Important Role In Denitrification Research Articles

Pyrrhotite-sulfur-limestone composite for high rate nitrogen and phosphorus removal from wastewater: Column study

Pyrrhotite-sulfur-limestone composite (PSLC) was prepared and PSLC autotrophic denitrification biofilter (PSLCAD) was constructed with PSLC particle (2–4.75 mm) in this study. During treating synthetic, municipal and industrial secondary effluent, PSLCAD showed good NO3−-N and PO43--P removal, and the highest TON (Total oxidized nitrogen) removal rate of PSLCAD was up to 1749.91 mg/L/d. At HRT 0.5 h, and influent NO3−-N 21.09 mg/L, TON removal rate was up to 1005.12 mg/L with effluent NO3−-N 0.10 mg/L. PSLCAD achieved effluent PO43--P below 0.2 mg/L when influent PO43--P was around 0.5 mg/L. HRT down to 0.5 h had no negative impacts on N removal. Effluent pH below 7 was harmful to denitrification performance of PSLCAD. TON removal rate increased with influent NO3−-N increasing, but influent NO3−-N over 103.55 mg/L decreased NO3−-N removal rate. In PSLCAD biofilter, the most dominant bacteria were Thiobacillus and Sulfurimonas, and they played the most important role in denitrification, but the abundance of heterotrophic denitrifiers was also quite high. PO43− was mainly removed through precipitate of Fe–P in PSLCAD. The synergistic effects between pyrrhotite and sulfur autotrophic denitrification were much enhanced, and that caused PSLCAD to achieve high rate N and P removal.

Performance and mechanism of chromium reduction in denitrification biofilm system with different carbon sources

In the process of biological reduction of Cr(VI), the type of carbon sources affects the rate and effect of Cr(VI) reduction, but its specific performance and influencing mechanism have not yet been explored. In this study, four denitrification biofilm reactors were operated under four common carbon sources (C6H12O6, CH3COONa, CH3OH, CH3COONa:C6H12O6 1:1) to reveal the impact of carbon sources on Cr(VI) reduction. Through preliminary experimental concentration research, 75 mg/L Cr(VI) was selected as the dosing concentration. In long-term operation, the composite carbon sources of CH3COONa and C6H12O6 demonstrated excellent stability and achieved an impressive Cr(VI) removal efficiency of 99.5 %. The following sequence was C6H12O6, CH3COONa, and CH3OH. Among them, CH3OH was less competitive and the system was severely unbalanced with lowest Cr(VI) reduction efficiency. The toxicity reactions, changes in EPS and its functional groups, and electron transfer revealed the reduction and fixation mechanism of chromium on denitrification biofilm. The changes in microbial communities indicated that microbial communities in composite carbon sources can quickly adapt to the high toxic environment. The proportion of Trichococcus reached 43.6 %, which played an important role in denitrification and Cr(VI) reduction. Meanwhile, the prediction of microbial COG function reflected its excellent metabolic ability and defense mechanism.

Nitrogen removal from low COD/N interflow using a hybrid activated sludge membrane-aerated biofilm reactor (H-MABR)

A hybrid activated sludge membrane-aerated biofilm reactor based on a two-stage simultaneous nitrification–denitrification (SND) process was built, and its utility for treating interflow with low chemical oxygen demand (COD)/total nitrogen (TN) (COD/N) was explored. The operating performance, functional microbial communities, and functional genes for nitrogen metabolism were evaluated at low COD/N (4–1.3). The reactor could achieve stable operation at COD/N = 4–1.5, and the removal efficiency of COD, TN, and ammonia nitrogen was stable at 90.30 ± 2.36 %, 85.69 ± 2.22 %, and 89.52 ± 6.06 %, respectively. The SND rates were 70.89 % and 50.75 % when influent COD/N was 2.0 and 1.7, respectively, indicating that SND makes an important contribution to nitrogen removal under these two COD/N conditions. Microbial analysis revealed that the sampling sites with a high abundance of denitrification genes in the outer ring experienced aerobic conditions, inferring that aerobic denitrification also plays an important role in denitrification.

