Horizontal Subsurface Flow Bed Research Articles

Assessing lab-scale hybrid wetland performance for pollutant and pathogen removal from high organic loading septage

A laboratory-scale three-staged hybrid constructed wetland (CW) system was operated for the treatment of septage. The Hybrid-CW, covering an area of 1.92 m2, consisted of a sludge drying bed (SDB) of 0.41 m2, a vertical sub-surface flow bed (VSSF) of 0.58 m2, and a horizontal sub-surface flow bed (HSSF) of 0.93 m2. The SDB, VSSF and HSSF beds were operated at an average hydraulic loading rate (HLR) 22.2, 5.2 and 3.2 cm d−1. With an average organic loading rate (OLR) of 28.8 g d−1 the Hybrid-CW confirmed effective removal of BOD5, COD, TS, and TSS by 99.2 ± 0.3% and 99.4 ± 0.2%, 96.4 ± 0.8% and 86.9 ± 2.4%, respectively. The correlation between effluent TS and TSS concentrations with influent concentrations indicated a dependence of TS and TSS removal on influent concentration. Average total coliform count in influent and effluent was observed as 6.3 × 107 and 1.6 × 103 CFU L−1respectively. Average helminthes eggs count in the influent and effluent was recorded as 5 × 107 and 2 × 104 eggs L−1 respectively. With an inlet TS loading of 177.5 g d−1, the hybrid CW system attained 96.4% TS removal and 87% TSS removal. The sludge drying bed, operating with a remarkably high TS loading of 861.9 ± 77.1 g m−2 d−1, avoided clogging and achieved >90% removal rates for total coliform and helminthes eggs along with around 60% for TS and BOD. Furthermore, the system exhibited excellent removal rates of over 99% for both total coliform (TC) and helminthes eggs (HE). Treated septage meets the Indian wastewater discharge criteria for BOD, COD, TS and TN.

Nutrient removal, methane and nitrous oxide emissions in a hybrid constructed wetland treating anaerobic digestate

A pilot hybrid constructed wetland (CW) planted with reeds (Phragmite australis) and rice (Oryza sativa L.) was designed to treat liquid anaerobic digestate in the Yangtze River Delta, China. The hybrid CW system was composed of four stages: two reed vegetated vertical subsurface flow beds (VSSF: U1 and U2) in sequence, followed by a reed vegetated horizontal subsurface flow bed (HSSF: U3) and a rice vegetated surface flow bed (SF: U4). The average loading rate of digestate was 3.6 m3 per day during the experimental period.The average concentrations of TN and TP in the influent were 379 ± 58 mg L−1 and 29.6 ± 9.2 mg L−1, while the average removal efficiencies of TN and TP were 94.6% and 88.4%. Both TN and TP removal efficiencies in the second VSSF containing zeolite gave the highest removal performance, in which the mass removal rates were 21.3 ± 8.0 g-N m−2 d−1 and 0.99 ± 0.69 g-P m−2 d−1, respectively. Similarly, the highest removal performance for COD was also observed in the second VSSF with a mass removal rate of 79.9 ± 72.4 g-COD m−2 d−1.On the other hand, the average CH4 and N2O fluxes were highest in the first VSSF, at 31.8 ± 12.9 mg m−2 h−1 and 3.7 ± 2.8 mg m−2 h−1, respectively. There was a significant linear relationship between CH4 flux and DOC concentration in the pore water as well as a correlation between N2O flux and TN concentration. Total GWP of the hybrid CWs, derived from CH4 and N2O emissions, was 792.4 kg CO2-eq, of which CH4 and N2O emissions accounted for 66.0% and 34.0%, respectively. Consequently, the hybrid CWs emitted on average 0.93 kg CO2-eq to remove 1 kg COD while the average EF of TNin was 0.34%, suggesting that the use of multistage hybrid CWs could be efficiency-wise and environmentally a promising strategy for anaerobic digestate treatment.

