Novel Denitrification Fuel Cell for Energy Recovery of Nitrate-N and TN Removal Based on NH4+ Generation on a CNW@CF Cathode.

NO3- is an undesirable environmental pollutant that causes eutrophication in aquatic ecosystems, and its pollution is difficult to eliminate because it is easily converted into NH4+ instead of N2. Additionally, it is a high-energy substance. Herein, we propose a novel denitrification fuel cell to realize the chemical energy recovery of NO3- and simultaneous conversion of total nitrogen (TN) into N2 based on the outstanding ability of NH4+ generation on a three-dimensional copper nanowire (CNW)-modified copper foam (CF) cathode (CNW@CF). The basic steps are as follows: direct and highly selective reduction of NO3- to NH4+ rather than to N2 on the CNW@CF cathode, on which negative NO3- ions can be easily adsorbed due to their double-electron layer structure and active hydrogen ([H]) can be generated due to a large number of catalytic active sites exposed on CNWs. Then, NH4+ is selectively oxidized to N2 by the strong oxidation of chlorine free radicals (Cl•), which originate from the reaction of chlorine ions (Cl-) by photogenerated holes (h+) and hydroxyl radicals (OH•) under irradiation. Then, the electrons from the oxidation on the photoanode is transferred to the cathode to form a closed loop for external power generation. Owing to the continuous redox loop, NO3- completely reduces to N2, and the released chemical energy is converted into electrical energy. The results indicate that 99.9% of NO3- can be removed in 90 min, and the highest yield of electrical power density reaches 0.973 mW cm-2, of which the nitrate reduction rates on the CNW@CF cathode is 79 and 71 times higher than those on the Pt and CF cathodes, respectively. This study presents a novel and robust energy recycling concept for treating nitrate-rich wastewater.

Novel 3D Pd-Cu(OH)2/CF cathode for rapid reduction of nitrate-N and simultaneous total nitrogen removal from wastewater.

Removal of NO3- is a challenging problem in wastewater treatment. Electrocatalysis shows a great potential to remove NO3- but selectively converting NO3- to N2 is facing a low efficiency. Here, a novel 3D Pd-Cu(OH)2/CF cathode based electrocatalytic (EC) system was proposed that can rapidly and selectively convert NO3- to NH4+, and further convert to N2 simultaneously. The special designs for the system include: Cu(OH)2 nanowires were firstly grown on copper foam (CF) with excellent conductivity that features high specific surface area in enhancing NO3- absorption and conversion to NO2-. Then, palladium (Pd) with a superior photons activation capacity was doped on the Cu(OH)2 nanowires to promote the reduction of NO2- to NH4+. Then NH4+ was quickly oxidized into N2 by active chlorine. Finally, total nitrogen (TN) could easily be removed completely via above exhaustive cycle reactions. The 3D Pd-Cu(OH)2/CF cathode exhibits a 98.8 % conversion of NO3- to NH4+ in 45 min with the reported highest removal rate of 0.017 cm-2 min-1, which is 19.4 times higher than that of CF. The converted NH4+ was finally exhaustively oxidized to N2 with a 98.7 % of TN removal in 60 min.

Efficient urine removal, simultaneous elimination of emerging contaminants, and control of toxic chlorate in a photoelectrocatalytic-chlorine system.

Urine, which is an important waste biomass resource, is the main source of nitrogen in sewage and contains large quantities of emerging contaminants (ECs). In this study, we propose a new method to efficiently remove urine, simultaneously eliminate ECs, and control the generation of toxic chlorate during urine treatment using a photoelectrocatalytic-chlorine (PEC-Cl) system. A type-II heterojunction of WO3/BiVO4 was used as a photoanode to generate chlorine radicals (Cl•) by decreasing the oxidation potential of WO3 valence band for the highly selective conversion of urine to N2 and the simultaneous degradation of ECs in an efficient manner. The method presented surprising results. It was observed that the amount of toxic chlorate was significantly inhibited by circumventing the over-oxidation of Cl- by holes or hydroxyl radicals (•OH). Moreover, the removal of urea nitrogen reached 97% within 90 min, while the degradation rate of trimethoprim in urine was above 98.6% within 60 min, which was eight times more than that in the PEC system (12.1%). Compared to the bare WO3 photoanode, the toxic chlorate and nitrate generated by the WO3/BiVO4 heterojunction photoanode decreased by 61% and 44%, respectively. Thus, this study provides a safe, efficient, and environmentally-friendly approach for the disposal of urine.

Efficient SO2 Removal and Highly Synergistic H2O2 Production Based on a Novel Dual-Function Photoelectrocatalytic System.

