An engineered PET depolymerase to break down and recycle plastic bottles | Hacker News Reader

An engineered PET depolymerase to break down and recycle plastic bottles

nature.com

250 pointsnkjoep85 comments

The exciting thing about this research is that the authors substantially exceeded the normal bar for publishing a scientific article (even one in Nature). They demonstrated industrial feasibility of their proposed enzymatic process by depolymerizing waste PET then repolymerizing it into a useful item (water bottle) that met industry standards for strength/ductility and coloration. I work in microbiology, and seeing research that connects basic biochemistry to actual industrial implementation (at the lab scale) is really impressive. There are pieces of their process that would need to be refined (and the cost of oil would have to rise) for this to be cost competitive with just manufacturing from virgin PET units, but it's a strong foundation. It's much closer to realization than most "researchers discover enzyme that does X and will revolutionize industry Y".

It doesn't need to be cheaper than virgin PET, it just needs to be cheaper than virgin PET plus the cost of landfill/"recycling". With more countries now refusing to be dumping grounds the cost of getting rid of old bottles stands to rise considerably.

...to the extent that such cost of landfill/recycling is internalized to the producer.

> ...to the extent that such cost of landfill/recycling is internalized to the producer.

It's enough for the costs to be internalized by consumers.

Do you expect a consumer's garbage bill to affect how many and what type of plastic bottles they buy?

> Do you expect a consumer's garbage bill to affect how many and what type of plastic bottles they buy?

Broadly speaking, I think people prefer to spend less money on equivalent goods. So, yes. I do.

I think the effect will probably be less strong than if the entire cost were concentrated at the point of purchase.

I don't mean in the "economics spherical cow" sense, but in the real world.

My garbage company here in Boulder(Western Disposal) determines your fees based on the bin size you have: bigger recycling or trash bins cost more than cheaper ones on an ongoing basis. Compost is free always though.

But really though, there have been weeks where we filled up our bin with lacroixs and beers and I stopped buying canned beverages because of that(for the next 7 days)

That's interesting. Here in Seattle, recycling is free but compost costs money.

They'd have to eat the cost of the more expensive plastic BEFORE their bills would go down, which means it is a decision most consumers will never know to make.

Recycling collection companies will pay them to take the old PET off their hands.

The US is doing more stateside recycling these days, and from this article it sounds like recycling bottles is actually a profitable business.

https://www.theatlantic.com/technology/archive/2015/12/what-...

That was written in 2015.

It is no longer the case https://www.npr.org/2019/08/20/750864036/u-s-recycling-indus...

You may be overestimating the cost of landfills.

Landfill costs (understandably) vary between countries. Got a lot of empty space? Cheap. Crowded? Not cheap at all.

Even in crowded countries, landfill land area is still a tiny percentage.

Its more environmental laws and how strict regulations are about seepage.

Unsorted waste cost is often regulated to be higher than what the immediate costs are. And landfills are banned in a lot of places so the unsorted waste has to be resorted and the uncrecycleable parts incinerated[1].

[1] eg see third column of table 3.1 at https://www.eea.europa.eu/themes/waste/waste-management/muni...

Some European countries have essentially banned landfills; waste must be either exported (expensive, increasingly difficult), recycled (expensive) or burned (expensive, controversial).

It needs to be cheaper than cost of ‘virgin PET’ including the taxes levied on the material itself, technically. I haven’t done the math to incorporate the tax duties of the hydrocarbons used to form non-recycled PET but I assume currently most countries levy no tax whatsoever on PET itself, regardless of source?

They're still at lab scale, though. Not pilot plant.

Here's the world's largest PET plastic bottle recycling plant as of 2014.[1] It serves Los Angeles. Plastic bottles go in, and pellets for injection molding of new bottles come out. Along with about 30% trash that came in with the old bottles. If this enzyme is useful, it could become one step in that process. Not clear how much of the cost is in depolymerizing vs. all the other steps required.

Separating bulk recycled materials is pretty much a solved problem technically, but the US was behind in this until shipping unsorted material to China stopped being an option. Robotic sorting is here. The first step was to apply the high-speed vision-based in-fight sorting technology used for fruits and vegetables to recycling. With multispectral imaging, different plastics that look similar to humans can be separated.[2] This isn't experimental; this is what sorts your recycling in most big US cities now. The last 5% of "quality control" sorting, which used to be done in China with cheap labor, is now done with vision-guided robots in the more advanced plants. SF got this last year. At the end of that line, sorted, de-labelled PET bottles are ready for the enzyme or heat processes.

