The company had already pushed the deadline back once from 21 October to the end of December.
The engineering, procurement and construction contract will add 240,000 cubic metres a day of capacity to the plant at Al-Khumrah.
The plant uses reverse osmosis technology.
Pre-qualified companies include Spain’s Cadagua, France’s Veolia and South Korea’s Doosan Heavy Industries & Construction.
The desalination plant upgrade will be followed by projects to improve the city’s wastewater plants at Bani Malik, Al-Escan, Al-Jamaa and two other plants, called A and C, located in the south central area of the city.