Writer Says Ventilation Eases Deterioration

I have enjoyed reading Pumper for several years. The story in the May 2010 issue, “Looking for Concrete Answers,” brings up a big concern of mine also.

I live and work in an environmentally sensitive mountain resort in South Central New Mexico. Following 20 years of building houses, I began manufacturing septic tanks in 1997. I installed the first advanced systems in my area in 1999, and began pumping operations in 2009. Advanced (aerobic) systems are now my primary business. I install and service two different brands and am familiar with many others.

In a multi-compartment design that locally we refer to as “dual primary/geoflow,” a two-compartment septic tank is followed by a second modified septic tank ending up with an 800-gallon third compartment “clarifier,” followed by a 400-gallon pump vault, and then ending in subsurface drip field disposal. In an installation, the level of deterioration progressed per compartment except the third compartment, which had no deterioration. A lid had not been fully seated, allowing the tiniest amount of venting. To deter additional deterioration, I installed a 2-inch vent line to all risers and the sewer cleanout.

I agree with many other Pumper readers that concrete inlet and outlet baffles are inferior to a PVC product. Care must be used to get the down piping lengths correct. Too long and too short yield bad results.

I switched from concrete to plastic distribution box installation after replacing a concrete box six years ago. Two hours on my belly in muck is not fun. I have not seen a healthy concrete distribution box on re-inspection. My conclusion to the problem of concrete deterioration: It’s all about ventilation, which can occur naturally through porous soils or through unobstructed vent pipes at the rooftop.

Jace Ensor

Mountain Top Inc.

President, New Mexico Onsite Wastewater Association

Ruidoso, New Mexico

Writer Elaborates on Concrete Deterioration

According to the Portland Cement Association, “ettringite (calcium sulfoaluminate), is found in all portland cement concretes.

“Any form of attack or disintegration of concrete by freeze-thaw action, alkali-silica reactivity (ASR), or other means, accelerates the rate at which ettringite leaves its original location in the paste to go into solution and recrystallizes in larger spaces such as voids or cracks. Both water and space must be present for the crystals to form. The space is often provided by cracks that form due to damage caused by frost action, ASR, drying shrinkage, or other mechanisms. Ettringite crystals in air voids and cracks are typically 2 to 4 micrometers in cross section and 20 to 30 micrometers long. Under conditions of extreme deterioration, the white ettringite crystals appear to completely fill voids or cracks.”

It would appear that the attack of hydrogen sulfide, which would condense to form a weak sulfuric acid, may lead to the migration or translocation of the ettringite found in concrete. This may be a byproduct and marker of deterioration, or part of the process.

In New Hampshire and Maine it is very common to see deteriorated concrete outlet baffle structures, and deteriorated concrete distribution boxes. It is logical to assume that gases originating in the soil disposal area would be the cause, or a significant contributor, to this phenomenon, as in most cases the rest of the tank is much less affected. Gases from the field area would likely accumulate in the distribution box and in the concrete outlet “chimney,” condensing there to form weak sulfuric acid, which would attack the concrete. This attack could lead to the translocation of ettringite. This ettringite would thus be a marker of the attack.

Sometimes water treatment is associated with this. We have high iron in many of our wells. The common iron removal system uses a salt brine. Often this salt brine backwash ends up in the septic system. There is overwhelming anecdotal evidence of poorly performing septic tanks associated with this. Black, murky, poorly stratified, emulsified tank contents with effluent strength in the 400 to 500 ppm of BOD and TSS is common. Some speculate that the salt may also play a part in the weakening. It is perhaps more likely that the higher strength effluent will lead to greater gas production during the natural decomposition in the disposal area that produces these gases.

(As a side note, this is not “hard” water, which is associated with calcium in the water. But the treatment process is similar.)

Tom Canfield

Thomas Canfield Earth Construction/

Earthcon Earth Construction

Rochester, N.H.