The 23-billion-baht project, about 95% complete, was stopped in its tracks two years ago after fierce local opposition and reports of irregularities in land acquisition and construction contracts. Twenty people, including three politicians and two senior officials, face charges filed by the National Counter-Corruption Commission.

Last year, the PCD hired two consultant firms, Black and Veatch Ltd and Progress Technology Consultants, to study alternatives to the suspended project.

The firms yesterday presented a study concluding the almost-complete facility was poorly built and would likely damage the environment when fully operational.

The study offered a series of options for dealing with the project, including finishing it as designed, modifying the existing system to reduce environmental damage, transforming the plant into a coastal research station, relocating the project from tambon Klong Dan to Bang Pu, or abandoning the project.

“Engineers, pollution control experts, independent academics, and local representatives have agreed that [modifying the system] is the most reasonable solution and we hope the policy-makers base their decisions on the study,” said Sangsant Panich, environmental expert of Progress Technology Consultants Co.

Under the recommended option, the government would pay an additional 5.6 billion baht to complete the unfinished part of the system, extend the water discharge pipeline from 3km to 10km offshore, upgrade the water treatment technology and install a recycled-water distribution system, he said.

The government has already invested more than 20 billion baht in the plant’s construction.

The study’s second option is relocating the plant, which would cost about 7.1 billion baht, while altering the project into a coastal research station, costing an estimated 1.6 billion baht, ranks third.

Modifying the system “will cause minimal impact on mussel farming because wastewater will be discharged 10km from shore and there will be zero discharge of wastewater into the sea during the dry season,” said Mr Sangsant.

He said the PCD could earn additional revenue by selling treated water for industrial and agricultural uses. Meanwhile, about 30,000 cubic metres of treated water would be drained into underground water sources for consumption in Bangkok.

“However, the department has to work hard to create consumer acceptance of using treated wastewater,” he said.

Klong Dan villagers, who have opposed the project for more than seven years, are threatening to renew protests.

Dawan Chantarahassadi, a protest leader, said the study was unacceptable  because it did not include alternatives proposed by the affected villagers. “The government will repeat the mistake if it agrees with the department’s proposal to modify the plant,” said Ms Dawan.

Local villagers want the project scrapped and the site turned into an aquatic animal research centre.

They want the government to build small plants in heavy-polluted areas rather than one massive plant that cannot serve local communities.