Water agency PUB has stepped up production at Singapore's desalination plants and local waterworks, after chicken droppings used to make fertilisers caused ammonia levels in the Johor River to spike beyond healthy levels.

The pollution lead to the temporary closure of the Singapore-run Johor River Waterworks as well as three other treatment plants in the Malaysian state, and cut off water supply to 1.8 million people there.

PUB said that water supply in Singapore will not be affected. It is monitoring the water quality in the river and will resume treatment "when the water quality is suitable".

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