Capstone: a hydrogen sulfide safety plan

This capstone brings the course together by building a hydrogen sulfide safety plan for an invented workplace. The site, numbers and people are fictional, chosen to show how the pieces fit. A real plan must be based on the actual site, its monitoring data and the law that applies, and written with the people who do the work.

The site

Tamarack Creek Utilities (invented) operates the wastewater system for a town of about 18,000 people in a western US state. Its assets include 11 sewage lift stations, a septage receiving station where vacuum trucks unload, and a treatment plant with a headworks building, grit chamber, aeration basins, an anaerobic digester and a sludge dewatering building. The utility has 24 field and plant staff, uses contractors for digester cleaning and major repairs, and dispatches crews from a central yard. Some staff also work part-time on the town's water system. Several lift stations are remote, and operators visit them alone on weekly rounds.

Over the past year, the utility has had three hydrogen sulfide alarms at lift stations, one operator treated for eye irritation after cleaning screens, a near miss when a vacuum truck driver unloaded acidic grease trap waste into the septage station, and repeated complaints of rotten-egg odor from residents near a force main discharge. The safety committee has asked for a written hydrogen sulfide safety plan.

Step 1: inventory and hazard assessment

The plan starts with an inventory of every location and task where hydrogen sulfide could be present, using monitoring data, datalogs, incident reports and operator knowledge.

The assessment identifies the main release triggers as opening wet wells and manholes, pump starts, cleaning, sludge handling, truck unloading and confined space entry. It also notes corrosion damage at older lift stations, where two fans and a fixed detector failed in the past year.

# Review one month of datalogs from four operators (invented data, 1-minute samples).
import random
random.seed(244)
def shift(peaks):
    data = [max(0.0, random.gauss(0.2, 0.15)) for _ in range(480)]
    for start, length, level in peaks:
        for i in range(start, start + length):
            data[i] = level * random.uniform(0.8, 1.2)
    return data
crew = {
    "Operator A": shift([(60, 12, 18), (300, 8, 6)]),
    "Operator B": shift([(120, 20, 4)]),
    "Operator C": shift([(200, 15, 35)]),
    "Operator D": shift([]),
}
for name, d in crew.items():
    twa = sum(d) / 480
    stel = max(sum(d[i:i+15]) / 15 for i in range(480 - 14))
    peak = max(d)
    flags = []
    if twa > 1: flags.append("TWA > 1 ppm")
    if stel > 5: flags.append("STEL > 5 ppm")
    if peak > 10: flags.append("peak > 10 ppm")
    print(f"{name}: TWA {twa:4.2f} ppm, max 15-min {stel:5.1f} ppm, peak {peak:5.1f} ppm  {', '.join(flags) or 'ok'}")

Operator A: TWA 0.73 ppm, max 15-min  13.9 ppm, peak  21.6 ppm  STEL > 5 ppm, peak > 10 ppm
Operator B: TWA 0.36 ppm, max 15-min   4.1 ppm, peak   4.7 ppm  ok
Operator C: TWA 1.32 ppm, max 15-min  35.9 ppm, peak  41.3 ppm  TWA > 1 ppm, STEL > 5 ppm, peak > 10 ppm
Operator D: TWA 0.21 ppm, max 15-min   0.3 ppm, peak   0.6 ppm  ok

The datalog review shows that short tasks drive the exposures. Two operators exceeded the ACGIH short-term limit during wet well work even though their shift averages were near or below 1 ppm, and one had a peak above 30 ppm. These peaks were never reported, because the alarms sounded only briefly.

Step 2: controls

Using the hierarchy of controls from lesson 9, the plan sets out controls for each high-risk location.

Source controls: add nitrate dosing at the station 4 force main to prevent septic conditions; schedule regular wet well cleaning to reduce sludge; and work with the town to reduce force main retention time at low flows.

Engineering controls: install continuous exhaust ventilation drawing from low level in the two worst wet wells; replace corroded fans and detectors with corrosion-resistant models; add fixed detection with beacons at each lift station door and remote alarms to the operations phone; install a sealed receiving connection at the septage station so trucks unload through a hose rather than an open grate; and enclose and ventilate the screenings conveyor.