The direct conversion of SO2 to SO3 is rather difficult for flue gas desulfurization due to its inert dynamic with high reaction activation energy, and the absorption by wet limestone-gypsum also needs the forced oxidation of O2 to oxidize sulfite to sulfate, which is necessary for additional aeration. Here, we propose a method to remove SO2 with highly synergistic H2O2 production based on a novel dual-function photoelectrocatalytic (PEC) system in which the jointed spontaneous reaction of desulfurization and H2O2 production was integrated instead of nonspontaneous reaction of O2 to H2O2. SO2 was absorbed by alkali liquor then oxidized quickly into SO42- by a nanorod α-Fe2O3 photoanode, which possessed high alkali corrosion resistance and electron transport properties. H2O2 was produced simultaneously in the cathode chamber on a gas diffusion electrode and was remarkably boosted by the conversion reaction of SO32- to SO42- in the anode chamber in which the released chemical energy was effectively used to increase H2O2. The photocurrent density increased by 40% up to 1.2 mA·cm-2, and the H2O2 evolution rate achieved 58.8 µmol·L-1·h-1·cm-2 with the synergistic treatment of SO2, which is about five times than that without SO2. This proposed PEC cell system offers a cost-effective and environmental-benign approach for dual purpose of flue gas desulfurization and simultaneous high-valued H2O2 production.

Sludge stabilization: Characteristics of the end-products and an alternative evaluative methodology.

The treatment, disposal and resource recovery of sewage sludge is a major bottleneck for the water and environmental remediation efforts in China. In this paper, sixteen sludge treatment plants using anaerobic digestion and aerobic composting as stabilization procedures were investigated and analysed. The organic degradation rates varied from 0.5% to 80.2% of different plants, showing the close relationship with raw sludge property and treatment process. The increment rate of humic-like substances ranged from 19% to 81% in different cases. It has been aware of that stabilization procedures coupled the degradation of simple organics (proteins, polysaccharides, lipids) with the synthesis of complex organics (humic-like substances). Therefore, an alternative methodology, considering the content of humic-like substances (no less than 150 mg/gVS) and the fluorescence complexity index (up to 5.0) in the end-products, was proposed to evaluate the stabilization level. Humic-like substances indicate the ecological value of the end-products. Fluorescence complexity index, combining the reduction of protein-like substances with the increment of humic-like substances, can predict the humification degree. The new criterion can be the supplementary of the current ones.

Innovative sludge pretreatment technology for impurity separation using micromesh.

In order to reduce the impacts on sludge treatment facilities caused by impurities such as fibers, hairs, plastic debris, and coarse sand, an innovative primary sludge pretreatment technology, sludge impurity separator (SIS), was proposed in this study. Non-woven micromesh with pore size of 0.40 mm was used to remove the impurities from primary sludge. Results of lab-scale tests showed that impurity concentration, aeration intensity, and channel gap were the key operation parameters, of which the optimized values were below 25 g/L, 0.8 m3/(m2 min), and 2.5 cm, respectively. In the full-scale SIS with treatment capacity of 300 m3/day, over 88% of impurities could be removed from influent and the cleaning cycle of micromesh was more than 16 days. Economic analysis revealed that the average energy consumption was 1.06 kWh/m3 treated sludge and operation cost was 0.6 yuan/m3 treated sludge.

Polyaniline/ß-MnO2 nanocomposites as cathode electrocatalyst for oxygen reduction reaction in microbial fuel cells.

An efficient and inexpensive catalyst for oxygen reduction reaction (ORR), polyaniline (PANI) and ß-MnO2 nanocomposites (PANI/ß-MnO2), was developed for air-cathode microbial fuel cells (MFCs). The PANI/ß-MnO2, ß-MnO2, PANI and ß-MnO2 mixture modified graphite felt electrodes were fabricated as air-cathodes in double-chambered MFCs and their cell performances were compared. At a dosage of 6 mg cm-2, the maximum power densities of MFCs with PANI/ß-MnO2, ß-MnO2, PANI and ß-MnO2 mixture cathodes reached 248, 183 and 204 mW m-2, respectively, while the cathode resistances were 38.4, 45.5 and 42.3 Ω, respectively, according to impedance analysis. Weak interaction existed between the rod-like ß-MnO2 and surficial growth granular PANI, this together with the larger specific surface area and PANI electric conducting nature enhanced the electrochemical activity for ORR and improved the power generation. The PANI/ß-MnO2 nanocomposites are a promising cathode catalyst for practical application of MFCs.

Influence of fermentation liquid from waste activated sludge on anoxic/oxic- membrane bioreactor performance: Nitrogen removal, membrane fouling and microbial community.

In order to investigate effects of waste activated sludge (WAS) fermentation liquid on anoxic/oxic- membrane bioreactor (A/O-MBR), two A/O-MBRs with and without WAS fermentation liquid addition were operated in parallel. Results show that addition of WAS fermentation liquid clearly improved denitrification efficiency without deterioration of nitrification, while severe membrane fouling occurred. WAS fermentation liquid resulted in an elevated production of proteins and humic acids in bound extracellular polymeric substance (EPS) and release of organic matter with high MW fractions in soluble microbial product (SMP) and loosely bound EPS (LB-EPS). Measurement of deposition rate and fluid structure confirmed increased fouling potential of SMP and LB-EPS. γ-Proteobacteria and Ferruginibacter, which can secrete and export EPS, were also found to be abundant in the MBR with WAS fermentation liquid. It is implied that when WAS fermentation liquid was applied, some operational steps to control membrane fouling should be employed.

