Carbon fixation via volatile fatty acids recovery from sewage sludge through electrochemical-pretreatment-based anaerobic digestion

Capturing the carbon in volatile fatty acids (VFA) produced from the anaerobic digestion (AD) of sewage sludge has the potential to not only provide economic benefits but also reduce greenhouse gas production. This study demonstrates a chemical-free method to collect VFA from an AD instead of methane that involves electrochemical pretreatment (EPT) of sludge. Experimental results show that applying 15 V EPT for 45 min enhances acidogenesis and selectively inhibits methanogenesis, leading to a substantial VFA accumulation (2563.1 ± 307.9 mg COD/L) and achieving 2.5 times more carbon fixation than via methane production. Interfacial thermodynamic analysis shows that EPT induces a decrease in both the repulsive electrostatic energy (from 152.9 kT to 12.2 kT) and the energy barrier (from 57.0 kT to 2.6 kT) in the sludge, leading to increased sludge aggregation and entrapment of microorganisms. Molecular docking sheds lights on how the methanogens interacts with the organic matter released from EPT (e.g., alanine-tRNA ligase), showing that these interactions potentially interfere with the proteins that are associated with the activities of the methanogens and the electron transfer pathways, thereby impeding methanogenesis. Integrating EPT into AD therefore facilitates the recovery of valuable VFA and the capture of carbon from freshwater sludge, providing notable economic and environmental benefits in sewage sludge treatment.

An optimized electro-fenton pretreatment for the degradation and mineralization of a mixture of ofloxacin, norfloxacin, and ciprofloxacin

The electro-Fenton process (EFP) is a powerful advanced oxidation process beneficial to treating recalcitrant contaminants, and there has been a continuing interest in combining this technology to enhance the efficiency of conventional wastewater treatment processes. In this work, an optimized EFP process is performed as pretreatment for the degradation and mineralization of three blank fluoroquinolones (FQs) drugs: ofloxacin (OFL), norfloxacin (NOR), and ciprofloxacin (CIP). The optimization of the experiment was carried out using a Box-Behnken experimental design. Faster and complete degradation of the drugs mixture was achieved in 90 min with 61.12 ± 2.0% of mineralization in 180 min, under the optimized conditions: j = 244.0 mA cm-2, [Fe2+] = 0.31 mM, and [FQs] = 87.0 mg L-1. Furthermore, a low toxicity effluent was obtained in 90 min of the experiment, according to bioassay toxicity with Vibrio fischeri. Five short-chain carboxylic acids, including oxalic, maleic, oxamic, formic, and fumaric acids, were detected and quantified, in addition to F- and NO3- inorganic ions. The inhibition of the reactive oxygen species with scavenger proof was also evaluated in this paper.

Optimizing waste activated sludge disintegration by investigating multiple electrochemical pretreatment conditions: Performance, mechanism and modeling

The complex and rigid floc structure often limits the reutilization of waste activated sludge (WAS). Electrochemical pretreatment (EPT) is one of the most effective technologies that can enhance WAS disintegration. But a comprehensive investigation into how multiple EPT conditions work was rarely reported. The study evaluated the effects of multiple EPT conditions, i.e., different electrolytes (NaCl, Na₂SO₄, and CaCl₂), electrolytes dosage (0 g/L, 0.5 g/L, 1.0 g/L, and 3.0 g/L), EPT current (0 A, 0.5 A, 1.0 A, and 3.0 A) and EPT time (0 min, 30 min, 60 min, and 90 min) on WAS disintegration. The results showed that NaCl was outstanding from other electrolytes in promoting more WAS disintegration. Besides, a relatively higher NaCl dosage, a higher EPT current, and a longer EPT time promoted more reactive chlorine species (RCS), thus enhancing WAS disintegration in terms of extracellular polymeric substances (EPS) structure destruction and biodegradable organic matter release. After EPT for 60 min at NaCl dosage of 1.0 g/L and current of 1.0 A, the EPS multilayer structure destruction, biodegradable organic matters release, and soluble chemical oxygen demand (SCOD) increase in the supernatant were enhanced by 17.2 %, 130.5 %, and 238.7 %, respectively. Then a predictive quadratic model was established and the impact significance of the above EPT factors for enhancing WAS disintegration followed dosage of NaCl > current > EPT time. Furthermore, response surface methodology (RSM) suggested NaCl dosage of 2.75 g/L, current of 2.0 A, and EPT time of 30 min were the optimal EPT conditions, bringing a 42.0 % increase in the net economic benefit of WAS treatment compared to without EPT.

