Free Water Surface Wetland Research Articles

Pilot-scale comparison of constructed wetlands operated under high hydraulic loading rates and attached biofilm reactors for domestic wastewater treatment

Four different pilot-scale treatment units were constructed to compare the feasibility of treating domestic wastewater in the City of Heraklio, Crete, Greece: (a) a free water surface (FWS) wetland system, (b) a horizontal subsurface flow (HSF) wetland system, (c) a rotating biological contactor (RBC), and (d) a packed bed filter (PBF). All units operated in parallel at various hydraulic loading rates (HLR) ranging from 50% to 175% of designed operating HLR. The study was conducted during an 8 month period and showed that COD removal efficiency of HSF was comparable (> 75%) to that of RBC and PBF, whereas that of the FWS system was only 57%. Average nutrient removal efficiencies for FWS, HSF, RBC and PBF were 6%, 21%, 40% and 43%, respectively for total nitrogen and 21%, 39%, 41% and 42%, respectively for total phosphorus. Removals of total coliforms were lowest in FWS and PBF (1.3 log units) and higher in HSF and RBC (2.3 to 2.6 log units). HSF showed slightly lower but comparable effluent quality to that of RBC and PBF systems, but the construction cost and energy requirements for this system are significantly lower. Overall the final decision for the best non-conventional wastewater treatment system depends on the construction and operation cost, the area demand and the required quality of effluent.

Phosphorus removal from agricultural runoff by constructed wetland

A free-water surface wetland covering an area of 2800 m 2 was operated from March 2002 to June 2004 for agricultural runoff treatment in the Dianchi Valley in China. In the wetland were grown Zizania Caduciflora Turez Hand-mazt and Phragmites australis ( Cav.) Trin.ex Steud. The instantaneous inflow rate was measured and the integrated flux was recorded by an ultrasonic flow instrument all year round. The average inflow rate, hydraulic loading rate (HLR) and hydraulic retention time (HRT) were kept at 242 m 3 d −1, 12.7 cm d −1 and 2.0 d, respectively. The annual average total phosphorus (TP) in the inflow was 0.87 mg L −1, and the corresponding removal efficiency was calculated to be 59.0%. Biannual plant uptake and removal by harvesting and seed transport was the main pathway for TP removal, while the influent TP load was 12.9 g m −2 year −1. Hydraulic retention time had a significant positive correlation with the removal of P ( r 2 = 0.88). Water temperature, inflow phosphorus load, inflow and hydraulic load rates were positively correlated with the removal of P. Inflow phosphorus concentrations were negatively correlated with the removal of P. It is shown that the free-water surface wetland was an effective and economical system for agricultural runoff treatment in lake regions.

Influence of vegetation on the removal of heavy metals and nutrients in a constructed wetland

A free water surface wetland was built to treat wastewater containing metals (Cr, Ni, Zn) and nutrients from a tool factory in Argentina. Water, sediment and macrophytes were sampled in the inlet and outlet area of the constructed wetland during three years. Three successive phases of vegetation dominance were developed and three different patterns of contaminant retention were observed. During the Eichhornia crassipes dominance, contaminants were retained in the macrophyte biomass; during the E. crassipes + Typha domingensis stage, contaminants were retained in the sediment and in the T. domingensis dominance stage, contaminants were retained in sediment and in the macrophyte biomass. Removal efficiency was not significantly different among the three vegetation stages, except for NH 4 + and i-P diss. Because of its highest tolerance, T. domingensis is the best choice to treat wastewater of high pH and conductivity with heavy metals, a common result from many industrial processes.

