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This study investigated the efficiency of 12 pilot-scale constructed wetlands (CWs) with different configurations on the removal of estrone and estradiol from raw domestic sewage. An orthogonal design was employed to evaluate the impact of four principal design parameters of CWs, including four wetland types, three substrates, three plant conditions, and three hydraulic loading rates, in summer and winter. A bench-scale anoxic simulation test was performed in the laboratory to clarify the photolysis, sorption, and degradation of estrogens. Estrogens were more effectively removed by the 12 CWs during summer. The experiment showed that target estrogens were efficiently removed by wetland substrate under anoxic conditions through exothermic sorption and degradation even in winter. This suggests that the inefficient removal in CWs in winter likely resulted from subsequent cleavage of a considerable amount of estrogen conjugates in influent due to insufficient decomposition at low temperatures. The transformation from estradiol to estrone could be driven by residual microbial activities not inhibited by azide, and the reversible process was then driven by active microorganisms but not solely abiotic redox reactions. Among the four design parameters, wetland-type was the most important and downward-vertical flow CWs performed best. [ABSTRACT FROM AUTHOR]

Occurrence and removal of six high-production high-volume neonicotinoids was investigated in 13 conventional wastewater treatment plants (WWTPs) and one engineered wetland. Flow-weighted daily composites were analyzed by isotope dilution liquid chromatography tandem mass spectrometry, revealing the occurrence of imidacloprid, acetamiprid, and clothianidin at ng/L concentrations in WWTP influent (60.5 ± 40.0; 2.9 ± 1.9; 149.7 ± 289.5, respectively) and effluent (58.5 ± 29.1; 2.3 ± 1.4; 70.2 ± 121.8, respectively). A mass balance showed insignificant removal of imidacloprid (p = 0.09, CI = 95%) and limited removal of the sum of acetamiprid and its degradate, acetamiprid-N-desmethyl (18 ± 4%, p = 0.01, CI = 95%). Clothianidin was found only intermittently, whereas thiamethoxam, thiacloprid, and dinotefuran were never detected. In the wetland, no removal of imidacloprid or acetamiprid was observed. Extrapolation of data from 13 WWTPs to the nation as a whole suggests annual discharges on the order of 1000-3400 kg/y of imidacloprid contained in treated effluent to surface waters nationwide. This first mass balance and first United States nationwide wastewater reconnaissance identified imidacloprid, acetamiprid, and clothianidin as recalcitrant sewage constituents that persist through wastewater treatment to enter water bodies at significant loadings, potentially harmful to sensitive aquatic invertebrates. [ABSTRACT FROM AUTHOR]

The present study investigated long-term treatment performance and nitrogen transformation mechanisms in tidal flow constructed wetlands (TFCWs) under 4, 8, and 12 °C temperature regimes. High and stable ammonium (NH4+-N) removal efficiency (93-96%) was achieved in our TFCWs, whereas nitrate (NO3--N) was accumulated at different levels under different temperatures. Quantitative response relationships showed anammox/amoA, (narG+napA)/amoA, and (narG+napA)/bacteria were the respective key functional gene groups determining 4, 8, and 12 °C NO3--N reduction. Pathway analysis revealed the contribution of these functional gene groups along a depth gradient. In addition, denitrification process increased, while anammox process decreased consistent with a rise in temperature from 4 to 12 °C. Furthermore, cold temperatures exhibited different effects on anammox and denitrification and their long-term acclimatization capacities changed with temperature. [ABSTRACT FROM AUTHOR]

Tidal flow constructed wetlands (TF CWs) have recently been studied as a sustainable technology to achieve enhanced nitrogen removal; however, the underlying mechanisms responsible for removing ammonium (NH4+) and nitrate (NO3-) have not been compared and quantified at the molecular level (genes) in controlled TF CWs. In this study, two TF CWs T1 (treating NH4+ wastewater) and T2 (treating NO3- wastewater) achieved high removal efficiencies for chemical oxygen demand (COD, 92 ± 2.7% and 95 ± 2.4%, respectively), NH4+/NO3- (76 ± 3.9% and 97 ± 2.2%, respectively), and total nitrogen (TN, 81 ± 3.5% and 93 ± 2.3%, respectively). Combined analyses revealed that the presence of simultaneous nitrification, anammox, and denitrification processes and the coupling of dissimilatory nitrate reduction to ammonium, ammonia oxidation, and anammox were the primary reason accounted for the robust treatment performance in T1 and T2, respectively. Results from stepwise regression analysis suggested that the NH4+ removal rate in T1 was collectively controlled by amoA, nxrA, and anammox, while the NO3- removal rate in T2 was governed by nxrA and narG gene. [ABSTRACT FROM AUTHOR]

