Nutrient Recovery from Anaerobically Digested Dairy Manure Using Dissolved Air Flotation (DAF)

Digestate application to soils located near anaerobic digesters can potentially contribute to nutrient loss to downstream freshwater ecosystems and associated eutrophication. However, it is cost-prohibitive to transport digestate to crop production areas further from digesters due to high water content. We evaluated the potential to recover phosphorus (P), as well as nitrogen (N) and potassium (K), from dairy manure digestate using dissolved air flotation (DAF) to separate fine solids. We measured flow rates and collected influent and effluent samples on 45 dates spanning 3.5 months from a DAF system fed by the post-screw press digestate on a Vermont dairy farm. Separated fine solids recovered during the initiation period contained 29 +/- 3% of N, 71 +/- 18% of P, and 11 +/- 2% of K from the influent. At full operational capability, the DAF system captured 36 +/- 2% of N, 85 +/- 12% of P, and 11 +/- 1% of K from the influent in the separated fine solids. These findings indicate that most P remaining in the post-screw press dairy manure digestate can be recovered using DAF, while remaining effluent has an expanded range of use options due to a 3-fold increase in the N:P ratio.