Combined anaerobic - aerobic bioelectrochemical system for ammonium sulfide and carbon removal from ethanol stillage
Polina Velichkova, Anatoliy Angelov, Svetlana Bratkova
AI summary
72% confidenceThis study integrated a two-chamber bioelectrochemical system (BES) with anaerobic and aerobic bioreactors to treat ethanol stillage, comparing five operating modes: no BES, MFC, and MEC at 0.6, 0.8, and 1.0 V. MEC at 1.0 V achieved 94.1% COD removal and 90.91% NH4+-N removal, with methane content rising from 57% (no BES) to 74% (MEC). Sulfate removal reached 100% in MEC modes versus 8.74% without BES. The cation exchange membrane enabled ammonium migration to the cathode where nitrification and denitrification occurred, improving both energy recovery and pollutant removal.
Key insights
Threshold Applied voltage directly controls ammonium removal efficiency: 1.0 V achieved 90.91% NH4+-N removal versus 53.58% without BES, a 1.7× improvement.
So what. Operators targeting high ammonia recovery should apply ≥0.8 V external voltage; below 0.6 V, removal plateaus near 67–69%.
Threshold Sulfate removal shows a sharp on/off transition: MFC achieved 72.71% SO4²⁻ removal, but all three MEC modes achieved 100% removal regardless of voltage (0.6–1.0 V).
So what. Sulfide oxidation at the anode is voltage-independent above ~0.6 V; MEC operation is sufficient for complete sulfate removal without voltage optimization.
Trade-off COD removal efficiency increases with applied voltage in MEC but plateaus in MFC: MEC1.0V achieved 94.1% versus MFC's 69.82%, a 1.35× gain.
So what. For maximum organic matter degradation, use MEC at ≥0.8 V; MFC mode offers minimal advantage over anaerobic digestion alone (69.82% vs 56.57%).
Threshold Methane content improves in all BES modes but saturates: MFC reached 70 vol.%, MEC0.6V 72%, and MEC0.8V/1.0V both 74%, suggesting a ceiling near 74%.
So what. Biogas quality improvement plateaus at MEC0.6V; higher voltages improve COD/NH4+ removal but not gas composition further.
Mechanism Current density scales linearly with applied voltage in MEC but remains low in MFC: MEC1.0V reached 1.5–2.9 A/m² versus MFC's 0.01–0.02 A/m², a 100–150× difference.
So what. MEC operation drives substantially higher electron transfer rates; if anode biofilm kinetics are limiting, MEC will unlock higher throughput than MFC.
Opportunity areas6
Specific gaps in this paper, with an experiment that would close each.
High No measurement of nitrite (NO2⁻) or N2O accumulation in cathode or aerobic reactor effluent despite nitrification/denitrification occurring.
Paper states 'nitrification in the zone of the aerobic bioreactor' and 'nitrate reduction takes place' at cathode, but provides no data on intermediate nitrogen oxides. N2O is a potent greenhouse gas and nitrite can inhibit methanogenesis.
Try. Measure NO2⁻ and N2O in aerobic reactor outlet and cathode effluent across 0.6–1.0 V range; quantify by ion chromatography and gas chromatography.
High No systematic variation of cathode pH or dissolved oxygen (DO) in aerobic reactor; both are known to control nitrification/denitrification selectivity.
Paper reports 'pH 7.5–7.6' as fixed input but does not report cathode pH or aerobic reactor DO. Nitrification rate and N2O yield are highly pH and DO sensitive.
Try. Vary cathode pH (6.5–8.5) and aerobic reactor DO (2–8 mg/L) at fixed 0.8 V; measure NH4⁺ removal, NO3⁻ accumulation, and N2O flux.
Medium No exploration of C/N ratio effects; ethanol stillage has fixed TKN (302–315 mg/L) but only one dilution tested.
Paper dilutes stillage 4× to achieve ~10 g/L COD but does not test whether lower or higher C/N ratios improve nitrogen removal or reduce N2O.
Try. Test 2×, 4×, and 8× dilutions (varying C/N from ~30 to ~120 on COD:TKN basis); measure NH4⁺ removal and NO3⁻/NO2⁻ accumulation at 0.8 V.
Medium No measurement of anode or cathode potential (vs. Ag/AgCl or SHE); only external voltage applied is reported.
Paper applies 0.6–1.0 V external voltage but does not report actual electrode potentials. Anode potential controls sulfide oxidation pathway (S⁰ vs. SO4²⁻); cathode potential controls denitrification selectivity (NO3⁻ → NO2⁻ → N2O → N2).
Try. Insert reference electrodes; measure anode and cathode potential vs. Ag/AgCl at each external voltage; correlate with sulfide speciation and nitrogen product distribution.
Medium No investigation of HRT trade-offs; fixed 11-day anaerobic and 6-day aerobic HRT may not be optimal for nitrogen cycling.
Paper states contact times but does not vary them. Shorter HRT may increase throughput but reduce nitrification/denitrification completeness; longer HRT may accumulate inhibitory ammonia.
Try. Vary anaerobic HRT (5–15 days) and aerobic HRT (3–9 days) at 0.8 V; measure NH4⁺ removal, NO3⁻ accumulation, and methane yield.
Medium No characterization of biofilm composition or electroactive bacteria (EAB) abundance on anode/cathode; mechanism of improved removal is inferred but not verified.
Paper attributes sulfide oxidation and COD removal to EAB but provides no 16S rRNA, qPCR, or microscopy data. Unclear whether Geobacter, Shewanella, or other taxa dominate.
Try. Sample anode and cathode biofilms at days 3, 7, and 11; perform 16S rRNA gene sequencing and qPCR for known EAB; correlate abundance with current density and removal efficiency.
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What they did
- System
- MEC
What worked
Keywords
Identifiers
- Journal
- Journal of Chemical Technology and Metallurgy
- Year
- 2023
Representative MEC — matched on the paper’s system type only, not its reactor or geometry.
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