Characteristics of rural domestic sewage discharge and their driving mechanisms: evidence from the Northern Region, China

The traits of rural domestic sewage emission are unclear,negatively affecting rural domestic sewage treatment and sewage management.This study used data from the Second National Pollution Source Census Bulletin to establish a data set.The spatial distribution characteristics and main factors influencing rural sewage discharge in the Northern Region were studied using spatial autocorrelation analysis and structural equations.The findings demonstrated that(l)a significant Spearman correlation between drainage water volume(DwV),chemical oxygen demand(COD),ammonia nitrogen(NH_(3)-N),total nitrogen(TN),and total phosphorus(TP)and that the correlation coefficients between DWV and COD,NH,-N,TNand TP were 0.87**,1.0**,0.99**,0.99**,respectively;(2)rural sewage discharge showed spatial autocorrelation,and rural domestic sewage discharge in the districts and counties with an administration was significantly higher than in the surrounding areas;and(3)social development was the main driver rural domestic sewage changes(path coefficient was 0.407**),and the main factors influencing rural domestic sewage discharge were the urbanization rate,years of education,and population age structure.This study obtained the spatial variation law and clarified the main influencing factors of rural domestic sewage to provide data support and a theoretical basis for subsequent rural sewage collection and treatment.Use of the Inner Mongolia Autonomous Region in northern China as a typical case,provides a theoretical foundation for scientific decision-making on rural domestic sewage treatment at the national and regional levels and offers new perspectives for managing pollutants.

Data-driven systematic analysis of waterborne viruses and health risks during the wastewater reclamation process

Waterborne viral epidemics are a major threat to public health.Increasing interest in wastewater reclamation highlights the importance of understanding the health risks associated with potential microbial hazards,particularly for reused water in direct contact with humans.This study focused on identifying viral epidemic patterns in municipal wastewater reused for recreational applications based on long-term,spatially explicit global literature data during 2000e2021,and modelled human health risks from multiple exposure pathways using a well-established quantitative microbial risk assessment methodology.Global median viral loads in municipal wastewater ranged from 7.92×10^(4)to 1.4×10^(6)GC L^(-1)in the following ascending order:human adenovirus(HAdV),norovirus(NoV)GII,enterovirus(EV),NoV GI,rotavirus(RV),and severe acute respiratory syndrome coronavirus 2(SARSCoV-2).Following secondary or tertiary wastewater treatment,NoV GI,NoV GII,EV,and RV showed a relatively higher and more stable log reduction value with medians all above 0.8(84%),whereas SARSCoV-2 and HAdV showed a relatively lower reduction,with medians ranging from 0.33(53%)to 0.55(72%).A subsequent disinfection process effectively enhanced viral removal to over 0.89-log(87%).The predicted event probability of virus-related gastrointestinal illness and acute febrile respiratory illnesses in reclaimed recreational water exceeded the World Health Organization recommended recreational risk benchmark(5%and 1.9%,respectively).Overall,our results provided insights on health risks associated with reusing wastewater for recreational purposes and highlighted the need for establishing a regulatory framework ensuring the safety management of reclaimed waters.

Characterization of the airborne bacteria community at different distances from the rotating brushes in a wastewater treatment plant by 16S rRNA gene clone libraries

Biological risks of bioaerosols emitted from wastewater treatment processes have attracted wide attention in the recent years. However, the culture-based analysis method has been mostly adopted for detecting the bacterial community in bioaerosols, which may result in the underestimation of total microorganism concentration as not all microorganisms are cultivable. In this study, oligonucleotide fingerprinting of 16S rRNA genes was applied to reveal the composition and structure of the bacterial community in bioaerosols from an Orbal oxidation ditch in a Beijing wastewater treatment plant （WWTP）. Bioaerosols were collected at different distances from the aerosol source, rotating brushes, and the sampling height was 1.5 m which is the common respiratory height of a human being. The bacterial communities of bioaerosols were diverse, and the lowest bacterial diversity was found at the sampling site just after the rotating brush rotating brush. A large proportion of bacteria in bioaerosols were affiliated with Proteobacteria and Bacteroidetes. Numerous bacteria present in the bioaerosols also emerged in water, indicating that the bacterial community in the bioaerosols was related to that of the aerosols＇ sources. The forced aeration of rotating brushes brought about observably distinct bacterial communities between sampling sites situated before and after the rotating brush. Isolation sources of closest relatives in bioaerosols clone libraries were associated with the aqueous environment in the WWTP. Common potential pathogens in bioaerosols as well as those not reported in previous research were also analyzed in this study. Measures should be adopted to reduce the emission of bioaerosols and prevent their exposure to workers.

