Upcycling Phosphorus Recovered from Anaerobically Digested Dairy Manure to Support Production of Vegetables and Flowers

Abstract

Dissolved air flotation (DAF) separates phosphorus (P)-rich fine solids from anaerobically digested dairy manure, creating opportunities to export surplus P to the marketplace as a bagged plant food product. Seedlings of tomato and marigold were amended at various volume per volume (v/v) ratios with plant foods consisting of fine solids upcycled (i.e., transformed into a higher quality product) by drying and blending with other organic residuals. A plate competition assay was conducted to assess the fine solids’ potential to suppress the plant pathogen Rhizoctonia solani. Plant foods were comprised of 2.0–2.1% N, 0.8–0.9% P and 0.6–0.8% K. Extractions indicated that plant foods contained a mixture of plant-available and slow-release P. At 6% v/v plant food, dry biomass of marigold and tomato were six-times greater than the unamended control and not significantly different from a market alternative treatment. Fine solids exhibited negligible potential to suppress R. solani. This study indicates that DAF-separated fine solids could be used to support horticulture, providing information for design of a circular economy approach to dairy manure nutrient management. Life cycle assessment and business model development for this nutrient recovery strategy are necessary next steps to further guide sustainability efforts.

Assessment of lime‐conditioned dairy manure fine solids captured using dissolved air flotation for fertilization in horticulture

“…Thermal drying slightly increased the fraction of water‐extractable P to 18%. This is consistent with previous reports that thermal drying increases the water‐extractable P fraction of DAF‐captured dairy manure fine solids (Porterfield, Joblin, et al., 2020) and manure, potentially by promoting mineralization of organic P (Toor et al., 2006). For as‐is fine solids, 85% of total P was extractable by 2% citric acid, indicating that the majority of P would become available to plants over time.…”

“…However, the total N concentration of the thermally dried fine solids increased by 52% on an as‐is basis due to concentration of the remaining N caused by water mass loss (Table 2). This is consistent with previous work on DAF‐captured dairy manure fine solids, which found that total N concentration increased slightly on an as‐is basis despite a 61% reduction in total N on a dry basis during thermal drying to 45% total solids (Porterfield, Joblin, et al., 2020). These results indicate that retaining N during dewatering of DAF‐captured dairy manure fine solids is a challenge for both thermal drying and lime‐assisted mechanical dewatering.…”

“…Each of the seven plant foods was blended into a professional grade peat+perlite (75:25) soilless substrate (ProMix BX) at 6% v/v based on findings from previous studies (Porterfield, Joblin, et al., 2020). A control group with no fertilizer amendment was also included.…”

“…We generated six plant foods by blending either 3, 4.5, or 6% m/m QL-or LKD-conditioned and dewatered DAFcaptured fine solids (93% v/v) with 5% v/v distiller's grain and whey permeate formulation and 2% v/v manure-derived biochar. This recipe has been shown to produce comparable tomato seedling biomass to a popular market alternative in a previous studying using thermally dried DAF-captured dairy manure fine solids (Porterfield, Joblin, et al, 2020). We also generated a thermally dried plant food using the same proportions and DAF-captured fine solids dried in an oven at 105 ˚C to achieve a similar total solids content to the mechanically dewatered fine solids (∼28% total solids).…”

“…Dissolved air flotation–separated fine solids have been shown to contain the majority (85 ± 12%) of P remaining in post‐screw press dairy manure digestate, but at <25% total solids, they are still too heavy to transport long distances (Porterfield, Faulkner, et al., 2020). Products made from DAF‐captured dairy manure fine solids thermally dried to 45% total solids have demonstrated substantial fertilization value for vegetables and flowers and could potentially be marketed as bagged plant food products for horticultural production (Porterfield, Joblin, et al., 2020). However, industrial‐scale thermal drying systems require substantial capital investment (Murray et al., 2008) and may not be feasible in colder climates where there is minimal surplus thermal output from digesters in the winter (USEPA, 2011), creating a need to identify alternative strategies for drying DAF‐captured fine solids.…”