A novel multistage anoxic/aerobic process with sludge regeneration zone (R-MAO) for advanced nitrogen removal from domestic sewage

To achieve advanced nitrogen removal from actual municipal sewage, a novel multistage anoxic/aerobic process with sludge regeneration zone (R-MAO) was developed. The reactor was used to treat actual domestic sewage and the nitrogen removal capacity of the sludge regeneration zone (R zone) was investigated during the long-term operation. The best performance was obtained at the R zone's Oxidation-Reduction Potential (ORP) of -50±30 mV and hydraulic residence times (HRT) of 1.2 hr. The average effluent COD, TN, NH4+-N and NO3−-N of the R-MAO process were 18.0±2.3, 7.5±0.6, 1.0±0.5 and 4.6±0.4 mg/L, respectively, with the corresponding removal efficiency of COD, TN and NH4+-N were 92.9%±1.0%, 84.1%±1.5% and 97.5%±1.1%. Compared to the sole MAO system, the TN removal efficiency of the R-MAO increased by 10.1%. Besides, under the optimal conditions, the contribution of the R zone in the R-MAO that removal COD, TN, NH4+-N and NO3−-N were 0.36, 0.15, 0.032 and 0.82 g/day. High-throughput sequencing results showed that uncultured_bacterium_f_Burkholderiaceae (5.20%), OLB8 (1.04%) and Ottowia (1.03%) played an important role in denitrification in the R zone. This study provided effective guidance for the design and operation of the R-MAO process in domestic sewage treatment.

The removal performance of nitrates in the novel 3D-BERS with GAC and diversity of immobilized microbial communities treating nitrate-polluted water: Effects of pH and COD/NO3--N ratio

In this work, a three-dimensional bioelectrochemical reactor system (3D-BERs) with granular activated carbon (GAC) was utilized to study the feasibility of simultaneous removal of nitrates by autotrophic-heterotrophic denitrification process under different pH levels. In this present study, it was found that when the influent COD/ NO3--N ratio ranged between 1.5 and 3.5, both autotrophic and heterotrophic denitrifying microorganisms played an important role in denitrification. The experimental results demonstrated that the highest removal efficiency of nitrates under the optimum COD/NO3--N ratio of 1.5 (98.62%) was achieved with an initial pH of 7.5 ± 0.4. Likewise, when the COD/NO3--N ratio of 3.5, the nitrates removal efficiency (81.12%) was achieved with an initial pH of 8.2 ± 0.3, respectively. Batch denitrification processes followed zero-order kinetics at various NO3--N concentrations obtained. The bacterial community structure and relative abundance of bacteria changed at the level of genes and the phylum of immobilized GAC particles. Moreover, the diversity of bacterial composition enhanced the removal of NO3--N at the inner surface (IS), and bottom surface (BS) of immobilized GAC carriers were Gammaproteobacteria, Bacilli, Proteobacteria, and Thauera. In general, this technique is more effective for enhancing the denitrification process in the 3D-BER system.

Pilot-scale constructed wetlands for swine wastewater treatment: Microbial community analysis in bacterioplankton and epiphyton and options for resource recovery

In this study, the abundance and structure of bacterial and denitrifying functional genes of epiphyton and bacterioplankton in pilot-scale surface flow constructed wetlands treating different ammonium nitrogen concentrations (∼0.2, 40, 170, and 230 mg/L) of swine wastewater were investigated by quantitative PCR. The gene abundance of denitrifiers in epiphyton outnumbered the gene abundance of denitrifiers in bacterioplankton (p < 0.01), e.g., nirK gene abundance: 1.6–4.5 × 107vs 0.23–1.5 × 104 copies/ng DNA. A nitrogen concentration-dependent microbial community structure was observed for epiphyton and bacterioplankton. Proteobacteria, Bacteroidetes, and Actinobacteria dominated at the phylum level in both epiphyton and bacterioplankton. The nirS gene abundance in both niches was negatively related to the dissolved oxygen and redox potential of wastewater, but it was positively related with the Bacteroidetes and Flavobacterium abundance. The results indicated that microbial community structure and abundance are distinct between epiphyton and bacterioplankton. This study also demonstrate that epiphytic biofilms play an important role in denitrification during the treatment of swine wastewater in constructed wetlands. Finally, since swine wastewater serves as an excellent source for nutrients, possible technological options including periphyton techniques for resource recovery from wastewater and future research prospective have been suggested.

