Hydrogen gas is another reaction byproduct. Methanogens are microorganisms that produce methane as a metabolic byproduct in hypoxic conditions. Diacetyl arises naturally as a byproduct of fermentation. However, most elemental selenium comes as a byproduct of refining copper or producing sulfuric acid. Paraffin wax is a byproduct of refined petroleum. Acetic acid is an example of one possible byproduct from hot copper oxidation. Ethanol is generally available as a byproduct of sugar production. Phenol is also a recoverable byproduct of coal pyrolysis. The CO that is captured is a high purity byproduct of nitrogen fertilizer production. The helium is an inert byproduct. Trichloroethylene is a major byproduct, which is separated by distillation. The land is contaminated with arsenic, reportedly a byproduct of farming. A major byproduct of pig production is slurry. Silica fume is obtained as byproduct from hot processes like ferrosilicon production. Stearin is obtained from animal fats created as a byproduct of processing beef.