Constructed wetlands operated as bioelectrochemical systems for improvement and control of wastewater treatment
, Marco Hartl
AI summary
72% confidenceThis thesis investigates constructed wetlands (CWs) operated as bioelectrochemical systems (CW-BES)—both as microbial fuel cells (CW-MFC) and microbial electrolysis cells (CW-MEC)—for wastewater treatment intensification. Eight meso-scale horizontal-flow systems receiving real urban wastewater were tested across four experiments: CW-MFC as a COD biosensor, contaminant removal in CW-MFC with varying hydraulic regimes and organic loading rates, microbial community shifts in CW-MEC versus CW-MFC, and organic micropollutant degradation. CW-MEC and CW-MFC+ achieved 18–22% higher ammonium removal and 7–13% higher COD removal versus conventional CW-control, with 10–17% improvement in removal of carbamazepine, diclofenac, and naproxen. Microbial activity increased 4–34% in CW-MFC+ relative to control.
Key insights
Trade-off Continuous hydraulic flow in CW-MFC+ produced significantly higher current density than intermittent flow, yet contaminant removal showed no statistically significant difference across flow regimes or organic loading rates (4.9–13.6 g COD/m²·day).
So what. Operators should use continuous flow to maximize electrical signal for biosensing or energy recovery, but should not expect proportional gains in COD or ammonium removal; optimize flow regime independently of contaminant removal targets.
Performance CW-MEC achieved 18% higher ammonium removal and 9% higher COD removal versus CW-control, while CW-MFC+ achieved 16% and 6% respectively, indicating MEC outperforms MFC for nitrogen removal despite both using the same electrode materials and HRT.
So what. If nitrogen removal is the priority, apply external voltage (MEC mode) rather than relying on passive MFC operation; the additional 2% ammonium removal gain justifies the energy input for nitrogen-rich wastewaters.
Mechanism Sphingobium yanoikuyae dominated CW-MEC anode and cathode communities and has not been previously reported in CW-MEC or other BES systems, suggesting electrolysis selects for novel electrochemically active bacteria distinct from those in MFC or conventional CW.
So what. Design CW-MEC systems expecting different microbial consortia than CW-MFC; do not assume MFC-derived microbial knowledge transfers to MEC operation, and investigate whether Sphingobium species are responsible for the superior ammonium removal.
Threshold CW-MFC biosensor response was asymmetric: 75–80% of COD increases were detected within 2–4 h, but the signal did not respond well to decreasing COD concentrations, limiting its use to alarm-only rather than continuous monitoring.
So what. If deploying CW-MFC as a biosensor, design it as a spike-detection alarm for contamination events (e.g., industrial discharge) rather than a real-time process control tool; invest in signal filtering or lag-compensation to handle concentration decreases.
Performance Organic micropollutant removal (carbamazepine, diclofenac, naproxen) improved 10–17% in CW-MEC and 5% in CW-MFC+ versus control, but ibuprofen removal was similar across all treatments, indicating selective enhancement of recalcitrant compounds without universal OMP degradation.
So what. Target CW-BES for treatment of persistent pharmaceuticals (carbamazepine, diclofenac, naproxen) but do not expect improvement for readily biodegradable compounds like ibuprofen; characterize the OMP substrate to predict whether bioelectrochemical enhancement will apply.
Contradictions1
Tensions the paper itself points to. Each side quotes the passage it comes from.
Within the paper Continuous flow produces significantly higher current density than intermittent flow, yet contaminant removal does not differ significantly between the two regimes.
One side'The CW-MFC+ fed with a continuous flow produced an extremely statistically significant higher current density than the CW-MFC+ with intermittent flow'
“The CW-MFC+ fed with a continuous flow produced an extremely statistically significant higher current density than the CW-MFC+ with intermittent flow”
Other side'Contaminant removal results showed no significant differences between tested organic loading rates... or different electrical connections.'
“Contaminant removal results showed no significant differences between tested organic loading rates (4.9±1.6, 6.7±1.4 and 13.6±3.2 g COD/m²ꞏday), hydraulic regimes (intermittent vs. continuous flow) or different electrical connections.”
Authors’ reading. The authors do not explicitly reconcile this. The implication is that current density (a measure of electron transfer rate) is decoupled from bulk contaminant removal efficiency under the tested conditions; continuous flow may enhance bioelectrochemical activity without translating to measurable improvements in COD or ammonium removal, possibly because conventional anaerobic pathways dominate removal kinetics.
Opportunity areas6
Specific gaps in this paper, with an experiment that would close each.
High No measurement of cathode potential (Ecat), anode potential (Ean), or cell voltage (Ecell) reported; operating window for nitrogen cycling versus energy recovery is not defined.
The paper states CW-MEC requires 'only an additional voltage of 0.2–0.8 V' but does not report actual potentials applied or measured, making it impossible to correlate electrode potential to ammonium removal or N2O accumulation.
Try. Vary applied voltage in CW-MEC from 0.2 to 1.0 V and measure ammonium removal, nitrite accumulation, and N2O emissions; identify the potential window that maximizes nitrogen recovery without N2O slip.
High No dissolved oxygen (DO) profile along the flow path; aerobic/anaerobic zonation and its effect on nitrification versus denitrification is not characterized.
The paper emphasizes 'pronounced natural redox gradient' in horizontal-flow CWs but provides no DO measurements at anode, cathode, or intermediate zones, limiting mechanistic understanding of nitrogen cycling.