On the basis of the fact that algae have the ability to volatilize substantial quantities of selenium (Se), we investigated the concept of including an algal pretreatment unit into a constructed wetland system for the removal of Se from river water entering the Salton Sea. Of six different algal strains tested, the most effective in terms of Se volatilization and Se removal from the water column was a Chlorella vulgaris strain (designated Cv). Cv removed 96% of Se (supplied as selenate) from the microcosm water column within 72 h, with up to 61% being removed by volatilization to the atmosphere. X-ray absorption spectroscopy revealed that the major forms of Se likely to be accumulated in an algal-wetland system are selenomethionine, a precursor of volatile Se formation, and elemental Se. Our results suggest that the inclusion of an algal pretreatment unit within a constructed wetland water treatment system should not only enhance the efficiency of Se removal but also significantly reduce the risk of the buildup of ecotoxic forms of Se by promoting the biological volatilization of Se. [ABSTRACT FROM AUTHOR]

Photosynthetic removal of CO2 from the atmosphere is an important planetary carbon dioxide removal mechanism. Naturally, an amount equivalent to all atmospheric carbon passes through the coupled plant-soil system within 7 years. Plants cycle up to 40% of photosynthesized carbon through their roots, providing a flux of C at depth into the soil system. Root-exuded carboxylic acids have the potential to supply 4-5 micromoles C hr-1g-1 fresh weight to the soil solution, and enhance silicate mineral weathering. Ultimately, the final product of these root-driven processes is CO2, present in solution as bicarbonate. This combines with Ca liberated by corrosion associated with silicate mineral weathering to enter the soil-water system and to produce pedogenic calcium carbonate precipitates. Combining understanding of photosynthesis and plant root physiology with knowledge of mineral weathering provides an opportunity to design artificial soils or to plan land use in ways that maximize removal and sequestration of atmospheric CO2 through artificially enhanced pedogenic carbonate precipitation. This process requires relatively low energy and infrastructure inputs. It offers a sustainable carbon dioxide removal mechanism analogous to the use of constructed wetlands for the passive remediation of contaminated waters, and is likely to achieve wide public acceptance. [ABSTRACT FROM AUTHOR]

A new development on treatment wetland technology for the purpose of achieving high rate nitrogen removal from high strength wastewater has been made in this study. The laboratory scale alum sludge-based intermittent aeration constructed wetland (AlS-IACW) was integrated with predenitrification, intermittent aeration, and step-feeding strategies. Results obtained from 280 days of operation have demonstrated extraordinary nitrogen removal performance with mean total nitrogen (TN) removal efficiency of 90% under high N loading rate (NLR) of 46.7 g N m-2 d-1. This performance was a substantial improvement compared to the reported TN removal performance in literature. Most significantly, partial nitrification and simultaneous nitrification denitrification (SND) via nitrite was found to be the main nitrogen conversion pathways in the AlS-IACW system under high dissolved oxygen concentrations (3-6 mg L-1) without specific control. SND under high dissolved oxygen (DO) brings high nitrogen conversion rates. Partial nitrification and SND via nitrite can significantly reduce the demand for organic carbon compared with full nitrification and denitrification via nitrate (up to 40%). Overall, these mechanisms allow the system to maintaining efficient and high rate TN removal even under carbon limiting conditions. [ABSTRACT FROM AUTHOR]

