[Note that this article is a transcript of the video embedded above.]

This is the Puente Hills Landfill outside of Los Angeles, California. The first truckload of trash was dumped here in 1957, and the trucks just kept coming. For more than five decades, if you threw something away in LA County, there’s a good chance it’s buried somewhere inside this mountain of waste. At its peak, Puente Hills was accepting around four million tons of trash every year, making it one of the largest landfills in the country. It closed in 2013, creating a time capsule of everyday life and consumption patterns over a span of 56 years. But Puente Hills is also a time capsule of landfill engineering itself. In 1976, right in the middle of its lifespan, sweeping federal regulations changed how we deal with solid waste forever.

You probably don’t think too much about where your trash goes, and that’s kind of the whole point of the solid waste industry: to make sure you have the ability to throw something away without it having a serious negative consequence on the environment or public health. There’s a larger conversation to be had about the amount of waste we generate and how much of it can be recycled or reused, but there is always going to be stuff that just doesn’t hold enough value to be kept. Trash is an inescapable element of the human condition. And, I think you’re going to be surprised how complicated that really is. When Puente Hills opened in the 50s, a landfill was pretty much just a hole in the ground where trash was dumped. By the time it closed, landfills were highly engineered holes where trash gets dumped. And I have a scale model of a landfill in the garage to show you how it all works. I’m Grady, and this is Practical Engineering.

There are lots of kinds of waste in this crazy world, but one of the biggest sources is just you and me throwing stuff in the trash. The technical term is municipal solid waste, since its collection is usually coordinated at the city level. There are a lot of ways to manage it once collected, but the most common by far is disposal in a landfill. And, one of the biggest parts of landfill engineering is just deciding where to put one in the first place. The main goal of a landfill is to maximize the volume of waste that can be stored there while minimizing the cost and the environmental impacts too, which turns choosing a suitable site into a giant geometry problem.

Digging a hole sounds like an obvious choice, but consider this: digging a hole is expensive, and not digging a hole is free. There are costs of excavating tons and tons of soil just to get it out of the way so it can be replaced with trash and costs of hauling away all that soil (since your goal is to maximize the volume on the site). Plus, you have to avoid the water table, any unsuitable geology, and the challenges of building and working deep below the surface of the earth. That’s why most landfills mostly build up into what sanitation professionals call the “air space.” Looking upward, it may seem like the sky is the limit, but anyone who’s built a tower of anything, let alone trash, knows better. The waste pile gets less stable as its height increases, requiring shallower slopes. And the pressures at the bottom go up too, which can lead to settlement and damage of facilities. Plus, there are visual impacts. The bigger the garbage heap, the bigger the eyesore, and people are only willing to look at a landfill so tall.

They can’t be too close to airports, because they attract birds that can interfere with planes. And they can’t be too close to homes, parks, playgrounds, and other places people congregate for obvious reasons. Of course, there’s floodplains and wildlife habitat to avoid as well. And you don’t just need a place to put the trash. You also need a scale house to weigh the trucks coming in and out, a shop and storage for the equipment, and sometimes a place for ordinary citizens to drop stuff off. Finally, you need a spot that can handle the huge increase in truck traffic coming and going, practically nonstop. Pretty much, if you can get a college degree in it, it’s going to come into play when siting a landfill: geology, geography, politics, archaeology, public relations, biology, every kind of engineering, and lot more.

But once you have your landfill, you can’t just start dumping trash. Let me show you why with a demonstration with some help from my shop assistants. I have my hole dug, and we’ll start adding some trash. So far, no major problems. But eventually, it’s going to rain. And you can’t immediately see the issue. Granted, this is more of a flood than a drizzle, but it gets the point across. All that water is going to filter through the garbage to the bottom of the hole, and, eventually, into the underlying soil. It might go without saying, but I’m going to say it anyway: We really don’t want garbage juice percolating into our soils. Mainly because it can contaminate sources of groundwater, but also because it can migrate well beyond the limits of the landfill, causing all sorts of environmental troubles. So, modern landfills use a bottom liner to keep waste separate from the underlying soils. Often this consists of a thick sheet of plastic, carefully tested and welded together into an impermeable membrane. Even the area between the plastic welds is tested using air pressure to make sure there are no leaks. Another option is thick clay soil compacted to create a watertight layer. In many cases, the two options are combined, so you end up with this intricate structure of different impermeable layers stacked together.

