Yangyel Lhaden

The new sewage treatment plant that replaced the wastewater stabilisation pond (WSP) in Babesa, Thimphu has not been able to treat the sewage as expected and is polluting Thimchhu.

The three-month trial of the sewage treatment plant started in December last year.

Sewage is treated with sewage feeding bacteria but due to the pandemic bacteria seedlings, which accelerate the growth of bacteria, could not be imported.

Thimphu thromde’s project manager with the Asian Development Bank, Kinlay Penjore, said that it might be due to low temperature as bacteria needed warmth to grow. “We don’t have the technical expertise for scientific examination but we’re hoping when days get warmer the growth of bacteria will accelerate.”

Currently, the sewage plant of capacity 12 million litres per day (MLD) sequencing batch reactor (SBR) depends on the natural biological process for the growth of bacteria.  Since the sewage cannot be treated completely it has failed to meet the effluent parameters of sewage treatment plant discharge standards of National Environment Commission (NEC).

This determines what quality of the water that can flow into the river.  The sewage plant has only met NEC standard of faecal coliform and pH out of the five parameters.

The treatment plant has not been able to lower the total suspended solid (TSS) or particles and debris floating on the surface of the sewage to the permissible threshold.

Kinlay Penjore said that SBR sewage plant globally did not use a filter but they were looking into placing rudimentary water filter. “It may be due to dietary habits of Bhutanese as TSS also consist of red chilli flakes.”

If the plant does not meet effluent parameters at the end of the trial period, the contractor will have to pay a penalty by fixing it, Kinlay Penjore said, “With the pandemic situation improving we’re looking into calling in an expert and importing bacteria seedlings.”

The sewage treatment plant in Langjophaka faced a similar problem and it was solved after sewage was fed with bacteria seedlings, Kinlay Penjore said.

“The sewage plant is mainly to reduce water pollution. If the plant fails to meet the effluent parameters it’s no better than directly dumping the sewage in the river,” Kinlay Penjore said.

Contract terms state that, after the completion if effluent parameters are not met within three months, the plant would not be fully commissioned.  The contractor will also be liable to re-work to meet the effluent parameters.  For the next five years, the contractor has to undertake the plant’s operation and maintenance.

Since December last year, sewer to water stabilisation pond was discontinued and diverted to the new SBR system.  Currently, only 6MLD of the total capacity is used every day. 

SBR functions with an extended aeration system, which pumps oxygen into sewage to help sewage-consuming bacteria to grow.  The plant also filters waste, extracts odour, and the final waste is treated with chlorine before it flows into the river.

The old treatment plant took about 45 days to complete the process of treatment whereas the new treatment plant takes about five hours. “The 1.75MLD WSP was built in the 1990s and had to treat more than four times its capacity. Even if Semtokha and Babesa are connected, which aren’t today, they would fill 8 MLD,” Kinlay Penjore said.

He said that the new treatment plant required a smaller area and fit in 200sqm.

The sewage plant will be inaugurated on February 21, coinciding with the birth anniversary of His Majesty The King.