British Columbia Mine Reclamation Symposia

Nitrogen removal from coal mine wastewater using a pilot scale wetland : Year 1 results Whitehead, Alan Joseph, 1952-; Kelso, Bryan W.; Malick, James G. 1989

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Proceedings of the 13th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation  NITROGEN REMOVAL FROM COAL MINE WASTEWATER USING A PILOT SCALE WETLAND - YEAR 1 RESULTS Alan J. Whitehead, M.Sc. Aquatic Biologist Norecol Environmental Consultants Ltd. Vancouver, British Columbia Bryan W. Kelso, M.Sc. Manager, Freshwater Group Environmental Protection, Conservation and Protection, Environment Canada West Vancouver, British Columbia James G. Malick, Ph.D. Executive Vice President Norecol Environmental Consultants Ltd. Vancouver, British Columbia 37 Proceedings of the 13th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation  38 ABSTRACT The release of nutrients, especially nitrate from blasting operations, is a significant environmental concern within the operation of open pit coal mines. Cost effective techniques have not yet been identified for the removal of nitrate from large discharges such as surface coal mines. Bench scale laboratory studies for nitrate removal from coal mine wastes have shown promising results (Norecol 1987a, 1987b). As a result of these earlier studies, a 3 year pilot wetland study was commissioned to study plant nutrient removal efficiency, survival, and growth relative to wastewater characteristics, and wetland maintenance requirements. An operating coal mine on Vancouver Island in British Columbia was selected as the study site. This paper gives the results of the first year of operation from August to December 1988. The wetland system removed an average of 87% of the total nitrogen over this time period. There was a slight increase in ammonia due to productivity within the wetlands, which resulted in a net mass removal of 98.2% of NO3-N from the waste water. Nitrate removal rates ranged from 0.251 g/m2.d in August to 0.113 g/m2.d in December. Proceedings of the 13th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation  INTRODUCTION Nutrient release, especially nitrogen, has been identified as a significant concern at surface coal mines in Canada. Discharges and seepage from open pits, stockpiles and waste rock dumps can contain large amounts of nitrate from mining operations. The main sources of nitrogen are blasting agents such as ammonium nitrate and fuel oil (ANFO) or slurry gel. From 1 to 6 percent of the total nitrogen in such explosives has been shown to enter the receiving environment from operating mines (Pommen 1983; K. Ferguson, pers. comm.). These nutrients when released into surface waters have the potential to degrade water quality and aquatic habitat (Nordin 1982). As a result, regulatory agencies are increasingly requiring additional monitoring and treatment of wastewaters at existing mines and permitting is becoming more difficult at proposed mines. Cost-effective techniques for nitrate removal from large wastewater streams have not been identified (E.P.A. 1975), especially at the scale necessary for a surface mine. Because of these factors there is a need to identify and evaluate alternatives to conventional treatment approaches. Wetlands have been identified as a promising treatment option for a variety of wastewaters, including sewage (Herskowitz et al. 1987; Steiner et al. 1987), food processing wastewater (Seidel 1976; Anon. 1988) and mine drainage (Kleinmann and Girts 1987). The promising results of laboratory studies aimed specifically at nitrate removal (Norecol 1987a, 1987b) have led to the establishment of a three-year pilot wetland study with the objective to construct and test the treatment efficiency of a field scale wetland at an operating surface coal mine. The study is also intended to provide information on wetlands operation and maintenance requirements as well as plant survival and growth relative to wastewater characteristics. This paper reports the findings of Year 1 of the pilot wetland study, with particular reference to nitrogen removal. METHODS Study Site The project is located at Quinsam Coal mine on Vancouver Island approximately 30 km west of Campbell River, British Columbia. The experimental wetland lies at approximately 290 m above sea level, within a 4 ha natural wetland vegetated by hardhack (Spiraea douglassi), sweetlgale (Myrica gale) and sedge (Carex sp.). The natural wetland is adjacent to the mine's main settling pond, that receives drainage from all disturbed areas of the mine, including the pit, waste rock dumps, coal processing facilities and haul roads. The settling pond was created during mine construction in 1987 by damming and flooding the upper portion of the natural wetland. The natural wetland drains into a tributary to Middle Quinsam Lake, Quinsam River system, which supports an important sport fishery and salmon hatchery. 