Pilot-scale Constructed Wetland Treatment System Research Articles

Performance of a hybrid pilot-scale constructed wetland system for treating oil sands process-affected water from the Athabasca oil sands

Mining leases in the Athabasca oil sands (AOS; near Ft. McMurray, Canada) produce large volumes of oil sands process-affected water (OSPW) that contain potentially problematic constituents requiring treatment prior to surface water discharge into receiving aquatic systems. The aim of this research was to identify constituents of concern (COCs) in OSPW sourced from an AOS external tailings facility and design a hybrid pilot-scale constructed wetland treatment systems (CWTS) to decrease concentrations of COCs and subsequently mitigate risks. COCs were identified based on comparisons to ambient water quality thresholds for the protection of aquatic life (i.e. Canadian Environmental Quality Guidelines [CEQGs], Alberta Environment Water Quality Guidelines [Alberta WQGs], and United States Environmental Protection Agency Water Quality Criteria [USEPA WQC]) and toxicity endpoints. Performance of the hybrid pilot-scale CWTS was evaluated by rate and extent of COC removal and change in toxicity as measured by an aquatic invertebrate Ceriodaphnia dubia. Following characterization of OSPW, specific COCs were identified as: naphthenic acids (NAs), oil and grease (O/G), metals/metalloids (Al, B, Cu, Ni, Se, and Zn), chloride, and total suspended solids (TSS). A hybrid pilot-scale CWTS was designed to promote treatment processes to alter (transfer and transform) COCs using sequential reducing and oxidizing wetland reactors and a solar photocatalytic treatment reactor using fixed film titanium dioxide (TiO2). Performance criteria were achieved as the CWTS decreased concentrations of NAs, O/G, and metals below protective thresholds and decreased toxicity to C. dubia. Results from this study provide proof-of-concept data to inform hybrid passive or semi-passive treatment approaches (i.e. constructed wetlands) that could be used to mitigate COCs contained in OSPWs.

Effects of Simulated Oilfield Produced Water on Early Seedling Growth After Treatment in a Pilot-Scale Constructed Wetland System

Seed germination and early seedling growth bioassays were used to evaluate phytotoxicity of simulated oilfield produced water (OPW) before and after treatment in a subsurface-flow, pilot-scale constructed wetland treatment system (CWTS). Responses to untreated and treated OPW were compared among seven plant species, including three monocotyledons: corn (Zea mays), millet (Panicum miliaceum), and sorghum (Sorghum bicolor); and four dicotyledons: lettuce (Lactuca sativa), okra (Abelmoschus esculents), watermelon (Citrullus lanatus), and soybean (Glycine max). Phytotoxicity was greater in untreated OPW than in treated OPW. Exposures to untreated and treated OPW enhanced growth in some plant species (sorghum, millet, okra, and corn) relative to a negative control and reduced growth in other plant species (lettuce, soybean, and watermelon). Early seedling growth parameters indicated that dicotyledons were more sensitive to test waters compared to monocotyledons, suggesting that morphological differences between plant species affected phytotoxicity. Results indicated the following sensitivity scale for plant species: lettuce > soybean > watermelon > corn> okra≈millet >sorghum. Phytotoxicity of the treated OPW to lettuce and soybean, although concentrations of COCs were less than irrigation guideline concentrations, suggests that chemical characterization and comparison to guideline concentrations alone may not be sufficient to evaluate water for use in growing crops.

Biogeochemical Process-Based Design and Performance of a Pilot-Scale Constructed Wetland for Arsenic Removal from Simulated Bangladesh Groundwater

A pilot-scale constructed wetland treatment system (CWTS) was designed and built to produce biogeochemical conditions promoting processes targeted for removal of arsenic from simulated Bangladesh groundwater. Two CWTS series were designed to promote coprecipitation and sorption of arsenic with iron oxyhydroxides under oxidizing conditions, and two series were designed to promote precipitation of arsenic sulfide and coprecipitation of arsenic with iron sulfide under reducing conditions. Arsenic removal performance was greater in series with oxidizing conditions than in series with reducing conditions (mean outflow concentrations of 64 and 108 μg L−1, respectively). Additions of zero-valent iron (ZVI) to oxidizing series and to reducing series enhanced arsenic removal (mean removal efficiencies of 72 and 42 %, respectively) compared to unamended series (27 and 20 %, respectively). Arsenic removal performance was significantly greater (α = 0.05) in the oxidizing series amended with ZVI than in the other series, with removal extents, efficiencies, and rate coefficients ranging from 6 to 79 μg L−1, 40 to 95 %, and 0.13 to 0.77 day−1, respectively. Results from this pilot-scale study demonstrate that a CWTS can decrease concentrations of arsenic in arsenic-contaminated water to below the World Health Organization (WHO) drinking water quality guideline of 10 μg L−1.

