Septic tank effluent retention time & net free area:

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).

If the effluent retention time is too brief in the septic tank, solid waste is pushed into the drainfield where that important component is clogged, leading to failure. We explain how we calculate or estimate the required septic tank retention time and we describe a related concept: net free area in the septic tank.

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 seldom 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 Septic Tank Retention time

Septic tank retention time is 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.

Definition of net free area or septic tank working volume

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.

What is the Required Septic Tank Effluent Retention Time Period?

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.

Typical design-retention time for septic tanks is 1-3 days in some sources or better, 2-3 days but there is not a single exact answer since tank sizes and shapes vary. The formula below gives a reasonable septic tank effluent retention time in days based on the tank volume and daily wastewater volume.

Watch out: for a shallow flat septic tank such as those used on rocky sites, the total tank volume may not be adequate and the retention time target should be increased.

Septic Tank Effluent Retention Time Formula

The necessary days of retention time R = the effective tank volume V_{(g Tank)} divided by the daily wastewater volume in gallons Vol_{(g Daily Wastewater)}.

R_Days = V_{(g Tank)} / Vol_{(g Daily Wastewater)}

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.

Research on Septic Tank Design Effluent Retention Time

• "SEPTIC TANK DESIGN", [PDF] Pan American Health Organization, World Health Organization (Who) Washington D.C., with offices in 27 countries, retrieved 2017/06/12, original source: www.bvsde.paho.org/texcom/desastres/oxfamstg.pdf
  Excerpt:
  
  Decide on a retention time (RT) of 1 to 3 days. Choose a long retention time to minimize cleaning frequency (running costs). Choose a short retention time to minimise tank size and initial cost. Larger tanks allow the sewage more time to digest and thus reduce the load on the drainage system. Smaller tanks (< 6m3) need longer retention times due to increased turbulence (i.e. 2 or 3 days.)
• Bounds, T. R. "DESIGN AND PERFORMANCE OF SEPTIC TANKS" [PDF] In Site Characterization and Design of On-Site Septic Systems. ASTM International, 1997.
  Abstract:
  
  More than forty million people in the United States currently use onsite wastewater disposal or decentralized sewerage collection and treatment that rely on septic tanks for primary treatment. There is a good reason why, in this age of advanced technology, the septic tank is still in use. It works.
  
  More than 45% of ultimate treatment can be accomplished in the septic tank. Advanced onsite and effluent sewer technologies have established their environmental importance by bringing highly reliable, affordable and permanent wastewater treatment to users worldwide. In short, passive—energy free—septic tanks provide the most cost efficient form of primary treatment available for non industrial sewage.
  
  Decentralized sewers and onsite alternatives have advanced us to a new era of wastewater treatment and management where designers must be able to rely on the many essential components of the system.
  
  System components must be designed and constructed with the same permanency and quality expected of any long-term option. Because the septic tank is an essential ingredient to the success of these systems, a new generation of structurally-sound, watertight septic tanks is evolving.
• Merino-solís, María L., Edgardo Villegas, José De Anda, And Alberto López-lópez. "THE EFFECT OF THE HYDRAULIC RETENTION TIME Time On The Performance Of An Ecological Wastewater Treatment System: An Anaerobic Filter With A Constructed Wetland." [PDF] Water 7, No. 3 (2015): 1149-1163.

Question: so what is a typical septic tank effluent retention time: hours or days?

I waited to read an actual septic effluent retention time but never found it in the article on this page.

Reply: 1-3 days

Joseph:

Fair question for which I thank you, and for which there is not a single "right answer" - rather there is a right concept. That's because tank shapes vary as well as volumes, as well as the rate of inflow of wastewater as well as, of course, the thickness of sludge and scum layer.

The design concept is that when the free area in the tank is small, down to say 1/3 or less, the sewage remains agitated and solids are pushed out into the fields, ruining them.

An effective retention time of 1-3 days is what's needed but I haven't seen a single design number.

...

Continue reading  at MEASURE SEPTIC TANK SCUM & SLUDGE or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

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EFFLUENT RETENTION TIME at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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