Most of us find waste to be viscerally repugnant. The smell and even the appearance of garbage revolts us. And yet we all generate it, in seemingly endless quantity.
In “developed” (rich) nations, we therefore have built extensive systems to get this repugnant material out of sight, and hence out of our minds, as quickly as possible. Sure, many of us dutifully separate our waste at source into the compartments required by the local waste authority, and sometimes that makes us feel good. But we’re left with a lot of questions:
1) Is the effort to separate waste at source, worth the bother? Not just the effort, but the energy to collect physically separated materials at source- is that a net environmental improvement?
2) What happens to the waste after we dispose it and it disappears from our consciousness?
3) We hear about waste from developed countries being “shipped overseas”. Is this a disposal strategy? Are we paying others to improperly dispose of our waste?
4) Wouldn’t we be better off to just burn the whole works? Or if that doesn’t sound appealing, couldn’t we do something “smarter” than burning, to harvest the energy contained in that waste for beneficial uses?
I’ve learned that “deep dives” into complex and important topics like this, tend to bore people and don’t get read. So I won’t be diving deep into this one- I’m leaving most of these complex questions unanswered! I just need to make a few quick points, based on decades of experience working on “waste to energy” type schemes of a bewildering variety of sorts. Because frankly, “waste to energy” and “waste to fuels” projects and proposals are popping up now like dog strangling vine on my farm. Whereas the dog strangling vine is just a pure green menace, waste to energy project proposal are seen as a chance to kill two birds with one stone- to deal with the smelly, unsightly, land-consumptive and potentially GHG emissive problem of waste landfills, while at the same time, making some energy that we need.
The problem here is that it sounds too good to be true. Because it’s not true. Or, more accurately, it is true in such a limited set of circumstances that it’s basically the exception rather than the rule.
Municipal solid waste (MSW) is an extremely heterogeneous mixture which varies in composition from place to place, from time to time, and as a result of the policies (or lack thereof) of the municipalities responsible for providing waste disposal as a public service. In places where waste source separation isn’t practiced at all, the waste stream contains a lot of materials that can be, should be, and in most other places, ARE recycled. In some cases, some of these materials aren’t source separated, but rather are separated manually or by machinery at the waste handling facility or transfer station. Let’s talk about groups of materials in general terms and consider them one by one.
Metals are sorted out because all metals are highly recyclable, and recycling them reduces GHG and toxic emissions dramatically relative to making fresh pure metals from their native ores. Of course only reduced metals in solid form (i.e. cans) are readily recovered by sorting.
To give you an idea: a typical aluminum can in 2014 weighed 15 grams. Aluminum has an embodied energy on the order of 200 MJ/kg, which means that a can has about 3 MJ of embodied energy associated with it. That’s about the same as the same can, 1/3 full of gasoline. Even with recycling, aluminum represents about 11.5 kg of CO2 emissions per kg. Anybody wasting aluminum cans needs to have their head examined if they also claim to care about the environment. Those thinking that it makes sense to turn aluminum, made from aluminum oxide by electrolysis, into hydrogen, are really energetic vandals.
There’s a lot of non-degradable, non combustible stuff in the average MSW stream. Lots of dirt, concrete, brick, rock, gypsum etc. from demolition.
There’s also a lot of glass and ceramic materials. The former are recyclable- the latter, aren’t. The best you can do with waste ceramics is grind them up and use them as a replacement for sand or gravel in concrete.
This is food and yard waste, diapers, pet waste and the like. And frankly the only thing that makes sense to do with this stuff is to remove it at source and not landfill it. When you landfill wet organics, you encourage anaerobic degradation which converts the waste into biogas- a nearly equal mixture of CO2 and methane, the latter being 86x worse than CO2 on the 20 yr horizon in terms of global warming potential (GWP).
Paper and Wood
Same deal- paper and wood need to be source separated, but not because of the risk of biodegradation primarily. While paper is compostable, it’s not readily degradable in a typical anaerobic landfill. I’ve personally seen newspapers which were buried in landfill100 yrs ago that were still perfectly legible. While some biodegradation does happen in landfill, landfills are not designed to be bioreactors- quite the opposite in fact.