Electrochemical pretreatment enhancing co-fermentation of waste activated sludge and food waste into volatile fatty acids: Performance, microbial community dynamics and metabolism

Waste activated sludge (WAS) has low biodegradability that restricts acidogenic fermentation (AF), thereby limiting the high-value volatile fatty acids (VFAs) production. This study investigated an alternative electrochemical pretreatment (EPT) approach that can facilitate AF of WAS and food waste (FW) and therefore enhance VFAs production. The results showed through introducing 50 % volatile solid basis of FW (containing massive chloride) into WAS, a 60-min EPT produced reactive chlorine species (RCS), which diffused into WAS-FW inner layers resulting in cell lysis, therefore significantly promoted and accelerated WAS-FW disintegration, contributing to more soluble and biodegradable dissolved organic matter (DOM). Then during the subsequent 15-day acidogenic co-fermentation (Co-AF), the residual RCS (approximate 5 mg Cl₂/L) also caused acidogenic bacteria (including Prevotella_7, Lactobacillus and Veillonella) gradually outcompeted methanogens due to their different tolerance to residual RCS. Consequently, the maximum VFAs yield of the WAS-FW Co-AF with EPT was 40.8 % higher than WAS-AF without EPT.

Freeze-thaw combined with activated carbon improves electrochemical dewaterability of sludge: analysis of sludge floc structure and dewatering mechanism

Freeze-thaw (F/T) and electrochemistry both are environment-friendly and efficient sludge treatment technologies. In this study, the sludge samples were frozen at - 15 °C, and 20% g/gTss activated carbon (AC) was added to the dissolved sludge. Finally, the uniformly mixed sludge was treated at a voltage of 15 V for 25 min. During the experiment, the effect of F/T on the floc structure was analyzed by a laser particle analyzer and scanning electron microscope. F/T treatment improved the dewatering performance of the sludge and promoted the aggregation of sludge flocs into larger particles either. At the same time, the median diameter (D50) increased from 45.27 to 128.94 μm. AC was added to the thawed sludge solution before electrochemical treatment (EP). The conductivity of AC enhanced the effect of EP, thereby cracking the sludge flocs. Therefore, the three-dimensional excitation-emission matrix (3D-EEM) intensity of tightly bound extracellular polymeric substances (TB-EPS) decreased significantly. The protein in TB-EPS decreased from 54 to 33%, and the D50 was also reduced to 105.3 μm. The final specific resistance of filtration and water content were reduced by 96.39% and 32.17%, respectively. The dehydrated cake elemental analysis showed that increased AC improved the sludge cake's combustion efficiency significantly. Moreover, the preliminary economic analysis indicated that the cost of this research was low, which implied the potential application value of combined treatment.

Electrochemical pretreatment (EPT) of waste activated sludge: Extracellular polymeric substances matrix destruction, sludge solubilisation and overall digestibility

Anaerobic digestion (AD) of waste activated sludge (WAS) is widely used for stabilisation, mass reduction and resource recovery. However, AD performance is often hampered by the poor digestibility and slow degradation rate for WAS. A series of bench-scale tests was conducted using innovative electrochemical pretreatment (EPT) method to enhance the destruction of the extracellular polymeric substances (EPS) matrix and anaerobic digestibility. The results showed that the EPS matrix destruction was enhanced by 15-30% after EPT for 0.5 h at voltages of 8-15 V. The highest improvement in hydrolysis rate and overall digestibility was achieved at EPT of 15 V for 1.5 h, which was determined as 59% (from 0.27 to 0.43 d^-1) and 28% (from 105 to 134 mL CH₄/g VSS_added) respectively, compared with the WAS treatment without EPT. The economic analysis showed 1 h, 12 V EPT was more economically feasible.