Biotic phase micropollutant distribution in horizontal sub-surface flow constructed wetlands

The distribution of micropollutants in biotic phases of horizontal sub-surface flow (HSSF) constructed wetlands was investigated. 88 diverse micropollutants (personal care products, pharmaceuticals and illicit drugs) were monitored for in full-scale HSSF steel slag and gravel beds to assess their fate and behaviour during tertiary wastewater treatment. Of the studied micropollutants 54 were found in receiving and treated wastewaters. Treatment reduced concentrations of several micropollutants by >50% (removal range −112% to 98%) and resulted in changes to the stereo-isomeric composition of chiral species. For example, stereo-selective changes were observed for 3,4-methylenedioxymethamphetamine (MDMA) and atenolol during HSSF constructed wetland treatment for the first time. Analysis of sludge present within the HSSF beds found 37 micropollutants to be present. However, concentrations for the majority of these micropollutants were not considered high enough to suggest partitioning into sludge was a contributing mechanism of removal. Nevertheless the preservative methylparaben was found at 2772mgbed−1. Its daily removal from wastewater of 3.4mgd−1 indicates partitioning and accumulation in sludge contributes to its removal. Other micropollutants found at high levels in sludge (relative to their overall removals) were the antidepressants sertraline and fluoxetine, and the metabolite desmethylcitalopram. Furthermore, process balances indicated uptake and metabolism by Phragmites australis (Cav.) Trin. ex Steud did not contribute significantly to micropollutant removal. However analysis of plant tissues evidenced uptake, metabolism and accumulation of recalcitrant micropollutants such as ketamine and carbamazepine. It is considered that the rate of uptake was too slow to have a notable impact on removal at the 14h hydraulic retention time. Despite evidence of other removal mechanisms at play (e.g., partitioning into sludge and plant uptake), findings indicate biodegradation is the dominant mechanism of micropollutant removal in HSSF constructed wetlands.

Treatment performance and macrophytes growth in a restored hybrid constructed wetland for municipal wastewater treatment

This work evaluated depuration performance and macrophyte plants growth in a full-scale hybrid-constructed wetland (H-CW) (1000 population equivalent) treating municipal wastewater. The plant was activated in June 2010 after a restoration of an existent H-CW composed of a horizontal subsurface flow (HSSF) bed vegetated with evergreen xerophile species (Prunus laurocerasus, Pittosporum spp., Elaeagnus angustifolia), connected with a sedimentation pond vegetated with Lemna spp. To enhance the depuration performances, in both sections of the plant the original vegetation was removed and the HSSF bed was vegetated with Phragmites australis, whereas in the pond, a floating treatment wetland vegetated with Iris pseudacorus was installed. The depuration performance in treating total nitrogen (TN), nitrate nitrogen (NO3-N), ammonia nitrogen (NH4-N), total phosphorus (TP), orthophosphate (PO4-P) and chemical oxygen demand (COD) was weekly monitored between August 2010 and August 2011. Results indicated median concentration abatements for the entire system of 74.3% for TN, 62.1% for NH4-N, 77.7% for NO3-N, 29.6% for TP, 37.4% for PO4-P, and 46.7% for COD. In addition, at the end of the first growing season (June 2010–November 2010), good adaption to the system was detected for both P. australis and I. pseudacorus, which produced respectively 3.9±2.2kgm−2 and 3.7±1.0kgm−2 of above-ground dry biomass, with average above-ground N uptakes of 62.4±35.6gm−2 and 69.8±19.0gm−2 and average above-ground P uptakes of 4.6±2.6gm−2 and 7.8±2.1gm−2. The results indicate that a H-CW composed of a sequence of HSSF bed and FTW represents an effective application for abating TN, NH4-N, NO3-N and COD concentrations from municipal wastewater.

Methanogenic activity of accumulated solids and gas emissions from planted and unplanted shallow horizontal subsurface flow constructed wetlands

