Investigating CH4 and N2O emissions from eco-engineering wastewater treatment processes using constructed wetland microcosms

Factors Influencing Gaseous Emissions in Constructed Wetlands: A Meta-Analysis and Systematic Review

Constructed wetlands (CWs) are an eco-technology for wastewater treatment and are applied worldwide. Due to the regular influx of pollutants, CWs can release considerable quantities of greenhouse gases (GHGs), ammonia (NH₃), and other atmospheric pollutants, such as volatile organic compounds (VOCs) and hydrogen sulfide (H₂S), etc., which will aggravate global warming, degrade air quality and even threaten human health. However, there is a lack of systematic understanding of factors affecting the emission of these gases in CWs. In this study, we applied meta-analysis to quantitatively review the main influencing factors of GHG emission from CWs; meanwhile, the emissions of NH₃, VOCs, and H₂S were qualitatively assessed. Meta-analysis indicates that horizontal subsurface flow (HSSF) CWs emit less CH₄ and N₂O than free water surface flow (FWS) CWs. The addition of biochar can mitigate N₂O emission compared to gravel-based CWs but has the risk of increasing CH₄ emission. Polyculture CWs stimulate CH₄ emission but pose no influence on N₂O emission compared to monoculture CWs. The influent wastewater characteristics (e.g., C/N ratio, salinity) and environmental conditions (e.g., temperature) can also impact GHG emission. The NH₃ volatilization from CWs is positively related to the influent nitrogen concentration and pH value. High plant species richness tends to reduce NH₃ volatilization and plant composition showed greater effects than species richness. Though VOCs and H₂S emissions from CWs do not always occur, it should be a concern when using CWs to treat wastewater containing hydrocarbon and acid. This study provides solid references for simultaneously achieving pollutant removal and reducing gaseous emission from CWs, which avoids the transformation of water pollution into air contamination.

A stable simultaneous anammox, denitrifying anaerobic methane oxidation and denitrification process in integrated vertical constructed wetlands for slightly polluted wastewater

Anaerobic ammonium oxidation (anammox), denitrifying anaerobic methane oxidation bacteria (DAMO) have received great attention for their excellent performance in nitrogen removal. However, not much study focused on the co-existence of anammox, DAMO, and denitrification in constructed wetlands, not to mention the advantage of their application in mitigating the necessary byproduct nitrous oxide (N₂O), methane (CH₄) from the biodegradation process. In this study, the result indicated the construction of integrated vertical constructed wetlands (IVCWs) contributed to the high-efficient stable simultaneous anammox, DAMO and denitrification (SADD) process for the nutrients removal, with denitrification being the least contributor to nitrogen reduction. Besides the succession of SADD process was largely the driver for the variation of N₂O, CH₄ emission. The structural equation method (SEM) further suggested that the three biological pathways of qnorB/bacteria, archaea/qnorB, and anammox/nirK accounted for the N₂O production, as were top-controlled by mcrA/DAMO in IVCWs. Besides the anammox-associated nitrifier denitrification was the main source for N₂O production. And that the trade-off effect between the CH₄ and N₂O production was exerted by the DAMO, while the influence was far from satisfactory under the methane constraints.

Effects of plant diversity on carbon dioxide emissions and carbon removal in laboratory-scale constructed wetland

Previous studies have shown that plant diversity can enhance methane (CH₄) emission and nitrogen purification efficiency in constructed wetlands (CWs), but effect of plant diversity on carbon dioxide (CO₂) flux and carbon removal efficiency in CWs is unknown. Therefore, we established four plant diversity levels (each level containing 4, 3, 2, and 1 species, respectively) in laboratory-scale wetland microcosms fed with simulated wastewater. Results showed that plant species richness enhanced CO₂ emissions (84.7-124.7 mg CO₂ m^-2 h^-1, P < 0.01), carbon fixation rate (P < 0.05), and microbial biomass carbon (P < 0.001), but did not improve carbon removal (P > 0.05). The presence of Pontederia cordata increased CO₂ emissions, carbon fixation rate of belowground, and microbial biomass carbon (P < 0.05), whereas the presence of Phragmites australis only enhanced CO₂ emission (P < 0.05). However, the presence of Typha orientalis or Lythrum salicaria did not show an influence on CO₂ emissions and carbon removal (P > 0.05). Hence, our study highlights the importance of plant diversity in mediating CO₂ emission intensity and carbon processes but not carbon removal in CWs.

