Case history on the reduction

Transcription

1 Case history on the reduction of chlorides from mine water by Srikanth Muddasani, Kathleen Lagnese, Kashi Banerjee and Carla Robinson Mine water generated from underground coal mining operations contains both dissolved and particulate solids. Dissolved solids primarily consist of sodium, calcium, magnesium, potassium, chlorides and sulfates. When discharged to a receiving stream without treatment, these constituents create a potentially toxic environment for aquatic life. A case study is presented to discuss how a centralized treatment plant treats mine water from six locations to meet discharge limitations for chlorides. Located in West Virginia, this facility consistently achieves less than the National Pollutant Discharge Elemenation System (NPDES) permit limitation of 218 mg/l chlorides in the discharge while generating almost zero liquid waste. The dissolved solids concentration in the influent ranges between 5,000 and 10,000 mg/l, with chloride concentrations of 1,000 to 2,000 mg/l and sulfate concentrations of 2,000 to 6,000 mg/l. The mine water is treated using advanced treatment technology to produce clean water for discharge or reuse. The treatment process comprises aeration, softening, filtration, reverse osmosis (RO) reject softening, evaporation, crystallization, final effluent remineralization and sludge dewatering. The solid waste generated in the treatment process is landfilled on site. The leachate is sent back to the facility s thermal treatment process. Because no liquid waste leaves the property, this plant is termed a zero liquid waste (ZLW) facility. The advanced water treatment system was built to meet a new 218 mg/l regulatory limitation imposed by the West Virginia Department of Environmental Protection for chlorides discharged to surface waters. The system is designed to treat a maximum flow of approximately 795 m 3 /h (5 million gpd) of mine water. The treatment process utilizes RO membrane treatment to achieve the effluent criteria, and evaporation and crystallization technology to manage the RO concentrate from the water treatment process. As a result, Srikanth Muddasani, Kathleen Lagnese, Kashi Banerjee and Carla Robinson are process engineer, senior process engineer, senior technical director, and marketing manager, Veolia Water Technologies, Pittsburgh, PA, srikanth.muddasani@veolia.com. the system produces clean water for discharge while generating zero liquid waste. The effluent water can be used for industrial purposes or discharged to a receiving stream. A water impact index (WIIx) study indicated that discharge of this high-quality water into the Mine-SORB TM by FloLogix A Breakthrough in Water Treatment Chemistry! Mine-SORB by FloLogix is a next generation chemistry platform developed to solve many of today s most difficult water treatment challenges. Highly effective for problem contaminants including iron, aluminum, sulfates, TSS and selenium, Mine-SORB s dry blended formulation is non-hazardous and extremely cost effective. Mine-SORB integrates easily with most existing water treatment equipment, helps reduce total water treatment costs, and insures discharge compliance. Key Mine-SORB Advantages Include: Excellent for removal of iron, aluminum, sulfates, TSS and certain forms of selenium in a single treatment step Improves water quality Reduces total treatment cost Lowers solids/sludge production Formulation can be customized for specific applications or requirements Visit us at FloLogix.com or call today to learn how Mine-SORB can help solve your water treatment challenges. We will exceed your expectations! Mınıng engıneerıng MARCH

