The Why, When and How of Hydraulic Load Testing

An HLT will answer three basic questions about a system. Here’s what you need to know.
The Why, When and How of Hydraulic Load Testing

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While hydraulic load testing (HLT) of onsite wastewater systems isn't a requirement during the sale of a home, it's been a common practice in many areas. Besides time-of-transfer requirements, HLT can be used to check systems for other reasons, says Ray Erb, president of Thomas H. Erb & Sons in Lititz, Pennsylvania.

The test simulates real conditions to determine if the effluent is moving through the system properly, and if the soil dispersal area is absorbing the effluent as it should. “In the case where the home owner would like to increase the size of their home with the addition of a bedroom and no previous soil testing data for the site is available, we have been allowed to use the results of HLT for verification that the existing system will handle the increase in sewage flows,” he says.

An HLT, he adds, will answer three basic questions about the soil treatment area: how much can it absorb in 24 hours; can it handle the amount for which it was designed; and can it absorb the peak amount that could be produced over 24 hours. “The inspector should have a clear goal in mind and should be able to answer the question, ‘Why am I conducting this test?’” Erb says. “If they cannot answer that question, then they should not be conducting the test.”

NAWT’s Standard Operating Procedure says an HLT is appropriate for time-of-transfer inspections, building permit applications, or system evaluations if the structure has been vacant for more than seven days, since normal operating conditions cannot be observed. It should also be used if conditions have changed (increased flow, new graywater sources introduced), or if there have been any remediation or repairs of the soil treatment area in the previous 30 days (chemical additives, soil fracturing, root removal, a crushed outlet line, a fouled or damaged D box) or a seepage pit that has received less than average flow in the last 24 hours. Another indication for an HLT would be a pretreatment tank liquid level that is below the outlet pipe. That could be caused by a leaking tank and putting more liquid than expected into the soil.

NAWT also says an HLT should not be done on systems less than 6 months old or if there are any signs of failure of the soil treatment area, such as fully saturated beds or trenches or effluent on the surface.

There are a few ways to conduct an HLT. Some states have developed their own guidelines, but NAWT’s SOP lists these steps:

  • Record inspection port liquid levels, if any, in reference to the as-built drawings or site plan.
  • Using a dedicated hose with backflow prevention, begin introducing water at the specified point downstream from the septic tank.
  • Record the test start time.
  • Monitor and record the liquid levels in the inspection ports every 30 minutes for two hours. If the liquid rises to the critical level (top of the media in all laterals or the bed) before the end of the two hours, stop adding water and return the following day.
  • Assuming the liquid level did not rise above the critical level, turn off the water after the desired volume is delivered.
  • If the system has a gravity soil treatment area, wait 30 minutes for stabilization. Then record and document the liquid levels in all inspection ports.
  • Return in 24 hours and repeat the test, first recording the liquid levels in the inspection ports.
  • Walk the entire soil treatment area and surrounding areas to verify that no adverse conditions have resulted from the test (such as surfacing effluent, saturated soils or downslope breakouts).
  • Clean and sanitize all test equipment before returning it to your service vehicle.
  • Secure all inspection port covers, clean-out caps, and any other system access points that were opened for the test.
  • Clean any residuals that might contaminate the surrounding area of the system site.
  • Document all findings and test results into a formal report for filing with the appropriate permitting authority.

Getting the liquid into the system requires some judgement, according to Erb. “Introduction of liquid to a system that is very small or has a large accumulation of liquid in it will be much more critical than a system that is large and has no liquid in it at the start of the test.”

How much liquid to use is a common source of debate and guidelines vary. In Erb’s state, the Pennsylvania Septage Management Association follows the NAWT SOP – the system’s peak daily flow as determined by the regulatory agency. For Erb, that number comes from the state Department of Environmental Protection at the time of design (400 gpd for one to three bedrooms, plus 100 gpd per additional bedroom). The National Sanitation Foundation recommends using the estimated average daily flow.

Erb cautions that average daily flow may change. “It may have been tested with a projected flow of a family of two. If they sell the home to a buyer with four people in the family, the average daily flow will change and the soil treatment area may not absorb the additional sewage flow.”

While there is science behind a hydraulic load test, there is obviously still some art involved in the procedure.


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