A trommel (from the Dutch word for drum, "trommel") is a screened cylinder used to separate materials by size - for example, separating the biodegradable fraction of mixed municipal waste or separating different sizes of crushed stone.
Portable trommels (also called portable trommel screens) are often used in the production of organic products from various types of waste.
For example, excavation contractors may screen their site debris into two fractions; a saleable topsoil for farms, nurseries and site-work, as well as cleaned rock for aggregates or landscaping work. This allows the contractor to resell their waste, instead of incurring the cost of sending it for disposal.
The same principal applies to the production of compost, sand/gravel, lumber mill by-products and municipal waste.