From BioCycle Magazine
September 2000, Page 31
HANDLING THE FLOW
FEEDSTOCK COMPOSITION AT COMPOSTING SITES
Widely varying proportions of grass, leaves and brush pose challenges for yard trimmings composting sites.
Cary Oshins and Dave Block
WHEN yard trimmings facilities are getting established, one of the first questions is: How much grass, leaves and brush will arrive, and when? This question leads to others: How do operations deal with variations in flow? Can they affect that flow, and if so, why and how?
To answer these questions, we called facilities around the country and inquired about throughput and processing. (All of the sites surveyed were composting facilities, not just mulch grinding operations.) We also compiled survey information that focussed on yard trimmings. Altogether, a picture was formed of what comes into sites. Around the country, and even in the same region, there are wide variations in feedstock composition.
PICK A NUMBER, ANY NUMBER
Most sites, whether public or private, more or less have to accept what comes and be able to deal with it. Yet some facilities seem to favor or ignore certain feedstocks. On an annual basis, grass comprises zero to 70 percent of incoming materials, leaves from zero to over 90 percent, and brush and woody material zero to near 90 percent. These ranges are wide even within regions. This variation is shown in Table 1.
Patterns are more difficult to discern because, of the 33 facilities that provided data, 19 reported yard trimmings throughput and composition in weight (tons) while the remaining facilities reported in volume (cubic yards). After converting cubic yards to tons using simple conversion factors, the average composition of yard trimmings nationally calculates to approximately 30 percent grass, 30 percent brush, and 40 percent leaves (Table 2). However, there are few “average” facilities that handle this well proportioned mix (Mecklenburg County, North Carolina has the honor of being the most average). The data in Table 2 show some differences among regions. Generally, leaves are a large proportion of yard trimmings in the Northeast. Moving south and west, brush and wood become more prominent. The grass load increases from east to west. Again, as Table 1 cautions, wide variations exist in yard trimmings compositions among facilities within all regions
While more sites reported in tons, that does not mean they all have scales. Scales are expensive and there has to be a reason for them — usually tip fees. However, many states require recycling to be reported in tonnage, and some facilities use state-approved conversion figures to translate cubic yards to tons. But if you’re not weighing everything on a scale, how do you know how much you take in? Count vehicles? Estimate volume on the pad?
“To tell you the truth, I really don’t have a precise figure on how much we get of each material. That’s why we’ve developed this new system,” explains Tim Bollinger, manager of the Lehigh County, Pennsylvania yard trimmings composting facility. “We take stuff from half a dozen different municipalities. With our old system, the driver was supposed to get out of his truck and record the source and truck size on a clipboard in a shed. We were lucky if one guy in ten actually did this. So we were left guessing. Now everyone gets coupons — different coupons for different size trucks. All they have to do is roll down their window and drop the coupon in a deposit box on the way in. We are getting much better cooperation. Next year, we hope to expand it to our satellite sites as well.”
At the municipal composting facility in Islip, New York, various proportions of grass, leaves and woody material are estimated at the scale station and the unloading area. Many sites put all incoming materials through a grinder, so they don’t separate brush and leaves. In addition, loads are often mixed, but get placed in one category or another. For example, Omaha, Nebraska’s composting facility reported 60 percent grass and 40 percent leaves. “There are enough leaves and brush mixed in with that grass year round that we could compost it straight without mixing any other browns in with it,” says Gordon Anderson, who directs the composting operation.
While the estimates may be rough, having a good sense of the proportions of yard trimmings components is important to composters, both from a processing and an end product standpoint. “During the grass season, we’ll use leaves and/or shredded wood as bulking material, and that will be a process management issue,” says Stuart Buckner, director of environmental services in Islip. “At certain times of year, we’ll want to keep the leaf material separate from the wood waste for the purpose of providing product that certain customers want. While we have several hundred accounts with landscaping contractors, who don’t care about what yard trimmings went into the finished product, we also have a golf course that doesn’t want fibrous material included.”
TAKING WHAT COMES
Almost all sites indicated that the reason for the mix of materials they receive is because “that’s just what’s out there.” Weather, climate and collection methods have more to do with the relative amounts of different yard trimmings components than recipe needs or regulations. “We have to handle what comes in when it gets here,” says Emily Johnson of Decatur, Alabama’s composting facility. “Last year, when there was a large area cleared for development, we got a whole lot of extra woody material. Aside from that, we are pretty consistent from year to year.”
“We would never limit a (clean) material,” states Sharon Barnes of Barnes Nursery in Ohio. “We have never had a situation of too much grass, but if we did, we would not limit the grass, but find a way to handle the material without odor.” As a private facility, refusing material would mean taking money out of the nursery’s pocket.
The recycling approaches taken by municipalities and counties can have a significant impact on the yard trimmings composition that composters take in. For example, Islip does not receive the large surges in grass loads that it once did since a grasscycling initiative reduced volume by 40 percent. Grass pick up switched from weekly collection of bags to seasonal collection. That put an end to debagging and allowed for separate processing of grass; the large proportion that does come in segregated does not get ground. Grass pileups on the pad are no longer a problem. “Once grass gets backlogged in the receiving area and you can’t process it for several days, problems only get worse,” says Buckner. “Now we can process it within several hours.”