Anaerobic processes play an important role in horizontal subsurface flow (HSSF) constructed wetlands (CW) and methane generation contributes to overall greenhouse gas emissions. Plants and bed depth are two main factors that can influence anaerobic processes but the effect of plants’ presence and species on methane emissions on shallower HSSF (0.3m bed depth) has not been studied. We measured CH4 and CO2 emissions from a pilot plant constituted of five HSSF units in parallel with different plant species (CW1-UN: unplanted, CW2-JE: Juncus effusus, CW3-IP: Iris pseudacorus, CW4-TL: Typha latifolia L. and CW5-PA: Phragmites australis). Shallow HSSF beds showed high methane emissions (averaging 440mgCH4/m2*d) even at low loading rates (<5.0gBOD5/m2*d). Differences in mean emissions between units were not significant. However, the unplanted unit showed lower methane emissions during cold periods and higher emissions during warmer periods than planted units. Temperature was the main variable determining CO2 and CH4 emission for all units, except for CO2 emissions in CW2-JE unit. Temperature models explained 63–98% of seasonal variability and predicted zero emissions of CH4 and CO2 at temperatures of 9±1°C and 7.6±1.3°C, respectively. Organic matter accumulation (volatile solids, VS), specific methanogenic activity (SMA) and methane potential (MP) of accumulated solids were also determined. None of these parameters showed significant differences among CW units, except the higher values of MP in CW2-JE unit. While VS content and minimum SMA clearly increased with operation time, maximum SMA and MP remained stable. Potential methane emissions can be estimated from the product of surface density of VS and SMA, the measured emissions fitting well in the estimated range from batch assays.

Design and performance evaluation of a highly loaded aerated treatment wetland managing effluents from a food processing industry in Denmark

Treating highly loaded industrial effluents are challenging when treatment wetlands (TWs) are the choice selected for treatment. If TWs are selected as the solution, passive systems demand relatively large areas and may generate operational limitations and are not flexible in case of new treatment demands since no process adjustments can be made once the system is built. New types of intensified TW, such as aerated systems, have opened new possibilities in the field of the Wetland Technology and have shown capacity to treat several types of wastewater (WW). Aerated wetlands have been built across the USA and several European countries, but not a single system has been built in Denmark as to date. Thanks to an EU funded project and the cooperation of several partners under a consortium with the acronym HIGHWET, a new system is being built at the premises of a food processing factory in the vicinity of Faarup Denmark. The system will treat the WW generated at the plant and consists of a pretreatment system in the form of an anaerobic reactor, followed by two 1 m deep vertical flow beds, one of them aerated while the other bed is not aerated. Following these beds, the plant has two horizontal sub-surface flow beds fitted with aeration. For improving the P removal, media with high P binding capacity is are being used as filling material. In additiuon and for research purposes, the influent pump well plant is fitted with a dosing system in order to obtain pollutant loads at will so performance limits and kinetic constants can be established from the performance of the plant.

The impact of a pulsing water table on wastewater purification and greenhouse gas emission in a horizontal subsurface flow constructed wetland

Artificial pulsing of water table is often used in horizontal subsurface flow (HSSF) constructed wetlands (CW) to enhance removal of BOD, COD, NH4 and total N utilization by bacteria. In 2008–2010 and during the experimental session in October–November 2012 we studied the impact of a fluctuating water table on the water purification efficiency and GHG emissions in the 216m2 HSSF bed of a hybrid CW commissioned in 2002 in Paistu, Estonia. For comparison, water purification data from a previous study (Öövel et al., 2007) have been used. Isotopologue studies have been conducted in order to distinguish between denitrification and nitrification as source processes of N2O.Short-term (one month) and short-range (up to 35cm) fluctuations in the water table enhanced CO2 and N2O emission whereas the median values varied from 0.5 to 246μg CH4-Cm−2h−1 and 1.5 to 8.3μgN2O-Nm−2h−1, respectively. There was a significant (p<0.05) negative correlation between water table depth and CO2 (R2=0.53) and N2O (R2=0.35) emissions. Due to the plant cover disturbance in the outflow section of the HSSF bed, CH4 emission did not show any significant difference between the high and low water table phase, and the correlation between water table depth and CH4 emission was positive.The impact of pulsing water table on water purification was minor: high water table enhanced the TOC removal and inhibited the TN removal, whereas the TP concentrations showed remarkable variations but no significant differences between the inflow and outflow values. Likewise, pH and dissolved O2 values, and NH4+-N removal were not significantly influenced by water table manipulations.The value of δ15NbulkN2O, δ18O-N2O and site preference (SP) of N2O in water samples varied from −2 to 32 ‰, between 41 and 78 ‰, and from 15 to 41 ‰, respectively. There was a significant positive correlation between the δ18O-N2O vs SP N2O values (R2=0.77). This correlation, the range of isotopologue values and relatively low N2O emission corroborate that the main source of N2O fluxes in the studied HSSF CW bed is denitrification.

