Compost Material Handling Options
of the NaturTech Composting System
December 5, 2001
Jim McNelly
In simple terms, composting methodologies rely, to varying degrees, on two actions; agitation and aeration. The two are often mistaken for each other, as some operators claim that they “turn their pile” to provide aeration, while data shows that a windrow actually consumes its oxygen in as little as 30 minutes, making the aeration value of windrow-turning insignificant. A rotating tube digester may be described as a composting process that is 95% agitation and 5% aeration while an aerated static pile may be defined as 5% agitation and 95% aeration. But at only 24 to 72 hours retention and high capital cost, such rotating tubes are used only for initial conditioning, not composting.
Since mixing and agitation are more expensive that blowers and air, finding the “sweet spot” of minimal agitation and maximum aeration is the goal of the NaturTech Composting System. Two key claims of Jim McNelly’s composting patents, of which the NaturTech Composting System is an example, are the methodologies of first filling digesters or curing bins from the top using either a front-end loader or conveyor, and secondly agitating, re-conditioning and or re-mixing the material outside the bin. NaturTech may be described as a system that is 75% aeration and 25% agitation. The NaturTech agitation practices might be considered to be “precision mid-course maneuvers”. Infrequent as they may be however, these agitation events are critical to achieving optimum rate of decomposition and a uniform finished compost product.
Although it is often referred to as an “enclosed aerated static pile”, the NaturTech process is markedly different from conventional aerated static piles that are characterized by no turning once the pile is formed, hence the defining term “static”. This lack of agitation is often required because the piles are built upon one-time use, perforated plastic pipe. There is also the difficulty of properly forming a static pile with a porous wood chip base and a 12” wood chip insulation layer. Aerated static piles are a practice that has been largely abandoned due to problems associated with lack of homogeneity such as channeling, odors, incomplete digestion, and lack of uniformity in the finished compost.
NaturTech is also distinguished from windrow systems with agitation taking place in-situ, or “in place” in the windrow. NaturTech’s concept of “moving the compost to the mixer” instead of “moving the mixer through the compost” requires specialized equipment and material handling practices designed to keep costs low. At first look, some operators see a large number of containers and think that material-handling costs will be high. In actuality, however, NaturTech’s containerized composting approach greatly reduces material handling requirements, providing some of the lowest operating costs of any modern composting system.
Once the critical initial mix is achieved, subsequent agitation usually does not require cycling again though the batch mixer. If the material is retained in the primary composting digesters for fifteen days or less, it is typically re-mixed by dumping and conveyor re-filling at seven days, achieving increased throughput capacity of 20% to 25% as a result of restoring volume lost to settling and volatilization. On large operations, multiple containers can be combined into fewer ones, freeing a container for increased facility capacity.
The main reason to agitate a second time in the batch mixer, however, is to add water. The first five days of composting are the most active, generating the most heat and requiring high volumes of air to lower temperatures. This process has the effect of removing moisture, often to levels below 50%, which can inhibit the subsequent curing phase. Many forced aeration systems have been abandoned because the compost simply dries out too much to continue decomposition. Reintroducing moisture is nearly impossible in tunnel reactors, static piles, or containerized systems that do not use this patented NaturTech feature of external re-mixing. Even an auger routing its way through a composting batch from the top is not a true homogenization process. It may agitate, but it does not blend. Weighing the water using the scale on the mixer and thoroughly homogenizing it is far more reliable than having a person standing on top of a windrow with a garden hose and a watch.
NaturTech uses a centralized tipping and filling area, sharing the act of transferring the compost with the composting process itself. Conventional composting relies on front-end loaders and dump trucks to move compost from the mixing area to the windrows and again back from the windrows to the screening area. The typical dump truck load is approximately 20 cubic yards, or ten tons. The maximum payload for a truck to go on the road is around 22 tons. NaturTech systems often eliminate the need for a dump truck on site.
Small NaturTech composting systems under 20 tons per day typically use 40 cubic yard (20 ton) digesters and conventional roll-off trucks. These small digesters can also be used as delivery vehicles, eliminating the need for dump trucks to deliver compost as well. To upgrade to 50 cubic yard (25 ton) digesters, custom roll-off trucks are required and the containers may weigh too much to transfer off site.
For the 25 ton and larger digesters, NaturTech uses heavy-duty multi-axle trailers or intermodal handling equipment such as side-loaders or gantry cranes. It is in these larger systems that the NaturTech system achieves its best economies of compost throughput to capital cost. The way these efficiencies are achieved is similar to the mining industry by utilizing off-road trailers and heavy duty tippers that can hold up to 55 tons, making a “cycle” of moving compost from one point to another close to three times more efficient than dump trucks, saving labor, operating and capital costs.
