Mayor Kirk Caldwell on Monday signed into law a bill that bans nearly all plastic bags distributed at Oahu retail checkout counters starting Jan. 1, 2020.

Honolulu has had a retail plastic bag ban since July 1, 2015, but critics called foul because a provision in the law allowed for distribution of thicker bags of 2.25-­ thousandths of an inch thick. Environmental groups argued that the thicker bags are worse for sea turtles and other ocean creatures than the standard “T-shirt” bags that were banned in 2015.

Bill 59 (2016), which Caldwell signed Monday, omits that provision by creating a new definition of what’s to be banned: “plastic film bags,” which are defined as those “made out of thin, flexible sheets of plastic with a thickness of 10 mils or less,” which would include the thicker bags that are currently allowed.

The law will be implemented in two parts. The thicker bags will be allowed at checkouts through Dec. 31, 2019, under the stipulation that customers are required to be charged at least 15 cents, beginning July 1, 2018. Paper bags would also cost 15 cents, with the idea that retailers should get some compensation.

The absolute ban on plastic film bags would then take effect in 2020. “You’re going to have to come in with your own reusable bag, just like the other counties have, just as most of Europe and parts of Asia have,” Caldwell said.

The original bill introduced by City Councilman Brandon Elefante had called for a complete ban by 2018. But as late as last month, at least one draft of the bill allowed the thicker bags to be distributed indefinitely for a fee.

The bill appeared to be ready to go at the Council’s meeting in early June but was pulled at the last minute by Public Works Committee Chairwoman Carol Fukunaga, who said merchants and other business interests wanted their concerns addressed in her committee.

While environmental interests were unhappy with the decision to defer a final vote, Fukunaga worked with parties from both sides, among them Lauren Zirbel, executive director of the Hawaii Food Industry Association, and Stuart Coleman, Hawaii manager for the Surfrider Foundation.

On Monday, Environmental Services Director Lori Kahikina commended Fukunaga for bringing the parties together to hash out a compromise.

Kahikina said about 10,000 businesses will be affected by the law, which will be enforced largely through self-policing.

Zirbel said her members’ main objection to the previous version was the definitions. “The substance of the bill didn’t change at all; it was more about clarifying the definitions so that it could be properly implemented by the retail side,” she said. “The way that it was defined, it would have lumped in some things that nobody intended to ban, such as a lot of the woven reusable bags.”

The definitions now make it clear that it’s the “film” plastic bags that are being phased out, she said.

Members of the nonprofit group Beach Environmental Awareness Campaign Hawaii said that while they’re happy the bill is becoming law, they will continue to fight exemptions, including bags distributed for prepared food.

The group’s president and co-founder, Suzanne Frazer, said that during a July 5 cleanup at Ala Moana Regional Park, 63 percent of the plastic bags collected were from takeout food establishments.