CH4 flux and methanogen community dynamics from five common emergent vegetations in a full-scale constructed wetland

To investigate the effects of emergent plants on CH₄ efflux and elucidate the key factors responsible for these effects, annual monitoring of CH₄ emissions and methanogen community dynamics in a full-scale constructed wetland (CW) was conducted. Five emergent plants (Typha orientalis, Cyperus alternifolius, Arundo domax, Iris pseudacorus, and Thalia dealbata) commonly used in CWs were selected for investigation. The greatest CH₄ flux (annual mean 19.4 mg m^-2 h^-1) was observed from I. pseudacorus, while the lowest CH₄ flux (7.1 mg m^-2 h^-1) was observed from Thalia dealbata. The CH₄ flux from five emergent plants showed marked seasonal variation. Total nitrogen (TN) and total phosphorous (TP) were weakly correlated with CH₄ emissions, whereas total carbon (TC) and root biomass of plants were positively correlated with CH₄ emissions. Quantitative real-time PCR (q-PCR) analysis indicated that the gene abundance of eubacterial 16S rRNA, particulate methane monooxygenase (pmoA) and methyl coenzyme M reductase (mcrA) significantly differed among plant species. Differences in TC, root biomass, and dissolved oxygen (DO) caused by plant species were potential factors responsible for differences in methanogens, methanotrophs, and CH₄ emissions. Methanobacteriaceae, Methanoregulaceae, Methanomicrobiaceae, and Methanosarcinaceae were the dominant families of methanogens. The pathways of methanogenesis from the five emergent plants differed, with the main pathway being hydrogenotrophic, while both hydrogenotrophic and acetotrophic methanogens were involved in A. domax. Redundancy analysis (RDA) further indicated that emergent plant types had a profound influence on the methanogenic communities. Taken together, these results suggest emergent plant species can significantly influence CH₄ fluxes in CW through microbial communities, biochemical pathways for methanogenesis, TC, and DO. Furthermore, plant species in CWs should be considered an important factor in evaluating greenhouse gases emission. Finally, it is necessary to effectively manage CWs vegetation to maximize their environmental benefits. Graphical abstract ᅟ.

Particle-scale visualization of the evolution of methanogens and methanotrophs and its correlation with CH4 emissions during manure aerobic composting

Methane (CH₄) emissions are a major environmental concern in composting facilities. Therefore, this study initially visualized the dynamic distribution and quantity of methanogens and methanotrophs in composting particles during manure aerobic composting using fluorescence in situ hybridization-confocal laser scanning microscopy (FISH-CLSM) and quantified their correlation with CH₄ emissions. The visualization results showed that methanogens existed inside the particles, while methanotrophs clustered in the outer layer; a facultative anaerobic zone existed in between. The quantification results of integral optical density of methanogens and methanotrophs per unit particle area (U_gen and U_oxi, respectively) indicated that, in the cooling phase, CH₄ generation and oxidation could still be high and could strike a balance if the initial organic matter content of composting materials is high, while both could be extremely low if the content is low. A strong linearity between U_gen obtained by FISH-CLSM and methyl-coenzyme M reductase copy number obtained by quantitative polymerase chain reaction analysis (R² = 0.88) was observed, which justified the effectiveness of the FISH-CLSM method and demonstrated that macro-scale CH₄ emissions were essentially an accumulation of particle-scale CH₄ emissions. CH₄ emissions were equal to 3.3297 × 10⁷U_gen - 3.1814 × 10⁶U_oxi - 3902.9900 (R² = 0.98). Overall, the results showed that methanogens exerted more influence on CH₄ emissions than methanotrophs. Combining these results with CH₄-generation and -oxidation kinetics may help illustrate CH₄-emission mechanisms, improve particle-scale CH₄-emission models, and thereby provide theoretical guidance for operation optimization and emission reduction in composting processes.