In wetlands, a variety of biotic and abiotic processes can contribute to the removal of organic substances. Here, we used compound-specific isotope analysis (CSIA), hydrogeochemical parameters and detection of functional genes to characterize in situ biodegradation of benzene in a model constructed wetland over a period of 370 days. Despite low dissolved oxygen concentrations (<30 μM), the oxidation of ammonium to nitrate and the complete oxidation of ferrous iron pointed to a dominance of aerobic processes, suggesting efficient oxygen transfer into the sediment zone by plants. As benzene removal became highly efficient after day 231 (>98% removal), we applied CSIA to study in situ benzene degradation by indigenous microbes. Combining carbon and hydrogen isotope signatures by two-dimensional stable isotope analysis revealed that benzene was degraded aerobically, mainly via the monohydroxylation pathway. This was additionally supported by the detection of the BTEX monooxygenase gene tmoA in sediment and root samples. Calculating the extent of biodegradation from the isotope signatures demonstrated that at least 85% of benzene was degraded by this pathway and thus, only a small fraction was removed abiotically. This study shows that model wetlands can contribute to an understanding of biodegradation processes in floodplains or natural wetland systems. [ABSTRACT FROM AUTHOR]

To perform a general assessment of treatment efficiency, a mass balance study was undertaken for two types of constructed wetlands (CWs), planted gravel filters and plant root mat systems, for treating VOC (benzene; MTBE) polluted groundwater under field conditions. Contaminant fate was investigated in the respective water, plant, and atmosphere compartments by determining water and atmospheric contaminant loads and calculating contaminant plant uptake, thereby allowing for an extended efficiency assessment of CWs. Highest total VOC removal was achieved during summer, being pronounced for benzene compared to MTBE. According to the experimental results and the calculations generated by the balancing model, degradation in the rhizosphere and plant uptake accounted for the main benzene removal processes, of 76% and 13% for the gravel bed CW and 83% and 11% for the root mat system. Volatilization flux of benzene and MTBE was low (<5%) for the gravel bed CW, while in the root mat system direct contact of aqueous and gaseous phases favored total MTBE volatilization (24%). With this applied approach, we present detailed contaminant mass balances that allow for conclusive quantitative estimation of contaminant elimination and distribution processes (e.g., total, surface, and phytovolatilization, plant uptake, rhizodegradation) in CWs under field conditions. [ABSTRACT FROM AUTHOR]

One of the few pathways for environmental transformation of polychlorinated biphenyls (PCBs) is microbial dechlorination under anaerobic conditions, which is reported to occur in contaminated sediments of rivers, lakes and harbors. The goal of this work was to determine whether PCB dechlorination occurs in built waste treatment environments. We analyzed a large database on PCB congener concentrations in effluents and some influents of facilities in the Delaware River Basin. Positive matrix factorization was used to identify the sources of PCBs and to look for evidence of dechlorination. Seven factors were resolved from the data set of 89 congeners in 645 samples. Two of the resolved factors represented dechlorination signals. One of these was dominated by PCBs 4 and 19 and represents an advanced stage of dechlorination of Aroclors to di- and trichlorinated congeners. This dechlorination signal was most prevalent in effluents from sites with contaminated groundwater and from wastewater treatment plants (WWTPs) that serve combined sewers or treat landfill leachate. The other dechlorination signal appeared to represent an intermediate stage of dechlorination, because it was dominated by two coeluting groups of tetrachlorinated congeners: PCBs 44 + 47 + 65 and 45 + 51. This partial dechlorination signal was most prevalent in the 40 WWTPs with separate (sanitary) sewer systems, where it often comprised more than 20% of the PCBs in the effluents. Both dechlorination signals were present in WWTP influents, but were not observed in stormwater runoff, suggesting that dechlorination occurs in sewers. This work represents the first convincing evidence of PCB dechlorination occurring outside of contaminated aquatic sediments or anaerobic digesters. The results suggest that PCBs are dechlorinated by anaerobic bacteria in sewers, landfills, and contaminated groundwater. These two dechlorination signals comprise about 19% of the total loads of PCBs to the Delaware River from the sampled dischargers. [ABSTRACT FROM AUTHOR]

