Aerobic digestion of food waste as a precursor for energy and resource recovery technology
Date
2021
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
One potential solution to manage food waste is the use of on-site aerobic bio-digesters. The technology utilizes a combination of a microbial consortium made up of various bacteria and fungi, warm water, and mechanical aeration to liquefy the food waste into a slurry. The overall goal of the thesis is to evaluate the aerobic digestion process as a potential method of food waste disposal. Currently, the effluent from the bio-digester is discharged to existing sewer infrastructure and treated at local wastewater treatment plants (WWTPs). In order to assess the impact of the aerobic digester effluent on receiving WWTP, we visited three local WWTP and conducted interviews with current and former WWTP superintendents and operators. All interview participants commented that even though the organics (COD) and solids (TSS) contents of digestate are very high, the current flow rates generated from these digesters are insignificant compared to the total influent flow to WWTPs. They all agreed that based on the current scale and distribution of the biodigesters, they would have no noticeable impact on their plants and treatment processes. ☐ This research also aimed to better understand the microbial activity and its effect on the solubilization of the food waste during the aerobic degradation process. A series of experiments using three food materials (apples, cucumbers, and pasta) were conducted using a laboratory scale digester to simulate the operation of the BioHiTech® bio-digester. Abiotic control experiments were compared to microbial-seeded experiments to determine if the presence of microorganisms positively affected the solubilization of solids, organics, nutrients, and carbohydrates. The results of these experiments showed that, for all three food materials, the presence of microorganisms increased the release of original solid materials into solution, suggesting that the microbial activity enhances the degradation of food waste and the liquefaction process. Pasta, which was the only food that was pre-cooked prior to digestion, exhibited the greatest increase in total solids concentrations, suggesting that cooked food materials may be more susceptible to microbial decomposition than uncooked fresh produce. Similarly, total dissolved solids (TDS) concentrations in the digestate of all three foods increased in the presence of microorganisms with the pasta (cooked) exhibiting the greatest increase in TDS in comparison to fresh apple and cucumber. On the other hand, the presence of microorganisms decreased TCOD concentrations in the digestate for all three food materials. These decreases in COD concentrations in the presence of microorganisms suggest the consumption of organic constituents by the microorganisms to fuel their growth and metabolism. Similarly, carbohydrate and total inorganic nitrogen (TIN) concentrations in digestates from microbial experiments were substantially lower than that in the control digestates. Biosynthesis calculations were performed to estimate the theoretical consumption of carbohydrates for the removed nitrogen, assuming the consumed TIN concentrations were incorporated into biosynthetic reactions exclusively. These calculations showed that in the absence of microbial consumption, the microorganisms would facilitate the solubilization and accumulation of these target constituents in the digestate. ☐ Understanding the effects of the microbial community within the digester is critical for the widespread application of this technology as a pretreatment method to improve anaerobic digestion and waste valorization. Optimizing this process will help reduce GHG emissions and promote a circular economy with less waste and pollution. ☐ It is clear from the results that the microorganisms present within the digester contribute to both the release and consumption of food waste constituents. The results also indicated that the residence time and the food waste composition can significantly affect the environment within the digester. These variables will need to be optimized to maximize the release of valuable constituents, such as, organics, nutrients, and macromolecules.
Description
Keywords
Aerobic digestion, Biogas, Food waste, Food waste disposal, Resource recovery