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1 WCW16 Annual Conference & Exhibition October Calgary AB wcw16.wcwwa.ca Arsenic removal from groundwater via adsorption: Comparing theoretical performance with performance monitoring & pilot testing results Mark Burger*, P.Eng. Kerr Wood Leidal Associates Ltd., A Still Creek Drive, Burnaby, BC V5C 6G9 Irfan Gehlen, P.Eng. Kerr Wood Leidal Associates Ltd., A Still Creek Drive, Burnaby, BC V5C 6G9 Corresponding Author*, ABSTRACT Adsorptive media is commonly used for arsenic removal from groundwater in small drinking water systems. Several arsenic removal media products are available on the market, and all of them will work under the right conditions. However, site-specific requirements may favour the selection of some media over others. Water quality analysis can help in selecting a short-list of products that are expected to be most effective, and pilot testing can identify which of the shortlisted materials achieves the best performance for arsenic removal and media life. Other sitespecific conditions such as backwash water availability, backwash flow requirements, and disposal options for exhausted media may also affect media selection and vessel sizing. To provide insight into how different adsorptive media perform in the real world, this presentation will share performance data from an operating water treatment plant and from pilot testing a community in northern British Columbia. Some discussion on manganese removal will also be included because manganese is often found in arsenic-contaminated groundwater and was at the two sites that will be discussed. Manganese must often be removed upstream of arsenic removal because it can interfere with the performance of some arsenic removal media. Monitoring of manganese concentrations during pilot testing and full-scale WTP operation should, therefore, be conducted alongside monitoring of arsenic upstream and downstream of each treatment step. Keywords: groundwater, arsenic, manganese, pilot testing. INTRODUCTION Arsenic and manganese are present in groundwater in many regions of Canada and around the world. Just as small communities are more likely than larger cities to rely on groundwater for their drinking water supplies, small water systems are often the ones presented with the challenge of installing water treatment equipment to remove arsenic and manganese. This paper summarizes the results obtained during pilot testing in a community in northern British Columbia, Canada. The community water system provides domestic water to approximately sixty homes. Two wells which draw water from the same aquifer are currently in operation and provide disinfection by sodium hypochlorite. These wells have a raw water arsenic concentration of approximately mg/l, which exceeds the Guidelines for Canadian Drinking Water Quality (GCDWQ) Maximum Acceptable Concentration (MAC). Raw water

2 manganese concentrations are approximately 0.1 mg/l, which exceeds the GCDWQ aesthetic objective. There are currently no treatment processes installed to reduce the concentration of these parameters. A feasibility study conducted in 2015 concluded that building a new water treatment plant to remove arsenic and manganese from the existing groundwater source was the best option for providing the community with water that meets the GCDWQ. Pilot testing was recommended to determine the most suitable adsorptive media for the removal of manganese and arsenic and to help predict filter media life. METHODOLOGY Materials Pilot test equipment consisted of the following key materials: Six (6) 200 mm diameter pressure vessels, each complete with a single opening 100 mm wide on the top of the vessel and a manifold to collect filtered water or distribute backwash water; Six (6) control head valves that direct flow through the vessels and are capable of reversing the flow through the filter during backwash. Control head valves also provided instantaneous flow rate through each filter in gallons per minute. The control head valves are also capable of initiating automatic timed backwashes with a manual on-demand backwash override; Flow meters to measure total flow that has passed through each filter; Gate valves located upstream of each vessel to control flow entering the vessel. There was also a ball valve installed upstream of all equipment to control overall flow entering the pilot test assembly; and Sample taps were installed downstream of each filter to allow for grab samples to be collected. Equipment Set-Up The pilot test equipment was located in the community s fire hall, approximately 750 m away from the groundwater wells and pump house. Chlorinated water from the distribution system was used to supply the pilot testing equipment. The water was first routed through one of two manganese removal filters installed in parallel (labelled F-1 and F-2) before being blended and split between four arsenic removal filters running in parallel (labelled F-3, F-4, F-5, and F-6). Details of the media installed in each filter are shown in Table 1. Table 1: Details of Media in Pilot Test Filter Filter ID Purpose and Configuration Media Description F-1 Manganese Removal manganese greensand F-2 Manganese Removal pyrolusite (manganese dioxide) F-3 Arsenic Removal activated alumina F-4 Arsenic Removal titanium dioxide F-5 Arsenic Removal titanium dioxide F-6 Arsenic Removal ferric oxide

