Using a green compostable bag to line your compost pail is incredibly convenient.
Oakland’s composting service doesn’t accept the bags; Alameda County calls them “contaminants.”
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In San Francisco, Recology accepts the bags, but there’s a catch: They generally get picked out, by machine or by hand.
The reason: They gum up sorting machines, and are also hard to distinguish from regular plastic bags.
Green compost bags hang from an excavator bucket during composting intake at Recology Blossom Valley Organics in Vernalis (San Joaquin County) on Aug. 31. The bags can gum up sorting screens.
Noah Berger/For the S.F. Chronicle
Here’s what happens in San Francisco:
Trucks from Recology — which handles the city’s bins — haul compost to the San Francisco Transfer Station & Hazardous Waste Facility. There, the contents are fed into a large cylinder with four-inch holes that filters out big materials — the first of a series of screenings. Some compostable bags plug up the screens, and others get screened out.
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Further screening occurs at Recology’s facility in San Joaquin County, where the compost gets left in heaps to biodegrade. Here, workers also pick out anything that looks like plastic, both manually and with screens.
That includes composting bags, unless they’ve broken open and are in small enough pieces to make it through the screens.
They’re a “challenging material” to work with, Recology spokesperson Robert Reed said by email.
Excavators move compost at Recology Blossom Valley Organics.
Noah Berger/For the S.F. Chronicle
The ‘messy middle’
But people like the bags. They are stronger and less leaky than other options like paper liners. Nonetheless, in addition to the physical challenges in the screening process, there are questions over whether the green compostable bags — generally known as bioplastics — add real value to the compost, which Recology sends primarily to local farms, or leave microplastic residue.
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Research appears to be scant on the impacts of bioplastics on compost, soil quality and plant growth — and the National Organic Program said that it was unclear what benefits compostable bags could provide to compost.
From San Francisco’s perspective, the ultimate goal of composting outweighs the inconvenience of pulling them out. The city doesn’t want to risk confusing people, and turning them off of composting, according to Hilary Near, commercial Zero Waste senior coordinator for the San Francisco Environment Department.
“It’s a messy middle,” said Near.
Compostable bags are made of plant-based biopolymers, and they can also contain products and additives that allow them to resemble synthetic plastic, according to a National Organic Program technical report.
According to the Biodegradable Products Institute, a prominent certifier for compostable products in North America, products that it certifies must meet a minimum benchmark of being at least 90% degradable in an industrial facility.
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However, the designation does not require that they leave no microplastics behind.
While Near said she didn’t discount the issue of compostable bags leading to microplastics in compost, she doesn’t see these microplastics as nearly as big a problem as the issue of distinguishing microplastics from regular plastic bags.
Starting next June, people in San Francisco and indeed all of California will not be able to buy compostable bioplastic bags anyway: Assembly Bill 1201 will require that bags with those labels meet federal standards set by the USDA for what goes into compost, and certified compostable products — whether they be bags or cups — don’t meet that standard.
What’s even more confusing for consumers is that “biodegradable” does not have a standard associated with it, whereas “compostable” does, Near said.
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An excavator takes in a shipment of material to be composted at Recology Blossom Valley Organics in Vernalis.
Noah Berger/For the S.F. Chronicle
If you want to keep using the green bioplastic bags, go for one labeled as “compostable.” State law requires that any product labeled with the word “compostable” meet a certain set of standards, as certified by BPI or other groups. (These particular standards apply only to industrial compost facilities — so if you’re looking to put the green bag in your home compost, don’t, since it won’t break down unless it’s treated under high temperatures at an industrial scale.)
The Biodegradable Products Institute said that its certification “evaluates the ingredients and finished product to ensure the material breaks down completely and does not persist.” The group says that “conventional plastic is the real contamination issue, and unfortunately there is a lot of it because California does not have strong labeling requirements to identify certified compostable products easily.”
Recology specifically notes that BPI-labeled bags are allowed in compost, and it prohibits those just labeled as “biodegradable.” Even so, the compostable bags are still likely to get plucked out.
Non-compostable materials fall from a conveyor at Recology’s facility in the San Joaquin Valley.
Noah Berger/For the S.F. Chronicle
Alternative options
So for the avid composter looking to ease up the system, what can you do?
According to Will Brinton, a soil scientist who has studied compostable bags for decades, the thickness of the bioplastic bags is an important factor. Choosing thinner bags could get them through the composting process more easily.
Reed, the Recology spokesperson, said he puts a paper towel at the bottom of his compost pail to control odor. Then he puts in scraps — banana peels, eggshells, etc. Each night, he empties the pail and rinses it.
“Do that and you will wake up to a fresh kitchen every morning. No bag required. And more economical,” Reed said by email.
Another choice, Brinton said, is organic materials — use paper liners or bags that will deteriorate much faster.
Recology spokesman Robert Reed examines finished compost at Recology’s Blossom Valley Organics facility in Vernalis on Aug. 31. Reed said he lines his compost pail with a paper towel and empties it and rinses the pail each night.
Noah Berger/For the S.F. Chronicle
(Experts note that for paper, just avoid anything that may have silicone in it, like many types of parchment paper.)
Brinton acknowledges that it’s going to be messier than a bioplastic. It’s clumsy, and may soak through.
But for this, “messy is good,” Brinton said.
Kate Galbraith (kgalbraith@sfchronicle.com) contributed to this report.