Metal Removal from Water Discharges by a Constructed Treatment ...

Eng. Life Sci. Full Paper

Metal Removal from Water Discharges by a Constructed Treatment Wetland By E. A. Nelson*, W. L. Specht, and A. S. Knox DOI: 10.1002/elsc.200620112 A full-scale constructed wetland treatment system consisting of four pairs of wetland cells (3.2 ha total area) with water flowing through a pair of cells in series prior to discharge was investigated. A retention basin provided stable water flow to the system. Water retention time in the wetland system was approximately 48 hours, and the wetland cells operated at circumneutral pH. Vegetation development within the cells has been excellent. Copper removal efficiency was greater than 75 % from the start-up of the system, while mercury efficiency improved with maturation of the treatment cells. Sampling of the water course through the wetlands conducted during the fourth year of operation validated continued performance, and assessed the fate of a larger suite of metals present in the water. Copper and mercury removal efficiencies were still very high, both in excess of 80 % removal from the water after passage through the wetland system. Mercury removal continued along the entire water course through the system, while copper was removed almost immediately upon entering the wetland cells. Lead removal from the water by the system was 83 %, zinc removal was 60 %, and nickel was generally unaffected. Organic carbon in the water was also increased by the system and reduced the bioavailability of some metals. Operation and maintenance of the system continued to be minimal, and mainly consisted of checking for growth of the vegetation and free flow of the water through the system. The system was entirely passive, relying on gravity as the power source of water flow. No reportable permit exceedances have been experienced since the wetland began treating an outfall discharge.

1 Introduction The ability of natural wetlands to improve many aspects of water quality has been recognized for many years. This natural process has been utilized in many different forms and applications to use constructed treatment wetlands for the purpose of water quality improvement [1–4]. One aspect of the natural wetland functions that has been capitalized on is the biogeochemical cycling and storage processes that occur in the systems. Heavy metal retention by constructed and natural wetlands has been effectively used in mining regions of the USA and Europe to reduce levels of Cu, Zn, Ni, Pb, and other metals in runoff and drainage [5, 6]. They are equally effective at treating stormwater runoff with high metal concentrations [7–9]. Removal of metals from the water occurs by two primary mechanisms. Adsorption of the metal ion to organic matter and/or clay particles begins removal immediately. In surface flow wetlands, the vegetation growing in the cell will continuously add new organic matter to the sediment and provide new adsorption sites, essentially renewing the media. This removal is especially effective on divalent metals, such as copper, lead, and zinc [10]. Precipitation of the ion as an insoluble salt is the other means of removal. As part of this method, maintenance of an anaerobic soil matrix can increase removal by precipitation of metals as sulfides. This is an outgrowth of the reduction of sulfate by sulfate-reducing – [*]

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E. A. Nelson (author to whom correspondence should be addressed, e-mail: [email protected]), W. L. Specht, A. S. Knox, Savannah River National Laboratory, Environmental Sciences & Technology Dept., Bldg. 773–42A, Aiken, South Carolina 29803, USA.

bacteria present in this environment. Actual removal of metals from the water depends on several factors, including the loading rate, the hydrological character of the wetland (retention time, water depth, flow rate, etc.), and the ecological characteristics (vegetation, soil matrix, nutrients, etc.). Some of these variables can be adjusted after construction to finetune the performance of the wetland. A regulated outfall (A-01) by the Federal National Pollutant Discharge Elimination System (NPDES) at the Savannah River Site (SRS) receives process wastewater discharges and receives stormwater runoff. Routine monitoring indicated that copper concentrations were regularly higher than the regulatory permit limit and the water routinely failed biomonitoring tests. Other chemicals (e.g., lead, chlorine, mercury, etc.) were occasionally higher than the permit limit. A series of studies revealed that the copper was coming from a wide variety of sources and was also elevated in stormwater runoff. The end result of these analyses was that nearly 3,785 m3 of water needed to be treated routinely each day, and during storms up to 75,000 m3 would need to be treated. Conventional treatment systems for metal removal (e.g., ion exchange, chemical precipitation, etc.) proved to be very expensive for the volume of water that needed to be treated and the extremely low concentrations that must be achieved in the water before it was released to the stream. The search for more cost-effective alternatives resulted in constructed wetlands being considered as an alternative. Constructed wetlands are widely used to treat both domestic and industrial wastewater and have been effective in treating metal containing waters from acid mine drainage. Preliminary evaluations showed that a wetland system might achieve the required level of treatment at the lowest cost for

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Eng. Life Sci. 2006, 6, No. 1

Eng. Life Sci. Metal Removal by Wetlands construction and operation. A pilot study was conducted using mesocosms to confirm that the design concept would provide the required treatment. After treatment in the mesocosms, effluent copper concentrations were routinely below permit limits, even though the influent concentrations varied widely [11]. During the mesocosm research phase the wetland system was also effective at reducing total mercury [12]. Wetland mesocosm systems must always take into account the flow and retention time approximations of scaling when they are compared to full scale constructions [13]. Removals of divalent metals in wetlands are reported in the range of 50 % to over 95 %, and vary with individual metal being assessed [1–9]. The results of the preliminary mesocosm WTS at SRS indicated that the planned system was at the upper end of the range for copper [10]. The focus of this research was to determine the metal removal process and performance of the full-scale wetland system and how this performance varied with age of the system. This information will enable us to provide better input for the design of future wetland treatment systems.

