BioCycle January 2008, Vol. 49, No. 1, p. 24
A look at how composting facilities can increase processing capacity without adding to the size of the composting pad. Insights offer guidance to yard trimmings sites considering expanding into food residual streams.
As businesses mature, sights generally turn to further growth and expansion. Nothing is different with a composting business, except that expansion generally means a larger physical footprint. However, there are several ways to add processing capacity, and therefore revenue, without increasing the acreage of a composting pad. For instance, adding food waste to a yard and wood debris composting operation would allow for more throughput, because food waste is a fast-acting source of nitrogen made up primarily of water, with a massive reduction ratio.
Another way to add capacity without enlarging the existing footprint is to change the method of composting, such as from windrows to trapezoidal aerated static piles (ASP). ASP utilizes space more efficiently than windrows, since it eliminates the need for wide aisles and turn around, and because the individual piles have larger dimensions. The following interviews with composting operators who have successfully added capacity show benefits and drawbacks of various methods of expansion. In an effort to exclude climactic considerations (arid vs. temperate), the focus is on facilities in Pennsylvania.
AgRecycle, in Pittsburgh, Pennsylvania, expanded capacity without leaving the windrows behind. It added food waste to what was primarily a yard and wood residual composting operation, increasing capacity while maintaining the pad size. However, AgRecycle is permitted and ready to expand the acreage of its pad size when needed. The general compost permit in Pennsylvania under which AgRecycle operates has been changing with regards to acceptable feedstocks. Initially, AgRecycle was only allowed to accept preconsumer food waste, but recent developments have opened up access to postconsumer food as well. In addition, more food processors have become interested in the sustainability movement, notes Carla Castagnero of AgRecycle, and thus are seeking to compost their organic waste streams. The motivation for adding capacity was therefore two-fold – new regulations and an increasing supply.
To maintain the same carbon to nitrogen ratio as before (between 25 and 30:1), AgRecycle is limiting food waste to roughly 25 percent of the mix. Because AgRecycle receives large quantities of wood debris, it hasn’t run into the trouble that facilities processing primarily yard trimmings sometimes encounter when adding food waste – the potential of storm water to cause leachate problems. Wood debris has a low moisture content and sufficiently absorbs the liquid.
The equipment fleet at AgRecycle is mobile, allowing existing grinders, screens and turners to be easily transported to its different composting sites (in Pittsburgh and in Washington County), as well as various pads at each site. As a preprocessing step, to better handle received materials, AgRecycle is having Roto-Mix custom manufacture a mixer to blend food waste with waxed corrugated cardboard, which will then be discharged directly into the windrows. This equipment is scheduled to be online at the end of January.
Garland Smith at Roto-Mix demonstrated the piece of equipment mixing waxed corrugated with food waste. “AgRecycle was very interested in testing waxed cardboard, to ensure that it could be well mixed,” says Smith. “In our demonstration the cardboard was ground to the preferred size in a matter of minutes and ready to discharge into the compost windrows.” Roto-Mix will mount the unit on a truck, but not for use as a collection truck on this project. It may set up mixers as collection vehicles in the future.
Castagnero explains that there have been some minor operational changes at AgRecycle since increasing the food residuals intake. As part of an active maintenance program that wasn’t necessary before, operators of each site must more closely monitor windrows that contain food residuals. Recipes are also now windrow-by-windrow, specific to quantities and types of food waste included. The general permit allows 6,000 cubic yards (cy) of material to be composted per acre at any given time. Because food is primarily water, it reduces to almost nothing when decomposing. When AgRecycle adds pure vegetative waste to a windrow, such as from a potato processing factory, there is no trace of the feedstock within three weeks. The windrows can therefore be reloaded with food residuals repeatedly, each time being turned and shaped into a new windrow.
Castagnero’s advice to composting facilities that want to add capacity by accepting food residuals: “Never take an odor problem,” she says. “In other words, take food waste while it is still fresh. Be committed to daily or every other day pick ups and deliveries, and mix the food waste into your windrows immediately. Even for a supermarket, have daily or every other day collection, which might only be three to four yards, and will require a smaller receptacle (as opposed to filling a big dumpster). That’s what is most important.”