3 Operation and Monitoring The pilot test equipment began operation on December 17, 2015 and operated continuously until May 13, The water system operator monitored the pilot testing equipment daily and recorded flow meter readings and free chlorine residual entering the filters. Grab samples for arsenic & manganese analysis were taken every two to three weeks for the duration of the pilot test. Samples were analyzed by a third-party laboratory (CARO Analytical Services, Richmond, BC). RESULTS Flow Rates The gate valves in the pilot test installation were throttled to provide flows close to the target flow rates listed in Table 2. The target flow rates were obtained using guidance from the manufacturers, who recommend a range of surface loading rates (flow rate per unit surface area of filtration). These surface loading rates were multiplied by the actual surface area of the pilot test filters to obtain the target flow rates listed below. However, actual flow rates recorded in the vessels varied over time as pressure drops in various filters rose between backwashes. The water system operator monitored the pilot test set up daily and adjusted the valves to adjust flow rates as needed. The flow rates displayed on each control valve and on the totalizers were recorded daily. The range in flow measured by these flowmeters and the average flows measured by the totalizers are summarized in Table 2. Table 2 also includes details of the manufacturer s recommended design criteria for the media, often expressed as a flow per unit area (surface loading rate) or empty bed contact time (EBCT), which is the hydraulic retention time of the water in a volume equal to the volume of the filter bed (media + void spaces). Filter ID Target Flow Rate L/min (USgal/min) Table 2: Actual Flow Rates for Filters during Pilot Test Average Flow Rate over Pilot Test Period (L/min) Median Flow Rate over Pilot Test Period (L/min) Target Surface Loading Rate (m 3 /m 2 *h) Target EBCT (min) Flow Rate Range Recorded from Control Head Flowmeters (L/min) F (1.5) F (3.5) F (1.0) F (1.5) F (1.0) F (1.5)

4 Manganese Results One raw water grab sample was taken on December 18, Raw water manganese concentration was mg/l, which was in the same range as historical water quality data. Upon receipt of initial results in January 2016, additional samples for manganese were included to be collected downstream of the arsenic removal filters. This was done because initial observations showed some high level of manganese passing through the manganese removal filters. The high levels passing through were later attributed to inconsistent and/or insufficient chlorine residual in the distribution system water fed to the pilot plant. The manganese concentration results from the pilot test are summarized below in Table 3. Table 3: Treated Water Manganese Concentration During Pilot Test Sample Date Filter F-1: manganese greensand Treated Water Manganese Level (mg/l) Filter F-2: pyrolusite Treated Water Manganese Level (mg/l) 18-Dec Jan Jan Feb Feb Mar Mar Apr Apr N/A 11-May-16 (16:45) 11-May-16 (18:15) May May N/A Manganese results from filters F-3 to F-6 indicated that some additional manganese removal occurred in those filters, but that the majority of manganese removal took place in filters F-1 and/or F-2. As shown in Table 3, there were instances in filter F-1, and to a lesser degree in Filter F-2, where the downstream manganese concentration was higher than expected, and in some cases exceeded the GCDWQ aesthetic objective. These occurrences were somewhat random and

5 seemed to be linked to the need for a higher chlorine residual and do not suggest a permanent performance issue or exhaustion of the media. What they do suggest is that the chlorine residual in the feed water to the pilot plant was not always sufficient to allow the absorptive media to perform as efficiently as possible. This will not be an issue in the proposed full-scale water treatment plant because the chlorine dose is adjusted and injected prior to the water distribution system. Bed Volumes Unlike manganese removal media, arsenic removal media cannot be regenerated and therefore becomes exhausted after a finite amount of water has been treated through the vessel. For comparative purposes, the performance of different arsenic removal media is usually evaluated based on bed volumes of media treated. As with the determination of Empty Bed Contact Time, a bed volume is defined as the total volume taken up by the media including void spaces. Since the adsorption process depends on arsenic attaching itself to available adsorption sites along the surface of the media, a filter bed that contains twice as much media, for example, will be able to treat twice as much water before exhaustion. With the knowledge that the size of a filter bed will vary based on water demands in each application, media manufacturers often measure their media performance based on the number of bed volumes it can treat. Arsenic Results The arsenic concentration results for filters F-3 to F-6 from the pilot test grab samples are summarized below in Table 4. A raw water arsenic sample was taken on December 18, The arsenic concentration measured was mg/l, which is similar to historical data. Arsenic concentrations were also measured downstream of filters F-1 and F-2. All results of these samples suggest that no significant removal of arsenic occurs in either of manganese removal media; therefore they have not been summarized in Table 4. As shown in Table 4, water samples passed through Filters F-4, F-5, and F-6 are below the GCDWQ maximum acceptable concentration (MAC) for arsenic. The treated water arsenic concentration as a bed volumes treated in each filter is summarized graphically in Figure 1. Results from Filter F-3 only met the GCDWQ MAC for arsenic in the December 18, 2015 sample, and exceeded the GCDWQ MAC for samples taken on and after January 6, Because the filter F-3 exhausted quickly after the start of the pilot test, samples were not taken downstream of this filter after February 23, 2016 and are not discussed further in this paper. The performance of the activated alumina media in filter F-3 is in line with previously published studies (AwwaRF, 2005). Therefore the time to media exhaustion was expected to be shorter than for the media installed in filters F-4, F-5, and F-6.

