Landfill Airspace and Waste Density: The Big Picture
Save airspace or cut costs? Through integration of surcharging, compaction, ADC, decomposition, and a variety of other elements, successful landfill managers are finding ways to have both.
I’m an advocate of maximizing waste density in landfills. It’s a vital part of any landfill’s operation. But if hard-hitting compaction is the only bullet in your landfill’s airspace arsenal, you are for the most part, unarmed. There’s more to airspace management and waste density than just pounding on trash.
A landfill that focuses on compaction and ignores all the other issues that impact overall waste density will be about as effective as an orchestra trying to play a concert with only one instrument. Being in tune, whether for landfills or orchestras, means getting all the parts working together.
In an effort to move our industry beyond the airspace argument that’s stuck between "Tastes Great" and "Less Filling," I’d like to take a look at some other important issues related to airspace. Some are passive. They’re as automatic as the sun coming up in the morning. All you have to do is adjust your operation to take advantage of them. Others, including compaction, are a lot tougher. There are no shortcuts. It’s like an exercise program: If you want it to work, you have to get in there and sweat it out - every day.
Here’s the good news. There are some relatively easy ways to increase overall waste density in your landfill.
The moisture content of garbage has a tremendous effect on density. Wet garbage is much easier to compact and will yield higher densities than will dry garbage. In fact, prior to rules that prohibited the addition of moisture, many landfills sprayed water on the garbage as part of the compaction process. To visualize this, crumple a piece of dry paper into a ball. Next, take a piece of paper, wet it, and crumple it into a ball. The increased compaction seen with this simple demonstration mirrors what occurs in a landfill.
The amount of moisture present in the waste can vary dramatically from one landfill to another. Depending on the type of waste, moisture content can be 20-80%. Similarly, the local climate affects moisture content. I’ve worked on some landfills that received less than 7 in. of rainfall per year and others that received nearly 9 ft. annually. As you might expect, under these varying conditions, the moisture content of the waste and its impact on settlement can cover quite a range.
Moisture content, which can impact day-to-day compaction density, can also affect the long-term settlement that comes from decomposition.
Bacteria want the three ingredients necessary for decomposition: lots of organic material, moisture, and warm temperatures. That’s what your landfill has...more or less.
The problem is that most landfills have a lot of variation in terms of moisture. Poor surface drainage, overuse of daily and intermediate cover, leftover construction materials, and seasonal weather changes make the environment inside your landfill anything but consistent. And so, even though your landfill is decomposing, it’s not necessarily doing so at a consistent or accelerated rate.
Imagine baking a cake on an open fire. It might be burnt on the outside and still raw on the inside. Sure it’s edible, but barely. From bacteria’s perspective, this is what many landfills look like. Yes, there are exceptions. Landfills with sandy soil and abundant rainfall often provide the perfect environment for bacteria. In those cases, decomposition can occur quickly. But for the majority of landfills, the best way to accelerate decomposition is to use less dirt. In many cases, cover soils, particularly soils with low permeability, provide a barrier inside the landfill. This often leads to dry pockets of waste and reduced transport of moisture and gas. The result is slow decomposition.
What’s the best way to decrease soil use? If you needed to reduce the fat in your diet, you’d eat less fat. Similarly, to reduce soil use at your landfill, use less soil. Of course, you can’t eliminate all soil any more than you can stop eating. The goal is to go for the right stuff.
The most effective way to cut back on your intake of daily cover soil is to use more alternative daily cover (ADC). Again, the goal here is to reduce the amount of soil and thereby reduce the occurrence of dry pockets in the landfill.
Under our current "dry tomb" rules, you can’t add water to the landfill. Too bad. Adding moisture to the landfill can give decomposition a big boost and provide you with lots of settlement - fast.
As an example, the Yolo County Landfill near Davis, CA, has been operating a bioreactor test cell for several years. Leachate is reintroduced into the double composite–lined test cell and, as a result, gas production and settlement have accelerated. Based on the county’s survey data, at its deepest point (50 ft. deep), the test cell has settled approximately 6 ft. in 29 months. This is compared to 1 ft. of settlement in a twin (control) cell that has not received leachate.
|Figure 1. Control and Enhanced Cell Settlement|
Depth of Fill
The depth of your landfill has a huge impact on how fast and how much it will settle. On average, taking into account both garbage and cover soil, every vertical foot of filled landfill exerts approximately 0.4 psi of ground pressure. Based on that, a 100-ft.-deep landfill would exert somewhere in the range of 40 psi on the lowest lift of waste. These kinds of pressures are significant in terms of their ability to increase waste density. As a point of reference, consider that a D9R dozer with 22-in. track shoes exerts less than 18 psi of ground pressure.
