Leiyu Feng

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.

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.

Long-Term Effects of Copper Nanoparticles on Wastewater Biological Nutrient Removal and N2O Generation in the Activated Sludge Process

The increasing use of copper nanoparticles (Cu NPs) raises concerns about their potential toxic effects on the environment. However, their influences on wastewater biological nutrient removal (BNR) and nitrous oxide (N(2)O) generation in the activated sludge process have never been documented. In this study the long-term effects of Cu NPs (0.1-10 mg/L) on BNR and N(2)O generation were investigated. The total nitrogen (TN) removal was enhanced and N(2)O generation was reduced at any Cu NPs levels investigated, but both ammonia and phosphorus removals were not affected. The mechanism studies showed although most of the Cu NPs were absorbed to activated sludge, the activated sludge surface was not damaged, and the released copper ion from Cu NPs dissolution was the main reason for TN removal improvement and N(2)O reduction. It was also found that the transformation of polyhydroxyalkanoates and the activities of ammonia monooxygenase, nitrite oxidoreductase, exopolyphosphatase, and polyphosphate kinase were not affected by Cu NPs, whereas the decreased metabolism of glycogen and the increased activities of denitrification enzymes were observed. Further investigation revealed that Cu NPs increased the number of denitrifiers (especially N(2)O reducing denitrifiers) but decreased nitrite accumulation. All these observations were in correspondence with the enhancement of TN removal and reduction of N(2)O generation.

Short-chain fatty acid production from different biological phosphorus removal sludges: the influences of PHA and Gram-staining bacteria.

Recently, the reuse of waste activated sludge to produce short-chain fatty acids (SCFA) has attracted much attention. However, the influences of sludge characteristics, especially polyhydroxyalkanoates (PHA) and Gram-staining bacteria, on SCFA production have seldom been investigated. It was found in this study that during sludge anaerobic fermentation not only the fermentation time but also the SCFA production were different between two sludges, which had different PHA contents and Gram-negative bacteria to Gram-positive bacteria (GNB/GPB) ratios and were generated respectively from the anaerobic/oxic (AO) and aerobic/extended-idle (AEI) biological phosphorus removal processes. The optimal fermentation time for the AEI and AO sludges was respectively 4 and 8 d, and the corresponding SCFA production was 304.6 and 231.0 mg COD/g VSS (volatile suspended solids) in the batch test and 143.4 and 103.9 mg COD/g VSS in the semicontinuous experiment. The mechanism investigation showed that the AEI sludge had greater PHA content and GNB/GPB ratio, and the increased PHA content accelerated cell lysis and soluble substrate hydrolysis while the increased GNB/GPB ratio benefited cell lysis. Denaturing gradient gel electrophoresis profiles revealed that the microbial community in the AEI sludge fermentation reactor was dominated by Clostridium sp., which was reported to be SCFA-producing microbes. Further enzyme analyses indicated that the activities of key hydrolytic and acids-forming enzymes in the AEI sludge fermentation reactor were higher than those in the AO one. Thus, less fermentation time was required, but higher SCFA was produced in the AEI sludge fermentation system.

Alkyl polyglucose enhancing propionic acid enriched short-chain fatty acids production during anaerobic treatment of waste activated sludge and mechanisms

Adding alkyl polyglucose (APG) into an anaerobic treatment system of waste activated sludge (WAS) was reported to remarkably improve the production of short-chain fatty acids (SCFAs), especially propionic acid via simultaneously accelerating solubilization and hydrolysis, enhancing acidification, inhibiting methanogenesis and balancing carbon to nitrogen (C/N) ratio of substrate. Not only the production of SCFAs, especially propionic acid, was significantly improved by APG, but also the feasible operation time was shortened. The SCFAs yield at 0.3 g APG per gram of total suspended solids (TSS) within 4 d was 2988 ± 60 mg chemical oxygen demand (COD) per liter, much higher than that those from sole WAS or sole WAS plus sole APG. The corresponding yield of propionic acid was 1312 ± 25 mg COD/L, 7.9-fold of sole WAS. Mechanism investigation showed that during anaerobic treatment of WAS in the presence of APG both the solubilization and hydrolysis were accelerated and the acidification was enhanced, while the methanogenesis was inhibited. Moreover, the activities of key enzymes involved in WAS hydrolysis and acidification were improved through the adjustment of C/N ratio of substrates with APG. The abundance of microorganisms responsible for organic compounds hydrolysis and SCFAs production was also observed to be greatly enhanced with APG via 454 high-throughput pyrosequencing analysis.

Enhanced Bio-hydrogen Production from Protein Wastewater by Altering Protein Structure and Amino Acids Acidification Type

Enhanced bio-hydrogen production from protein wastewater by altering protein structure and amino acids acidification type via pH control was investigated. The hydrogen production reached 205.2 mL/g-protein when protein wastewater was pretreated at pH 12 and then fermented at pH 10. The mechanism studies showed that pH 12 pretreatment significantly enhanced protein bio-hydrolysis during the subsequent fermentation stage as it caused the unfolding of protein, damaged the protein hydrogen bonding networks, and destroyed the disulfide bridges, which increased the susceptibility of protein to protease. Moreover, pH 10 fermentation produced more acetic but less propionic acid during the anaerobic fermentation of amino acids, which was consistent with the theory of fermentation type affecting hydrogen production. Further analyses of the critical enzymes, genes, and microorganisms indicated that the activity and abundance of hydrogen producing bacteria in the pH 10 fermentation reactor were greater than those in the control.