A Cost-Effectiveness Analysis of Seminatural Wetlands and Activated Sludge Wastewater-Treatment Systems

A cost-effectiveness analysis was performed to evaluate the competitiveness of seminatural Free Water Surface (FWS) wetlands compared to traditional wastewater-treatment plants. Six scenarios of the service costs of three FWS wetlands and three different wastewater-treatment plants based on active sludge processes were compared. The six scenarios were all equally effective in their wastewater-treatment capacity. The service costs were estimated using real accounting data from an experimental wetland and by means of a market survey. Some assumptions had to be made to perform the analysis. A reference wastewater situation was established to solve the problem of the different levels of dilution that characterize the inflow water of the different systems; the land purchase cost was excluded from the analysis, considering the use of public land as shared social services, and an equal life span for both seminatural and traditional wastewater-treatment plants was set. The results suggest that seminatural systems are competitive with traditional biotechnological systems, with an average service cost improvement of 2.1-fold to 8-fold, according to the specific solution and discount rate. The main improvement factor was the lower maintenance cost of the seminatural systems, due to the self-regulating, low artificial energy inputs and the absence of waste to be disposed. In this work, only the waste-treatment capacity of wetlands was considered as a parameter for the economic competitiveness analysis. Other goods/services and environmental benefits provided by FWS wetlands were not considered.

Evaluation of constructed wetlands by wastewater purification ability and greenhouse gas emissions

Domestic wastewater is a significant source of nitrogen and phosphorus, which cause lake eutrophication. Among the wastewater treatment technologies, constructed wetlands are a promising low-cost means of treating point and diffuse sources of domestic wastewater in rural areas. However, the sustainable operation of constructed wetland treatment systems depends upon a high rate conversion of organic and nitrogenous loading into their metabolic gaseous end products, such as N2O and CH4. In this study, we examined and compared the performance of three typical types of constructed wetlands: Free Water Surface (FWS), Subsurface Flow (SF) and Vertical Flow (VF) wetlands. Pollutant removal efficiency and N2O and CH4 emissions were assessed as measures of performance. We found that the pollutant removal rates and gas emissions measured in the wetlands exhibited clear seasonal changes, and these changes were closely associated with plant growth. VF wetlands exhibited stable removal of organic pollutants and NH3-N throughout the experiment regardless of season and showed great potential for CH4 adsorption. SF wetlands showed preferable T-N removal performance and a lower risk of greenhouse gas emissions than FWS wetlands. Soil oxidation reduction potential (ORP) analysis revealed that water flow structure and plant growth influenced constructed wetland oxygen transfer, and these variations resulted in seasonal changes of ORP distribution inside wetlands that were accompanied by fluctuations in pollutant removal and greenhouse gas emissions.

The effects of deep zones on wetland nitrogen processing

Twelve research wetlands were operated under varying conditions at a site west of the city of Phoenix. These were constructed as a triplicated design, with zero, one, two and three internal deep zones, all containing an inlet distribution and an outlet collection deep zone, together comprising 12.5-35% of the wetland areas. The water supply was partially nitrified effluent from a city wastewater treatment plant. Total nitrogen was reduced by about 50%, from inflow concentrations between 6 and 8 mg/L. Speciation of the inflow was approximately 25% organic nitrogen, 25% ammonium nitrogen and 50% nitrate nitrogen. Typical outflow concentrations were about 1.2 mg/L organic, 0.5 mg/L ammonium and 0.0-2.5 mg/L nitrate. Rate constants for total nitrogen were 15-20 m/year at 20 degrees C, and 20-30 m/year for nitrate, which agree well with other project reports. Temperature factors averaged 1.100 for total nitrogen, and 1.184 for nitrate. There were no differences in the internal hydraulics with deep zone numbers. Deep zone numbers in the wetlands did not affect nitrogen treatment performance. No differences with deep zone numbers were found for temperature, dissolved oxygen, pH, or nitrogen removals or rate constants. In conjunction with other reported results, there appears to be no large treatment benefit or detriment of incorporating internal deep zones in free water surface wetlands.