Just how much food waste can be added to a yard/wood residuals composting facility without expanding the acreage of the composting pad size? “I do not believe there is any way that you can quantify how much food waste can be added to yard waste on a consistent percentage or tonnage basis,” explains Castagnero. “There are just too many variables regarding both the composition of the yard waste (woody or leaf based, vs. summer with grass added to the mix) and of the food waste (large discards like heads of lettuce, vs. more condensed feedstock such as batches of baby food or potato soup).” When the windrows are fairly woody, AgRecycle will reload them with food residuals two or three times during the active composting process. However, less woody windrows will only have food introduced once.
In fact, according to Castagnero, it is even difficult to say conclusively that adding food waste accelerates the process of composting yard trimmings. In the summertime, during drought-prone months, the process may be accelerated by moisture-laden food residuals because the piles are on the drier side, without much grass coming in to the facility. During the winter and periods of heavy precipitation, or in the fall with wet leaves, food waste may actually slow down the composting process. “When the other feedstocks are already wet, food can fill up the pore spaces in the pile and deter the chimney effect of having enough oxygen in the piles to keep them properly aerobic with sufficiently high oxygen numbers,” says Castagnero. “All of this is to say that a composting site that accepts food waste cannot be passively managed. It must be actively managed with temperature, moisture and oxygen readings done routinely – if not, you may end up with one big mess on your hands in addition to creating unnecessary malodors.”
For AgRecycle, adding food residuals was sufficient for increasing capacity, without switching to trapezoidal ASP. It operates on a large scale, and tip fees are relatively low (due in part to cheap fees in nearby Ohio), making the economics of ASP unreasonable. Also, although actively managing windrows with food residuals is time intensive, AgRecycle is strictly a composting operation and can afford the time. Two Particular Acres, a small farm in Montgomery County, Pennsylvania, on the other side of the state, has a different scenario, finding ASP to be the preferred route.
Two Particular Acres
Two Particular Acres received Pennsylvania’s first on-farm composting permit in 2003. The general farm permit allows 3,000 cy/acre, on up to five acres, at any given time. A subcategory of that permit allows for processing a variety of materials, as part of a farming operation, such as food residuals. However, food residuals are limited to 500 tons/year (tpy), a number that seems to be based on what one grocery store is expected to produce annually. According to Ned Foley, who owns Two Particular Acres with his wife, this limitation may be deterring other farmers from applying for composting permits. “The investment needed for composting equipment is major, and for such a small amount of intake, it might not seem worthwhile,” he explains.
The Foleys have made their operations successful however, switching from composting in windrows to ASP in February 2006. The economics are different in eastern Pennsylvania, with higher tip fees than in western Pennsylvania. The reason was multifaceted: “The initial motivation was the rising price of diesel,” says Foley. “Second, and most prevalent, was expanding my operation without expanding the acreage of the composting pad.” At Two Particular Acres, the windrows were only about five feet tall. By switching to ASP, the space in between was eliminated, and the piles are now 10 to 12 feet high. Foley estimates that he can take four times more material per year, if he chose to do so. In addition, ASP requires significantly less management time than windrows, making it an ideal choice if composting is secondary to a farming operation.
“The third reason for switching to aerated static piles was for the ability to take more challenging feedstocks, like food waste,” he explains. Suburban sprawl has been encroaching on the farm fields of Two Particular Acres, which made receiving food residuals an issue (due to odor). Before the switch to ASP, Two Particular Acres had planned on stopping food waste loads, not only because of odor concerns, but also problems with plastics contamination. Now when food waste is dropped off, it is mixed in and under control within 30 minutes.
The Pennsylvania Department of Environmental Protection recently awarded Two Particular Acres $91,893 under its Compost Infrastructure Development Grant Program. These funds will be used to purchase an Air Lift Separator (Hawker Corporation), in order to better remove plastics from the finished compost. “The Air Lift Separator is the most affordable option, and I found it to be very effective in a demonstration,” says Foley. In addition, he plans to purchase a truck scale to more accurately calculate the weight of loads, because supermarkets, restaurants and hotels serviced by the facility are interested in quantifying a precise diversion rate. Besides help from the grant, Foley hopes to free up money by selling his large tractor, since the ASP requires a much smaller, less expensive and more efficient tractor.
For the small scale of Two Particular Acres, start-up costs for ASP were manageable, with an inexpensive blower and consulting fees. (Foley worked with Peter Moon of O2Compost to design the ASP system.) The piles are positively aerated, with a foot of finished compost on top to serve as a filter for odors. However, the tubes are set up for negative aeration, a function the Foleys have utilized occasionally, mostly to pull heat down to the bottom of the pile in the winter. Negative aeration can be used with a biofilter for odor control, but this has not been necessary for Two Particular Acres yet. In fact, the Foley’s limit negative aeration because the vapors pulled through are corrosive and can damage the aeration system.