The remaining problems involve cost. It's all do-able.

[1] https://youtu.be/vAr4BZM_Tzk?t=25

[2] https://youtu.be/k2XYA3bXPpo

Wow interesting, thanks for sharing. So should we be trying to recycle plastics again? The news on this changed a lot in the past years. Also there was that whole thing about a bottle cap/ring vs. the bottle itself... do you know what the status is on that?

I think this is already big enough to be the biggest thing about PET for the next few years.

Even if their enzyme is only good enough for 30% round trip efficiency, the benefit of simply doing away with discarded PET is enormous.

Imagine PET having to lay in the garbage dump not for 20-30 years, but for just few month after a day or two in a chemical reactor.

I will add some more comments soon about sorting, and the situation in USA (not going to improve IMHO)

For many applications, it's not necessary to depolymerize PET and repolymerize it. Just grinding it into flakes, cleaning the flakes, melting them down to liquid, and re-casting as pellets is enough.

I don't have access to the article right now either, but I would like to note to the "this enzyme is going to eat all our stuff!" commenters that, in general, most enzymes don't work very effectively in ambient conditions. The abstract doesn't go into detail into how they acheived the "90% in 10h" metric, but I would assume this was in a precisely controlled solution, incubated at an optimal temperature, with fresh enzyme that hadn't had a chance to start denaturing. You aren't going to be able to spray this on the plastic siding of someone's house and watch it wither away (even if home siding was PET and not, IIRC, vinyl or something.)

I guess I can't rule out at that some recombinant bacterium expressing this enzyme might escape the lab and start causing PET objects to get moldy or develop a patina if left out too long [0]. But that would require that this enzyme somehow benefits the bacterium enough that natural selection wouldn't favor dropping the gene for it.

[0] Hrm, there's a thought - what if your refrigerator needed to be refrigerated?

This all sounds sensible and reasonable, but just at this exact moment, it's hard to feel totally confident that something bioengineered can't possibly create a plague inadvertently. And I'm assuming that covid-19 isn't in fact an example.

It made an impression on me, something I read on HN not too long ago, comparing an epidemic to a nuclear reaction - it's really hard to create a critical mass, and when it's dispersed that's it; the really acute problems are local and temporary, despite peoples' fears of invisible radiation and contamination. But once a global epidemic gets going, essentially the whole world may have reached critical mass and has to be diluted. Maybe obvious when stated, but presenting the comparison in the context of the fears people have of nuclear weapons/power is the point.

The probability is low, but the stakes in the near future of biotech seem higher than anything else people can mess with.

It's interesting, I've been helping plastic situation by buying shredded plastic scrap and turning it into filament for 3d printers, you need to mix it with virgin plastic (30-70 or 40-60)

Here's the project details: https://medium.com/endless-filament/make-your-filament-at-ho...

Communities can setup their own machines and create filament from scrap plastic, this is good enough for 3d printing where your life doesn't depend on it.

The exciting thing is that there is real progress in increasing temperature stability of enzymes. That's the point of the article, engineering an enzyme that is still active above the glass transition temperature of the substrate. I remember a paper where they tried random mutations all over some enzyme, it was just mindless make-work for an army of graduate students. This one is different.

I can't read the article due to paywall. How did they achieve this thermostability? Supercharging the surface?

Site-saturation mutagenesis in the active site followed by introduction of disulphide bridges.

Man. I would have guessed that adding disulfide bridges would decrease catalytic conversiom rates by making the protein too rigid.

For a layman like me, what does this enzyme decompose the plastic into?

I think that it decomposes the plastic back into the monomers that it is made up of. At which point it could be polymerized again to create new plastic.

Monomers, the basic building block of plastic. You can then re-polymerize them.

From the abstract:

Poly(ethylene terephthalate) (PET) is the most abundant polyester plastic, with almost 70 million tons manufactured annually worldwide for use in textiles and packaging3. The main recycling process for PET, via thermomechanical means, results in a loss of mechanical properties4. Consequently, de novo synthesis is preferred and PET waste continues to accumulate. With a high ratio of aromatic terephthalate units—which reduce chain mobility—PET is a polyester that is extremely difficult to hydrolyse5. Several PET hydrolase enzymes have been reported, but show limited productivity6,7. Here we describe an improved PET hydrolase that ultimately achieves, over 10 hours, a minimum of 90 per cent PET depolymerization into monomers, with a productivity of 16.7 grams of terephthalate per litre per hour (200 grams per kilogram of PET suspension, with an enzyme concentration of 3 milligrams per gram of PET).