Maybe you still see a problem with this solution on its own. Now when it rains, the landfill just fills up with water. This causes issues with stability and settlement. It causes garbage to decompose more quickly, leading to odor and temperature problems. Plus, you just can’t work on top. There’s no way for trucks to unload trash on top of a garbage swamp. So we need a way to get the garbage juice out, without letting it flow into the soil below. By the way, garbage juice isn’t a technical term. It’s actually called leachate, so I’ll use that from here on out. And all modern landfills have sophisticated leachate collection systems to keep the waste as dry as possible and avoid the issues I mentioned. Usually, this consists of a system of perforated pipes covered in a layer of sand, draining to sumps, and eventually leading out of the waste.

I built a little leachate collection system in my model landfill using a small tube so you can see this in action. Now my clay bathtub has a drain. When the rain comes, the water that makes its way into the waste is able to flow out of the landfill, keeping it from becoming a swampy mess. This is a little simplified compared to a real landfill. I’ve made a video all about French Drains, which is much closer to what a leachate collection system consists of if you want to learn more after this. Obviously, in my example, the leachate system has to penetrate the bottom liner, which can be a potential source of leaks. So these penetrations are sealed really carefully in the real world, or the collection system just uses pumps and risers that run up the slope of the landfill to the top, so no penetration system is necessary.

Of course, now you have a stream of leachate you have to deal with. Actually, leachate management is one of the biggest costs of running a facility like this. Some landfills send it off to a treatment plant that can clean it up. Some have ways to treat it on-site with settling ponds, evaporation, biological treatment, and even plants that can consume and convert landfill leachate into waste that’s easier to dispose of (maybe even back into the landfill itself).

Finally, the bottom of our landfill has all the necessary pieces, but the work doesn’t stop there. Remember that volume is everything in a landfill. For as much effort goes into finding a location and building the infrastructure, it’s essential that we get the most trash in here as possible. You probably know this, but municipal garbage just isn’t that dense. Maybe you’ve had to smash a few more bags in the can because you missed the collection one week. If so, you know there’s usually a lot of room for densification. The trucks that collect garbage usually have a way to compact it to make more room in the box before needing to be emptied. But once the trash is at the landfill, there’s still an opportunity for compaction. Landfills often use massive roller compactors with enormous teeth and giant blades to grade out and compress waste and get as much as possible into the site. It saves money, and it’s good stewardship of the space. But density isn’t the only challenge with day-to-day operations.

Despite what you’ve heard, landfills are kind of gross. I mean, that’s their whole point is to accept the stuff we don’t want to put anywhere else. But putting it all in one place creates a lot of problems: pests, odors, windblown waste, fires, birds, and more. So to mitigate some of that, most places require that the garbage be covered up at the end of every day. This “daily cover” can take a lot of forms. The basic approach is just to put a layer of soil over the top of the working face at the end of the day.

When I do this in my model, you get a sense of the problem. All that clean daily cover is taking up precious space in the landfill. One option is to trim it back off each morning before trucks start arriving, but that’s a sisyphean task of just moving tons and tons of soil around each day. Other alternatives for daily cover are tarps, or just holding back certain types of waste that are more inert like foundry sand, foam, paper, and shredded tires. They’re going in anyway, so you might as well use them on top to cover the more disagreeable stuff overnight. Those alternatives can also help avoid leachate getting perched within the waste, encouraging it to continue downward to the collection system.

