Composting Processing Equipment
Once green waste has been received at the site it needs to be processed mechanically to sieve or screen out the smaller fraction from the large, chop it up into smaller particles, and mix it. This may be tailored to suit the particular product which the operator seeks to achieve, and will assist in the composting process itself.
Consistency and homogeneity of the end product will only be achieved by use of the right processing plant. During processing the water content may also be optimised by controlled addition where necessary.
This section covers the common types of equipment which are used for the processing of green waste for composting and mulch production.
Grinding Equipment
Grinding or milling green waste and other wastes accelerates the decomposition process by reducing particle size, exposing more surface area of the material to the activity of micro-organisms. It aids in mixing different wastes, such as wastes which are high in nitrogen with those which are low in nitrogen. Also, in the case of aerobic composting, more surface area is exposed to oxygen. However, if the material is ground too fine, it can compact to the extent that the passage of air is impeded and oxygen is not able to penetrate the mass. This delays the rate of aerobic decomposition and can result in anaerobic conditions.
There are several types of grinders available for grinding green waste, and these fall into several categories. These include hammermills, slow speed shredders, and chipper disks. The tonnages of material ground per hour varies depending on the size and design of the equipment, the nature of the material being ground, and the desired particle size. An important feature for grinding green waste is for the grinder to have a wide throat which will accept brushy material and logs. Some systems reportedly process up to 150 tonne per hour, but amounts of 20 to 30 tonne per hour are more common (Glenn, 1990). Operating costs for grinders, including depreciation, may run to as much as several hundred pounds per hour.
Hammermills have 18 or more swinging or fixed hammers that grind the material. These hammers may weigh from a few pounds to over 50 pounds. Hammers need to be resurfaced or turned frequently to maintain a hard, efficient grinding surface. These grinders may also incorporate screens or bars and sieves which determine the particle size of the finished material.
A tub grinder, has a rotating cylindrical hopper, is a from of hammermill system commonly used for green waste. These either have an integral power unit or use a PTO shaft to transfer power from an auxiliary power unit or a tractor. Prices for self-powered models range from £40,000 to £100,000; for PTO powered models, from £15,000 to £50,000.
Slow-speed shredders and chipper disks do not use swinging hammers. Slow-speed shredders may use a system of slowly rotating augers to shred the material. Augers also need to be resurfaced to maintain an effective grinding surface, but resurfacing is less frequent than with swinging hammers. Chipper disks use a high speed, rotating, "toothed" disk. Both are generally self powered. Prices may range from £20,000 to £300,000. 
Each type of grinding equipment has advantages and disadvantages. Slow-speed shredders, for example, are usually not damaged by large pieces of metal, which may contaminate green waste, but tub grinders may suffer extensive damage. On the other hand, tub grinders generally produce a smaller and more uniform particle size. If boiler fuel is being produced, uniformity of particle size is not as an important a consideration and chippers may be the more appropriate equipment.
Mixing Equipment
For co-composting green waste with nutrient amendments, such as manures or sewage sludge, mixing equipment may be useful, or even just to ensure a correct balance between woody material and green (eg grass cuttings). Mixing green waste and nutrient amendments before placing them in a composting system ensures good contact between materials high and low in nitrogen, thus permitting more uniform composting. The equipment may use an auger system to mix the material. Mixing equipment is built in two styles: continuous feed units or batch type units. Continuous feed machines may process from 10 to 300 tonne per hour, while batch systems have capacities of 4 to 20 cubic yards per batch. These machines range in price from £5,000 to £80,000. If a windrow turner or other compost turning system is used at the site, separate mixing equipment may not be needed.
Compost Turners
These machines are used to aerate and mix the composting material, thereby accelerating the composting process. Several types of machines have been designed specifically for turning compost. These fall into two broad categories: machines which move through the free-standing windrow and machines which move through compost contained in a bin. In addition to these machines, front-end loaders are used for turning compost because they are usually readily available at composting sites. Front-end loaders do not mix the compost as uniformly nor accelerate the composting process as much as compost turners do.
The compost turners which move through free-standing windrows are either self-propelled or are powered by a PTO. The PTO-driven models are pushed or pulled by a tractor or other prime mover. These machines use several designs to aerate and mix the material. One design has a moving, elevating face with tines to lift, aerate and mix the material. A second type of design utilises a rotating drum, with tines, that moves through the material, and a third type of design uses a paddle system to move the material. The height and width of the windrows must not exceed the capacity of the machine design. Maximum height and width appear to be about 3 metres by 7 metres respectively. Self contained units cost from £75,000 to £170,000, and PTO-driven units cost from £40,000 to £55,000.
Compost turners which move down an enclosed bin are generally of two designs: a rotating shaft with paddles or a rotating drum with tines. These units have integral power units and usually are built on a concrete slab or some other hard surface. The turners travel down rails on the side walls as they aerate and mix the compost. They are designed to move the compost down the bin and expel finished compost each day. The total residence time of the material in the bin may be 21 days or more, depending on the material and the design and operation of the system. These systems may be enclosed in a building or covered by a pole barn style structure to protect them from rain. The systems may be relatively inexpensive (under £100,000) or extremely expensive, depending on the design and capacity.
Screening Equipment
Screens may be used to remove soil, sand and fines in green waste prior to grinding or they may be used in the finishing step to produce a uniform, high  quality compost product. Decisions on the size of the screen opening to be used are usually based on the intended market for the material. Screens for processing the finished compost are used to remove debris and oversized material. Two types of screens are commonly used in green waste composting operations: rotating, trommel screens and vibrating screens. Volumes processed through these screens may range from 15 to 400 cubic yards per hour, and the cost ranges from £12,000 to £300,000.
Suppliers of this plant can be identified on our supplier matrix.
With acknowledgements to the Florida Co-operative Extension Service,11/91 Circular 1027. (http://edis.ifas.ufl.edu/HE027 ).
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