A typical large refinery costs billions of dollars to build and millions more to maintain. It runs 24/7, employs up to 2,000 people and occupies as much land as several hundred football fields. There are 159 refineries in the U.S. with a capacity for distilling 17,177,371 barrels a day. A typical refinery turns about half of a 42-gallon barrel of crude oil into gasoline. Essentially, refining breaks crude oil down into its various components, which then are reconfigured into new products. When the oil arrives at the refinery, it is transferred to a fractions. The simplest refineries stop at this point. Most in the United States, however, reprocess the heavier fractions into lighter products to maximize the output of the most desirable products, like gasoline, diesel, and jet fuel. The type of crude oil dictates the level of processing necessary. There are a number of processes that take heavy, low-valued feedstock and change it into lighter, higher-valued output. A catalytic cracker, for instance, uses the gasoil (heavy distillate) output from crude distillation as its feedstock and produces additional finished distillates (heating oil and diesel) and gasoline. Sulfur removal is accomplished in a hydrotreater. A reforming unit produces higher-octane components for gasoline from lower octane feedstock that was recovered in the distillation process. A coker uses the heaviest output of distillation, the residue or residuum, to produce a lighter feedstock for further processing, as well as petroleum coke. Once products are refined, they are stored until they can be delivered to various markets such as gas stations, airports and chemical plants. In addition to making the oil-based products, refineries must also treat the wastes involved in the processes to minimize air and water pollution.