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Residential Septic Tank Sludge and Scum - what pathogens and contaminants do they contain?
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• Residential Septic Tanks - Septic Tank Sludge and Scum - what does it contain?
• What are the Pathogens and Contaminants in Sewage

This page 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.

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." This document is a chapter of Inspecting, Testing, & Maintaining Residential Septic Systems an online book on septic systems.

© Copyright 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

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

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.

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.

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)

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)

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.

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.

Gases 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 me 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. Septic system safety is discussed further at "More Information" below.

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Technical Reviewers & References

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Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

• Measuring Septic Sludge & Scum Levels in Septic Tanks - Measuring the Level of Accumulated Solids, Sludge and Floating Scum in Treatment Tanks
• Septic Contaminants, identifying - what are the Nitrogen contaminants produced by septic systems and what can be done about them if they escape initial treatment?
• Safety: Septic System, Septic Tank, & Cesspool Safety Warnings for Septic Inspectors, Septic Pumpers, and Homeowners
• Sewage & Septic Backups and what to do about sewage spillage in buildings
• Septic Flood Response when and what to do to a septic system after area flooding

More Information on Building Diagnostic Inspections and Repairs

• Additives & Chemicals: Septic System Maintenance - Are they even needed? Are they even legal?
• Contamination in Buildings how to test buildings for bacterial contamination from Sewage and Septic backup, remediation, & references
• Don't Flush these things into a septic system: a list of what's ok and what's not ok to put into septic tanks and building drains
• Maintenance: Ten Steps to Keeping a Septic System Working, suggestions from the U.S. EPA, edits and additions by DJF
• Measuring Septic Sludge & Scum Levels in Septic Tanks - Measuring the Level of Accumulated Solids, Sludge and Floating Scum in Treatment Tanks
• Safety: Septic System, Septic Tank, & Cesspool Safety Warnings for Septic Inspectors, Septic Pumpers, and Homeowners
• Septic Contaminants, identifying - what are the Nitrogen contaminants produced by septic systems and what can be done about them if they escape initial treatment?
• Septic Flood Response when and what to do to a septic system after area flooding
• Sewage & Septic Backups and what to do about sewage spillage in buildings
• Sewage Contaminants potentially released from onsite septic systems and wastewater disposal systems
• Tank Location - How to Find the Septic Tank, how deep will the cover be, how to document its location
• Tank Pumping Frequency Guide for Septic Tanks: When, How, What to Watch For when pumping or cleaning septic tanks


• The Septic System Information Website home page for this topic


• Water Softeners maintenance, adjustment, and water softener effect on drinking water and on septic systems
• Septic Systems Inspection, Testing, & Maintenance- online textbook. Detailed how to inspect, maintain, repair information
• The Home Buyer's Guide to Septic Systems
• Home & Outdoor Living Water Requirements
• Septic Tank Capacity vs Usage in Daily Gallons of Wastewater Flow, calculating required septic tank size, calculating septic tank volume from size measurements
• How Big Should the Leach Field Be? - table of soil percolation rate vs. field size
• Septic System Drainfield Absorption System Biomat Formation - what leads to drain field clogging and expensive drainfield repairs
• Table of Required Septic & Well Clearances: Distances Between Septic System & Wells, Streams, Trees, etc.


Pennsylvania State Fact Sheets relating to domestic wastewater treatment systems include

• Pennsylvania State Wastewater Treatment Fact Sheet SW-161, Septic System Failure: Diagnosis and Treatment
• Pennsylvania State Wastewater Treatment Fact Sheet SW-162, The Soil Media and the Percolation Test
• Pennsylvania State Wastewater Treatment Fact Sheet SW-l64, Mound Systems for Wastewater Treatment
• Pennsylvania State Wastewater Treatment Fact Sheet SW-165, Septic Tank-Soil Absorption Systems
• Document Sources used for this web page include but are not limited to: Agricultural Fact Sheet #SW-161 "Septic Tank Pumping," by Paul D. Robillard and Kelli S. Martin. Penn State College of Agriculture - Cooperative Extension, edited and annotated by Dan Friedman (Thanks: to Bob Mackey for proofreading the original source material.)

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