Commercial Composting - How It Works
Brought to you by Lincolnshire recycling and composting company, Down To Earth Recycling LtdThere are four main types of composting system:
- turned windrows;
- static piles;
- covered-aerated static piles;
- in-vessel composting.
The system used depends on the type and amount of organic material to be composted, available land area, the odour-sensitivity of the site, financial resources available, the desired end product and the length of time the material can remain in-process.
In turned windrow composting, the material is arranged in long rows (windrows) that are aerated by convective air movement, diffusion, and periodic mechanical turning that exposes the material to oxygen. The raw materials are mixed and aerated with front-end loaders or windrow turners, turning frequently during the first period of high oxygen demand and heat generation, and then turned less frequently as the composting process proceeds.
Static piles are also formed in the shape of windrows but may be higher and wider. The term static piles refers to the fact that these piles are not turned, or are turned only several times a year, generally using a front end loader. Static systems tend to be less expensive in terms of manpower and equipment than windrows, but require more land area because the material decomposes more slowly and stays on the site longer.
Covered-aerated, static-pile composting is useful for co-composting yard waste with sludge or manures. A forced aeration system is placed under the piles to enable a minimum oxygen level throughout the composting mass.
In-vessel, bin or closed-reactor composting takes place in partially or entirely enclosed containers in which environmental conditions are closely controlled. The principles of operation are essentially the same as for windrow and static pile systems as material is piled (in a container) and aerated by turning or forced air. In-vessel systems are more Space efficiency is an advantage of this system and the operators have greater process controls. However, in-vessel composting is much more expensive than other methods.
Co-composting is the composting of a mixture of materials such as sludge cake and green waste. High quality compost products can be produced by co-composting, provided that the primary material is carefully selected and reasonably consistent in quality to ensure that a suitably open texture is maintained. In addition to sewage sludges, co-composting is also suitable for industrial organic sludges, such as those generated by the food industry, and by brewing and fermenting activities.
Partly processed compost Partly composted material
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 material from the large, chop it up into smaller particles, and mix it. This may be tailored to suit the particular end product, 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.
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. Different wastes are mixed, such as wastes which are high in nitrogen with those which are low in nitrogen. There are several types of grinders available for grinding green waste; 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.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. The 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, and is a form of hammermill system commonly used for green waste.
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. Chipper disks use a high speed, rotating "toothed" disk.
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 material such as grass cuttings. Mixing green waste in this way before placing it in a composting system ensures good contact between materials high and low in nitrogen, to make a more uniform compost. The equipment may use an auger system to mix the material. 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. |  Compost Turner at Down To Earth Recycling's site |
