Sea-Tac officials say the airport handled 23,200 passengers per acre in 2013. That’s more than other old urban airports, such as in Boston, which handled 18,880 per acre, and Newark, N.J., which handled 20,590.

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Traffic at Seattle-Tacoma International Airport is growing faster than expected, and officials say expansion is a top priority.

In a memo to Port of Seattle commissioners, airport director Mark Reis noted that last year Sea-Tac was the fastest-growing large hub airport, The News Tribune reported. It handled nearly 37.5 million passengers last year, a jump of 7.7 percent.

Two near-term expansions are about to begin: a new International Arrivals Facility at the airport’s south end and a remodeled and expanded North Satellite Terminal.

But officials say that won’t be enough to accommodate the 66 million passengers projected to use Sea-Tac annually two decades from now. The Port commission is expected to decide this year among options that include an expansion of the existing North Satellite; a new terminal north of the existing North Satellite, at what is now an airport parking lot; a second South Satellite terminal; a long, dogleg extension of the existing A Concourse; or an expansion of the main terminal to the north and east.

“Due in large part to a strong regional economy and projected slight decline in airfares over the long-term, airport activity is projected to reach 66 million annual passengers and 540,000 annual aircraft operations by 2034,” Reis wrote. “Strong near-term growth and significant increases in peak hour aircraft operations, passenger volumes and vehicles on the roadways presents formidable facilities planning challenges.”

According to Sea-Tac officials, the airport handled 23,200 passengers per acre in 2013. That’s more than other old urban airports, such as in Boston, which handled 18,880 per acre, and Newark, N.J., which handled 20,590.

The planned International Arrivals Facility, to open in 2019, will increase the processing capacity for international passengers by 60 percent and increase international gates from 11 to 18.

The North Satellite project will expand the area from 12 gates to 20, more than double the space for vendors and restaurants, create a rooftop lounge for Alaska Airlines’ elite passengers, and bring the whole facility up to date. The terminal will become the center of Alaska Airlines’ passenger activity at Sea-Tac. The expansion is set for completion in summer 2020.

Beyond that, the airport says that over the next two decades it will need 35 additional gates for domestic travel; nine for international, wide-bodied airliners; and new taxiways and runway bypasses, to increase Sea-Tac’s capacity from the current 88 landings and takeoffs per hour to 120 per hour.