SINGAPORE - Forty years after Singapore embarked on a cleanup of the Singapore River, the Prime Minister says that keeping the river clean is a relentless effort and people should help by refraining from littering.

"The river cleanup is never done," said Mr Lee Hsien Loong on Sunday (Nov 26). "Singaporeans still must learn not to drop litter and trash on the ground, which washes into drains and the river, and has to be caught with booms along the way."

He was speaking at a 60th anniversary event for Malay daily Berita Harian. The newspaper's celebration includes a five-day public exhibition by the Singapore River, honouring pioneers' efforts in the cleanup, which started 40 years ago.

PUB says that its contractors retrieve an estimated 200 kg to 400kg of litter a day from the 3.2km-long river, just between Kim Seng Road Bridge and the Esplanade Bridge. They use vessels such as flotsam removal crafts.

"The amount of litter has not significantly changed from five years ago," the national water agency told The Straits Times, in response to queries.

Besides leaves and twigs, litter collected includes plastic bags and plastic bottles.

It added that litter in the vicinity of the Singapore River - which includes popular spots such as Clarke Quay, Boat Quay and Robertson Quay - can be washed into the drains by rain and find its way into the river.

The National Environment Agency, which takes care of the rest of the river upstream of Kim Seng Road Bridge to Alexandra Canal, was unable to respond on the amount of litter it retrieves.

In his speech, PM Lee recalled the state of the Singapore River prior to 1977: "The water was black, notoriously smelly and toxic". A blind telephone operator who worked at his mother's law firm always knew when to get off the bus because he could smell the river when the bus crossed it.

PM Lee added that it took much coordination, planning and a "determination and political will" to remove the pollution.

Over 10 years, Singapore dredged up the polluted riverbed and removed pig farms and other pollutive industries, he said.

There was also a need to sewer up all premises in the catchment, relocate squatters to HDB flats, street hawkers to new hawker centres, and shipyards to Pasir Panjang, he added.

Back in the day, "the water was black, notoriously smelly and toxic", he said, adding that a blind telephone operator who worked at his mother's law firm always knew when to get off the bus because he could smell the river when the bus crossed it.

Dr Albert Winsemius, the chief economic adviser back then, even had a bet with the river cleanup team, believing that the river would not sustain life, recalled PM Lee.

"He recommended that we turn the Singapore River into a sewage system by covering it up," he said.

But when Dr Winsemius visited Singapore after the cleanup was completed, he was amazed, but perhaps not unhappy, that he had lost his bet, said PM Lee.

"We want to keep on beautifying the river and transform ourselves into a City of Gardens and Water with our ABC (Active, Beautiful and Clean) Waters programme," he added.

Chief executive of water solutions company Aquayana, Mr Hassan Ahmad, said: "I think our generation has started to be complacent."

He was referring to how younger Singaporeans waste water in everyday activities such as washing cars.

A humanitarian relief worker of 17 years who has conducted activities in China, India and North Korea, Mr Hassan added that clean water is key to survival and development.

On how PM Lee said there is still a need for Singaporeans to learn not to litter, Mr Hassan added that there could be more platforms to show the impact of such litter on the water supply. He added that as "habits die hard", there also has to be education at all levels.

"As (former national water agency PUB chairman) Tan Gee Paw said, you don't need a PhD to solve the issue, it's more of will power, to rally the people to solve it together," Mr Hassan said. Mr Tan was involved in carrying out the river cleanup project, and is now special adviser to the Centre for Liveable Cities.

Berita Harian editor Saat Abdul Rahman, who also spoke at the event on Sunday, said: "Besides commemorating our anniversary, we are also celebrating 40 years since the call by founding prime minister Lee Kuan Yew to clean up the then ailing Singapore River."

He said: "Flowing through the heart of Singapore, the Singapore River and what it represents is an integral part of the lives of Singaporeans." He added: "Similarly, Berita Harian is also the heartbeat of the inclusive Malay community and has played a significant role in the community's progress over the years."

Minister for the Environment and Water Resources Masagos Zulkifli, and Senior Minister of State for Defence and Foreign Affairs, Dr Maliki Osman, were also present at the event.