Sulfide Corrosion in a Low-Velocity Interceptor Reach
A 900 mm concrete interceptor carries an average dry-weather flow of 0.18 m3/s at a slope giving roughly 0.55 m/s velocity. Upstream, a 3.5 km branch built at 0.0012 slope has velocities near 0.45 m/s. Crews report strong H2S odor at three manholes and crown corrosion with exposed aggregate and softening mortar in the 600 mm branch. The interceptor has no drop structure where the branch enters, and the branch invert sits 0.4 m above the interceptor crown, creating a small cascade. Treatment plant staff note the receiving flow has low dissolved oxygen and a long detention time in the upstream trunk. Average wastewater temperature is 22 C in summer. There is a 90-day window before the annual flow increases and the corrosion survey must be repeated.
1. Diagnose the mechanism producing the crown corrosion and rank the contributing factors.
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The corrosion is classic sulfide-driven microbiologically induced corrosion. Under anaerobic, low-velocity conditions in the branch and trunk, sulfate-reducing bacteria generate dissolved sulfide. At the cascade into the interceptor the flow is turbulent and reaerated, so H2S strips into the headspace. On the moist concrete crown above the waterline, Thiobacillus-type bacteria oxidize H2S to sulfuric acid, attacking the cement matrix and exposing aggregate.
Ranked contributors:
- Long detention and low DO in the upstream trunk (main sulfide generator).
- Low velocity in the 600 mm branch (0.45 m/s) keeping flow anoxic and allowing solids/sulfide accumulation.
- The 0.4 m cascade at the junction, which provides the release point for H2S into the headspace.
- Warm temperature (22 C) accelerating biological kinetics.
- No ventilation or odor-control at the affected manholes.
Velocity in the branch is below the commonly used self-cleansing target of about 0.6 m/s, so the branch is both a generator and a place where gas accumulates.
2. Recommend a prioritized 90-day mitigation plan that fits the inspection window, distinguishing quick operational fixes from capital work.
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Quick operational (within 30 days):
- Install or restore manhole ventilation and add temporary chemical dosing (e.g., ferrous salt or nitrate) at the upstream trunk to suppress dissolved sulfide.
- Increase flushing in the 600 mm branch to reduce solids and detention; document flush volumes and frequency.
- Deploy H2S monitoring/loggers at the three odor manholes to establish a baseline before and after dosing.
Structural/hydraulic (planning within 90 days, construction may follow):
- Evaluate a drop structure or vortex insert at the branch-to-interceptor junction to reduce free-fall and control gas release.
- Consider increasing branch slope or a relief/parallel alignment to raise velocity toward 0.6 m/s.
- Line the corroded crown with an acid-resistant liner after structural assessment confirms remaining wall thickness.
Sequencing matters: characterize remaining concrete strength and sulfide loading before committing to lining, because lining over an active corrosion cell without controlling the source will fail. Repeat the corrosion survey at the end of the window to confirm the trend reversed.