Emerging organic contaminant removal in a full-scale hybrid constructed wetland system for wastewater treatment and reuse

A full-scale hybrid constructed wetland (CW) system based on three stages of different wetlands configurations showed to be a very robust ecotechnology for domestic wastewater treatment and reuse in small communities. It consisted of a 317-m2 vertical subsurface flow (VF), a 229-m2 horizontal subsurface flow (HF), and a 240-m2 free water surface (FWS) CWs operating in series. VF and HF wetlands were planted with Phragmites australis and the FWS contained a mixture of plant species. An excellent overall treatment performance was exhibited on the elimination of conventional water quality parameters (98–99% average removal efficiency for TSS, BOD5 and NH4–N; n=8), and its final effluent proved to comply with existing Spanish regulations for various reuse applications. The removal of studied emerging contaminants, which included various pharmaceuticals, personal care products and endocrine disruptors, was also very high (above 80% for all compounds), being compound dependent (n=8). The high rates were achieved due to high temperatures as well as the differing existing physico-chemical conditions occurring at different CW configurations, which would allow for the combination and synergy of various abiotic/biotic removal mechanisms to occur (e.g. biodegradation, sorption, volatilization, hydrolysis, photodegradation). While aerobic metabolic pathways and solids retention are enhanced in the VF bed, other removal mechanisms such as anaerobic biodegradation and sorption would predominate in the HF bed. At last, photodegradation through direct sunlight exposure, and less importantly, sorption onto organic matter, seem to take an active part in organic contaminant removal in the FWS wetland.

Performance of a pilot-scale constructed wetland for stormwater runoff and domestic sewage treatment on the banks of a polluted urban river

To examine the performance of a constructed wetland system on stormwater runoff and domestic sewage (SRS) treatment in central east China, two parallel pilot-scale integrated constructed wetland (ICW) systems were operated for one year. Each ICW consisted of a down-flow bed, an up-flow bed and a horizontal subsurface flow bed. The average removal rates of chemical oxygen demand (CODCr), total suspended solids (TSS), ammonia (NH4(+)-N), total nitrogen (TN) and total phosphorus (TP) were 63.6, 91.9, 38.7, 43.0 and 70.0%, respectively, and the corresponding amounts of pollutant retention were approximately 368.3, 284.9, 23.2, 44.6 and 5.9 g m(-2) yr(-1), respectively. High hydraulic loading rate (HLR) of 200 mm/d and low water temperatures (<15 °C) resulted in significant decrease in removals for TP and NH4(+)-N, but had no significant effects on removals of COD and TSS. These results indicated that the operation of this ICW at higher HLR (200 mm/d) might be effective and feasible for TSS and COD removal, but for acceptable removal efficiencies of nitrogen and phosphorus it should be operated at lower HLR (100 mm/d). This kind of ICW could be employed as an effective technique for SRS treatment.

Carbon dioxide emissions from horizontal sub-surface constructed wetlands in the Mediterranean Basin

Constructed wetlands (CWs) are widely used natural-like systems for wastewater treatment where organic matter is removed through carbon dioxide (CO2) emissions. Several studies have been conducted regarding emissions and the sequestration of CO2 in CWs in the Northern Hemisphere; however, to the best of our knowledge, no studies have been performed in the Mediterranean Basin. This work quantified daily and cumulative CO2 emissions from a full-scale CW horizontal subsurface flow (HSSF) bed during semiarid Mediterranean spring climate conditions. The average daily CO2-C that was released in the atmosphere during the first 50 days ranged from approximately 17.5% to 32.6% of the C that was removed from wastewater. Considering both the Phragmites australis aerial part dry matter production (0.83kgm−2) and the average CO2-C emissions, after 50 days of vegetative regrowth, the HSSF bed was demonstrated to act as a CO2 sink. The cumulative CO2 efflux was 452.15±50.40CO2gm−2 and 276.02±12.07CO2gm−2 for vegetated and unvegetated sites, respectively.

