The Pollution Control Department will sue Rajburi Ethanol Co for allowing molasses wastewater to leak into the Mae Klong River, killing many giant stingrays and other aquatic life.
Director-general Wijarn Simachaya said on Friday testing had shown that molasses wastewater leaked from the plant into the river in Ban Pong district of Ratchaburi, polluting it and killing aquatic life, including many native giant stingrays, in Ratchaburi and Samut Songkhram provinces from Oct 1 to 7.
He said Rajburi Ethanol earlier admitted there was a leak from its last pond of treated molasses wastewater at 8.50am on Sept 30. Many agencies had subsequently tested the water in the river.
Tests found that dissolved oxygen in the lower part of the river amounted to 1.0-2.8 milligrammes per litre from Oct 4 to 10. This was below the level measured there in the same period last year.
Biochemical oxygen demand (BOD) in Samut Songkhram from Bang Khonthi district down to the estuary ranged between 11 and 28 mg per litre on Oct 7.
Molasses wastewater leaked from Rajburi Ethanol on Sept 30 continued to accumulate in the Mae Klong River at least until Sept 7. It took 6-8 hours for the water mass in the river to flow from Ban Pong district, Ratchaburi, to Bang Khonthi district, Samut Songkhram, and a high tide during the period blocked the water mass from reaching the sea.
Consequently, some of the molasses wastewater settled in the riverbed and high levels of free ammonia killed aquatic animals, including stingrays, which were unable to discharge ammonia from their bodies during the time of low dissolved oxygen, Mr Wijarn said.
Tests to simulate the condition of the river during the pollution period showed that the level of free ammonia soared 18 times and caused the sudden death of fish, he said.
The Pollution Control Department would take legal action against the plant, he said.
A total of 45 giant stingrays were found dead in the area in early October.