Is added to the top, and the end-product is removed from the bottom. Batch composters (including Carousel, Vera, and nearly all of the site-built composters worldwide) are actually two or more composters that are filled and then allowed to cure without the continuous addition of new potentially pathogen contaminated excrement. Alternating concrete double-bins are the most common batch system, although several systems use polyethylene 55-gallon drum that contain the process.