Rotary drum reactors (RDR) have been used since the early 1970's by the composting industry for pretreating and separating organic fractions from municipal solid waste (MSW) prior to composting. The RDR process consists of a long, horizontal rotating vessel followed by screens for separating the organic fraction. RDR systems not only separate organic materials from mixed MSW, but they also increase the speed of composting. Gasses and odors are treated using biofilters. Operating the RDRs with a reduced retention time can potentially have both positive and negative effects. On the one hand, fewer organics may be recovered due to insufficient time for the larger organic materials to break down enough to fit through the screen holes. Composting facilities that plan to install RDR systems may wish to consider a smaller RDR in conjunction with an anaerobic digester as an alternative to the larger RDR required for aerobic composting alone.