Biological nutrient removal with low nitrous oxide generation by cancelling the anaerobic phase and extending the idle phase in a sequencing batch reactor

Although wastewater biological nutrient removal can be achieved by alternating the anaerobic-oxic-anoxic phases, significant amount of nitrous oxide (N2O) is generated in oxic phases, where ammonia-oxidizing bacteria (AOB) rather than heterotrophic denitrifiers are the main contributors. Here a new efficient strategy to remarkably reduce N2O generation was reported. It was found that by cancelling the anaerobic phase and extending the idle phase the N2O generation was reduced by 42% using synthetic wastewater, whereas the total nitrogen and phosphorus removals were unaffected. The mechanistic investigations revealed that the cancelling of anaerobic phase benefited heterotrophic denitrifiers instead of AOB to be responsible for nitrogen removal in the oxic phases, increased the ratio of total nitrogen removal driven by external carbon source, and decreased nitrite accumulation. Quantitative real-time polymerase chain reaction and fluorescence in situ hybridization analyses further showed that the new strategy increased the number of N2O reducing bacteria but decreased the abundance of glycogen accumulating organisms, with N2O as their primary denitrification product. It was also determined that the ratio of nitric oxide reductase activity to N2O reductase activity was significantly decreased after anaerobic phase was cancelled. All these observations were in accord with the reduction of N2O production. The feasibility of this strategy to minimize the generation of N2O was finally confirmed for a real municipal wastewater. The results reported in this paper provide a new viewpoint to reduce N2O generation from wastewater biological nutrient removal.

Stimulating short-chain fatty acids production from waste activated sludge by nano zero-valent iron

An efficient and green strategy, i.e. adding nano zero-valent iron into anaerobic fermentation systems to remarkably stimulate the accumulation of short-chain fatty acids from waste activated sludge via accelerating the solubilization and hydrolysis processes has been developed. In the presence of nano zero-valent iron, not only the short-chain fatty acids production was significantly improved, but also the fermentation time for maximal short-chain fatty acids was shortened compared with those in the absence of nano zero-valent iron. Mechanism investigations showed that the solubilization of sludge, hydrolysis of solubilized substances and acidification of hydrolyzed products were all enhanced by addition of nano zero-valent iron. Also, the general microbial activity of anaerobes and relative activities of key enzymes with hydrolysis and acidification of organic matters were improved than those in the control. 454 high-throughput pyrosequencing analysis suggested that the abundance of bacteria responsible for waste activated sludge hydrolysis and short-chain fatty acids production was greatly enhanced due to nano zero-valent iron addition.

Immobilizing photogenerated electrons from graphitic carbon nitride for an improved visible-light photocatalytic activity.

Reducing the recombination probability of photogenerated electrons and holes is pivotal in enhancing the photocatalytic ability of graphitic carbon nitride (g-C3N4). Speeding the departure of photogenerated electrons is the most commonly used method of achieving this. To the best of our knowledge, there is no report on suppressing the recombination of photogenerated electron–hole pairs by immobilizing the electrons with ester functional groups. Here, for the first time the mesoporous g-C3N4 (mpg-C3N4) was integrated with polymethyl methacrylate, a polymer abundant in ester groups, which showed a photocatalytic activity unexpectedly higher than that of the original mpg-C3N4 under visible-light irradiation. Experimental observations, along with theoretical calculations, clarified that the impressive photocatalytic ability of the as-modified mpg-C3N4 was mainly derived from the immobilization of photogenerated electrons via an electron-gripping effect imposed by the ester groups in the polymethyl methacrylate. This novel strategy might also be applied in improving the photocatalytic performance of other semiconductors.

Enhancing anaerobic digestion of waste activated sludge by pretreatment: effect of volatile to total solids

ABSTRACT In this study the effect of volatile to total solids (VS/TS) on anaerobic digestion of waste activated sludge (WAS) pretreated by alkaline, thermal and thermal–alkaline strategies was studied. Experimental results showed that the production of methane from sludge was increased with VS/TS. When anaerobic digesters were fed with sludge pretreated by the thermal–alkaline method, the average methane yield was improved from 2.8 L/d at VS/TS 0.35 to 4.7 L/d at VS/TS 0.56. Also, the efficiency of VS reduction during sludge anaerobic digestion varied between 18.9% and 45.6%, and increased gradually with VS/TS. Mechanism investigation of VS/TS on WAS anaerobic digestion suggested that the general activities of anaerobic microorganisms, activities of key enzymes related to sludge hydrolysis, acidification and methanogenesis, and the ratio of Archaea to Bacteria were all increased with VS/TS, showing good agreement with methane production.