Removal of selected pharmaceuticals from domestic wastewater in an activated sludge system followed by a horizontal subsurface flow bed — Analysis of their respective contributions

Seventy-three commonly administered pharmaceuticals from twelve different therapeutic classes were investigated at a municipal wastewater treatment plant in northern Italy featuring a conventional activated sludge system (full-scale) and a polishing horizontal subsurface flow bed (pilot plant). Removal of these micro-pollutants by the two systems was assessed in order to evaluate their respective contributions. Mean concentrations and standard deviations were calculated and found to differ for the compounds detected, ranging from few ng/L to over 1165ng/L in the secondary effluent and from 11 to 533 in the polished effluent. Eighteen compounds were consistently below the detection limit and the remaining 55 compounds were found at a minimum of one sampling point.Average removal efficiencies of both treatment steps and in treatment train as a whole are evaluated and discussed, highlighting the difficulties in predicting the fate of pharmaceuticals in both an activated sludge system and a horizontal subsurface flow bed. Comparison between the observed average removal efficiencies and those reported in the literature was also carried out for the pharmaceuticals of interest, and the discrepancies that emerged are discussed. The investigated constructed wetland did show efficacy in removing some of these compounds, and it contributed to the overall removal efficiency of each therapeutic class. Indeed, evaluation of the specific mass loadings of each class of PhC detected in the raw wastewaters, secondary and polished effluent evidences that the investigated constructed wetland is able to further reduce the load of micropollutants, which could become a necessity, especially where the receiving water body is an effluent-dominant river and mitigation measures of the discharge impact are required to protect and safeguard the aquatic environment.

Nutrient Removal from Polluted River Water by Combining Vertical and Horizontal Subsurface Flow Constructed Wetlands

A lab-scale hybrid constructed wetland system was constructed to purify polluted river water. The system was composed of a first stage of the vertical subsurface flow filter, followed by a second stage of horizontal subsurface flow bed. Both beds used furnace slag with a size of 4-60 mm for the main layer. The system was continuously fed. Different depths of unsaturated layer (0 cm, 15 cm and 30 cm) in vertical filter were tested. The unsaturated layer of 30 cm in vertical filter presented the most effective ammonia removal of 89.1%, while lowest NO3--N removal rate of 74.1% for the system. High TN removal efficiencies (77.3%-81.0%) could be observed during operation of three depths. The removals of COD and TP were in the range of 97.1%-98.4% and 76.4%-88.9%, respectively.

Analysis of the reclamation treatment capability of a constructed wetland for reuse

A research project was conducted during 2008-2009 in Portugal to evaluate the potential of reclaimed water from constructed wetlands for irrigation reuse. A 21 month monitoring campaign was set up in a Filtralite-based horizontal subsurface flow bed. Results showed a significant fluctuation of the hydraulic loading rate that has influenced the hydraulic retention time and the wastewater characteristics over time and, therefore, the removal efficiencies for BOD5, COD, TSS, nitrogen and phosphorus were lower than the reported values for CW performance. If the hydraulic loading rate could be properly controlled the treatment performance, as well as the quality of the reclaimed water, can be improved considerably. The effluent concentrations of conductivity (EC), BOD5, COD, TN, K, Ca, Mg and phytotoxic elements (Na, Cl and B), showed a suitable quality for irrigation reuse according to different international standards, although it is necessary to improve the removal of phosphorous and a final disinfection must be implemented to decrease the pathogenic content.

Removal and accumulation of Cu, Ni and Zn in horizontal subsurface flow constructed wetlands: Contribution of vegetation and filling medium

This study investigated the accumulation and removal of Cu, Ni and Zn in two horizontal subsurface flow constructed wetlands for domestic wastewater treatment, which differ by shape, presence of macrophytes and water depth. Between March and December 2007, the three metals were measured in the influent and effluents of the two systems. Average percentage removal rates were extremely low for Cu (3% and 9% in the two beds) and higher for Zn and Ni (between 25 and 35%). Under higher Zn influent concentrations, it was found to be between 78–87%, which is in agreement with other literature data. During the peak standing crop season (August), biomasses of the different parts of Phragmites australis (stems, leaves and flowers, roots and rhizomes) were analysed in terms of weight and heavy metal concentration in order to assess heavy metal distribution among the tissues. It was found that the plants contribute to total heavy metal removal to a lesser extent than the filling medium. Aboveground tissues remove 34% of Cu, 1.8% of Ni and 6.2% of Zn % and, once harvested, their disposal does not appear to pose a problem for the environment. If heavy metals are present at high concentrations in the horizontal subsurface flow bed influent, over time, their accumulation in the filling medium could necessitate special care in the bed's management to avoid release into the surrounding environment.

