Diverting food waste from landfills to composting or anaerobic digestion can reduce greenhouse gas emissions, enable the recovery of energy in usable forms, and create nutrient-rich soil amendments. However, many food waste streams are mixed with plastic packaging, raising concerns that food waste-derived composts and digestates may inadvertently introduce microplastics into agricultural soils. Research on the occurrence of microplastics in food waste-derived soil amendments is in an early phase and the relative importance of this potential pathway of microplastics to agricultural soils needs further clarification. In this paper, we review what is known and what is not known about the abundance of microplastics in composts, digestates, and food wastes and their effects on agricultural soils. Additionally, we highlight future research needs and suggest ways to harmonize microplastic abundance and ecotoxicity studies with the design of related policies. This review is novel in that it focuses on quantitative measures of microplastics in composts, digestates, and food wastes and discusses limitations of existing methods and implications for policy.

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.

Ecological sanitation (EcoSan) systems capture and sanitize human excreta and generate organic nutrient resources that can support more sustainable nutrient management in agricultural ecosystems. An emerging EcoSan system that is implemented in Haiti and several other contexts globally couples container-based household toilets with aerobic, thermophilic composting. This closed loop sanitation system generates organic nutrient resources that can be used as part of an ecological approach to soil nutrient management and thus has the potential to contribute to Sustainable Development Goals 2 (zero hunger), 6 (clean water and sanitation for all), and 13 (climate change solutions). However, the role of organic nutrient resources derived from human excreta in food production is poorly studied. We conducted a greenhouse experiment comparing the impact of feces-derived compost on crop production, soil nutrient cycling, and nutrient losses with two amendments produced from wastewater treatment (pelletized biosolids and biofertilizer), urea, and an unfertilized control. Excreta-derived amendments increased crop yields 2.5 times more than urea, but had differing carry-over effects. After a one-time application of compost, crop production remained elevated throughout all six crop cycles. In contrast, the carry-over of crop response lasted two and four crop cycles for biosolids and biofertilizer, respectively, and was absent for urea. Soil carbon concentration in the compost amended soils increased linearly through time from 2.0 to 2.5%, an effect not seen with other treatments. Soil nitrous oxide emissions factors ranged from 0.3% (compost) to 4.6% (biosolids), while nitrogen leaching losses were lowest for biosolids and highest for urea. These results indicate that excreta-derived compost provides plant available nutrients, while improving soil health through the addition of soil organic carbon. It also improved biogeochemical functions, indicating the potential of excreta-derived compost to close nutrient loops if implemented at larger scales. If captured and safely treated through EcoSan, human feces produced in Haiti can meet up to 13, 22, and 11% of major crop needs of nitrogen, phosphorus, and potassium, respectively.

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.

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.