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

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

Dissolved air flotation (DAF) has shown potential to substantially improve phosphorus (P) mass balance on dairy farms by capturing P associated with fine solids from liquid manure, enabling new management options. However, at <25% total solids, further dewatering is necessary to facilitate export of recovered fine solids off farm for use in bagged or bulk products. Physical conditioners such as quicklime (QL) and lime kiln dust (LKD) are commonly used to enhance mechanical dewatering of biosolids, but their effect on the properties and fertilization value of DAF-captured manure fine solids has not been documented. We generated plant foods using DAF-captured dairy manure fine solids conditioned with 3, 4.5, and 6% m/m QL or LKD and dewatered using a benchtop press for comparison with thermally dried fine solids. Tomato (Solanum lycopersicum L.) seedlings were grown in a soilless substrate amended with 6% v/v plant food and in an unamended control. Thermally dried and LKD plant foods produced significantly greater seedling biomass than QL plant foods and the unamended control. Quicklime- and LKD-conditioned fine solids contained approximately 30× and 10× less water-extractable P than thermally dried fine solids, respectively, likely due to precipitation of Ca-P minerals. The elevated pH (≥10) of the lime-conditioned fine solids could have also suppressed plant growth. These effects limit horticultural applications but could be beneficial in agricultural field settings where slow-release P is desirable. Research beyond this preliminary assessment is needed to determine the practicality and sustainability of the approach along with longer-term nutrient bioavailability.

Second-Generation Phosphorus: Recovery from Wastes towards the Sustainability of Production Chains

Phosphorus (P) is essential for life and has a fundamental role in industry and the world food production system. The present work describes different technologies adopted for what is called the second-generation P recovery framework, that encompass the P obtained from residues and wastes. The second-generation P has a high potential to substitute the first-generation P comprising that originally mined from rock phosphates for agricultural production. Several physical, chemical, and biological processes are available for use in second-generation P recovery. They include both concentrating and recovery technologies: (1) chemical extraction using magnesium and calcium precipitating compounds yielding struvite, newberyite and calcium phosphates; (2) thermal treatments like combustion, hydrothermal carbonization, and pyrolysis; (3) nanofiltration and ion exchange methods; (4) electrochemical processes; and (5) biological processes such as composting, algae uptake, and phosphate accumulating microorganisms (PAOs). However, the best technology to use depends on the characteristic of the waste, the purpose of the process, the cost, and the availability of land. The exhaustion of deposits (economic problem) and the accumulation of P (environmental problem) are the main drivers to incentivize the P’s recovery from various wastes. Besides promoting the resource’s safety, the recovery of P introduces the residues as raw materials, closing the productive systems loop and reducing their environmental damage.