Airspace Management Is Resource Management

Among the many things landfill managers do, managing airspace is a top priority-after all, it is a landfill’s primary resource. To do it well, this singular task requires expertise in such different disciplines as surveying, finance, civil engineering, regulatory compliance, machine operation, recycling, and an understanding of the biological, chemical, and mechanical processes of decomposition.

And yet no manager-regardless of his or her talent-can do all of these things without help. So the best managers know how to train, delegate, and monitor these activities, bringing together a team that can make it all work.

A landfill’s performance in regard to airspace utilization can be monitored by periodically quantifying how much airspace is being consumed in relation to the inbound waste tonnage. For example, a landfill that receives 100,000 tons of waste per year and consumes 180,000 cubic yards of airspace (total, including trash, cover soil, etc.) has an airspace utilization factor (AUF) of 0.56 tons (of waste) per cubic yard (of airspace). While this is equivalent to an effective density of 1,111 pounds per cubic yard, by tradition, it is more commonly expressed as an AUF of 0.56.

When it comes to tracking landfill performance, AUF is very important. It is an all-in, overall airspace factor, reflecting the airspace consumption rate, among other things. Most often, AUF is calculated on an annual basis, although some landfills, typically larger ones, will calculate AUF more frequently.

The formula for calculating AUF is a simple one. It is simply the total tons of waste received divided by the total volume of landfill airspace consumed during the same time period. For example, if your landfill received 50,000 tons of waste and during that same time period consumed 100,000 cubic yards of landfill airspace, then your AUF is .5 tons per cubic yard, or an AUF of .5.

In the United States, AUF is most commonly expressed in these units, tons per cubic yard. AUF can also be expressed in other units, however, such as pounds per cubic yard, kilograms per cubic meter, etc. The key here is that individual landfills simply be consistent in how they express AUF to avoid confusion. AUF is an important measure of landfill performance.

Tracking AUF
We recently surveyed a number of landfills and found some interesting trends. The first was that 75% of the landfills responding to our survey check their AUF on a quarterly or annual basis. This is by far the most typical scenario. But some landfills-17.5%-check their AUF every few years…or not at all.

If this sounds extreme, consider that the remaining 7.5% check their AUF every day-using on-board GPS.

This leads us to the next question on our survey: How does your landfill team monitor grade for surveying? Based on our survey, it appears that the eyes have it. In our survey, 47.2% of our respondents indicated that Operator’s Eyeball is the most common method of grade control. In my experience, this is true-and it represents a reasonable method of grade control.

There are many skilled operators who can do a great job of maintaining grade-simply because they have a good eye. Operators who can make grade by eye are very valuable to the landfill team. But no matter how skilled an operator is, there are benefits to having a more precise form of making grade.

The results of our survey indicate that 30% of the responding landfills use ground-based survey (e.g., laser, transit, or total station) for maintaining grade on a daily basis. Economical yet effective, ground-based surveying is tied for second place with GPS. Our survey split out GPS into two categories: backpack GPS-where an individual surveyor is able to check grade and set stakes using a GPS system-and onboard GPS.

Individual (Backpack) GPS
GPS surveying units are rapidly replacing traditional ground-based surveying methods because of ease of use …and the fact that a single person can effectively use GPS, whereas typical ground-based systems require two people.

Machine-Mounted GPS
Onboard GPS, while not yet standard equipment, is becoming a much more common option. Using very accurate GPS, the machine operators can closely monitor the effort put into every area across the active cell, enabling them to achieve optimal compaction density. Information regarding the desired grade and shape of each day’s cell can be transmitted to an onboard display on the machine…and the machine’s position is sent back to the in-office computer database. This provides real-time survey control for the operator-and a continuously updated topographic map of each day’s cell. You may have guessed that this allows for a daily calculation of AUF.

Are there real benefits to these systems? It appears so. The results of our survey indicate that landfills with onboard GPS typically achieve a higher AUF than those that rely solely on the operator’s eye. This does not take away the importance of having a good eye, but does show the importance of using every available resource to improve the operation.

So, does the use of onboard GPS increase density? No, not in a literal sense, but when used properly, it can provide real-time feedback on performance-a very important part of process improvement.

Every landfill should be tracking its AUF in one form or another. AUF can show overall performance from year to year and it can identify to some degree, trends in how your airspace consumption rate may be changing. But AUF has limitations. One of those limitations is its inability to provide detail. Consider a landfill that achieves an AUF of 0.50. This would indicate that the landfill consumes 100,000 cubic yards of airspace for every 50,000 tons of waste landfilled.

But what it does not show is how that 100,000 cubic yards of airspace was consumed. For example, that AUF of 0.6 could indicate a (poor) waste density of 1,100 pounds per cubic yard, and a (great) cover ratio of 10:1. Conversely it could indicate a (great) waste density of 1,500 pounds per cubic yard and a (poor) cover ratio of 2:1. AUF is an excellent benchmark for tracking overall performance, but it does little to identify specifics of operational performance.