Gantry cranes are expensive, with some models over two million dollars new, and are viable only for composting facilities that share land and equipment at existing intermodal container handling facilities. The great advantage of cranes is that they can stack containers up to five high, although the scaffolding necessary to couple the airlines and temperature sensors are safety and logistical concerns. Side-loader container handlers that operate like giant fork lifts are more practical in some circumstances, being capable of not just moving containers from the mixing area and back, but for stacking containers up to three high. They cost up to $750,000 new, although used units under $200,000 that can stack small containers two-high are often available.
Transfer trailers come in three configurations, transfer trailers, tipper trailers, and the “swing thru” trailer. All three trailers have multiple axles capable of handling the weight of either 40’ 40 ton digesters or 45’ high cube 55-ton digesters, making each trailer a custom unit. Transfer and tipper trailers have a winch system that grabs the corners of the container, lifts it, then slides the dove-tail trailer tipped at seven degrees under the trailer until it is balanced, then winches it further onto the front of the trailer. Transfer trailers can cost from $50,000 up to $150,000.
The tipper trailer is a transfer trailer that can also dump its load. It is equipped with “outriggers” like a backhoe capable of stabilizing the trailer and balancing it while compost is dumped out the rear at 55 degrees. This trailer can be taken to a surge bin with a stacking conveyor, or coupled with a hopper with a stacking conveyor. It can even dump on the ground, forming windrows. Tipper trailers can cost from $70,000 up to $250,000.
The “swing-thru” trailer is a piece of equipment used for moving containers on-site at container handling terminals. It works by straddling a container lengthwise, reaching out, picking up a container by its side, then lifting and placing it on the trailer. It can pick up and set down containers either direction. A significant advantage of the swing-thru trailer is that it can also stack containers two-high, requiring only ten-foot aisles. This can be an advantage when placing a container onto a tipping platform; especially a side-tipper as the tipper can be elevated to begin with, reducing the cost of building the tipper. The swing-thru trailer is the preferred transfer system for sites with limited available space. The swing-thru system can cost from $140,000 up to $200,000.
The transfer, tipper, and swing-thru trailers all require a separate semi tractor, often with a heavy-duty rear axle that can cost up to $100,000. Transfer trailers can be used to deliver containers full of compost to customers, setting a lockable, clean container on the site where it can be opened and unloaded using skid-steer loaders or wheelbarrows. When empty, the transfer trailer can return and pick up the empty container, perhaps delivering a second load of compost. This transfer trailer delivery system can avoid contamination and clean up problems associated with dump truck delivery systems.
The swing-thru and side-loader systems can place one way or returnable containers onto train cars, where weight is not the concern it is for road travel. If there is a suitable tipper at the other end, or a mobile tipper is transported along with a train load of containers of compost, the NaturTech system can achieve the lowest delivery costs imaginable, going hundreds, even thousands of miles to reach markets rather than tens of miles.
One of the greatest causes of failure in windrow composting systems is the lack of homogeneity and uniformity in the initial composting mixture. Front-end loader layering and mixing with a windrow turner is imprecise and too dependent on operator judgment. Only a controlled batch-mixer with a scale or a continuous flow mixer with a high degree of process control using automated hopper feeders can provide uniform recipe management. It is generally accepted by most composting professionals that controlled initial mixing is very important for all composting operations, but it is absolutely indispensable to systems that rely on forced aeration and temperature feedback. The lack of porosity or a mismanaged carbon to nitrogen ratio can be disastrous to a composting system that relies on forced air for oxygen and temperature control. We will not sell a NaturTech composting system without a controlled mixing system available.
To make the mixing stage as efficient as possible, conveyors are used to transfer the relatively small mixer batches into the containers. At two to eight mixer batches per container, it is important to be efficient with the front-end loader to keep the “cycle time” of filling a container to a minimum. Avoiding jamming of the discharge system is essential, as augers or chain conveyors such as those used in agricultural mixing operations do not work as well with composting feedstocks with irregular sized ingredients and wood chips. The viscosity and bridging properties of raw compostable materials is also a challenge, requiring cleats or ribs on a belt conveyor.
