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Septic Effluent Retention time and Effective Septic Tank Volume InspectAPedia®  -     What is Effluent Retention Time? How Retention Time Affects Septic System Life

This document answers the question: what is septic tank retention time, or "effluent retention time" and explains the importance of this concept. Septic tank maintenance requires that the tank be pumped at the appropriate interval (given in our table of septic tank pumping frequency). Technical review by industry experts has been performed and is ongoing - reviewers are listed at "References."

© Copyright 2010 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.

© Copyright 2010 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use the links at page left to navigate this document or to go to Other Website Topics. Green links show where you are in our document & website.

EFFLUENT RETENTION TIME - Septic Tank Effluent Retention time and Effective Septic Tank Volume - Why pump a septic tank before it is "full" of solids and grease?

This article explains that if septic tank cleaning is done too infrequently the liquid volume area in the tank becomes too small and effluent does not stay long enough in the septic tank. In short: retention time is the time that septic effluent spends in the septic tank before flowing out to the drainfield. If a septic tank is not pumped frequently enough retention time becomes short and the drain field will have a much shorter life. Solids moving into the drain field will clog it.

A septic tank with too little net free area or free volume for liquid effluent will not permit solids to settle out of the sewage. The result is that the waste in the tank remains agitated during system use, forcing solids into the absorption system - a condition which can spell the quick and costly ruination of a septic leach field or other absorption system.

Longer retention time in the septic tank permits effluent to separate into floating scum, settled sludge, and clarified effluent. It's clarified effluent not sewage that we want to discharge into the absorption system. Use of this information at other websites, in books or pamphlets for sale is reserved to the author.

Definition of Retention time: the length of time that effluent remains in the septic tank before moving out to the absorption system or leach field. In order for a septic tank to function properly, adequate liquid volume must be maintained to allow for sufficient "settling time" or "retention time" which permits solids to either settle out as sludge or join the floating-scum layer at the top of the tank. Baffles in the tank prevent the floating scum from leaving the tank, an event which would lead to rapid failure of the soil absorption system or leach field. When pumping is too infrequent, even if the tank is not totally clogged with solids, the reduced liquid volume in the tank (the "net free area") reduces settlement or "retention time" and grease coagulation time.

Net free area or effective septic tank working volume: The "net free area" (my term) or "effective septic tank volume" is the actual tank interior volume minus the space occupied by settled sludge and floating scum. In addition to the requirements that the sludge level be sufficiently below the baffle and tank outlet, a sludge level which has reached more than 20% of the septic tank volume is unacceptable and such a tank needs to be cleaned.

Retention time requirement: the retention time necessary for a septic tank to function properly depends on the size of the septic tank and the daily wastewater volume or flow. The necessary days of retention time = the effective tank volume divided by the daily wastewater volume in gallons. Example: in a 1000 gallon septic tank with an effective tank volume before cleanout of 600 gallons, and with a family placing 600 gallons of wastewater into the tank each day, the tank needs one day of retention time. This means that if the wastewater flow exceeds 600 gallons a day or if the sludge and scum levels increase to reduce the tank volume below 600 gallons, then we're pushing floating debris into the leach field.

This condition means that the septic tank contents remain agitated when the system is used, forcing small floating solids and grease out of the tank and into the absorption system, shortening the life of that component. If a property owner waits until a system or septic tank is blocked before having the tank pumped, it is quite likely that the leach fields have already been damaged. This is why various authorities set guidelines on tank pumping/cleaning based on scum layer thickness and sludge thickness, and this is why those figures require cleaning of the septic tank well before it is "full" or nearly full of solids and floating scum.

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Technical Reviewers & References

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• Additional technical contributors & reference sources for this article are listed below.

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SEPTIC TANK PUMPING SCHEDULE
  TANK PUMPING FREQUENCY TABLE
  WHY DOES A TANK NEED PUMPING?
  EFFLUENT RETENTION TIME
SEPTIC TANK PUMPING PROCEDURE
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Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

• Our recommended books about building design, inspection, and repair, and about indoor environment testing, diagnosis, and cleanup are at the InspectAPedia Bookstore.
• The Septic System Information Website home page for this topic
• Septic Systems Inspection, Testing, & Maintenance- online textbook. Detailed how to inspect, maintain, repair information
• The Home Buyer's Guide to Septic Systems
• Septic Tank and Septic System Maintenance - online articles giving details of many septic system maintenance and repair topics.
• Septic Tank Pumping Guide: When, Why, How to pump the septic tank
• Tank Location - How to Find the Septic Tank, how deep will the cover be, how to document its location
• 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
• Septic System Design Basics: Choosing Septic Tank Size, Absorption System Size
• 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.
• Ten Steps to Keeping a Septic System Working, suggestions from the U.S. EPA, edits and additions by DJF
• 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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