Recycling Garbage with High-Powered Drives

As environmental concerns continue to mount, engineers are tasked with developing innovative solutions to reduce landfill space. Some engineers have been successful in reducing use of landfill space by up to 90 percent and, in the process, combating pathogens in the trash and automatically retrieving ferrous and non-ferrous metals for recycling.

One such example of this landfill innovation can be found on the island of Aruba, where landfill space is at an absolute premium. There, an innovative process and complex control system is being used to convert unsorted household garbage into a safe, inert material called Fluff, which is similar to cellulose insulation used in the attic of a house.

"On an island, solutions that may work on the mainland are not always workable," says Mark Brown, president of WastAway. "If you are on the continental U.S. and have a unique technology that fails, you can truck the refuse to another landfill until the problem is resolved. But on an island, there is no bypass capability and there is a need for a facility that runs reliably and can handle all of the garbage that they produce."

Working in conjunction with the U.S. Army Corp. of Engineers in the 1990s to process waste from military bases, WastAway developed a technology that could fully process garbage loaded into the system in 20 minutes. The resulting "Fluff" can then be used as a peat moss replacement in agricultural applications, as a fuel source to make electricity from steam or as a material component in various building products.

Fluff is a valuable commodity in Aruba, because topsoil is a resource in short supply. The island currently has a large, uncovered landfill which is the target for the initial use of Fluff - to cover the landfill. Eventually, vegetation will be planted in the Fluff to improve the appearance of the area.

"The current facility on Aruba employs three WastAway lines designed to process 50 percent of the island's waste. Since the lines have been installed, Aruban sanitation officials have learned that humans on the island may not be generating quite as much waste as originally thought," says Brown. "As a result, phase two of the project will increase the size of the facility, with the goal now being the ability to process all of the waste on the island."

Powering the WastAway Process

As advanced as the WastAway system is, at its core are time-proven technologies such grinders, shredders, belts with magnets and Eddy currents to remove ferrous and non-ferrous materials from the garbage input into the system. Also central to the system is a hydrolyzer, which is similar to an autoclave that would be used to sterilize surgical instruments.  

"The hydrolyzer in the WastAway system is unique because it operates in a continuous flow mode rather than in a batch process. Therefore, material can constantly move under pressure through the hydrolyzer," says Brown. "Material moves in and out of the high-pressure steam environment without the need to constantly pressurize and de-pressurize the vessel."

The WastAway system uses a series of operations, from cell to cell, that gradually breaks down the garbage. In essence, it functions as an automated processing line with the garbage loaded into a hopper on the front end. From that point, the garbage is automatically processed without human intervention. The shredders, grinders and equipment used to process the garbage are PLC-controlled; variable speed drives are used on conveyor belts to move the garbage through the system.

"Mitsubishi Electric's Q series PLC controls the process, conveyors and all of the I/O throughout all three lines at the plant in Aruba," says Kurtis Ullein of Mitsubishi Electric Automation Inc. "All of the drives in the system, from 300 hp down to 3 hp, are controlled by a CC-Link industrial network connected to the PLCs.

Two Q series PLCs and two Q Series Safety PLCs are used per line, which provide safety monitoring for the conveyors, hoppers, door switches and grinders. All safety operations are achieved using CC-Link Safety and remote I/O safety blocks. Another CC-Link: IE network communicates between the Q-PLC and the QS Safety PLCs to coordinate both functions. Ethernet connectivity in the PLCs and CC Link: IE network enables remote monitoring and the ability to make programming changes over the Internet.

Variable frequency drives were selected for use in the project because of the shredders' and grinders' high power requirements - which can run up to 300 hp. The need to control the speed of the conveyors was also a determining factor. For example, if the garbage input starts to back up at a downstream hopper, the VFDs provide a way to automatically slow down the line upstream to balance the capacity of the system and prevent overloads in any single area.

Design Challenges

"Both the scope and timing of the project, due to design delays and contracting issues on the island of Aruba, provided the major challenge," according to Brown. "Once the team started on the project, we had to be operational within seven months from quotation to a fully operational facility including construction of the building. The schedule only allowed eight weeks to deliver the complete control system and electrical panels." Within this eight-week period, only five of those weeks were allotted to commissioning and making the system fully operational after getting live power in the facility.

Another difficulty was designing and fabricating the control panels according to Netherland-Antilles electrical specifications and delivering the system in eight weeks for shipment. The complete system has more than 30 electrical panels and enclosures.

"Part of the challenge we encountered was creating a flexible control system that allowed us to tune the process as we rolled out the system," says Mike King, owner and president of Motion Control Express, the integrator that helped develop the system on Aruba.  "We couldn't optimize the components ahead of time, so we needed the capability to make adjustments as part of rolling the plant out."

With the lines spread out over a large area in a 42,000 sq. ft. building, diagnostic tools need to identify if a section, a shredder or Eddy current drive, for example, has a failure. The system communicates status through the plant using the HMIs, and also provides automatic shutdown to keep garbage from piling up. The design goal was to be able to direct maintenance personnel within one foot of any diagnostic issue, and pinpoint specific solutions. To do this, the control system dynamically informs the plant operators what the system needs in order to complete the task. This approach to maintenance compresses the training requirements for new operators and also helps point to potential issues before a fault occurs.

Starting with Simulation

WastAway engineers used SolidWorks to develop a model of the complete Fluff control system as part of the design process. These models were then imported into PCs as bitmap files and animated to compress the timing of the software development.

"Due to the tight timeframe, we wanted to look on the computer screen and watch the system cycle properly before moving onsite," says King. "We were able to simulate the process that would appear on the HMI screen using a simulator from Mitsubishi and GT SoftGOT products (software visualization tools)."

SolidWorks was used to verify the dimensions and develop the system's complex process model. Motion Control Express, working as the project's software developer, used this input to replicate the mechanical system inside the Mitsubishi HMI and animate it to watch the different gates move and verify the timing of the signals to ensure proper operation before arriving at the plant location.

When bringing the system up, the engineering team wanted to be sure it was done step by step, carefully verifying the process before making steam vessels, for example, operational. The entire project is programmed using Mitsubishi's GX Developer software package as PLC ladder logic code.

From WastAway's central location in Tennessee, the Aruba project can be fully controlled through a remote connection to the HMI. "Anything that can be controlled in Aruba, we will have that capability back in Tennessee to help address any current and future issues with the system," says Brown.