It had been caught four times for polluting Singapore's sewers with toxic industrial used water. But that did not stop printing company Tat Seng Packaging Group from illegally discharging copper-tainted water yet again in September 2016.

Yesterday, national water agency PUB said the recalcitrant offender has been fined $12,000 for its most recent breach of the law.

The company pleaded guilty to three counts under the Sewerage and Drainage (Trade Effluent) Regulations and was fined in January $4,000 on each count. Another charge was taken into consideration during sentencing. The company could have been fined up to $15,000 on each count.

It is among 18 companies fined a total of $112,500 between last November and February for their pollutive acts. Each was fined between $3,500 and $13,500.

Mr Maurice Neo, PUB's director of water reclamation network, said the agency takes a serious view of such offences, as public sewers convey used water to water reclamation plants for treatment before its safe release into the environment, or for further purification into NEWater.

"Hazardous substances in the sewage endanger the lives of our staff, upset our treatment processes and negatively impact the quality of feedstock for NEWater production," he said.

At high concentrations, regulated metal substances and chemical compounds are toxic to the microorganisms which fuel the treatment process at the plants, and hence can disrupt operations at water reclamation plants.

But The Straits Times understands that measures are in place to ensure that the safety of NEWater is not compromised.

Tat Seng Packaging Group's misdeed came to light when PUB's sensors detected abnormality in the quality of incoming used water at its Kranji Water Reclamation Plant - one of four that treat used water.

Investigations found that the company had discharged black and turbid industrial used water into the public sewer.

"Water quality tests showed that the discharge from Tat Seng's premises contained quantities of copper, a regulated metal, that exceeded the allowable limit by as much as three times," PUB said.

Tat Seng Packaging Group did not reply to The Straits Times' queries by press time yesterday.

Finest Gold and Silver Refinery, another of the 18 companies which were fined, said it deployed additional staff and machines to monitor its factory's waste-water treatment process after it was found to be discharging used water with high metal content.

The electroplating company had faced three counts of illegally discharging industrial used water containing nickel and copper that exceeded the allowable limit between Dec 1 and Dec 14, 2015, and was fined $13,500.

"The company treated these cases seriously," said Finest Gold and Silver Refinery managing director Lim Seng Kuan. He added that the factory in Woodlands was eventually shut down a year ago due to poor business conditions.