6 Table 4: Treated Water Arsenic and Bed Volumes Treated for Most Recent Sample Sample Date Filter F-4: titanium dioxide Filter F-5: titanium dioxide Filter F-6: ferric oxide Treated Cumulative Treated Cumulative Treated Cumulative Water Bed Water Bed Water Bed Arsenic Volumes Arsenic Volumes Arsenic Volumes Level Treated Level (mg/l) Treated Level (mg/l) Treated (mg/l) 18-Dec Jan , , , Jan , , , Feb , , , Feb , , , Mar , , , Mar , , , Apr , , , Apr , , , May , , , May-16 N/A N/A , , Filtered Water Arsenic Concentration (mg/l) GCDWQ Maximum Acceptable Concentration ,000 10,000 15,000 20,000 25,000 30,000 35,000 40,000 45,000 Adsorption Filter Bed Volumes Filter F-4: titanium dioxide Filter F-5: titanium dioxide Filter F-6: ferric oxide Figure 1: Filtered Water Arsenic Concentrations vs. Bed Volumes Treated

7 DISCUSSION Measurement of Performance in Bed Volumes The arsenic breakthrough curves shown in Figure 1 follow a somewhat linear trend. Based on the data collected during the pilot test, filter F-4 (titanium dioxide) achieved arsenic breakthrough beyond mg/l after approximately 36,000 bed volumes have been treated. Filter F-5, with similar titanium dioxide media provided by a different manufacturer than Filter F- 4, achieved arsenic breakthrough beyond mg/l after treated approximately 30,000 bed volumes. Filter F-6, filled with iron oxide media achieved arsenic breakthrough beyond mg/l after treating between 36,000 and 39,000 bed volumes. Removal of Interference Chemicals Manganese is known to affect the performance and media life of both titanium dioxide and ferric oxide based arsenic absorption media. For this reason, a separate upstream process to remove manganese is recommended and was installed as F-1 and F-2 in the pilot test. The manganese removal media requires a certain level of chlorine to regenerate itself. At this time, the water system s free chlorine residual is between 0.2 mg/l and 0.3 mg/l. Some chlorine dosed into the system is being used to oxidize manganese and arsenic. An arsenic speciation sample of the chlorinated was taken from the water system on October 7, 2014 and showed that almost all arsenic was in its oxidized form As (V). However, once a new WTP is built, a slightly higher chlorine dose will be required to regenerate the manganese media. Based on the influent manganese concentration at the water source, it is estimated that the chlorine demand required to regenerate the media is a minimum of 0.3 mg/l. This regeneration demand is in addition to other chlorine demands (e.g. to oxidize manganese and arsenic) in the source water and the free chlorine dose required to achieve primary disinfection and residual to maintain secondary disinfection. During the pilot test, the operator measured the chlorine residual entering the pilot equipment. There were some days where the chlorine residual has dropped to levels close to zero. If there are periods of time where water with lower residuals or an absence of residual is produced at the pump house, this water may make it to the pilot test equipment at a later time. At these times, there may be insufficient residual to regenerate the manganese media, which may cause temporary periods of manganese breakthrough downstream of the manganese removal filters and subsequent exposure to manganese in the arsenic removal filters. Manganese greensand (filter F-1) has higher regeneration requirements than pyrolusite (filter F-2), therefore it is expected that higher amounts of oxidant would be required for filter F-1 (Wirth, 2013). This is corroborated with the pilot test results, which show more instances of temporary manganese breakthrough in vessel F-1 compared to vessel F-2. These periods of manganese exposure, while temporary, may shorten the media life of both titanium dioxide and ferric oxide based arsenic removal media. In the future full-scale WTP, the chlorine dosing will be revised to provide a higher chlorine dose upstream of the manganese removal filters to ensure chlorine demands for filter regeneration are being met. The data from this pilot test and also from experience elsewhere highlights the importance of maintaining manganese removal equipment and providing an adequate chlorine residual for oxidation and regeneration if arsenic removal media life is to be maximized.

