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Description

The desire for food waste diversion from municipal solid waste (MSW) landfills is steadily increasing. Recently, through current EREF funding, our research group at the University of South Carolina (USC) has demonstrated that hydrothermal carbonization (HTC) may serve as a sustainable and effective approach to manage diverted food waste, resulting in the generation of a high value, energy-rich, coal-like material that has an energy content as high as 33,750 J/g dry solids. Carbonization of food wastes presents a unique opportunity to not only generate a valuable energy source, but to also recover valuable soluble nutrients and produced chemicals from the process water that are otherwise unrecovered or unavailable. The potential revenue expected from the recovery of these resources, coupled with previously documented generation of a high-value coal material, increases the sustainability and economic viability of HTC, providing a compelling argument for continued evaluation of food waste carbonization. The goal of this project is to explore the sustainable and economically beneficial recovery of these resources. The specific objectives of this project include: 

  1. Determine the hydrothermal carbonization operational parameters (e.g., reaction time, reaction temperature) that result in the greatest fraction of extractable nutrients and valuable chemicals (e.g., acetic acid and 5-HMF) in the process water for subsequent sale. 
  2. Evaluate nutrient and chemical (e.g., acetic acid and HMF) recovery techniques. 
  3. Conduct an economic analysis of simultaneous energy, nutrient, and chemical recovery from food wastes converted via hydrothermal carbonization to provide operational guidance. 

Laboratory-scale experiments conducted to determine the fate of nutrients during the carbonization of food wastes and mixed food and packaging wastes under different operating conditions are described. At all evaluated reaction times and temperatures, the majority of nitrogen, calcium, and magnesium remain integrated within the solid-phase, while the majority of potassium and sodium reside in the liquid-phase. The fate of phosphorus is dependent on reaction times and temperatures, with solid-phase integration increasing with higher reaction temperature and longer time. Results from the leaching experiments suggest that, at least in the short term, tightly bound nitrogen within the solid matrix is unlikely to be released, while almost all of the phosphorus present in the solids generated when carbonizing at 225 and 250 oC is released. At a reaction temperature of 275 oC, smaller fractions of the solid-phase total phosphorus are released as reaction times increase, likely due to increased solids incorporation. Up to 0.96% and 2.61% of nitrogen and phosphorus-based fertilizers (accounting for recovery in the liquid and solid-phases), respectively, in the US can be replaced by nutrients integrated within hydrohcar and liquid-phases generated from the carbonization of currently landfilled food wastes. 

Liquid phase nutrient recovery from the process water using struvite precipitation was evaluated using laboratory-scale experiments and equilibrium modeling. Nutrient recovery via struvite is viable. Adding either MgCl2 or MgSO4 and a base is necessary because of the low pH and Mg2+ levels in the HTC process water. Because of the low P:N ratio in the HTC process water, adding phosphate is needed to maximize nitrogen nutrient recovery. A chemical equilibrium model can reasonably predict the amount of struvite formed and can be used as a basis for the amount of chemicals (i.e., Mg2+, base, and/or phosphate) to be added to the system. Fitting experimental titration behavior of the HTC process water is necessary to obtain the required amount of added base. A strategy of pH adjustment with periodic Mg2+ and base addition recovered more struvite than a single dose of Mg2+ and base in a batch system. Refinement of the operational strategy to include pH control and chemical addition programming in a flow through system during larger scale implementation is recommended. 

Laboratory experiments aimed at evaluating the recovery of acetic acid and 5-HMF using liquid-liquid extraction techniques were also conducted. Concentrations of acetic acid and 5-HMF in the process water are significant. When carbonizing to maximize acetic acid concentrations, higher reaction temperatures and longer reaction times are needed. Although liquid-liquid extraction methods to extract acetic acid from deionized water were shown to be feasible, the same methods failed when applied to HTC process water. Investigation of alternative approaches (e.g., membrane separation) is necessary. In contrast, maximizing 5-HMF generation requires lower reaction temperatures and shorter reaction times. A 90% 5-HMF extraction efficiency from the process water was achieved using tetrahydrofuran and sec-butanol with the addition of salt. 

Nutrient and 5-HMF recovery from the HTC processing of food waste and mixed food and packaging wastes is technically feasible. A previous project also demonstrated energy recovery from the carbonization of the same materials is technically feasible. An initial assessment of the costs and benefits associated with eight different operational strategies for resource recovery was performed. System costs (e.g., chemical requirements) and benefits (e.g., selling of hydrochar, nutrients, chemicals) were normalized per mass of wet food waste processed and reaction time. Results from this analysis indicate that conducting HTC for resource recovery can be economically viable. Using the hydrochar for energy and recovering nutrients from the HTC process water can be more economically attractive than composting and anaerobic digestion of food wastes. Results also suggest that recovering 5-HMF via liquid-liquid extraction is not currently economically viable. Investigation of alterative liquid-phase resource recovery approaches is needed to improve the economics of this recovery. 

A flowchart that provides guidance on carbonizing food wastes for resource recovery is detailed. After specifying the desired resources to recover, the flowchart specifies the HTC processing conditions that maximizes the production of the resource. 

Overall, results associated with this EREF funded project indicate that recovering resources from food wastes (and mixed food and packaging wastes) using HTC has promise; significant levels of resources can be recovered that can be economically attractive and, under some conditions, achieve greater profits than composting and anaerobic digestion of food wastes. This work provides the basis for large-scale implementation of this process. As resources become more limited, it will likely become more important to recover resources from food-related waste. HTC is a novel process that can economically and efficiently accomplish this. 

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