By Kayla Hanson, P.E.
NOTE: By reading this article along with the “Gravity Grease Interceptor Design” story and passing the associated online exam, you will receive a certificate for 2 PDHs. Please check your state licensure board on specific PDH acceptance requirements.
Precast concrete septic tanks and grease interceptors are designed to be strong, durable, watertight and resilient. Watertightness depends on many factors and starts with the raw materials used to manufacture concrete, but structural design, mix design, manufacturing, curing, handling and installation all play key roles. A low water-cementitious material ratio can lead to low porosity, high density and low permeability, which all work together to enhance concrete’s watertightness.
ASTM C1227, “Standard Specification for Precast Concrete Septic Tanks,” and ASTM C1613, “Standard Specification for Precast Concrete Grease Interceptor Tanks,” discuss design requirements, manufacturing practices and performance requirements for both monolithic and sectional precast concrete septic tanks and grease interceptors prior to installation or backfill. IAPMO Z1000, “Prefabricated Septic Tanks,” and IAPMO Z1001, “Prefabricated Gravity Grease Interceptors,” discuss requirements for design, materials, performance, testing and markings for septic tanks and grease interceptors. ASTM C1719, “Standard Test Method for Installed Precast Concrete Tanks and Accessories by the Negative Air Pressure (Vacuum) Test Prior to Backfill,” is also a valid test. Tank watertightness tests shall be performed in accordance with the applicable sections of the aforementioned standards or the requirements set forth by the authority having jurisdiction, whichever is more stringent.
Watertightness testing is imperative for underground storage structures including, but not limited to, septic tanks and grease interceptors as they are designed, manufactured and installed to keep their contents contained until exiting the proper outlet as well as to keep groundwater and other external substances out of the tanks entirely.
Vacuum tests should not be performed on-site after backfilling, as no industry standard exists for this situation. If a vacuum test were to be performed on a backfilled septic tank or grease interceptor and an issue were to arise, it would be difficult to identify the cause and locate and repair the affected area. Additionally, other loads such as soil and groundwater have the potential to cause a system failure during a vacuum test of a backfilled tank if the loads are not taken into account.
First things first
Before beginning a vacuum test, ensure the tank has reached sufficient strength to undergo the test. The concrete should reach at least 75% of its design compressive strength prior to testing. It is imperative that the concrete has developed enough tensile and compressive load capacity prior to inducing loads by vacuum.
Prior to beginning a watertightness test, perform a visual inspection of the tank to locate any potential issues that could cause problems during the test. These issues should be resolved before conducting a vacuum test on the tank.
Tank vacuum testing
Appropriate personal protective equipment must be worn during the test. Care must also be taken to keep testing personnel a safe distance away from any pressurized tank. Vacuum testing should be performed on a tank surrounded by other precast concrete tanks. In the rare case that a tank experiences sudden structural failure during a vacuum test, fragments of the tank could be propelled into the air, creating a dangerous situation for those in the vicinity.
The tank must be assembled with joint sealants. In addition, the lid or top half of the tank must be in place before beginning the test. Once the empty tank is assembled, determine to which tank opening the vacuum device will be affixed. All other access points, including inlet and outlet openings, must be temporarily, yet thoroughly, sealed.
After properly sealing the tank, attach the vacuum device to the unsealed access point. Because the device will draw air from the tank and measure the resultant negative pressure, the vacuum device must be capable of drawing a minimum vacuum pressure of 7 inches of mercury (Hg) and must employ a calibrated gauge, mercury manometer or water manometer accurate to within 0.2 inches Hg. However, if a vacuum gauge is used, its range may not exceed 0-to-10 inches Hg. When possible, two gauges should be used to ensure one is not malfunctioning.
To prepare the vacuum test, slowly and steadily begin introducing negative pressure into the tank – up to 4 inches Hg – using the affixed vacuum device. It is imperative to allow the pressure to stabilize before beginning the test period. Once the pressure reading is stable, begin timing a five-minute test duration. The test is completed at the end of the period.
- Pass: If the pressure in the tank remains at 4 inches Hg for the entire five-minute test duration, the tank passes the test.
