Lignite versus. bituminous GAC for biofiltration a case study

Transcription

1 The city of Wilmington, N.C., initiated a study to compare lignite granular activated carbon (GAC) to bituminous GAC for biofiltration at the city s surface water treatment plant. The plant uses conventional filtration with preozonation and intermediate ozonation to treat water from the Cape Fear River. For this study, the existing GAC media in two of the plant s full-scale dual-media filters was replaced one with lignite GAC and the other with bituminous GAC. The two new filters and one control filter containing the original GAC were operated BY ISSAM NAJM, MICHAEL KENNEDY, AND WILLIAM NAYLOR and monitored for 3 months. The first three months were used to exhaust the adsorptive capacity of the new GAC and biologically acclimate it. During the subsequent months, water quality samples were collected from various points throughout the treatment plant and from the effluent of each of the three filters. The samples were analyzed for various physical, chemical, and biological constituents. Quarterly GAC samples were also collected from each filter and analyzed for biomass content. The results indicated that the two types of GAC performed equally well in terms of removing turbidity, dissolved organic matter, color, assimilable organic carbon, and total aldehydes. The bacterial counts in the effluent of the two filters and on the GAC were also similar. Lignite versus bituminous GAC for biofiltration a case study T he Sweeney Water Treatment Plant (WTP) in Wilmington, N.C., treats Cape Fear River water using a series of water treatment processes that include ozonation and biofiltration. The biofiltration process serves two purposes: () it meets the filtration requirements for turbidity and filter run time, and (2) it removes organics via adsorption and biodegradation. The biofilters are dual-media filters with granular activated carbon (GAC) over silica sand. GAC was chosen over anthracite because it provides an additional barrier against chemicals that cause taste and odor in the water as well as greater surface area for bacterial attachment. The plant currently uses bituminous-based GAC, which has been in place since 997. The GAC has performed well for filtration and removal of the biodegradable organic matter (BOM). The city s water department was interested, however, in determining whether other types of GAC material would perform as well as or better than the bituminous GAC. If GAC made from other material performs as well in a biofiltration role, the city s options for GAC suppliers would increase substantially. To answer these questions, a side-by-side full-scale study was conducted to compare lignite GAC to bituminous GAC in a biofiltration mode. One benefit of lignite GAC over bituminous GAC is its lower density. This translates into a lower backwash water flow rate compared with bituminous GAC, which would in turn translate into savings in pumping energy and wasted water. This study was conducted between June 2 and. The objectives of the study were to 94 JANUARY 25 JOURNAL AWWA 97: PEER-REVIEWED NAJM ET AL