Fate and distribution of arsenic in a process-designed pilot-scale constructed wetland treatment system

The fate and distribution of arsenic in simulated groundwater was determined in a pilot-scale constructed wetland treatment system (CWTS) designed to promote specific biogeochemical processes for arsenic removal. Two CWTS series were designed to promote co-precipitation and sorption of arsenic with iron oxyhydroxides under oxidizing conditions, and two series were designed to promote precipitation of arsenic with sulfide and co-precipitation of arsenic with iron sulfide under reducing conditions. Measured conditions in the CWTS were within ranges favorable for the targeted processes. Arsenic removal was significantly greater (α=0.05) in an oxidizing series amended with zero-valent iron (ZVI) than in the other series, with removal extents, efficiencies, and rate coefficients ranging from 6 to 79μgL−1, 40 to 95%, and 0.13 to 0.77d−1, respectively. The majority of inflow arsenic retained in the first reactor of each series partitioned to the sediment (88–99%), while the remainder partitioned to Typha latifolia. A greater percentage of inflow arsenic was retained in the sediment of the first reactor of the two oxidizing series (20 and 13%) than in the first reactor of the two reducing series (6 and 7%). Addition of ZVI enhanced arsenic removal from the aqueous phase in both oxidizing series and reducing series and increased the percentage of inflow arsenic partitioned to sediment. A vertical concentration gradient developed over time in the sediment, with 74–85% of sediment-bound arsenic accumulated in the upper 6cm and the remaining percentage below 6cm. Results from this study demonstrate that a CWTS can decrease the concentration of arsenic in simulated groundwater to below the World Health Organization (WHO) drinking water quality guideline of 10μgL−1 primarily by transferring arsenic from the aqueous phase to the sediment.

Water depths and treatment performance of pilot-scale free water surface constructed wetland treatment systems for simulated fresh oilfield produced water

Water depth may enhance performance of constructed wetland treatment systems (CWTSs) for renovation of complex mixtures such as fresh oilfield produced waters (FOPWs) that contain constituents such as oil and divalent metals (i.e. Cd, Cu, Ni, and Zn). The purpose of this research was to evaluate the influence of water depth on treatment performance of a free water surface pilot-scale CWTS for renovating simulated FOPW. A CWTS was designed with an oil-water separator and wetland cells with fixed hydraulic retention time (HRT), surface area, volume, water depth, hydraulic loading rate (HLR), and mass loading rate (MLR) as treatment variables. Replicated wetland series were constructed with different water depths and four cells for each series. For wetland series with water depths of 15 and 23cm, rates of removal of oil marker compounds (0.44–1.17day−1) were greater than rates of removal for wetland series with water depths of 46 and 56cm (no removal–0.12day−1). As water depth decreased (i.e. from 56 to 15cm) and sediment redox potential increased (i.e. from −250 to 234mV) in the wetland series, concentrations of oil marker compounds decreased in outflows of the wetland cells. For wetland series with water depths of 46 and 56cm, rates of removal for Cd, Cu, Ni, and Zn (0.31–1.11day−1) were greater than rates of removal for wetland series with water depths of 15 and 23cm (0.003 and 0.23day−1). As water depth increased (i.e. from 15 to 56cm) and sediment redox potential decreased in the wetland series, Cd, Cu, Ni, and Zn concentrations decreased in outflows of the wetland cells. From these results, water depth either enhanced or decreased treatment performance of a CWTS depending on the targeted constituent (i.e. oil, Cd, Cu, Ni, and Zn) due to changes in the sediment redox potential and dissolved oxygen content in the wetland cells. Data from this study indicated that sequential wetland cells could enhance the treatment of complex mixtures like FOPW by establishing specific conditions in each wetland cell targeting removal of different classes of constituents.

