December 2025 Vol. 80 No. 12
Features
Fighting odor, corrosion at the source
By Kerry Koressel
Odor and corrosion are persistent challenges in wastewater collection systems – issues that stem primarily from hydrogen sulfide (H₂S), a gas formed when organic matter decomposes under low-oxygen conditions. Within sewer networks, this process intensifies in stagnant or low-velocity sections and in systems with high biological oxygen demand (BOD), such as those receiving industrial or food-processing waste.
Hydrogen sulfide gas not only produces the telltale, “rotten egg” odor that leads to public complaints, but it also oxidizes into sulfuric acid when exposed to sulfur-oxidizing bacteria. This acid aggressively attacks concrete and metal structures, accelerating infrastructure deterioration. The combination of odor control and asset protection has, therefore, become a top concern for utility managers and engineers responsible for long-lived, resilient sewer networks.
Historically, most municipalities have relied on chemical dosing – using nitrates, oxidizers or bioxide – to suppress hydrogen sulfide formation. Others deploy charcoal filters or air scrubbers to mask odors escaping from manholes. While effective in the short term, these methods come at a price: recurring material costs, staff time for monitoring and refills, and the added burden of treating chemical residuals downstream.
The reactive nature of chemical treatment also fails to address the underlying issue – how system hydraulics and oxygen dynamics contribute to H₂S generation in the first place. Over time, the expense and environmental footprint of chemical feed systems have driven many utilities to seek engineering-based, passive solutions that tackle the problem at its source.
PASSIVE, SUSTAINABLE DESIGN
A more sustainable approach is to design the flow-path so that H₂S generation and release are minimized, and the infrastructure itself becomes part of the control strategy. Instead of dosing chemicals or ventilating manholes, engineering the hydraulic structure – especially drop shafts, steep grade sewers and transitions – can suppress gas formation and release. Many of the same hydraulic and atmospheric factors that encourage hydrogen sulfide formation – low dissolved oxygen, turbulence and splash zones – can be mitigated through smarter flow design.
Engineers in Lincoln, Neb., took the flow-design approach in 2009 when they encountered odor complaints during a major wastewater expansion. In the city’s multi-phased Salt Valley Relief Trunk Sewer project, a new sewer extension needed to connect to an existing system located at a higher elevation. Rather than accept excessive flow velocity or resort to air-phase treatment, the design team introduced a vortex-style drop structure to manage the transition.
“We needed to create a drop condition but also avoid corrosion and gases that can cause foul odors,” said James Burroughs, P.E., senior project engineer with Olsson Associates. “This was especially important because the drop is located near residential neighborhoods.”
The design’s controlled spiral flow created a downdraft that captured odorous gases and pulled fresh air into the wastewater, increasing dissolved oxygen levels and reducing odor complaints.
HARNESSING HYDRAULICS
Innovative hydraulic solutions, such as the IPEX Vortex Flow Insert used by the city of Lincoln, demonstrate how physics can naturally manage odor and corrosion without chemicals.
The underlying science of this hydraulically driven solution rests on three key elements:
- Controlled spiral flow. The wastewater flows around a decreasing radius, to achieve a supercritical velocity, and directs the flow in a tight spiral as it descends the drop.
- Negative air-core formation. As the spiral flow hugs the walls and the center void becomes a low-pressure zone, odorous gases (H₂S and others) are drawn downward into the flow rather than being vented at the top.
- Air entrainment and oxidation. At the base of the drop, flow enters a submerged energy-dissipation pool, where the previously drawn-down air (and associated gases) are re-entrained into the wastewater, increasing dissolved oxygen (DO). The higher DO amount suppresses anaerobic conditions and reduces H₂S. In doing so, it also reduces odor and extends infrastructure life.
Importantly, these systems have no moving parts, no chemical feed, and no energy input, relying solely on gravity and hydraulics.
In Fort Wayne, Indiana, for the massive 3RPORT (Three Rivers Protection & Overflow Reduction Tunnel) project, in 2019, nine drop shafts were constructed, ranging from 23- to 91-million-gallons-per-day (MGD) flows and a drop of 175 feet.
The engineering firm chose this solution because it provided both odor control and energy dissipation without chemical treatments. As the case study notes:
“With no moving parts and requiring virtually no maintenance … the spiral flow design … traps odorous gases and sucks them downward toward the bottom where they are entrained back into the sewage flow.”
Because the system oxidizes sulfides prior to downstream treatment, the overall wastewater quality improves, and the treatment plant load may be reduced.