Characterization of microbes and denitrifiers attached to two species of floating plants in the wetlands of Lake Taihu.

Biofilms are often observed at the solid-water interface. The leaves of many floating macrophytes have characteristics of both terrestrial plants and submerged macrophytes, because, in general, their upper and lower surfaces are exposed to air and water, respectively. However, little is known about the biofilms attached to floating plants. We investigated biofilms attached to the leaves, stems and roots of the floating plants Nymphoides peltata (in summer and winter) and Trapa natans (in summer) in the Gonghu Bay of Lake Taihu. Bacteria and algae were major components of the biofilm on the leaves of the two species of plants. In addition, 454 pyrosequencing analysis of bacterial 16S rRNA genes revealed that Proteobacteria was the dominant phylum, followed by Bacteroidetes, Firmicutes, Chloroflexi, Acidobacteria, and Verrucomicrobia. Cluster analysis showed that bacterial communities from the same plant source were clustered into the same group. A total of 677 genera were detected, and 47 genera were shared by all samples. Nitrifiers, including Nitrosomonas, Nitrosococcus and Nitrospira were detected in this study. Seven denitrifying genes (napA, napG, nirS, nirK, cnorB, qnorB and nosZ) were used to detect the abundance of denitrifiers. Genes nirK, nirS cnorB and nosZ were the four most abundant genes in all samples. Our results demonstrated that cultivation of floating plants in water column could enlarge the area for biofilm growth, and biofilms might play an important role in denitrification in eutrophic water.

Water Quality and Three-Dimensional Fluorescence of Stormwater Runoff from Lined Bioretention Field Cells

Bioretention cells have become one of the most popular devices in urban stormwater runoff management for their high efficiency in reducing runoff volume and pollution loads. However, bioretention cells are unstable in nutrient pollutant removal, as is verified by many cases, especially at low temperatures. Dissolved organic matters (DOMs) plays an important role in denitrification, but the three-dimensional fluorescence characteristics of stormwater runoff in bioretention field cells are poorly understood, so to improve the performance of bioretention cells in nutrient pollutants control, the authors determined that it was necessary to clarify the characteristics of three-dimensional fluorescence of stormwater runoff from bioretention cells and explain their relationship to water quality. To be applicable to urban landuse in mountainous cities, two field lined bioretention cells were constructed, and, from March to April in 2017, while temperatures were still low, water quality and three-dimensional fluorescence of stormwater runoff were analyzed during low rainfall intensity precipitation events. Analysis results showed that the effluent pollutant concentrations had low fluctuation after purification by the bioretention cells, and both cells showed relatively stable removal rates for NH4+-N and total phosphorus (TP; i. e., 78.38%-95.03% and 72.04%-76.04%, respectively). Analysis of fluorescence spectra showed that the DOMs in the runoff from the two cells was mostly made up of protein and humic substances, mainly of biological origin or aquatic bacterial metabolites. Both cells performed well in the removal of protein at I, protein at Ⅱ and fulvic acid (i. e., the removal rates could reach 57.33%-61.30%, 29.82%-31.28%, and 35.55%-43.16% respectively). Correlation analyses between water quality and DOM showed that total Nitrogen (TN), TP and total organic carbon (TOC) were positively correlated with the protein content in runoff from the biorentention cells, while NO3--N and NH4+-N were negatively correlated with protein content. Meanwhile, TN was negatively correlated with DOMs in regions Ⅳ and Ⅴ of the cells.

