Looking for Concrete Answers

As they evaluate a pattern of septic tank corrosion, experts seek the aid of pumpers who discover deterioration during cleaning and maintenance work

For more than 20 years, pumpers and installers have popped the lids on precast tanks to occasionally find eroded outlet baffles, a flaky white substance engulfing exposed concrete aggregate and deteriorating and collapsing walls. Distribution or drop boxes crumbled when touched.

Now experts are working to unravel the mystery of why this deterioration doesn’t affect every tank, and when it does attack, doesn’t always follow the same patterns. Hydrosulphuric acid, a naturally occurring byproduct of the thiobacillus bacteria in septic tanks, was deemed the clear culprit. Though until recently, no one questioned if extenuating circumstances were accelerating the deterioration process.

In January, the Wisconsin Onsite Water Recycling Association held a panel discussion on concrete deterioration at its annual 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.”

It won’t be easy. It’s a difficult problem with no simple answer, mainly because of innumerable variables and, where it exists, conflicting research. Consequently, concerned individuals are asking pumpers for help.

ELIMINATE THE OBVIOUS

The industry has many more structurally sound concrete tanks than deteriorating ones. Some concrete degradation results from poor manufacturing practices, yet scientists trying to create a uniform concrete code fail because the parameters change from location to location. Quality, however, is controllable, and Ausen and the Wisconsin Precast Concrete Association 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,” Ausen says. “We also would like inspectors and WOWRA to specify only certified tanks. That is huge, because it rules out construction variances and enables us to look more closely at corrosive causes.”

Evidence suggests degradation 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,” Ausen says. “The turbulence churns and releases the gas, which corrodes the tank and can travel through the effluent lines and drop en mass into d-boxes.”

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

Concrete corrosion is sporadic, occurring in specific areas throughout the country. Ausen sees it in northeast Wisconsin and the Indiana-Ohio Valley region. Kim Aldrich, Yamhill County (Oregon) regulator, notices deteriorated d-boxes in locations with higher levels of 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. The second common denominator Aldrich sees is strong household cleaners used to remove hard-water stains. Researching over-the-counter products, she learned that some contain hydrochloride-based chemicals. “Because I inspect just d-boxes and drain lines — pumpers pump and inspect the tanks — I have only half the equation,” she says. The affected systems are at least 20 years old. Aldrich reports her findings in the Oregon association’s newsletter and solicits responses.

ONLINE DATABASE

Ausen’s second objective is creating a statewide online database for pumpers to report their findings, then going nationwide, if necessary. “I want to assure pumpers that they will meet no resistance when coming forward,” Ausen says. “They should not take corroded tanks lightly.”

Since most pumpers won’t know who manufactured the tanks, Schambureck recommends they tell customers to call the installer. “We can easily manage the problem and report the specifics,” she says.

Pumpers 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,” Ausen says. “If corrosion reaches the structural rebar, we’re talking a possible total collapse. That’s scary.”

Knowing the corrosion pattern will help pumpers gauge the degree of degradation. According to Schambureck, 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 byproduct of hydrogen sulfide gas attacks is ettringite, a flaky, chalky, gypsum-like substance. By the time corrosion reaches structural steel, pumpers will see an orange-red stain (rust) seeping through the ettringite.

“If you see that color, proceed with extreme caution, as you have no way to gauge the degree of corrosion,” Ausen says. “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 a National Association of Wastewater Transporters-certified pumper and inspector, Dawn Long of American Septic Service in Sierra Vista, Ariz., has documented more than 2,500 septic tanks. Of those, only 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 tanks, from the 1960s and ’70s, lacked d-boxes, but Long sees deterioration on newer systems with them.

THEORIES

Hydrogen sulfide gas is heavier than oxygen, yet some experts suggest that inadequate tank ventilation is to blame. While no definitive data exists that creating an exhaust system cures the corrosion problem, 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,” Ausen says. “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. The Canadian precaster, who used coatings for years, stated that 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 disposers possibly increasing sulfide levels. Aldrich reports homeowners and house painters using muriatic acid as a cleaner, while Long assumes Arizona residents are using strong household cleaners to remove hard water stains.

“Homeowners won’t admit to pouring anything harmful down the drain,” Long says. “However, muriatic acid is a common cleaner here.” Ausen even found one instance of homeowners cooking crystal methamphetamine. Muriatic acid is a major byproduct of the process.

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 a thousand different things may need evaluation, Ausen is focusing on anecdotal evidence from everyone in 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 (if possible). E-mail reports to aaron@dalmarayconcreteproducts.com and copy Aldrich at aldrichk@co.yamhill.or.us.



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