Yinguang Chen

Nitrogen-doped carbon nanotubes as efficient and durable metal-free cathodic catalysts for oxygen reduction in microbial fuel cells

Recently, the application of microbial fuel cells (MFCs) with cost-effective and long durable cathodic catalysts to generate electricity sustainably, has drawn much attention. This study investigated the use of nitrogen-doped carbon nanotubes (NCNTs) as the cathodic catalyst for oxygen reduction in MFCs to produce electricity efficiently and durably. The obtained maximum power density was 1600 ± 50 mW m−2, which was higher than the commonly used platinum (Pt) catalyst (Pt/C) (1393 ± 35 mW m−2). Also, the drop percentage of power densities with NCNTs was lower than with Pt/C over 25 cycles, indicating that MFCs with NCNTs as the cathodic catalyst could generate electricity more durably than those with Pt/C. Further investigation of the mechanisms revealed that MFCs with the bamboo-shaped and vertically aligned NCNTs had lower internal resistance and higher cathode potentials. Rotating ring-disk electrode voltammogram, Raman microspectroscopy and X-ray photoelectron spectroscopic analyses suggested that NCNTs possessed a higher electrocatalytic activity for the oxygen reduction reaction (ORR) via a four-electron pathway in neutral pH phosphate buffer solution (PBS). Cyclic voltammograms on NCNTs and Pt/C electrodes before and after a continuous potentiodynamic swept in neutral PBS demonstrated that NCNTs had a better durability for cathodic ORR than Pt/C, which drove MFCs with NCNTs to generate electricity durably.

New Sludge Pretreatment Method to Improve Methane Production in Waste Activated Sludge Digestion

During two-phase sludge anaerobic digestion, sludge is usually hydrolyzed and acidified in the first phase, then methane is produced in the second stage. To get more methane from sludge, most studies in literature focused on the increase of sludge hydrolysis. In this paper a different sludge pretreatment method, i.e., pretreating sludge at pH 10 for 8 d is reported, by which both waste activated sludge hydrolysis and acidification were increased, and the methane production was significantly improved. First, the effect of different sludge pretreatment methods on methane yield was compared. The pH 10 pretreated sludge showed the highest accumulative methane yield (398 mL per g of volatile suspended solids), which was 4.4-, 3.5-, 3.1-, and 2.3-fold of the blank (unpretreated), ultrasonic, thermal, and thermal-alkaline pretreated sludge, respectively. Nevertheless, its total time involved in the first (hydrolysis and acidification) and second (methanogenesis) stages was 17 (8 + 9) d, which was almost the same as other pretreatments. Then, the mechanisms for pH 10 pretreatment significantly improving methane yield were investigated. It was found that pretreating sludge at pH 10 caused the greatest sludge hydrolysis, acidification, soluble C:N and C:P ratios, and Fe(3+) concentration with a suitable short-chain fatty acids composition in the first stage, which resulted in the highest microorganism activity (ATP) and methane production in the second phase. Further investigation on the second phase microorganisms with fluorescence in situ hybridization (FISH) and scanning electron microscopy (SEM) indicated that there were much greater active methanogenesis Archaea when methane was produced with the pH 10 pretreated sludge, and the predominant morphology of the microcolonies suggest a shift to Methanosarcina sp. like.

Long-term effect of ZnO nanoparticles on waste activated sludge anaerobic digestion.

The increasing use of zinc oxide nanoparticles (ZnO NPs) raises concerns about their environmental impacts, but the potential effect of ZnO NPs on sludge anaerobic digestion remains unknown. In this paper, long-term exposure experiments were carried out to investigate the influence of ZnO NPs on methane production during waste activated sludge (WAS) anaerobic digestion. The presence of 1 mg/g-TSS of ZnO NPs did not affect methane production, but 30 and 150 mg/g-TSS of ZnO NPs induced 18.3% and 75.1% of inhibition respectively, which showed that the impact of ZnO NPs on methane production was dosage dependant. Then, the mechanisms of ZnO NPs affecting sludge anaerobic digestion were investigated. It was found that the toxic effect of ZnO NPs on methane production was mainly due to the release of Zn(2+) from ZnO NPs, which may cause the inhibitory effects on the hydrolysis and methanation steps of sludge anaerobic digestion. Further investigations with enzyme and fluorescence in situ hybridization (FISH) assays indicated that higher concentration of ZnO NPs decreased the activities of protease and coenzyme F(420), and the abundance of methanogenesis Archaea.