Effect of organic carbon on the production of biofuel nitrous oxide during the denitrification process

The carbon source plays an important role in denitrification for nitrogen removal from wastewater. In this study, the denitrification performance and nitrous oxide (N2O) generation in four sequencing batch reactors (SBRs) fed with methanol, ethanol, sodium acetate and glucose were investigated. The maximum N2O generation was achieved when glucose was used as the carbon source, with a N2O conversion ratio of 56%. The high conversion ratio was contributed from the organic carbon of glucose and the glucose-acclimated denitrifiers. The nitrite accumulation and N2O generation during denitrification with glucose as the carbon source increased with increasing chemical oxygen demand to nitrogen ratios in the range of 2–8. The microbial community and their relative abundances varied greatly in the four reactors, and a low abundance of Thauera was found in the glucose-fed SBR, which might contribute to the greater N2O production. Practical strategies for N2O generation from the denitrification process using glucose as the carbon source were proposed so as to achieve energy recovery from nitrogen in wastewater.

A nitrogen removal system to limit water exchange for recirculating freshwater aquarium using DHS–USB reactor

ABSTRACTThis study proposes a biological nitrogen removal system for freshwater aquaria consisting of a down-flow hanging sponge (DHS) and an up-flow sludge blanket (USB). DHS–USB systems can perform nitrification and denitrification simultaneously, reducing ammonia (NH3) and nitrate (NO3−) toxicity in the water. The performance of the system was evaluated using on-site fresh water aquaria at ambient temperature (23–34°C) over 192 days. NH3 and nitrite (NO2−) were maintained at a detection limit of 0.01 mg N L−1 and NO3− was maintained below 10 mg N L−1, despite limited water exchange. The 16S rRNA gene of microorganisms from the sludge retained in the bioreactors was sequenced to identify the microbial communities present. Microbial community analysis revealed that ammonia oxidizing archaea (AOA), Ca. Nitrososphaera and Nitrosopumilus, played an important role in nitrification in the DHS reactor, while denitrifying bacteria Thauera played an important role in denitrification in the USB reactor. The proposed DHS–USB system is a promising technological advancement in the development of lower maintenance aquaria.

Revealing the bacterial profile of an anoxic-aerobic moving-bed biofilm reactor system treating a chemical industry wastewater

This study aimed at the characterization of the microbial community diversity of a moving bed-biofilm reactor (MBBR) system by pyrosequencing the 16S rRNA gene. The MBBR process comprised two stages (anoxic and aerobic) for organic matter and nitrogen removal from a pesticide-producing industry wastewater. Over 15,000 sequence reads from each reactor were analysed after 400 days of operation. Despite similar values of bacterial richness, the aerobic reactor exhibited a more equitable distribution of the bacterial groups, leading to higher values of diversity. Proteobacteria was observed to be the most dominant classified phylum in both reactors, accounting for more than 80% of the reads, while Alphaproteobacteria was preponderant at the class level. As observed for both phylum and class levels, the genus diversity was less pronounced in the anoxic reactor, which was highly dominated by very few genera. Among them, Hyphomicrobium (21%) was the most frequent one. These organisms possibly played an important role in denitrification, as COD abatement in the non-aerated reactor was mainly coupled to nitrite/nitrate reduction. The aerobic tank was dominated by the genus Methylobacillus (13%). Thiobacillus corresponded to the second most relevant genus in both reactors. As expected, no nitrifying bacteria were detected in the anoxic biofilm. In the aerated tank, however, ammonium oxidizers were represented by the Nitrosomonas genus, accounting for 2.5% of the entire bacterial population. Nitrospira- and Nitrobacter-like nitrite oxidizers coexisted in the aerobic reactor. The former was almost 10 times more frequent than the latter, despite the occurrence of salt shock loading events.

