X. Wang, S. Lu, B. Li
University of Connecticut,
United States
Keywords: electrochemical–biological coupling, inner-layer biofilm activation, energy-efficient wastewater treatment
Summary:
1. Introduction Biofilms are central to biological wastewater treatment but are limited by inefficient oxygen transfer. Oxygen penetrates only the outer layer of 100–200 µm, leaving the inner-layer of biofilms oxygen-depleted and metabolically inactive. This impedes pollutant removal and forces wastewater treatment plants (WWTPs) to implement over-aeration, resulting in high energy consumption. Meanwhile, microbial respiration and organic breakdown generate CO₂ that accumulates within the biofilms, further suppressing metabolism and aggravating biofilm aging and detachment. To address these problems, we developed an innovative CO₂-reduction-backed nourished biofilm (CBNB) system, where an electrode is coated with a hydrophilic polyamide (PA) membrane supporting biofilms. The membrane–electrode interface electrochemically enhances oxygen availability while in situ converting bacterial respiration-derived CO₂ into easily assimilable 1C or 2C intermediates (e.g., HCOOH), sustaining microbial activity throughout the entire biofilms. By simultaneously controlling oxygen delivery and CO₂ conversion, CBNB redefines biofilm stability and activity in wastewater treatment, establishing a novel electro-biological paradigm beyond conventional technologies. 2. Results and Discussion Electrochemical Interface Reviving Inner-Layer Biofilm Activity. A compact CBNB (≈20 mL) was developed by integrating an electrode (with Sn catalyst), a hydrophilic PA membrane, and biofilms growing on the PA membrane. The CBNB enables uniform oxygen transfer and electron distribution. After operating for 8 hours in real wastewater, the CBNB maintained stable electrochemical performance and formed a uniform 54 ± 6 µm biofilms, twice as stable as conventional biofilms, demonstrating successful revival of inner-layer biofilms and enhanced structural integrity. Enhanced Treatment Efficiency through Electro–Microbial Synergy. The CBNB achieved 32% higher organic removal than conventional biofilms under identical operation. The electrode–PA–biofilm interface senses local oxygen demand and adjusts oxygen transfer and CO₂ reduction. In the inner-layer of biofilms, respiration-derived CO₂ is electrochemically converted to HCOOH, resolving nutrient depletion and sustaining microbial activity, while in the outer-layer of biofilms exposed to wastewater, the electrode self-regulates to prevent biofilm aging. The CBNB produces a steady current of 20±1 mA cm⁻², more than twice that of similar microbial electrochemical systems (MES), reflecting more efficient pollutant degradation. Mechanistic Insight into Electrochemical–Microbial Coupling. Modeling simulations of CO₂–O₂ transport showed that in situ CO₂ reduction stimulates microbial oxidation in the inner-layer of biofilms, bolstering treatment capacity to 0.27 kg COD m⁻³ h⁻¹, compared with 0.20 kg COD m⁻³ h⁻¹ for conventional biofilm systems, while reducing oxygen demand from 1.8 mg O₂ L⁻¹ in conventional biofilm systems to 1.2 mg O₂ L⁻¹, and cutting aeration energy from 0.45 to 0.28 kWh per m³ of wastewater treated, demonstrating that the electrochemical–microbial coupling in CBNB enables energy-saving and high-efficiency wastewater treatment. 3. Significance and Impact The CBNB technology uniquely integrates electrochemical CO₂ reduction with microbial metabolism in biofilms treating wastewater, enabling real-time adjustment of oxygen supply and nutrient transfer across the entire biofilms. Unlike existing MES technologies that require obligatory anaerobic environments, CBNB functions stably in normal wastewater conditions. This distinct adaptability boosts contaminant removal efficiency and broadens the scalability of electrochemical technologies in wastewater treatment, paving the way for high-performance biofilm-based solutions.