For tracking performance, AUF is fine. But to improve performance you must dig deeper. You must examine the performance of individual machines as they perform specific tasks. We’ll start by looking at landfill compaction equipment.

Compaction Equipment
Does is seem odd that we’d use high-tech GPS technology simply to monitor the performance of a compactor rolling back and forth across a layer of trash? If it does, perhaps you aren’t aware of the science that also goes onto the nuts and bolts process of waste compaction.

In our survey, we also solicited information regarding the use of landfill compactors.

First, as you might expect, the number of compactor hours (i.e., machine operating hours) increases as the inbound tonnage increases, a logical finding.

But when we looked a bit further, we also found that the AUF tends to increase-slightly-as the daily tonnage increases. This may be a result of larger landfills typically having larger compactors…or perhaps may include some settlement, since larger landfills are also typically deeper than smaller landfills. But regardless of the factors, the results indicate that the more tonnage a landfill receives, the higher the AUF.

Yes, managing airspace is much more technical than just having a landfill compactor run back and forth across the waste. But that doesn’t mean the process of operating a landfill compactor is not technical too. Here is a little something extra to think about next time you’re watching your compactor do its thing. This has to do with flow rate through a system-in this case, the waste-handling system.

Theory of Constraints
The theory of constraints (TOC) is a management tool based on the assumption that all processes are limited by one or more constraints in the system. In the context of the waste handling process, the constraint (i.e., bottleneck) is typically the individual step of compacting waste with the compactor.

If we were to look at the waste-handling process as simply a pipeline, it would throughput (i.e., flow) varying from one point to another. This throughput could be measured in tons per hour. Please note that the flows shown in Figure 1-while typical for some landfills-are intended as examples only. Also, this example is based on a single bulldozer and a single compactor, but in a real analysis, multiple machine configurations would be considered.

Offsite access roads (2,000 tph)-The rate that inbound waste could flow to the landfill is typically quite high, limited most often by traffic congestion. In this example, we’ll assume that the potential inbound flow of waste at this point could be two thousand tons per hour.

Entrance/scale (500 tph)-The entrance facility, including the scale, will have a maximum flow potential, based on many factors, including: the number of scales, whether or not the system is automated (i.e., RFID), the payload of inbound vehicles (transfer trucks will allow a higher flow rate), number of lanes, the hours of operation, and various other factors. We’ll set the potential flow rate for this stage at 500 tons per hour.

Onsite access roads (1,000 tph)-After passing through the entrance/scale facility, waste vehicles must travel the onsite access roads to the active tipping area. The flow rate along the access roads is most significantly affected by the condition of the roads. Steep grades, sharp turns, and icy or muddy conditions are the most common factors limiting flow along the onsite access roads. Let’s assume that the onsite roads are properly designed and maintained, and that flow is fairly high: approximately 1,000 tons per hour.

Unloading at Tipping Pad (240 tph)-As waste vehicles reach the tipping pad, they reach the first obvious constraint. Depending on the dimensions (mostly the width) of the tipping pad, only a limited number of vehicles can unload at a given time. In this example, we’ll make a few assumptions to determine the flow rate:

* Width of tipping pad: 160 feet
* Width required for each vehicle: 20 feet
* Number of vehicles able to dump at one time: 8
* Average unload time, including cleanout: 10 minutes
* Average payload of each vehicle: 5 tons

Based on these parameters, each slot can allow six dumps per hour at 5 tons per dump. Thus, each slot can process 30 tons per hour. With eight slots, the unloading flow at the tipping pad is 240 tons per hour.

Pushing and Spreading (420 tph)-Once waste has been dumped at the tipping pad, it is pushed, usually by a bulldozer, to the active face, where it is spread in a thin layer. In most cases, the production rate of the bulldozer exceeds the flow capacity of the tipping pad. Our studies of landfill production rates indicate that a dozer’s typical cycle time (push, spread, return) is around one minute. If we assume that a D8 size bulldozer will, under normal conditions, be able to push an average of 7 tons per push, then the dozer could potentially push 7 tons per minute x 60 minutes …or 420 tons per hour. This exceeds the rate that waste is being dumped on the pad. Thus at this point, the bulldozer is not the limiting constraint, because it can process up to 420 tph.

Compacting (150 tph)-A landfill compactor, depending on the size, can effectively process between 60 and 200 tons per hour. In this situation, we’ll assume the landfill has a fairly large compactor (greater than 100,000 pounds) and that the optimum production rate is 150 tons per hour.

This optimum rate is that which balances the cost of operating the machine versus the savings (in airspace $) gained by achieving good compaction.