What this means is that deep landfills have the unique opportunity to gain waste density by the sheer weight of the landfill. No fuss, no muss, and it’s free. Of course, the deeper the landfill and the more time it has to exert its load, the more settlement you’ll see.
With that in mind, you might consider adjusting your closure sequencing plan so that you place all but the final lift of waste and postpone closure for a while to allow gravity to do its work.
Taking this concept one step further, many landfills coordinate the timing and placement of their temporary soil stockpiles with landfill closure. This is called surcharging, and it refers to the placement of an additional load (soil, in this case) on top of the landfill.
Over time, waste will settle from decomposition, physical settlement, and consolidation. This type of "passive" settlement can be accelerated by the strategic placement of soil stockpiles on unclosed areas of the landfill footprint.
The most effective time to place soil stockpiles is when given sections of the landfill have nearly reached final grade but have not been closed or received the placement of final cover. That way, when the stockpiles are removed, you can go back and place more waste, then cap and close the area.
Depending on lots of factors (e.g., moisture, depth, and type of waste) the settlement gained by surcharging can be significant. In my experience I’ve seen a 10-ft. soil stockpile (on an average 50-ft. depth of waste) yield up to 4 ft. of settlement over a one-year period. Increasing the depth of waste, depth of soil stockpile, and/or allowing more time would yield greater settlement, of course.
In many cases, placing soil stockpiles to accelerate landfill settlement is not an added cost. If your landfill’s excavation/filling plan requires some soil to be stockpiled, it might not cost any more to just place it on top of previously filled waste. To take this a step further, purposefully building stockpiles to solely gain settlement can make economic sense. In fact, while such a decision should be based on an economic comparison of the cost of placement versus the value of additional airspace, when you actually run the numbers, you might be surprised to find how favorable they are.
For example, suppose that it costs $10/yd.3 to provide airspace based on the cost to construct a liner, a leachate collection and removal system (LCRS), a final cover, and so on. Conversely, using an example in which a 10-ft.-deep stockpile gains 4 ft. of settlement, here are the numbers: If it costs $1/yd.3 to place the soil and $1/yd.3 to remove it, the soil costs are $2/yd.3 Next, it follows that for every cubic yard of stockpile placed, we gain 0.4 yd.3 of landfill airspace. Remember, though, that it’s not just the volume of soil, but also the depth of soil that impacts settlement.
Dividing the cost to place and remove soil ($2/yd.3) by 0.4 indicates that the "added" airspace costs only $5/yd.3 In this example, airspace gained by surcharging with stockpiles costs half as much as airspace gained by building more liner.
Keep in mind that every landfill is unique in terms of how much settlement a given soil stockpile will provide. Along those lines, you might consider placing a test stockpile on top of the current active-fill footprint. Track your own settlement versus stockpile depth versus time.
If you conduct a settlement test, it is important that the area be closely surveyed before placing the stockpile and after it’s removed. Also, scraper-load counts should be recorded (loads in and loads out) just to ensure that all of the soil is accounted for.
Finally, it’s a good idea to run the idea of surcharge stockpiles by the design engineer prior to placement, just to make sure that the extra loading won’t damage the liner and/or LCRS.
Years Until Closure
When it comes to landfill settlement, time is your friend. Thus, if you can find ways to allow more time for portions of the landfill to settle before they’re closed, you’ll be able to get more settlement and more "free" airspace.
This isn’t to say that you should postpone closure indefinitely. Certainly there are other issues to consider, such as leachate generation, gas control and collection, closure funding, and rules that require you to close portions of the landfill within a certain period of time from when waste hits final grade. But even taking these things into account, there are often practical ways to adjust the fill sequence plans to allow more time for settlement without creating other problems in the process.
Often the key is to stop filling a lift (or even half a lift) shy of final grade. This step ensures that after some settlement occurs there will be adequate depth for placing a normal lift to bring you to final grade.
How much will waste settle? It depends. Depth and moisture content of waste are two of the most important factors. In my experience, I’ve seen landfills that were +100 ft. deep settle 1-2 ft. per year for several years. Again, the best data are based on your own site. You might be able to track settlement by comparing historical topo maps of the landfill. Some landfills install settlement plates and survey them on a regular basis. However you do it, getting a picture of how much and how fast your landfill settles can be very useful.
Eliminate the Supporting Structure
- ADC. Many landfills are saving airspace, saving soil, getting recycling credit, and making money by using chipped woodwaste and greenwaste as ADC.
- Road Surfacing. It might not be the yellow brick road, but some landfill wizards have found that chipped woodwaste and greenwaste work well temporarily for roads. They can absorb large quantities of water and provide a fairly durable base.
- Compost. Chipped woodwaste and greenwaste can become part of your compost mix. The chips are often used as a bulking agent where the compost contains sludge from a wastewater treatment plant or a cannery.