Efficient Polyhydroxyalkanoates Production from a Waste-Activated Sludge Alkaline Fermentation Liquid by Activated Sludge Submitted to the Aerobic Feeding and Discharge Process

It was reported in our previous publication that the accumulation of short-chain fatty acids (SCFA) was significantly enhanced when waste-activated sludge (WAS) was anaerobically fermented at pH 10.0 (Yuan, et al., Environ. Sci. Technol. 2006, 40, 2025-2029). In this paper, the production of polyhydroxyalkanoate (PHA) by activated sludge with an aerobic feeding and discharge (AFD) process was investigated by the use of WAS alkaline fermentation liquid as the carbon source. It was observed that compared with other PHA synthesis processes reported in the literature, the AFD process showed the highest PHA production. The PHA content in sludge reached 72.9% when activated sludge was submitted to the AFD process. This was the highest PHA content obtained so far by activated sludge using wastes as the renewable carbon source. Although nitrogen and phosphorus were released into the WAS alkaline fermentation liquid, their presence did not affect PHA synthesis, which indicates that it is unnecessary to remove the released nitrogen and phosphorus, and the fermentation liquid can be used directly for PHA production. The accumulated PHAwas mainly composed of 3-hydroxybutyrate (3HB) (73.5 mmol C%), 3-hydroxyvalerate (3HV) (24.3 mmol C%), and 3-hydroxy-2-methylvalerate (3H2MV) (2.2 mmol C%). Further investigation showed that SCFA rather than protein and carbohydrate in the alkaline fermentation liquid made the main contribution to PHA production. The PHA produced from WAS alkaline fermentation liquid had a molecular weight of 8.5 x 10(5) Da and a melting point of 101.4 degrees C. Analysis using the 16S rRNA gene clone library revealed that gamma-Proteobacteria, alpha-Proteobacteria, and beta-Proteobacteria were the dominant microorganisms in the PHA production system.

Waste activated sludge hydrolysis and short-chain fatty acids accumulation under mesophilic and thermophilic conditions: Effect of pH

The effect of pH (4.0-11.0) on waste activated sludge (WAS) hydrolysis and short-chain fatty acids (SCFAs) accumulation under mesophilic and thermophilic conditions were investigated. The WAS hydrolysis increased markedly in thermophilic fermentation compared to mesophilic fermentation at any pH investigated. The hydrolysis at alkaline pHs (8.0-11.0) was greater than that at acidic pHs, but both of the acidic and alkaline hydrolysis was higher than that pH uncontrolled under either mesophilic or thermophilic conditions. No matter in mesophilic or thermophilic fermentation, the accumulation of SCFAs at alkaline pHs was greater than at acidic or uncontrolled pHs. The optimum SCFAs accumulation was 0.298g COD/g volatile suspended solids (VSS) with mesophilic fermentation, and 0.368 with thermophilic fermentation, which was observed respectively at pH 9.0 and fermentation time 5 d and pH 8.0 and time 9 d. The maximum SCFAs productions reported in this study were much greater than that in the literature. The analysis of the SCFAs composition showed that acetic acid was the prevalent acid in the accumulated SCFAs at any pH investigated under both temperatures, followed by propionic acid and n-valeric acid. Nevertheless, during the entire mesophilic and thermophilic fermentation the activity of methanogens was inhibited severely at acid or alkaline pHs, and the highest methane concentration was obtained at pH 7.0 in most cases. The studies of carbon mass balance showed that during WAS fermentation the reduction of VSS decreased with the increase of pH, and the thermophilic VSS reduction was greater than the mesophilic one. Further investigation indicated that most of the reduced VSS was converted to soluble protein and carbohydrate and SCFAs in two fermentations systems, while little formed methane and carbon dioxide.