The effects of oxygen and metal oxide catalysts on the reduction reaction of NO with lignite char during combustion flue gas cleaning

The development of lignite-char-supported metal oxide catalyst for reduction of nitric oxide (NO) is investigated in this paper. The characteristics of NO reduction by copper and iron oxide catalysts supported on activated lignite chars (ALC) was studied using a fixed-bed reactor at 300°C. The results showed that the impregnation of Cu on ALC resulted in higher catalytic reactivity during NO reduction compared with that of Fe. Chemisorption of O2 and NO on Cu/ALC catalyst was found to play an important role in denitrification. Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) analyses showed that chemically adsorbed oxygen facilitates the formation of C(O) complex and oxidation of Cu0 to Cu+ for Cu/ALC catalyst. The C(O) intermediates and C* production formed due to the fact that C/O2 reaction promoted the reduction of NO. It is suggested that the catalytic reaction of NO in this case comprised of C/O2 reaction, C(O)/NO reaction and formation of N2 and CO2. Cu seemed to have significantly promoted the C(O) formation and CO oxidation compared with Fe. The catalytic reactivity of Cu species for C(O) formation and CO oxidation followed the order of Cu0>Cu+>Cu2+. Fe3O4 was believed to be the active phase in Fe catalyst. The oxygen and char-supported metal catalysts significantly promoted C/NO reaction, and therefore may lead to a lower operation temperature of NOx removal.

Nitrate removal from high strength nitrate-bearing wastes in granular sludge sequencing batch reactors

A 6-L sequencing batch reactor (SBR) was operated for development of granular sludge capable of denitrification of high strength nitrates. Complete and stable denitrification of up to 5420mgL(-1) nitrate-N (2710mgL(-1) nitrate-N in reactor) was achieved by feeding simulated nitrate waste at a C/N ratio of 3. Compact and dense denitrifying granular sludge with relatively stable microbial community was developed during reactor operation. Accumulation of large amounts of nitrite due to incomplete denitrification occurred when the SBR was fed with 5420mgL(-1) NO3-N at a C/N ratio of 2. Complete denitrification could not be achieved at this C/N ratio, even after one week of reactor operation as the nitrite levels continued to accumulate. In order to improve denitrification performance, the reactor was fed with nitrate concentrations of 1354mgL(-1), while keeping C/N ratio at 2. Subsequently, nitrate concentration in the feed was increased in a step-wise manner to establish complete denitrification of 5420mgL(-1) NO3-N at a C/N ratio of 2. The results show that substrate concentration plays an important role in denitrification of high strength nitrate by influencing nitrite accumulation. Complete denitrification of high strength nitrates can be achieved at lower substrate concentrations, by an appropriate acclimatization strategy.

NirN Protein from Pseudomonas aeruginosa is a Novel Electron-bifurcating Dehydrogenase Catalyzing the Last Step of Heme d1 Biosynthesis

Heme d1 plays an important role in denitrification as the essential cofactor of the cytochrome cd1 nitrite reductase NirS. At present, the biosynthesis of heme d1 is only partially understood. The last step of heme d1 biosynthesis requires a so far unknown enzyme that catalyzes the introduction of a double bond into one of the propionate side chains of the tetrapyrrole yielding the corresponding acrylate side chain. In this study, we show that a Pseudomonas aeruginosa PAO1 strain lacking the NirN protein does not produce heme d1. Instead, the NirS purified from this strain contains the heme d1 precursor dihydro-heme d1 lacking the acrylic double bond, as indicated by UV-visible absorption spectroscopy and resonance Raman spectroscopy. Furthermore, the dihydro-heme d1 was extracted from purified NirS and characterized by UV-visible absorption spectroscopy and finally identified by high-resolution electrospray ionization mass spectrometry. Moreover, we show that purified NirN from P. aeruginosa binds the dihydro-heme d1 and catalyzes the introduction of the acrylic double bond in vitro. Strikingly, NirN uses an electron bifurcation mechanism for the two-electron oxidation reaction, during which one electron ends up on its heme c cofactor and the second electron reduces the substrate/product from the ferric to the ferrous state. On the basis of our results, we propose novel roles for the proteins NirN and NirF during the biosynthesis of heme d1.