There are numerous ways to fill a container from the mixer, but they all are variations on either the front-end loader or conveyor themes. The open-top NaturTech digesters allow for either conveyor belt or front-end loader feeding. If a conveyor is used, it should be versatile if the mixer is mobile, meaning capable of being moved with the mixer alongside stationary containers, filling them without moving the containers. The front-end loader system is very basic and effective, but still requires a small discharge conveyor at the mixer to take material from the mixer to a bunker where the loader can scoop it up. If the mixer is stationary, the conveyor should be able to reach the center of the digester. For small systems under 20 tons per day, an operator standing on a mobile ladder with a rake can level the crown formed by the conveyor. Moving the digester under a stationary conveyor can fill the container.
For most systems, the conveyor should have a diffusing secondary spreader-belt capable of evening the load across the top of the bin. Even larger systems should consider an additional shuttling, or telescoping, transfer conveyor that can fill a container without having to position an operator tying up the truck to move the container. NaturTech also offers a “dumpster-veyor” apparatus that gradually moves a container under the conveyor using a rail, trolley and cable winch system that can be integrated with level sensors for automated filling.
In all cases, the ability to divert raw compost from the mixer to a staging area for loading into the container with a front-end loader should be designed for contingencies and surge capacity. The largest systems are designed with multiple mixers that share transfer conveyors with reversing shuttle belts capable of filling two containers without stopping the mixes, thereby minimizing the cycle time of filling a container and placing it on the aeration grid.
Once composting is finished, or it is time to re-mix or transfer to a windrow or curing bin, the bins must be unloaded. On small NaturTech systems with 40 cubic yard digesters, containers can be moved around the yard and dumped using conventional roll-off trucks. For the larger 80 and 110 cubic yard digesters and curing bins, unloading containers up to 60 tons is more of a challenge. No roll-off system currently made is designed to handle such large containers. Consequently, there are two options, stationary tippers and customized tipper-trailers.
Stationary tippers are fixed units that dump either to the end or sideways, depending on the site requirements. They cost anywhere from $50,000 to $300,000 depending on the length of the container, the angle of the dump (55 degrees minimum) weight of the load, and whether the semi-tractor is dumped with the container. There typically needs to be a discharging hopper and conveyor system placed in an appropriately sized dumping pit. Various surge bins capable of holding at least an entire container worth of material may be required to enable the truck to unload quickly. These surge bins are often of a “walking floor” design and can cost between $50,000 and $200,000.
From the discharge conveyor at the end of the surge bin or pit, a screening facility may be positioned, saving the need for an additional hopper-feeding system to couple with the screen. The discharge conveyor should also be able to move like a radial stacking conveyor that can be used to fill dump trucks for transfer off-site or to fill additional containers. Similar to the mixer system, a radial stacking conveyor should be able to load directly to a surge bin or stockpile area for processing at a later time.
On small operations, less than 60 tons per day, the same hopper feeder that is used at the mixer can be shared with the tipper. Some operations share the engine and hydrostatic power supply from the truck to power the mixer, screen, conveyors, and tipper. The mixer can be modified to roll up onto the truck or trailer and further used as a collection vehicle picking up feedstock from various sources. The US Navy facility added a “J” lifter to raise 60 and 90 gallon totes into the mixer hopper, diffusing it throughout the bed by briefly turning on the augers. The Mississippi Topsoils facility uses its mixer to collect thickened biosolids from underneath the discharge of a centrifuge, recording the weight, and transferring the solids to the composting area where bulking materials are added.
Stationary tippers can be used to automate the process of supplying bulking materials such as wood chips, shavings, and inoculant into the mixer. If difficult-to-handle materials such as biosolids are loaded into the mixer using a front-end loader, weigh feed belts or belts controlled by the load cell scale on the mixer can be programmed to add the appropriate weights of easier-to-handle supplemental ingredients. A container hopper and conveyor system to feed the mixers can reduce labor, indoor storage space, and front-end loader requirements. In one design, a 900-ton per day facility could be operated with a single front-end loader. The tippers are re-loaded with full containers of bulking materials when necessary, again using containers for handling bulking materials, often located outside the building. This is especially advantageous when screening compost and re-mixing, as a dumping, stockpiling, and re-loading step is eliminated.
Trailer tippers are desirable for moving raw compost to curing areas if curing is taking place in windrows. The tipper trailer can dump a partial load, walk forward, and dump again, leaving a nearly perfect windrow. Containerized composting is not just an advantage in reducing building requirements; it is an advantage in reducing material handling and on-site transfer costs. Careful selection of the transfer, tipping, mixing, and discharging equipment, especially if a sloped building configuration is available to enable gravity to assist in material transfer, can save many dollars per ton in operations efficiency.