Conditions and baseline
An anonymous sewer network recorded an undosed downstream mean gas-phase H₂S reading of 29.7 ppm. The 30% magnesium hydroxide liquid (MHL) operating period began in October 2025 and the Suppressor period began on 30 April 2026.
Method
We assessed 1-minute gas-phase H₂S logger readings at two wet-well locations across separate undosed, MHL and Suppressor operating periods. The downstream point is the final wet well before the treatment plant; the reference point is upstream.
Outcomes achieved
Downstream mean gas-phase H₂S was 29.7 ppm undosed, 5.3 ppm during MHL operation and 1.5 ppm during Suppressor operation. These outcomes were achieved in separate historical operating periods.
Project dosing figures:
- 30% magnesium hydroxide liquid (MHL)
- ≈175–190 L/day; treatment reach ≈1.95–2.0 km from a mid-network dose point
- Suppressor
- Representative operating volume ≈140 L/day; treatment reach ≈3.6–3.65 km from an upstream dose point
Project context and application
Results are drawn from Critical Elixirs network monitoring and operational case records. Daily volumes are representative operating rates, rather than a daily consumption ledger. The Suppressor mean spans dose adjustments and is not exclusively a measurement at 140 L/day.
Flow, temperature and operating conditions varied between periods; this is an operational case study, not a season-matched controlled trial. Gas-phase means describe the monitoring periods, not peak concentrations or personal exposure. Worker-exposure assessment and confined-space entry require their own controls and measurements. Treatment and cost comparisons for a new network should use its actual duty, delivery records and downstream response.
