MADRID, Dec 10 (Reuters) - 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)