Electrochemical decolorization of liquid digestate from coffee waste biomass using a boron-doped diamond anode
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2024-03-18 |
| Journal | International Journal of Environmental Science and Technology |
| Authors | Haibin Chen, Gen Yoshida, Fetra J. Andriamanohiarisoamanana, Ikko Ihara |
| Institutions | Kobe University |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis study successfully demonstrated the use of Boron-Doped Diamond (BDD) electrochemical oxidation (EO) for treating liquid digestate derived from spent coffee grounds (SCG), aiming to create a high-quality medium for microalgae cultivation.
- Superior Anode Performance: The BDD anode exhibited significantly faster and more effective decolorization compared to conventional Ti/Pt and Ti/IrO2 anodes, attributed to its wide electrochemical window and high generation of physisorbed hydroxyl radicals (·OH).
- Decolorization Achievement: The EO process reduced the platinum-cobalt (Pt-Co) color value by up to 98.7%, achieving the target light-permeable standard (Pt-Co value < 200) necessary for efficient light penetration in microalgae bioreactors.
- Organic Load Reduction: A high Chemical Oxygen Demand (COD) removal rate of 84.1% was achieved, reducing the final COD concentration to 350 mg/L.
- Nutrient Retention: Crucially, the BDD EO process retained 87.4% of the Ammonium Nitrogen (NH4-N), minimizing nutrient loss compared to indirect oxidation methods that generate hypochlorite.
- Fenton Enhancement: Increasing the initial Fe2+ concentration enhanced the Fenton reaction, leading to improved COD removal, although excessive Fe2+ concentrations had mixed effects on color due to the formation of colored iron complexes.
- Medium Suitability: The resulting electrochemically oxidized liquid digestate contained sufficient concentrations of NH4-N and PO4-P (520 ± 60 mg/L and 62 ± 14 mg/L, respectively) for use as an economical, nutrient-rich medium for microalgae growth.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Initial SCG Digestate Color | 11000 ± 850 | Pt-Co value | Raw digestate concentration |
| Target Decolorization | 200 | Pt-Co value | Standard for light-permeable medium |
| Color Removal Efficiency (BDD) | 98.7 | % | Achieved at 270 min (1.5 A) |
| Initial SCG Digestate COD | 7600 ± 640 | mg/L | Raw digestate concentration |
| Final COD (BDD) | 350 | mg/L | After EO treatment (84.1% removal) |
| Initial SCG Digestate NH4-N | 1520 ± 240 | mg/L | Raw digestate concentration |
| NH4-N Retention (BDD) | 87.4 | % | Minimal degradation (12.6% loss) |
| Anode Material | Boron-Doped Diamond (BDD) | N/A | Optimal material tested |
| Standard Operating Current | 1.5 | A | Used for primary comparison tests |
| Current Range Tested | 0.6 to 2.0 | A | Investigated for kinetic effects |
| Inter-Electrode Gap | 0.5 | cm | Reactor setup parameter |
| Immersed Electrode Area | 35 | cm2 | Reactor setup parameter |
| Operating Temperature | 35 | °C | Maintained via water bath |
| Fe2+ Concentration Range | 0.1 to 0.8 | mM | Tested for Fenton reaction enhancement |
Key Methodologies
Section titled âKey MethodologiesâThe liquid digestate was prepared and treated using a multi-step process involving physical separation, membrane filtration, and electrochemical oxidation (EO).
- Digestate Pre-treatment: Raw Spent Coffee Grounds (SCG) digestate was filtered through a mesh, followed by two centrifugation steps (4347Ăg for 15 min) to separate suspended solids (SS).
- Membrane Filtration: The resulting solution underwent membrane filtration (0.2 ”m pore size) to obtain the liquid digestate, which reduced NH4-N by 27.4% and COD by 42.1%.
- Solution Preparation: The liquid digestate was diluted two-fold for the EO experiments.