Pyrosequencing Reveals the Key Microorganisms Involved in Sludge Alkaline Fermentation for Efficient Short-Chain Fatty Acids Production

Short-chain fatty acids (SCFAs) have been regarded as the excellent carbon source of wastewater biological nutrient removal, and sludge alkaline (pH 10) fermentation has been reported to achieve highly efficient SCFAs production. In this study, the underlying mechanisms for the improved SCFAs production at pH 10 were investigated by using 454 pyrosequencing and fluorescent in situ hybridization (FISH) to analyze the microbial community structures in sludge fermentation reactors. It was found that sludge fermentation at pH 10 increased the abundances of Pseudomonas sp. and Alcaligenes sp., which were able to excrete extracellular proteases and depolymerases, and thus enhanced the hydrolysis of insoluble sludge protein and polyhydroxyalkanoates (PHA). Meanwhile, the abundance of acid-producing bacteria (such as Clostridium sp.) in the reactor of pH 10 was also higher than that of uncontrolled pH, which benefited the acidification of soluble organic substrates. Further study indicated that sludge fermentation at pH 10 significantly decreased the number of methanogenic archaea, resulting in lower SCFAs consumption and lower methane production. Therefore, anaerobic sludge fermentation under alkaline conditions increased the abundances of bacteria involved in sludge hydrolysis and acidification, and decreased the abundance of methanogenic archaea, which favored the competition of bacteria over methanogens and resulted in the efficient production of SCFAs.

Response of anaerobic granular sludge to a shock load of zinc oxide nanoparticles during biological wastewater treatment.

The increasing use of zinc oxide nanoparticles (ZnO NPs) in consumer and industrial products highlights a need to understand their potential environmental impacts. In this study, the response of anaerobic granular sludge (AGS) to a shock load of ZnO NPs during anaerobic biological wastewater treatment was reported. It was observed that the extracellular polymeric substances (EPS) of AGS and the methane production were not significantly influenced at ZnO NPs of 10 and 50 mg per gram of total suspended solids (mg/g-TSS), but they were decreased when the dosage of ZnO NPs was greater than 100 mg/g-TSS. The visualization of EPS structure with multiple fluorescence labeling and confocal laser scanning microscope revealed that ZnO NPs mainly caused the decrease of proteins by 69.6%. The Fourier transform infrared spectroscopy analysis further indicated that the C-O-C group of polysaccharides and carboxyl group of proteins in EPS were also changed in the presence of ZnO NPs. The decline of EPS induced by ZnO NPs resulted in their deteriorating protective role on the inner microorganisms of AGS, which was in correspondence with the observed lower general physiological activity of AGS and the death of microorganisms. Further investigation showed that the negative influence of ZnO NPs on methane production was due to their severe inhibition on the methanization step.

Enhancing Electrocatalytic Oxygen Reduction on Nitrogen-Doped Graphene by Active Sites Implantation

The shortage of nitrogen active sites and relatively low nitrogen content result in unsatisfying eletrocatalytic activity and durability of nitrogen-doped graphene (NG) for oxygen reduction reaction (ORR). Here we report a novel approach to substantially enhance electrocatalytic oxygen reduction on NG electrode by the implantation of nitrogen active sites with mesoporous graphitic carbon nitride (mpg-C3N4). Electrochemical characterization revealed that in neutral electrolyte the resulting NG (I-NG) exhibited super electrocatalytic activity (completely 100% of four-electron ORR pathway) and durability (nearly no activity change after 100000 potential cyclings). When I-NG was used as cathode catalyst in microbial fuel cells (MFCs), power density and its drop percentage were also much better than the NG and Pt/C ones, demonstrating that the current I-NG was a perfect alternative to Pt/C and offered a new potential for constructing high-performance and less expensive cathode which is crucial for large-scale application of MFC technology.

Waste Activated Sludge Fermentation for Hydrogen Production Enhanced by Anaerobic Process Improvement and Acetobacteria Inhibition: The Role of Fermentation pH