Treatment of ammonia in pilot-scale constructed wetland systems with clinoptilolite

Clinoptilolite was investigated as a sorptive medium for use in constructed wetland treatment systems (CWTSs) based on its affinity for ammonia and high surface area for attachment of periphytic biofilms. Results from a batch sorption experiment indicate that the clinoptilolite studied has an affinity for ammonia described by the Freundlich equation q=0.72Ce0.57 for equilibrium ammonia-N concentrations from 0.07 to 30.1mg/L. During a 10-day sampling period, a pilot-scale CWTS planted with Schoenoplectus californicus and containing 1000g clinoptilolite removed significantly more (p-value=8.8×10−3) ammonia-N (mean outflow 4.5mg/L, standard deviation=4.1) than a control system containing no clinoptilolite (mean outflow 8.6mg/L, standard deviation=2.7). Nitrification was detected in samples of clinoptilolite from the treated CWTS using nitrifying bacteria activity reaction tests (n-BARTs). Ammonia removal was not affected by addition of clinoptilolite to a pilot-scale CWTS planted with Typha latifolia, and nitrification activity was not detected in samples of clinoptilolite. These data illustrate that clinoptilolite can increase ammonia removal and nitrifying activity in CWTSs receiving ammonia concentrations equal to or greater than ∼6–10mg/L when conditions required to support nitrification including hydrosoil redox are provided.

Seasonal Performance of a Hybrid Pilot-Scale Constructed Wetland Treatment System for Simulated Fresh Oil Field-Produced Water

Seasonal changes (e.g., temperature, photoperiod, microbial activity) can affect treatment performance of constituents (oil, Cd, Cu, Ni, and Zn) in a constructed wetland treatment system (CWTS). The purpose of this research was to measure seasonal differences in performance of a hybrid free water surface CWTS for renovating simulated fresh oil field-produced water (FOPW) for surface water discharge and irrigation. The hybrid CWTS was constructed outdoors and consisted of an oil/water separator and two pilot-scale wetland series with four cells each. Oil removal activity (oil-specific agar plates) as well as acid-volatile sulfide (AVS) and simultaneously extractable metal concentrations were measured in the laboratory. For this experiment, water temperatures ranged from 0 to 30.2 °C in the wetland cells. There were no measurable seasonal differences in performance during this study. From inflow to outflow, the hybrid CWTS decreased oil marker compound concentrations by ≥99 % and Cd, Cu, Ni, and Zn concentrations by ≥98 %. There were no significant (p ≥ 0.5634) differences in biodegradation activity of oil nor in production of AVS during the year-long study, which supported our findings of consistent removal of the targeted constituents. With a hydraulic retention time of 4 days, oil marker compound and Cd concentrations in the treated simulated FOPW were greater than surface water discharge criteria, but did not cause toxicity to Ceriodaphnia dubia and Pimephales promelas (e.g., decreased survival or reproduction). Data from this pilot-scale hybrid CWTS indicated that performance goals for the removal of targeted constituents from simulated FOPW could be achieved throughout a seasonal study.

Design and Performance of Pilot-Scale Constructed Wetland Treatment Systems for Treating Oilfield Produced Water from Sub-Saharan Africa

Pilot-scale constructed wetland treatment systems (CWTSs) were designed and built to decrease concentrations of constituents of concern in water simulated to match characteristics of water produced from specific oilfields in sub-Saharan Africa. The oilfield produced water has low ionic strength (704–1,370 mg L−1 total dissolved solids) and contains Fe, Mn, Ni, Zn, and oil and grease (O&G). To treat these constituents, biogeochemical pathways were targeted in the design of two subsurface flow (SSF) CWTS series planted with Phragmites australis and a free-water surface (FWS) series planted with Typha latifolia. These systems were designed for prevailing conditions at the sub-Saharan site studied. Concentrations of O&G, Fe, Mn, Ni, and Zn in outflow from the SSF series met use criteria for irrigation and livestock watering. For the FWS series, outflow concentrations of O&G, Fe, and Mn met use criteria for irrigation and livestock watering, and Ni concentrations met use criteria for livestock watering. Both SSF and FWS series were effective in reducing concentrations of O&G in the produced water investigated with >98% efficiency. The high-removal efficiency is attributed to achieving aerobic conditions in the wetland cells. Both SSF and FWS series reduced concentrations of Fe and Mn but with a wider range of efficiency compared with O&G removal. The removal of Ni and Zn could be increased by the addition of organic matter, such as plant detritus, to wetland cells to promote reducing conditions and dissimilatory sulfate reduction.