An efficient strategy of phosphorus recovery: Electrochemical pretreatment enhanced the anaerobic fermentation of waste activated sludge

The anaerobic fermentation (AF) of waste activated sludge (WAS) with an electrochemical pretreatment (EPT) was investigated to determine its correlation with the release of phosphorus and the disintegration of WAS. The sludge was pretreated by holding under 4.5 V for 60 min, followed by AF for 9 days. Untreated sludge was used as the control group (no-EPT). Results showed that, with pretreatment, the total dissolved P (TDP), orthophosphate (PO₄^3--P) and organic P (OP) reached the maximum values of 7.30 mg/L, 4.77 mg/L and 2.35 mg/L on day 8, respectively, which were approximately 5.3, 9.2 and 2.7 times greater than that in the control group. The analysis of soluble chemical oxygen demand (SCOD), protein and polysaccharides showed that the EPT promoted the disintegration of sludge, thereby enhancing the P release. The SCOD reached 1625 mg/L on day 6 in pretreatment experiment, which was about 9.8 times greater than that in control group. Additionally, the EPT contributed to fewer metal ions in sludge supernatant. This mechanism might have been due to the anions accumulating in the supernatant from the greater degree of sludge collapse after EPT, which caused the released metal ions to combine with anions to form insoluble compounds. In conclusion, EPT could be a promising method for the dissolution of sludge and the recovery of phosphorus from WAS under AF. Besides, the economic benefit evaluation showed the potential value of EPT for P recovery.

Integration of vertically-ordered mesoporous silica-nanochannel film with electro-activated glassy carbon electrode for improved electroanalysis in complex samples

Vertically-ordered mesoporous silica-nanochannel films (VMSF) with highly ordered nanochannels, uniform and adjustable pore size, ultra-thin thickness, and high porosity, have attracted considerable attention in analysis, molecular separation, catalysis, and nanomaterial synthesis. However, their widespread applications in practical electrochemical sensing are largely limited by the poor adhesion to common electrode materials, especially the lack of highly active substrate electrode to equip mechanically stable VMSF. Herein, we report a facile strategy to fabricate VMSF on widely used sensing electrodes without the use of any chemical adhesive for developing superior VMSF based electrochemical sensors. We demonstrate that simple electrochemical polarization (anodic polarization and subsequent cathodic reduction) to activate glassy carbon electrode (GCE) can generate a suitable surface environment allowing direct growth of stable VMSF on such pre-activated GCE (p-GCE) via electrochemically assisted self-assembly (EASA). Compared to traditional VMSF electrodes with ITO or organosilane grafted GCE as substrate, the developed VMSF/p-GCE exhibits much higher electrochemical response to four redox biomarkers (norepinephrine, dopamine, tryptophan, and uric acid). In-depth insights on mechanisms of the high electrochemical activity and incorporation stability of VMSF/p-GCE are made. We further demonstrate the VMSF/p-GCE can be employed to detect dopamine in real serum samples with exceptional sensitivity, low detection potential, as well as superior anti-interference and anti-fouling performance. In addition, high selectivity is realized as the common co-existing interference substances (ascorbic acid-AA and uric acid-UA) do not interfere with the detection.

Dewaterability enhancement and sulfide mitigation of CEPT sludge by electrochemical pretreatment

Dewatering and sulfide control are the key challenges in treating chemically enhanced primary treatment (CEPT) sludge. In this study, an electrochemical pretreatment (EPT) approach with the input of 10 V/800 mA was explored for simultaneously improving the dewaterability of CEPT sludge and eliminating its sulfide production. The effects of different electrode materials (carbon and titanium) and EPT durations (from 5 to 15 min) were documented to reveal the underlying EPT mechanism. EPT with titanium electrodes (titanium-EPT) led to limited improvement in dewaterability and sulfide control. EPT with carbon electrodes (carbon-EPT) for 15 min, however, led to decreases in capillary suction time and specific resistance in filtration of over 80% and the suppression of about 99% of hydrogen sulfide (H₂S_(g)) production over 5 days of anaerobic storage. Analysis of the characteristics of treated CEPT sludge revealed that carbon-EPT disintegrated sludge flocs with ∼70% reduction in sludge particle sizes and release of aromatic and tyrosine protein-like substances, thus enhancing sludge dewaterability. The sulfur balance in the liquid and gaseous phases showed that most of the sulfur-containing compounds remained in the solid phase as aliphatic sulfur and sulfonic acid after carbon-EPT, thereby mitigating sulfide emission. While the pattern of sulfur distribution in sludge with titanium-EPT was dominated by sulfide, it was similar to the control sample. Reduction in bacteria associated with sulfide production (i.e., Lachnospiraceae) in CEPT sludge after carbon-EPT also contributed to sulfide elimination. This study demonstrates that EPT can be a superior option for simultaneously enhancing the dewaterability of CEPT sludge and mitigating its sulfide production.