Basically, plastics are polymers (many units) of repeating building block monomers (single unit). The single unit is terephthalate, a " dimethyl-ester that is a major starting material for polyester fibers and coatings." So you're breaking down the already formed polymers into the building blocks that can be redeployed.

the article only says hydrolase. which means using water to break bonds. breaking bonds in plastic gives you smaller plastic.

PET is formed via the condensation of ethylene glycol (ethane-1,2-diol, aka EG) and terephthalic acid (1,4-benezenedicarbyoxylic acid, aka BDC). In the process, a proton (H, a hydrogen) is lost from the carboxylic acid (COOH -> COO) and a hydroxyl is lost from the alcohol (R-OH -> R). The H and OH combine to form a water (hence "condensation") and leave, and an ester bond (RCOO-R') is formed. Amide bonds (RCONR'), the backbone proteins and materials like Kevlar, spider silk, and Nylon, are similar, but less flexible and harder to break.

This condensation reaction (and others) is reversible. If you heat PET in boiling water, you can begin to depolymerize the polymer into its constituent monomers by having a water molecule consumed in the reaction (RCOOR' + H2O -> RCOOH + HO-R').

However, PET is a rigid plastic that acts as a barrier material (that's why it's used for bottles and packaging), and has a high glass transition temperature (the temperature at which polymer chain mobility becomes broadly possible -- the polymer is in equilibrium, while it is kinetically trapped below the Tg). This inhibits the breakdown of PET, particularly at low temperature.

Hence, the sentence from the abstract:

> With a high ratio of aromatic terephthalate units—which reduce chain mobility—PET is a polyester that is extremely difficult to hydrolyse.

Aromatic rings are just that -- rings. They don't flex a lot and can't undergo a lot of thermal motion, can't change conformation, etc.

> Here we describe an improved PET hydrolase that ultimately achieves, over 10 hours, a minimum of 90 per cent PET depolymerization into monomers, with a productivity of 16.7 grams of terephthalate per litre per hour (200 grams per kilogram of PET suspension, with an enzyme concentration of 3 milligrams per gram of PET).

> We also show that biologically recycled PET exhibiting the same properties as petrochemical PET can be produced from enzymatically depolymerized PET waste, before being processed into bottles, thereby contributing towards the concept of a circular PET economy.

So, they depolymerize the PET, an subsequently repolymerize the terephthalic acids to produce new PET. I haven't clicked through to find if they recycle the EG as well, though.

Yours was a wonderful summary – I've actually favorited this comment. Thank you.

See the end of the article, patents were taken out starting 2017, and there is a company https://carbios.fr/en/profile/

What does this mean for PET already in landfills?

Can we trawl through our landfills for old PET and bring it back to (perpetual) life with this process?

It would not make economic sense. You'd spend far more money and CO2 output moving mountains of garbage and trying to sort than would ever be recovered by recycling some scraps of plastic. The supply chain of incoming recyclable material is very important to make any recycling process work well.

But yes, it would be possible.

Would spraying garbage or landfill with bacteria that could produce this enzyme work? At least at slowly reducing volumes of plastic in landfills into liquid-y form?

A better place to start would be the Great Pacific garbage patch:

https://en.wikipedia.org/wiki/Great_Pacific_garbage_patch#Si...

That's an above ground source for trillions of pieces of plastic.

"Most debris consists of small plastic particles suspended at or just below the surface, evading detection by aircraft or satellite."

Collecting that is going to be a massive headache.

Don't have access to the full article so I've only read the abstract, but what's the feasibility of deployment here?

https://sci-hub.tw/downloads-ii/2020-04-09/c8/10.1038@s41586...

Its got to be cost-effective. That's the gold standard. Is it cheaper than not breaking the plastic down? E.g. other solutions - burning, burying...

Not a moral argument. Just a practical one - the free market isn't going to use this unless it has a short payback period.

There's complicated stuff for figuring out if it's useful:

- Energy in vs energy out

- The price fluctuation of the underlying feedstocks

- Scalability - (chemistry is 1000x harder to scale things than comp sci.)

Can’t plastic already be melted down and reshaped?

Now the question is how to collect plastic in the wild

It can be reshaped, but the plastic will not have the same mechanical and optical properties as "virgin" PET, so it can't be used for the same applications.

Yes, but for plastics like PET that downgrades the plastic. This process breaks it into monomers so the recycling can be truly circular.