Acceleration of Denitrification in Turbid Rivers Due to Denitrification Occurring on Suspended Sediment in Oxic Waters

High suspended sediment (SPS) concentration exists in many rivers of the world. In the present study, the effects of SPS concentration on denitrification were investigated in airtight chambers with sediment samples collected from the Yellow River which is the largest turbid river in the world. Results from the nitrogen stable ((15)N) isotopic tracer experiments showed that denitrification could occur on SPS in oxic waters and the denitrification rate increased with SPS concentration; this was probably caused by the presence of low-oxygen microsites in SPS. For the water systems with both bed-sediment and SPS, the denitrification kinetics fit well to Logistic model, and the denitrification rate constant increased linearly with SPS concentration (p < 0.01). The denitrification caused by the presence of SPS accounted for 22%, 38%, 53%, and 67% of the total denitrification in systems with 2.5, 8, 15, and 20 g L(-1) SPS, respectively. The activity of denitrifying bacteria in SPS was approximately twice that in bed-sediment, and the denitrifying bacteria population showed an increasing trend with SPS concentration in both SPS and bed-sediment, leading to the increase of denitrification rate with SPS concentration. Furthermore, the denitrification in bed-sediment was accelerated by increased diffusion of nitrate from overlying water to bed-sediment under agitation conditions, which accompanied with the presence of SPS. When with 8 g L(-1) SPS, approximately 66% of the increased denitrification compared to that without SPS was attributed to denitrification on SPS and 34% to agitation conditions. This is the first report of the occurrence of denitrification on SPS in oxic waters. The results suggest that SPS plays an important role in denitrification in turbid rivers; its effect on nitrogen cycle should be considered in future study.

Temporal shifts in diversity and quantity of nirS and nirK in a rice paddy field soil

Denitrification is an important part of the nitrogen cycle in the environment, and diverse bacteria, archaea, and fungi are known to have denitrifying ability. Rice paddy field soils have been known to have strong denitrifying activity, but the microbes responsible for denitrification in rice paddy field soils are not well known. Present study analyzed the diversity and quantity of the nitrite reductase genes ( nirS and nirK) in a rice paddy field soil, sampled four times in one rice-growing season. Clone library analyses suggested that the denitrifier community composition varied over sampling time. Although many clones were distantly related to the known NirS or NirK, some clones were related to the NirS from Burkholderiales and Rhodocyclales bacteria, and some were related to the NirK from Rhizobiales bacteria. These denitrifiers may play an important role in denitrification in the rice paddy field soil. The quantitative PCR results showed that nirK was more abundant than nirS in all soil samples, but the nirK/ nirS ratio decreased after water logging. These results suggest that both diversity and quantity changed over time in the rice paddy field soil, in response to the soil condition.

Identification of Novel Betaproteobacteria in a Succinate-Assimilating Population in Denitrifying Rice Paddy Soil by Using Stable Isotope Probing

Rice paddy soil has been shown to have strong denitrifying activity. However, the microbial populations responsible for the denitrification have not been well characterized. In this study, we employed Stable Isotope Probing (SIP) to study succinate-assimilating denitrifiers in soil microcosms amended with nitrate and (13)C-succinate. Microbial populations represented in (12)C- and (13)C-DNA fractions were different based on denaturing gradient gel electrophoresis (DGGE) of the PCR-amplified 16S rRNA gene fragment. A nearly full-length 16S rRNA gene was also amplified, cloned, and sequenced from (13)C-DNA fraction. Both PCR-DGGE and clone library analyses revealed that Burkholderiales and Rhodocyclales dominated the succinate-assimilating population in denitrifying soil after 24 h incubation. Among these, novel Betaproteobacteria, possibly within the order Rhodocyclales, represented 43% of the clones obtained. Nitrite reductase genes, nirS and nirK, were also amplified and cloned from the (13)C-DNA fraction. While most nirK clones in this study were similar to the nirK sequences from Rhizobiales, a majority of the nirS clones were similar to the nirS sequences from Burkholderiales and Rhodocyclales, consistent with the 16S rRNA gene analysis. These groups of bacteria, including the novel Betaproteobacteria, may play an important role in denitrification in rice paddy soil.

