• E-mail
• Print
• Comment
• Font Size
• Digg
• del.icio.us
• Discuss article

Enclosed Aerated Static Piles and "in-Vessel" Semantics

Posted on: Saturday, 6 August 2005, 03:01 CDT

WHEN the U.S. Environmental Protection Agency adopted its pathogen reduction standards for biosoiids composting, horizontal agitated bed composting was categorized as windrow composting. This meant these in-vessel systems had to meet the 55C consecutive day time/temperature requirement for 15 days. Other in-vessel technologies, such as vertical reactors, fell into the "three days at 55C'' time/temperature requirement.

At the time, this regulatory status led to a casual debate about whether agitated bed systems were "in-vessel" or windrows contained within concrete walls. Over time, it didn't really matter as long as the systems performed as the operators and regulators expected.

A few years ago, a casual debate evolved at the BioCycle offices when working on our annual composting equipment and systems directory. An increasing number of composting projects were using aerated static piles enclosed in plastic bags (either Ag-Bag or CTI (now Versa Corp.) systems. The question was: should we characterize these systems as in-vessel as they completely contain the composting pile? Would that be confusing as typically composting "vessels" are rigid structures made from concrete or metal?

The questions became easier to answer when several states gave permits. to sites composting food residuals in piles that had to be contained, i.e., they could not be on a bare surface and they also had to control vector attraction and leachate. In California, several fairly large composting facilities were permitted to take food residuals using the AgBag or Versa systems. A site in Minnesota and another in Massachusetts received similar permits.

More recently, a new technology was introduced in North America using the Gore Cover composting system. This is essentially an enclosed aerated static pile. In this case, piles are constructed over concrete floors with aeration channels, then covered with a fabric made from Gore-Tex. In Washington State, a large-scale composter was permitted to take residential and commercial organics (it had been receiving the latter, but in fairly small volumes) as long as it installed a contained composting system. In this case, the company installed the Gore system.

The Gore installations in North America include several composting biosolids and several processing yard trimmings and source separated organics. All include forced aeration. Explains Modern Composting Technologies, "the Gore-Tex membranes keep precipitation from entering the windrow, but allow the CO^sub 2^ produced during the composting process to escape. In addition, the membrane acts as a physical barrier against gaseous substances escaping from the composting material. During the composting process, a fine film of condensation usually is formed on the inside of the cover. The condensation contributes to the suppression of odors and other gaseous substances. Condensation of the water vapor on the inside surface of the cover minimizes dehydration of the biomass, allowing a normal evolution of the composting process without the requirement for additional water."

Modern Composting Technologies also includes descriptions of the Versa 0 and Ag-Bag systems: "A particular type of forced aeration system for windrow composting is based on the utilization of large plastic bags, with diameters of 8- to 12 feet (2.4 to 3.6 m) and lengths of up to 500 feet (150 m), into which the material is forced by means of a press. The bags are equipped with two perforated plastic pipes, connected to a blower for the supply of air. In addition, the bags have a pattern of openings in the upper portion to allow for the release of gases.

"The pipes are inserted into the biomass with the same unit that is used to compress and spread the plastic sheet. The bag is formed during the progressive withdrawal of the press at the time that the material to be composted is loaded. The first and second phases of the process are conducted sequentially-in the same bag and last about eight weeks." -N.G.

Copyright J.G. Press Inc. Jul 2005

Source: BioCycle

More News in this Category