> An engineered PET depolymerase to break down and recycle plastic

Wrong title, and it's not for all plastics. We all complain here about journalism taking the wrong conclusion and I'm just pointing out the factually wrong message in the title.

I prefer this title, I clicked on it. I might not have with the original.

But it doesn't break down all plastics, just for PET. The title should match the article according to the HN rules.

How many people know what PET is? I'm not sure I would have got the reference. The original title has too much jargon.

So? It's not for all plastics, there are so many different kinds and they're all different! This is basically only used in food containers, not products like your TV or phone case.

Also, that's a false argument. We shouldn't dumb things down to incorrectness just so people understand them. It's not hard to look up 'PET plastic', and I see enough complex article titles on HN that I don't think that applicable to our community.

How's that a false argument? It's reasonable to avoid jargon in communication where possible - if you want to communicate to a wider audience. In this case I don't think it adds much. Once you click through you get the details.

But the mods seem to agree with you, so I'm in the minority opinion it seems.

"PET" isn't jargon, it's a designation/name, much like "relational database" or "Phoenix framework". It's a name and type of a plastic, and to replace it with "plastic" alone makes the headline wildly inaccurate, like if I said "this new medicine cures cancer in 90 percent of patients" but it was just about some rare cancer that only 100 people get a year and couldn't be applied to other cancers.

Depolemerize is jargon, admittedly.

90% PET plastic maybe. It seems PET is not the most common plastic.

"Polyethylene" might be a good compromise, I believe it's in more common usage outside of the US and would still be fairly understandable (and searchable) to those within it.

But it isn't for plain polyethylene (HDPE or LDPE). It's specifically for PET. I suggest "PET plastic bottles" in place of just "plastic".

"An enzyme decomposes 90% of plastic in 10h" is more succinct and readable than the real article title, and it's not wrong. A depolymerase is a kind of enzyme [0].

[0] https://en.wiktionary.org/wiki/depolymerase

I didn't have any trouble understanding it, but I guess we'll just dumb it down into the factually wrong message...

We've changed the submitted title ("An enzyme decomposes 90% of plastic in 10h") to that of the article.

i don't want this anywhere near my home please

Because?

It might be extremely toxic.

It’ll eat everything inside!

I have a large number of plastic things inside my home that I hope do not become decomposed.

Garbage, sure, decompose away.

It does not multiply like you expect in a movie. So the amount it can decompose is limited by quantity of the enzyme.

Enzymes are proteins. They're applied to powderized plastic in a bio-reactor. It's unlikely that they'd be able significantly attack bulk plastics before they themselves get broken down.

Wait until some organism eats them and learns to reproduce.

My parents have a neighbor whose house is literally made of plastic, aside from a few structural supports. You could spray this on his house and 10 hours later, goodbye house ( assuming it is PET plastic, I have no idea.)

Was it 3-D printed?

I was going to say that it probably isn't PET, but a bit of Googling quickly turns up articles about people making houses out of recycled plastic. If that's what it is, it could well be PET.

More like big, hollow 2x4 beams of plastic arranged vertically and interlocked. Could be recycled, I don't know for sure.

Don't be histrionic, the PET they did this with wasn't in large chunks, it was processed into a powder (with particle sizes between 200-500 micrometers). This enzyme would be less harmful to their house than rubbing it with acetone.

I suspect that your parents' neighbor would already be unhappy if someone smashed their house into micron-size particles, heated it to 155˚F and kept it there for many hours.

At that point, not having the PET broken down into monomers is a pretty lousy silver lining.

(Seriously though, it needs fairly particular conditions to work.)

If you're heating the whole house to 65 degrees, as required, that is probably the least of their worries.

And it's not like the ability to dissolve plastic is new; there are plenty of solvents which will do this for various plastics.

The research article say they use the enzyme at 65°C/70°C (150°F/160°F). Also, usually the reaction speed decrease exponentially with the temperature. So your home items are probably fine.

Say, at 18C - does it still work? Maybe 100 times slower, but still work? Or you have to reach 65C for it to evens start working?

Sounds like a weapon

That’s alarming. Sounds like an effective way to destroy every modern manufactured good.

It only destroys the goods if you let them soak in the solution for hours. How does that make it dangerous? If someone wanted to just destroy goods, fire is way more effective.

It’s a solution that needs controlled pressure and 60+C temperature in a bioreactor...

There are many more effective chemicals if your goal is just to destroy, chlorine trifluoride for example

How do you feel about gasoline and matches?

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