Pollutants in agricultural irrigation return flow (tailwater) constitute a significant nonpoint source of pollution in intensive agricultural regions such as the Central Valley of California. Constructed wetlands (CWs) represent a feasible mitigation option to remove pollutants including pesticides in the tailwater. In this study, we evaluated two CWs in the Central Valley for their performance in removing pyrethroid and organophosphate insecticides under field-scale production conditions. Both CWs were found to be highly effective in reducing pyrethroid concentrations in the tailwater, with season-average concentration reductions ranging from 52 to 94%. The wetlands also reduced the flow volume by 68-87%, through percolation and evapotranspiration. When both concentration and volume reductions were considered, the season-average removal of pyrethroids ranged from 95 to 100%. The primary mechanism for pyrethroid removal was through sedimentation of pesticide- laden particles, which was influenced by hydraulic residence time and vegetation density. Temporal analysis indicates a potential efficiency threshold during high flow periods. The season- average removal of chlorpyrifos ranged 52-61%. The wetlands, however, were less effective at removing diazinon, likely due to its limited sorption to sediment particles. Analysis of pesticide partitioning showed that pyrethroids were enriched on suspended particles in the tailwater. Monitoring of pesticide association with suspended solids and bed sediments suggested an increased affinity of pyrethroids for lighter particles with the potential to move further downstream before subjectto sedimentation. Results from this study show that flow-through CWs, when properly designed, are an effective practice for mitigating hydrophobic pesticides in the irrigation tailwater. [ABSTRACT FROM AUTHOR]

This study used anaerobic slurry assays and intact core incubations to quantify potential rates of anammox (anaerobic ammonia oxidation) in sediments along the flow path of a surface flow constructed wetland receiving secondary treated sewage effluent. Anammox occurred at two of the four sites assayed with a maximum rate of 199.4 ± 18.7 μmol N·m-2·hr-1 (24% of total N2 production) at the discharge end of the wetland. Denitrification was the major producer of N2, with a maximum rate of 965.3 ± 122.8 μmol N·m-2.hr-1 at site 2. Oxygen was probably the key regulator of anammox activity within the studied CW. In addition to anammox, we found evidence that nitrifier-denitrification was potentially responsible for the production of N2O. Total production of N2O was 15.1% of the total gaseous N produced. Limitations to the methodology for quantifying anammox in CW's are outlined. This study demonstrated that denitrification is not the only pathway for gaseous production in constructed wetlands and that wetlands may be significant sources of greenhouse gases such as N2O. [ABSTRACT FROM AUTHOR]

Removal efficiencies and elimination kinetics of 13 pharmaceuticals and personal care products (PPCPs) and BOD5, TSS, and ammonium were evaluated in a pilot vertical subsurface-flow constructed wetland (VFCW) and compared with those obtained by a sand filter. On the basis of the observed removals, the PPCPs studied were grouped in relation to their removal efficiency into (i) PPCPs very efficiently removed, that is, >95% removal in one of the systems (caffeine, salicylic acid, methyl dihydrojasmonate, CA-ibuprofen, hydrocinnamic acid, oxybenzone, ibuprofen, OH-ibuprofen); (ii) PPCPs moderately removed, with removals between 70 and 90% in the two systems (naproxen, diclofenac, galaxolide, and tonalide); and finally (iii) PPCPs poorly removed, with elimination rates of <30% (carbamazepine). At design hydraulic loading rate (HLR) the planted VFCW and the nonplanted SF exhibited similar PPCP removal efficiencies, but the VFCW was less sensitive to removal decline in overloading conditions (up to 2 times the design HLR). Moreover, under a clogging simulation, the HLR and the presence of vegetation were some of the key aspects affecting the PPCPs, BOO5, and ammonium removal from domestic wastewater. The VFCW evaluated was more efficient in terms of removal efficiency and loading rate for most of the PPCPs studied in comparison to constructed wetlands of other configurations (i.e., horizontal subsurface flow) and SF (nonplanted). The shorter hydraulic residence time (a few hours) in VFCW compared to that in other CW configurations ranging from days to weeks makes VFCWs a very appropriate wastewater treatment option in space-limited areas. [ABSTRACT FROM AUTHOR]

The behavior of linear alkylbenzene sulfonate (LAS) and sulfophenyl carboxylate (SPC) biointermediates in a pilot subsurface flow constructed wetland (SFCW) is reported for the first time. The effects of wetland configuration and operation on their treatment efficiency were investigated. The pilot SFCW constituted by eight beds of 55 m² with different aspect ratios (1 x 1; 1.5 x 1; 2 x 1; 2.5 x 1), two water depths (i.e., 0.47 and 0.27 cm) at 5 cm below surface and two medium sizes (i.e., D80 = 10 mm and 3.5 mm) planted with Phragmites sp. That SFCW pilottreats urban wastewater (i.e., 200 inhabitants) and was operated at four hydraulic loading rates (HLRs) (20, 27, 36, and 45 mm d-1). Influent and effluent sampling was carried out from May 2001 to January 2002 with a weekly pattern. Main results were as follows: (i) water depth has a major influence on the performance of SFCW for the LAS removal, and HLR shows significant effect on SPC evolution; (ii) water temperature has a significant effect on the LAS evolution; (iii) biodegradation of LAS and SPC can occur under sulfate-reducing environment and mixed conditions (i.e., sulfate-reducing and denitrification), but aerobic respiration cannot be excluded; and (iv) C13 LAS homologues were generally removed in higher extent than the shorter alkyl chain counterparts. ln the most appropriate conditions, LAS and SPC can be biodegraded up to 71% and 11%, respectively, in the pilot SFCW evaluated. [ABSTRACT FROM AUTHOR]