Alkali-assisted membrane cleaning for fouling control of anaerobic ceramic membrane bioreactor.

In this study, a chemically enhanced backflush (CEB) cleaning method using NaOH solution was proposed for fouling mitigation in anaerobic membrane bioreactors (AnMBRs). Ex-situ cleaning tests revealed that NaOH dosages ranging from 0.05 to 1.30mmol/L had positive impacts on anaerobic biomass, while higher dosages (>1.30mmol/L) showed inhibition and/or toxic impacts. In-situ cleaning tests showed that anaerobic biomass could tolerate much higher NaOH concentrations due to the alkali consumption by anaerobic process and/or the buffering role of mixed liquor. More importantly, 10-20mmol-NaOH/L could significantly reduce membrane fouling rates (4-5.5 times over the AnMBR with deionized water backflush) and slightly improve methanogenic activities. COD removal efficiencies were over 87% and peaked at 20mmol-NaOH/L. However, extremely high NaOH concentration had adverse effects on filtration and treatment performance. Economic analysis indicated that 12mmol/L of NaOH was the cost-efficient and optimal fouling-control dosage for the CEB cleaning.

[Diversity of operation performance and microbial community structures in MBRs and CAS processes at low temperature].

In this paper, the performance of membrane bioreactors (MBR) and conventional activated sludge (CAS) processes at low temperature was investigated by analyzing their effluent quality and microbial viability. 454 high-throughput pyrosequencing was also applied to study the microbial community structures. The results of three systems (two MBRs: R1 with high sludge concentration, R2 with low sludge concentration, and one CAS: R3) showed that the average removal rate of NH4(+) -N was 99.7%, 99.7% and 59.7%, respectively, and the average removal rate of TN was 85.2%, 56.1% and 58.8%, respectively. R2 showed the highest specific ammonium uptake rate (SAUR), followed by R1 and R3; R1 showed the highest specific nitrate uptake rate (SNUR), followed by R2 and R3. It could be concluded that MBRs with high sludge concentration had a better performance of nitrogen removal under low temperature operation. 454 high-throughput pyrosequencing analysis revealed that the microbial richness was R2 > R3 > R1, and the microbial diversity was R2 > R1 > R3 at 97% sequence identity. The microbial structure and bacterial abundance were quite different between MBR and CAS systems. The dominant nitrifier in this research was Nitrospira, and the total relative abundance of nitrifiers in R1, R2, R3 was 1.22%, 1.64% and 0.15%, respectively. The Zoogloea, Thauera, Comamonadaceae and Comamonas might be the dominant denitrifers in this study, and the total relative abundance of denitrifier in R1, R2, and R3 was 5.8%, 4.52% and 15.21% respectively. MBR's characteristics of long solid retention time, high sludge concentration and low total nitrogen loading well supported the accumulation of nitrifiers and denitrifiers, and improved the performance of biological nitrogen removal at low temperature.

Correlating microbial community structure and composition with aeration intensity in submerged membrane bioreactors by 454 high-throughput pyrosequencing.

For understanding of the microbial community structure and composition under different aeration intensities, 454 high-throughput pyrosequencing was applied to analyze the 16S rRNA gene of bacteria in two submerged membrane bioreactors (MBRs) under low (R(L)) and high aeration (R(H)) conditions. In total, 7818 (R(L)) and 9353 (R(H)) high-quality reads were obtained, and 1230 (R(L)) and 924 (R(H)) operational taxonomic units (OTUs) were generated at 3% cutoff level, respectively. 454 pyrosequencing could also reveal the minority bacteria that were hardly detected by the conventional molecular methods. Although the core populations were shared with highly functional organization (>80%), clear differences between the samples in the two MBRs were revealed by richness-diversity indicators and Venn analyses. Notably, microbial diversity was decreased under high aeration condition, and the evolution of the populations was observed mainly in the shared OTUs. Moreover, specific comparison down to the class and genus level showed that the relative abundances of ß-Proteobacteria and γ-Proteobacteria in the R(H) community were respectively decreased by 41.5% and 66.6%, consistent with the observed membrane fouling mitigation during the reactor operation. It was also found that Nitrospira and Nitrosomonas, being nitrite oxidizing bacteria (NOB) and ammonium oxidizing bacteria (AOB), were the dominant phylogenetic groups at the genus level of both reactors, and that the high ratio of NOB to AOB populations well supported the complete ammonium oxidation performance in the two reactors. Although some populations of NOB and AOB decreased with the increase of aeration intensity, the functional stability of the nitrification process was less affected, probably due to the low influent substrate concentration and the high level of functional organization.