Existing forms and changes of nitrogen inside of horizontal subsurface constructed wetlands

Horizontal zeolite subsurface constructed wetland system (HZCWs) and horizontal limestone subsurface constructed wetland system (HLCWs) were applied to the removal of nitrogen in lightly polluted wastewater, and the existing forms, changes, and removal mechanism of nitrogen in the constructed wetlands were analyzed. The results indicated that compared with HLCWs, HZCWs exhibited better nitrogen removal effect, and the maximum removal rates of ammonia nitrogen and total nitrogen could reach 96.97 ± 5.29 and 93.12 ± 3.35%, respectively. Besides, analysis of the removal effect on nitrogen in different existing forms on different substrate heights in the constructed wetlands showed that variation of nitrogen removal efficiency had certain regularities, which were related to the interior construction features of the wetland systems, and agreed with the regularities in the changes of the influential factors such as DO inside of the wetlands. In addition, degradation mechanism of pollutions was also analyzed, and the results indicated that the quantity of microorganisms and enzymes, including FDA, catalase, and urease, on the surface of the substrates had significant influence on the removal regularities and effects of the major pollutions in constructed wetlands.

Assessment of low concentration wastewater treatment operations with dewatered alum sludge-based sequencing batch constructed wetland system

Competition of volatile fatty acids between anoxic denitrification and anaerobic phosphorus release is prominent. Therefore, low concentration wastewater has restricted effects on nitrogen and phosphorus removal. The purpose of this study is to treat dormitory sewage with a biochemical oxygen demand (BOD) ranging from 50 to 150 mg/L using dewatered alum sludge-based sequencing batch constructed wetland system. Vegetation in the wetland system was chosen to be Phragmites australis. Three parallel cases were carried out to assess impacts due to different hydraulic retention time (HRT) and artificial aeration. The results showed that this system is effective in removing total nitrogen (TN), ammonia nitrogen (NH₃-N) and total phosphorus (TP) under different HRT. However, nitrous oxide (N₂O) emission poses to be the greatest challenge in the high HRT cases. Artificial aeration could reduce N₂O emission but is associated with high operational cost. Results indicate that dewatered alum sludge-based sequencing batch constructed wetland system is a promising bio-measure in the treatment of low concentration wastewater.

Effects of carbon sources and COD/N ratio on N2O emissions in subsurface flow constructed wetlands

A set of constructed wetlands under two different carbon sources, namely, glucose (CW) and sodium acetate (YW), was established at a laboratory scale with influent COD/N ratios of 20:1, 10:1, 7:1, 4:1, and 0 to analyze the influence of carbon supply on nitrous oxide emissions. Results showed that the glucose systems generated higher N₂O emissions than those of the sodium acetate systems. The higher amount of N₂O-releasing fluxes in the CWs than in the YWs was consistent with the higher NO₂^--N accumulation in the former than in the latter. Moreover, electron competition was tighter in the CWs and contributed to the incomplete denitrification with poor N₂O production performance. Illumina MiSeq sequencing demonstrated that some denitrifying bacteria, such as Denitratisoma, Bacillus, and Zoogloea, were higher in the YWs than in the CWs. This result indicated that the carbon source is important in controlling N₂O emissions in microbial communities.