Characterization and source analysis of indoor/outdoor culturable airborne bacteria in a municipal wastewater treatment plant

The potential health risks of airborne bacteria emission from a wastewater treatment process have been concerned. However, few studies have investigated the differences in community structure between indoor and outdoor bacteria. In this work, the characterization of airborne bacteria was studied in a municipal wastewater treatment plant in Beijing, China. Two indoor（i.e., fine screen room and sludge dewatering house） and two outdoor（i.e., aeration tank and control site） sampling sites were selected. An Andersen six-stage impactor was used for collecting culturable airborne bacteria in the air, and Illumina MiSeq sequencing was conducted to track the emission source of the culturable airborne bacteria. The results indicate that, compared with the outdoor aeration tank site, the concentrations of culturable airborne bacteria in the indoor fine screen room with poor ventilation were more than ten times higher and the particle size was about twice as large. The community structures of indoor and outdoor culturable airborne bacteria were obviously different. Enterobacteriaceae and opportunistic pathogens were detected in indoor culturable airborne bacteria, with wastewater and sludge dewatering machine identified as the primary sources. Conversely,Enterobacteriaceae and opportunistic pathogens were not detected in outdoor culturable airborne bacteria. Outdoor high wind speed might have resulted in rapid dilution and mixing of culturable airborne bacteria generated from the aeration tank with the ambient air. The results of the present research suggest that covering pollution sources, increasing ventilation rates, and using protective measures for personnel should be implemented to decrease the exposure risk to indoor culturable airborne bacteria.

Sulfur dioxide and o-xylene co-treatment in biofilter：Performance, bacterial populations and bioaerosols emissions

Sulfur dioxide（SO_2） and benzene homologs are frequently present in the off-gas during the process of sewage sludge drying. A laboratory scale biofilter was set up to co-treat SO_2 and o-xylene in the present study. SO_2 and o-xylene could be removed simultaneously in a single biofilter. Their concentration ratio in the inlet stream influenced the removal efficiencies. It is worth noting that the removal of SO_2 could be enhanced when low concentrations of o-xylene were introduced into the biofilter. Pseudomonas sp., Paenibacillus sp., and Bacillus sp. were the main functional bacteria groups in the biofilter. Sulfur-oxidizing bacteria（SOB） and o-xylene-degrading bacteria（XB） thrived in the biofilter and their counts as well as their growth rate increased with the increase in amount of SO2 and o-xylene supplied. The microbial populations differed in counts and species due to the properties and components of the compounds being treated in the biofilter. The presence of mixed substrates enhanced the diversity of the microbial population. During the treatment process, bioaerosols including potentially pathogenic bacteria, e.g., Acinetobacter lwoffii and Aeromonas sp., were emitted from the biofilter. Further investigation is needed to focus on the potential hazards caused by the bioaerosols emitted from waste gas treatment bioreactors.

Flocculating characteristic of activated sludge flocs:Interaction between Al^(3+) and extracellular polymeric substances

Aluminum flocculant can enhance the flocculating performance of activated sludge.However,the binding mechanism of aluminum ion（Al 3＋） and extracellular polymeric substances（EPS） in activated sludge is unclear due to the complexity of EPS.In this work,threedimensional excitation emission matrix fluorescence spectroscopy（3DEEM）,fluorescence quenching titration and Fourier transform infrared spectroscopy（FT-IR） were used to explore the binding behavior and mechanism between Al 3＋ and EPS.The results showed that two fluorescence peaks of tyrosineand tryptophan-like substances were identified in the loosely bound-extracellular polymeric substances（LB-EPS）,and three peaks of tyrosine-,tryptophanand humic-like substances were identified in the tightly boundextracellular polymeric substances（TB-EPS）.It was found that these fluorescence peaks could be quenched with Al 3＋ at the dosage of 3.0 mg/L,which demonstrated that strong interactions took place between the EPS and Al 3＋.The conditional stability constants for Al 3＋ and EPS were determined by the Stern-Volmer equation.As to the binding mechanism,the-OH,N-H,C=O,C-N groups and the sulfurand phosphorus-containing groups showed complexation action,although the groups in the LB-EPS and TB-EPS showed different behavior.The TB-EPS have stronger binding ability to Al 3＋ than the LB-EPS,and TB-EPS play an important role in the interaction with Al 3＋.

