Nutrient recovery abates methane emissions from digestate storage

Farm-based anaerobic digestion is a promising source of renewable natural gas (RNG), but the carbon intensity of RNG is influenced by methane (CH 4 ) emissions from digestate storage. This study evaluated whether a nutrient recovery system could minimize digestate CH 4 emissions from uncovered storage. Nutrients were concentrated in the solid fraction, which contained 82% of the Phosphorus (P) and 39% of the Nitrogen (N) with only 7% of the water. Laboratory results showed that the CH 4 emission potential from digestate was 113 ± 6 mL g −1 volatile solids (VS), which was reduced to zero in the liquid fraction after nutrient separation. Measurements from the 0.4-ha uncovered outdoor liquid storage showed CH 4 emissions were low, with annual emissions being 1.5% of the CH 4 collected by the biodigester. Storage emissions could be further reduced by ensuring that all manure passed through the nutrient recovery system. Overall, the combination of biogas and nutrient recovery reduces the carbon intensity of RNG and improves nutrient distribution. • Nutrient recovery reduced CH 4 emissions from uncovered digestate storage • Reduced VS concentration by 92% and eliminated CH 4 potential from liquid digestate • Annual CH 4 emissions from outdoor liquid storages were 1.5% of biogas CH 4 production • Outdoor storage emissions would be lower if not for presence of some unprocessed manure • On a mass basis, 82% of P and 39% of N was field applied in the solid fraction with only 7% of water