- Landscaping. It can be used around the landfill as a ground cover and a landscape material. You might be able to give it away to homeowners and landscape contractors. You might even be able to sell it. To create a more attractive product, the wood chips can be dyed red, brown, green, etc.
- Fuel. It’s becoming more difficult, but you might still be able to find a market for chipped woodwaste for use as a fuel product. You’ll have to meet certain specs in regard to size, moisture, and so on; however, many landfills are making it work and making it pay.
- Landfilling. If all else fails, you could landfill it. No, this isn’t recycling, but in terms of volume reduction, chipped woodwaste and greenwaste landfilled with incoming waste take up a lot less space than they do in their original forms.
The passive steps to better density are available to most landfills, often for little or no additional cost. In addition, there are a number of active steps that can pump more life (and more airspace) into your landfill.
The most significant gain in long-term effective density will result from an aggressive effort to increase compaction density and reduce the use of cover soil. Other factors may play a part, but in terms of overall waste density in your landfill, these are the Big Momma and Big Daddy.
At most landfills, compaction ranks number one or two in terms of its potential to affect landfill airspace. The following checklist can be used to tune up your landfill’s compaction.
My, What Big Teeth You Have. When it comes to landfill compaction, teeth make a difference. Regardless of the kind of compactor you have, it’s the teeth that do the compacting. For optimal performance, make sure that your compactor’s teeth aren’t worn out. Also, if your site has cohesive soils, keeping the compactor on the trash (and off the dirt) will keep its wheels from plugging.
Flatter Is Better. There’s more than one way to add teeth to your compactor. One of the easiest is to flatten the slope that the compactor works on. Working flat means more tooth penetrations because your compactor will move faster on a flat slope than it will on a steep slope. A flatter slope means more speed, more speed means more wheel revolutions per day, more revolutions means.… Do you get the picture?
Consider the comparison between two identical machines working in similar conditions (see example in Table 1). Both machines are working in MSW and both are equipped with 60-in.-diameter wheels with 48 teeth per wheel.
|Table 1. Compactor Comparisons|
Compactor No. 1
Compactor No. 2
Working Gear Range
Wheel Revolutions (per eight-hour day)
Tooth Penetrations (per eight-hour day)
Obviously there is an incentive to work on flatter slopes, especially in this example where flatter slopes offer the advantage of 620,000 more tooth penetrations per day - free.
There are differing opinions in the landfill industry regarding what slope is best for cell construction. Some people contend that it is best to have the compactor work on a 3:1 slope. And certainly there are situations when flat slopes don’t make sense. My experience, however, has shown that flatter slopes generally are safer, are easier to work, and yield better compaction density.
Bigger Is Better. If you think weight doesn’t matter, you’ve obviously never had a horse step on your foot. In the compaction game, bigger compactors will generally produce greater density. Of course, you can’t ignore other factors, such as waste segregation, thin lifts, and multiple passes. But in an apples-to-apples comparison, heavier machines will achieve higher density. With that in mind, you still need to match the machine to the tonnage.
Match the Machine to the Tonnage. Since bigger compactors get better density, shouldn’t all landfills buy 100,000-lb. compactors? No, no, no. You must match the machine to your site’s tonnage. Every compactor has a certain production rate that is optimum.
For example, a Cat 826 will generally be most efficient and most cost-effective when handling approximately 60 tph. Thus, if your site takes in 900 tpd, you’d need 15 hours per day with an 826 - or 7.5 hours per day with two 826s. At some point it’s better to upgrade to a larger machine. Conversely, if your site receives just 90 tpd of waste, an 826 would have to work only 1.5 hours per day. In that case it’s probably better to downsize to a smaller machine.
It is difficult to define a "typical" refuse density for landfills because of the wide variation in wastestream, equipment types, refuse-cell construction methods, and climates. There are ranges that most machines will achieve, however. Table 2 shows some typical density ranges for various types and sizes of landfill equipment.
Table 2. Typical Density Ranges for Various Landfill Equipment
Density Range (lb./yd.3)*
* Will vary depending on such variables as waste type, wheel design, operator skill, etc.
**Based on achieving the optimum density. At many sites, machines may handle more tonnage, but usually at the expense of wasted airspace.
Note: The data shown in Table 2 are typical but should not be considered accurate for all sites. Also, keep in mind that the practical tons per hour and the resulting density will vary considerably, depending on whether or not the machine doing the compacting must also push garbage from the tipping area to the face and spread it. Along those lines, having the dozer push waste to the cell, thus enabling the compactor to spend more time compacting, would increase either density or the compactor’s practical production (measured in tph) at the same density.
Cover Soil Reductions in cover-soil use represent the least expensive (and potentially most dramatic) opportunities to increase the overall waste density at most landfills. There are two steps to reducing cover-soil use. The first is related to cell geometry. The second is to actually reduce the amount of soil used-primarily through the use of ADC.
