June 26, 2006 | General

Knowledge Is Power In Compost Process Control

BioCycle June 2006, Vol. 47, No. 6, p. 36
Operator Forum examines new generation of process control equipment – from wireless data transmitters to software programs that optimize composting parameters.
Nora Goldstein

Composting piles are teeming with information about themselves, offering details in the form of temperature, oxygen, carbon dioxide, odors, moisture, density and more. The badge of a seasoned operator is someone who can walk up to a pile, sniff, touch and squeeze and know pretty much what is going on at that particular moment.
Components of that information pool form the basis of state and federal composting regulations, product quality assessments, air quality requirements and optimum facility management. For example, the U.S. Environmental Protection Agency’s 40 CFR Part 503 requirements for pathogen and vector attraction reduction during composting are based on time and temperature parameters. Specifications for compost used by some state Departments of Transportation require a stability measurement. And every compost operator certification program requires proficiency in understanding the relationships between temperature, oxygen, and moisture levels in compost process control.
One of the first tools to assist operators in capturing the information offered up by the compost pile was hand-held temperature probes. With the advent of the aerated static pile composting technology came blower systems designed to activate when temperatures in the pile reached a certain set point. More sophisticated composting technologies and controls led to more sophisticated temperature feedback aeration systems. Eventually, desktop computing gave birth to opportunities to manage the composting process from a “control room” using data obtained from temperature and oxygen probes and moisture readings. System vendors can log into a facility’s system and assist operators with process control and conduct equipment and software diagnostics.
New data retrieval, transmission and management hardware and software keep arriving on the compost process control scene. This Operators’ Forum highlights several technology developments and innovations.
In 2000, I had the opportunity to tour a yard trimmings composting facility in Puyallup, Washington, operated by Land Recovery Inc. (now Pierce County Recycling, Composting and Disposal). The aerated windrow composting operation was housed in a building. While walking on the composting floor, I noticed probes in the windrows with a “box” on top. Data from the probes was collected on a network, then onto a desktop PC with process control software. The wireless temperature probes had tiny radio transmitters and single chip computers (see “Plugging Into The Composting Process,” December 2000 for complete details). Land Recovery opted for the wireless probes because continual movement of equipment and material made it impractical to run temperature probe cables on the floor, and installing them along the ceiling would have made them too inaccessible.
That first generation wireless compost probe technology, designed and installed by Engineered Compost Systems, has evolved over the past few years. Today, several companies market wireless monitoring systems for composting operations. The radio network topologies used in these wireless systems fall into several categories: 1) Point-to-point; 2) “Basic” MESH (Multipoint Enhanced Signal Handling); and 3) “Advanced” MESH. With the first category, the communication is always directly between a probe and the master RF (radio frequency) device. The range is limited to line-of-sight of the radios, and possibly reduced by RF interference from structures or machines. In the second category, the communication can either be point-to-point, or relayed through one or more repeater MESH nodes. This extends range and reduces the impacts of interference. In the third category, all of the probes can serve as a MESH node to provide a myriad of communications’ paths. This method further reduces the effects of interference. Technologies have been incorporated with some of the wireless units to control fan speeds on blowers based on temperature or oxygen data transmitted.
Offerings in the marketplace include the following (presented in alphabetical order with information provided by the vendor):
Engineered Compost Systems: ECS’ TeleProbe System features a Basic MESH topology; repeater MESH nodes can be used to extend the line-of-sight distance of 3,000 feet, or to overcome a point of interference. Multiple RF Masters can be networked and implemented at a single site. The TeleProbes are designed to be left in the composting biomass, exposed to the elements, on a continuous basis. The probes measure temperatures at two depths of the pile, and transmit this data, along with battery strength and a sample counter. The RF Master can either function as a stand-alone data acquisition platform, or be integrated into the real-time control system, explains Tim O’Neill of ECS. “Real-time feedback is used to automatically adjust fan speeds, control the volume of air to a given composting zone or vessel, direction of air flow, and the percentage of the air that is recirculated or fresh. All that is controlled automatically based on settings established by the operator.” The RF Master also stores the process data in non-volatile memory, then downloads on demand to the operator’s PC interface to the process control system. O’Neill adds that the RF technology goes to sleep when it isn’t communicating. “It is quick to reboot, and doesn’t lose data,” he says. “We estimate its battery life will easily exceed three years.”
Green Mountain Technologies: GMT introduced a battery-powered probe that automatically transmits temperature readings to a wireless base station within a 400-foot range (point-to-point technology). The data is logged internally by the base station and retrieved periodically by GMT’s Windrow Manager software (see below for a description). The probe has a weatherproof head, and is built to withstand the corrosive conditions of composting. It has a long battery life because very little power is used to transmit data to the base unit.