Administrative controls: a written procedure for each high-risk task; a waste acceptance policy at the septage station requiring drivers to declare waste types and banning acidic wastes from being mixed with septage; confined space permits for all wet well, manhole, digester and tank entries; a lone-worker check-in system for lift station rounds, with check-ins before entering and after leaving each station; and pre-job briefings for contractor work.

Personal protective equipment: personal hydrogen sulfide monitors for every field and plant worker; positive-pressure SCBA at the plant and in each crew truck; supplied-air breathing apparatus with escape cylinders for digester and sludge tank cleaning; and fit testing, medical evaluation and training as HSE-240 requires.

Step 3: detection and alarms

The plan sets alarm set points based on the most protective values rather than OSHA's older limits, which apply to this general industry work as a 20 ppm ceiling. The utility adopts the ACGIH values as its internal limits: personal monitors have a low alarm at 5 ppm, a high alarm at 10 ppm, a TWA alarm at 1 ppm and a STEL alarm at 5 ppm. Fixed detectors alarm at 5 ppm and 10 ppm and start ventilation at 5 ppm. Every alarm has a defined response: low alarm, stop work and move to fresh air; high alarm, evacuate to the briefing point and report. Monitors are bump tested daily using docking stations at the yard and plant, which also download datalogs weekly for review.

Step 4: training and competence

All field and plant staff complete hydrogen sulfide training meeting ANSI/ASSP Z390.1-2024, including hands-on SCBA and detector practice and drills, with refreshers at the interval set in the program, plus confined space training (HSE-211), respiratory protection training with fit testing (HSE-240) and site-specific training on each station's hazards, detectors and procedures. Vacuum truck drivers delivering to the septage station must show proof of hydrogen sulfide training and attend a site orientation. Supervisors complete additional training on the plan, datalog review and incident investigation.

Step 5: emergency response

The emergency plan follows the sequence in lesson 10. Each lift station has a laminated card at the door with its address, the nearest briefing point, the location of SCBA in the crew truck, and emergency numbers. For confined space entries, a standby rescue team with SCBA and a retrieval system is on site whenever an IDLH atmosphere is possible. The utility has an agreement with the town's fire department for confined space rescue, but because the fire hall is 15 minutes away, the plan does not rely on it for entries where knockdown is possible. Rescue drills are held twice a year at a lift station and at the digester.

# Annual plan check: training renewals and drills due (invented roster).
from datetime import date
today = date(2026, 10, 24)
roster = {
    "Operator A": date(2024, 3, 10), "Operator B": date(2023, 11, 2),
    "Operator C": date(2025, 6, 18), "Operator D": date(2023, 9, 30),
}
renewal_years = 3
for name, trained in roster.items():
    due = date(trained.year + renewal_years, trained.month, trained.day)
    days = (due - today).days
    status = "OVERDUE" if days < 0 else ("due within 90 days" if days <= 90 else "current")
    print(f"{name}: trained {trained}, renewal due {due} -> {status}")

Operator A: trained 2024-03-10, renewal due 2027-03-10 -> current
Operator B: trained 2023-11-02, renewal due 2026-11-02 -> due within 90 days
Operator C: trained 2025-06-18, renewal due 2028-06-18 -> current
Operator D: trained 2023-09-30, renewal due 2026-09-30 -> OVERDUE

Step 6: records and review

The plan requires records of the inventory and assessments, detector bump tests and calibrations, datalog reviews, ventilation and fixed detector maintenance, training and fit testing, permits, drills, alarms, incidents and corrective actions. The safety committee reviews the plan every year and after any significant incident, alarm pattern or change in operations. Indicators include the number of alarms by location, datalog peaks above limits, overdue bump tests, training currency, drill results and corrective actions completed.

Reviewing the plan

A good test of a plan is to walk through a realistic scenario. Suppose an operator on a lone round at station 9 opens the wet well hatch and her monitor alarms high at 85 ppm. Under the plan, she steps back upwind, closes the hatch if it can be done from a safe position, leaves the station, reports by phone, and does not re-enter. The supervisor checks the fixed detector trend remotely, confirms the exhaust fan is running, and sends a two-person crew with SCBA to investigate. The alarm is logged, the datalog reviewed and the cause, perhaps a failed nitrate pump upstream, investigated and fixed. If any step in that story is unclear or impossible in practice, the plan needs work.