Water Supplies Department releases investigation report on incident of unpleasant odour in drinking water in Kwai Chung and Tsuen Wan
The Director of Water Supplies set up an independent Investigation Team headed by the Assistant Director of Water Supplies (Mechanical and Electrical) to look into the cause of the incident and recommend improvement measures.
The investigation revealed that the incident was caused by maintenance works carried out in the eastern compartment of the Tsuen Wan Fresh Water Service Reservoir (TWFWSR). The TWFWSR is a service reservoir with two compartments (eastern and western compartments) separated by a concrete division wall. When maintenance works are carried out in one compartment, the other compartment can continue to supply water. There is a gap of about 2.5 metres high between the top of the division wall and the ceiling of the service reservoir, and the gap is sealed off with plastic curtains for air insulation. However, there are two access manholes with cat ladders on the top of the division wall to allow access to the service reservoir. The gap on top of the division wall at the locations of the access manholes and cat ladders has not been completely sealed off with plastic curtains for air insulation.
During the maintenance works in the eastern compartment of the TWFWSR carried out by a maintenance contractor of the WSD last November, a protective coating material that was approved for use in potable water installation works was applied to the floor slab of the compartment. The coating material releases volatile organic compounds (VOCs) during curing. The investigation revealed that the contractor had not adopted appropriate measures to seal off the two openings without plastic curtains for air insulation at the locations of the access manholes and cat ladders. In addition, the ventilation arrangement was not able to totally extract the VOC fumes from the service reservoir. The VOC fumes passing through the two openings got into the western compartment, which was in operation, and dissolved in the drinking water that was stored in it and then distributed to customers. The Investigation Team also considered that the staff of the WSD lacked awareness of the possible impact of the maintenance works on drinking water quality.
The WSD collected a water sample from the outlet pipe of the western compartment of the TWFWSR after the incident for a rapid toxicity test. The test result was negative, i.e. non-toxic. The water sample was also tested for the 15 health-related VOC parameters in the World Health Organization (WHO) Guidelines for Drinking-water Quality and all results complied with the requirements of the WHO guidelines. As VOCs have a low odour detection threshold, a very low VOC concentration in the drinking water would still be perceivable by customers despite the fact that the VOC concentration in the drinking water was compliant with the requirements of the WHO guidelines. However, this should not cause any health hazard.
To prevent recurrence of similar incidents, the Investigation Team has recommended the following short- and long-term improvement measures, and the short-term measures have already been implemented by the WSD:
(i) For maintenance works carried out in one compartment of a service reservoir with another compartment in operation for supplying water, all openings between the ceiling of the service reservoir and the division wall should be completely sealed off;
(ii) The use of materials with high VOC contents (including those in the raw materials) for maintenance works inside service reservoirs under condition (i) above should be stopped immediately;
(iii) Contractors should be required to conduct a Water Quality Impact Assessment (WQIA) by relevant professionals before carrying out construction or maintenance works that may affect drinking water quality. The WQIA should be duly vetted and approved by senior professionals of the WSD;
(iv) Regular training for front-line staff to enhance their awareness of the possible risk on drinking water quality posed by construction and maintenance works should be strengthened;
(v) The arrangements for handling water quality incidents, which include the collection of water samples for testing and the release of information to the affected customers and the media, should be reviewed and further enhanced; and
(vi) Guidelines should be provided to property management companies to assist them to take prompt and appropriate contingency measures in response to water quality incidents.
The unsatisfactory performance of the maintenance contractor concerned has been reflected in its performance appraisal by the WSD.
The investigation report is available on the WSD's website at www.wsd.gov.hk/filemanager/en/share/pdf/kwai_chung_and_tsuen_wan_incident_report_e.pdf.
Ends/Wednesday, January 31, 2018
Issued at HKT 16:50
Issued at HKT 16:50