Analysis of constructed wetland performance for irrigation reuse

Most of the wastewater treatment systems in small rural communities of the Cova da Beira region (Portugal) consist of constructed wetlands (CW) with horizontal subsurface flow (HSSF). It is believed that those systems allow the compliance of discharge standards as well as the production of final effluents with suitability for reuse. Results obtained in a nine-month campaign in an HSSF bed pointed out that COD and TSS removal were lower than expected. A discrete sampling also showed that removal of TC, FC and HE was not enough to fulfill international irrigation goals. However, the bed had a very good response to variation of incoming nitrogen loads presenting high removal of nitrogen forms. A good correlation between mass load and mass removal rate was observed for BOD(5), COD, TN, NH(4)-N, TP and TSS, which shows a satisfactory response of the bed to the variable incoming loads. The entrance of excessive loads of organic matter and solids contributed for the decrease of the effective volume for pollutant uptake and therefore, may have negatively influenced the treatment capability. Primary treatment should be improved in order to decrease the variation of incoming organic and solid loads and to improve the removal of COD, solids and pathogenic. The final effluent presented good physical-chemical quality to be reused for irrigation, which is the most likely application in the area.

AN EXPERIMENTAL THREE-STAGE HYBRID CONSTRUCTED WETLAND SYSTEM FOR REMOVAL OF ORGANICS AND NUTRIENTS FROM DOMESTIC WASTEWATER

An experimental three-stage subsurface flow constructed wetland system was investigated for the removal of nutrient and organic matter from primary-treated domestic wastewater. Built in the Turkey Scientific and Technical Research Council-Marmara Research Center, Gebze, Turkey (Istanbul), the system consisted of a vertical flow-gravel filtration in the first stage, a horizontal-subsurface flow bed vegetated with Iris in the second stage, and a vertical-subsurface flow bed vegetated Phragmites in the third stage. Wastewater was recycled from the vertical bed to the horizontal one at a ratio of 1:2 and 1:1. The study aimed at evaluating the effects of the loading rates and seasonal changes, effects of recycling and rapid drainage, plant uptake, and the effectiveness of the system in the removal of organic matter and nutrients. The average removal efficiencies were high in summer periods. The results show that the average removal efficiencies increased depending on decrease in hydraulic loading rate and increase in recycle ratio. The relationships between the loading rate and the removal rate were linear.

Emission of greenhouse gases from constructed wetlands for wastewater treatment and from riparian buffer zones

We measured N2O, CH4 and CO2 fluxes in horizontal and vertical flow constructed wetlands (CW) and in a riparian alder stand in southern Estonia using the closed chamber method in the period from October 2001 to November 2003. The average rates of N20, CH4 and CO2 emission from the riparian gray alder stand were from -0.4 to 58 microg N2O-N m(-2) h(-1) and 0.1-265 microg CH4-C m(-2) h(-1), 55-61 mg CO2-C m(-2) h(-1), respectively. The average N2O-N emission from the microsites above the inflow pipes of horizontal subsurface flow (HSSF) CWs was 6.4-31 microg N2O-N m(-2) h(-1), whereas the outflow microsites emitted 2.4-8 microg N2O-N m(-2) h(-1). In vertical subsurface flow (VSSF) beds the same value was 35.6-44.7 microg N2O-N m(-2) h(-1). The average CH4 emission from the inflow and outflow microsites in the HSSF CWs differed significantly ranging from 640 to 9715 and from 30 to 770 microg CH4-C m(-2) h(-1), respectively. The average CO2 emission was somewhat higher in VSSF beds (140-291 mg CO2-C m(-2) h(-1)) and at inflow microsites of HSSF beds (61-140 mg CO2-C m(-2) h(-1)). The global warming potential (GWP) from N2O and CH4 was comparatively high in both types of CWs (4.8 +/- 9.8 and 6.8 +/- 16.2 t CO2 eq ha(-1) a(-1) in the HSSF CW 6.5 +/- 13.0 and 5.3 +/- 24.7 t CO2 eq ha(-1) a(-1) in the hybrid CW, respectively). The GWP of riparian alder forest from both N2O and CH4 was relatively low (0.4 +/- 1.0 and 0.1 +/- 0.30 t CO2 eq ha(-1) a(-1), respectively), whereas the CO2-C flux was remarkable (3.5 +/- 3.7 t ha(-1) a(-1). The global influence of CWs is not significant. Even if all the global domestic wastewater were treated by wetlands, their share in the trace gas emission budget would be less than 1%.