2 WHY SETTLE FOR METAL? Monongahela River watershed may improve the water quality for downstream water users. The waste produced in the treatment process, including softening sludge and salt, is disposed of in an onsite landfill. Currently, the plant is treating at an average flow of 490 m 3 /h (3.1 million gpd) of mine water with an average influent chloride concentration of 1,530 mg/l and an average effluent chloride concentration of 11 mg/l. Design basis versus current feed Table 1 shows the design versus current average, while Table 2 shows the key effluent Table 1 Design assumptions. Get the advantages of Hayward s industry-leading thermoplastic Flow Control products and solutions! Ability to handle harsh chemicals and environments Outstanding long-term performance in highly corrosive applications Materials of construction that prevent contamination of sensitive fluids Economical life cycle cost vs. metal and traditionally specified materials Hayward Flow Control Products and Solutions are designed and suitable for use in key piping systems and applications such as: Electro-winning solutions Chemical handling, production and distribution Mineral extraction Water treatment, transmission and disposal For more information on Hayward Flow Control products and solutions, call or visit us online at haywardflowcontrol.com Hayward is a registered trademark of Hayward Industries, Inc Hayward Industries, Inc. FLV Series Bag and Cartridge Filters Parameter Design flow, GPM Design 3,500 Max (1750 Min, 2,300 Avg) Current avg feed ( 3 ) 2142 ph, S.U Temperature, F Calcium, mg/l Magnesium, mg/l Chlorides, mg/l 1,500 1,530 Sulfates, mg/l 5,500 2,700 1 Iron as Fe+ 2, mg/l Parameter Design Current average feed ( 3 ) Manganese, mg/l Aluminum, mg/l Alkalinity, mg/l as CaCO 3 TDS (estimated), 10,000 7,900 mg/l Notes: 1 Average value based on the data collected between Aug. 20 Sept. 5, Average value based on the data collected during startup (March 19 May 31, 2013). 3 Other average values were based on the data collected between Jan. 21 May 31, Thermoplastic Valves Actuation & Controls Strainers Filters Bulkhead Fittings & Tank Accessories Pumps 52 MARCH 2015 Mınıng engıneerıng

3 water quality requirements. The differences between the design basis and the current average feed water concentrations of iron, manganese, aluminum, sulfate and total desolved solids (TDS) are related to the fact that pretreatment is performed at some of the six mine locations prior to discharging to the centralized water treatment plant. The design basis was developed assuming no pretreatment at the upstream sources. The existing pretreatment systems, however, do not significantly impact the feed water chloride concentration. As a result, it is very close to the original design value. The treatment process consists of the treatment steps described in this article. Figure 1 Aerial photo of centralized mine water treatment plant located in West Virginia. Mınıng engıneerıng MARCH

4 Figure 2 Full flow softening clarifier. Figure 3 Multimedia filters. Raw water pretreatment Equalization. The influent to the treatment system is water from three mines collected at six different sites. As noted previously, pretreatment for metals removal is performed at some of these sites prior to conveyance to the water treatment plant. Two separate pipelines convey the water to the 11,360 m 3 (3 million gal) raw water feed tank shown in the upper right corner of Fig. 1. Backwash water from the multimedia filters is also directed to the raw water tank, along with discharges from various collection sumps at the site. Residence time in the tank is approximately 12.5 hours at maximum design flow s. Jet mixing accomplished by a grid at the bottom of the tank prevents total suspended solids (TSS) from settling out in the tank and promotes equalization of the influent to the treatment process. Aeration. From the raw water tank, the water is pumped to an aeration tank, where the air oxidizes dissolved iron to form a ferric hydroxide precipitate. Effluent from this aeration tank overflows into a second aeration tank where the ph can be increased with caustic soda (NaOH) to promote the precipitation of dissolved manganese, if needed (this is not the case currently). Table 2 Effluent water quality requirements. Parameters Maximum effluent concentration Chlorides, mg/l <218 1 TDS, mg/l <150 2 ph 6-9 Minimum hardness, 50 mg/l as CaCO 3 Note: 1 NPDES permit requirement for outfall discharge. 2 Consent decree requirement applied to product water prior to remineralization. Softening. The second aeration tank overflows into an adjacent full flow softening tank (Fig. 2), where water is lime soda ash softened. Based on the design, softening is required prior to RO to reduce calcium hardness and to avoid potential calcium sulfate precipitation at high recovery. The softening tank is equipped with a draft tube reactor to ensure thorough solids contact with the recycled sludge and treatment chemicals. The softening tank effluent flows to a solids contact clarifier that removes the precipitated solids. A majority of the precipitated sludge is recycled back to the crystallization tank at a specified sludge recycle ratio to promote crystal growth kinetics and improved settling. The excess sludge is wasted from the system and pumped to the sludge holding tank. Aluminum precipitation and filtration. The clarifier effluent flows into an aluminum precipitation tank where the ph is adjusted to 6.5 to 7.2 S.U. for removal of aluminum. Based on the design mentioned, aluminum removal is required to avoid potential downstream RO fouling. Sodium hypochlorite 54 MARCH 2015 Mınıng engıneerıng