Investigation on mechanism of phosphate removal on carbonized sludge adsorbent

For the removal of phosphate（PO43-） from water, an adsorbent was prepared via carbonization of sewage sludge from a wastewater treatment plant： carbonized sludge adsorbent（CSA）. The mechanism of phosphate removal was determined after studying the structure and chemical properties of the CSA and its influence on phosphate removal. The results demonstrate that phosphate adsorption by the CSA can be fitted with the pseudo second-order kinetics and Langmuir isotherm models, indicating that the adsorption is single molecular layer adsorption dominated by chemical reaction. The active sites binding phosphate on the surface are composed of mineral particles containing Si/Ca/Al/Fe. The mineral containing Ca, calcite, is the main factor responsible for phosphate removal. The phosphate removal mechanism is a complex process including crystallization via the interaction between Ca2＋ and PO43-; formation of precipitates of Ca2＋, Al3＋, and PO43-; and adsorption of PO43-on some recalcitrant oxides composed of Si/Al/Fe.

Flow field and dissolved oxygen distributions in the outer channel of the Orbal oxidation ditch by monitor and CFD simulation

In the Orbal oxidation ditch, denitrification is primarily accomplished in the outer channel. However, the detailed characteristics of the flow field and dissolved oxygen （DO） distribution in the outer channel are not well understood. Therefore, in this study, the flow velocity and DO concentration in the outer channel of an Orbal oxidation ditch system in a wastewater treatment plant in Beijing （China） were monitored under actual operation conditions. The flow field and DO concentration distributions were analyzed by computed fluid dynamic modeling. In situ monitoring and modeling both showed that the flow velocity was heterogeneous in the outer channel. As a result, the DO was also heterogeneously distributed in the outer channel, with concentration gradients occurring along the flow direction as well as in the cross-section. This heterogeneous DO distribution created many anoxic and aerobic zones, which may have facilitated simultaneous nitrificafion-denitrification in the channel. These findings may provide supporting information for rational optimization of the performance of the Orbal oxidation ditch.

Microbial aerosol particles in four seasons of sanitary landfill site:Molecular approaches,traceability and risk assessment

Landfill sites are regarded as prominent sources of bioaerosols for the surrounding atmosphere.The present study focused on the emission of airborne bacteria and fungi in four seasons of a sanitary landfill site.The main species found in bioaerosols were assayed using high-throughput sequencing.The Source Tracker method was utilized to identify the sources of the bioaerosols present at the boundary of the landfill site.Furthermore,the health consequences of the exposure to bioaerosols were evaluated based on the average daily dose rates.Results showed that the concentrations of airborne bacteria in the operation area(OPA)and the leakage treatment area(LTA)were in the range of(4684±477)–(10883±1395)CFU/m^(3) and(3179±453)–(9051±738)CFU/m^(3),respectively.The average emission levels of fungal aerosols were 4026 CFU/m^(3) for OPA and 1295 CFU/m^(3) for LTA.The landfill site received the maximum bioaerosol load during summer and the minimum during winter.Approximately 41.39%–86.24%of the airborne bacteria had a particle size of 1.1 to 4.7μm,whereas 48.27%–66.45%of the airborne fungi had a particle size of more than 4.7μm.Bacillus sp.,Brevibacillus sp.,and Paenibacillus sp.were abundant in the bacterial population,whereas Penicillium sp.and Aspergillus sp.dominated the fungal population.Bioaerosols released from the working area and treatment of leachate were the two main sources that emerged in the surrounding air of the landfill site boundary.The exposure risks during summer and autumn were higher than those in spring and winter.

