Online

O'Brien, Brendan J.

Rubenstein School of Environment and Natural Resources

2019

M.S.

Organics recycling is increasing in New England as multiple states have enacted laws to divert organic materials, including food scraps and food processing residuals, away from landfills. Anaerobic digesters on dairy farms represent an attractive approach to food waste recycling because existing infrastructure is in place and co-digestion of dairy manure with food waste can increase renewable biogas production. In addition, anaerobic digestion results in effluents that can be separated into solid and liquid residual materials, or 'digestates'. Screw-press separated solids consist of lignocellulosic biomass resistant to microbial degradation during anaerobic digestion. These separated solids are typically recycled on farms as animal bedding before returning to the digester, whereas remaining liquid digestates are typically spread as fertilizer for nearby feed crops or pasture fields. Within this model, anaerobic digestion is not a nutrient management solution and repeated land application of digestate nutrients can create eutrophication risk over time. Alternative models are needed where digestate materials are converted into valuable products to be sold off-farm, enabling the removal of nutrients to help meet nutrient management goals. In this thesis, I address two research questions related to the pursuit of such alternative models. First, how do physicochemical characteristics of digestate materials vary across full-scale systems in the region, including systems with and without food waste as a substantial proportion of feedstock, and how do these variations affect the potential for conversion of digestates into valuable products (e.g., soil amendments)? Second, can separated digestate solids be used for commercial cultivation of gourmet oyster mushrooms (Pleurotus ostreatus) to produce food for human consumption, providing synchronous nutrient recovery and food production? Results from my first research chapter indicate that increasing food waste inputs (and thus diversification of feedstock recipes) will likely increase the variability of some solid and liquid digestate characteristics and can result in greater contamination with synthetic particles, with implications for nutrient recovery efforts and associated products. My second research chapter shows that screw-press separated digestate solids can offset non-local substrate ingredients to a degree while achieving oyster mushroom yields comparable to commercial recipes. Furthermore, this strategy could divert nutrients away from land adjacent to digesters and directly into safe, nutritious, protein-rich food for humans, while also producing a useful spent mushroom substrate product.