Paper is, however, of fairly high value for recycling, particularly cardboard and and paperboard. Recycling corrugated cardboard is an environmental no brainer. Cardboard is a high value material and even businesses which really don’t care about the environment at all, collect cardboard separately for recycling because it reduces their waste disposal costs. And wood, meaning lumber and the like rather than yard waste, can always be made into paper. Wood demolition waste is already harvested for this purpose in some locales, if it can be properly source separated. And of course if there’s a surplus, both can be burned as a solid biofuel.
Plastics are nearly 100% of fossil origin. While they can be recycled, how much they are actually recycled depends on the nature of the plastic, the nature of the source collection (which determines how clean and how hard it is to sort), whether it’s a thermoplastic or a thermoset such as those used in composite materials and rubber (thermosets are basically not recycled, because recycling them requires the breaking of chemical bonds which don’t come so easily unstuck), and of course, what purposes the waste can be put to. Recent reports put the average of plastic recycling around 9%, which is far from stellar.
PET, the material used in beverage bottles, is very easily recycled mechanically and is fairly easy to source separate. However, just like with metals, we don’t generally recycle PET bottles back into PET bottles. Rather, we make PET bottles into things like carpet fibre, which don’t care about thinks like leachable content, colour and transparency quite as much as a clear food grade beverage bottle does.
However, the largest volume plastics are polyethylene (PE), polypropylene (PP) and polystyrene (PS). These materials, though readily mechanically recycled, are often used in the form of films, foams or thin-walled goods which can come back from consumers very mixed, dirty, coloured and otherwise hard to separate. And while you can make SOME goods out of PE/PP blends, most uses require quite pure material. Accordingly MOST PE and PP and most PS foam are not in fact recycled, but rather are landfilled.
There are myriad of other plastics, used either alone or in layers with other materials to provide the desired properties. Some like polyvinylchloride (PVC), are at once extremely valuable and useful and also basically a bomb, waiting to go off when you do the wrong thing at the end of life of that plastic material. And there are a myriad of uses for all those plastics, varying from inarguably dumb single uses like tie hangers or individual wrappers for plastic cutlery, to convenient but questionable uses like plastic grocery bags, to life-saving uses like IV bags, catheters, oxygen tubing, disposable syringes and the like.
When you compare the LCA data for plastics against materials they compete against in the marketplace for similar uses (such as paper, glass, aluminum or natural fabrics), plastics tend to come out on top. They use less energy and water, weigh less, have superior properties, and can be recycled. They can also sometimes save giant amounts of other waste- the thin PE wrapping on English cucumbers comes instantly to mind. This wrapping reduces the waste of cucumbers from field to table by at least 50%- a reduction in the mass and impact of waste of around a thousand fold. My friend Chris DeArmitt is a great source of the research into this topic- he knows more about it than just about anyone.
But of course, we all love to hate plastics. With some good reasons, and some bad ones. They are over-exploited in packaging. They have become so extraordinarly inexpensive that they have come to exemplify “cheap”, non-durable, consumptive and wasteful by virtue of the many dumb uses we’ve come up for them- which sadly people in the marketplace have rewarded by buying. Many plastic products are optimized for cost and aesthetic function, not for recyclability. And they enable convenience that suddenly seems mandatory- something you can’t opt out of without trying very hard indeed.
Energy From Waste to the Rescue!
The usual pitch for a “waste to energy” scheme is as follows:
- get rid of the cost and inconvenience of source selection
- eliminate the problem of methane generation in landfill
- offset some fossil burning
- save precious land by reducing landfilling
Who couldn’t love those things!
The Devil in the Details
As usual the devil lurks in the details.
First of all, we really need source separation for a few reasons. First, getting people involved in source separation helps them focus on minimizing waste generation. Second, it improves recoveries (dramatically) of the highest value, most energetically favourable materials to recycle, i.e. metals, carboard/paperboard etc.
Secondly, waste to energy isn’t the highest value use of the wet organic content. That material contains some energy, but it also contains a lot of water. Burning, gasifying or pyrolyzing waste (heating it up in the absence of oxygen), requires us to boil off all that water, and that takes a lot of energy. In net terms, most of the energy in the wet organic fraction, is needed just to provide the energy to boil off the water it contains. In net terms therefore, MSW which has been source separated for recyclables in even a prefunctory way, doesn’t contain ANY net energy of biological origin.
There’s an alternative which doesn’t mind the water. It’s anaerobic digestion, to produce biogas. That’s what we do in Toronto with our green bin waste.