Composition,dispersion,and health risks of bioaerosols in wastewater treatment plants:A review

Bioaerosols are defined as airbome particles(0.05-100 um in size)of biological origin.They are considered potentially harmful to human health as they can contain pathogens such as bacteria,fungi,and viruses.This review summarizes the most recent research on the health risks of bioaerosols emitted from wastewater treatment plants(WWTPs)in order to improve the control of such bioaerosols.The concentration and size distribution of WWTP bioaerosols;their major emission sources,composition,and health risks;and considerations for future research are discussed.The major themes and findings in the literature are as follows:the major emission sources of WWTP bioaerosols include screen rooms,sludge-dewatering rooms,and acration tanks;the bioaerosol concentrations in screen and sludge-dewatering rooms are higher than those outdoors.WWTP bioacrosols contain a variety of potentially pathogenic bacteria,fungi,antibiotic resistance genes,viruses,endotoxins,and toxic metal(loid)s.These potentially,pathogenic substances spread with the bioaerosols,thereby posing health risks to workers and residents in and around the WWTP.Inhalation has been identified as the main exposure route,and children are at a higher risk of this than adults.Future studies should identify emerging contaminants,establish health risk assessments,and develop prevention and control systems.

Characteristics of greenhouse gas emission in three full-scale wastewater treatment processes

Three full-scale wastewater treatment processes, Orbal oxidation ditch, anoxic/anaerobic/aerobic （reversed A^2O） and anaerobic/anoxic/aerobic （A^2O）, were selected to investigate the emission characteristics of greenhouse gases （GHG）, including carbon dioxide （CO2）, methane （CH4） and nitrous oxide （N2O）. Results showed that although the processes were different, the units presenting high GHG emission fluxes were remarkably similar, namely the highest CO2 and N2O emission fluxes occurred in the aerobic areas, and the highest CH4 emission fluxes occurred in the grit tanks. The GHG emission amount of each unit can be calculated from its area and GHG emission flux. The calculation results revealed that the maximum emission amounts of CO2, CH4 and N2O in the three wastewater treatment processes appeared in the aerobic areas in all cases. Theoretically, CH4 should be produced in anaerobic conditions, rather than aerobic conditions. However, results in this study showed that the CH4 emission fluxes in the forepart of the aerobic area were distinctly higher than in the anaerobic area. The situation for N2O was similar to that of CH4： the N2O emission flux in the aerobic area was also higher than that in the anoxic area. Through analysis of the GHG mass balance, it was found that the flow of dissolved GHG in the wastewater treatment processes and aerators may be the main reason for this phenomenon. Based on the monitoring and calculation results, GHG emission factors for the three wastewater treatment processes were determined. The A^2O process had the highest CO2 emission factor of 319.3 g CO2/kg CODremoved, and the highest CH4 and N2O emission factors of 3.3 g CH4/kg CODremoved and 3.6 g N2O/kg TNremoved were observed in the Orbal oxidation ditch process.

An innovative integrated system utilizing solar energy as power for the treatment of decentralized wastewater

This article reports an innovative integrated system utilizing solar energy as power for decentralized wastewater treatment, which consists of an oxidation ditch with double channels and a photovoltaic （PV） system without a storage battery. Because the system operates without a storage battery, which can reduce the cost of the PV system, the solar radiation intensity affects the amount of power output from the PV system. To ensure that the power output is sufficient in all different weather conditions, the solar radiation intensity of 78 W/m 2 with 95% confidence interval was defined as a threshold of power output for the PV system according to the monitoring results in this study, and a step power output mode was used to utilize the solar energy as well as possible. The oxidation ditch driven by the PV system without storage battery ran during the day and stopped at night. Therefore, anaerobic, anoxic and aerobic conditions could periodically appear in the oxidation ditch, which was favorable to nitrogen and phosphate removal from the wastewater. The experimental results showed that the system was efficient, achieving average removal efficiencies of 88% COD, 98% NH 4 ＋ -N, 70% TN and 83% TP, under the loading rates of 140 mg COD/（g MLSS·day）, 32 mg NH 4 ＋ -N/（g MLSS·day）, 44 mg TN/（g MLSS·day） and 5 mg TP/（g MLSS·day）.