Our knowledge about the effectiveness of constructed wetlands in retaining agricultural nonpoint-source pesticide pollution is limited. A 0.44-ha vegetated wetland built along a tributary of the Lourens River, Western Cape, South Africa, was studied to ascertain the retention, fate, and effects of spray drift-borne azinphos-methyl (AZP). Composite water samples taken at the inlet and outlet during five spray drift trials in summer 2000 and 2001 revealed an overall reduction of AZP levels by 90 ± 1% and a retention of AZP mass by 61 ± 5%. Samples were collected at the inlet, outlet, and four platforms within the wetland to determine the fate and effect of AZP in the wetland after direct spray drift deposition in the tributary 200 m upstream of the inlet. Peak concentrations of AZP decreased, and the duration of exposure increased from inlet (0.73 µg/L; 9 h) via platforms 1 and 4 to outlet (0.08 µg/L; 16 h). AZP sorbed to plants or plant surfaces, leading to a peak concentration of 6.8 µg/kg dw. The living plant biomass accounted for 10.5% of the AZP mass initially retained in the wetland, indicating processes such as volatilization, photolysis, hydrolysis, or metabolic degradation as being very important. AZP was not detected in sediments. Water samples taken along two 10-m transects situated perpendicular to the shore indicated a homogeneous horizontal distribution of the pesticide: 0.23±0.02 and 0.14±0.04 µg/L (n = 5), respectively. Both Copepoda (p = 0.019) and Cladocera (p = 0.027) decreased significantly 6 h postdeposition and remained at reduced densities for at least 7 d. In parallel, the chlorophyll a concentration showed an increase, although not significant, within 6 h of spray deposition. The study highlights the potential of constructed wetlands as a risk-mitigation strategy for spray drift-related pesticide pollution. [ABSTRACT FROM AUTHOR]

The effectiveness of a nonvegetated lab-scale subsurface flow constructed wetland for wastewater treatment had been evaluated with the feed ammonium concentration of ∼20-40 mg of NH[sub 4, sup +]-N L[sup -1] and a hydraulic retention time of ∼10 d. The present system had a nitrification zone plus a sulfur/limestone (S/L) autotrophic denitrification zone followed by an anaerobic polishing zone and was operated with and without aeration. The wetland had only 80% organics removal and no net nitrogen removal when there was no artificial aeration. However, almost 100% organics removal and ∼81-90% total inorganic nitrogen (TIN = NH[sub 4, sup +]-N + NO[sub 2, sup -]-N + NO[sub 3, sup -]-N) removal were achieved when the oxic zone of the system was aerated with compressed air. S/L autotrophic denitrification contributed 21-49% of total NO[sub 3, sup -]-N removal across the whole wetland and 50-95% across the S/L column. TIN and NH[sub 4, sup +]-N in the effluent were always <5.5 and < 0.7 mg L[sup -1], respectively, when the feed had NH[sub 4, sup +]-N ≤ 35 mg L[sup -1]. Sulfate removal of ∼53-69% was achieved in the anaerobic polishing zone. The position of the S/L column was changed (1.78, 2.24, and 2.69 m from the inlet), and no remarkable difference in nitrogen removal was observed. However, without the S/L column, TIN removal decreased to ∼74%, and the effluent NO[sub 3, sup -]-N increased about two times (9.13 mg of N L[sup -1]). The present study has demonstrated the possible use of S/L autotrophic denitrification for nitrate removal in a constructed wetland. [ABSTRACT FROM AUTHOR]