In Lincoln, the Salt Valley Phase V extension connected a new sewer extension to a higher-elevation trunkline. Rather than allow an extremely steep slope and excessive velocity (which would further strip oxygen and generate H₂S), the design employed a vertical drop of about 14 feet within a 48-inch line. The team selected the spiral drop structure that created a controlled downward flow path, entrained air into the wastewater (raising the amount of dissolved oxygen) and suppressed turbulence and splash emissions.
Meanwhile, installation was efficient. In just two hours, the custom‐designed unit tied into both ends of the 48-inch pipe, peak flow was 8.4 MGD, and the manhole drop occurred in a standard 10-foot diameter structure.
This case highlights how hydraulic intervention can effectively mitigate odor and corrosion while simplifying construction and reducing long-term maintenance costs.
LONGEVITY AND SIMPLICITY
Compared to other hydraulic drop or flow-control structures, this type of flow insert delivers a fundamentally different level of odor and corrosion control as it:
- Targets the cause, not just the symptoms. Traditional drops focus on slowing wastewater velocity and reducing splash, but they allow hydrogen sulfide gas to escape. The system prevents gas release entirely by managing the interaction between air and wastewater inside the drop.
- Creates a true negative air column. Its spiral geometry forms a controlled downdraft that draws odorous gases downward instead of letting them vent upward – something conventional drops and energy-dissipation structures cannot achieve.
- Re-entrains and oxidizes hydrogen sulfide. As gases are pulled into the flow, air is simultaneously drawn in, increasing the amount of dissolved oxygen. This oxidizes H₂S into a more stable, water-soluble form, reducing both odor and corrosion at the source.
- Combines odor control and energy dissipation. Most hydraulic drops handle only flow velocity. The system controls both hydraulics and gas chemistry, reducing turbulence while neutralizing odor and corrosion potential.
- Smaller footprint and easier installation. Competing hydraulic drop structures often require large vaults, deaeration chambers, or mechanical aids.
- Completely passive and energy-neutral. No power, no moving parts, no chemicals, and virtually no maintenance – unlike systems that rely on pumps, scrubbers, or aeration equipment.
- Long-term performance and cost savings. Made from corrosion-resistant PVC with a 50-year design life, it delivers decades of odor and corrosion protection with payback typically within a year – far outperforming other hydraulic or mechanical alternatives.
By design, a gravity-driven hydraulic solution shifts odor and corrosion control from an ongoing operational cost to a one-time design/installation cost.
For municipalities, the benefits are threefold:
- Extended sewer life due to reduced corrosion as concrete and metal structures last longer.
- Lower maintenance costs: fewer chemical purchases, fewer air treatment systems, fewer odor complaints).
- Improved wastewater quality (higher DO, lower sulfides, reduced downstream treatment burden).
The upfront installation – often completed in less than a day – pays for itself within a year for utilities currently reliant on chemical feed systems.
As urban areas grow, sewer systems age and municipalities are pressed to meet sustainability goals, the choice becomes clearer. Rather than chasing the next chemical or mechanical fix, the smarter path is to design infrastructures that control odor and corrosion by default – through hydraulics.
About the Author: Kerry Koressel is the Central District Manager – Municipal for IPEX USA LLC, where he oversees municipal piping systems strategy, business development and project support across the central U.S. region.
FOR MORE INFORMATION:
Ipex USA, 800-463-9572 ipexna.com/en-us
FREQUENTLY ASKED QUESTIONS
How much hydrogen sulfide can passive systems actually remove?
Removal rates vary with system conditions, but field studies and performance data indicate that up to 85 percent of hydrogen sulfide can be reduced through vortex-style designs. The key factors influencing performance include the flow rate, drop height, sewage temperature, and the concentration of H₂S entering the structure.
Will a vortex system clog or require regular cleaning?
No – when properly designed within its flow range, the system is self-cleaning. The internal hydraulics create small eddy currents in the dissipation pool, preventing sediment buildup. Routine inspection every few months is typically sufficient, aligning with standard municipal maintenance schedules.
How much drop and flow are required for this type of design to work?
Generally, a minimum 6-foot drop and at least 300,000 gallons per day of flow are needed for effective air entrainment and gas capture. Larger flows or drops improve performance and efficiency.
How does the cost compare with chemical treatment systems?
Passive systems are typically a higher upfront investment, but they have no ongoing chemical or energy costs, no moving parts and minimal maintenance. Many municipalities report full payback within a year or less by eliminating recurring chemical expenditures.
Are these systems customizable, or are they standard sizes?
Each installation is engineered to site-specific conditions – flow rate, drop height and manhole diameter. Because no two sewer networks are identical, vortex systems are custom designed to ensure optimal hydraulic performance.
How long do these inserts last?
Units constructed from PVC offer strong resistance to corrosion from sulfuric acid and abrasion. Conservatively, they’re expected to last 50 years or more in typical sewer environments.
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