Molecular analysis of microbial population transition associated with the start of denitrification in a wastewater treatment process

The objective of this study is to determine the bacteria playing an important role in denitrification by monitoring the molecular dynamics accompanying the start of denitrification. cDNA reverse-transcribed from 16S rRNA was amplified with fluorescent labelled primer for terminal restriction fragment length polymorphism (T-RFLP) analysis and an unlabelled primer for cloning analysis. The terminal restriction fragments (T-RFs) that increased in association with the start of denitrification were determined. These T-RFs were identified by in silico analysis of 16S rRNA sequences obtained from cloning. As a result, it was clearly observed that the bacteria belonging to the genera Hydrogenophaga and Acidovorax increased in number after the start of denitrification. It was demonstrated that T-RFLP analysis targeting 16S rRNA is appropriate for the daily monitoring of a bacterial community to control wastewater treatment processes. Combination of the results of T-RFLP analysis and 16S rRNA clone library indicated that the bacteria belonging to the genera Hydrogenophaga and Acidovorax play an important role in denitrification. The results of this study provide new insight to the 16S rRNA level of active denitrifying bacteria in wastewater treatment processes.

Three-dimensional structure of manganese superoxide dismutase from Bacillus halodenitrificans, a component of the so-called “green protein”

A so-called “green protein” has been purified from a moderate halophilic eubacterium, Bacillus halodenitrificans (ATCC 49067), under anaerobic conditions. The protein, which might play an important role in denitrification, dissociates mainly into two components after exposure to air: a manganese superoxide dismutase (GP-MnSOD) and a nucleoside diphosphate kinase. As a first step in elucidating the overall structure of the green protein and the role of each component, the 2.8-Å resolution crystal structure of GP-MnSOD was determined. Compared with other manganese dismutases, GP-MnSOD shows two significant characteristics. The first is that the entrance to its substrate channel has an additional basic residue—Lys38. The second is that its surface is decorated with an excess of acidic over basic residues. All these structural features may be related to GP-MnSOD’s high catalytic activity and its endurance against the special cytoplasm of B. halodenitrificans. The structure of GP-MnSOD provides the basis for recognizing its possible role and assembly state in the green protein.

Biological treatment of leachate from solid waste landfill sites—Alterations in the bacterial community during the denitrification process

Nitrite accumulation is frequently observed in leachate treatment plants. Generally, it has been attributed to unfavourable process conditions during the nitrification process. Production and utilization of nitrite during leachate treatment were investigated in an activated sludge bioreactor and an aerobic packed bed/anoxic activated sludge reactor. Under the conditions used, nitrite was accumulated by incomplete nitrification during the start-up period of the treatment only. On the other hand, high amounts of nitrite were built-up within the denitrification step. These high nitrite concentrations (more than 200 mg l −1 NO − 2) were measured during the start-up period of the denitrification process and at TOC N-ratios less than 1.5. Nitrite accumulation and nitrite utilization could be correlated with changes in the microbial community present. During the start-up period (about 20 days), most of the bacteria isolated from the denitrification tank were able to reduce both nitrate and nitrite. Later at a TOC N-ratio of 3.5, a significant shift in the activated sludge biocommunity was observed. After 40 days, less than 0.1% of the isolated bacteria were capable to reduce nitrite. From the results obtained, it is concluded that the composition of the biocommunity plays a more important role in denitrification of leachate than previously considered.