Co-Regulations of Spartina alterniflora Invasion and Exogenous Nitrogen Loading on Soil N2O Efflux in Subtropical Mangrove Mesocosms

Both plant invasion and nitrogen (N) enrichment should have significant impact on mangrove ecosystems in coastal regions around the world. However, how N2O efflux in mangrove wetlands responds to these environmental changes has not been well studied. Here, we conducted a mesocosm experiment with native mangrove species Kandelia obovata, invasive salt marsh species Spartina alterniflora, and their mixture in a simulated tide rotation system with or without nitrogen addition. In the treatments without N addition, the N2O effluxes were relatively low and there were no significant variations among the three vegetation types. A pulse loading of exogenous ammonium nitrogen increased N2O effluxes from soils but the stimulatory effect gradually diminished over time, suggesting that frequent measurements are necessary to accurately understand the behavior of N-induced response of N2O emissions. With the N addition, the N2O effluxes from the invasive S. alterniflora were lower than that from native K. obovata mesocosms. This result may be attributed to higher growth of S. alterniflora consuming most of the available nitrogen in soils, and thus inhibiting N2O production. We concluded that N loading significantly increased N2O effluxes, while the invasion of S. alterniflora reduced N2O effluxes response to N loading in this simulated mangrove ecosystem. Thus, both plant invasion and excessive N loading can co-regulate soil N2O emissions from mangrove wetlands, which should be considered when projecting future N2O effluxes from this type of coastal wetland.

Methane and carbon dioxide emissions from constructed wetlands receiving anaerobically pretreated sewage

The aim of this research was to determine methane and carbon dioxide emissions from a hybrid constructed wetland (CW) treating anaerobically pre-treated sewage. The CW was constituted of two horizontal flow (free water surface followed by a subsurface) units. A long-term study was carried out as both CW units were monitored for three campaigns in Period 1 (0.9-1.5years after start-up), and four campaigns in Period 2 (4.5-5.8years after start-up). The closed chamber method with collecting surfaces of 1810cm(2) was used. For this system, variability due to position in the transverse section of CW, plant presence or absence and recommended sampling period was determined. Overall methane emissions ranged from 96 to 966mgCH4m(-2) d(-1), depending on several factors as the operation time, the season of the year and the position in the system. Methane emissions increased from 267±188mgCH4m(-2)d(-1) during the second year of operation to 543±161mgCH4m(-2)d(-1) in the sixth year of operation. Methane emissions were related to the age of the CW and the season of the year, being high in spring and becoming lower from spring to winter. Total CO2 emissions ranged mostly from 3500 to 5800mgCO2m(-2)d(-1) during the sixth year of operation, while nitrous oxide emissions were below the detection limit of the method.

Emissions of NO and N2O in wetland microcosms for swine wastewater treatment

Nitric oxide (NO) and nitrous oxide (N2O) emitted from wetland systems contribute an important proportion to the global warming effect. In this study, four wetland microcosms vegetated with Myriophyllum elatinoides (WM), Alternanthera philoxeroides (WA), Eichhornia crassipes (WE), or without vegetation (NW) were compared to investigate the emissions of NO and N2O during nitrogen (N) removal process when treating swine wastewater. After 30-day incubation, TN removal rates of 96.4, 74.2, 97.2, and 47.3 % were observed for the WM, WA, WE, and NW microcosms, respectively. Yet, no significant difference was observed in WM and WE (p > 0.05). The average NO and N2O emissions in WE was significantly higher than those in WM, WA, and NW (p < 0.05). In addition, the emission of N2O in WE accounted for 2.10 % of initial TN load and 2.17 % of the total amount of TN removal, compared with less than 1 % in the other microcosms. These findings indicate that wetland vegetated with M. elatinoides may be an optimal system for swine wastewater treatment, based on its higher removal of N and lower emissions of NO and N2O.