In this study an efficient strategy, i.e., controlling the fermentation pH at constant pH 10, for significantly increasing hydrogen yield from waste activated sludge (WAS) via the improvement of anaerobic process (sludge solubilization, hydrolysis, and acidification) and inhibition of hydrogen consumption by acetobacteria was reported. Without addition of pure hydrogen producer and nutrient source, the effect of different constant pH in the range of pH 4-11 on hydrogen production from WAS was compared with that of different initial pH. The maximal hydrogen yield was observed respectively at constant pH 10 and initial pH 10, but the former was 47.8% higher than the latter (26.9 versus 18.2 mL per gram volatile suspended solids) and much greater than that reported in literature. Then, the mechanisms for constant pH 10 resulting in remarkably higher hydrogen production than initial pH 10 were investigated. It was observed that constant pH 10 fermentation showed much higher solubilization of sludge main particulate organic matters, hydrolysis of solubilized organic materials and acidification of hydrolyzed products, which were of benefit to the hydrogen production. Also, there was more acetic but less propionic acid in the constant pH 10 test, which was in correspondence with the theory of fermentation type affecting hydrogen production. Moreover, in the reactor of initial pH 10 the produced hydrogen was readily converted to acetic acid, but no obvious hydrogen consumption was observed in constant pH 10 reactor. Further investigation of microorganisms with enzymes analysis and fluorescence in situ hybridization (FISH) indicated that the activity and growth of acetobacteria in the reactor of constant pH 10 was much lower than those in initial pH 10 reactor.

Dilemma of Sewage Sludge Treatment and Disposal in China

S sludge, the most important byproduct of biological wastewater treatment, is considered an important source of secondary pollution in aquatic environments, linked to health problems and even deaths in humans. In 2012, China generated more than 68.5 billion metric tonnes of wastewater, and this is expected to rise to 78.4 billion metric tonnes in 2015. The amount of sewage sludge would increase accordingly, from 30 million metric tonnes (at a moisture content of 80%) in 2012 to 34 million metric tonnes in 2015. Historically, over 80% of the sludge has not been treated and disposed of effectively and safely, and this poses a great threat to the environment, particularly because of the ubiquitous use of combined systems for municipal wastewater, industrial wastewater, and rainwater treatment. Thus, it is extremely important to set up separate drainage systems to improve the efficiency and effectiveness of sludge treatment and disposal. The proportion of industrial wastewater entering wastewater treatment plants (WWTPs) in China is often as high as approximately 35.0%. Treatment of this industrial wastewater leads to sewage sludge containing heavy metals such as Zn, Cr, and Pb, and persistent organic pollutants (POPs), such as polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs), seriously limiting options for disposal. For example, this type of waste cannot be applied to land as soil conditioner. Also, the organic content of the sewage sludge is usually low (less than 50%) because most of the drainage systems, especially in southern China, combine municipal wastewater, and rainwater, resulting in a high proportion of grit and other inorganic matter being incorporated into the sludge. As a result, commonly used techniques for sludge treatment, such as anaerobic digestion and aerobic composting are ineffective. Anaerobic digestion/aerobic composting and land application have been widely recommended in China as the preferred method for the treatment and disposal of sewage sludge. However, the output from sludge treatment by anaerobic digestion or aerobic composting is of poor quality because of the low organic matter content. In 2013, there were about 2600 sludge treatment plants in China, but only about 60 plants had adopted anaerobic digestion processes and just 10−30 of them were actually operating, resulting in wasted infrastructure and treatment facilities. The treated sludge contains large amounts of nutrients, such as N, P, K, and residual organic material, and has the potential to be used as soil conditioner and fertilizer during land application. However, the high concentrations of heavy metals, POPs, and grit in the sludge, mainly from industrial wastewater and rainwater, have greatly limited its use. In the future, when making plans for the construction of wastewater treatment and recycling facilities in China, the separate treatment of municipal wastewater, rainwater, and industrial wastewater should be considered to eliminate the problem of sewage sludge treatment and disposal. Even if this proposal were to be adopted, sludge treatment and disposal would remain a challenge. In China, most residential districts have no separate discharge systems for municipal wastewater and rainwater, and only a few industrial parks have been built with individual wastewater treatment facilities. The construction of separate drainage pipeline networks and facilities for industrial wastewater treatment should be prioritized, though it is a daunting task. The safe treatment and disposal efficiency of sewage sludge in developed cities of China is expected to be 80%. Meanwhile, there has been disproportionately low investment in sludge treatment and disposal (5.6 billion USD/year) compared with wastewater treatment (68.8 billion USD/year) in China, unlike in developed countries where there are approximately equal levels of investment in sludge disposal and wastewater treatment. More investment in sludge treatment and disposal is urgently needed in China. If not, the investment in wastewater treatment could be in vain as the pollutants would re-enter the environment through sewage sludge. Overall, China still faces many challenges in solving the problem of sewage sludge treatment and disposal.