5 Figure 4 Reverse osmosis system. is also added here to inhibit bacterial growth in the downstream filters. The aluminum precipitation tank overflows into an adjacent concrete multimedia filter feed (MFF) tank. Based on economic considerations, an MMF system was selected instead of ultrafiltration. The MMF system consists of seven vertical multimedia filters operated in parallel (Fig. 3). Each filter vessel is 3.6 m (12 ft) in diameter and 2 m (6 ft) high. Filtration of suspended solids is provided by a 1-m (3- ft) deep bed comprised of layers of garnet, sand and anthracite with a gravel underdrain. A filter aid is used to enhance the removal of suspended and colloidal particles. The MMF system effluent is pumped to the RO feed tank, and filter backwash water is directed to raw water feed tank. Reverse osmosis membrane system The RO feed tank effluent is pumped through a 5-micron cartridge filtration system to remove any fine colloidal particles. Antiscalant and sodium bisulfite are added prior to RO to protect the membranes from scaling and from residual free chlorine that can damage the membranes. The single-pass, reverse-osmosis process is a three-stage system design with 18 x 9 x 4 7 M array. The main objective of the RO system is removal of chlorides and other dissolved solids present in the water. The RO system consists of five parallel skids, each sized for 25 percent of the design flow with one standby unit. The RO system is designed to operate at a flux rate of GFD (17-20 LMH). The RO system is designed to operate at a high water recovery rate of 85 percent. The clean permeate produced by the RO system is sent to the product water tank where it combines with distillate from the evaporation/crystallization process prior to being remineralized. Figure 5 RO reject softening clarifier. Figure 6 Evaporator and crystallizer. Final effluent remineralization. The final effluent water is remineralized using carbon dioxide and lime water to meet the effluent requirement for total hardness (50 mg/l as CaCO 3 ), which protects the aquatic life in the receiving stream. The treated water flows into Hibbs Run, a tributary to Buffalo Creek that ultimately discharges to the Monongahela River. In addition, final effluent water can also be sent to onsite truck loading stations for reuse in other industrial operations. RO reject softening. An evaporator feed tank collects the RO reject as well as leachate from the onsite landfill for subsequent processing through the facility s thermal Mınıng engıneerıng MARCH

6 Figure 7 Feed water conductivity. treatment process. The evaporator/crystallizer system is designed to handle corrosive materials with high salt concentrations. However, it must first be treated to prevent fouling of the heat transfer surfaces within the evaporator. Treatment consists of chemical softening in a second RO reject softening tank followed by solids settling in a second flocculating clarifier, acidification, preheating and deaeration/ decarbonation. Sludge from the reject softening process is sent to the sludge holding tank. Evaporator and crystallizer The evaporator concentrates the RO reject by removing water in an energy-efficient, economical manner prior to the crystallization system. The evaporator is a concentric falling film unit divided into two sections, a low concentration side and a high concentration side. The split design reduces the overall power consumption of the system by allowing a portion of the evaporation to occur at a Table 3 Estimated waste generation in treatment process based on current average and design. Waste Softening sludge (on a 100% dry basis) Salt (on a 100% dry basis) Total waste generated (on 100% dry basis) Design Current average 6,666 lb/hr 2,500 lb/hr 17,500 lb/hr 8,460 lb/hr 24,166 lb/hr 10,960 lb/hr lower boiling point rise than the final concentration leaving the evaporator. T h e evaporator operates as a mechanical v a p o r recompression system. Vapors created by concentrating the feed brine are recompressed by the evaporator fan to a higher pressure and, consequently, a higher temperature. These vapors are then recycled back into the heater shell to provide the heat source for concentrating the brine. This design eliminates constant steam use from auxiliary sources. The vapor that condenses on the outside of the evaporator heater tubes leaves the evaporator shell side and gravity drains to the distillate tank. The evaporator distillate combines with distillate from the crystallizer unit and is pumped through the feed preheater for heat transfer to incoming brine. This cools the distillate, which is then conveyed to the product water tank. The high-solids brine from the evaporator is sent to the crystallizer feed tank for further concentration in the crystallizer unit. The crystallizer has a vapor body, a recirculation pump and a forced circulation heat exchanger. Similar to the evaporator, the vapors created by concentrating the slurry within the crystallizer are recompressed by fans. Heat from the mechanically recompressed vapors is transferred through the tubes of the heater to the brine. The concentrated brine collects in the vapor body and is recirculated through the heater. As the evaporation process continues, the concentration of the brine contained in the vapor body increases. As the concentration increases, the solution becomes supersaturated and salts precipitate from solution, resulting in a brine slurry. A slipstream of the recirculating crystallizer slurry is sent to two centrifuges for dewatering. The two centrifuges are located in a dewatering building as are two filter presses dedicated to the softening sludge. The dewatered salt cake produced by the centrifuges is disposed in the on-site landfill together with the dewatered softening sludge filter cake. The liquid from the centrifuges is returned to the crystallizer feed tank. Sludge dewatering. Wasted sludge from the full flow softening and RO reject softening process is combined and dewatered using two plate-and-frame filter presses housed in the dewatering building. The filter cake produced by the presses is collected in trucks for transport to the on-site landfill for disposal. Operating results and discussion The effluent water quality requirements for TDS and chlorides were provided in Table 2. Operating data for conductivity, as well as recent sampling results for chlorides and sulfates, were used to evaluate the treatment plant performance in achieving these treatment 56 MARCH 2015 Mınıng engıneerıng