Comparison of carbon balance in Mediterranean pilot constructed wetlands vegetated with different C4 plant species

This study investigates carbon dioxide (CO2) and methane (CH4) emissions and carbon (C) budgets in a horizontal subsurface flow pilot-plant constructed wetland (CW) with beds vegetated with Cyperus papyrus L., Chrysopogon zizanioides (L.) Roberty, and Mischantus × giganteus Greef et Deu in the Mediterranean basin (Sicily) during the 1st year of plant growing season. At the end of the vegetative season, M. giganteus showed the higher biomass accumulation (7.4 kg m(-2)) followed by C. zizanioides (5.3 kg m(-2)) and C. papyrus (1.8 kg m(-2)). Significantly higher emissions of CO2 were detected in the summer, while CH4 emissions were maximum during spring. Cumulative CO2 emissions by C. papyrus and C. zizanioides during the monitoring period showed similar trends with final values of about 775 and 1,074 g m(-2), respectively, whereas M. giganteus emitted 3,395 g m(-2). Cumulative CH4 bed emission showed different trends for the three C4 plant species in which total gas release during the study period was for C. papyrus 12.0 g m(-2) and ten times higher for M. giganteus, while C. zizanioides bed showed the greatest CH4 cumulative emission with 240.3 g m(-2). The wastewater organic carbon abatement determined different C flux in the atmosphere. Gas fluxes were influenced both by plant species and monitored months with an average C-emitted-to-C-removed ratio for C. zizanioides, C. papyrus, and M. giganteus of 0.3, 0.5, and 0.9, respectively. The growing season C balances were positive for all vegetated beds with the highest C sequestered in the bed with M. giganteus (4.26 kg m(-2)) followed by C. zizanioides (3.78 kg m(-2)) and C. papyrus (1.89 kg m(-2)). To our knowledge, this is the first paper that presents preliminary results on CO2 and CH4 emissions from CWs vegetated with C4 plant species in Mediterranean basin during vegetative growth.

How the novel integration of electrolysis in tidal flow constructed wetlands intensifies nutrient removal and odor control

Intensified nutrient removal and odor control in a novel electrolysis-integrated tidal flow constructed wetland were evaluated. The average removal efficiencies of COD and NH4(+)-N were above 85% and 80% in the two experimental wetlands at influent COD concentration of 300 mg/L and ammonium nitrogen concentration of 60 mg/L regardless of electrolysis integration. Effluent nitrate concentration decreased from 2.5mg/L to 0.5mg/L with the reduction in current intensity from 1.5 mA/cm(2) to 0.57 mA/cm(2). This result reveals the important role of current intensity in nitrogen transformation. Owing to the ferrous and ferric iron coagulant formed through the electro-dissolution of the iron anode, electrolysis integration not only exerted a positive effect on phosphorus removal but also effectively inhibited sulfide accumulation for odor control. Although electrolysis operation enhanced nutrient removal and promoted the emission of CH4, no significant difference was observed in the microbial communities and abundance of the two experimental wetlands.

Intensified nitrogen and phosphorus removal in a novel electrolysis-integrated tidal flow constructed wetland system

A novel electrolysis-integrated tidal flow constructed wetland (CW) system was developed in this study. The dynamics of intensified nitrogen and phosphorus removal and that of hydrogen sulphide control were evaluated. Ammonium removal of up to 80% was achieved with an inflow concentration of 60 mg/L in wetland systems with and without electrolysis integration. Effluent nitrate concentration decreased from 2 mg/L to less than 0.5 mg/L with the decrease in current intensity from 1.5 mA/cm(2) to 0.57 mA/cm(2) in the electrolysis-integrated wetland system, thus indicating that the current intensity of electrolysis plays an important role in nitrogen transformations. Phosphorus removal was significantly enhanced, exceeding 95% in the electrolysis-integrated CW system because of the in-situ formation of a ferric iron coagulant through the electro-dissolution of a sacrificial iron anode. Moreover, the electrolyzed wetland system effectively inhibits sulphide accumulation as a result of a sulphide precipitation coupled with ferrous-iron electro-dissolution and/or an inhibition of bacterial sulphate reduction under increased aerobic conditions.