This stage of the waste-handling process represents the most limiting constraint, with a production rate lower than all other stages.

Covering (1,000 tph)-Once the waste has been properly graded and compacted, it is covered with soil or some type of alternative daily cover. In this example, we’ll assume that the cell is covered with soil. In terms of potential throughput, the covering process has a high production rate. In this example, we’ll assume that the onsite equipment could transport/spread enough soil to cover a cell containing 8,000 tons of waste (in an 8-hour shift)…thus accommodating a waste flow rate of 1,000 tons per hour.

In a condensed format, this perspective of the waste-handling process shows that the limiting constraint occurs at step five-compacting, where the rate is limited to 150 tons per hour.

What does this mean? Well, let’s first step back and look at the overall system. In order to streamline landfill operations, many managers will look for improvement in all areas. This means improving the production rate of the scale attendant, the scraper operator, the equipment maintenance crew …and a host of other activities. This is fine, and will result in some efficiency improvement and cost savings, but it may not necessarily improve the overall waste-handling process.

However, the theory of constraints acknowledges that the flow through any system will be limited by one or more constraints. And by increasing the flow rate at those points, the overall flow rate will increase. In our example, the primary constraint is the compactor.

With that in mind, developing a massive entrance facility, or purchasing oversize bulldozers makes little sense-because these are not the constraints. Improving the production rate in those areas could be as fruitless as increasing an 18-inch diameter pipe to 24 inches…when downstream everything still has to flow through a 4-inch pipe.

Focus on the constraints in order to improve the overall system. In our example, the most dramatic improvement would occur by increasing the production rate of the compactor. A short list of ways to increase compactor production includes working flat, spreading thin, covering breaks and lunch, increasing length of run, and using a well-maintained machine with aggressive teeth.

If you’ve been in the waste industry for any time at all, you know that these concepts are common knowledge. However, after spending 30-plus years evaluating the efficiency and effectiveness of landfill operations, I can testify that they are not universally applied.

Even though our discussion on the theory of constraints was only an example, it was a realistic example. At most landfills, the compactor is the limiting constraint-and increasing the productivity of the compactor will do more to increase overall efficiency than any other single task.

It is for that reason that those who manufacture landfill compactors put so much effort into building effective, dependable machines. When it comes to improving landfill operations, compaction is king.

The machine is working flat, as opposed to working on a slope. This will result in decreased machine stress, less operator fatigue, and, most significantly, more tooth penetrations into the waste.

Because of the wheel cleaners, this machine’s teeth are clean and effective. Bomag’s wheel cleaner system ensures that the teeth actually function as teeth, even in wet, clayey conditions that could cause plugging on machines that don’t have cleaners.

Of important note is the number of teeth. You’ll note that the waste is well compacted and the compactor wheels are riding well up on the surface of the trash. But even so, the teeth are achieving full penetration. This is typical of all compactors-the ability to achieve full tooth penetration. Along that line, to achieve maximum compaction, this machine is providing big teeth, lots of teeth…and they are clean and available to do their job.

Heavy Equipment Maintenance/Service
In line with the theory of constraints, and our focus to increase throughput at that point, it follows that top priority be given to keeping the compactor working efficiently on a full-time basis. In addition to adjusting your crew’s schedule so that the compactor isn’t parked during breaks and lunch, basic machine maintenance is also vital.

A good maintenance program begins with a comprehensive walk-around inspection. Your crew should have an accurate pre-op checklist of items to check on every machine-including the compactor. This checklist should include all safety components such as backup alarms, brakes and rearview cameras. It must also include all of the daily checkpoints recommended by the manufacturer. Toss the checklist you borrowed from the road department. You know…the one that asks heavy equipment operators to inspect the air brakes, trailer lights, and turn signals. Create a checklist that applies to your machines.

Initiate a program to ensure that all servicing is performed at the appropriate time. If your operators share some of the maintenance duties with the mechanic, take time to train them to do it right.

Team Training
OK, we’ve discussed GPS and the theory of constraints, both topics being a bit more technical than one would expect for achieving our ultimate goal of packing more trash into less space. Yes, technical but important. But on a perhaps more practical note, this technical information must be understood-and accepted-by the landfill crew. The heavy equipment operators play a vital role in the successful management of the management of airspace. This is worth saying again: Theirs is a vital role. If the operators aren’t onboard, all the GPS and process theory in the world won’t matter.

So provide training for the crew. Explain the value of airspace. Put it in terms everyone can understand. It may be a diminishing asset to the accountants, but for your landfill crew, it’s a cubic yard of airspace that’s worth $5 or $10 or $20 if we sell it…and nothing if it’s carelessly wasted.

And then explain how their decisions and actions control how well airspace is utilized.

In the landfill industry airspace is it. It’s what we manage. It’s what we sell.