Study demonstrates the impact of corrosion and effective means of treatment.
Sulfuric acid poses a danger to people working in the sewers. It can attack eyes, throats and lungs. Two municipal workers in Texas died due to H2S exposure earlier this year. In extreme cases, when you have anaerobic conditions, the formation of methane gas can occur and result in explosions.
Thiobacillus oxidizes hydrogen sulfide gas to form sulfuric acid, and hydrogen sulfide may also combine with oxygen to form polythionic acids — a weak form of sulfuric acid.
Municipalities not only have to be concerned about odors from wastewater systems, but from the corrosion caused by Thiobacillus bacteria that oxidizes hydrogen sulfide gas to form sulfuric acid, which corrodes concrete sewer pipes and structures and shortens the life span of the systems.
“Hydrogen ions in the acid attack calcium hydroxide in the hydrated Portland cement,” says Joost Goossens, co-author of an Evoqua Water Technologies study on the impact of corrosion. “The calcium ions and sulfate ions combine to form gypsum, a soft corrosion product. Additionally, ettringite may form. Gypsum and ettringite expand, placing stresses on the concrete and resulting in the loss of concrete aggregate materials. Due to the softness of both products, they are easily washed away by the scouring action of wastewater flow. The generally accepted method for preventing corrosion in the collections system is to short circuit the first step in the above process through chemical treatment of the wastewater to either prevent sulfides from forming or remove existing sulfides.”
Cities already chemically treat wastewater to remove noxious smells, but Evoqua’s 2015 study, A Novel Test Method for Measurement of MIC in a Wastewater Collection System, authored by Goossens and Tim Matheis, demonstrated the impact of corrosion. The study compared two sections of sewer lines owned by Sanitation District No. 1 in Boone, Campbell and Kenton counties in Kentucky, suburbs of Greater Cincinnati.
The two-year study evaluated the effect of H2S at two sites 7.6 miles apart. One was treated as part of an odor control program (continuous pumping) and the other was untreated. The untreated site had a net loss in material of 5.4 percent and a reduction in compressive strength of 13 percent. The treated site had a net loss in material of only 0.2 percent and no decrease in compressive strength, and effectively reduced the formation of sulfuric acid and corrosion in the neighboring collections system.
The results, stresses Eric Hansen, Evoqua’s municipal services product manager, showed that “the best protection against corrosion is a comprehensive control program that includes hydrogen sulfide monitoring, controlled chemical dosing, and performance monitoring and control. This investment will protect your system from dangerous conditions and expensive repairs down the road.”
The study notes a 1991 U.S. EPA report to Congress that estimates a national cost, in 1991 dollars, for sewer rehabilitation at $6 billion. In addition to the direct cost of replacing corroded infrastructure, there are hidden costs in lost time and labor that are diverted to emergency and repair activities, which are not spent on core wastewater collections system operations.
The EPA report was based on the severity and impact of concrete corrosion as surveyed by CSDLAC, AMSA and WPCF in 89 cities. It found that 32 cities “reported sewer collapses, of which 81 percent of the collapses are believed to be due to hydrogen sulfide corrosion. Furthermore, the study indicated that almost 70 percent of the 61 respondents experienced hydrogen sulfide corrosion at the treatment plant.”
Once corrosion begins and is neglected, concrete pipes (often under roads), weaken and experience additional pressure from daily traffic. This can result in sewer collapses.
“We’re at the cusp of municipalities starting to look at this problem proactively,” Matheis says. “‘Am I going to spend a small amount every year to maximize the life of my sewer systems as opposed to waiting for them to fail and spend millions to repair or reconstruct them?’ Usually, in the U.S., people just let it go and when it fails, they’ll fix it. But it is very disruptive to the community because of construction and overflows.”
The attacks usually occur at force main discharges, areas of turbulent flow, and other areas conducive to the release of H2S from wastewater.
For the study, concrete coupons were placed at the discharges of two force mains, similar in terms of potential for sulfide generation and H2S release. The treated site received a nitrate double-salt solution to prevent the formation of sulfide in the wastewater. Concrete test samples were compared at six-month intervals to assess the impact on the strength and integrity of the material.
The samples deployed downstream of the treated site were exposed to an average of 3.6 ppmv hydrogen sulfide, and showed a 0.2 percent reduction in mass and a 9 percent increase in compressive strength. The primary conclusion from the trial is that minimal corrosion of concrete occurred at hydrogen sulfide concentrations below 5 ppmv.
According to Evoqua, the chemical used for treatment is one of many different formulations available on the market under a variety of trade names and was selected as the best option from a stable of a dozen different odor control chemicals.
The treatment “demonstrated (that) effective sulfide control at other sites within the municipality’s treatment area is well suited for sulfide control in wastewater collections systems with anaerobic retention times greater than two hours,” Goossens says. “It’s also safe to handle, as it is a nonhazardous chemical. You don’t have to treat every inch of your system — you have it at a few key points.”
The calcium nitrate salt solution contained 3.5 pounds of nitrate oxygen per gallon, which the study notes is used safely and effectively in over 600 municipalities to control hydrogen sulfide-related odors and corrosion on a daily basis.
It was released via the wet well of the pump station upstream of the control point, with dosing set at a constant average rate of 32 gpd.
“You could visually see the corrosion of the coupons in the untreated line,” Goossens says.
The treatment, applied at certain points along the line, costs about $75 per day and uses about 30 gallons of chemicals. Evoqua is currently conducting another study in Florida as part of a follow-up looking into chemical treatment and comparing it with the cost of sewer line replacement.