Anaerobic digestion, solid-liquid separation, and drying of dairy manure: Measuring constituents and modeling emission

Abstract Anaerobic digestion (AD) and solid-liquid separation (SLS) can increase operational flexibility at livestock facilities, but they can also affect environmental impacts during downstream manure handling. In this study, manure was characterized before and after AD, SLS, and drying. The measured data were used as inputs to models to estimate greenhouse gas (GHG) and ammonia (NH3) emissions during manure storage and land application. Nine dairy farms were sampled between each processing component to evaluate total solids (TS), volatile solids (VS), chemical oxygen demand (COD), total nitrogen (TN), total ammoniacal nitrogen (TAN), total phosphorus (TP), and total potassium (TK). AD systems with co-digestion have higher VS reduction than AD systems processing only dairy manure. SLS data indicate that both screw presses and centrifuges achieve higher separation efficiencies (mass in the solids) for TS and VS than for the other manure constituents. The farm with centrifugation achieves the highest separation efficiency for TP. TAN and TK are not well concentrated in the solids fraction for any processing system. TAN remains entirely within the liquid fraction, showing that each constituent has its own separation profile. Drying manure results in moisture, VS, and TAN losses. Since TAN stays with the liquids, these losses are negligible. When analyzing modeling results, most GHGs are emitted during storage as methane. However, land application is the major emitter of nitrous oxide and NH3. Both AD and SLS can reduce GHG emissions, with the combined AD and SLS scenario achieving the highest reduction (41%). AD increases NH3 emissions during storage due to the mineralization process during digestion. SLS alone can achieve significant GHG emission reductions (38%) even greater than AD when using actual performance data from operating systems. Both AD and SLS have the potential to reach higher GHG and NH3 emission reductions with improved technology efficiencies and management.