Toxic gas rose to a dangerous level last month in a sewage treatment plant being built by a company whose chairperson was Hong Kong justice minister Teresa Cheng’s husband, Apple Daily has learnt.

Ventilation faults initially led to the detection of a foul odour on Aug. 24 inside a dehydration building at Yuen Long’s San Wai Sewage Treatment plant, a person familiar with the situation told Apple Daily. The concentration of hydrogen sulfide was found to reach an “alert” level of 1 part per million.

Three days later, hydrogen sulfide in the same area rose further to 11.6 ppm, which was deemed “dangerous.” The factory’s gas detection alarm system did not go off on either occasion, the source said.

Hydrogen sulfide was a toxic gas, former university chemistry lecturer Kenneth Kwong said. A concentration of 1 ppm could cause a strong smell. Exposure to 10 ppm might lead to eye sores and vomiting. A gas concentration in excess of 100 ppm was capable of leading to death in one to four hours of exposure, while inhaling over 500 ppm could kill within a short time, Kwong said.

The HK$2.57 billion (US$331.6 million) plant is under construction by a joint venture consisting of Analogue Holdings Ltd, Degremont and China Harbour. Analogue is headed by Cheng’s husband Otto Poon. Works began in 2016 and are scheduled for completion near the end of this year.

Project consultants detected a strong smell during inspections to the dehydration facility on the two days, according to the government’s Drainage Department, which was overseeing the project, in a reply to Apple Daily’s enquiry.

Investigations showed that the Aug. 24 incident took place after contractors erred when adjusting the fan and valve systems, causing traces of hydrogen sulfide to surface, the department said.

In the Aug. 27 event, factory workers detected 13 ppm of hydrogen sulfide and used a portable device to trigger an alert signal. They also turned up the ventilation and 15 minutes later, the concentration of the toxic gas went down to a safe level. The factory’s ventilation system was found to have been switched off to carry out a test on fire services, the department said.

The project consultants had instructed the contractors to improve their work procedures on ventilation and to activate a gas detection warning system in the building, the department said, adding it would include the incidents in its assessment report.

Analogue said the joint venture had reported the incidents to the Drainage Department and could not comment further, citing limitations under contract terms.

The Labour Department said that its own investigations showed the plant was testing and tuning its gas detection system, and as a result the alarm was not functioning yet. The levels of hydrogen sulfide were below 15ppm, the limit for short exposure under occupational health standards, it noted.

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