Practical results of a water budget estimation for a constructed wetland

An experimental water treatment plant was established to verify the effectiveness of constructed wetlands to improve water quality in the Venice Lagoon watershed. The wetland comprised three different subsystems, ranging from a riparian swamp to a marsh ecosystem. As a first step, monitoring was conducted over three years to evaluate the efficacy and efficiency of the system. Here, we report an analysis of the water budget resulting from routinely collected hydraulic and meteorological data. We used independent estimates of the water-budget terms, rather than budgetary residual estimation, because we wanted to estimate the water-budget error. Surface-water inflow, surface-water outflow, and direct precipitation were measured. Daily potential evapotranspiration values were estimated using the FAO Penman Monteith equation; runoff was estimated using the USDA Soil Conservation Service curve number model; indirect precipitation that flows toward the wetland via subsurface flow was estimated from the soils field capacity; and seepage was estimated using Darcy's law. The objectives of the research were to establish the best way to develop a water budget useful for application and design purposes, to determine the term that most influences the water-budget error, and to perform a sensitivity analysis on the parameters affecting this term. Surface flow dominated the wetland system, precipitation and evapotranspiration contributed about 10% to the water budget, and changes in water storage and the seepage flow contributed less than 5% to the water budget. The seepage flow term had the highest uncertainty, and the frequency of ground-water level measurements had the greatest impact on the water-budget error (ranging from 10.0%-28.6%). Therefore, in a free-water surface wetland with a shallow ground-water system, the main effort in field measurement should be to ensure a measurement frequency of less than five days.

Performance and Cost Comparison of a FWS and a VSF Constructed Wetland System

The biodegradation of municipal solid waste (MSW) was investigated in simulated bioreactor landfills under aerobic conditions. The bioreactors were operated to determine Two constructed wetland systems, treating domestic wastewater, are compared in terms of performance and costs. One is a free water surface (FWS) wetland system located in Pompia, Crete, south Greece, and the other one is a vertical subsurface flow (VSF) wetland system located in Gomati, Chalkidiki, north Greece. The FWS system is designed for 1200 p.e. Its construction cost was €305,000, and the capital, operation and maintenance cost was €22.07 p.e.−1 yr−1 or €0.50 m−3 of influent. The VSF system is designed for 1000 p.e. Its construction cost was €410,850, and the capital, operation and maintenance cost was €36.81 p.e.−1 yr−1 or €0.56 m−3 of influent. Both systems achieved high removal rates for BOD5, COD, TSS, TKN, phosphorus, TC, and FC, which makes them ideal for small communities in the Mediterranean region.

Lake eutrophication management modeling using dynamic programming

Lake eutrophication problems have received considerable attention in Taiwan, especially because they relate to the quality of drinking water. In this study, steady-state river water quality and lake eutrophication models are solved using dynamic programming algorithms to find the nutrient removal rates for eutrophication control during dry season. The kinetic cycle of chlorophyll-a, phosphorus and nitrogen for a complete-mixed lake is considered in the optimization framework. The Newton-iterative technique is adopted to solve the nonlinear equations for the steady-state lake eutrophication model. The optimization framework is applied to Cheng-Ching Lake in southern Taiwan. Several nutrient loading scenarios for eutrophication control are studied. Optimization results for nutrient removal rates and corresponding wastewater treatment capacities of each reach of the Kao-Ping River define the least cost approach to lake eutrophication control. A natural purification method, structural free water surface wetland, is also suggested to save more investment and improve river water quality at the same time.

Removal efficiency of a constructed wetland for wastewater treatment according to vegetation dominance

