Residential Septic Tank Sludge and Scum
what pathogens and contaminants do they contain?
     


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Sewage pathogens: This septic/sewage information article provides a general discussion of the contents: contaminants, pathogens, components of typical residential septic tank sludge and scum and cites several hazards related to septic tanks and septic tank sewage contents. We also provide links to more detailed information in articles about nitrogen contamination, how to inspect and test and clean up sewage contamination in buildings, and what to do about a septic system after it has been flooded.

Anyone working on or around or owning a septic tank should be sure to see SEPTIC & CESSPOOL SAFETY. Citation of this article by reference to this website and brief quotation for the sole purpose of review are permitted. Use of this information at other websites, in books or pamphlets for sale is reserved to the author. Technical review by industry experts has been performed and is ongoing - reviewers welcomed and are listed at "References."

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What Makes Up Septic Sludge and Septic Scum in Residential Septic Tanks?

Watch out: sewage spills contain contaminants that can cause serious illness or disease. Disease causing agents in raw sewage include bacteria, fungi, parasites, and viruses and can cause serious illnesses including bacterial infections, Tetanus, Hepatitis A, Leptospirosis, infections by Cryptosporidium & Giardia and gastrointestinal diseases. For a detailed list of the pathogens found in common household wastewater such as a septic tank and drainfield, see also our discussion of pathogens in sewage at SEWAGE PATHOGENS in SEPTIC SLUDGE: what makes up the contents of residential sewage? (SEWAGE & SEPTIC CONTAMINANTS)

Components of Sewage Entering and Leaving the Septic Tank

Sewage, or "blackwater" from a typical residential building contains a variety of inorganic and organic substances contained in feces-fecal residue, urine, and food wastes. Included are digested food, skin cells from the intestinal lining, bacteria (coliform, other), other organic waste and debris which may have entered the septic system such as food waste or waste from a garbage grinder; cellulose (dissolved toilet tissue); Nitrogen, ammonia, nitrite, nitrate, phosphorous, sulfate, grease.

First (detailed sewage pathogen lists follow), stated simply, and according to various sources such as the Utah DEQ

"The major contaminant discharged from septic systems is disease-causing germs. These germs (bacteria and viruses) - can cause many human diseases.

Another contaminant discharged from septic systems is nitrogen in the form of nitrate. If the nitrate level of drinking water is too high, infants, up to the age of six months old, can develop a fatal disease called blue baby syndrome (methemoglobenemia).

Additionally, if toxic chemicals are disposed in a septic system, they can percolate through the drainfield and into the ground water."

Minnis (see references at the Septic Systems Home Page) cites total solids as 300-1200 mg/L, subcategorized into dissolved fixed volatile, suspended fixed volatile, and settleable. Also see SEWAGE CONTAMINATION in BUILDINGS and
SEWAGE CONTAMINANTS in FRUIT / VEGETABLES.

What are the Components of Raw Sewage

Jantrania & Gross (see references at the Septic Systems Home Page) list the following as characteristics of raw sewage

  1. Total suspended solids: 155-330 mg/L
  2. 5-Day BOD: 155-286 mg/L
  3. Total coliform bacteria: 108 to 1010 CFU/100mL
  4. Fecal coliform bacteria: 106 to 108 CFU/100mL
  5. Ammonimum-nitrogen, N4-N: 4-13 mg/L
  6. Total nitrogen: 26-75 mg/L
  7. Total phosphorus: 6-12 mg/L

(The complete list is in their book)

What are the Components of Septic Tank Effluent

Jantrania & Gross (see references at the Septic Systems Home Page) list the following as characteristics of septic effluent as it leaves the septic tank (where only limited treatment has occurred).

  1. Total suspended solids: 38-85 mg/L
  2. 5-Day BOD: 118-189 mg/L (this is about a 40% reduction from the level of the entering sewage)
  3. Fecal coliform bacteria: 106 to 107 CFU/100mL (note that this little or no reduction over the level of coliform in the entering sewage)
  4. Ammonimum-nitrogen, N4-N: 30-50 mg/L (note that this is considerably higher than their number for raw sewage)
  5. Total nitrogen: 29-63 mg/L
  6. Total phosphorus: 8 mg/L

(The complete list of components of septic tank discharged effluent is in their book)

Details about the effects of key septic or wastewater constituents on soils and water and the environment are at WASTEWATER BIOCOMPATIBILITY.

What is Found in Settled Septic Tank Sludge

To be complete, a conventional septic tank contains settled sludge solids at its bottom, a floating grease/scum layer, and a central volume of liquid effluent and dissolved solids.

Because it is difficult to chemically separate individual sewage components, septic "sludge" is measured in the amount of oxygen needed to support the consumption of the waste by microbes (bacteria and other) - biochemical oxygen demand or "BOD".

Total solids in this waste (if measured by weighing what's left if sewage has all of its water content removed) are broken down into: - total suspended solids (able to be removed from effluent by use of a 2.0u filter) - total dissolved solids (dissolved in the liquid and thus pass through the filter)

Solid residue can also be broken down into a volatile solids portion (which is consumed When a sample is ignited at 550 degC) and fixed solids portion which remains after This process.

Settleable solids, that is solids that settle out of the septic effluent, are defined as those particles which will settle out of the sewage after a specific time period.

What Contaminants are Found in Floating Septic Tank Scum

Oil and grease in sewage will, in a septic tank and given enough time, will rise to the top of the tank and join the floating scum layer there. In residential sewage the oil and grease will be primarily from animal or vegetable fats.

What Gases are Found in the Septic Tank

Methane Gas Hazards in septic tanks:

Finally, not really a direct component of septic sludge or floating scum are the gases, including combustible methane gas, produced by decaying organic matter including sewage.

Readers have sent us reports of fires, explosions, and even deaths associated with accidental igniting of methane gas over a septic tank or asphyxiation caused by entering or falling into a septic tank.

See METHANE GAS HAZARDS and other septic system gas explosion or asphyxiation hazards such as hydrogen sulfide. Also see SEPTIC & CESSPOOL SAFETY where we describe septic methane gas asphyxiation and explosion hazards.

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