2 Materials and Methods 2.1 The Treatment Facility The SRS was established as a federal facility in the early 1950’s, and is administered by the United States Department of Energy. The 80,000 hectare facility is located near Aiken, South Carolina, USA. The wetland treatment system was built to treat a permitted outfall near the main administration area. The design provided for a stormwater retention basin that would be used to manage the amount of water going to the wetland treatment cells, moderating the effects of stormwater surges and providing additional water to keep the wetland flooded during dry periods. The treatment cells consisted of four pairs of 0.4 hectare wetland cells with water flowing from one cell (A cell) to a second cell (B cell), then to the discharge point (see Fig. 1). The cells have been previously described in detail [14, 15]. Normal water depth in the cells is maintained at 30.5 cm. A geosynthetic liner and compacted clay layer was installed under each wetland cell to prevent infiltration of the water in the cells into the groundwater. A 60 cm layer of sandy soil was placed on top of the liner complex to support growth of vegetation. The soils in the wetland cells were amended with organic matter, fertilizer and gypsum at the time of construction and are vegetated with giant bulrush (Schoenoplectus californicus). Water retention time in the wetland system is approximately 48 hours, depending on the flow rate, and the wetland cells operate at circumneutral pH. Vegetation development within the cells was excellent, surpassing 2.5 kg/m2 of dry aboveground biomass and a density of over 140 shoots per m2. The bulrush plants provide a con-


Figure 1. Schematic diagram of the wetland treatment system and sample locations. Arrows indicate direction of flow.

tinuing source of organic material to the sediments where bacteria and fungi decompose the plants and maintain anoxic (no oxygen) conditions in the hydric soil. The combined effects of the organic matter and anoxic soil conditions work to capture and immobilize the metals in the soils, with yearly growth, dieback, and decomposition of the plants keeping the soil ecosystem functioning year after year to remove metals and toxicity from the water. Construction of the system began in January 2000, and construction of the entire system was completed in the early fall of 2000. The establishment of vegetation in the system and the removal of copper and mercury during the initial two years of operation have been previously published [15].

2.2 Sampling and Analysis Routine monitoring samples are collected at a compositing sampler at the compliance point for monthly reporting. As part of the separate research effort, monthly grab water samples were collected from numerous other locations from the inflow to the system through the discharge to the receiving stream from April through September 2004. Water samples were collected from the old compliance point, at the entrance into the wetland cells (after the retention basin), after passage through each of the first wetland cells (A cells), after passage through each of the second wetland cells (B cells), and at the discharge to the stream. A total of eleven water samples were collected on each monthly sampling date in 2004. Samples are analyzed by ICP-MS for cations, by ion chromatography for anions, by the new EPA methods 1630 and 1631 for low level detection of total mercury and methylmercury, and for carbon by an O.I. Analytical 1020A Total Organic Carbon Analyzer. All samples were analyzed as unfiltered samples.


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Eng. Life Sci. Full Paper

3 Results and Discussion Copper removal efficiency was excellent from the start-up of the system, while mercury efficiency improved with maturation of the treatment cells during the first two years of operation. Methylmercury was only a small component of the total mercury discharged from the system. Water samples collected at irregular intervals during the third year of operation were analyzed for a wider suite of metals and anions. Copper and mercury removal continued to be excellent (82 % and 86 %, respectively), and the expanded analysis showed that lead was reduced by 89 % and zinc by 47 % from the influent concentration. Sampling of the water course through the wetlands conducted during the fourth full year of operation validated continued performance during systematic monthly sampling, and assessed the fate of the larger suite of metals present in the water. Copper and mercury removal efficiencies were still very high, both in excess of 80 % removal from the water after passage through the wetland system. Copper reduction continued to occur primarily during passage of water through the first wetland cell. After passage through the second wetland cell, copper concentration is generally near the detection limit of analysis (see Fig. 2). Previous sampling of the sediments to identify the distribution of copper deposition was conducted. This indicated that most of the copper was found in the upper 5 cm of the sediments and organic layer in the first half of the A cell [16]. Most of the divalent metals are rapidly removed from the water and held shortly after making contact with treatment wetlands. More recent sediment sampling results are reported in this issue [17].

only 0.5 % of the influent mercury or 3.5 % of the total discharge mercury. This is an extremely low rate of methylation, and is likely attributable to the large sulfate pool available in the sediment [18]. Mercury removal from the water continued along the entire path of water movement through the wetland cells. Average lead removal from the water by the system was 83 % in 2004, average zinc removal was 60 %, and nickel was generally unaffected. Most other metals analyzed during the six monthly samplings either showed no trend during passage through the wetland cells or were present in such low levels that no pattern was discernable.