A free water surface wetland was built to treat wastewater containing metals (Cr, Ni and Zn) and nutrients from a tool factory in Santo Tomé, Santa Fe, Argentina. Eichhornia crassipes became dominant and covered about 80% of the surface throughout the first year, and decreased progressively until its disappearance. When water depth was lowered Typha domingensis steadily increased plant cover and attained 30% of the surface by the end of the study. While E. crassipes was dominant, the wetland retained 62% of the incoming Cr and 48% of the Ni. NO3- and NO2-, were also removed (65% and 78%, respectively), while dissolved inorganic phosphate (i-P(diss)) and NH4+ were not removed. Zn was below 50 microg l(-1) in both the influent and effluent. Metal concentration in the sediments did not increase and retention was mediated through macrophytes uptake. During the period of E. crassipes decline the wetland retained 49% of the incoming Cr, 45% of Ni, 58% NO3-, 94% NO2-, 58% NH4+ and 47% i-P(diss). Cr, Ni and Zn in the bottom sediment increased in the inlet but not in the outlet. Since T. domingensis became dominant, retention was 58% Cr, 48% Ni and 64% i-P(diss), while 79% NO3-, 84% NO2- and 13% NH4+ were removed. Metals in the bottom sediment increased in the inlet. In spite of the significant growth of E. crassipes at the beginning, T. domingensis remained after most of the transplanted macrophytes had disappeared. Macrophyte disappearance could be related to the overall toxicity of several environmental constrains as high pH and conductivity, metal concentration, and sulphide presence.

Heavy Metal Behaviour in an Experimental Free Water Surface Wetland in the Venice Lagoon Watershed

Wetlands are effective in the treatment of polluted surface water. A semi-natural wetland pilot plant was established to verify the pollutant abatement effectiveness of the Venice Lagoon inlet water. The unique conditions of this brackish environmental site are: (1) a high concentration of carbonate and low concentrations of sulphides, and (2) the abundance of organic matter in sediments. The goal of this study was to examine how these characteristics influence the metal mobility in the wetland and to compare our results to published literature. The data collected were limited with respect to statistical analysis because the metal concentrations were often below the detection limits. Therefore, we chose to perform non-parametric analyses. To analyse the relationships among heavy metal concentrations and the physical and chemical wetland variables, and to investigate the processes of metal removal, we performed a multivariate statistical analysis and a Spearman correlation analysis. The results indicated that the reduced and basic conditions of the sediments seen in the Venice Lagoon environment facilitated the removal of metals due to the formation of insoluble compounds with sulphides and carbonates.

Experiences with constructed wetland systems in Korea

In spite of the low temperature during the winter season and the high land environment, the wetland treatment system is gaining popularity in Korea because of its lower construction cost and simplicity in operation and maintenance. Many different types of wetland treatment systems have been built during the last 10 years, among which the free water surface wetland has been predominant. Most of the large-scale systems are government projects for improving the water quality of the streams flowing into the estuary dikes and reservoirs. The covering plants used in this system are different in different areas but cattails and reeds or their combinations are common. Constructed wetlands in Korea can be characterized by their shallow depths and short hydraulic residence times. There is no established flow pattern and configuration rules for constructing wetlands, but many efforts have been made with a view to improving their ecological function. Flow control is the most difficult problem in designing a riverbed or riparian wetland. There have been scores of flow rate control devices developed for wetlands, but none of them guarantee wetlands’ safety against flooding. In earlier wetland construction, the building materials were mainly soil. Recently, strong and durable building materials such as rocks, gravel beds, concrete and steel are used at vulnerable places to protect them from erosion. Our investigation indicated that the wetland system would be an appropriate technology because it is not only cheaper to construct, but also requires less maintenance work. However, we suffer from the reduced effectiveness in performance during the winter. We need to evaluate the partial treatment accomplished during 6 to 7 months per year.