Industrial Telemetry, Inc.: ITI uses the “advanced” MESH technology in its patented radios that are mounted onto temperature probes. The distance between probes, or between a probe and the base station, can be as much as a half-mile. The radios support both transceiver and repeater functions in the same module. Using ITI’s scheduling module, the radios can be made to sleep when not transmitting data, thus extending battery life. (The same scheduling module supports a variety of user-determined polling schedules.) ITI introduced the BioMESH Windrow Control system, a software program designed to track the building and managing of windrows with data received from the MESH radios in the field. More recently, the company has developed an I/O (input/output) mapping system that incorporates a “Scadamesh 424” board. “The board supports four analog in, two analog out and four digital I/O ports,” explains Louis DeSilvio, president of ITI. “Each board also has a processor, memory and a PLC on a chip. This technology, in a composting application, enables the operator to sample temperatures and actually regulate the RPM of the fans to provide a more consistent temperature.” He adds that the memory capabilities of the Scadamesh technology can be used to establish a host of process controls based on data obtained from sensors in the piles. “You can set up program #1 to be negative pressure aeration, and program #2 to be positive pressure. Commands are programmed in for the PLC to toggle between the two.”
Reotemp Instruments: Reotemp’s Eco-Mesh Wireless Compost Monitoring System has three components – computer software, a base unit and remote temperature probes. The Eco-Mesh probes use a MESH network acting as both “transmitters” and “repeaters,” passing data from one probe to the next, all the way back to the base unit. The user-friendly software is designed to control, organize and retrieve data from the wireless probes, which are assigned a “customized” location I.D., e.g., Row #1 100-feet out. The software records up to four temperatures for each probe, as well as ambient temperature. “We decided to add in multiple sensors within each probe to get as much information as possible,” explains Nathan O’Connor, Compost Product Manager of Reotemp. “Multiple sensors allow compost operators to create temperature maps by location and depth within each pile. They can also detect any temperature anomalies.” The base unit acts as both a transmitter (by sending commands) and a receiver (by collecting data). The base unit and remote probes have a range of over 700 feet (line of sight) to the nearest probe. The operator can set up a scheduler for “polling” the probes (activating them to transmit data). The Eco-Mesh rechargeable batteries will last 35 days if polled hourly and considerably longer if polled less frequently. Reotemp is offering a trial period with its wireless monitoring package. “A qualified facility will receive a base unit and about three remote probes with software,” says O’Connor. “We are currently setting up three compost facilities with 45 day trials – enough to go through the composting cycle.” He adds that Reotemp, a market pioneer in compost temperature measurement for over 20 years, has “combined our expertise with that of Unitorq Wireless to create a highly reliable, easy-to-use monitoring system.”
Based on the literature and interviews with vendors, it appears that one significant advantage of this latest generation of technology is the ability to collect data and control blowers without having to run wires over great distances. “Savings on the wiring costs alone can be significant at large composting facilities,” says O’Neill. “We have installed our technology at a large aerated static pile composting facility in British Columbia. There is an access road on both sides of the pile, so there wasn’t a good place to come up with wires from underground, e.g., through a wall, that wouldn’t be subject to loaders running them over. In this case, the RF technology was the only way to go, with savings realized by not having electrical installation costs.”
Optimizing battery life on remote probes is an important consideration with wireless transmission technologies. Where wires are run for the blower system, tapping into that power source can enhance use of battery-powered MESH radios. Industrial Telemetry has taken advantage of that power source with its BioMESH package. “If we wanted to take the temperatures from six sensors in a windrow and average them, then trigger a fan on or off (analog in probes, but digital out on fans), we could do that with our original BioMESH board and the 1 milliwatt ‘RealMESH’ radio, since the fans’ radios would be line-powered and ‘on’ all the time with no battery constraints,” says DeSilvio. “We could then use the battery-powered RealMESH radios in the temperature probes with the sleep cycles already there to conserve power and still provide temperature readings when needed. Because of the efficiency, the savings in electricity for the fans alone could pay for the system over time.”
What separates composting and other organics processing technologies from more traditional manufacturing and industrial systems is the variation in raw materials. There isn’t, for example, one type of biosolids with standard characteristics. Yard trimmings vary by tree species and region. As a result, compost recipes need to be adjusted, as do assumptions about the rate of decomposition, the volume of air necessary to optimize that rate, and so forth.
“It is always a learning process,” says Tim O’Neill. “You start out with feedstock assumptions based on lab data or experiences at other facilities, and then make adjustments with the real life situation.” ECS’ process control packages provide the company with dial-up access or a web connection to a facility’s control system. “For the first few months, we connect fairly regularly to the facility, talking to the operator about specific details,” he adds. “It is a very efficient way to tune up a system and train the operator. While we start out as the ‘expert’, overtime that switches and the operators become the experts and train us, adding to our knowledge base about performance of our equipment and control systems.”
