How to calculate airspace utilization factor for a landfill cell

Airspace utilization factor shows up in every capacity report, but half the sites I've worked with calculate it differently and never notice until the permit renewal meeting. Here's the version that ties back to your survey data.

What AUF is measuring

Airspace utilization factor (AUF) is the ratio of waste tonnage accepted to the volume of airspace it took to put that waste in the ground. It's usually expressed as pounds per cubic yard, sometimes tons per cubic yard if you're working in bigger numbers. The permit's original capacity projection was built on an assumed AUF, often somewhere in the 1,200 to 1,500 lb/cy range depending on waste stream and compaction equipment. Your actual AUF is what you get once real tonnage meets real survey volume.

The two don't have to match. If your actual AUF comes in below the design figure, you're burning airspace faster than the permit math assumed, and the remaining-life number on your capacity report is optimistic. If it comes in above, you've got more runway than the original design gave you credit for.

The formula and where the numbers come from

AUF = (tons of waste accepted x 2,000) / cubic yards of airspace consumed

Tons comes off the scale house, usually totaled over the same period as your survey interval. Airspace consumed is the volume difference between two topographic surfaces of the cell, typically your current month's drone or aircraft survey against last month's.

This is the part that trips people up: airspace consumed isn't just waste volume. It includes daily cover, intermediate cover, any settlement that happened in between, and whatever subsidence occurred on waste placed in prior months. If you're comparing your AUF to the permit's design assumption, check which convention the original engineering report used, gross airspace (waste plus cover) or waste-only. Comparing a gross AUF to a waste-only design figure will make your utilization look worse than it is, and the reverse makes it look better than it is. Pick one convention and stick with it every month so the trend line means something.

In-place waste density is a related but narrower number, tons of waste placed divided by cubic yards of waste volume only, with cover soil backed out. It tells you how well your compactor is doing its job. AUF tracks how your permit's airspace budget is being spent, cover and all. A QA/QC team pulling a core sample for density is checking compaction. AUF is checking permit consumption, a different question with a different answer most months.

A worked example

Say your scale house logged 42,000 tons in September, and your survey shows 58,000 cubic yards of airspace consumed over the same cell during that period.

AUF = (42,000 x 2,000) / 58,000 = 1,448 lb/cy

If your permit's design AUF was 1,350 lb/cy, you're running ahead of plan, using less airspace per ton than projected. If the design figure was 1,500 lb/cy, you're consuming airspace faster than the permit assumed, and worth a line in your next capacity update before the regulator asks why the remaining-life projection moved.

Run this every survey cycle, not once a year. A single month's AUF can swing on weather, waste mix, or a placement area that got extra daily cover because of odor complaints. The trend across six or twelve cycles, tracked against the same cell boundary each time, is the number that actually tells you something.

Getting the volume side right

The tonnage half of this equation is easy, it's sitting in your scale house software already. The airspace half depends entirely on getting two comparable surveys, same cell footprint, same datum, close enough in time that settlement and new fill don't get tangled together. That's the part a monthly flight handles for you. Landfill Volume runs that surface comparison every month and hands you the consumed volume figure so the AUF math above takes five minutes instead of a half day in CAD.

If you're ready to stop reconstructing survey volume by hand each cycle, take a look at what a monthly photogrammetric surface gets you.