Excess of water quality standards in tap water

The cause of the accident was verified in Water Works Bureau for exceeding water quality standards at Takatsuka Reservoir on Thursday, January 27.
Based on these results, we have taken measures to prevent recurrence.

At 19:00 on Thursday, January 27, 2022, an accident occurred at the Takatsuka Reservoir (Izumi Ward) sent from Kosuzume Purifiction Plant to supply tap water containing "chloric acid" with a concentration exceeding the national standard of 0.6 mg/L. (The concentration of chloric acid is up to 0.93 mg/L)
The water supply areas affected by the impact were part of Totsuka Ward, Seya Ward and Izumi Ward, with approximately 36,000 water supply units.

In Kosuzume Purifiction Plant, rainwater (approximately 40 m3) accumulated in the work passage was pumped and discharged into rivers via wastewater treatment facilities in the site to repair pipes that transport sodium hypochlorite, a disinfectant, from the chemical tank to the injection point.
When pumping rainwater, water quality such as residual chlorine concentration, odor, and pH were inspected, and no abnormalities were observed. However, during the release, it was confirmed that sodium hypochlorite, which had leaked from the piping, was mixed in the collected rainwater, and the pumping of rainwater and the discharge to rivers were stopped.
Due to the suspension of river discharge, water was about to overflow from the wastewater treatment facility, so it was decided to urgently return the water from the wastewater treatment facility to raw water and treat it while monitoring the water quality during the water purification process. . This returned water contains chloric acid.

（１） Cause

①Fundamental cause: Accumulated rainwater was put into a wastewater treatment facility in the treatment process of a water purification plant.

②Secondary causes: Returning the water from the wastewater treatment facility, which is not originally performed, to the raw water

（２） Measures to prevent recurrence

Based on the lessons learned from this accident, it was stipulated in the operation manual of the water purification plant that water outside the route leading to the wastewater treatment facility should not be filled.

In addition, thorough guidance was given to water purification plant staff to (2) not return water from wastewater treatment facilities to raw water, and (3) dispose of rainwater collected in the facilities separately. In addition, repair work will be carried out so that rainwater does not accumulate in the work passage, and regular patrols will be carried out.

The results of the above verification and measures to prevent recurrence were reported at the Standing Committee of City Counsil on Wednesday, February 16.
In the future, we will do our utmost to prevent recurrence so that similar accidents do not occur again.
We apologize to the citizens for their concerns and inconveniences.