What is a Hydraulic Load Test and do I need one?
What does a Hydraulic Load Test (HLT)
Why do an HLT?
What is a Hydraulic Load
What is an HLT testing?
Will an HLT hurt my absorption
How much water is used?
What about precipitation?
What about snow?
What about peak flow vs. normal
What are reasonable expectations for a
The HLT mimics the normal operating conditions in the absorption area. The HLT process is similar
to a perk test. During an HLT, a known volume of clear liquid is introduced into the absorption area. The level of
liquid in the absorption area is recorded both before and after the water is introduced. The inspector returns the
following day and repeats the process.
The HLT is used to verify that the absorption area can receive and transmit to the soil
environment the volume of liquid which the system is expected and intended to be able to handle on a peak-flow
When a system has been unused for more than 7 days
because of vacations, seasonal use or the structure has simply been vacant, the normal operating conditions
in the absorption area are not present for the inspector to observe. The longer the system has been unused,
the longer it will take for normal conditions to return.
The NOF/PSMA Standards provide the opportunity to
reestablish normal conditions to enable a complete inspection or an additional test can be performed. Either
method is acceptable, however, time is often the deciding factor in selecting the process to be
When a system displays the unintended
condition of liquid standing in the aggregate, the absorption area is not performing as intended.
When the level of liquid in the aggregate is above the holes in the distribution laterals, this unusual
condition is of such concern that additional investigation is needed. The additional investigation called for
in the NOF/PSMA Standards is a test known as a Hydraulic Load Test or HLT.
Some landowners resist a request from a buyer for an HLT to be conducted on their system. At
first blush, this concern seems understandable, however, upon investigation it is seen to be
“It’s gonna blow out the system,” is a commonly stated misconception.
Pennsylvania, for example, the flow rate is four hundred gallons for one to three bedrooms. One hundred gallons are
added for each additional bedroom. A five bedroom house must have a system that is able to receive and release to
the soil environment six hundred gallons.
Is it unfair for the buyer to expect that the system he may own should be capable of
properly handling the volume of sewage that the structure generates?
“We’ve had a lot of wet weather which caused the system to be full of water,” is another popular
but unfounded reaction from property owners, either when they first learn an HLT is necessary or at the conclusion
of the HLT. Again, the rainfall equivalent table sheds light on the situation.
For the aggregate to be saturated for its entire depth, which
is usually 10 or 12 inches, three unusual and unexpected things must all have
significant and unprecedented amount of precipitation must have fallen in the immediate past few
None of the precipitation ran off, none
became trapped in the soil matrix above the aggregate and all of the water percolated immediately to
The water DID NOT LEAVE the
absorption area by moving into the underlying soil
as it is expected to but rather accumulated in the aggregate.
“But there was snow…” Yes,
however, snow accumulates at a far greater rate than rain. On average, 10 in. of fallen snow is equivalent to 1 in.
of rain. While there are heavier and lighter “snows,” the melting process, runoff, direct evaporation of snow, and
reduced permeability of frozen soil all combine to spread the conversion of snow to water over time, diminish the
actual amount of snow available to infiltrate and reduce the actual amount of water that can infiltrate. The issues
regarding the behavior of rain all come into play when the melt water eventually enters the soil.
Is it unfair for the
buyer to expect that the system he may own should work properly in rain, sleet, or snow?
Septic systems permitted since
the early 1970s are designed to handle a peak flow based on the number of bedrooms in the house. A system’s
treatment capacity is, by regulation, over designed! Peak flows are not “typical” flows; they are intended to be
above the norm. If systems were designed solely to handle the normal flow, then systems would regularly fail when
the family comes over for a birthday party, thanksgiving, a picnic or to watch the super bowl!
A HLT is done to
replicate peak flow conditions. Indeed, only the peak flow is a known
value since it is based on the number of bedrooms. The current occupant’s use pattern cannot be used to control the
future occupant’s use patterns. Use differs from occupant to occupant based on a host of characteristics or
patterns that may be attributable to the number of adults and children and their respective ages and genders. Use
patterns may also be influenced by ethnic, dietary, or other very real, however, unpredictable characteristics or
practices of those future occupants!
For all of these reasons, the
volume of clean water used in the HLT is based on the prevailing regulation and it is introduced much like a peak
load would be introduced to the system. This is why the load is not introduced over a period of 24 hours in a
series of incremental loadings. Indeed, modern perc tests used to determine the size of a brand new septic system
is conducted on a very compressed time frame of just four hours.
Is it unfair for the
buyer to expect that the system can handle the flows from a celebration or when all the friends and relatives come
over for the house warming party?