Effects of plant species on soil microbial processes and CH4 emission from constructed wetlands

Methane (CH(4)) emission from constructed wetland has raised environmental concern. This study evaluated the influence of mono and polyculture constructed wetland and seasonal variation on CH(4) fluxes. Methane emission data showed large temporal variation ranging from 0 to 249.29 mg CH(4) m(-2) h(-1). Results indicated that the highest CH(4) flux was obtained in the polyculture system, planted with Phragmites australis, Zizania latifolia and Typha latifolia, reflecting polyculture system could stimulate CH(4) emission. FISH analysis showed the higher amount of methanotrophs in the profile of Z. latifolia in both mono and polyculture systems. The highest methanogens amount and relatively lower methanotrophs amount in the profile of polyculture system were obtained. The results support the characteristics of CH(4) fluxes. The polyculture constructed wetland has the higher potential of global warming.

Nutrient removal and greenhouse gas emissions in duckweed treatment ponds

Stormwater treatment ponds provide a variety of functions including sediment retention, organic and nutrient removal, and habitat restoration. The treatment ponds are, however, also a source of greenhouse gases. The objectives of this study were to assess greenhouse gas (CH(4), CO(2) and N(2)O) emissions in duckweed treatment ponds (DWPs) treating simulated stormwater and to determine the role of ammonia-oxidizing organisms in nutrient removal and methanogens in greenhouse gas emissions. Two replicated DWPs operated at a hydraulic retention time (HRT) of 10 days were able to remove 84% (± 4% [standard deviation]) chemical oxygen demand (COD), 79% (± 3%) NH(4)(+)-N, 86% (± 2%) NO(3)(-)-N and 56% (± 7%) orthophosphate. CH(4) emission rates in the DWPs ranged from 502 to 1900 mg CH(4) m(-2) d(-1) while those of nitrous oxide (N(2)O) ranged from 0.63 to 4 mg N(2)O m(-2) d(-1). The CO(2) emission rates ranged from 1700 to 3300 mg CO(2) m(-2) day(-1). Duckweed coverage on water surface along with the continued deposit of duckweed debris in the DWPs and low-nutrient influent water created a low dissolved oxygen environment for the growth of unique ammonia-oxidizing organisms and methanogens. Archaeal and bacterial amoA abundance in the DWPs ranged from (1.5 ± 0.2) × 10(7) to (1.7 ± 0.2) × 10(8) copies/g dry soil and from (1.0 ± 0.3) × 10(3) to (1.5 ± 0.4) × 10(6) copies/g dry soil, respectively. The 16S rRNA acetoclastic and hydrogenotrophic methanogens ranged from (5.2 ± 0.2) × 10(5) to (9.0 ± 0.3) × 10(6) copies/g dry soil and from (1.0 ± 0.1) × 10(2) to (5.5 ± 0.4) × 10(3) copies/g dry soil, respectively. Ammonia-oxidizing archaea (AOA) appeared to be the dominant nitrifiers and acetoclastic Methanosaeta was the major methanogenic genus. The results suggest that methane is the predominant (>90%) greenhouse gas in the DWPs, where the relatively low stormwater nutrient inputs facilitate the growth of K-strategists such as AOA and Methanosaeta that may be responsible for ammonia removal and greenhouse gas emissions, respectively.

Life cycle assessment of vertical and horizontal flow constructed wetlands for wastewater treatment considering nitrogen and carbon greenhouse gas emissions