- Retest: If at any time during the test the pressure drops below 4 inches Hg, the tank may be retested by resetting the pressure, allowing it to stabilize at 4 inches Hg and restarting the five-minute test duration. The tank must maintain 4 inches Hg for the entire period for the tank to pass the test.
- Inspect the equipment and ensure the connection and access points are properly sealed. Oftentimes, the equipment seal is the source of the leak.
- Inspect the tank and listen for a hissing sound caused by air seeping in. If the tank is leaking and the source cannot be located, spray soapy water inside the tank, assemble and reseal the tank, and repeat the vacuum test. Once the test is complete, open the tank and look for bubbles, which will form at the site of the leak.
- If needed, the manufacturer can specify repair material and procedures for the tank before retesting.
In addition to verifying a septic tank or grease interceptor’s watertight integrity, vacuum testing can also be used to validate watertightness and structural integrity of other precast products, like installed manholes.
Septic tank hydrostatic testing
The septic tank must be assembled and sealed before the hydrostatic test may commence. Fill the tank with water to its operable level – typically taken as the effluent invert – and let the tank stand undisturbed for 24 hours. Some water absorption into the interior concrete surface may occur, and the water level may drop as a result. Restore the tank to its initial water level after 24 hours, if necessary. Once the tank is refilled, the one-hour test duration may begin.
- Pass: If the water level remains constant for the entire period, the septic tank passes the test.
- Retest: If the water level falls during the period, restore the tank to its initial water level. Once the tank is refilled, begin timing another one-hour test duration. The tank must maintain the water level for the entire one-hour period to pass the test.
- If at any time during the test period the water level drops, the tank may be repaired per the manufacturer’s recommendations in accordance with ASTM C1227 and the test procedure may be restarted from the beginning.
Grease interceptor hydrostatic testing
The grease interceptor must be assembled and sealed before the hydrostatic test begins. Fill the tank with water to its operable level – typically the effluent invert – and let the tank stand undisturbed for 8-to-10 hours. If the water level does not drop throughout the 8-to-10-hour duration, timing of the one-hour test period may begin. If the water level does fall during the 8-to-10-hour period as a result of moisture being absorbed by the interior concrete surface, refill the tank to its initial water level and let the tank sit for another 8-to-10 hours. Afterward, there should be no further drop in the water level, and timing of the one-hour test period may begin.
- Pass: If the water level remains constant for the entire one-hour test period, the grease interceptor passes the test.
- Retest: If at any time during the 8-to-10-hour test period the water level drops, the grease interceptor may be repaired per the manufacturer’s recommendations in accordance with ASTM C1613 and the test procedure may be restarted from the beginning.
Beyond the test methods: Vacuum testing
Vacuum testing is a relatively quick watertightness integrity test that uses easily transportable equipment and a straightforward procedure. These attributes make vacuum testing the watertightness test method of choice for many manufacturers. Because the test requires very little space, it can be performed in the yard or even on-site.
Vacuum testing is also a conservative approach to watertightness testing and is more stringent than hydrostatic testing. Tanks in service do not operate in a theoretical vacuum, nor do they experience perfectly uniform maximum pressure on all four sides, the lid and the base. However, these considerations validate the performance ability and strength of precast concrete tanks. Hydrostatic testing ultimately applies very little pressure to the upper portions of tank walls.
Beyond the test methods: Hydrostatic testing
Filling the tank with water up to the top of the tank, into the riser or to the top of the riser is unnecessary and is not specified in the aforementioned standards. Typically, filling the tank to the flow line of the outlet is sufficient.
NPCA Plant Certification critical section requirements
Watertightness testing is necessary and beneficial for many underground storage and conveyance vessels, including septic tanks and grease interceptors. NPCA Plant Certification requires watertightness testing to be demonstrated in accordance with the applicable section(s) of ASTM C1227, ASTM C1719, IAPMO/ANSI Z1000 for septic tanks; and ASTM C1613, ASTM C1719, IAPMO/ANSI Z1001 for grease interceptors; or the requirements set forth by the authority having jurisdiction, whichever is more stringent. A minimum of one test per year on a septic tank produced in each septic tank form used at the plant must be performed and documented. The same testing frequency applies to grease interceptors.
Kayla Hanson, P.E. is NPCA’s director of technical services.