Performance of a pilot-scale constructed wetland treatment system for selenium, arsenic, and low-molecular-weight organics in simulated fresh produced water

A by-product of petroleum extraction, produced waters (PWs) containing selenium (Se), arsenic (As), and low-molecular-weight organics (LMWOs) may be generated. Pilot-scale constructed wetland treatment systems (CWTSs) were designed and built to evaluate the removal of these constituents from simulated fresh PW (SFPW). Study objectives were to characterize a fresh PW and determine the constituents of concern (COC); formulate an SFPW; design and build a pilot-scale CWTS for SFPW; and measure performance (i.e., COC removal rates and extents). The treatment goals for this study were to decrease Se concentration in SFPW from approximately 50 g/L to less than 5 g/L via microbial reduction; decrease As concentration in SFPW from approximately 20 g/L to less than 5 g/L via iron coprecipitation; and decrease LMWO concentration in SFPW from approximately 25 mg/L to less than 1 mg/L via biodegradation. To determine COC removal rates and extents and environmental factors, measurements included analysis of Se, As, LMWOs, dissolved oxygen, conductivity, pH, oxidation-reduction potential, alkalinity, hardness, and temperature. Mean outflow Se concentrations ranged from less than 1 to 47.1 g/L. Mean outflow As concentrations ranged from 5.7 to 9.5 g/L, and the mean outflow LMWO concentrations were less than 1 mg/L for all treatments and the untreated control. Organic carbon amendments had a significant effect on Se removal and no effect on As or LMWO removal. This pilot-scale study illustrates that CWTSs can enhance Se removal from SFPW and that removal can be achieved to meet stringent discharge limits. More research is needed to advance the techniques of As removal in CWTSs designed to simultaneously target Se.

Biogeochemical process approach to the design and construction of a pilot-scale wetland treatment system for an oil field-produced water

Using a process-based approach, a pilot-scale constructed wetland system was designed and built for treating water produced from an oil field in sub-Saharan Africa. The characteristics of the oil field-produced water were compared with water quality guidelines for irrigating crops and watering livestock to identify constituents of concern (COC) requiring treatment. The COC identified in the produced water include oil, grease, and metals (Zn, Ni, Fe, Mn). A pilot-scale constructed wetland treatment system was then designed and built based on biogeochemical pathways (i.e., sorption, oxidation, and reduction) for transferring and transforming the identified COC to achieve target concentrations meeting water quality guidelines. The pilot-scale treatment system consisted of three series of wetland cells, with four cells in each series. Two series of subsurface flow wetland cells were constructed with each cell having a two-layer hydrosoil of pea gravel and medium-size gravel planted with Phragmites australis. In addition, a series of free water surface wetland cells was constructed, with each cell containing sandy hydrosoil and planted with Typha latifolia. The design allows adjustment of parameters (i.e., hydraulic retention time and organic content of the hydrosoil) to promote the conditions needed to achieve treatment of COC through the identified biogeochemical pathways. This study provides an example of the design and construction of a pilot-scale wetland treatment system using a process-based approach.

Treatment of Selenium in Simulated Refinery Effluent Using a Pilot-Scale Constructed Wetland Treatment System

