerobic organisms capable of liquifying anticipated amounts of putresible materials in wastewater, the drain field should be sized to support a community of aerobic soil microorganisms capable of decomposing the anaerobic septic tank's effluent into aerobic water. [...] The biofilm on the walls of the drain field trenches will use atmospheric oxygen in the trenches to catabolize organic compounds in septic tank effluent.

MeCDC (2013) says regarding "biomat" films that form in septic fields (p. 23): "Anaerobic organisms living in the biomat digest suspended organic matter contained in the effluent before it reaches the soil, where digestion of dissolved components occurs by aerobic or facultative bacteria." The same source adds (p. 37): "Treatment occurs in 2 phases, anaerobic digestion in the septic tank & aerobic and anaerobic digestion in the disposal area and filtration in the surrounding soils."

Dickert (2010/2015):

Many different soil microbes will act to filter and cleanse the liquid effluent before the harmful bacteria in it has a chance to reach ground water. These soil microbes require oxygen to function optimally, and perform less effectively in compacted and/or saturated soils. This is why it is recommended to keep excessive traffic off the drain field to avoid over-compaction of the soils. It is also recommended to keep excessive moisture from flowing over the drain field.

Invertebrate animals in drain fields?

Septic Maxx (2015) says: "Soil contains a variety of microbes that rely on the organic material in the wastewater for sustenance. Bacteria, algae, protozoa, fungi, rotifers, and nematodes are all present in a typical septic system. Aerobic bacteria are the most effective at breaking down materials in wastewater. This type of bacteria relies on oxygen to survive. [...] soil that is oversaturated with water is not a good filter, as it blocks oxygen".

Wikipedia ("Septic drain field"): "A drain field may be designed to offer several separate disposal areas for effluent from a single septic tank. One area may be 'rested' while effluent is routed to a different area. The nematode community in the resting drain field continues feeding on the accumulated biofilm and fats when the anaerobic septic tank effluent is no longer available."

Geomatrix (2016), advertising the SoilAir system, says: "SoilAir has been proven to increase the pathogen removal rate of traditional leach fields[...]. SoilAir increases predatory micro-organisms such as nematodes; which will consume pathogens such as fecal coliform."

MeCDC (2013) says (p. 3): "The microbes associated with septic systems are bacteria, fungi, algae, protozoa, rotifers, and nematodes. Bacteria are by a wide margin the most numerous microbes in septic systems." This quote doesn't explicitly distinguish septic tanks from drain fields, but I would guess the nematodes and rotifers mostly dwell in drain fields, since the septic tank is anaerobic?

Of course, nematodes would be present in lawn soil even without a septic drain field, but some of the above quotes make it sound like the effluent in septic drain fields increases the food available to lawn nematodes.

Seepage pits

tribalv (2016): "Some older septic systems were built with a seepage pit rather than a leaching field. [...] Most seepage pits are built so that at least four to six feet of soil covers the top of the pit. Because the pit is buried so deeply into the ground, most of the waste water processing is done by anaerobic bacteria." Because seepage pits are mostly anaerobic, I presume they contain basically no invertebrates. I haven't heard much about seepage pits, so I assume they're not very common?

Metabolism in septic tank vs. drain field

At a very crude level, it appears that septic tanks don't contain invertebrates, while drain fields contain some. What fraction of organic matter flushed in toilets or washed down sink drains is eaten in septic tanks vs. drain fields?

I don't have much information on this, but MeCDC (2013) reports (p. 24) that the "% Removal In A Septic Tank" of "BOD (Biochemical Oxygen Demand)" is "15% to 50%". BOD is a measure of the amount of microbe-edible organic matter in the water (Wikipedia "Biochemical ..."). So it looks like usually less than half of microbe-edible organic matter is removed within the septic tank?

MeCDC (2013) is more explicit about this point on p. 38: "About 1/2 the pollutants are removed by anaerobic digestion and/or retained in the septic tank, and the remaining constituents are metabolized by microbes in and adjacent to the disposal area in both aerobic and anaerobic conditions."

Stimulating plant growth on the septic drain field?

Wikipedia ("Septic tank") says regarding water released into septic drain fields: "The remaining impurities are trapped and eliminated in the soil, with the excess water eliminated through percolation into the soil, through evaporation, and by uptake through the root system of plants and eventual transpiration or entering groundwater or surface water."

User "humic" on Yahoo Answers says: "The on-site system partially treats the water, and the soil structure and bacteria finish cleaning it, with the water eventually evaporating, being taken up by plants, or percolating down to the groundwater table."

