July 14, 2008 | General

Nuts & Bolts: Mixers

BioCycle July 2008, Vol. 49, No. 7, p. 37
Some composting facilities purchase mixers that offer size reduction and blending – especially when receiving loads of food scraps and corrugated – while others select a unit for mixing only.
Nora Goldstein

IN THE early years of food scraps composting, it was fairly standard practice to unload a truck of produce and other materials onto a bed of wood chips, and then scoop that up and add it to a windrow. The first blend took place with a windrow turner. We would hear stories about watermelons rolling out of wind-rows while they were being turned.
While many operations still use that materials handling approach, a growing number of sites are premixing the food scraps, often with corrugated and soiled paper, before adding it to a windrow. In some cases, bulking agent is added to the mixer as well, especially when loading the mix directly into a vessel or drum. “We think food waste is going to be a boon to the composting industry, not only for recycling and keeping organics out of the landfill, but for our company as well,” says Garland Smith of Roto-Mix. “It is opening up more markets for us.”
Recently, Roto-Mix sold a horizontal mixer with three augers to AgRecycle, a source separated organics composting facility in Pittsburgh, Pennsylvania. AgRecycle rolled out a collection and composting service for grocery stores, restaurants, hotels, a convention center and other generators in late 2007. Pre and postconsumer food scraps, soiled paper and corrugated (waxed and unwaxed) and approved compostable products are accepted. “Food, unlike feedstocks such as yard trimmings and wood debris, has to be handled immediately and correctly at the composting site,” says Carla Castagnero, President of AgRecycle. “One of the major differences is water content. We preblend all food because we can’t just throw it in the pile and hope it decomposes. That can lead to leachate issues and the windrows going anaerobic. Food waste is very dense, so preprocessing it before it goes into the windrow is crucial.”