39 Proceedings of the 13th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation  40  Wetland Design and Operation An 800 m2 area of the natural wetland below the settling pond (Figure 1) was enclosed in July 1988 with a plastic lined earthen berm to create a hydrologically isolated experimental system. A ridge of higher ground within the enclosure divided the experimental wetland into two distinct basins. Baffles made of logs were installed in each basin to distribute water flow. Woody shrubs within the enclosure were cleared to favour the growth of sedge. Cattail ( Typha sp.), which did not occur naturally at the site, were also interplanted to increase the diversity of marsh species. The water supply was piped by siphon from the settling pond. The nitrate content of the influent to the experimental wetland was increased to nominal 10-15 mg/L by dosing with fertilizer grade potassium nitrate (KNOa) to simulate conditions found at other mines (K. Ferguson pers. comm.). Water depth averaged 0.1 m and ranged between O and 0.3 m, depending on local micro-relief. The inflow was adjusted to yield a theoretical retention time of 4 days. Effluent flowed over a weir into the natural wetland. Sampling and Analyses Triplicate water grab samples were collected weekly between August and December 1988 at the inlet of each basin and at the outlet weir. The samples were iced and shipped within 24 hours to the laboratory for determination of NO3-N, NO2-N, NH4-N (NH3 and NH4+), total organic N (TON) and total-N (TN), as well as pH, suspended solids (SS), total organic carbon (TOC) and conductivity. Water quality analysis were carried out according to Standard Methods (APHA 1985). Temperature was measured in the field at the time of sampling. A complete description of the sampling program is available in Norecol (1989). Flows were measured at the inlet and outlet of the experimental enclosure. Precipitation data was obtained from a rain gauge located at the minesite, less than 1000 m from the wetland. Calculations The water budget was calculated from flow measurements and precipitation. Exfiltration and evapotranspiration (ET) were calculated as a unit, by subtraction. (ET will be quantified in subsequent years.) Mass loadings were calculated by multiplying average concentrations and flow volumes for the beginning and end of each period for which data was available (usually weekly). Mass removal efficiency was estimated over each sampling interval and for the entire experimental period. RESULTS Water Budget Inflow averaged 20.3 m3/d, but was variable due to varying hydraulic head in the water supply siphon as a result of water level changes in the settling pond. The inflow decreased in late Septmember, eventually ceasing for seven days in early October, during which period the wetland partially dried up. Above average inflows (maximum 30.2 m3/d) occurred in late October with the onset rainy weather. Proceedings of the 13th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation  41  Proceedings of the 13th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation  42 Outflows during dry weather were significantly less than inflows, indicating that losses to exfiltration and ET accounted for 3.6-8.6 m3/d or approximately 18-43% (average 30.5%) of the piped inflow. During rainy weather, outflows approximated piped inflows, reflecting the added precipitation inputs. Continuing water losses after the onset of the vegetation dormant season in late October, when evapotranspiration would have been negligible (Kadlec 1986), suggested that a large fraction of the water loss was due to exfiltration. Leakage through the berm was also identified as a cause of water loss from the experimental wetland (Norecol 1989). Water Quality The water quality in the settling pond effluent on August 24,1988, before nitrate dosing is shown in Table 1. The pH was circumneutral and suspended solids content was 2 mg/L. Total nitrogen concentration was 0.316 mg/L, consisting primarily of TON (0.31 mg/L); NH4-N (0.006 mg/L) was the only detectable inorganic N. Total P concentration was 0.022 mg/L, of which about half occurred in paniculate