For more than 20 years, onsite professionals have popped the lids on precast tanks, sometimes to find eroded outlet baffles, a flaky white substance engulfing exposed concrete aggregate, and deteriorating and collapsing walls.

Now experts are working to find out why this deterioration affects only a few tanks, and when it does attack, why it doesn’t follow a consistent pattern. Hydrosulphuric acid, a byproduct of the Thiobacillus bacteria in septic tanks, is clearly the culprit, though until recently no one asked if specific circumstances were accelerating deterioration.

The Wisconsin Onsite Water Recycling Association held a panel discussion on concrete deterioration at its 2010 conference. The Oregon Onsite Wastewater Association uses its newsletter as a platform for pumpers to report findings.

“We need pumpers, plumbers, inspectors and precasters to talk about what’s going on,” says Aaron Ausen, vice president of WOWRA and a civil engineer specializing in concrete engineering. “We can’t keep this a secret. We need to reveal the definitive problems and hopefully find the solution.”

There is no simple answer, mainly because there are many variables and research, where it exists, is conflicting.

Eliminate the obvious

Some concrete degradation comes from poor manufacturing practices, yet scientists creating a uniform concrete code fail because the parameters change from location to location. Quality is controllable, and Ausen and the Wisconsin Precast Concrete Association have selected a board to create a stringent yet attainable code for all precasters in the state. A third party would conduct mandatory testing.

“We agreed unanimously on our course,” says Ausen. “We also would like inspectors and WOWRA to specify only certified tanks. That rules out construction variances and enables us to look more closely at corrosive causes.”

Evidence suggests erosion caused by hydrogen sulfide occurs when there is extra turbulence in tanks. “It’s prevalent in pump tanks, but not in gravity-flow systems or holding tanks,” says Ausen. “The turbulence churns and releases the gas, which corrodes the tank and can travel through the effluent lines and into distribution boxes.”

However, installer-designer Sue Schambureck of Madson Tiling and Excavating in Manitowoc, Wis., has replaced many corroded tanks from gravity-flow systems. She believes gas coming back from the drainfield contributes to deterioration.

Concrete corrosion occurs in specific areas of the country. Ausen sees it in northeast Wisconsin and the Indiana-Ohio Valley region. Kim Aldrich, a regulator in Yamhill County, Ore., notices deteriorated distribution boxes in areas with high sulfur and iron in the soil. “Before I publish any levels, I must study more cases to ensure that I wasn’t encountering an anomaly,” she says. “However, the soil pH of around 6 is not acidic enough to cause the destruction.”

High iron levels produce hard water, and a second commonality Aldrich sees is household cleaners used to remove hard-water stains. She has learned that some contain hydrochloride-based chemicals. “Because I inspect just distribution boxes and drain lines, I have only half the equation,” she says. The affected systems are at least 20 years old.

Online database

Ausen’s second objective is to create a statewide online database, and eventually a national one where professionals can report their findings. “I want to assure pumpers that they will meet no resistance when coming forward,” says Ausen. “They should not take corroded tanks lightly.”

Since most professionals won’t know who manufactured the tanks, Schambureck recommends they tell customers to call the installer. Practitioners also should notify the local health authority, because the system could be failing. “Unfortunately, some inspectors brush it off as no big deal, but it is,” says Ausen. “If corrosion reaches the structural rebar, we’re talking a possible total collapse. That’s scary.”

Knowing the corrosion pattern helps in gauging the degree of degradation. Schambureck says the first area to corrode in a gravity system is the tank outlet. “Sometimes we can see right through the wall to the soil,” she says. “The center wall of a two-compartment tank and the ceiling corrode next. In pump tanks, we see the center wall deteriorate before the outlet because the gases aren’t returning from the drainfield.”

The result of hydrogen sulfide attacks is ettringite — a flaky, chalky, gypsum-like substance. By the time corrosion reaches structural steel, an orange-red stain (rust) shows through the ettringite. “If you see that color, proceed with extreme caution, as you have no way to gauge the degree of corrosion,” says Ausen. “Another red flag is when the rotten egg odor is strong enough to make your eyes water. Pump the tank very slowly, and watch for the formation of cracks.”

As an inspector certified by the National Association of Wastewater Transporters, Dawn Long of American Septic Service in Sierra Vista, Ariz., has documented more than 2,500 degraded septic tanks. Of those, 20 of 225 tanks built with concrete baffles had the baffles in tact, though there was almost no aggregate deterioration on the tank walls. Most of those systems, from the 1960s and 1970s, lacked distribution boxes, but Long sees deterioration on newer systems that do have them.

Theories abound

Some experts suggest that inadequate tank ventilation is to blame. While no definitive data exists that creating an exhaust system cures corrosion, they recommend installing a vented manhole cover with a charcoal filter.

“A Canadian precaster told me that house vents are insufficient if tanks are more than 15 feet away,” says Ausen. “Long distances enable wastewater to churn in the lateral. In Ontario, they install tanks closer to the house and vent every riser. It appears to work for them.”

Another study by Long of 278 corroded tanks revealed that all were vented properly. She intends to put a sulfur gas meter in a newly installed tank for a week, then install a vent with charcoal filter and note any differences.

Other professionals suggest spraying a sealant on tank walls at the onset of corrosion. However, researchers found that one scratch or imperfection in the coating allows hydrogen sulfide gas to reach the aggregate.

A Canadian precaster who used coatings for years, says they were ineffective. Ausen fears that as coatings corrode, they may release harmful chemicals into the groundwater. “It’s better to put something in the concrete mix rather than coat tank walls,” he says.

Ausen hears theories about bleach mixing with certain chemicals in the tank and decomposing food from garbage disposals possibly increasing sulfide levels. Aldrich reports homeowners and house painters using muriatic acid as a cleaner.

Long suggests that latex paint in the tank accelerates deterioration. “In two documented cases, the degradation was markedly different, with ettringite hanging like stalactites,” she says. “We also saw that the rear wall of a 1996 tank had eroded one inch. That’s significant for a 12-year-old tank.”

More evidence needed

While many factors may need evaluation, Ausen is focusing on anecdotal evidence from the field. Reports should contain as much relevant information as possible, such as tank location, manufacturer, age of tank, number of compartments, extent of deterioration, applicable parameters and photographs. E-mail reports to aaron@dalmarayconcreteproducts.com and copy Aldrich at aldrichk@co.yamhill.or.us.