Microalgae Cultivation in Electrochemically Oxidized Anaerobic Digestate from Coffee Waste Biomass
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2023-09-20 |
| Journal | Journal of the Japan Institute of Energy |
| Authors | H. P. Chen, Gen Yoshida, Fetra J. Andriamanohiarisoamanana, Ikko Ihara |
| Institutions | Kobe University |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research investigates the use of Boron-Doped Diamond (BDD) electrochemical oxidation (EO) to pre-treat dark, nutrient-rich anaerobic digestate from coffee waste, enabling its use as a highly effective medium for microalgae cultivation.
- Core Value Proposition: Successful integration of BDD-based EO into a circular economy model, transforming highly colored liquid digestate (LD) into a light-permeable, nutrient-retaining medium (ELD) for high-yield microalgae biomass production.
- Decolorization Performance: The BDD anode achieved rapid and effective decolorization, removing up to 85% of the color density (at 475 nm) within 3 hours of treatment.
- Nutrient Retention: The EO process was highly selective, retaining the majority of essential nutrients: approximately 80% of ammonium nitrogen (NH4-N) and 99% of phosphate phosphorus (PO4-P).
- Optimal Medium: The best microalgal growth (for C. sorokiniana NIES-2173) was achieved using the 10 times diluted, 2-hour electrochemically oxidized liquid digestate (ELD).
- Growth Enhancement: The optimal ELD condition yielded a final cell density of 1.28 * 107 cells/ml, significantly outperforming all tested diluted LD media due to improved light permeability.
- Mechanism: The BDD anodeâs high overpotential generated reactive oxidizing species (e.g., OH radicals), effectively degrading color-causing organic compounds (melanoidins, humus) without substantially oxidizing the ammonium or phosphate ions.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Initial COD (LD) | 7200 ± 440 | mg/L | Anaerobic Digestate Characteristic |
| Initial Color Density | 9500 ± 750 | Pt-Co unit | Anaerobic Digestate Characteristic |
| Initial NH4-N (LD) | 1260 ± 230 | mg/L | Anaerobic Digestate Characteristic |
| EO Anode/Cathode | BDD | - | Boron-Doped Diamond plates |
| Electrode Area (Immersed) | 35 | cm2 | EO Reactor Setup |
| Inter-electrode Gap | 0.5 | cm | EO Reactor Setup |
| EO Current Intensity | 1.5 | A | Constant current condition |
| EO Temperature | 35 | °C | Maintained by water bath |
| Maximum Color Removal | 85 | % | Achieved after 3 h EO treatment |
| NH4-N Retention (EO) | 80 | % | After 3 h EO treatment |
| Optimal ELD Treatment Time | 2 | h | For best microalgae growth |
| Optimal Dilution Ratio | 10 | times | For ELD cultivation |
| Highest Specific Growth Rate | 0.79 ± 0.11 | d-1 | 2 h ELD condition |
| Highest Final Cell Density | 1.28 * 107 | cells/ml | 2 h ELD condition (21 d cultivation) |
| Microalgae Cultivation Light | 150 | ”mol photons m-2 s-1 | Continuous illumination |
Key Methodologies
Section titled âKey Methodologiesâ- Digestate Preparation: Coffee waste was anaerobically digested at 37 °C for 3 weeks to produce the raw digestate.
- Liquid Digestate (LD) Isolation: The raw digestate was filtered (mesh), centrifuged (6000 rpm, 15 min, twice), and then microfiltered (0.2 ”m membrane) to obtain the solid-free Liquid Digestate (LD).
- Electrochemical Oxidation (EO): 200 mL of LD was treated in a 500 mL undivided cell reactor equipped with BDD anode and BDD cathode plates (50 cm2 each).
- EO Operation: The solution was stirred at 800 rpm, maintained at 35 °C, and subjected to a constant current of 1.5 A. Reaction times were varied from 1 to 5 hours to produce Electrochemically Oxidized Liquid Digestate (ELD).
- Medium Preparation: LD and ELD were diluted (5x, 10x, 20x) to create the cultivation media, primarily to increase light permeability.
- Microalgae Cultivation: Chlorella sorokiniana NIES-2173 was cultivated in 20 mL medium under continuous light (150 ”mol photons m-2 s-1) at 25 ± 1 °C for up to 28 days.
- Performance Monitoring: Growth was tracked by measuring optical density (OD) at 680 nm and cell density. Nutrient concentrations (NH4-N, PO4-P, COD) were analyzed before and after cultivation.
Commercial Applications
Section titled âCommercial ApplicationsâThe successful application of BDD electrochemical oxidation for digestate treatment and subsequent nutrient recovery opens pathways for several industrial and environmental applications:
- Wastewater Treatment and Decolorization: BDD reactors can be integrated into facilities handling highly colored industrial effluents (e.g., food processing, textile, paper mills) where conventional methods struggle to achieve adequate light transmission or organic removal.
- Sustainable Nutrient Recovery: Provides an energy-efficient alternative to energy-intensive methods like ammonia stripping, allowing for the recovery of valuable nitrogen and phosphorus from agricultural or municipal digestate streams.
- Biofuel and Biomass Production: Enables the use of low-cost, nutrient-rich waste streams (like coffee digestate) as a primary growth medium for microalgae, reducing reliance on expensive synthetic fertilizers for biofuel feedstock or high-value chemical production.
- Circular Economy Integration: Facilitates a closed-loop system where waste biomass is digested for biogas (energy), and the resulting liquid digestate is treated and reused to grow microalgae (biomass), minimizing environmental discharge and maximizing resource utilization.
- BDD Electrode Technology: Demonstrates the superior stability and oxidative power of BDD electrodes in complex, high-organic load environments, validating their use in advanced oxidation processes (AOPs) for recalcitrant pollutant removal.
View Original Abstract
Anaerobic digestate contains rich nutrients, such as nitrogen and phosphorus, which could be reused in microalgae cultivation. However, a clear growth medium is required for the cultivation to facilitate light permeable condition. The aim of this work was to investigate microalgae cultivation in electrochemically oxidized liquid digestate from coffee waste biomass. After removing the solid fraction of the digestate through microfiltration, the liquid digestate was treated by electrochemical oxidation using a boron-doped diamond anode. The liquid digestate (LD) and electrochemically oxidized liquid digestate (ELD) were used as media for microalgae cultivation.The effects of dilution from 5 to 20 times of the LD and reaction time from 1 to 5 h of the ELD on microalgae growth were also investigated. The results showed that the electrochemical oxidation had little influence on ammonium concentration in the digestate, whereas a color removal of up to 85% was observed. The ELD showed better microalgal growth performances than diluted LD, based on the data from optical density at 680 nm and cell density. The 10 times diluted, 2 h ELD achieved the best growth performance (additional optical density of 1.5 (-)) in all conditions. Our experiments proved that the ELD as a highly light permeable medium, better improved the growth performance of C. sorokiniana cultivation when compared with the LD medium.