Assessment of Cryptosporidium Removal from Domestic Wastewater Via Constructed Wetland Systems

Constructed wetlands have been recognized as offering a removal treatment option for high concentrations removal of chemical and biological contaminants in domestic wastewater. The enteric protozoan parasite Cryptosporidium is considered one of the highly resistant to treatment and highly infectious organisms to humans and animals. Moreover, some species of Cryptosporidium are known to have a zoonotic nature. In this investigation a pilot scale for domestic wastewater treatment system was used, consisting of the following steps in series: (1) up-flow anaerobic sludge blanket (UASB) reactor, (2) free water surface (FWS) wetland unit, and (3) sub-surface flow (SSF) wetland unit. This treatment system was fed with domestic wastewater to assess its efficiency in removing Cryptosporidium oocysts. The obtained Cryptosporidium oocysts were detected and enumerated by two different staining techniques ‘acid fast trichrome (AFT) and modified Ziehl Neelsen (MZN) stains’. Polymerase chain reaction (PCR) technique was also used to detect Cryptosporidium DNA in wastewater samples. Results revealed that anaerobic treatment (using UASB reactor) could remove about 53.1% of Cryptosporidium oocysts present in raw wastewater. The in-series connection between the two wetland units allowed complete elimination of Cryptosporidium oocysts as the first (FWS) wetland unit removed 95.9% of the oocysts present in anaerobically treated wastewater and the remaining portion of oocysts was completely removed by the second (SSF) wetland unit. Cryptosporidium oocysts were detected in 95.8% of raw wastewater samples with a mean count of 43.8 oocysts/l when AFT stain was used while they were detected in only 87.5% of raw wastewater samples with a mean count of 35.6 oocysts/l when MZN stain was used. Polymerase chain reaction (PCR) technique was able to detect Cryptosporidium DNA in only 45.8% of raw wastewater samples. Positive PCR results were only achieved in wastewater samples containing 52 oocysts or more per liter.

Dealing with nitrogen in subtropical Australia: Seven case studies in the diffusion of ecotechnological innovation

This paper describes seven case studies in which ecotechnological approaches are being used to reduce the discharge of nitrogen to the environment in three local government jurisdictions in the moist subtropical zone of eastern Australia. Both technical performance and factors relating to acceptance of the technologies are examined. Three of the technologies have survived early setbacks to achieve increasing acceptance. Composting toilets and reed beds (sub-surface flow wetlands) have achieved takeoff in the on-site wastewater context in the Lismore City Council area where the level of adoption (based on 2004–2005 figures) for both technologies was approximately 30% of newly approved systems. In both cases, the level of adoption has been favoured by local scientific studies which have confirmed claims regarding technical performance and by regulations which encourage the reduction of nitrogen discharge to the environment. Free water surface (FWS) wetlands are installed as polishing devices at three of the 10 sewage treatment plants (STPs) in Lismore and the adjoining Byron and Richmond River Shires. Early problems arising from inexperience at design, construction and management of these wetlands have been overcome as local familiarity has increased. This confidence has been a factor in the decision to support a major upgrade to the West Byron constructed wetland system and to install a further three wetlands locally. A further two technologies each have one full-scale system in operation. A 24 ha multipurpose wetland regeneration project for effluent polishing and transpiration, acid sulfate soil management, wetland regeneration, and carbon sequestration, recently installed at the end of the West Byron STP treatment train, is reducing total nitrogen concentrations from approximately 4–1 mg/L. A landfill leachate treatment system incorporating pond, macrophyte zones, horizontal flow gravel filter and single pass sand filter is reducing NH 4-N and TKN concentrations by 95% and 84%, respectively. The final two technologies described are associated with the reuse and/or treatment of municipal wastewaters. The annual crops hemp and kenaf have both exhibited high N uptake rates in irrigated crop trials. However, their short growing season has resulted in rejection as production crops at one STP in the Byron Shire. The recent realization that the perennial pasture grass Setaria sphacelata can perform the role of wetland macrophyte has given rise to the concept of the “wet-and-dry-land” cell. Still at the pilot stage, this ecotechnology may have application in areas with distinct wet and dry seasons.

