Courtesy : www.mswmanagement.com
Composting technology latest
The management of solid wastes, particularly municipal solid waste (MSW), has undergone substantial changes over the last 30 years. Most of the changes have taken place in industrialized countries and address issues related to the protection of public health and of the environment as well as resource conservation.
The typical components of MSW include paper products, various types of plastics, metals, glass, biodegradable matter such as garden and yard trimmings, kitchen and food waste, and other miscellaneous components. Advances have been made in the storage, collection, and processing for most of these materials to divert them from disposal in landfills. One of the more important developments in the waste processing industry has taken place in the treatment of the biodegradable components.
Currently, there are close to 3,500 composting facilities dealing with yard waste in the US alone (Simmons, P.; Goldstein, N.; Kauffman, S.M.; Themelis, N. J.; and Thompson, J. “The State of Garbage in America”; BioCycle; April 2006). The growth in the use of composting throughout the world has led to the development and implementation of many innovations in the equipment and systems used to process and stabilize the materials. The challenges become more complex when the materials are highly putrescible (such as food wastes) and the processing facilities are located relatively close to human populations.
Because of the interest in the application of composting and due to the relatively large number of developments in the technical areas, the authors decided to publish a book (Modern Composting Technologies) on the latest developments in the equipment and systems being used primarily in Western Europe and in North America. This article provides a brief overview of the equipment and systems discussed in the book.
Processing Equipment
Size Reduction
In general, the types of size-reduction units include grinders, shredders, and chippers. Another type of size-reduction unit, described in Modern Composting Technologies, is the Archimedean screw. This design originally was used in wagons to break up or mix animal feed. As indicated in the book: “The shattering/mixing trailers used for the composting process are characterized by having from two to four Archimedean screws.”
Screening
Screening is a unit operation that involves the separation of a mixture of materials with different particle size distributions into two or more fractions (each fraction having a discrete spectrum of particle sizes) by means of one or more sets of screen openings. The material that remains on the screening surface generally is known as the oversize (or plus) material. Material passing through the screen openings is generally referred to as the undersize (or minus) material. The screening surface may be fabricated using plastic, cloth, wire mesh, perforated steel plate, or steel bars.
In general, industrial screening processes can be conducted when the material is wet or dry. However, some materials are difficult to screen efficiently if they are too wet. There are several types of screens; the most common units used in the composting industry are deck (flat), disc, grizzly, orbital, star, and trommel screens.
Air Classification
Air classification has not been widely utilized in composting operations, particularly in North America. This situation has started to change as equipment specifically designed to process compost has entered the market.
Air classifiers generally are used for refining the finished product, in particular to remove contamination after size reduction or after screening. It is usually one of several steps used to remove plastics from compost. Hand sorting and screening still remain important components in producing compost that is nearly free of plastics.
In-Vessel Systems
In-vessel systems can be divided into two main categories: vertical and horizontal bioreactors. Horizontal bioreactors can be further subdivided into horizontal reactors composed of channels, cells, containers, or tunnels. Another type of reactor is the rotating drum. The rotating drum generally is placed before the first phase of the composting process to “condition” the feedstocks (to size-reduce and mix the material) as well as initiate the composting process.