A company statement said that as of 6.30pm today, the threshold odour number at the Sungai Semenyih water treatment plant had fallen from 9 at 12.30pm to 7 at 3pm and 2 at 6.30pm.

The plant can only operate when the readings fall to 0 over three consecutive tests.

However, readings at the Bukit Tampoi plant have remained consistently high since noon today. The 6.30pm and 3pm reading both came in at 9, with the 12.30pm reading 8.

Both plants suspended operations yesterday evening after pollution was detected in Sungai Semenyih, the raw water source for both these plants.

The plants’ suspension has left Air Selangor unable to produce the daily 602 million litres of water required to supply more than 300,000 residents in the Petaling, Hulu Langat, Kuala Langat and Sepang districts.

The disruption is the second in the span of a month for Selangor residents after pollution detected in Sungai Gong, Rawang, caused unscheduled water cuts for several days in early September.