Septic tank cleanout timing based on objective data:

Determining septic tank cleanout frequency based on physical inspection of the septic tank scum & sludge layers. This article explains when to pump the septic tank and sludge and scum layer thicknesses.

We explain that making these basic observations when a septic tank is pumped, and knowing how long it has been since the previous pumpout can tell us more accurately when the septic tank is going to next need cleanout. This approach can save you money in pumping costs or it may save you money by properly protecting the life of the septic system drainfield.

We discuss the floating scum and sludge layer thicknesses and their measurement for evaluating the septic tank net free area and overall condition.

How the Septic Tank Scum Layer or Sludge Layer Determine Pumpout Need

A Photographic Guide to Pumping a Septic Tank

This article series provides a step by step, photo-illustrated guide to opening, pumping, and inspecting septic tanks, how a conventional septic tank is located, opened, pumped out, cleaned, and inspected. This guideline is intended for septic pumping tank truck operators and as general information for homeowners or septic service companies concerned with septic system care.

Readers should see Septic Tank Pumping Frequency Guide for a table explaining when to schedule a septic cleanout. 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 reviewers are welcome and are listed atReferences or Citations .

How to Determine If and When a Septic Tank Needs to Be Pumped

Standard Method 1: Pump the septic tank based on usage level - using the septic tank pumping tables

The most common method used to determine when a septic tank should be pumped is to use a tank pumping frequency table which suggests the tank cleanout interval as a function of tank size and the level of usage of the system, such as the number of occupants in the building.

We provide a table to give a septic pumping schedule
at SEPTIC TANK PUMPING SCHEDULE. Refer to the table to see the recommended pumping frequency for the particular septic system being considered. For example, (from the septic tank pumping table), a 1000 gallon septic tank serving a home with five occupants should be cleaned out about every two years. If the tank has only a 500 gallon capacity, it needs to be cleaned about every six months!

Why a septic tank pumping table may be wrong - other septic tank pumping schedules

Septic tank shape affects its pumping needs.

What septic tank pumping tables may not recognize is the effect of tank shape on the need to pump it out. Most septic tanks are 4ft. to 6ft. deep. But if the tank in question is a "low boy" or shallow septic tank (used in areas where there is not enough soil depth to bury a conventional septic tank) it may need to be pumped more often.

Septic tank usage affects its pumping schedule.

Septic tank pumping tables presume a number of occupants based usually on the number of bedrooms in a home. If your home has more occupants or other reasons for heavier septic system use the tank needs to be pumped more often.

Alternative Method 2: Use actual septic tank conditions to decide its next pump out time

By inspecting the conditions in the tank at pumping time you can decide if your septic tank should be pumped more or less often than called for in the pumping schedules. The thickness of the floating scum layer and bottom sludge layer determine how much net free area remains in the septic tank. The size of the net free area in the septic tank in turn determines

the EFFLUENT RETENTION TIME - which means that we want effluent to stay in the tank long enough for new solids and greases to separate out before effluent leaves the septic tank.

Pump the tank based on sludge/scum thickness

It is in fact quite possible to MEASURE SEPTIC TANK SCUM & SLUDGE thicknesses in the septic tank.

The exact point at which a septic tank needs to be pumped could be determined by opening the septic tank and measuring the sludge and scum levels in the tank. As long as the sludge and scum layers are minimal (just a few inches of thickness) or in other words, as long a the "net free area" in the tank (see below) is adequate, the tank does not need to be pumped.

See "EFFLUENT RETENTION TIME"

and MEASURE SEPTIC TANK SCUM & SLUDGE for details of these procedures.

Depending on the size of the tank and its dimensions, as the sludge and scum layers become thick the net free area remaining in the tank becomes so small that there is inadequate time for solids to separate into sludge and scum layers. At this point, even though the septic system appears to be "working" its remaining life is in jeopardy because suspended solid waste is being pushed into the drain field.

The volume of net free area in a septic tank determines the time available for this solid and grease separation. That time is referred to as "septic effluent retention time." Short effluent retention time means that the tank sewage remains agitated which in turn means floating and suspended solids are being pushed into and thus are clogging the drain field.

Method 3: there are other reasons people pump septic tanks

There are other reasons besides time or sludge/scum layer thickness that lead people to pump septic tanks out, such as diagnosing septic system problems, odors, backups, or inspecting / repairing septic tank baffles.

We list and discuss these in detail at REASONS FOR PUMPING THE SEPTIC TANK.

Question: when does a 900 gallon septic tank need pumping

(Aug 9, 2011) A home owners question said:
I had my septic tank inspected recently by a pumping company and was advised that the tank "should" be pumped (was recommended). The 900 gallon tank was pumped three years ago. The result of the inspection showed one (1)inch sludge and two (2)inch scum. I don't know what the criteria is as to when a tank requires pumping. Can anybody tell me what the criteria is when a 900 gallon tank requires pumping?

Reply: it's not just the tank volume, it's the tank dimensions that we look at to see the net free area of effluent

H.O. that was very little sludge and scum thickness. Based on those levels, if your usage level remains the same, you can pump less often. Please take a look at SEPTIC TANK PUMPING SCHEDULE for a table of septic tank pumping frequency recommendations based on tank size and number of occupants in the building.

It's always safer (and more profitable) for a pumping company to say "pump the tank every year..." or whatever, but you should be fine if you start with the pumping frequency in the recommended table, adjusting the actual pumping rate up or down in frequency based on the scum and sludge layer thickness observed each time the tank is pumped.

Also see SEPTIC TANK PUMPING REASONS

(Aug 11, 2011) "A home owner's" response. said:
Thanks ... for your advise. My wife and I are permanent occupants of our house (year around residence) and it seemed to me that the sludge or scum depth did not warrant the pumping of the tank except that the pumping company believes that a client may not know when a tank needs pumping therefore "recommends" pumping. The tank is gravity fed and I am treating the it monthly with enzyme in order to keep the drain field "healthy / operable. I have two subsequent questions and would appreciate receiving your advice.

(1)Based on the same tank capacity and same usage/load does a "single compartment" tank requires more frequent pumping than a "two compartment" tank?

(2)Is there a diffence between the performance of a "single compartement" tank and that of a "two compartment" tank?

Reply:

You don't need septic treatments, and some can be harmful or even illegal, depending on where you live. Pumping the tank on schedule is the No. 1 thing you can do to extend drainfield life. Plus being sure groundwater is not flooding the fields.

Good questions 1 and 2. I haven't seen an answer phrased quite as you ask. In general a 2 compartment tank does a better job of separating sludge from effluent, and is more necessary with some designs such as aerobic systems. But it's the tank size or volume, total size, combined with usage that chiefly determines pumping frequency.

I could SPECULATE that in a 2-compartment tank of the SAME TOTAL CAPCACITY as a one-compartment tank, while the 2-compartment version is generally a superior design, as it improves sludge and scum separation from the effluent and thus extends drainfield life (you can also get this benefit with a septic tank outlet filter).

But this tank might have smaller volume to retain solids and thus might need more frequent pumping.

...

Continue reading  at WHEN NOT TO PUMP A SEPTIC TANK or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• SEPTIC TANK INSPECTION PROCEDURE
• SEPTIC TANK PUMPING MISTAKES
• SEPTIC TANK PUMPING REASONS
• SEPTIC TANK PUMPING SCHEDULE - table of septic tank pumping frequency recommendations based on tank size and number of occupants
• SEPTIC TANK SAFETY
• WHEN NOT TO PUMP A SEPTIC TANK

Suggested citation for this web page

WHEN to CLEAN THE SEPTIC TANK at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to SEPTIC SYSTEMS

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