7 Figure 8 Product water conductivity. goals. TDS removal The consent decree limit for TDS is 150 mg/l. Figure 7 illustrates the variation of actual feed water conductivity measurements recorded from Jan. 21, 2014 to May 31, 2014, relative to the design influent concentration. The current average feed water conductivity is 10,270 µs/cm, compared to the design assumption of 13,000 µs/cm. Figure 8 illustrates product water conductivity level measured prior to the remineralization process, as well as the current final discharge conductivity measured after the remineralization process. The required TDS concentration prior to remineralization is less than 150 mg/l. The average product water conductivity before remineralization is 67 µs/cm (approximately 42 mg/l TDS). The average final effluent conductivity after remineralization is 140 µs/cm (approximately 88 mg/l TDS). The remineralization process adds dissolved calcium and carbonate ions to the water that are responsible for the increase in conductivity. Chloride removal The principal driver for this project was to meet the NPDES effluent discharge chloride limit of less than 218 mg/l. Figure 9 shows the current feed chloride concentration versus current effluent concentration as measured in grab samples collected and analyzed in the facility s onsite lab from Aug. 20-Sept. 5, The current average feed water chloride concentration is 1,530 mg/l and average final effluent water chloride concentration is 11 mg/l. Waste generated The wastes generated in the treatment plant (softening sludge filter cake and salt cake) are disposed in an onsite landfill. Table 3 shows the estimated amount of softening sludge and salt produced based on the design feed and the current average feed (Table 1). The waste generated currently is less than designed, resulting in less frequent dewatering cycles and/or centrifuge use. Conclusion The centralized mine water treatment plant located in West Virginia was built to meet a new regulatory limit of <218 mg/l for chlorides. The advanced water treatment system utilizes state-of-the-art membrane technology to achieve the chloride limit and energy-efficient Figure 9 Feed water chloride versus final effluent chloride. evaporation and crystallization technology for brine management. The plant performance consistently exceeds the design requirements. Solid waste generated in the treatment process is disposed in an onsite landfill and the leachate generated at the landfill is sent back to the facility s thermal treatment process. Because no liquid waste leaves the property, this facility is termed a ZLW facility. n Acknowledgment The authors thank Chris Miller and Alex Seago of Veolia North America s operations team at this facility, as well as Charles Blumenschein, Michael Pietropaoli and Robert Zick of Veolia Water Technologies for their contributions to this work. Reference J. Swearman, R. Zick, M. Pietropaoli and C. Robinson, Zero liquid waste: A regulatory driven approach to mine water treatment, IWC, San Antonio, TX, November Mınıng engıneerıng MARCH