ECS markets the CompTroller, an automated composting control and monitoring system. Primary components are operator PC software, control nodes, process sensors (e.g., the wireless probes) and variable speed blower drives. The software allows the user to select many process control settings, view real-time data and store and recall information. The software also can control the variable speed blower drives, and monitor the biofilter and leachate tank conditions. One area of expertise, reflected in the automated control package, is control and implementation of reversing aeration – alternating automatically between positive or negative mode depending on pile conditions.
“We didn’t invent reversing aeration, but I think how we implement and control it is somewhat unique,” explains O’Neill. “We’ve refined air handling in terms of being able to combine reversing and recirculating into one package and to control the amount of fresh air in or the temperature of exhaust air out.” The concept behind reversing aeration is that anytime air is run in one direction through a compost pile, the point at which air comes in is cooler than the place where it exits because the air heats up as it runs through the pile. The initial contact point is close to ambient, whereas the temperature at the exit point could be upwards of 80°C, although 65° to 70° C is more common. “There could be upwards of a 60° range,” he says. “Some areas could be below thermophilic temperatures while others are much hotter, and higher temperatures produce more odors. By reversing air flow at a set temperature differential between the sensor at the bottom of the probe and the sensor at the top of the probe, the temperature gradient starts working in the other direction. That keeps the pile much more uniform in terms of temperature and humidity, because the pile also tends to be much drier where air comes in than where it exits. Drying a pile out is one of the most common ways to reduce biostabilization rates.”
While it is valuable to take moisture and oxygen readings early on in the compost process to gain empirical data on how much fresh air is needed to maintain a reasonable oxygen and moisture level – and make recipe adjustments as needed – O’Neill doesn’t see continuous monitoring of those parameters as necessary once composting is underway. “If the basic settings don’t change that much once you get to know your feedstocks and recipe, then oxygen and moisture won’t vary that much from batch to batch. Occasional readings are good to be sure you are operating in the right range, but normally only temperatures can change dramatically in any single day, which makes them the best parameter to monitor on a frequent basis.”
Green Mountain Technologies introduced its Windrow Manager software package several years ago and recently began selling an upgrade (Version 2.0). The software collects, stores and graphs temperature and oxygen data; tracks PFRP and VAR days for EPA 503 regulations; emails compliance reports to regulators; shifts and combines compost batches between windrow bays; and supports the capability to manage tasks and feedstocks in the field with GMT’s PocketPC. Data from temperature and oxygen probes are recorded by a hand-held computer, then transmitted to a desktop PC.
“The concept behind the product is to give operators a tool they can take into the field and get a lot of data quickly,” says Michael Bryan-Brown of GMT. “This includes temperature monitoring at two different depths, plus oxygen levels. All that information is brought together as a person is walking along the windrow. The benefit is there is a physical relationship of the data to what the person is smelling and feeling. The computer takes data and consolidates it into a report that can be accessed later – relating it to the initial recipe, temperature data, stability measurements and lab results.”
An operator makes some assumptions when developing a recipe for the feedstocks at hand. Windrow Manager tracks process “performance” from start to finish, recording how temperature and oxygen levels responded over the cycle of that windrow. At the end, the operator can look at the compost quality and analyze how all the information tied together. “And when you get a product that looks really good and your customers are happy with, you can go back and figure out what you did with that windrow,” notes Bryan-Brown. “In many circumstances, an operator would have recordings of temperature data, but not the whole picture. What Windrow Manager does is keep a definitive record of each windrow, consolidating that information and making it accessible again.”
Most recently, GMT has been working with Woods End Laboratories to develop a specific module that inputs compost stability data directly into the Windrow Manager program. Woods End’s Solvita kit measures the respiration rate of compost via ammonia and carbon dioxide release in the air stream. The levels are indicated on a color strip, numbered from one to eight, which relates to how far along the compost is in the process. For example, a Solvita 3 reading (orange-colored/immature) indicates that 50 percent of the oxygen in the compost has been consumed by microbes.
Woods End has developed a Digital Color Reader (DCR), a portable device that reads the color of a Solvita CO2 and ammonia test kit paddle and provides a digital output. “What we are developing is a module that allows Solvita data to input directly into the Windrow Manager,” explains Bryan-Brown. “Data strips on the Solvita paddle are put in the DCR, which converts the reading into a digitized form. Now information based on a stability index can be correlated with length of time in a windrow, temperature data, feedstock recipes and so forth.” According to Woods End, any kind of digital input system from any composting technology should be able to make use of the DCR information, thus allowing differing monitoring systems to incorporate Solvita stability readings in any way that is useful.