Seasonal decomposition of Typha latifolia in a free-water surface constructed wetland

Litter decomposition has important consequences on constructed wetlands because it is related to the autochthonous production of organic matter, clogging rates in surface-flow wetlands, and terrestrialization in free-water surface wetlands. Decomposition rates of Typha latifolia were quantified during both winter and summer in a free-water surface constructed wetland using the litter bag technique. Air-dried leaves and stems of Typha were incubated in triplicate submerged litter bags of 25 μm, 100 μm, 1 mm and 5 mm mesh size, and also in control litter bags without mesh. Total mass loss and nitrogen remaining (percentage) were measured after 1 day, 3 days, 8 days, 20 days, 58 days and 90 days of submersion. With the exception of the control bags in winter, no significant differences were found in both variables among the different mesh sizes. Meso- or macro-fauna did not play any role in plant decomposition. Decomposition rates were significantly different between winter and summer when considering each mesh size separately. Decomposition rates from adjusted exponential models ranged from 0.0014 day −1 to 0.0026 day −1 in winter (5 °C), and from 0.0043 day −1 to 0.0052 day −1 in summer (20 °C). From these rates we estimated that 31% of the initial mass of plant detritus would remain in the system after 1 year.

Performance of a stratified sand filter in removal of chemical oxygen demand, total suspended solids and ammonia nitrogen from high-strength wastewaters

A stratified sand filter column, operated in recirculation mode and treating synthetic effluent resembling high-strength dairy wastewaters was studied over a 342-d duration. The aim of this paper was to examine the organic, total suspended solids (TSS) and nutrient removal rates of the sand filter, operated in recirculation mode, under incrementally increasing hydraulic and organic loading rates and to propose a field filter-sizing criterion. Best performance was obtained at a system hydraulic loading rate of 10 L m −2 d −1; a higher system hydraulic loading rate (of 13.4 L m −2 d −1) caused surface ponding. The system hydraulic loading rate of 10 L m −2 d −1 gave a filter chemical oxygen demand (COD), TSS, and total kjeldahl nitrogen (TKN) loading rate of 14, 3.7, and 2.1 g m −2 d −1, respectively, and produced consistent COD and TSS removals of greater than 99%, and an effluent NO 3-N concentration of 42 mg L −1 (accounting for an 86% reduction in total nitrogen (Tot-N)). As the proportional surface area requirement for the sand filter described in this study is less than the recommended surface area requirement of a free-water surface (FWS) wetland treating an effluent of similar quality, it could provide an economic and sustainable alternative to conventional wetland treatment.

Effect of incubating plant cuticles in soil on their sorptive capabilities

Phosphorus (P) in agricultural runoff may cause eutrophication of recipient waterbodies. As one of the key waterbodies to be protected in China, the Dianchi Lake is highly sensitive to pollution sources, so agricultural runoff of the region must be treated before entering into the lake. A free water surface (FWS) wetland was operated from March 2002 to June 2004 for agricultural runoff treatment in the Dianchi Valley. The wetland occupied 2800 m 2, and was planted with Phragmites austrails and Zizania caduciflora. Traditionally the macrophyte components of such systems had been thought to be insignificant sinks for major contaminants such as phosphorus. However, we reported high rates of plant uptake of phosphorus where P was present at relatively low concentration level, suggesting that macrophytes may well be critical to achieving low residual phosphorus concentrations in final effluents from such systems. The average inflow rate was 242 m 3 ,d -z all year round; the hydraulic loading rate (HLR) was 12.7 cm .d" and the hydraulic retention time (HRT) was 2.0 days. It is shown that with the average TP of 0.87 mg 9 L ~ in the inflow all-year round, the corresponding removal efficiency is 59.0 %. About 58 % of TP was removed by plant uptake and by harvesting and seed transport, which was undertaken biannually, when the influent TP load was 12.9 g 9 m -2. Hydraulic retention time (1.7-3.8 days) has significant correlation with P removal efficiency (a=0.05). Water Temperature, inflow P load, inflow, inflow P concentration, and hydraulic load rate have little influence on the P removal. It is shown that the FWS wetland is an effective and economical system for agricultural runoff treatment in the lake region.

