It said it was not due to sewage treatment operations.
In a statement, IWK said the problem was identified during a joint inspection carried out by Lembaga Urus Air Selangor (LUAS) and the National Water Service Commission (SPAN) at the Bandar Putera Mahkota sewage water treatment plant following a report in the morning.
Following the odour, it said, the team had to temporarily stop the effluent discharge to enable cleanup operations.
“At 7pm, there was no (more) odour. SPAN, LUAS and IWK personnel conducted an inspection at the sewers around the area and found out that there was illegal waste disposal with solvent odour.
“This incident is similar to the incident that occurred last September.”
IWK said it could not treat the illegal waste as it was only able to treat sewage, It said it lodged a report at a nearby police station.
The statement regretted that various parties were pointing fingers at IWK over the incident “when it was actually caused by those who were irresponsible to dispose of the illegal waste there”.
It said the operations and maintenance of all 7,000 plants under IWK were being carried out in accordance with the specifications and procedures laid out by the authorities.
The Sungai Semenyih water treatment plant was forced to halt its operations yesterday following the incident, which affected more than 320,000 consumers in 348 areas in Petaling, Hulu Langat, Kuala Langat and Sepang.
Water treatment plant still closed
In a statement, Pengurusan Air Selangor Sdn Bhd (Air Selangor) said the water treatment plant will remain closed as it was still emitting an unpleasant odour with a threshold odour number (TON) of three as of 6.30am today.
It said the TON reading needs to reach the zero level to reopen.
IWK and LUAS are actively carrying out cleaning work, it added.
In another development, the Bukit Tampoi water treatment plant, which also receives raw water from Sungai Semenyih, was also forced to cease its operations from 3.30pm yesterday, affecting 336,930 accounts in 366 areas.
Among the affected areas in Kuala Langat district are Kampung Bukit Changgang, Kampung Labohan Dagang, Kampung Olak Lempit, Kampung Orang Asli Bukit Bangkong, Bukit Tampoi, Kampung Selangor Dredging, Kampung Sungai Manggis, Kilang ISF Dengkil, Kompleks Perabot Olak Lempit, Pusat Serenti, RRM Bukit Changgang, Telor Datok and Taman Ambar, the statement said.
Other affected areas include Taman Arked, Taman Changgang Jaya, Taman Seri Dagang, Taman Seri Manggis, Taman Seri Telok Datok and Taman Topaz.
“Emergency water supply continues to be provided to the affected areas, with priority given to consumers and critical and emergency premises, including hospitals. Eleven local service centres are still operating around the clock.
Air Selangor said it expected the recovery to take some time.