Woods End’s DCR innovation eliminates interpretation of the results as influenced by the reader’s color bias. Furthermore, the whole numbers on the color strip represent an incremental scale of carbon dioxide (e.g., a #6 is actually two times less CO2 than a #5), similar to the incremental scale developed to represent logarithmic pH. “When we developed the Solvita test, we decided to use a similar numerical system [as pH] to relate exponential numbers to a simple, visually discernible range of colors,” explains Will Brinton of Woods End. “The new approach provides the real numbers, so instead of a five or a six, for example, the reading would be 5.23, which can then also be directly converted to its equivalent CO2 concentration. And the repeatability is very high. We can estimate more accurately the amount of CO2 produced between certain lengths of time, e.g., every four or eight hours, after a pile is turned.” He adds that users of the Solvita test have “grown up and gotten used to” what the readings mean. “This isn’t a complete platform change. We’ve simply added more muscle to it.”
The discovery that the Solvita readings relate directly to in-vessel carbon dioxide concentrations resulted somewhat serendipitously this spring, according to Brinton. In a project funded by the Maine Technology Institute (Maine’s technology advancement agency), Woods End and Maine Aquaculture Associates installed a GMT Earth-Tub to test composting of hatchery wastes. “We noticed that Solvita readings taken on the enclosed samples were pretty much equivalent – and opposite – to the oxygen readings we were making on the contents of the Earth Tub,” he says. “This led us to speculate that the Solvita test also could be used to represent air depletion on a more real-time basis. Our digital innovation helped make this much more doable.”
Generally speaking, investments in process control technologies not only enable operators to more easily comply with regulator and end user requirements, relate end product quality (or the lack thereof) with historical process data and make process adjustments as needed, they ultimately result in labor and cost savings. Wireless temperature probes, for example, reduce the need for a technician to continually “walk the piles” taking readings and entering data. In some applications, there is a payback on elimination of wiring costs.
Another significant cost savings, noted by several vendors during interviews, is also the ability to lower utility costs related to aeration. “One advantage with computer-controlled aeration systems and installation of variable frequency drives on blowers is that blowers stay on continuously, with the amount of air flow determined by pile temperature,” says Bryan-Brown. “And refinements are being made continuously, such as gradually opening and closing valves to lower temperatures. What typically happens in facilities without aeration control is that a cycle timer turns the blower on and off every 10 or 20 minutes. Each time the blower is turned on and off, there is a big spike in power consumption.”
ORGANIX Inc. of Walla Walla, Washington is under contract with Three Mile Canyon Farms in Boardman, Oregon to manage the farm’s composting operations. There are 80,000 animal units within Three Mile Canyon’s operations (three dairies and heifer, steer and calf feedlots) generating about 2,000 tons/day of manure. While a portion of the manure is land applied on acreage surrounding the farm operations, another portion is composted in windrows on a 200-acre site. The manure is mixed with yard debris that Organix manages under contract for the city of Spokane, Washington.
There are an average of 200 windrows at any one time, each 1,000 to 1,500 feet long, says Del McGill, president of Organix. “After managing the site for several years, it became evident that we needed a data management tool to track the volume of material handled and ensure regulatory compliance and customer satisfaction with the end product. I needed to know how much manure was coming from which operation, what type of manure was in which rows, how long the material had been there and whether we were meeting PFRP (Process to Further Reduce Pathogens), which is necessary for organic certification. And I needed that information at my fingertips. We had begun a process to develop our own software to meet our needs, but we knew that was going to take some time.”
Instead, McGill selected the Windrow Manager software from Green Mountain Technologies. The day we spoke, Organix was cleaning out the calf farm and incorporating that material with green waste into windrows going into Aisle 23 and Aisle 24 in the Windrow Manager program. “The load sheet shows how many truckloads are coming from each corral, which makes it simple to do our reporting to comply with the CAFO (confined animal feeding operation) regulations,” he adds. “We also have a 2,500 acre organic farm, and make bedding for the dairies, so we need to track temperatures to ensure we get pathogen kill for both of those end uses.”
An employee “walks” the wind-rows with the aid of an ATV “4-wheeler.” He plugs the Pocket PC into the sensors and downloads the information. Temperatures are taken every 100 feet. That information is then uploaded into the computer, and McGill can manage the windrows from one centralized point. “I can get a quick vision if a windrow is through PRFP, if an aisle is empty and so forth,” he notes. “It also helps me direct the windrow turning equipment, as we have a 16-foot Frontier turner for the early part of the process, and a 12-foot HCL turner to finish the process. That way, we can avoid cross contamination of the piles.”

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