What Is a Hydraulic Load Test, and When Is It Used?

In many situations, it’s important for installers to put a system through its paces to determine proper baseline operation.
What Is a Hydraulic Load Test, and When Is It Used?
Jim Anderson, Ph.D., is an emeritus professor at the University of Minnesota Department of Soil, Water and Climate and recipient of the pumping industry’s Ralph Macchio Lifetime Achievement Award. Email Jim questions about septic system maintenance and operation at editor@pumper.com.

Interested in Onsite Systems?

Get Onsite Systems articles, news and videos right in your inbox! Sign up now.

Onsite Systems + Get Alerts

As many of you are aware, I have been involved with numerous education programs and efforts over the years. Probably more than any others is the inspection program begun by the National Association of Wastewater Technicians. One of the inspection procedures we discuss in that program is called the “operation test.” The purpose of this test is to determine whether wastewater moves through the system as designed. This fits with the operation level of inspection the program was designed to address.

An operation test consists of the inspector running a set amount of water (100 gallons is recommended) into the septic tank and then making sure the water flowed through the system to the final soil treatment and dispersal area. If the water ends up where it is supposed to be, the system is determined to be operating satisfactorily. This does not speak to whether all the individual components of the system are in satisfactory condition, just that the wastewater moves as it should. Each individual component needs to be evaluated for sound operating condition before the system passes the inspection.


There was nothing magical about the 100 gallons. It was felt by practitioners that this amount represented a good “wash day’s” worth of water and was enough to make sure the water went where it should without endangering the system.
Several affiliated state associations and individual class attendees maintained that this level of test was not sufficient, and instead a hydraulic load test, or HLT, should be conducted on the system. There were procedures being used in the affiliated states; but there was a desire to standardize the approach and make that information available to anyone who wished to incorporate it into their procedures.

Just like any efforts to standardize procedures within the industry, it was a long, difficult process to agree on procedures, and there are still states and individuals who don’t agree with the procedures. But there is a document available through NAWT with the detailed procedures.

The purpose of a HLT is to “determine if an absorption area can satisfactorily receive and allow to pass into the soil/environment the daily volume of sewage effluent that the prevailing regulatory authority assigns to a structure based on occupancy, number of bedrooms or other regulatory factors.” The test involves introducing the estimated average daily flow into the system to see if the system accepts the effluent without surfacing or other problems. Not every system should have a HLT conducted.


The standards specify a HLT should be performed if, during the course of an inspection, any of these conditions occur:

• Less than 24 hours’ volume capacity in cesspool or seepage pit.

A note on seepage pits and cesspools: The states I work in don’t allow these systems, so they must be replaced because they are not considered treatment systems. In some states, these systems are allowed, either through grandfathering or under special geologic and hydrologic conditions.

• Structure is vacant for more than seven days.

There was quite a bit of discussion about what the time period should be, and it was decided if the structure is reoccupied for 14 continuous days, the system can be reevaluated and the test avoided.

• When the treatment tank, cesspool or seepage pit has been pumped less than 30 days prior to the inspection.

A provision is made that if the structure is occupied for 30 continuous days, the system can be reevaluated and the test avoided. This is also true for situations where the flow increases either through new graywater sources being added to the system within the previous 30 days or if any other water source is added.

• There is soil-fracturing activity within last 30 days.

If the structure is occupied for 30 continuous days, it can be reevaluated and the test avoided.

• When initial inspection reveals that, for whatever reason, the treatment tank’s liquid level is below the outlet pipe.

If the structure is occupied for 30 continuous days it can be reevaluated. This addresses the issue of the tank having been pumped or the system not being in continuous use. Other parts of the inspection should identify that the tank is leaky and should be replaced.

• When standing effluent is discovered in an absorption area or gravelless chamber, the inspector shall determine when an HLT is indicated.

This allows the inspector to evaluate distribution of the effluent to determine if the ponding is due to the way effluent is distributed, such as in a dropbox system.

• A broken or clogged pipe, a dysfunctional D-box or other condition that would result in atypical flows reaching all or part of the system.

After the condition is fixed or remedied, the system can be reevaluated.


When the inspector is informed that the existing system will be subjected to increased daily flows through increased occupancy or a change in the structure’s use, an HLT shall be recommended.

An HLT is not recommended for absorption areas that have been in use for less than 30 days. If a client elects to have an HLT performed on a newly installed system, the inspector shall fully explain to the client the purpose of an HLT, the type of conditions it is intended to explore, and the limitations related to testing, such as newly installed or never used systems.

If you are interested in the detailed testing procedures, go to www.nawt.org and review the HLT document.


Comments on this site are submitted by users and are not endorsed by nor do they reflect the views or opinions of COLE Publishing, Inc. Comments are moderated before being posted.