form; ortho-P was below the 0.001 mg/L detection limit. Table 2 summarizes the changes in water quality through the wetland over the experimental period. Average TN concentrations declined from 6.9 mg/L in the influent (dominated by NO3) to 1.4 mg/L (dominated by nitrate and organic N) and 1.2 mg/L (dominated by organic N and NH3) after passing through the upper and lower wetland basins, respectively. Influent TN and nitrate concentrations were usually between 6 and 14 mg/L, but declined below this range on 3 occasions due to dosing problems, resulting in lower average values and the high standard deviations shown in Table 2, Nitrate exhibited marked decreases from an average of 6.41 mg/L in the influent (92% of TN), to 1.11 mg/L (59% of TN) and 0.186 mg/L (11% of TN) at the outlets of the upper and lower basins, respectively. The NO3-N fraction of TN decreased as the water passed through the wetland, indicating assimilation by vegetation and/or conversion to reduced forms via clenrtrification. Although the vegetation pathway was not assessed this year, a certain amount of NO3-N was undoubtedly assimilated by the plants. Increases in NO3-N and NH4-N concentrations suggest that denitrification was also taking place, particularly in the upper basin. Average N02-N concentrations increased from 0.007 mg/L in the influent to 0.023 mg/L in the upper basin effluent, and decreased to 0.014 mg/L after passing through the lower basin. Average NH4-N concentrations increased from 0.045 mg/L (0.7% of TN) in the influent to 0.361 mg/L (19% of TN) and 0.847 mg/L (48% of TN) after passing through the upper and lower basins, respectively. Organic N content declined from 0.519 mg/L (7.5% of TN) in the influent, to 0.466 mg/L (25% of TN) in the upper basin effluent, but increased to 0.912 mg/L (52% of TN) in the lower basin effluent. Similar trends were evident for TOC and SS (Table 2), suggesting net removal and net export, respectively, of organic matter from the upper and lower wetland basins. Conductivity declined in both basins, from an average of 400 umhos/cm in the influent to 301 umhos/cm and 247 umhos/cm in the upper and lower basin effluents, respectively, reflecting both the decrease in dissolved inorganic and increase in organic constituents as the water passed through the wetland. Proceedings of the 13th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation  TABLE 1 Water Quality In Quinsam Coal Mine Settling Pond Effluent Before Nitrate Enrichment of the Wetland Influent, August 24,1988 (One Sample Only) 43 Proceedings of the 13th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation  44  Proceedings of the 13th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation  Nitrogen Mass Removal Table 3 and Figure 2 show the N mass removals over the August 24 to December 7,1988, sampling period. Approximately 15.3 kg of the estimated 17.6 kg of total N added to the wetland (both basins) were removed from the wastewater over the entire period. This represents a net total N removal efficiency of 87%, and an average removal rate of 0.182 gN/m2d. Between late August and late October, total N removal efficiency averaged 0.217gN/m2.d, and with cooler temperatures in November and early December declined to 0.142 gN/m2.d. Nitrogen removal during the first week of December was 87.4%, when frost occurred nightly and daytime water temperatures remained below 40C. Nitrate mass removal efficiency averaged 98% over the entire study period, representing a net removal rate of 0.191 g NO3-N/m2 d. The slightly higher NO3 than total N removal shown in Figure 2 reflects the net effect of NH4-N release within the wetland. Nitrate N removal until late October averaged 0.232 gNO3-N/m2.d, and declined to an average of 0.143 gNO3-N/m2.d in November and early December. Release of NH4-N followed a similar pattern, with average production rates declining from 0.016 gNH4-N/m2.d to 0.009 gNH4-N/m2.d after the onset of colder temperatures in late October. Average monthly N removal efficiencies were 99.8% in August (n = 1 ), 99.6% in September (n = 4), 98.0% in October (n = 4,95.3% in November (n=2), and 95.1 % in December (n = 1). DISCUSSION An objective of this pilot project is to determine the seasonal changes in the nitrogen removal capability of a wetland system. The results from the first year of operation, while preliminary, show that significant amounts of nitrogen can be removed in the wetland between summer and late autumn, including periods of intermittent freezing weather. The data also suggest that significant