But there’s still lots of energy in that MSW, right?
Yes. Even moreso if you don’t source separate the waste…
Sadly, that energy is waste plastic. All of which is of fossil origin.
MSW is therefore, in net calorific (heat content) terms, almost entirely a fossil fuel.
Finally, when you heat up waste materials to burn, gasify or pyrolyze them, you carry out chemical reactions which can produce emissions of significant toxicity, carcinogenicity, and leachate toxicity. It is not uncommon for a waste incinerator to dramatically reduce the volume, but to much less dramatically reduce the mass of feed waste materials- but to also render that material leachate toxic whereas the feed material wasn’t leachate toxic. That means that the process of burning “mobilizes” species that can leave in the (nearly inevitable) leachate which must be collected from the bottom of the landfill and treated prior to being disposed. That adds both cost and environmental impact that could be avoided. Incineration has to be made less energy efficient to cope with these toxic materials, both as a result of manipulating combustion conditions to avoid generating the worst species, and by virtue of energy used to scrub out or adsorb/desorb the toxic materials that are produced.
Waste to Fuels- Incineration in a Sexy Green Dress
Of course people have pictures in their heads of incineration that are very unpleasant, and some of that is unfair to incineration. Modern incinerators can generate quite clean exhausts, well scrubbed of toxic chemicals- if at the cost of generating only a fraction of the energy contained in the feedstock as a result. But that’s not the problem.
The problem is the plastic.
The problem is the energy derived from the fossil origin materials in the waste stream.
The problem is the needless dumping of fossil CO2 into the atmosphere.
Since the net energetic value in the waste is of fossil origin, the waste itself is, in net terms, a fossil fuel.
Accordingly, some greenwashing is needed to recondition the image of waste to energy schemes, by dressing incineration up in a sexy green dress called “waste to fuels”.
By converting part of the energy and maybe even some of the embodied carbon in the MSW feed, into a new fuel, proponents hope to you won’t see incineration under its new clothes.
These schemes are varied, but they usually involve converting waste to simpler chemical by means of endothermic (heat-consuming) reforming reactions by processes called pyrolysis or gasification, which differ only in degree, energy input and desired suite of products. In both cases, heat, usually generated by burning part of the waste feedstock or part of the products or byproducts, is used to break big molecules into smaller ones. Sometimes, oxygen or air is added to the feed to produce some of that energy right in the reactor, and sometimes it is produced outside the reactor and transferred into it via heat exchangers. Sometimes solid materials, often inorganic constituents of the waste itself, are used to help transfer this heat.
The typical strategy is to make either a very light gaseous material called synthesis gas, which is typically a mixture of carbon monoxide, carbon dioxide, hydrogen, sometimes methane, along with water vapour, nitrogen, and acid gases like hydrogen chloride, hydrogen sulphide etc. (remember, waste is very heterogeneous- and it’s going to contain some PVC, some brominated fire retardants, some fluoropolymers…a host of nasty molecules can result when you break these bigger molecules down). The syngas can then either be burned for energy in a turbine after some basic cleanup, or it can be cleaned up much more thoroughly and converted over catalysts to molecules like hydrogen, methanol or Fischer-Tropsch liquids (waxes, diesel etc.). These materials, including the hydrogen, are generally intended for use as fuels. Yields vary depending on the feedstock and process and conditions, but let’s be clear: a considerably smaller fraction of the energy in the feed material is converted to these secondary fuels than would have been obtained if you simply burned the waste in an incinerator.
What happens to the fossil CO2? It all ends up in the atmosphere. When hydrogen is the product, all that CO2 goes to the atmosphere directly, and more CO2 is released per kg of hydrogen produced than if you started with fossil methane instead. That we can say with certainty just from looking at the nature of the feeds: plastics, the major energetic content of MSW, have a typical formula of (-CH2-), whereas methane has a formula of CH4. The higher the C:H ratio of the feed, the higher the CO2:H2 ratio in the products. The result might be better than coal, but is definitely worse than methane. The only difference between the two is that fossil methane comes along with a burden of production and distribution methane leakage that the plastic waste doesn’t. Depending on where you’re making the “black” hydrogen from fossil methane, that leakage can vary between a significant and a very significant CO2e impact, given methane’s GWP of 86x CO2 on the 20 yr timeframe. Sorry folks, but that alone isn’t going to get you my sympathy for making hydrogen from garbage. It’s going to be close to a wash, at best.
