Anaerobic digestion in a closed system is an alternative treatment for liquid manure which produces biogas, approximately 60% methane and 40% carbon dioxide.  Digestion also reduces odors and the chemical oxygen demand (COD) of the manure.  In Europe, high COD levels have been associated with disruption of the microbial ecosystem in soils due to reduced oxygen levels during rapid degradation of manure.  Within the digester, mixing occurs by circulation of biogas and injecting a small amount of oxygen into a central tube reduces hydrogen sulfide.  During digestion, the characteristics of the manure are changed so that when sprayed as a fertilizer, it is less “sticky” and absorbs more quickly into the ground.  While the initial investment in a digester may be high, as the capacity of a plant increases the cost of treatment per ton of manure decreases.  Costs may also be offset by charging tipping fees for co-substrates and by producing biogas for use in the operations or sale to a power grid.