Exceeding water quality standards for tap water

Regarding the water quality standards at the Takatsuka Reservoir that occurred on Thursday, January 27, the cause of the accident was verified in Water Works Bureau.
Based on these results, measures were taken to prevent recurrence.

At 19:00 on Thursday, January 27, 2022, an accident occurred to supply tap water containing "chloric acid" at the Takatsuka Reservoir (Izumi Ward), which is being supplied from Kosuzume Purifiction Plant at 19:00 on Thursday, January 27, 2022. (Chloric acid concentration is 0.93 mg/L)
The affected water supply areas were Totsuka Ward, Seya Ward and Izumi Ward, with approximately 36,000 water units.

In Kosuzume Purifiction Plant, rainwater (approximately 40 m3) accumulated in the work passage was pumped up in the work passage to repair pipes that transport sodium hypochlorite, a disinfectant, from the chemical tank to the injection site, and then released it to the river via the wastewater treatment facility.
When pumping rainwater, water quality such as residual chlorine concentration, odor, and pH were inspected, and no abnormalities were found. However, during the release, it was confirmed that sodium hypochlorite leaked from the pipe was mixed in the pumped rainwater, sodium hypochlorite that had been leaked from the pipe, so we stopped pumping rainwater and discharge it into rivers.
Due to the suspension of river discharge, water was almost overflowing from the wastewater treatment facility, so we decided to urgently return the water from the wastewater treatment facility to raw water and process it while monitoring the water quality during the water purification process. The returned water contains chloric acid.

（１） Causes

①Fundamental causes: The stagnant rainwater was put into a wastewater treatment facility in the treatment process of the water purification plant

②Secondary Cause: Returning water from a wastewater treatment facility that was originally not performed, to raw water

（２） Measures to Prevent Recurrence

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

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

The above verification results and measures to prevent recurrence were reported to the Standing Committee of City Counsil on Wednesday, February 16.
In the future, we will do our utmost to prevent similar accidents from occurring again.
We apologize to the citizens again for their concerns and inconvenience.