Can You Make it Worse?
Sure. You can do the source separation, separate out the waste plastic, and then gasify it. That’s even worse!
Why is it worse?
Because the alternative would be to simply landfill the waste plastic.
Waste plastic degrades when left in the environment, losing mechanical properties and fragmenting into smaller and smaller pieces. Those degradation processes however are driven by two things: oxygen, and sunlight. Sunlight provides the high energy needed to break the bonds in these synthetic materials, that natural processes like biodegradation were not evolved to break apart. And oxygen can react with the polymers both in the dark and in the light.
What happens when we bury plastics in a landfill? Those degradation mechanisms simply stop. There’s no more driving force for the molecules to fall apart, so they don’t. Waste plastics in a proper anaerobic (covered) landfill are stable for millenia. They don’t break down into microplastics. They don’t leach into the groundwater. They just stay there. Their environmental impact simply ends.
As does their fossil carbon content. It stays there. Sequestered. Durably.
What We Should Do Instead
We should stop believing in ideological fantasies of “circular economy”. We should instead begin to think about optimal recycle. And we should focus on making lots and lots of truly renewable and low emissions energy, because as we do that, not only will we feel less need for fuels made from garbage and waste plastic, we will also increase the optimal amount of recycle- because we will reduce the environmental impact arising from the energy used to drive recycling.
What does an optimal recycling system look like for plastic waste?
Here’s my view:
1) You start with good public policy. Policy which looks holistically at the use and end of life of products, using reliable, disinterested 3rd party LCAs as a guide. Stop making decisions on the basis of the “natural is better” fallacy, like the totally idiotic decision to replace polypropylene drinking straws with waxed paper ones. The waxed paper ones are inferior in function, alter the taste of the beverage, aren’t durable, can’t be re-used, use more water and create more emissions in manufacture and transport than their PP cousins, and yet neither PP nor paper degrade in a landfill. The only, minor benefit of mandating paper straws is that the paper ones degrade a little faster when they’re deposited as litter. Litter makes up perhaps 1% of our disposed waste packaging materials in developed nations. Doing worse with 99% of a product to partially mitigate the impact of 1% of its end of life disposal is not sensible.
Better still: don’t offer a straw of any kind unless it is asked for.
2) Mandate “deposit return” for goods which otherwise don’t end up recycled well. This would apply to goods ranging from beverage bottles to cellphones. Users are quite willing to return goods for cash if there’s cash to be had. This generates cleaner, better sorted waste streams for either re-use or recycling.
3) Maximize mechanical recycling of plastics. And don’t concern yourself about the fact that most recycling- of plastics and of metals and other materials, is really “down-cycling”. Just as we don’t recycle pure copper wire into copper wire again, we don’t recycle PET bottles into bottles again. Why not? Because down-cycling copper wire to copper pipe, and PET bottles to carpet fibre, makes more sense energetically.
4) When you have a stream of mixed plastics that are partially degraded, you can do a limited amount of chemical down-cycling to make materials such as waxes, asphalt extenders, printing inks and the like. Doing this makes sense, but will only be a limited endpoint for the plastics.
5) Most of the degraded, mixed and dirty plastics will end up being useless after they’ve been optimally recycled. How should we deal with them? We should bale them and then bury them in properly constructed landfills. They represent the cheapest, lowest impact, lowest risk post-consumer fossil carbon sequestration strategy imaginable. All you have to do is not burn them.
Finally: if you don’t have space for landfill, you have two choices: work harder at steps 1-4, or pay somebody else to landfill the waste plastic for you.
Disclaimer: this is not a thorough examination of the topic, it has been kept brief so that people might read it. This is an enormously complex topic involving the interrelation of society, technology and values, environmental impact, decarbonization and economics.
I’m not at all saying that there can never be a “waste to X” or even a “waste to energy” scheme that makes environmental sense. I’m specifically attacking waste plastic or MSW to energy or fuels schemes, because I think it’s quite clear they aren’t good waste management practice and aren’t in the interest of decarbonization either.
If you don’t like what I’ve said, that’s OK. If you think I’ve materially erred, that’s entirely possible as I’m human and make mistakes like anyone. Provide good references demonstrating where I’ve gone wrong and I’ll correct my piece with gratitude for your input.