Woodwaste and Greenwaste Processing
You’ve got to know your costs.
Processing woodwaste and greenwaste has become a common landfill activity, one that varies in complexity depending on whether the end product is ADC, mulch, compost, or (biomass) fuel.
Producing each of these requires that one follow certain steps, but the number, type, and sequence of those steps can vary widely based on the feedstock and end product.
However, even though feedstock, equipment, and product specifications can vary widely, in the end these all boil down to classic textbook examples of material handling. And, in that regard, the reduction of steps generally equates to less cost and faster throughput.
The study of material flow, like other disciplines, has its own language. And terms such as lean flow, value stream, and inventory may be important for one wanting a thorough understanding of the science of material handling. Yes, efficient material handling is a science—but it’s not rocket science.
All of the terms and intricacies of material management can be boiled down to two common goals: efficient throughput and minimal cost.
Let’s start by looking at a typical operation where a landfill is diverting greenwaste and woodwaste, processing it for compost and alternative daily cover (ADC). Here are the steps for this example:
- Waste vehicles, including those carrying woodwaste, arrive at the tipping pad.
- If the spotter is able to identify vehicles with woodwaste, they will be directed to dump at the edge of the tipping pad.
- Loads that are not identified by the spotter and end up dumped at the active face are picked up by a loader with a grapple bucket and moved to the edge of the tipping pad.
- Periodically, throughout the day, using a loader and truck, an operator will load and transport the woodwaste to the wood processing area located near the back corner of the landfill property.
- In order to maintain adequate workspace, at the end of the day these loads are pushed into a large stockpile of unprocessed material—the U-stockpile.
- Approximately once per week, another loader begins removing woodwaste from the stockpile and feeding it into a grinder.
- As the processed material comes off the conveyor, the loader moves the material into a series of windrows.
- Then, over the next three to four weeks, the windrows are turned and watered as the material goes through the composting process.
- Once this is completed, the windrowed material goes through a screening process. This may be done with a trommel or a shaker screen.
- During the screening process, the material is separated into two stockpiles, with the finer compost material going into the C-stockpile and the oversize material going into the O-stockpile.
- Material in the C-stockpile is left to cure for a while, and then eventually loaded into trucks and shipped offsite as compost.
- Material in the O-stockpile is loaded into trucks and hauled to the active landfill face, where it is dumped and spread as ADC.
If you’ve been keeping count, these materials (compost and ADC) have been handled eight to 10 times…not counting the windrow turning. At some facilities, the process can be even more complicated if woodwaste is separated and processed for (biomass) fuel or mulch. Similarly, some presorting may be required to remove trash that would damage the grinder or end up contaminating the final product(s).
And of course, every time those materials are pushed, loaded, hauled, processed, dumped, mixed, or spread, there is a cost. The landfill manager’s job is to evaluate the process and make sure that the steps and associated costs represent the best overall use of resources. It is essentially a cost/benefit analysis of the process.
Surprisingly, these processes can evolve, one step at a time, from a good idea (e.g., using greenwaste as ADC) into a complex and very costly chain of activities that takes on a life of its own. Along the way those incremental handling costs accumulate, but because they are gradually absorbed into the landfill budget, managers often have no idea of their true cost.
For the manager who wants to get a handle on those handling costs, he/she must do two things:
- Determine the incremental costs
- Affirm that the process fits into the overall waste management hierarchy for the landfill
It’s like captaining a large ship. Step 1 is to make sure that all of the components such as engine, propeller, and sonar are all working at maximum efficiency. Step 2 is to affirm that the ship is pointed in the right direction. Why steam toward Panama at maximum efficiency if the ship’s destination is Hawaii?
Here’s an example of how to address Steps 1 and 2.
Determine Incremental Costs
This refers to the cost of each step in the process. Normally, this is expressed in a cost per ton or cost per cubic yard. Either can work, just be consistent to avoid confusion. In this example, we’ll work with cost per ton. Keep in mind that there are various paths you can follow to get to a unit cost. This is just one example.
At every point where the material is handled, you’ll need to determine the hourly cost. In this example, we’ll evaluate one of the many steps: grinding. At the grinding stage, you’ll need to know the cost of the operator, the loader, and the grinder. Your payroll department should have information on the cost of the operator. Remember: The operator cost should include all taxes, insurance, benefits, etc. What you want is the loaded or “all-in” cost.
Your finance or fleet services department may have information on machine cost. If not, try contacting the machine manufacturer…or calculate your own costs by using a format similar to what’s shown in the Caterpillar Performance Handbook (see the section on Owning and Operating Costs). You can purchase one of these from you local Cat dealer.
OK, let’s say you come up with the following:
Loader: $76 per hour
Grinder: $122 per hour
Operator: $38 per hour
Thus, the total hourly cost of grinding material is $236 per hour.
Next, you must determine the production rate. Let’s assume it takes three-and-a-half hours to process a pile of material that you estimate contains 154 tons of material. You may have inbound scale records…or simply have to come up with an estimate based on truckloads.
In any event, this works out to a production rate of 44 tons per hour. Remember, this is just an example.
With a cost of $236 per hour, and a production rate of 44 tons per hour, this single step—grinding—costs $5.36 per ton.
This same type of calculation should be performed for every step of the process. When combined, you’ll have a very clear picture of what the incremental and total costs are.
Review Overall Plan
Once you’ve identified the processing costs, you can step back and evaluate the big picture. Do you need to compost the material? Is it cost-effective? Is it required by regulation? Is it simply a political or public perception activity? Does your landfill need ADC? If so, how much does it cost versus how much does it save? These are vital questions—and they are your questions.
Your landfill will have its own unique answers. Determining the “why” in regard to woodwaste or greenwaste processing—compared with the associated cost—can provide you with fresh insight into whether or not your ship is headed in the right direction.
After coming up with an accurate picture of the processing costs, we’ve seen landfills shift from producing (biomass) fuel to ADC. Others have replaced their compost operation with one that produces mulch. Some have found that landfilling was the overall lowest cost option and have dropped wood processing altogether.
Sometimes these types of changes are obvious and simple. Other times they can be very political and controversial.
Either way, when it comes to processing greenwaste and woodwaste, one thing’s for sure: If you don’t know your costs, it’s impossible to manage effectively…and you just might end up in Panama.
Author's Bio: Neal Bolton is a consultant specializing in landfill operations and management.
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