Digester Gas Production Troubleshooting
Amana Farms consists of 6,750 acres of crops, a 2,400-head cow/calf herd, a 4,000-head cattle feed lot, and a 1.69 million gallon anaerobic digester that is set up at the feedlot to digest manure generated at the farm, combined with waste from local industries. The primary purpose of digesting these wastes is to generate methane gas which is used to fuel engine generators that produce electricity that is sold on the Amana grid to power the Amana colonies. The facility was designed to generate 2,600 kW electricity. However, since its installation in August 2008, biogas production from the digester has been far below expectations. Consequently, electrical power generation has ranged from only 300 kW to 1,100 kW. Amana Farms retained HR Green to determine the cause(s) of the low gas production and provide recommendations to improve performance. HR Green first recommended analysis of the ingredients being fed to the digester to determine if toxins were hampering the anaerobic process, and whether sufficient carbon was being supplied to the digester.
Test results revealed the following. First, the carbon content of influent wastes was severely deficient. Second, the digester was overloaded with ammonia, thus upsetting the carbon-nitrogen ratio. Third, with the exception of ammonia, no toxins were present in sufficient quantities to adversely impact the digester. Finally, the industrial wastes were not high enough in carbon content to warrant the volumes being fed.
Given these results, HR Green recommended that new substrates should be identified, tested and if the results are acceptable be introduced to the digester. HR Green recommended the parameters that should be tested and provided guidelines with regard to interpreting test results. In general the new sources should be low in ammonia, be within the acceptable pH range and be rich in carbon. Amana Farms has since identified multiple potential substrates and full scale testing of these substrates is continuing.