8 Media Performance at Other sites Data from other similar water treatment plants from previous experience and published studies have been collected and summarized in Table 5 for comparison with the pilot test results. Table 5: Projected Number of Bed Volumes to Media Exhaustion from Other Studies Study / Data Nazko Water Treatment Plant, Nazko, BC (Burger & Gehlen, 2015) New Jersey Field Study; continuous operation (Bang et al., 2005) New Jersey Residential Study; intermittent operation (Bang et al., 2005) New Jersey Comparative Media Study (Bang et al., 2011) New Jersey Comparative Media Study (Bang et al., 2011) Datong Basin Study, China (2015) (Hu et al., 2015) Water Research Foundation / NSF Challenge Water Study (AwwaRF, 2005) Water Research Foundation / NSF Challenge Water Study (AwwaRF, 2005) Influent Arsenic Concentration (mg/l) Media Used Number of Bed Volumes to Exhaustion (i.e. downstream arsenic concentration reaches 0.10 mg/l) to ferric oxide Approximately 27, to Titanium dioxide 45, to Titanium dioxide Test stopped at 32,000 bed volumes with no breakthrough observed to Titanium dioxide 41, to Granular ferric oxide 58, Titanium dioxide 2, Bayoxide E33 (ferric oxide) 1.0 MetSorb G (titanium dioxide) 10,000 Test stopped at 8,000 bed volumes with no breakthrough observed. Note in Table 5 that water sources with significantly higher influent arsenic concentrations (e.g. Datong Basin Study, NSF Challenge Water Study) result in shorter times to media exhaustion. In addition to the data projected in Table 5, note that technical data sheets for popular titanium dioxide and ferric oxide products project media life of 42,000 bed volumes and 25,000 bed volumes, respectively. Based on the performance from other sites, the pilot test data collected indicates the following: Titanium dioxide-based and ferric oxide-based media perform similarly for the water source in this community and appear to be trending towards having similar times to media exhaustion. This is in line with another study of source water at higher ph (Bang et al., 2011). The projected media life for this water source is within the range of previously published studies of expected media life for similar source waters.

9 The lower influent arsenic concentration in this water source compared to many of the other sites discussed in previously published studies did not seem to have an effect on lengthening media life. CONCLUSIONS Based on the scope and findings of this study noted above, we conclude that: 1. the performance of the manganese removal media during the pilot test appears to be affected by the variability in free chlorine residual available for regeneration of the filter media, therefore, it is important that the minimum chlorine residual for regeneration always be met; 2. temporary exposure of manganese to the arsenic removal media filters may reduce the time to arsenic media exhaustion; 3. filter F-3 (activated alumina) had the shortest media life, having exhausted within a few weeks of starting the pilot test; 4. filter F-4 (titanium dioxide) and Filter F-6 (ferric oxide) have performed similarly in terms of arsenic removal and bed volumes treated; and 5. filter F-5 (titanium dioxide, different manufacturer than F-4) provided similar arsenic removal media life as filters F-4 and F-6, but with a longer Empty Bed Contact Time, which would require larger filter bed volumes at full-scale. REFERENCES AWWA Research Foundation (2005). Adsorbent Treatment Technologies for Arsenic Removal. Bang, S., Patel, M., Lippincott, L., Meng, X. (2005). Removal of arsenic from groundwater by granular titanium dioxide adsorbent. Chemosphere, 60: Bang, S., Pena, M.E., Patel, M., Lippincott, L., Meng, X., Kim, K. (2011). Removal of arsenate from water by adsorbents: a comparative case study. Environmental Geochemistry and Health, 33: Burger, M.S. & Gehlen, I.J. (2015). Nazko WTP: a reliable and low-maintenance treatment solution for arsenic removal. Environmental Science and Engineering Magazine, September- October 2015, Vol. 28(5): Hu, S., Shi, Q., Jing, C. (2015). Groundwater arsenic adsorption on granular TiO 2 : Integrating Atomic Structure, Filtration, and Health Impact. Environmental Science and Technology, 49(16): Wirth, M. (2013). The Magic of Manganese Dioxide: What It Is and Why You Should Care. Water Conditioning and Purification, March 2013.