Two Dead After Toxic Gas Exposure in Confined Space at Singapore Waterworks

On 23 May 2024, three contractor workers collapsed inside a water treatment process tank at a waterworks facility in Singapore after inhaling hydrogen sulphide — a colourless, highly toxic gas with a distinctive rotten-egg smell. One worker died that day. Five days later, a second worker succumbed to his injuries. The third survived after intensive care treatment and was discharged in late June 2024.

This incident is a devastating reminder that confined spaces are among the most dangerous environments in any workplace. They demand rigorous permit-to-work systems, continuous atmospheric monitoring, and trained rescue teams on standby — none of which can be improvised on the day.

The Incident

At approximately 11:15 AM on 23 May 2024, the three workers were found unconscious inside a water treatment process tank at the Choa Chu Kang Waterworks. They had been performing routine cleaning of the tank when they collapsed.

Initial investigations indicated that the workers had inhaled hydrogen sulphide (H2S), a gas that is typically emitted when sludge is drained from water treatment tanks. On the day of the incident, hydrogen sulphide levels at the time and location were higher than normal, as measured by the H2S sensors installed at the facility.

All three workers were rushed to hospital. One worker was pronounced dead. The other two were admitted to the intensive care unit (ICU). Tragically, on 28 May 2024, a second worker also died from his injuries, bringing the death toll to two. The third worker was discharged from hospital on 22 June 2024.

PUB — Singapore’s national water agency — immediately issued a board-wide safety time-out and carried out comprehensive checks on its processes and safety procedures, both at Choa Chu Kang Waterworks and across all PUB facilities.

The Investigation

The Ministry of Manpower launched a thorough investigation into the incident. A stop-work order was issued for the Choa Chu Kang Waterworks, and work at the facility remained halted pending the completion of the investigation.

In a written reply to parliamentary questions, the Ministry of Sustainability and the Environment confirmed that PUB implemented a safety timeout for all similar operations in confined spaces and commenced a comprehensive review of safety measures associated with such work.

The Workplace Safety and Health Council subsequently issued a specific advisory on confined space workplace fatal injuries in the second half of 2024, highlighting the ongoing risks of such work environments and recommending strengthened safety measures for confined space operations across all industries.

What Went Wrong

Confined space incidents in Singapore almost always trace back to the same critical failures. While the full investigation report for this specific incident has not been publicly released, the following systemic issues are common in such tragedies:

1. Inadequate atmospheric monitoring before and during entry. Hydrogen sulphide is produced when organic matter — like the sludge in water treatment tanks — decomposes. H2S levels can spike unpredictably, especially during draining or agitation. Continuous real-time monitoring with personal gas detectors is essential, not just a pre-entry check.
2. Failure to implement a proper Permit-to-Work (PTW) system. A confined space entry permit must document the hazard assessment, atmospheric test results, ventilation requirements, emergency rescue plan, and the names and qualifications of every person entering the space. If any condition changes, the permit must be re-evaluated.
3. No rescue plan or trained rescue team on standby. When a worker collapses in a confined space, the natural instinct is for colleagues to rush in to help — which often leads to multiple casualties. A trained rescue team with self-contained breathing apparatus (SCBA) must be on standby outside the confined space at all times during entry operations.
4. Insufficient ventilation of the confined space. Mechanical ventilation must be used to continuously supply fresh air and dilute or remove toxic gases. Relying on natural ventilation in a tank environment is never sufficient.
5. Workers not equipped with appropriate respiratory protection. When working in environments where toxic gases may be present, workers must have access to and be trained in the use of supplied-air respirators or SCBA — not just dust masks or basic respirators.

