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Wastewater treatment tailored to the tropics

The new wastewater treatment plant on the Thai island Koh Phi Phi utilises natural processes, is odourless - and designed like a butterfly.

Supported by Danish disaster relief, the local authorities assisted by COWI are introducing a completely new technology on Koh Phi Phi that is the result of the whole issue of wastewater management.

The tsunami on Boxing Day 2004, caused extensive damage to the island's water and wastewater systems.

Even before this latest disaster, the wastewater situation had been proving a plague to residents and tourists alike.

Old septic tanks

Punkham Kittithornkul, mayor of Koh Phi Phi, explains that the island has only old septic tanks made of concrete for processing wastewater from toilets, kitchens and bathrooms.

The wastewater treatment plant on Koh Phi Phi consists of a siphon-driven inlet tank, three types of planted subsurface flow constructed wetlands (gravel filters), a surface flow constructed wetland, a polishing pond and a storage tank for reuse purposes.

Gravel filters

The main treatment process takes place in the gravel filters. Three different sizes of crushed stone, all shipped to Koh Phi Phi from the mainland, are used in the gravel filters.

The treatment utilises natural processes: when the wastewater passes through the filter, a layer of bacteria on the aggregate devours the pollutants in the wastewater.

"Constructed wetlands and the polishing pond function well in the tropics, in part because of the high temperatures, which give optimal conditions for the microorganisms that break down the pollutants in the wastewater. The temperature on Koh Phi Phi is 28°-32°C all year round," explains COWI engineer Henrik Lynghus.

Wastewater treatment plant

Tourist consumption of water is straining reserves on the island. The mayor points out that about 2,000 people live on Koh Phi Phi, and prior to the tsunami about one million tourists visited the island each year. And they all had to share 12 km² of land and 6000 m³ of water from the reservoir.

Recycling of wastewater is the obvious solution. Project manager Carsten Laugesen considers it a waste to simply release wastewater into the sea.

The wastewater treatment plant has the capacity to treat 400 m³ of water daily, which he plans to supply to the island's hotels and restaurants.

Recycled wastewater can be supplied for uses such as watering gardens and flowers.

By Mikkel Rye Christensen  
Published: 8.5.2006

The butterfly filter

The wastewater treatment plant must not be an eyesore and must be odour-free. Therefore the plant and piping is being laid underground. The underground gravel filter will be concealed by a landscaped park complete with flowers, pavilions and paths.

The crowning glory of the treatment plant is its design, innovatively depicting a flower and a butterfly, with wastewater inflow through the petals and outflow through the butterfly's antennae.

LAST UPDATED: 17.09.2016