A few states, such as Pennsylvania and New Jersey, limit the proportion of grass that a site can accept to 25 percent so that it will not drop below a 1:3 ratio of grass to leaves and brush. “We manage seven sites in this county,” says John Haas, recycling coordinator for Ocean County, New Jersey, “but only one of them can accept grass. Even there, the grass can only go to some of the windrows, due to buffer requirements. Haulers call us in the morning to ask if we are accepting grass. If not, we will direct them to private sites in the region.”
The Islip site is required by the state to record the percentages of yard trimmings components it receives, although the only limit is on the total volume of materials, notes Buckner. Segregated, chipped wood residuals are excluded from the count because they are used as bulking materials.
MATERIALS RECYCLER OR PRODUCT PRODUCER?
Feedstock composition has grown in importance as professionals in the composting industry have pushed to shift the paradigm to the “compost factory,” a manufacturing facility where yard trimmings are the feedstocks and compost and mulch are the products. The problem is that the generators of the feedstocks often view the composting facilities as alternatives to disposal. Hence, unlike other factories, supplies just show up, not necessarily in the proportions they are needed, and the compost factory has to look at those inputs and make the best of them.
How do yard trimmings facilities that focus on producing a range of high quality products approach this variability? They look to other types of materials besides yard trimmings. “If we need extra nitrogen to balance a recipe, we bring in manures or food processing residuals,” says Bob Kelly of Seacoast Compost in Exeter, New Hampshire.
This is echoed by Jack Hoeck of Rexius in Eugene, Oregon. “We produce six different mulches and a variety of composts, potting soils and soil blends. For example, we are using the soil food web concept to produce a fungally-dominated soil designed specifically for potting evergreens. We have to take what comes in the dropoff programs or municipal overflows, so then we go and get manures, agricultural residuals and urban lumber to balance our recipes.”
MANAGING THE FLOW
Mary Matava of Agri Services in Vista, California notes that the company’s El Corazon composting facility in Oceanside started taking other feedstocks about five years ago. For example, gypsum wallboard and yard trimmings are ground together in a Morbark grinder, which reduces dust from the wallboard. Adding ten percent gypsum to the mix also saves the landscaper or farmer the additional costs of purchasing and applying gypsum.
Sometimes, large volumes of any one yard trimmings component pose a processing challenge. One way to deal with receiving a large influx of woody material, especially if it is separated, is to process it alone and add mulch to the product line. To handle large loads of rancid grass, Islip uses its water truck to spray the piles with a diluted chemical oxidant solution that suppresses odors. Like many composters, Living Earth Technologies in Dallas, Texas keeps plenty of sawdust and other carbon sources available to cover or blend with grass, notes Mark Rose, company president.
Seasonal leaf collections typically bring huge volumes to composters, who have the option of stockpiling them or processing more aggressively (see chart). At Barnes Nursery, incoming leaves are kept separate from other yard trimmings. If compost will be needed soon, the leaves are run through the grinder. If composting can take longer, Barnes simply runs over the leaf pile with a turner and saves money by not grinding. Either way, processing begins soon. “It gives us flexibility, plus it keeps the size of the piles manageable, because so many leaves come in at one time,” she says. “The only time our site has had an odor problem was when we put leaves in static piles in the fall and cut into them in March. Now we windrow them so that the compost is finished by the time people are opening their windows for spring.”
While the details vary widely from site to site, the basic processes of grinding, turning, combining, screening and materials handling mean that the same sorts of equipment can be found at most sites. The only exceptions are small sites handling only leaves, or not accepting anything that requires grinding. To process both segregated and combined yard trimmings components most efficiently, large operations may find it helpful to have a variety of grinding equipment. In Islip, Buckner uses a CBI grinder for mulch production and to grind mixed yard trimmings, a Diamond Z tub grinder for mulch to back up the CBI during high-volume times, and a Finn horizontal grinder used primarily for leaves and as a backup. Living Earth has eight composting sites. To serve them, the company owns a Peterson Pacific horizontal grinder and seven tub grinders that can be moved around as needed.
Given the right processing methods and resources, composters can turn virtually any yard trimmings combination into a quality finished product. “All material breaks down to the same basic elements,” notes Barnes, whose yard trimmings are 85 percent brush, ten percent leaves and five percent grass. “Even though we start with a lot of wood, we compost for eight to nine months and turn anywhere from every five days to every two to three weeks, depending on how busy we are. If we work our material all the way down the process, we’ll have a 15:1 to 17:1 C:N ratio, and you could tell very little difference between our compost and leaf compost, other than a little variation in the texture. The difference is that if we did have much more grass, we wouldn’t have to process as long or as intensively.”
www.jgpress.com
BIOCYCLE | IN BUSINESS | COMPOST SCIENCE |
HOME