Figure 3. Mercury removal from water during passage through the wetland treatment system.

Influent metal concentrations into the wetland cells were highly variable during the six sampling events of 2004, and affected the removal efficiency of the cells. When influent concentrations were higher, removal percentage of each metal was also higher. Average overall removals for each metal during the four years of operation are summarized in Fig. 4. Copper and mercury removal have been very similar over the four years of analysis. Lead concentration in the influent is generally very low, but still removed very efficiently.

Figure 2. Copper removal from water during passage through the wetland treatment system.

Mercury continued to be removed during water movement through both the first and second of the wetland cells (see Fig. 3). Methylmercury concentration in the effluent from the wetland system continued to be very low, averaging less than 0.26 ng/L. This was equivalent to methylation of 28


Figure 4. Average removal of metals from influent during passage through the wetland cells during four years of operation.



Eng. Life Sci. Metal Removal by Wetlands Zinc removal was more variable, based primarily on influent concentration, and therefore, overall removal efficiency was less. Nitrate entering into the wetland cells (average 1.73 mg/L) is almost immediately removed from the water and generally no nitrate is discharged from the A cells. Phosphate level in the water after passage through the wetlands is generally slightly reduced to unaffected (average 0.94 mg/L). Based on the high vegetative productivity of the wetland cells, nitrogen retention in the sediments and organic components of the wetland system are very efficient. Productivity rate of the vegetation in the B cells is unaffected, possibly due to the large floating growth of the nitrogen fixing plant Azolla caroliniana along the edge of many of the cells. The wetland cells are anaerobic and the sediments have negative redox potentials. As a result, manganese and iron mineral phases in the sediments have been reduced to soluble forms and increase in the water during passage through the wetland system (see Fig. 5). Average effluent concentration of iron was 476 lg/L and of manganese was 123 lg/L. Solubilization of iron and manganese from soils under anaerobic conditions in wetlands is not uncommon [19]. The discharges are seasonal in nature, with higher levels present in the effluent during the warmer months. These metals are rapidly oxidized and deposited on the rock discharge from the wetland cells, as indicated by analysis of the periphyton at the discharge to the receiving stream.

in the regulatory copper limit through application of a Water Effects Ratio (WER). This high organic material concentration is also responsible for the ability of the effluent to pass acute toxicity testing using the U.S. Environmental Protections Agency methodology on all sample dates.

4 Conclusions Sampling of the water course through the wetlands conducted during the fourth year of operation validated continued performance, and assessed the fate of a larger suite of metals present in the water. Copper and mercury removal efficiencies were still very high. Mercury removal continues along the entire water course through the system, while copper is removed almost immediately upon entering the wetland cells. Lead removal from the water by the system was 83 %, zinc removal was 60 %, and nickel was generally unaffected. Nitrate entering into the wetland cells is almost immediately removed from the water, while potassium and phosphate are only slightly reduced. The high negative redox potentials in the cells results in release of manganese and iron from the soil mineral. Total organic carbon in the water is also increased by the system, and reduces toxicity of the effluent. The wetland treatment system has very low operation and maintenance costs associated with it (less than $ 50,000 US per year), and these mainly consists of checking for growth of the vegetation and free flow of the water through the system. The system is entirely passive, relying on gravity as the power source of water flow. No reportable permit exceedances of metals, toxicity, or other regulated parameters have been experienced since the wetland began treating the outfall discharge.

Acknowledgements This research was conducted and the document was prepared in connection with work under U.S. Department of Energy contract DE-AC09–96SR18500. Received: June 28, 2005 [ELS 112] Received in revised form: December 5, 2005 Accepted: December 13, 2005 Figure 5. Manganese increase in the water during passage through the wetland treatment system.

References Water discharges at SRS are typically very soft, with low hardness and buffering capacity. Total organic carbon in the water is also increased by the wetland system due to the high additions of organic matter to the system and the normal decompositional processes. Levels of total organic carbon generally doubled during passage through the wetlands to 7.01 mg/L. High organic ligand levels in the water reduce the toxicity of some metals resulting in a three-fold increase


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