Maximizing Methane Production Potential of Farm Waste

Methane Production Potential of Farm Waste

The rate of methane production is typically expressed as the volume of gas produced (measured in cubic feet) per pound of organic solids (volatile solids) destroyed. Livestock manure typically consists of around 80% organic matter. One gallon of liquid manure consisting of 8% solid matter can potentially produce around 3,75 cu.ft. of digester gas, which in terms of methane production, equates to about 2,5 cu.ft. Methane. For every pound of volatile/organic solids digested in an efficiently-operating digester, approximately 10-13 cu.ft. of gas is produced. Considering that roughly half of the organic solids added to the digester will be destroyed, and that methane will make up between 1 half to three-quarters of the gas generated, every pound of total manure solids fed into the digester, will produce around 5 cu.ft. of digester gas, which equates to 3 cu.ft. of methane gas. The size of the digester will determine the output capacity, with between 0.75-2.50 cu.ft. of digester gas (0.5-1.5 cu.ft. methane) for each cubic foot of volume within the digester being possible. The livestock species generating the waste will also influence gas production to some degree. The table below shows the expected gas production from waste produced by dairy cows, beef cattle and swine.

Daily Waste and Methane Production by Dairy, Beef and Swine per 1000 Pounds of Animal Weight

Item Dairy Beef Swine

----------------------------------------------------------------------------------- Raw manure (lb.) 82.0 60.0 65.0 Total solids (lb.) 10.4 6.9 6.0 Volatile solids (lb.) 8.6 5.9 4.8 Methane potential (cu.ft.)* 28.4 19.4 18.6 -------------------------------------------------------------------------------------- * Based on 65 percent of gas being methane Source: Purdue University, Cooperative Extension Service

Toxins Present in Waste

Livestock waste can contain a number of substances that can negatively affect methane production when they are present in significant concentrations, the most common of which is ammonia as it forms a major component of animal urine. For optimal methane production, ammonia concentrations should not be higher than 1500 ppm (see table below). If ammonia concentrations exceed this level, then it is recommended that water is added to the waste to dilute it.

Effect of Ammonia Concentration on Methane Production

Concentration (mg/l of Ammonia-N) Effect --------------------------------------------------------------------------------------------------------------- 5 - 200 Beneficial 200 - 1000 No adverse effect 1500 - 3000 Possible inhibition at higher pH Values Above 3000 Toxic ---------------------------------------------------------------------------------------------------------------- Source: Purdue University, Cooperative Extension Service Other toxins that can inhibit methane production include cleaning disinfectants and antibiotics. Therefore waste collected from farrowing buildings should not be added to the digester. By the same token, rumensin, an antibiotic routinely used for dosing cattle, should not be given to livestock whose waste will be used to produce methane.

Benefits of Adding Crop Residues

For optimal methane production, the carbon to nitrogen (C:N) ratio of waste fed into the digester should ideally be 20:1 (ie. 20 parts carbon to 1 part nitrogen). However, one of the key limitations in using livestock waste is its high nitrogen content in comparison to its carbon content. Mixing crop residues — which typically have low nitrogen and high carbon levels — can be very useful for ensuring a better C:N ratio, and consequently, for improving methane production in the digester. Featured Image By Ryan Thompson/U.S. Department of Agriculture, [CC BY 2.0], via Flickr

Reference: Methane Generation from Livestock Waste, Purdue University, Cooperative Extension Service