State deputy police chief SAC Che Zakaria Othman said the police were expecting to make the arrests soon.
“We are already on the trail of those behind this illegal act and we hope to arrest and charge them in Seremban.
“My officers are quite positive about the case,” he said at the site where the waste was dumped.
However, he did not say how many suspects were expected to be picked up or where they were from.
The dumping of the waste, believed to be liquid and semi-solid organic compounds, disrupted operations at the Sungai Semenyih and Bukit Tampoi water treatment plants on Sunday.
This led to a water cut to 309,867 account holders of Air Selangor in 274 areas in the Petaling, Hulu Langat, Kuala Langat and Sepang districts.
The supply is expected to resume in stages from today until Friday.
SAC Che Zakaria said based on preliminary findings, the waste dumped along the banks of Sg Batang Benar near here had been brought in from another state.
He declined to elaborate.
“We have opened an investigation paper and are working with our counterparts from elsewhere including from Bukit Aman.
“We will be able to reveal more once we detain the culprits,” he said, adding that the case was being investigated under Section 430 of the Penal Code for “mischief to works of irrigation or by wrongfully diverting water”.
Those found guilty can be jailed between five and 30 years or fined, or both.
State environment, health, cooperatives and consumerism committee chairman S. Veerapan was also present to monitor clean-up works being carried out in the area.
He said the authorities were waiting for results from the Chemistry Department to verify the type of waste dumped.
“We have yet to establish if it was scheduled or domestic waste and if it was the main cause of the contamination of Sg Semenyih.
“We are expecting the results from the Chemistry Department in the next few days,” he said.
Veerapan said the waste was dumped in an area measuring 15m by 20m.
He appealed to the public to notify the authorities if they came across any dumping of waste-related activities.
“It is difficult to catch the culprits redhanded, as they will pick secluded areas.
“As such, we will also look into putting up CCTVs in certain areas,” he said adding that the incident was the fifth illegal dumping case reported in the state since December last year.