- Electrochemical Reactor Setup: Experiments were conducted in a 500 mL flask reactor containing 400 mL of solution, maintained at 35 °C with magnetic stirring (800 rpm).
- Electrode Configuration: A BDD anode (Sumitomo Electric Industries) was used as the primary electrode and compared against Ti/Pt and Ti/IrO2 anodes. The cathode was BDD in all tests, with an inter-electrode gap of 0.5 cm and an immersed area of 35 cm2.
- Oxidation Process: The EO was performed under constant current conditions, typically at 1.5 A. The effect of current density was investigated across a range of 0.6 A to 2.0 A.
- Fenton Reaction Study: FeSO4·7H2O was added to the digestate at concentrations ranging from 0.1 mM to 0.8 mM to evaluate the synergistic effect of the electro-Fenton process on degradation.
- Performance Analysis: Decolorization was monitored via absorbance decay at 475 nm and converted to the Pt-Co color value. COD and nutrient (NH4-N, PO4-P) concentrations were measured using standard methods and ion chromatography.
Commercial Applications
Section titled âCommercial ApplicationsâThe BDD-based electrochemical oxidation process is highly relevant for industries requiring efficient wastewater treatment, resource recovery, and the production of specialized cultivation media.
- Industrial Wastewater Treatment: Applicable for treating highly colored and refractory industrial effluents, particularly from food and beverage processing (e.g., coffee, brewing, molasses) that contain complex organic compounds like melanoidins and caramel.
- Resource Recycling and Circular Economy: Provides a viable pathway for converting high-volume agricultural and food waste digestate into a valuable, marketable resource (microalgae growth medium), reducing disposal costs and environmental impact.
- Microalgae and Aquaculture: Directly supports sustainable microalgae biomass production by supplying a clear, nutrient-rich medium (high NH4-N and PO4-P) that maximizes light utilization and growth efficiency, essential for biofuel or high-value chemical production.
- Advanced Oxidation Processes (AOPs): Utilizes BDD electrodes, known for their stability and capacity to generate powerful hydroxyl radicals (·OH), making the technology suitable for general environmental remediation and the mineralization of persistent organic pollutants (POPs).
- Electrochemical Reactor Design: The findings inform the design and optimization of industrial-scale electrochemical reactors, emphasizing BDD material selection and optimal current density control for balancing degradation rate and energy consumption.
View Original Abstract
Abstract Liquid digestate can be used to provide nutrients for microalgae cultivation but the medium needs to be clear and colorless. The aim of this work was to use liquid digestate from coffee waste biomass to produce a light-permeable medium for microalgae cultivation. A boron-doped diamond anode was applied for electrochemical decolorization of the digestate. The electrochemical oxidation process reduced the platinum-cobalt color value by up to 97% and the chemical oxygen demand by 84.1%. After electrochemical oxidation, 87.4% of the ammonium nitrogen (NH 4 -N) was retained. Decolorization of the spent coffee grounds liquid digestate was compared with that of dairy cow manure liquid digestate. It took 90 min longer to fully decolorize the spent coffee grounds liquid digestate compared with the dairy cow manure liquid digestate. The boron-doped diamond anode performed better in the decolorization than Ti/IrO 2 and Ti/Pt anodes. The effects of the initial Fe 2+ concentration and current on the electrochemical oxidation process were also evaluated. Increasing the initial Fe 2+ concentration enhanced the Fenton reaction and chemical oxygen demand removal. A higher current enhanced the electrochemical decolorization process and side reactions. Electrochemical oxidation using a boron-doped diamond anode is a promising method for producing an appropriate medium for microalgae cultivation because it promotes decolorization of liquid digestate and retains most of the NH 4 -N.
Tech Support
Section titled âTech SupportâOriginal Source
Section titled âOriginal SourceâReferences
Section titled âReferencesâ- 1999 - Handbook of chlorination and alternative disinfectants