GHG EMISSIONS REDUCTION FROM AN ANAEROBIC DIGESTER IN A DAIRY FARM: THEORY AND PRACTICE

Abstract. Despite documented and theoretical models showing the benefit of an anaerobic digester (AD), the actual implementation of digesters has been low. An AD located in the Pacific Northwest dairy with 6,200 heads of Holstein milking cows was studied. GHG emissions from a modified plug flow AD running under average or optimal conditions were compared to the base case which was an anaerobic lagoon prior to AD installation. IPCC Tier-2 model was used to estimate the GHG emissions from different waste management strategies. The results showed that the AD reduced the GHG emissions by 47.2% (±4.2%) compared to anaerobic lagoon. The same AD running under its peak performance (optimal condition) would have reduced GHG by 72.7%. The real AD could not operate at its peak performance year round because of three main reasons: i) the cows were allowed to be in outdoor drylot during summer months from which collected manure was not fed into AD, ii) Problems encountered in the AD operation, and iii) gensets were not able to use 100% of the produced methane. For every 1% increase in manure fed into AD, GHG emissions were reduced by 0.96%. If the digester was running 100% of time and if 90% of manure could have been used, the implementation of AD would have reduced the GHG emission by 86.4% compared to anaerobic lagoon. It was concluded that a robust design of AD could reduce or even eliminate operational problems and may improve overall AD acceptance.