Electrochemical pretreatment for stabilization of waste activated sludge: Simultaneously enhancing dewaterability, inactivating pathogens and mitigating hydrogen sulfide

Stabilization of waste activated sludge (WAS) is an essential step for the disposal or reuse. In this study, WAS stabilization via electrochemical pretreatment (EPT) at 0-15V was evaluated for simultaneous dewaterability enhancement, pathogen removal and H₂S mitigation. The mechanism underlying EPT was investigated and discussed based on the changes in the physicochemical (e.g., particle size, zeta potential, hydrophobicity and extracellular polymeric substances) and biological characteristics (i.e. cell morphology, and distribution and percentages of live/dead cells) of WAS with different EPT voltages. The results revealed that EPT disintegrated WAS flocs and disrupted the cell walls leading to a reduction in particle size (by up to 50%), increased release of extracellular and intracellular substances (by up to 4 times) to facilitate WAS stabilization. With EPT at 15V, the capillary suction time of WAS decreased by 42%, and the concentrations of E. coli and indicator pathogens (Salmonella spp. and Streptococcus faecalis) fell by nearly 5 log₁₀ reaching U.S. EPA hygienization levels. Furthermore, EPT at 12V or higher suppressed the amounts of dissolved sulfide and H₂S_(g) produced from the WAS under anaerobic conditions by over 99%. This study demonstrates the feasibility of EPT for simultaneous WAS dewaterability enhancement, pathogen inactivation and H₂S mitigation, providing a one-step alternative for sludge stabilization.

Electrochemical pretreatment of yard waste to improve biogas production: Understanding the mechanism of delignification, and energy balance

In the present study, electrochemical pretreatment with a pair of graphite electrode was conducted to pretreat yard waste prior to anaerobic digestion. The Response Surface Methodology was employed to optimize the pretreatment conditions. To determine the mechanism of delignification physical and chemical properties of untreated and pretreated yard waste were investigated. In the subsequent anaerobic digestion of pretreated yard waste, the ultimate biogas production of 446 mL/g VS was achieved in comparison to the untreated yard waste of 287 mL/g VS on 35th day of anaerobic digestion. A net energy gain of 4.75 kJ/g VS (Output energy of 5.73 kJ/g VS - Input energy of 0.98 kJ/g VS) and net profit of 518 rupees (US$ 7.4) per 1 ton of yard waste indicates the applicability of electrochemical pretreatment for pilot scale.

Polycyclic aromatic hydrocarbon-emulsifier protein produced by Aspergillus brasiliensis (niger) in an airlift bioreactor following an electrochemical pretreatment

An emulsifier protein (EP) was produced and easily separated from oil-contaminated water as an economical substrate when Aspergillus brasiliensis, pretreated in a solid state culture with a controlled electric field, was used in an airlift bioreactor. The hydrocarbon-EP comprised 19.5% of the total protein, its purification enhanced the specific emulsifying activity (EA) seven times. The influence of operational conditions (pH and salt concentration) on the EA were assessed to characterise the emulsion stability. The EA was increased by 19% in alkaline environments (pH 7-11), but it was not affected by the presence of salt (0-35 g L^-1). On the other hand, preheating the EP samples (60 °C) enhanced the EA by 2.5 times. Based on analysis of its EA, this EP can be applied as a bioremediation enhancer in contaminated soils.