Ideally, a landfill will last for decades, slowly filling up by packing as much waste as possible. Throughout the course of operating a landfill, there’s constant testing of groundwater, surface water, leachate, air quality and more to make sure they’re not exceeding limits. Landfills are usually built in smaller cells so you don’t have to manage this huge area of waste all at once. A cell fills up, you put soil over the top (called interim cover), and start a new one within the landfill. But eventually, you reach the top of the airspace, and the landfill reaches the end of its useful life. And closing a landfill is not an easy job. Of course, you have to cover all that waste up, creating a mountainous sealed tomb of garbage. That final cover has to keep water out, to reduce the volume of leachate you’re having to collect and treat over time. But it also has to keep the garbage in, and not just the garbage itself, but anything else that comes with it like smells and leachate and pests. And it has to do it basically forever. So, just like the bottom liner, the final cover over a landfill is usually a system of multiple layers, including compacted soil, membranes, and fabrics. And then you have to get the grass to grow, to protect the soil from erosion and damage over time. I don’t have time to wait for grass to grow in my demo, so I’m cheating a little bit.

But the fun isn’t quite over yet. The waste may be sealed up, but that doesn’t mean it’s inert. In fact, there’s a lot of chemistry and biology happening inside a landfill, and a lot of those reactions generate gases like methane and hydrogen sulfide that can create pressure, heat, smells, greenhouse effects in the atmosphere, and the potential for explosions. So, one of the steps in landfill closure is to install wells that can collect the gases from the waste. Usually, these consist of vertical pipes connected to a blower that constantly draws air to a collection point. There’s a lot that goes into these systems too. You can’t pull too hard, or you might draw oxygen into the landfill, changing the reactions and microbiological processes, and creating a potential for a fire within the waste. Plus the gas includes a lot of humidity, so managing condensation creates another liquid stream that has to be collected and treated. Once it’s collected, the landfill gas can be flared, combusting it into less environmentally harmful constituents. Another option is to put it to beneficial use to create heat or even electricity. The Puente Hills landfill I showed earlier has a gas-to-energy facility that’s been running since 1987, and even though the landfill is now closed, it currently provides enough electricity to power around 70,000 homes.

Once a landfill is closed, there’s not a lot you can do with it after that. It’s a big, sealed up, mountain of trash, after all. Owners are generally required to look after a closed landfill for at least 30 years afterwards, inspecting for leaks, monitoring the air and water, and repairing any damage. Those costs have to be built into the rates they charge, since there’s not a lot of benefit (or revenue) after closure. But, with all that open space and carefully-maintained landscaping, one option that many landfill operators are trying out is parks. And I love this idea. They say, “We’re willing to put our money where our mouth is and invite the public to spend time here, to enjoy this place that used to be, you know, one of the least enjoyable places you can imagine.” Puente Hills in California has big plans, including trails on the slopes, biking, slides, gardens and more. It looks like it will be a really nice place to visit when it’s done. And it also puts the whole concept of landfills in perspective.

Of course, we have a lot of room for improvement in how we think about and manage solid waste in this world. Landfills seem like an environmental blight, but really, properly designed ones play a huge role in making sure waste products don’t end up in our soil or air or water. It’s not possible to landfill waste everywhere. Many places are too densely populated or just don’t have enough space. But where they are, the environmental impacts are relatively small. Just consider the resources that go into them. I pay about 20 dollars a month, probably a little on the low end of the national average, and that buys me 64 gallons (about a quarter of a cubic meter) of space in a municipal landfill per week. Of course, I don’t fill the can every week, and that trash gets compacted. But still, do that for a decade, and your 20 bucks a month has paid for the volume of a modest apartment. It’s covered the cost of building the lining and collection systems, the environmental monitoring, the daily operations, the closure, the gas collection, and the maintenance for at least three decades afterwards and for your trash to stay there effectively forever. It’s (almost) free real estate, not that you’d want to live there. But my point is: landfills are a surprisingly low-impact way to manage solid waste in a lot of cases. I hope the future is a utopia where all the stuff we make maintains its beneficial value forever, but for now, I am thankful for the sanitary engineers and the other professions involved in safely and economically dealing with our trash so we don’t have to.