Construction is scheduled to begin in October 2014, with completion set for April 2017.
The work will be undertaken with loan assistance from the Japanese government.
Hitachi and Veolia will be responsible for the design, delivery, and test operation of mechanical and electrical facilities, and ArabCo will be in charge of civil engineering, construction and on-site installation work.
This new pretreatment contract follows on from an EPC order that the three companies received earlier this year on the water desalination plant.
The Basrah desalination plant will be the largest facility of its kind in Iraq, capable of supplying 199 000 cubic metres of drinking water per day.