Life cycle assessment (LCA) is used to compare the environmental impacts of vertical flow constructed wetlands (VFCW) and horizontal flow constructed wetlands (HFCW). The LCAs include greenhouse gas (N(2)O, CO(2) and CH(4)) emissions. Baseline constructed wetland designs are compared to different treatment performance scenarios and to conventional wastewater treatment at the materials acquisition, assembly and operation life stages. The LCAs suggest that constructed wetlands have less environmental impact, in terms of resource consumption and greenhouse gas emissions. The VFCW is a less impactful configuration for removing total nitrogen from domestic wastewater. Both wetland designs have negligible impacts on respiratory organics, radiation and ozone. Gaseous emissions, often not included in wastewater LCAs because of lack of data or lack of agreement on impacts, have the largest impact on climate change. Nitrous oxide accounts for the increase in impact on respiratory inorganic, and the combined acidification/eutrophication category. The LCAs were used to assess the importance of nitrogen removal and recycling, and the potential for optimizing nitrogen removal in constructed wetlands.

Greenhouse gas emissions from surface flow and subsurface flow constructed wetlands treating dairy wastewater

Agricultural wastewater treatment is important for protecting water quality in rural ecosystems, and constructed wetlands are an effective treatment option. During treatment, however, some C and N are converted to CH(4), N(2)O, respectively, which are potent greenhouse gases (GHGs). The objective of this study was to assess CH(4), N(2)O, and CO(2) emissions from surface flow (SF) and subsurface flow (SSF) constructed wetlands. Six constructed wetlands (three SF and three SSF; 6.6 m(2) each) were loaded with dairy wastewater in Truro, Nova Scotia, Canada. From August 2005 through September 2006, GHG fluxes were measured continuously using transparent steady-state chambers that encompassed the entire wetlands. Flux densities of all gases were significantly (p < 0.01) different between SF and SSF wetlands changed significantly with time. Overall, SF wetlands had significantly (p < 0.01) higher emissions of CH(4) N(2)O than SSF wetlands and therefore had 180% higher total GHG emissions. The ratio of N(2)O to CH(4) emissions (CO(2)-equivalent) was nearly 1:1 in both wetland types. Emissions of CH(4)-C as a percentage of C removal varied seasonally from 0.2 to 27% were 2 to 3x higher in SF than SSF wetlands. The ratio of N(2)O-N emitted to N removed was between 0.1 and 1.6%, and the difference between wetland types was inconsistent. Thus, N(2)O emissions had a similar contribution to N removal in both wetland types, but SSF wetlands emitted less CH(4) while removing more C from the wastewater than SF wetlands.

Microbial biomass, activity and community composition in constructed wetlands

The aim of the current article is to give an overview about microbial communities and their functioning but also about factors affecting microbial activity in the three most common types (surface flow and two types of sub-surface flow) of constructed wetlands. The paper reviews the community composition and structural diversity of the microbial biomass, analyzing different aspects of microbial activity with respect to wastewater properties, specific wetland type, and environmental parameters. A brief introduction about the application of different novel molecular techniques for the assessment of microbial communities in constructed wetlands is also given. Microbially mediated processes in constructed wetlands are mainly dependent on hydraulic conditions, wastewater properties, including substrate and nutrient quality and availability, filter material or soil type, plants, and different environmental factors. Microbial biomass is within similar ranges in both horizontal and vertical subsurface flow and surface flow constructed wetlands. Stratification of the biomass but also a stratified structural pattern of the bacterial community can be seen in subsurface flow systems. Microbial biomass C/N ratio is higher in horizontal flow systems compared to vertical flow systems, indicating the structural differences in microbial communities between those two constructed wetland types. The total activity of the microbial community is in the same range, but heterotrophic growth is higher in the subsurface (vertical flow) system compared to the surface flow systems. Available species-specific data about microbial communities in different types of wetlands is scarce and therefore it is impossible make any general conclusions about the dynamics of microbial community structure in wetlands, its relationship to removal processes and operational parameters.

Maximizing pollutant removal in constructed wetlands: should we pay more attention to macrophyte species selection?