Rapid detection of multiple class pharmaceuticals in both municipal wastewater and sludge with ultra high performance liquid chromatography tandem mass spectrometry

This work described the development, optimization and validation of an analytical method for rapid detection of multiple-class pharmaceuticals in both municipal wastewater and sludge samples based on ultrasonic solvent extraction, solid-phase extraction, and ultra high performance liquid chromatography-tandem mass spectrometry quantification. The results indicated that the developed method could effectively extract all the target pharmaceuticals （25） in a single process and analyze them within 24 min. The recoveries of the target pharmaceuticals were in the range of 69%-131% for wastewater and 54%-130% for sludge at different spiked concentration levels. The method quantification limits in wastewater and sludge ranged from 0.02 to 0.73 ng/L and from 0.02 to 1.00μg/kg, respectively. Subsequently, this method was validated and applied for residual pharma- ceutical analysis in a wastewater treatment plant located in Beijing, China. All the target pharmaceuticals were detected in the influent samples with concentrations varying from 0.09 ng/L （tiamulin） to 15.24 μg/L （caffeine）; meanwhile, up to 23 pharmaceuticals were detected in sludge samples with concentrations varying from 60 ng/kg （sulfamethizole） to 8.55 mg/kg （ofloxacin）. The developed method demonstrated its selectivity, sensitivity, and reliability for detecting multiple-class pharmaceuticals in complex matrices such as municipal wastewater and sludge.

Fate and degradation of nonylphenolic compounds during wastewater treatment process

In order to explore the biodegradation behavior of nonylphenolic compounds during wastewater treatment processing, two full-scale wastewater treatment plants were investigated and batch biodegradation experiments were conducted. The biodegradation pathways under the various operational conditions were identified from batch experiments： shortening of ethoxy-chains dominated under the anaerobic condition, whereas oxidizing of the terminal alcoholic group prevailed over the other routes under the aerobic condition. Results showed that the anoxic condition could accelerate the biodegradation rates of nonylphenolic compounds, but had no influence on the biodegradation pathway. The biodegradation rates of nonylphenol （NP） and short-chain nonylphenol polyethoxylates （NPnEOs, n： number of ethoxy units） increased from the anaerobic condition, then the anoxic, finally to the aerobic condition, while those of long-chain NPnEOs and nonylphenoxy carboxylates （NPECs） seemed similar under the various conditions. Under every operational condition, long-chain NPnEOs showed the highest biodegradation activity, followed by NPECs and short-chain NPnEOs, whereas NP showed relatively recalcitrant characteristics especially under the anaerobic condition. In addition, introducing sulfate and nitrate to the anaerobic condition could enhance the biodegradation of NP and short-chain NPnEOs by supplying more positive redox potentials.

Continuous desulfurization and bacterial community structure of an integrated bioreactor developed to treat SO_2 from a gas stream

Sulfide dioxide（SO2） is often released during the combustion processes of fossil fuels. An integrated bioreactor with two sections, namely, a suspended zone（SZ） and immobilized zone（IZ）, was applied to treat SO2 for 6 months. Sampling ports were set in both sections to investigate the performance and microbial characteristics of the integrated bioreactor. SO2 was effectively removed by the synergistic effect of the SZ and IZ, and more than 85%removal efficiency was achieved at steady state. The average elimination capacity of SO2 in the bioreactor was 2.80 g/（m3·hr） for the SZ and 1.50 g/（m3· hr） for the IZ. Most SO2 was eliminated in the SZ. The liquid level of the SZ and the water content ratio of the packing material in the IZ affected SO2 removal efficiency. The SZ served a key function not only in SO2 elimination, but also in moisture maintenance for the IZ. The desired water content in IZ could be feasibly maintained without any additional pre-humidification facilities. Clone libraries of 16 S r DNA directly amplified from the DNA of each sample were constructed and sequenced to analyze the community composition and diversity in the individual zones.The desulfurization bacteria dominated both zones. Paenibacillus sp. was present in both zones, whereas Ralstonia sp. existed only in the SZ. The transfer of SO2 to the SZ involved dissolution in the nutrient solution and biodegradation by the sulfur-oxidizing bacteria.This work presents a potential biological treatment method for waste gases containing hydrophilic compounds.

Characteristics of sewer biofilms in aerobic rural small diameter gravity sewers

Small diameter gravity sewers(SDGS)are extensively used to collect rural sewage as they are low in cost and quick to construct.However,the characteristics of biofilms in rural SDGS are still not clear.In this study,biofilms characteristics of aerobic rural SDGS were investigated using simulations in a lab under different flow conditions and slopes.Results indic ated that the average thickness of aerobic rural SDGS biofilms was in the range of 350-650μm,decreasing at locations with variable flow and high slopes.Protein was the most abundant substance in extracellular polymeric substance of SDGS biofilms.The most abundant bacteria,Proteobacteria,Actinobacteria,and Bacteroidetes,and functional bacteria showed different distributions when analyzed through Illumina HiSeq sequencing of 16 S rRNA.The relative abundances of denitrifying bacteria,nitrite-oxidizing bacteria,and sulfate-reducing bacteria(SRB)were lower during variable flow than during stable flow.High slopes(15‰)decreased SRB presence,which could be used to mitigate H2 S accumulation in aerobic SDGS.Overall,this study describes the characteristics of aerobic rural SDGS biofilms and provides valuable suggestions for the optimal design of SDGS based on these characteristics.