Cell Geometry. Cover-soil reduction begins with an understanding of cell geometry. Obviously, a wide variety of cell shapes could be created with a given quantity of refuse. However, there would also be a wide variation in the resulting surface areas (requiring cover material) of the various cells. As you might expect, there is a cell geometry that yields the minimum surface area for a given quantity of refuse. To find the optimum geometry, start by defining how wide the daily cell should be. The cell width is typically based on the unloading space needed to accommodate trash trucks and self-haul vehicles.
Once the width is set, various combinations of cell depth and advance are available. Thus, it is a simple matter to choose the depth, based on minimizing the surface area of the cell. The optimum geometry, which varies depending on the quantity of refuse, is usually boiled down to cell depth.
|Figure 2. Soil Requirement Based on Varying Cell Depths|
Thus, for a given landfill, the optimum cell depth might be 13 ft. (see Figure 2). If ADC is used on the face in lieu of soil, however, our perspective changes. Instead of seeking to minimize the overall surface area of the cell, our goal is to minimize the surface area on the top and side of the cell-those areas that require soil cover. But because we're using ADC on the face, we're not concerned with its size. That's why the optimum cell is much deeper when you use ADC. Figure 2 shows that with ADC, deeper is better.
Use Less Soil. Do you want to make more money? Then use less soil. If your landfill is typical, 25-33% of its available airspace will be filled with soil. In other words, if you kept out the dirt, your landfill would last 25-33% longer. Stated another way, you could reduce your liner costs by 25-33% by eliminating soil. Good waste compaction and proper cell construction can reduce the amount of soil used. So too can a cautious (even stingy) equipment operator. But if you really want to lower the boom on excessive soil use, start using an ADC. ADC comes in many forms; the number of materials continues to grow. The main benefit of ADC is that it keeps dirt out of the landfill. In the landfill business, whatever saves space makes sense.
Tracking Your Progress: Measure the Effective Density
All of the issues related to overall waste density and airspace, including settlement, decomposition, compaction, and ADC, can be boiled down and described in two words: effective density. And the ultimate effective density of every landfill will be based to a great extent on how management deals with both the passive and active issues discussed earlier.
Landfills often measure their performance by stating the waste density they achieve. For example, the industry standard is 1,200 lb./yd.3 But in order to determine how much waste a landfill can receive, one must also consider cover soil, settlement, construction material, ADC, and other factors that affect how landfill airspace is used. When all is said and done, a cubic yard of airspace in a landfill will actually contain (on average) considerably less than the 1,200 lb. of waste listed in this example. Waste and its related waste density are only part of the equation.
But effective density relates to the all-inclusive waste density of a landfill. It reflects the site's overall use of airspace by taking into account all materials that go into the landfill. In terms of effective density, your landfill might actually be placing (on average) 900 lb. of waste into each cubic yard of landfill airspace. In this regard, effective density gives a truer picture of how your landfill is utilizing its airspace.
One of the difficulties with effective density is that the individual values of the ingredients that comprise it (e.g., waste density, cover ratio, settlement, construction materials, and ADC) might not be known. Thus, while it's easy to calculate the effective density, by flying the site, then checking the total airspace volume used and comparing it to the incoming tonnage, it's not clear how well the landfill is doing in each area.
As an example, consider a landfill that is currently operating with an effective density of approximately 1,000 lb./yd.3 Is it doing a good job or not? It's hard to say, for as can be seen in Table 3, this value could represent various combinations of waste density and cover-soil ratio.
For example, an effective density of 1,000 lb./yd.3 could result from:
- a waste density of 1,500 lb./yd.3with a cover ratio of 2:1,
- a waste density of 1,400 lb./yd.3 with a cover ratio of 2.5:1,
- a waste density of 1,250 lb./yd.3 with a cover ratio of 4:1, or
- a waste density of 1,100 lb./yd.3 with a cover ratio of 10:1.
A reasonably good, effective density doesn’t necessarily mean that its components are good. But by measuring one of them - waste density or cover-soil use - the other can be determined.
So if effective density is a stew made up of lots of different ingredients, how can we determine which parts need improvement? The answer is found by collecting as much information as possible. For example, if you conducted an onsite density test and determined the waste-only density, the cover-soil ratio would become obvious. Or if you accurately determine cover-soil use, waste density would fall out (of Table 3).
Also, by tracking effective density over time, you can begin to get a handle on how some of the passive factors (e.g., settlement) play into your use of airspace.
Getting the most out of your landfill’s airspace is a complex chore. But by addressing the factors one at a time, you can squeeze a lot more waste into your landfill.
Author's Bio: Neal Bolton is a consultant specializing in landfill operations and management.