inhibition of N removal by low temperatures may not occur as early in the winter as previously thought. Modifications to the dosing system are planned during Year 2 to extend the operating period and monitoring of the experimental wetland beyond early December and allow quantification of nitrogen removal during the winter and early spring. It is hoped that the information thus generated will allow N removal efficiencies to be estimated on a seasonal basis and thus facilitate the cost effective design of wetland treatment systems. Other program alterations will investigate exfiltration as well as loss and gain of N from the atmosphere. CONCLUSIONS The following conclusions can be made from the first year's results: 1. A modified natural wetland system can remove up to 87.0% of the total N mass added via simulated surface coal mine drainage water. 2. Nitrate removal rates of 0.217 gNO3-N/m2.d can be achieved during the late summer/early autumn and 0.143 gNO3-N/m2.d during late autumn. 3. Ammonium-N and organic N can be released from a modified natural wetland receiving nitrate-enriched mine drainage. 45 Proceedings of the 13th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation  46  Proceedings of the 13th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation  47 Proceedings of the 13th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation  48 ACKNOWLEDGEMENT This study was supported by the Federal Panel on Energy Research and Development (PERD), Office of Energy Research and Development (OERD) Task -2, Oil Sands, Heavy Oil and Coal, and Supply and Services Canada. The technical assistance of Jack Cann and the staff at Quinsam Coal Ltd. is also gratefully acknowledged. REFERENCES American Public Health Association (A.P.H.A). 1985. Standard Methods for the Examination of Water and Wastewater. American Public Health Association, Washington, D.C. 20005. 1268 p. Anonymous. 1988. Waste water to wetlands. Sustainable Development, Vol. 9 No. 2. p. 5 Environmental Protection Agency (E.P.A.). 1975. Process Design Manual for Nitrogen Control. EPA/625/1 -75-007. United States Environmental Protection Agency, Office of Technology Transfer, Washington, D.C. (Personal communication). Ferguson, Keith. Head, Mining, Mineral and Metallurgical Process Division, Environmental protection, Environment Canada, West Vancouver, British Columbia (Personal communication). Herskowitz, J., S. Black and W. Lewandowski. 1987. Listowel Artificial Marsh Treatment Project, pp. 247-254 JQ: Aquatic Plants for Water Treatment and Resource Recovery, K.R. Reddy and W.H. Smith (Eds.). Magnolia Publishing Inc., Orlands, Florida. Kleinman, R.L.P. and M.A. Girts. 1987. Acid mine water treatment in wetlands: an overview of an emergent technology, pp. 255-261 in: Aquatic Plants for Water Treatment and Resource Recovery, K.R. Reddy and W.H. Smith (Eds.). Magnolia Publishing Inc., Orlands, Florida. Nordin, R.N. 1982. The effects on water quality of explosives used in surface mining. Volume 2: Effects on algal growth. Unpublished Ms. British Columbia Ministry of Environment. Norecol Environmental Consultants Ltd. 1987a. Potential Coal Mine Wastewater Treatment Options. Manuscript M587-03, Environmental Protection, Conservation and Protection, Environment Canada. Norecol Environmental Consultants Ltd. 1987b. Potential Coal Mine Wastewater Treatment Options. II: Emergent Aquatic Plants. Manuscript MS 87-05, Environmental Protection, Conservation and Protection, Environment Canada. Norecol Environmental Consultants Ltd. (in press). Evaluation of Wetlands for Nutrient Removal from Coal Mine Wastewater: Pilot Scale. Annual Report-Year 1. Proceedings of the 13th Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1989. The Technical and Research Committee on Reclamation  Pommen, LW. 1983. The effect on water quality of explosives use in surface mining. Volume 1: Nitrogen sources, water quality and prediction, and management of impacts. Tech. Rep. 4, British Columbia Ministry of Environment. Seidel, K. 1976. Macrophytes and Water Purification. In: Biological Control of Water Pollution. J. Tourbier and R.W. Pierson, Jr. (Eds). University of Pennsylvania Press, pp. 109-121. Steiner, G.R., J.T. Watson, D.A. Hammer and D.F. Harker, Jr. 1987. Municipal Wastewater Treatment with Artificial Wetlands - A TVA/Kentucky Demonstration, pp. 923-932 IQ: Aquatic Plants for Water Treatment and Resource Recovery, K.R. Reddy and W.H. Smith (Eds.). Magnolia Publishing Inc., Orlands, Florida. 49 

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