Try. Install DO probes at inlet, anode, middle, cathode, and outlet; correlate DO profile to ammonium removal, nitrite accumulation, and microbial community composition across the three BES in series.
High No measurement of nitrite (NO2--N) or nitrous oxide (N2O) accumulation; nitrogen mass balance is incomplete and N2O emissions are not quantified.
Ammonium removal is reported but the fate of nitrogen (complete denitrification to N2 versus incomplete denitrification to N2O or accumulation as NO2-) is not tracked, preventing assessment of environmental impact.
Try. Measure NO2--N in liquid samples and N2O in headspace gas at each BES transect; calculate nitrogen recovery efficiency and N2O yield (mol N2O / mol NH4+ removed).
Medium Hydraulic retention time (HRT) is not explicitly stated; organic loading rate (OLR) is given but residence time is needed to assess whether nitrogen removal is kinetically limited or thermodynamically driven.
The paper reports OLR values (4.9–13.6 g COD/m²·day) but does not state HRT, making it impossible to compare with other CW-BES studies or predict performance at different flow rates.
Try. Report HRT for each experiment; vary HRT from 2 to 10 days while holding OLR constant and measure ammonium removal kinetics and microbial community response.
Medium No measurement of catholyte pH or pH profile; pH is known to affect nitrification, denitrification, and ammonia speciation (NH3 vs NH4+), yet is not reported.
The paper mentions 'redox gradient' but does not report pH, which is critical for understanding whether ammonia recovery (as NH3 gas) or nitrification-denitrification is the dominant removal pathway.
Try. Measure pH at anode, cathode, and outlet; vary catholyte pH from 6.5 to 8.5 and measure ammonium removal, nitrite accumulation, and ammonia volatilization.
Medium Microbial community analysis is limited to 16S rRNA gene sequencing; functional genes for nitrification (amoA, nxrA) and denitrification (nirK, nirS, nosZ) are not quantified.
The paper identifies Sphingobium yanoikuyae as dominant in CW-MEC but does not confirm whether this organism is responsible for ammonium oxidation, nitrite reduction, or other nitrogen transformations.
Try. Perform qPCR for nitrification and denitrification functional genes; correlate gene abundance to ammonium removal rate and nitrite/N2O accumulation.
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What they did
- System
- MEC
- Substrate
- real wastewater
- Inoculum
- mixed culture from real urban wastewater
What worked
Abstract
The objective of this work was the improvement and control of wastewater treatment using constructed wetlands operated as Microbial Fuel Cells (CW-MFCs) and Microbial Electrolysis Cells (CW-MECs). For this purpose, eight meso-scale experimental systems were constructed. The first experiment investigated the use of CW-MFC as a bioindicator, showing that it could be used as a qualitative alarm tool for sudden COD increases. The following three experiments investigated the removal of conventional contaminants as well as organic micropollutants (OMPs) using duplicates of CW-MEC, closed-circuit CW-MFC, open-circuit CW-MFC and conventional CW-control. Results showed that CW-MEC and CW-MFC+ increased the removal of COD (7-13%) and ammonium (18-22%) when compared to the control systems. Regarding OMPs, carbamazepine, diclofenac and naproxen removal was increased by 10-17% in CW-MFC+ and CW-MEC when compared to the control, while ibuprofen removal was similar amongst treatments. Additionally, a microbial activity analysis showed that activity was 4-34% higher in CW-MFC+ as compared to CW-control, and a microbial community analysis indicated that anode and cathode communities in CW-MEC were significantly different tq other treatments, seemingly due to the effects of electrolysis. In CW-MFC+ only cathode communities were different. probably due to sampling issues at the anodes. El objetivo de esta tesis es la mejora de la eficiencia y control del tratamiento de aguas residuales mediante humedales construidos operados como sistemas bioelectroquímicos (CW-BES), tanto en su configuración de celdas microbianas de combustible (CW-MFC) como en configuración de celdas microbianas electrolíticas (CW-MEC). Para este propósito, ocho CW-BES a escala experimental de mesocosmo fueron construidos. Durante el primer experimento se investigó el uso de CW-BES operados como celdas CW-MFC como herramienta de bioindicación de la carga orgánica. En los subsiguientes experimentos se abordó el estudio de la mejora del tratamiento de contaminantes convencionales y microcontaminantes mediante la operación de CW-BES y CW-MFC y se comparó con la eliminación de los mismos con sistemas de humedales control (CW-Control). Los resultados mostraron que los CW-MEC y los CW-MFC incrementaban la eficiencia de tratamiento entre un 7-13% y entre el 18-22% respectivamente a la eliminación de la materia orgánica y el amonio. En relación a la eliminación de microcontaminantes, la eliminación de carbamacepina, diclofenaco y naproxeno mejoró entre un 1 O y un 17% en los CW-BES comparado con los sistemas control, mientras que la eliminación de ibuprofeno no mostró diferencias. El estudio de la actividad microbiana en los sistemas experimentales mostró que ésta aumenta entre un 4 y un 34% en los CW-BES. El estudio de las comunidades microbianas en los sistemas experimentales mostró diferencias significativas tanto en zona anódica como catódica de los CW-BES, comparado con los sistemas experimentales control.
Keywords
Identifiers
- Journal
- Unknown journal
- Year
- 2020
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