Effective and reliable treatment is needed to treat selenium (Se) in petroleum refinery effluents to meet water discharge limits established under the Clean Water Act through the National Pollutant Discharge Elimination System. As a byproduct of petroleum refining processes, effluent containing Se may be produced due to geochemistry of the formations from which petroleum is derived. Pilot-scale constructed wetland treatment systems (CWTSs) were designed and built at Clemson University to evaluate removal of Se from simulated refinery effluent (SRE). For this study, objectives were to: (1) chemically and physically characterize a specific petroleum refinery effluent for simulation and assess Se as a constituent of concern, (2) design and conduct bench-scale experiments to measure Se removal in response to organic carbon additions, (3) design and build a pilot-scale CWTS using information from the bench-scale experiments, and (4) measure the performance of the pilot-scale CWTS in terms of rate and extent of Se removal in response to organic carbon additions following a period of maturation and acclimation. The treatment goal (i.e., Se removal) for this study was to decrease the Se concentration in SRE from ∼50 μg/L to less than 5 μg/L. Samples were collected from the pilot-scale CWTS inflow and the outflow from each wetland cell. In order to determine rates and extents of Se removal and factors that may influence performance, parameters measured from the sampling locations included elemental analysis of Se, dissolved oxygen concentration, conductivity, pH, alkalinity, hardness, and temperature. Outflow Se concentrations ranged from 3.4 to 9.8 μg/L, depending on the experimental treatment, with an organic carbon addition to the simulated effluent inflow. This pilot-scale study illustrates that CWTSs can enhance Se removal from a simulated petroleum refinery effluent and that performance (i.e., removal of constituents of concern) can be achieved to meet stringent discharge limits.

Characterization of flue gas desulfurization particulates in equalization basins

Particulates in pilot-scale flue gas desulfurization (FGD) scrubber water were characterized by powder X-ray diffraction, scanning electron microscopy with energy dispersive spectroscopy, and size analysis based on Stokes’ Law after settling in an equalization basin of a pilot-scale constructed wetland treatment system. Three sources were interpreted for specific particle types identified in samples analyzed: FGD wet scrubbing processes, coal combustion byproducts, and uncombusted material from coal. Gypsum (CaSO 4 · 2H 2O) from the FGD scrubbing process comprises ∼95% of the particulates. Iron oxide particles and cenospheres in the particulate samples are interpreted as coal combustion products. Particles interpreted as originating from unburned coal contain carbon and metals including Zn and Fe. The most abundant elements in the particulates analyzed are O, C, Ca, S, Fe, and Si, with maximum mean content of individual elements ranging from 13% to 70% among particle types. Less abundant elements include Al, K, Mg, Ti, and Mo, with maximum mean content from 0.1% to 3.8%.

Performance of a pilot-scale constructed wetland system for treating simulated ash basin water

A pilot-scale constructed wetland treatment system (CWTS) was designed and built to decrease the concentration and toxicity of constituents of concern in ash basin water from coal-burning power plants. The CWTS was designed to promote the following treatment processes for metals and metalloids: precipitation as non-bioavailable sulfides, co-precipitation with iron oxyhydroxides, and adsorption onto iron oxides. Concentrations of Zn, Cr, Hg, As, and Se in simulated ash basin water were reduced by the CWTS to less than USEPA-recommended water quality criteria. The removal efficiency (defined as the percent concentration decrease from influent to effluent) was dependent on the influent concentration of the constituent, while the extent of removal (defined as the concentration of a constituent of concern in the CWTS effluent) was independent of the influent concentration. Results from toxicity experiments illustrated that the CWTS eliminated influent toxicity with regard to survival and reduced influent toxicity with regard to reproduction. Reduction in potential for scale formation and biofouling was achieved through treatment of the simulated ash basin water by the pilot-scale CWTS.

Hydrosoil conditions in a pilot-scale constructed wetland treatment system for natural gas storage produced waters

A pilot-scale constructed wetland treatment system (CWTS) was designed and constructed for simulated fresh to hypersaline natural gas storage produced waters. The system incorporated three types of wetland reactors: freshwater reducing, freshwater oxidizing, and saltwater. Each type of reactor was designed specifically to promote biogeochemical reactions that transform or transfer targeted constituents. As the system acclimated, hydrosoil conditions developed, which sustained the biogeochemical reactions necessary for the function and performance of the system. The characteristics of hydrosoil in the freshwater reducing cells included redox conditions within the ideal range for sulfate reduction (150 to 250 mV), near-neutral pH, and relatively high concentrations of acid volatile sulfide (AVS). These conditions were favorable for removing metals from the simulated gas storage produced waters. In addition, fine-grained sediments in the freshwater reducing cells provided surfaces for binding metals, and organic matter supported sulfate-reducing bacteria by providing an energy source. As designed, the organic content and amount of AVS were elevated, and redox potential was less in the freshwater reducing cells than in the saltwater cells and freshwater oxidizing cells. The results of our study confirm that the hydrosoil of CWTSs can be designed to achieve specific conditions and functions, and that hydrosoil characteristics can be manipulated to attain the desired treatment performance.