The Cost

The cost of this incident extends far beyond any financial penalty:

• Two lives lost. Two workers — with families and futures ahead of them — were killed doing routine maintenance work.
• All tank cleaning operations halted nationwide. PUB suspended tank cleaning across all its facilities pending a full safety review, disrupting water treatment operations across Singapore.
• Parliamentary scrutiny. The incident was raised in Parliament, with questions directed at the responsible Minister — a level of public accountability that no organisation wants.
• Fines under the Workplace Safety and Health Act for breaches resulting in death now carry a maximum of SGD $50,000 per charge for a first offence (increased from $20,000 as of 1 June 2024). Employers face fines of up to SGD $500,000.
• Work Injury Compensation Act (WICA) claims for workplace deaths can amount to significant sums, depending on the worker’s age and earnings.
• Reputational damage to both the contractor and the facility operator.

Lessons Learned: Preventing Confined Space Fatalities

The following recommendations are general industry best practices for educational purposes only. Every workplace is different — consult a qualified safety professional before implementing changes specific to your situation.

Confined space deaths are preventable. Every single one. Here is what every organisation that operates or maintains confined spaces must do:

1. Identify and register every confined space on your premises. Tanks, vats, silos, manholes, trenches, tunnels, and enclosed drains are all confined spaces. If a space has limited entry/exit, is not designed for continuous occupancy, and has the potential for a hazardous atmosphere — it is a confined space.
2. Implement a robust Permit-to-Work system. No one enters a confined space without a valid permit. The permit must be signed by an authorised person who has verified atmospheric conditions, ventilation, rescue arrangements, and emergency procedures.
3. Conduct continuous atmospheric monitoring. Use calibrated 4-gas detectors (O2, LEL, CO, H2S) worn by every entrant. Monitor before entry, during entry, and continuously throughout the work. If any reading deviates from safe limits, evacuate immediately.
4. Station a trained rescue team outside the space. The rescue team must have SCBA, retrieval equipment (tripod and winch), and first aid capability. They must never enter the space without their own respiratory protection.
5. Train every worker who may encounter a confined space. Training must cover hazard recognition, gas detection equipment, emergency procedures, and the absolute rule: never enter a confined space to rescue a colleague without proper equipment and training.
6. Ventilate continuously. Mechanical ventilation must run throughout the work. Monitor ventilation effectiveness with atmospheric testing.

Recommended Best Practices

Industry best practices for confined space safety go beyond minimum regulatory compliance. Organisations committed to protecting their workers should consider the following:

• Confined space risk assessment — Conduct thorough risk assessments specifically for confined space operations, identifying atmospheric hazards, physical hazards, and engulfment risks. Assessments should comply with the Workplace Safety and Health (Confined Spaces) Regulations and produce actionable recommendations.
• Permit-to-Work system design and implementation — Design, implement, and regularly audit Permit-to-Work systems that are practical, effective, and fully compliant with the Ministry of Manpower’s expectations. PTW systems should cover confined space entry, hot work, height work, and other high-risk activities.
• Qualified safety personnel — Ensure a qualified Workplace Safety and Health Officer is available for confined space operations, either on a full-time or part-time basis.
• Safety audits — Conduct regular audits of your confined space management programme against regulatory requirements and industry best practices, identifying gaps before they lead to incidents.

For ongoing awareness and knowledge building, Sage Shield Academy offers online courses covering confined space awareness, risk assessment methodology, and workplace safety fundamentals. Note: Sage Shield Academy courses are for awareness and knowledge purposes only and do not constitute WSQ or ATO-issued certification.

If your organisation operates or maintains confined spaces and needs help with risk assessments, permit-to-work systems, or safety audits, Sage Shield Safety Consultants is one provider that offers specialised confined space safety services across Singapore.

Frequently Asked Questions

Sources

1. Ministry of Sustainability and the Environment — Written Reply to Parliamentary Questions on Choa Chu Kang Waterworks Incident (2024)
2. Workplace Safety and Health Council — “Confined Space Workplace Fatal Injuries in 2H2024” (March 2025)
3. Ministry of Manpower — Workplace Safety and Health National Statistics 2024
4. Ministry of Manpower — WSH Act: Liabilities and Penalties
5. Singapore Statutes Online — Workplace Safety and Health (Amendment of Penalties) Regulations 2024

Disclaimer: This article is for informational purposes only and does not constitute professional or legal advice. Every workplace is different — consult a qualified safety professional for advice specific to your situation.