Day and Silva (2013), p. 1: "Plant roots can help remove excess moisture and nutrients thereby making the purification of the remaining effluent more efficient." In addition (p. 2): "Use of fertilizer may be reduced for plants growing over a leach field because some of these salts [from wastewater] are forms of the plant nutrients nitrogen, phosphorus, and potassium."

Texas Cooperative Extension (2002) says (17m21s) regarding the "Conventional Drain Field" process that "Microbes in the soil feed on the waste, nutrients, and pathogens remaining in the wastewater. Grass growing on the soil surface then utilizes that water." The video also discusses a few other techniques that, if used, clearly allow for plant uptake of septic water, though I assume these methods are less common:

  • "Evapotranspiration Bed" systems (19m27s) use evaporation and transpiration (which implies plant uptake of some of the water).
  • "Subsurface Drip": "The drip lines are buried relatively shallow, so the soil can provide treatment, and landscape plants can use the nutrients and water" (23m18s).
  • "Spray Distribution": "These systems distribute wastewater across the soil surface, much like a traditional lawn-irrigation system" (24m2s).

In cases where plant roots can access water and nutrients in the drain field, does this increase plant growth? There's a humor book titled The Grass Is Always Greener over the Septic Tank (Bombeck 1995). Presumably "Septic Tank" should be replaced with "Septic Drain Field" in this title, but perhaps the basic idea is correct?

If so, then septic systems probably create some invertebrates indirectly, by stimulating more plant growth that will eventually feed invertebrates near the soil surface. This may be an argument for reducing home water use (at least during non-winter months when plants are growing), although maybe marginal reductions to home water use make basically no impact to drain-field plant growth because those plants already receive so much water that water is not growth-limiting for them? Maybe too much water would actually inhibit plant growth?

Another reason to reduce home water use is that its treatment and consumption often kill zooplankton in the source water (Tomasik "Water ..."), although rural households that use septic systems often get their water from a well, and deep well water is unlikely to contain many zooplankton.

Dickert (2010/2015) says: "The effluent in the septic drain field does affect growing conditions" for the plants on the drain field. However, these effects are not always stimulatory for plant growth; for example, pH and salinity can sometimes be elevated due to septic effluent (Dickert 2010/2015).

Wikipedia ("Aerobic ...") explains: "Some aerobic systems recycle the effluent through a sprinkler system, using it to water the lawn where regulations approve." On the other hand: "Stabilized forms of chlorine persist after the effluent is dispersed, and can kill plants in the leach field."

Fecal vs. plant-growth biomass

How does the possibility of somewhat increasing drain-field plant growth compare in magnitude with the direct invertebrate impact of decomposition of the waste sent to the septic system?

Wikipedia ("Human feces"): "On average humans eliminate 128 g of fresh [i.e., wet weight] feces per person per day". Lundie and Peters (2005) assume "an average of 2.1 persons per household in" "the Waverley Council area"a in Sydney, Australia (p. 277). A household of 2.1 people would produce 2.1 * (128 g per day) * (365 days per year) = 98 kg of feces per year.

As far as drain-field size, DoItYourself (2010/2012) says: "A typical home with a good percolation rate may require as little as 4500 square feet of field, or as much as 9000 square feet with a poor perc rate." The average of 4500 and 9000 is 6750 square feet ≈ 600 m2. Net primary productivity for typical temperate ecosystems is on the order of ~103 metric tons per km2 per year = ~1 kg per m2 per year (Tomasik "Net ..."). Suppose that septic effluent increases plant growth over the drain field by ~25%, to make up a number. Then the increase in biomass due to stimulated plant growth over the year is 0.25 * (1 kg/m2) * (600 m2) = ~150 kg. This is surprisingly close to the mass of feces decomposed per year, given the massive error bars on these numbers.

However, the marginal impact of washing additional water down the drain might be quite a bit smaller, assuming the size of the drain field remains constant, because plants can only use so much water and nutrients. However, if you have to expand your drain field to accommodate increased organic loads, then this probably will increase plant growth in proportion to the expanded field area.

Suppose a family adds a garbage disposal unit for removal of food waste down the drain. Lundie and Peters (2005) assume that "the average amount of food waste produced by a household in 1 year" is "182 kg (wet) per annum" (p. 277). Imagine that if the family didn't send the food scraps down the drain, they would compost them in an invertebrate-filled compost pile. Following are two example calculations using made-up numbers to illustrate the comparisons that need to be made in analyzing this situation from the perspective of invertebrate suffering.

