The Landfill Manager’s Guide to the Airspace Balance Equation

For the savvy landfill manager, settlement is like money in the bank … only better.

Here is a real-life example: A municipal landfill—one of our long-time clients—had listed a new liner in next year’s budget. The amount? Three million dollars.

We discovered this while conducting a comprehensive operational review—something we call a “CORE” Assessment—of this facility. We had been looking for ways to increase efficiency, improve safety, and reduce cost, and a $3-million line item was something that caught our eye.

“Yes,” they affirmed, “we are out of space and need to line the next phase next year.”

Over the years, we’ve learned to be skeptical; so, we started asking questions:

  1. When was the perimeter slope constructed?
  2. What was the design slope of the final grade?
  3. How much remaining airspace capacity does the landfill actually have?
  4. Where is that airspace located?
  5. How much will it cost to access it?
  6. How long can we stay on the existing footprint…before more liner is needed?

That was nine years ago. And this year, finally, that $3 million liner project is back on next year’s budget. This time, they really are out of space and do need to line the next phase.

In the intervening nine years, not only did they gain a lot of additional capacity—with the increased gate revenue it generated—but also, they were able to postpone that $3 million cost. When we calculated the savings, using a cost of (borrowing) money of 5% per year, we found that the landfill had saved approximately $1.4 million.

But this story really began nearly 40 years ago…when the first load of trash was dumped at this landfill and the organics began decomposing. This is a fairly old landfill; it is deep, and it is wet. This all adds up to a lot of settlement, which, of course, is airspace.

We were able to utilize that available airspace by refilling the perimeter slopes, which—over time—had settled.

So, what’s the deal? Does the typical landfill have massive airspace resources just sitting around, while the management team is focused on new liner projects? In our experience, many do … too many.

This comes from not thinking about the airspace balance equation.

“What,” you ask, “is the airspace balance equation?” It’s nothing technical—just some common sense arithmetic. It is simply:

AirI – AirF + AirR
AirI = Initial Airspace (cy)
AirF = Airspace Filled (cy)
S = Settlement (cy)
AirR = Remaining Airspace (cy)

It’s a simple concept, but like the frog in hot water, it often goes unnoticed—because settlement occurs gradually. You may not be consciously aware of it, but right now your landfill is working hard to create more airspace at all times.

According to EPA estimates, 1 million tons of MSW in-place will generate approximately 432,000 cubic feet of landfill gas (LFG) every day (during the active decomposition period).

Over the entire decomposition process, 1 ton of MSW will (eventually) produce nearly 500 pounds of LFG. Of course, the actual quantity and rate at which this conversion occurs depends on many factors, such as percent organics, moisture, time, etc. But what this generally means is that somewhere around 25% of the solid waste (tonnage) that goes into your landfill will come back out as LFG (tonnage).

This doesn’t mean that your landfill will settle a full 25% of its original depth, because there is some internal structure that stays in place—even as the organics convert from solid to gas. But some settlement does occur. This unrecognized settlement is what provided the additional nine years of capacity in our example.

So far we’ve talked about passive settlement that happens as a result of decomposition. But additional settlement can be gained by adding moisture or by surcharging.

An increasing number of landfills are recirculating leachate. This can defer leachate disposal costs, but will also accelerate the rate of decomposition—especially at dry-climate landfills.

Surcharging is the process of putting additional weight on the landfill. Often, this is done with soil stockpiles. Generally, the heavier the stockpile and the longer it stays in-place, the greater the resulting settlement. Settlement can also be enhanced by placing your green/wood waste operation on top of the landfill. Weight is weight, and loading the top of your landfill will produce some settlement.

Most of the effort that goes into generating and conserving landfill airspace is focused on landfill expansions and landfill compactors—both very important. But the landfill in this example saved $1.5 million and gained an additional nine years of capacity by taking advantage of something that was free.

You may be wondering, “Does my landfill have the same potential?” There’s a good chance it does. In our experience, hidden airspace as a result of settlement is not that hard to find…if someone looks for it. MSW_bug_web


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