The primary disadvantage Two Particular Acres has found with ASP, compared to windrow composting, is the need for a precise mixture. Although recipes are necessary for windrows as well, with aerated static piles there is no ability to change the pile once it has been set up, because it is not turned. “With windrows, it was more like making lasagna, adding layers and adjusting as you go along,” says Foley. With ASP there is no chance to add materials later, and if the recipe is not perfect, some material will be finished, while others will appear untouched. Foley has tried mixing the material and putting it back into an ASP a second time, but pad space is limited for such endeavors. Now the compost comes out of the ASP and goes into an early curing stage – a static, nonaerated pile for about a month. Then the material is screened and cured a second time. Each of these steps involves agitation, which is beneficial. “Agitation from moving the material to different piles is good,” he explains. “More agitation would be great, but it takes a lot of space and time.” Foley has experimented with different levels of agitation, such as turning the “static” pile every few weeks.
Although the Foleys were selling about half of their compost originally, they now use even more on their farm’s fields. “There was such an incredible crop response from using the compost on our fields that we just decided to put more of it back on our land,” he says. And because less is being sold, the finished compost doesn’t have to be quite as perfect either. Revenue has still increased, since they can take in four times the material.
Penn State University
Penn State University, in University Park, Pennsylvania, composts food residuals from campus food service outlets with leaves, cow manure (laden with sawdust), wood chips and switchgrass. Leaves are the primary source of carbon, with switchgrass as an extra boost. The facility aims for 50 percent carbon, by weight, in its recipe. Nadine Davitt, who operates the composting facility, explains that because food residuals are a source of nitrogen, it could be either a beneficial additive, or a detrimental one. “Typically leaf and yard trimmings facilities are carbon rich,” says Davitt. “If that is the case, I would agree that acreage might not need to be increased when adding food residuals. However, if carbon and nitrogen are balanced at the facility, then there might not be room for food residuals without the addition of a complementary carbon source.” The latter scenario could require additional pad space.
Space utilization is another option to look at when considering increasing capacity on an existing footprint, adds Davitt. “A small operation looking to grow could increase throughput by 80 to 85 percent on one acre by increasing windrow size from 5 feet by 10 feet to 6 feet by 16 feet. Larger windrows will increase space utilization efficiency. However, it should be noted that windrows can be constructed too large and potentially cause nuisance problems.”
As far as a ratio for increasing capacity by taking on food waste, Davitt agrees with Castagnero that it all depends on which feedstocks are used. “You really need to think about available nitrogen in the feedstocks,” she explains. “Grass has more nitrogen than food. If you are assuming grass is not a feedstock, then a ratio of approximately a half-ton of food to one ton of yard and leaf debris could be expected.” Periodic installments of food residuals into yard/wood debris windrows, as conducted at AgRecycle, would also serve to increase the amount of food processed on a given acre, she says. Still, the rate of decomposition has to do with many alterable conditions, such as carbon to nitrogen ratio, moisture, oxygen and porosity. “If a leaf and yard debris facility is rich in carbon, then the carbon to nitrogen ratio could be 35:1 or even 45:1, but by adding food, the ratio could be brought more in the line of 30:1, and decomposition should be more rapid,” says Davitt.
Composting operations at Penn Sate have changed over the years, responding to a host of initial challenges. It originally layered feedstocks, blending material with a turner, but this led to issues of leaching, vectors, odors and freezing. For the past four years, Penn State has been using a Kuhn Knight vertical auger mixer. “This practice allows us to blend feedstocks homogenously, reduce particle size and form windrows upon discharge from the mixer,” says Davitt. “Through batch mixing we are able to push the moisture content to the upper limits without leaching.” Besides almost eliminating leachate, other benefits include better vector control, faster optimum heat levels, a reduced turning schedule and improved odor control. Although batch mixing seems time intensive on the front end, Davitt explains that they see a savings in the end, and have more quality control on the finished product. She notes that a smaller operation could probably handle mixing food waste directly into windrows, but Penn State processes 4 to 11 tons of food waste daily, almost necessitating a premixing stage.