  1. Suppose the existing septic system can handle the increased loads of organic matter from sending food scraps as well as feces down the drain. Assume the increase in plant growth due to the extra water and organic matter is only (say) 1%, because there's already plenty of water and nutrients from the toilet, showers, and sinks. By using the garbage disposal unit, the family prevents invertebrate-filled composting of 182 kg of food waste per year and only increases invertebrate decomposition of grass in the yard by 0.01 * (1 kg/m2) * (600 m2) = 6 kg. Of course, the food waste sent down the drain will still support some invertebrates (rotifers, nematodes, etc.) in the drain field, but I'm assuming that 1 kg of organic matter decomposed via a septic system creates fewer invertebrates than 1 kg decomposed via a compost pile, especially considering that the septic tank is anaerobic?
  2. Suppose that the family, planning to use a garbage disposal unit in their newly built home, increases the drain-field size of their home by 300 m2. Now a bigger land area of lawn can use the water and nutrients flushed down the drain from the toilet, shower, etc. Assuming a 25% increase in plant growth on the 300 m2, the increase in grass production per year is 0.25 * (1 kg/m2) * (300 m2) = 75 kg. Now the comparison is between 182 kg of food waste composted vs. 182 kg of food waste sent to the septic system plus 75 kg of extra grass growth.b It's not obvious which of these scenarios creates more total invertebrates, although I would still incline toward disposal in the septic system because the grass-stimulating effects of septic effluent seem more theoretical (pending further information on this topic). If the 182 kg of food waste wouldn't be composted in the absence of a garbage disposal unit but would instead, e.g., be thrown in the trash to be decomposed mostly anaerobically in a landfill, it would then seem plausible that sending the food scraps down the drain, with the attendant required increase in drain-field size, would be the option that creates more total invertebrates.


Septic systems can sometimes contribute to eutrophication of surrounding water bodies. It's unclear to me what the net impact of this is on invertebrate populations, though I suspect there may often be an increase in total invertebrate numbers (Tomasik "How ..."). Either way, eutrophication presumably increases bacterial populations.

Wikipedia ("Onsite ..."): "Onsite wastewater treatment systems have also contributed to an overabundance of nutrients in ponds, lakes, and coastal estuaries, leading to the excessive growth of algae and other nuisance aquatic plants (USEPA, 1996b)."

Wikipedia ("Septic tank"):

Septic tanks by themselves are ineffective at removing nitrogen compounds that have potential to cause algal blooms in waterways into which affected water from a septic system finds its way. This can be remedied by using a nitrogen-reducing technology, or by simply ensuring that the leach field is properly sited to prevent direct entry of effluent into bodies of water.[...]

The soil's capacity to retain phosphorus is usually large enough to handle the load through a normal residential septic tank. An exception occurs when septic drain fields are located in sandy or coarser soils on property adjoining a water body. Because of limited particle surface area, these soils can become saturated with phosphates. Phosphates will progress beyond the treatment area, posing a threat of eutrophication to surface waters.

Sludge disposal

Septic tanks are typically cleaned out periodically. Strachan (2016): "The septic tank in a residential system should be inspected regularly for sludge buildup. The tank should be pumped by a licensed septic tank pumping contractor when necessary. Under normal use a septic tank should have its contents removed every three to five years."

This pumped-out sludge may support various organisms at its place of disposal. MeCDC (2013), pp. 18-19: "Anaerobic digestion which occurs in septic tanks represents an incomplete digestion. [...] The resulting sludge will readily decompose further when exposed to oxygen and aerobic bacteria. This generally will take place in a municipal sewage treatment plant or landfill if either of these places is used to dispose of sludge pumped periodically from septic tanks."c While mostly filled with bacteria, sewage-treatment plants contain some invertebrate animals. As far as landfills, I imagine they might contain some invertebrates near the surface? But plausibly invertebrates don't constitute a large fraction of decomposers in landfills.

Texas Cooperative Extension (2002) says regarding septic-tank solids (7m27s): "As much as 50% of these solids remain in the tank and decompose. The rest accumulates as sludge at the tank bottom and must be removed periodically. Still, the majority of the organic matter—solids, nutrients, and pathogens—remain in the wastewater as it leaves the septic tank." Thus, it sounds like a majority of organic-matter decomposition occurs within septic systems (whether in the septic tank or the drain field) rather than wherever the pumped-out sludge ends up.


  1. This is not a rural area, and the residents probably don't have septic systems there. However, I'm just using this as one estimate of the average number of people per household.  (back)
  2. I'm ignoring any increase in grass productivity on the original drain-field area on the assumption that this impact is minimal.  (back)
  3. Are landfills aerobic? My impression is that decomposition in landfills is generally mostly anaerobic?  (back)