MADRID - Abu Dhabi sovereign fund ADQ has joined buyout groups Blackstone and EQT for the second round of bids in the sale process of Spanish waste management company Urbaser, Spain's Expansion newspaper said on Tuesday.

Urbaser's owner, private equity firm Platinum, hired Citi and Santander as advisors for the sale earlier this year in a deal that could value the waste management firm at around 5 billion euros ($5.26 billion), sources told Reuters.

Expansion said a fourth candidate could join Blackstone, EQT and AQD in the second round.

The private equity firm has told bidders to start due diligence and prepare binding offers for the last week of February, a source told Reuters.

AQD did not immediately reply to a request for comment.

($1 = 0.9498 euros)

(Reporting by Emma Pinedo and Andrés González; Editing by Susan Fenton)