When composting food residuals, management needs to be more intense, focusing on keeping the facility site clean to prevent possible vectors, leachate and odors. “Depending on the source of the food, contamination such as bottles, cans and food packaging can be a factor,” says Davitt. “To an extent, processors need to learn how to deal with contaminants and decide on an acceptable level. Unfortunately, zero contamination is probably unrealistic.” Steps that Penn State has taken to curb contamination include hand picking, informational signage, a training video for food service employees and a vacuum system on the screen.
Davitt raised questions about using a mixer as a collection vehicle, based on several factors, including available dock space for maneuvering and the order in which feedstocks need to be loaded into the mixer – if dry materials need to be added first, with wet and dense food wastes added second, this could pose problems as a collection vehicle. This led to an inquiry to manufacturers about whether the order of loading materials was different depending on style of mixer, vertical or horizontal, and what the considerations were for using a truck-mounted mixer for collection (see sidebar).
There are several methods for increasing capacity at a composting site without expanding the footprint of the pad. Adding or increasing food waste intake is a viable option, since it rapidly reduces in size. However, as Nadine Davitt at Penn State points out, food waste is not ideal for everyone. It is high in nitrogen, which is beneficial at a yard and wood residuals facility that would like to balance out its carbon ratio, but harmful if a facility is already at an optimum carbon to nitrogen ratio. Food residuals are also primarily water, which could provide necessary moisture for dry feedstocks or at a facility in an arid climate, but could cause problems with leachate when mixed with already wet feedstocks, potentially causing the compost piles to become anaerobic. As Carla Castagnero at AgRecycle explains, food residuals can also quickly become putrid, and therefore are best collected frequently, and mixed with dry feedstocks prior to the composting stage. Choosing a mixer depends on materials being processed.
Another method for adding capacity is to switch to aerated static piles, as the Foleys did at Two Particular Acres. ASP is more space and time efficient, and is cost-effective on a small-scale. Switching to ASP can also alleviate odor problems associated with food residuals composting, as the piles are quickly formed and not agitated quite as frequently as windrows. Concerns with ASP relate to initial cost, which can be unreasonable for large-scale facilities, not to mention those with significant investments in windrow turning equipment.
Sidebar page 28
SEVERAL years ago, an article in BioCycle described the use of a mixer mounted on a truck as a food residuals collection vehicle. The article (“Trucks With An Appetite For Food Residuals,” January 2003) discussed a project at Purdue University, using the Kuhn Knight reel mixer. As composters and haulers explore collection options, the possibility of using a truck-mounted mixer is being evaluated by some projects. What type of mixer is best suited for this application depends in part on the kinds of food residuals collected.
“We did some trial work in the Northwest and discovered that if the food waste is preconsumer, and has uncooked vegetables and/or fruit, a horizontal mixer may not process (cut up) the fruit and vegetables, it will probably only mix it well,” says Chris Searles, Product Specialist at Kuhn Knight. “A vertical mixer may be better suited to process the fruit and vegetables into smaller particle size to allow them to decompose faster.” Postconsumer food waste, on the other hand, should be easier to process, as it tends to be cooked or in smaller pieces. For instance, the Kuhn Knight “Scrapper” is a horizontal mixer, often in a tow type model and self-contained for maneuverability, to mix various ingredients from several locations.
Another consideration, as discussed with Roto-Mix, is processing cardboard with food waste, since it is plentiful at restaurants and grocery stores. Although this may necessitate a shredder, it is also possible to have the mixer break it down. Cardboard is very absorbent, but depending on the end product, it may interfere with certification for use in organic farming (in some states waxed corrugated cardboard is prohibited as a feedstock if compost is to be approved for use in organic farming). Searles points out that cardboard, due to its absorption properties, “could help to alleviate the concern of liquid leaking out of the mixer when traveling on the roadway.”
John Paul, Ph.D. and President of Transform Compost Systems, views vertical mixers as an ideal tool for composting. Transform Compost Systems sells the Supreme Enviro-Processor, which Paul says will blend waxy cardboard, food residuals and green waste together quickly, and allow for accurate recipes. As discussed in the main article, accurate recipes are important when composting food waste, especially for processing in aerated static piles. “The mixer can also debag if the material is in plastic bags,” notes Paul. “The advantage here is that it debags, but the plastic is not shredded too small that it cannot be easily removed after the composting process by screening.” Although he does not recommend using the Supreme Enviro-Processor as a collection vehicle, since it is best suited as a mixer, it would theoretically be possible, if drier materials were added first, with wet food on top.