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Field Testing for Water-Tightness of Precast Concrete Tanks

February 13th, 2009

By Dan O’Connell, Castle Valley Consultants, SEO # 01462

The Pennsylvania Code Title 25 Chapter 73 Standards for Onlot Sewage Treatment Facilities and the associated Alternate System Guidance document, in their current form, require the ‘Tanks shall be water tight and constructed of sound and durable material not subject to excessive corrosion or decay.’ (Section 73.31(b)(1)). There is no current requirement for in field testing of treatment tanks for water tightness. However for the past couple years, Pennsylvania Department of Environmental Protection (DEP) has been revising both Chapter 73 and the Alternate System Guidance documents. One of the revisions was to include a requirement for in field testing, using either, water or vacuum testing of treatment tanks for water-tightness. The revised documents have been reviewed and commented on informally by outside agencies. A majority of the reviewing agencies all commented that the in field testing for water tightness should not be a requirement since the industry has designed and makes water tight tanks.

Background

As an SEO since 1980, I was first employed by a County Health Department to permit systems, and then by engineering firms to design systems, I’ve seen over 1000 tanks installed and have designed over 500 systems utilizing precast concrete tanks. I also agreed with the notion that treatment tanks (the majority of which are precast concrete tanks) are designed and manufactured so they do not leak – are water tight. Any surface water infiltration into tanks has typically been around the pipe penetrations or where the manhole lid risers meet the tank lids.

Photo A

In 2008, I had the opportunity to consult for a Township resident in a sewage system repair. As part of the repair permit application review it was noted that after the original permitted system had been installed the owner had requested the developer/builder extend the turnaround portion of the driveway. The installed driveway extensions infringed into the required 10 ft isolation distance between any tank within the sewage system and the driveway. (Section 73.13(‘C)((2)). (See Photo A) In order to issue the permit the permitting agency required either 1.) The tanks be re-located 10 ft off the driveway or 2.) That added portion of the driveway be removed. The property owner chose option 1.) Relocate the existing tanks 10 ft off the existing driveway.

Relocating an existing Septic Tank

Moving or relocation of installed existing tanks is a tricky procedure. The typical tank walls and bottom are only 2.5 to 3″ thick. Just touching the sidewall during excavation with a backhoe, can cause the side wall to crack and render the tank useless. In this situation, it was decided to try to move two tanks – a 1500 gallon aerobic treatment tank and a 1250 gallon pump tank. Since the tank manufacturer had been onsite and had offered that he probably wouldn’t try to move the tanks, I suggested that after the tanks were relocated, we follow the newly proposed DEP tank water-tightness testing and fill the tanks with water. The local permitting agency agreed and required the water-tightness testing.

The tanks are manufactured in two different configurations. The aerobic tank has two extra internal walls and in order to precast those walls with the tank, the tank mold is split in half. The joint between the two halves is a horizontal joint running around the entire circumference of the tank at the vertical midpoint of the wall. (See Photo B). The pump tank is a single compartment with four exterior walls, a bottom and a lid. The lid has a number of manhole access opening.  This tank is poured in two molds. The first mold contains the bottom and all four walls. The second mold is for the flat lid that will sit on top of the walls and includes all associated tank lid openings.

Photo B

After the original aerobic tank had been relocated and the water added, a 10″ long crack near the base of the tank was found. This was not a new crack, as evidenced by the discoloration of the concrete, but had been blocked from view by soil clinging to the tank wall. Our assumption is that it was cracked during the original installation process 2 years ago.  See Photo C A second aerobic tank was ordered.

The Contractor then switched his attention to the relocated pump tank. The tank was water tested and the wall to lid joint (immediately below the tank lid) leaked. See Photo D The lid was removed and two new rows of 1″ thick mastic was applied to the tank walls. See Photo E The lid was then reinstalled and the tank retested. The seam still leaked, so the contractor applied a thick layer of hydraulic cement to the entire joint area. See Photo F The cemented joint held. The tank manhole risers were placed on a layer of mastic and hydraulic cemented to the tank lid. The tank was filled with water to a level above the ‘riser to tank lid’ joint. The pump tank passed the water-tightness test.

Photo G

The new Aerobic tank arrived the next day and the contractor immediately hydraulic cemented the risers and the extra manhole lids in place. Water was then added to the tank. Before that tank was ¾ full, water started to seep out of a sidewall of the tank. It was along a vertical seam between the external sidewall and an internal wall. The tank was delivered with the seam in question smear coated with hydraulic cement but it still leaked. See Photo G

A second new aerobic tank was ordered. It also arrived with the same pattern of smeared of hydraulic cement on the walls. The tank was immediately filled with water and even though the manufacturer had stated he had water tested the tank at his yard, it also leaked at the same seam location as the first tank. See Photo H

A third tank was ordered but this time we asked for a tar coated tank, inside and outside. The tar coating is applied with a roller by the tank manufacturer, to all accessible internal walls and all external sidewalls.  Water testing and careful examination of the side walls revealed no leakage. The risers were set on a layer of mastic and sealed with hydraulic cement. A second water tests also revealed no tank leakage. See Photo I

The relocated tanks were then backfilled , the area regraded, seeded and mulched. See Photo J

Findings

  1. Water testing is an extremely cumbersome process. The water should be clean water (not effluent) since one may have to remove the water to reseal lids, pipe penetrations etc.
  2. When the tanks were removed from the hole they had to be emptied prior to removal. Then additional water was needed to tests the replacement tank.
  3. The quantity of water (in this case 1500+ gallons) is hard to handle. It would also take a while to fill the tank from a typical garden hose. The contractor did transfer water from one tank to another with a small electric sump pump, but that took up to 4 hours to fill the tank.
  4. Concrete pump tanks are only designed and manufactured to be water-tight to the height of the  pipe penetrations, since effluent levels would typically not be above the discharge pipe penetration. That was the reason behind manufacturers switching to the monolithic tanks, the joint is above the tank discharge pipe.
  5. The ‘lid to tank wall joint’ is not water-tight as delivered. This joint should be sealed with a layer of hydraulic cement.
  6. Tar coated tanks seem to have another layer of protection from leakage that uncoated tanks do not have. Tar coating is easily applied by the manufacturer and if the tar coating is scratched by lift chains during delivery, the tar could be recoated onsite by the contractor.
  7. Vibrations during the delivery process can adversely affect the seams and water tightness of the tanks.

Recommendations

  • All precast concrete tanks used in an on-lot sewage system should be tar coated (pitched) on the exterior walls.
  • Scratches in the tar coating should be re-coated before backfilling of installed tanks.
  • The regulations should allow for, but not require, in field water-tightness testing. If the SEO suspects a problem with the tank condition, a test may be appropriate, but not all tanks used in the system should have to be tested.
  • All tank joints should be sealed with a layer of mastic AND a coating of hydraulic cement.
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