Artificial Neural Network Modeling of Dissolved Oxygen in a Wetland Pond: The Case of Hovi, Finland

Artificial neural networks (ANNs) are flexible tools from neuroinformatics that have performed well in a number of hydrologic applications so far. They tend to be particularly useful when applied to complex processes, the details of which are not well understood. The dissolved oxygen regime in constructed wetland ponds is, in turn, such a complex process, governed by a considerable number of hydrologic, hydrodynamic, and ecological controls which operate at a wide range of spatiotemporal scales. This paper reports on the results from a study conducted to test the adequacy of artificial neural networks in modeling near-bottom concentrations of dissolved oxygen in the Finnish free water surface wetland at Hovi. Various different networks of the multilayer perceptron (MLP) type of ANN were developed. The application proved successful, and in particular it was observed that MLPs were able to “learn” the mechanism of convective oxygen transport quite well. The ANN was also used to determine the relative influe...

Wetland remediation of cyanide and hydrocarbons

Cyanide is a common constituent present in groundwater from historical aluminum industry landfills. Aluminum manufacturing produces wastes which contain cyanide, together with fluoride, a variety of metals, and some petroleum hydrocarbons. These leachates pose a moderate threat to receiving ecosystems and human health. Source control is virtually impossible, and physico-chemical removal processes are expensive and energy intensive. This pilot project investigated the use of free water surface wetlands for the reduction of complex and free cyanide and associated pollutants in water from a groundwater spring. Shallow basins, initially planted with cattail ( Typha latifolia) and bulrush ( Schoenoplectus tabernaemontani), subsequently converted to coontail ( Ceratophyllum demersum) and pondweed ( Potamageton spp.). Both total and free cyanide were effectively reduced during 7 d detention, by 56% and 88%, respectively. Gasoline range organics and diesel range organics were reduced by approximately 67%. These removals are lower bound estimates, because effluent concentrations were often below detection. First order areal removal rate constants were in the range 13–100 m/year for the various constituents. Preliminary, synoptic studies indicated little volatilization of the cyanide, but significant microbial degradation, and essentially no harmful by-products. The full-scale treatment wetland is entering the final design stages and is scheduled for construction in 2004.

Mosquito larvae density and pollutant removal in tropical wetland treatment systems in Honduras

Constructed wetlands offer a low-cost wastewater treatment option for tropical developing countries. The vast majority of published treatment wetland research has been conducted in temperate regions. Because the function of treatment wetlands is related to the environmental conditions, more research specific to the tropics should be completed. A six-cell free water surface (FWS) wetland mesocosm was constructed in Santa Rosa de Copán, Honduras that received input from an open-sewer/wastewater-impacted stream. Three cells were planted with Typha domingensis Pers., and three cells were left unplanted. Both planted and unplanted wetlands were constructed with three different surface areas to concurrently study different hydraulic retention times (HRTs) and hydraulic loading rates (HLRs). Results from 6 months of operation showed improved water quality and mosquito larvae populations affected by their specific environment. Five-day biochemical oxygen demand (BOD) removal appeared to be proportional to HRT and HLR, and BOD concentration in the effluent was higher in unplanted cells than in planted cells ( P < 0.05). BOD removal approached 60% for greater than 3.5 days HRT. Total phosphorus (P) removal and coliform bacteria removal were found to be correlated with pollutant load, and P removal was found to be correlated with HRT but not HLR. A second municipal wastewater FWS treatment wetland in Copán Ruinas, Honduras that had been established for 15 years, was also evaluated. BOD removal in the Copán Ruinas system was determined to be 93% with 2.6 day HRT. These pollutant removal rates are higher than would be expected in temperate regions. Mosquito larvae density was correlated with depth of the wetland. Mosquito larvae densities were higher in cells that were operating below design depth of 0.2 m ( P < 0.005). Results indicate that it is possible to design FWS wetlands in the tropics for effective BOD removal and reduced mosquito larvae development.