AgRecycle was looking for a machine that would enable them to size-reduce the corrugated cardboard and paper and blend it with the food scraps. “We have had machines that do one or the other very well, but we wanted one machine that could do both together,” adds Castagnero. “Our goal with our food composting program is to change the customers’ behavior as little as possible so that they will participate. If they are used to putting cardboard boxes in with their kitchen waste, we wanted them to be able to continue doing that.”
She met Garland Smith at a BioCycle Conference in the spring of 2007, and was talking about the company’s search for a mixer. “While a vertical mixer works well for size reduction, we believed that one of our horizontal mixers would be better for AgRecycle’s application,” recalls Smith. “In a vertical mixer, there is a lot of space between the knives, and the operator wouldn’t be able to reduce the particle size as quickly as they would be able to do with our horizontal unit that has the three augers. The middle auger actually works like a rotor, lifting material so that the side augers can get a hold of it.”
The horizontal mixer is from the company’s Hay Pro line that was designed for animal feed operations that use round bales of hay. “It just so happened that as we started getting more into servicing composting operations, we found it does a great job in those applications as well,” he adds. “For sites that have already size-reduced their feedstocks, they would use a rotor machine as a blender.”
An additional advantage to the horizontal unit is that it can be loaded quickly, says Castagnero. “The opening on a vertical mixer is smaller and higher up,” she notes. “The opening is larger on the horizontal unit so it is easier to load.”
Supreme International Ltd. also sees more of its mixers used in applications related to source separated organics composting. The company describes its “Enviro Series” line as three machines in one, says Al Brodie of Brodie Ag and Industrial. “Mixing is the first component. With our vertical mixers, 95 percent of the capacity of the tub is usable mixing space. The second component is particle size reduction. Supreme has patented interference knives that are mounted on the wall of the tub. The knives are on a retractable arm that can be moved in and out of the material in the tub, depending on how aggressive the operator needs to be to size reduce the feedstocks. This is a very aggressive machine when processing yard waste and packaged food waste.”
The third component is debagging, adds Brodie. “Projects want to be able to take materials in paper and plastic bags from curbside collection of leaf and yard waste and other source separated materials,” he explains. “They want to debag it so they don’t end up with 1- to 2-inch particle sizes of plastic after grinding. The Supreme mixer debags the material and the bags are more like 6-8 inches, and can be screened out very easily.”
The City of Toronto allows residents to set out their source separated organics in plastic bags (which are then placed in a curbside cart). While some of that material is directed to the city’s anaerobic digester, where bags are opened and removed during the hydropulping stage, much of the collected organics are processed and shipped out of the city to composting facilities. TRY Recycling, based in London, Ontario, handles a range of organic residuals at its processing facilities. The company tested its Supreme 900T vertical mixer to debag the curbside collected organics in Toronto.
“We used the mixer to open the bags, and then the material was screened through a trommel before being sent to a composting facility,” says Rick Vandersluis of TRY Recycling. “We use our mixer to process curbside yard waste, both to debag and for size reduction. We’ve also tried it with other materials we process, such as gypsum.” An advantage to using the mixer to open the plastic bags and for size reduction is that the plastic pieces aren’t too small, he notes. “We are able to capture more of the plastic after composting.” Brodie adds that the auger and knife designs on the vertical mixers prevent bags from getting tangled on the auger.
When looking for a mixer for a composting project that involves source separated food scraps, whether the food to be processed is raw or cooked is an important consideration. Chris Searles of Kuhn North America recalls a trial done in the Pacific Northwest with its Reel type mixer. “In that situation, we discovered that if the food waste is preconsumer and has uncooked vegetables and/or fruit, the reel design will mix the materials well, but may not cut up the fruit and vegetables,” he says. “A vertical mixer that has ‘chopping’ ability may be better suited to process the fruit into a smaller particle size to allow it to decompose faster. Postconsumer food waste is easier to process and decompose, because it is typically cooked and of a smaller particle size.”
He points out that that the horizontal axis of the company’s augers/reel provides more active and thorough mixing of food scraps and amendments. “Gravity plays a bigger role with a vertical mixer with regard to dense versus bulky materials,” explains Searles. “If a majority of the dense material is introduced into the mixer first, and then the bulky material on top, the materials may have a tendency to stay segregated through the mixing process. As the vertical augers try to lift that dense material up, the lighter material wants to ‘float’ on top, and not intermix. To overcome this, the operator may need to switch back and forth from bulky material to dense material with each loader bucket.”
While Patz Sales, Inc. manufactures a full line of mixing equipment, it is getting a lot of interest in its new Model 400 Stationary Single Screw Vertical Mixer with 140 to 180 cubic feet of capacity. The unit is designed for generators of smaller volumes of food waste, particularly universities and correctional facilities. “The single screw mixer has a capacity of about 5 cubic yards,” says Jim Reed of Patz Sales. “BW Organics in Texas is our biggest dealer in the composting field, and they have been getting a lot of orders for the Model 400.” It can be customized to match the user’s operation, and the welder steel screw has laser cut holes to accept knives. The cost for the unit is between $15,000 and $20,000, depending on options selected.
BW Organics manufactures rotary drum vessels for composting. “A few years ago, we visited Patz Sales to learn more about their equipment and see if it would fit our needs,” recalls Bernie Beers of BW. “We were looking for something small, as we couldn’t justify a $30,000 mixer for a $15,000 drum. We hounded Patz until they agreed to make the Model 400!”
The small mixer pairs up well with BW’s Model 824 rotary drum that has a capacity of 8 cubic yards/day. “Prepping the product is the whole success and secret to our system,” adds Beers. “That is why we recommend a small mixer so the operator can get the food and dry carbon material in the proper recipe before it is loaded into the drum. As we move into our larger capacity drums, we also scale up the capacity of the mixer.” In some cases, knives are added into the screw to size reduce larger materials, such as melons and cardboard.
Orders for the mixer-drum combination are coming primarily from the institutional and agricultural sectors – and all corners of the world. Recently, a system was shipped to the Galapagos Islands; another was sent to a university in Kuwait. Upcoming installations include a biosolids/cotton gin trash composting project in South Carolina and a farm digester in Washington State. “The dairy was marketing the material coming out of the digester as a compost, but the material hadn’t reached 55°C for the required amount of time,” says Beers. “We are building them a Model 1050 drum, which should be installed this summer.”
The Ottawa Valley Waste Recovery Centre (OVWRC) in Pembroke, Ontario launched a composting project for source separated municipal organics in November 2001. The facility receives about 6,000 metric tons/year of leaves, yard trimmings and household food scraps from five partner municipalities in its service area. Three of the five municipalities offer curbside collection using 86-gallon Schaefer carts. The other two accept the materials at recycling drop-off depots. “When organics collection was introduced six years ago, it was offered as a biweekly service,” says Sue Campbell, General Manager of OVWRC. “Food and yard waste commingled in the cart are collected along with fibers one week, and garbage and recyclable containers are collected on the alternate week. A waste audit done in 2006 found 60 to 64 percent diversion of recyclables and organics.”
At the composting facility, organics are presorted to remove any larger contaminants, loaded into a slow speed shredder, and then passed through a magnetic separator. The size-reduced materials are loaded into a horizontal Luck/Now mixer supplied by Engineered Compost Systems (ECS). Depending on the season, leachate from the composting process, or ground leaf and yard trimmings, may be added to the mixer. “We have a specific C:N ratio and a targeted moisture content,” says Chris Hoffman, a facility operator. “In the winter, we get mostly food waste in the carts, so we need to add leaf and yard waste we have on site for aeration purposes. In the summer, the food waste is mixed in with yard waste, which absorbs free liquid, so we may need to add leachate to increase the moisture content.”
He adds that the mixer does a good job of blending materials, but notes that contaminants such as textiles or stringy vines can get caught in the augers. Also, the facility has started receiving source separated organics from area parks, which arrive in compostable plastic bags. “If a lot of the bags get into the mixer in one shot, they wrap around the augers, which can be a challenge,” says Hoffman.
The blended material is loaded into ECS’ CV Composter vessels. OVWRC purchased 11 of the vessels, which have additional insulation and an enclosed and insulated aeration hallway to enable composting in extremely cold conditions. To load the units, an empty compost vessel is placed next to the mixer, which discharges onto an indexing conveyor. The full vessel is transported via a roll-off truck to an outdoor concrete pad and connected to the ECS aeration and control system. “We have a computer program that monitors temperatures in the vessels,” explains Steve Munro, Operations Supervisor. “Once the material achieves PFRP – 55°C for three nonconsecutive days – the vessel is moved to the leaf and yard waste composting pad and unloaded.” Certain times of the year, PFRP can be achieved in four to five days, adds Campbell. During cold weather, the retention time is longer. Compost that includes the food waste is kept separate from windrows of leaves and yard trimmings only. (Those materials are not processed in the vessels but are ground and go directly into windrows or are used as amendment in the food waste composting process).
Finished compost is screened and sold to residents and commercial users. Campbell says that recently, OVWRC passed an environmental assessment to add capacity to its landfill. “One consideration that went into our approval is that we have such an aggressive diversion program, and thus can make that expansion last for 40 years!”

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