Performance of pilot-scale constructed wetland treatment systems for flue gas desulfurization waters

Federal laws regarding ambient air quality are currently requiring industries to reduce emissions of sulfur and nitrous oxides. Coal-fired power plants have therefore begun implementing flue gas desulfurization (FGD) scrubbers that use a highly oxygenated water stream (calcium-carbonate-saturated water) to transform sulfur gases into soluble anion species (sulfite and sulfate). The chemical compositions of FGD waters are dependent on the FGD scrubber design, coal types burned, chemical additives, and scrubbing solution source. The FGD waters contain potentially toxic elements including arsenic, cadmium, chemical oxygen demand (COD), copper, mercury, selenium, chloride, sulfates, and zinc. Therefore, these waters must be treated before discharge into a receiving system because of constituents that can elicit toxicity. The specific objectives of this research were to (1) characterize FGD waters in terms of chemical composition and constituents of concern, (2) design constructed wetland treatment systems (CWTSs) for the remediation of constituents of concern in FGD waters, and (3) measure the performance of CWTSs for formulated and actual FGD waters based on discharge criteria established by the United States Environmental Protection Agency and regulated by National Pollutant Discharge Elimination System permits. The FGD waters are characteristically high in total dissolved solids (calcium, chloride, magnesium, and sulfate), are semineutral in pH, contain high concentrations of total suspended solids, and contain several potentially toxic constituents. Constituents of concern were identified as cadmium, COD, chloride, copper, mercury, selenium, and zinc. Pilot-scale CWTSs were designed based on biogeochemical data, and each system contained an equalization basin and two reducing and oxidizing wetland reactors in series. Three FGD waters were introduced in the pilot-scale CWTS, and the performance was assessed by measuring targeted constituents of concern (mercury and selenium) and the toxicity of pre- and posttreatment waters. Results from these studies indicate that mercury and selenium concentrations in FGD waters can be decreased using CWTSs, and, with an appropriate comanagement of low-ionic strength water for chloride concentrations, toxicity of posttreatment samples is decreased to acceptable discharge limits.

Constructed wetland systems for efficient and effective treatment of contaminated waters for reuse

Using a basic biogeochemical approach, constructed wetland treatment systems (CWTSs) can be designed to renovate contaminated waters for beneficial reuse. The purpose of this article is to present the fundamental design strategy for CWTSs for a variety of contaminated waters. In designing a CWTS, the contaminants of concern are identified in the water to be treated, and effective biogeochemical pathways by which the targeted constituents can be transferred or transformed are determined. Specific transfer processes in wetland cells of a CWTS include sorption, volatilization, precipitation (and settling), and bioconcentration. Transformation processes in the wetland cells include photolysis, hydrolysis, speciation and ionization, oxidation, reduction, and biotransformation. Physical models (pilot-scale CWTSs) are built according to the process-based design, and their performance is measured in terms of rate and extent of removal of targeted constituents as well as functional parameters indicating readiness to perform. Demonstration-scale systems may be used to provide additional site-specific data. Full-scale CWTSs are designed for site conditions, and performance is monitored as part of the operation and maintenance to ensure treatment. A variety of contaminated waters can be treated effectively and efficiently using well-designed CWTSs.

Design and performance of a pilot-scale constructed wetland treatment system for natural gas storage produced water

To test the hypothesis that water produced from natural gas storage wells could be treated effectively by constructed wetland treatment systems, a modular pilot-scale system was designed, built, and used for treating gas storage produced waters. Four simulated waters representing the range of contaminant concentrations typical of actual produced waters were treated, and the system’s performance was monitored. Freshwater wetland cells planted with Schoenoplectus californicus and Typha latifolia were used to treat fresh and brackish waters. Saline and hypersaline waters were treated by saltwater wetland cells planted with Spartina alterniflora and by reverse osmosis. Effective removal of cadmium, copper, lead, and zinc was achieved by the pilot-scale system. Results suggest that use of specifically designed constructed wetland treatment systems provides a flexible and effective approach for treating gas storage produced waters over a wide range of compositions.