ABU DHABI- Abu Dhabi has invited international firms to express interest in building one of the world's biggest water desalination plants as the capital of the United Arab Emirates boosts capacity to meet rising demand, a senior official said on Wednesday.

The project, estimated to cost between $600 million and $1.2 billion, will have capacity of 200 million gallons per day, Adil al-Saeedi, acting director of privatisation at the Abu Dhabi Water & Electricity Authority (ADWEA), told Reuters.

Companies are likely to be prequalified by the second quarter of this year with a developer for the project selected by the third quarter, he said.

The developer will own up to 40 percent of a special purpose vehicle that will sign a long-term agreement to sell water to ADWEA, which will directly or indirectly own the rest of the equity in the project.

Abu Dhabi's current water production capacity is around 960 million gallons per day from 10 desalination plants.

(Reporting by Stanley Carvalho; Editing by Andrew Torchia/Mark Heinrich) ((stanley.carvalho@thomsonreuters.com; + 9712 6444431; Reuters Messaging: stanley.carvalho.thomsonreuters.com@reuters.net))