While the positive role of macrophytes on removal efficiency in constructed wetlands has been well established, possible differences in performance between plants species of comparable life forms and sizes are much harder to demonstrate. We reviewed 35 experimental studies published in peer-reviewed journals and proceedings on the effect of macrophyte species selection on pollutant removal in SSFCW. The studies cover a wide range of macrophyte species, experimental approaches (from well-replicated microcosm experiments to comparison between full full-size constructed wetlands), climatic conditions (from tropical to cold-temperate) and types of effluent (domestic, industrial, etc.). Frequent methodological limitations in these studies compel caution in the interpretation of their results. Yet, the fact that the majority found some (occasionally large) differences in efficiency between plant species for one or more type of pollutant suggests that macrophyte species selection does matter. However, there is little generalization to be made that could help guide species selection for SSFCW, except for the exact conditions in which the experiments were done. For example, the same pair of species that was tested in different studies occasionally gave opposite results in terms of which one performs best. Also, most studies provided few insights on the mechanisms or plant properties that could explain the observed differences in plant species efficiency. Finally, we discuss other relevant research questions and approaches that could help better guide macrophyte species selection for CW.

Impact of COD/N ratio on nitrous oxide emission from microcosm wetlands and their performance in removing nitrogen from wastewater

Constructed wetlands (CWs) are considered to be important sources of nitrous oxide (N(2)O). In order to investigate the effect of influent COD/N ratio on N(2)O emission and control excess emission from nitrogen removal, free water surface microcosm wetlands were used and fed with different influent. In addition, the transformation of nitrogen was examined for better understanding of the mechanism of N(2)O production under different operating COD/N ratios. It was found that N(2)O emission and the performance of microcosm wetlands were significantly affected by COD/N ratio of wastewater influent. Strong relationships exist between N(2)O production rate and nitrite (r=0.421, p<0.01). During denitrification process, DO concentration crucially influences N(2)O production rate. An optimal influent COD/N ratio was obtained by adjusting external carbon sources for most effective N(2)O emission control and best performance of the CWs in nitrogen removal from wastewater. It is concluded that under the operating condition of COD/N ratio=5, total N(2)O emission is minimum and the microcosm wetland is most effective in wastewater nitrogen removal.

Greenhouse gas production and efficiency of planted and artificially aerated constructed wetlands

Greenhouse gas (GHG) emissions by constructed wetlands (CWs) could mitigate the environmental benefits of nutrient removal in these man-made ecosystems. We studied the effect of 3 different macrophyte species and artificial aeration on the rates of nitrous oxide (N(2)O), carbon dioxide (CO(2)) and methane (CH(4)) production in CW mesocosms over three seasons. CW emitted 2-10 times more GHG than natural wetlands. Overall, CH(4) was the most important GHG emitted in unplanted treatments. Oxygen availability through artificial aeration reduced CH(4) fluxes. Plant presence also decreased CH(4) fluxes but favoured CO(2) production. Nitrous oxide had a minor contribution to global warming potential (GWP<15%). The introduction of oxygen through artificial aeration combined with plant presence, particularly Typha angustifolia, had the overall best performance among the treatments tested in this study, including lowest GWP, greatest nutrient removal, and best hydraulic properties.

Nitrous oxide emission from polyculture constructed wetlands: effect of plant species

Loss of nitrogen from the soil-plant system has raised environmental concern. This study assessed the fluxes of nitrous oxide (N2O) in the subsurface flow constructed wetlands (CWs). To better understand the mechanism of N2O emission, spatial distribution of ammonia-oxidizing bacteria (AOB) in four kinds of wetlands soil were compared. N2O emission data showed large temporal and spatial variation ranging from -5.5 to 32.7 mg N2O m(-2) d(-1). The highest N2O emission occurred in the cell planted with Phragmites australis and Zizania latifolia. Whereas, the lower emission rate were obtained in the cell planted with P. australis and Typha latifolia. These revealed that Z. latifolia stimulated the N2O emission. Transportation of more organic matter and oxygen for AOB growth may be the reason. The study of AOB also supported this result, indicating that the root structure of Z. latifolia was favored by AOB for N2O formation.