Occurrence,distribution,and potential influencing factors of sewage sludge components derived from nine full-scale wastewater treatment plants of Beijing,China

Millions of tons of waste activated sludge（WAS） produced from biological wastewater treatment processes cause severe adverse environmental consequences. A better understanding of WAS composition is thus very critical for sustainable sludge management. In this work, the occurrence and distribution of several fundamental sludge constituents were explored in WAS samples from nine full-scale wastewater treatment plants（WWTPs） of Beijing, China. Among all the components investigated, active heterotrophic biomass was dominant in the samples（up to 9478 mg/L）, followed by endogenous residues（6736 mg/L）,extracellular polymeric substances（2088 mg/L）, and intracellular storage products（464 mg/L）among others. Moreover, significant differences（p 〈 0.05） were observed in composition profiles of sludge samples among the studied WWTPs. To identify the potential parameters affecting the variable fractions of sludge components, wastewater source as well as design and operational parameters of WWTPs were studied using statistical methods. The findings indicated that the component fraction of sewage sludge depends more on wastewater treatment alternatives than on wastewater characteristics among other parameters. A principal component analysis was conducted, which further indicated that there was a greater proportion of residual inert biomass in the sludge produced by the combined system of the conventional anaerobic/anoxic/oxic process and a membrane bioreactor. Additionally, a much longer solids retention time was also found to influence the sludge composition and induce an increase in both endogenous inert residues and extracellular polymeric substances in the sludge.

Temporal variation of microbial population in a thermophilic biofilter for SO_2 removal

The performance of a biofilter relies on the activity of microorganisms during the gas contaminant treatment process. In this study, SO2 was treated using a laboratory-scale biofilter packed with polyurethane foam cubes（PUFC）, on which thermophilic desulfurization bacteria were attached. The thermophilic biofilter effectively reduced SO2 within 10 months of operation time, with a maximum elimination capacity of 48.29 g/m^3/hr.Temporal shifts in the microbial population in the thermophilic biofilter were determined through polymerase chain reaction-denaturing gradient gel electrophoresis and deoxyribonucleic acid（DNA） sequence analysis. The substrate species and environmental conditions in the biofilter influenced the microbial population. Oxygen distribution in the PUFC was analyzed using a microelectrode. When the water-containing rate in PUFC was over 98%, the oxygen distribution presented aerobic–anoxic–aerobic states along the test route on the PUFC. The appearance of sulfate-reducing bacteria was caused by the anaerobic conditions and sulfate formation after 4 months of operation.

Effects of oxygen and water content on microbial distribution in the polyurethane foam cubes of a biofilter for SO_2 removal

The performance of a biofilter for off-gas treatment relies on the activity of microorganisms and adequate O_2 and H_2O. In present study, a microelectrode was applied to analyze O_2 in polyurethane foam cubes（PUFCs） packed in a biofilter for SO_2 removal. The O_2 distribution varied with the density and water-containing rate（WCR） of PUFCs. The O_2 concentration dropped sharply from 10.2 to 0.8 mg/L from the surface to the center of a PUFC with 97.20%of WCR. The PUFCs with high WCR presented aerobic–anoxic–aerobic areas.Three-dimensional simulated images demonstrated that the structure of PUFCs with high WCR consisted of an aerobic ＂shell＂ and an anoxic ＂core＂, with high-density PUFCs featuring a larger anoxic area than low-density PUFCs. Moreover, the H_2O distribution in the PUFC was uneven and affected the O_2 concentration. Whereas aerobic bacteria were observed in the PUFC surface, facultative anaerobic microorganisms were found at the PUFC core, where the O_2 concentration was relatively low. O_2 and H_2O distributions differed in the PUFCs, and the distribution of microorganisms varied accordingly.