Management of selenium (Se)-contaminated agricultural drainage water is one of the most important environmental issues in California. To evaluate the feasibility of utilizing constructed wetlands to remediate Se-laden drainage water and the role of biological volatilization in Se removal, 10 flowthrough wetland cells were constructed in 1996 in Corcoran, California. The monthly monitoring study from May 1997 to December 1999 showed that the vegetated wetlands were capable of significantly reducing Se from the inflow drainage water; an average of 69.2% of the total Se mass in the inflow was removed. Most of the Se was retained in sediment, and <5% of the Se was accumulated in plant tissues. Selenium volatilization was highest in the rabbitfoot grass wetland cell, where 9.4% of the Se input was volatilized over a 2-year period. Volatilization was greater in spring and summer than in fall and winter. For example, in May and June of 1998, 35 and 48%, respectively, of the Se entering the rabbitfoot grass cell was volatilized, whereas in the winter months, <5% was volatilized. The feasibility of using constructed wetlands for Se remediation, methods for the enhancement of Se volatilization, and the importance of considering potential Se ecotoxicity are discussed. [ABSTRACT FROM AUTHOR]

Aerobic wetlands are increasingly being included in mine water treatment systems which need to achieve low residual iron concentrations (<0.5 mg L[sup -1])in final discharges. Traditionally the macrophyte components of such systems have been thought to be insignificant sinks for major contaminants such as iron. However, we report high rates of plant uptake of iron where the latter is present at relatively low concentrations, suggesting that macrophytes may well be critical to achieving low residual iron concentrations in final effluents from such systems. The wetland macrophyte Phragmites eustralis was grown in waters with a range of iron concentrations (0-50 mg L[sup -1]). At an Fe supply of 1 mg L[sup -1] almost 100% of the Fe was taken up into plant tissues. The majority of iron was stored in and around the roots of the plants, which helps allay fears of possible release of contaminants during seasonal die-back of emergent shoots and leaves. The 1 mg L[sup -1] threshold also proved to be important in terms of plant growth, with significant inhibition (evident in root length and in dry weights of shoots and roots) in plants grown in waters with Fe above this concentration. No direct causal relationship between iron content in aerial tissues and growth inhibition was found, which strongly suggests that iron toxicity cannot explain these results. These results have implications for the design of mine water treatment wetlands, particularly with regard to initial establishment of vegetation and achieving sufficient Fe removal in "polishing" applications (i.e. where it is intended to remove the last few mg L[sup -1] of Fe). [ABSTRACT FROM AUTHOR]

Attempts to assess the effectiveness of a constructed wetland for control of sediments, nutrients, and pesticides originating from rainfall-induced edge-of-field runoff. In situ exposure bioassays; Retention of agricultural pesticide pollution; Comparison of results for typical dry and wet weather conditions; Toxicological effects of the wetland.

Analyzes the performance of treatment wetlands for petroleum industry effluents using criteria such as chemical oxygen demand, biochemical oxygen demand, metal toxicity and total suspended solids. Effectiveness of treatment wetlands in removing pollutants from wastewater; Factors affecting the efficiency of the treatment wetlands.

Describes the feasibility of using the constructed wetlands to treat acidic wastes containing hexavalent chromium (Cr(VI)) on a laboratory scale. Determination of the chemistry of Cr(VI); Procedure for acid digestions; Synthesis and characterization of chromium; Chromium removal.

Investigates the importance of wastewater treatment in constructed wetlands and chrysanthemums in European Union countries. Identification of the oxygen-dependent microbes in chrysanthemum plants; Influence of anaerobic organisms on wetland treatment systems; Removal of ammonia from wastewater.

Focuses on the construction of an artificial wetland in South Carolina that may provide key clues about how man-made marshes can eliminate mercury from surface water. Pilot tests of a smaller version of the wetland; Ability of wetland to reduce the mercury content of water passing through it.

Reports on the creation and management of a wetland system by P.R. Adler and colleagues that substantially increase plant removal of phosphorus compared with previous systems.

Reports that several parameters must be considered when designing an effective constructed wetlands for wastewater treatment.

Focuses on Edgar W. Garbisch and Joanna L. Garbisch's report on step-by-step procedures for successful tidal marsh construction and maintenance. Methods; Applicability.

Reports on M.B. Green and J. Upton's investigation of the application of constructed reed beds as a cost-effective tertiary water treatment for small communities. Results for five reed bed systems operated between 1987 and 1991.