High-solids reactor

Solid waste disposal is a serious problem in the United States and other developed countries. Solid waste can constitute valuable raw materials for commercial and industrial operations, however, and one of the challenges facing scientists is to develop an economically efficient method for utilizing it.

Although the concept of bacterial waste conversion is simple, achieving an efficient method for putting the technique into practice is difficult. The main problem is that efficiency of conversion requires increasing the ratio of solids to water in the mixture, and this makes mixing more difficult mechanically. The high-solids reactor was designed by scientists at the Solar Energy Research Institute (SERI) to solve this problem. It consists of a cylindrical tube on a horizontal axis, and an agitator shaft running through the middle of it, which contains a number of Teflon-coated paddles oriented at 90 degrees to each other. The pilot reactors operated by SERI had a capacity of 2.6 gal (10 l).

SERI scientists modeled the high solids reactor after similar devices used in the plastics industry to mix highly viscous materials. With the reactor, they have been able to process materials with 30–35% solids content, while existing reactors normally handle wastes with five to eight% solid content. With higher solid content, SERI reactors have achieved a yield of methane five to eight times greater than that obtained from conventional mixers. Researchers hope to be able to process wastes with solid content ranging anywhere from zero to 100%. They believe that they can eventually achieve 80% efficiency in converting biomass to methane.

The most obvious application of the high-solids reactor is the processing of municipal solid wastes. Initial tests were carried out with sludge obtained from sewage treatment plants in Denver, Los Angeles, and Chicago. In all cases, conversion of solids in the sludge to methane was successful, and other applications of the reactor are also being considered. For example, it can be used to leach out uranium from mine wastes: anaerobic bacteria in the reactor will reduce uranium in the wastes and the uranium will then be absorbed on the bacteria or on ion exchange resins. The use of the reactor to clean contaminated soil is also being considered in the hope is that this will provide a desirable alternative to current processes for cleaning soil , which create large volumes of contaminated water.

See also Biomass fuel; Solid waste incineration; Solid waste recycling and recovery; Solid waste volume reduction; Waste management

[David E. Newton ]

RESOURCES

PERIODICALS

"High Solids Reactor May Reduce Capital Costs." Bioprocessing Technology (June 1990).
"SERI Looking for Partners for Solar-Powered High Solids Reactor." Waste Treatment Technology News (October 1990).