Emission of N 2O, N 2, CH 4, and CO 2 from constructed wetlands for wastewater treatment and from riparian buffer zones

We measured nitrous oxide (N 2O), dinitrogen (N 2), methane (CH 4), and carbon dioxide (CO 2) fluxes in horizontal and vertical flow constructed wetlands (CW) and in a riparian alder stand in southern Estonia using the closed chamber method in the period from October 2001 to November 2003. The replicates’ average values of N 2O, N 2, CH 4 and CO 2 fluxes from the riparian gray alder stand varied from −0.4 to 58 μg N 2O-N m −2 h −1, 0.02–17.4 mg N 2-N m −2 h −1, 0.1–265 μg CH 4-C m −2 h −1 and 55–61 mg CO 2-C m −2 h −1, respectively. In horizontal subsurface flow (HSSF) beds of CWs, the average N 2 emission varied from 0.17 to 130 and from 0.33 to 119 mg N 2-N m −2 h −1 in the vertical subsurface flow (VSSF) beds. The average N 2O-N emission from the microsites above the inflow pipes of the HSSF CWs was 6.4–31 μg N 2O-N m −2 h −1, whereas the outflow microsites emitted 2.4–8 μg N 2O-N m −2 h −1. In VSSF beds, the same value was 35.6–44.7 μg N 2O-N m −2 h −1. The average CH 4 emission from the inflow and outflow microsites in the HSSF CWs differed significantly, ranging from 640 to 9715 and from 30 to 770 μg CH 4-C m −2 h −1, respectively. The average CO 2 emission was somewhat higher in VSSF beds (140–291 mg CO 2-C m −2 h −1) and at the inflow microsites of HSSF beds (61–140 mg CO 2-C m −2 h −1). The global warming potential (GWP) from N 2O and CH 4 was comparatively high in both types of CWs (4.8 ± 9.8 and 6.8 ± 16.2 t CO 2 eq ha −1 a −1 in the HSSF CW 6.5 ± 13.0 and 5.3 ± 24.7 t CO 2 eq ha −1 a −1 in the hybrid CW, respectively). The GWP of the riparian alder forest from both N 2O and CH 4 was relatively low (0.4 ± 1.0 and 0.1 ± 0.30 t CO 2 eq ha −1 a −1, respectively), whereas the CO 2-C flux was remarkable (3.5 ± 3.7 t ha −1 a −1). The global influence of CWs is not significant. Even if all global domestic wastewater were treated by wetlands, their share of the trace gas emission budget would be less than 1%.

Experience with establishment and operation of reed bed treatment for small communities in the UK

The design and development of constructed reed beds for secondary, tertiary and storm outflow treatment is described from the experience of a major water utility in the Midland region of the UK. There were only two trial sites in 1987, but 125 sites by June 1995. The problems and their resolution with the establishment ofPhragmites in gravel filled beds are described. Two secondary treatment systems with horizontal subsurface flow beds arranged in terraces show that dilute sewage can be treated to a good secondary standard, but that nitrification is likely to be only partially complete. The example of four tertiary treatment applications showed that, for beds sized at 0.7 to 1.2 m2/pe, effluents consistently averaged better than 5 mg/l BOD5 and 10 mg/l TSS. Three sites showed improved removal of ammonium nitrogen (N) after the first year, with better than 50% removal. One site, treating highly nitrified secondary effluent, showed a small net increase in ammonium-N even after 4 years. Samples taken by the regulatory authority from 79 sites confirm the ability of the system to meet tight standards for BOD5. A comparison made of the performance of storm reed beds during three storm events showed relatively consistent removal of BOD, TSS, ammonia nitrogen and TON.