2 compare the performance of lignite GAC to that of bituminous GAC in terms of their ability to reduce the BOM concentration in the water, determine whether lignite GAC filters can be backwashed at a lower rate than bituminous GAC filters without compromising filter performance in terms of turbidity and BOM removal, and collect long-term operational data on the ability of both GAC types to meet the filtration requirements for turbidity removal and adequate filter run times. DESCRIPTION OF THE SWEENEY WTP Figure is a schematic of the Sweeney WTP. Water drawn from the Cape Fear River is first preozonated at a transferred dose ranging from 2 to 6 mg/l. The water then splits between two trains a south plant and a north plant. The south plant is a 5-mgd (57-ML/d) conventional coagulation, flocculation, and sedimentation train with intermediate ozonation and dual-media GAC/sand filtration. The north plant has a capacity of mgd (38 ML/d) and includes coagulation and high-rate clarification, followed by intermediate ozonation and dual-media GAC/sand filtration. The filtered waters from the two trains then combine into the plant s clearwell before entering the distribution system. After preozonation, caustics, lime, or both are added to each train to raise the alkalinity of the water before coagulation. Alum and cationic polymer are then added at the rapid mix of each train. Intermediate ozone is used to meet the primary disinfection requirements of.5-log Giardia and 2-log virus inactivation. The transferred intermediate ozone dose varies with water quality and can range from.5 mg/l to >6 mg/l. Caustics, lime, or both, along with chlorine, phosphate, and fluoride are then added to the combined filter effluents from the south and north plants. Additional chlorine is added to the clearwell effluent before the point of entry to the distribution system. DESCRIPTION OF TESTING PROGRAM The testing program described in this article was conducted at the south plant (Figure ), which contains 2 dual-media GAC/sand filters. The GAC in the filters, which was installed in 997, is bituminous-based. In June 2, the GAC was removed from two of the filters (filters and 2) and replaced with fresh GAC. The GAC in filter was replaced with bituminous-based GAC 2 and that in filter 2 was replaced with lignite-based GAC. 3 FIGURE General schematic of the Sweeney water treatment plant Polymer Alum Caustic or lime Preozone Caustic or Lime Alum Polymer South Plant 5 mgd (57 ML/d) Coagulation and high-rate clarification Filter, which contained the old(er) GAC media, was used as the control filter in the study. The characteristics of the media in all three test filters are listed in Table. The new bituminous GAC installed in filter was virtually identical to that in filter. The one noted difference in the lignite GAC installed in filter 2 was its smaller effective size, which was measured at.74 mm, compared with.9. mm for the bituminous GACs in filters and. After the new GAC was installed in filters and 2 in June 2, the filters were operated under normal conditions for three months in an effort to exhaust a large fraction of the adsorptive capacity of the new GAC and to biologically acclimate the new GAC filters. Beginning in and continuing through, a full-scale monitoring program was conducted on filters,, and 2. In the rest of this article, filter is referred to as the control filter, filter as the bituminous filter, and filter 2 as the lignite filter. Table 2 lists the types and locations of the various parameters monitored in this study along with their mon- Flocculation Sedimentation Intermediate filtration GAC/sand ozonation Intermediate ozonation North Plant mgd (38 ML/d) GAC/sand filtration Caustic or lime Chlorine Phosphate Fluoride Chlorine Clearwell TABLE Characteristics of the filtration media in test filters,, and 2 Filter, Filter, Filter 2, Media Control Bituminous Lignite Sand Depth mm (in.) 35 (2) 35 (2) 35 (2) Effective size mm Uniformity coefficient Granular activated carbon Base material Bituminous, old Bituminous, new Lignite, new Depth mm (in.) 686 (27) 686 (27) 686 (27) Effective size mm Uniformity coefficient < NAJM ET AL PEER-REVIEWED 97: JOURNAL AWWA JANUARY 25 95

3 TABLE 2 Types, locations, and frequencies of parameters monitored in this study Type Parameter Location Frequency General water quality TOC, DOC, color, ph, alkalinity, Raw water; preozone effluent, settled water, Monthly parameters temperature, turbidity, filtered UV 254 intermediate ozone effluent, filter influent, absorbance, and HPC bacteria control filter effluent, bituminous filter effluent, and lignite filter effluent Operational parameters Chemical doses through the plant, Control filter, bituminous filter, and lignite filter Monthly as well as filter loading rate; filter backwashing rates and durations; and filter run time Biological parameters Aldehydes and AOC, control filter effluent, bituminous Monthly filter effluent, and lignite filter effluent Biomass density on the surface Three lateral locations at a depth of 52 mm Quarterly of the GAC (6 in.) below the surface of the media AOC assimilable organic carbon, DOC dissolved organic carbon, HPC heterotrophic plate count, TOC total organic carbon, UV 254 ultraviolet absorbance measured at 254-nm wavelength itoring frequency. General chemical and physical parameters were measured each month at various locations across the treatment train. Similarly, operational parameters were monitored monthly. These included the chemical doses at the plant as well as detailed filter run and backwashing information from the three test filters. Finally, and most important, biofiltration performance parameters were collected, including monthly filter influent and effluent samples, which were analyzed for aldehydes, assimilable organic carbon (AOC), and bacterial heterotrophic plate counts (HPC), as well as quarterly GAC samples, which were analyzed for biomass density on the surface of the media. The biomass samples were collected from three locations across each filter and about 52 mm (6 in.) below the surface of the media. ANALYTICAL METHODS As shown in Table 2, several water quality parameters were measured. Total organic carbon (TOC) and dissolved organic carbon (DOC) were measured using a TOC analyzer. 4 DOC samples were filtered TABLE 3 using a.45-µm filter paper. 5 The filter papers were prewashed with 3 ml of deionized water. After an additional 3 ml of deionized water was filtered through the paper, it was collected and used as a DOC blank that was subtracted from the sample DOC result. All DOC samples were acidified after filtration. Color was measured using a spectrophotometer, 6 and water ph was measured using a ph meter. 7 The alkalinity was measured using method 232B (Standard Methods, 998). Ultraviolet light absorbance at 254-nm wavelength (UV 254 ) was measured using the spectrophotometer. HPC bacteria were enumerated using method 925B (Standard Methods, 998) with standard plate agar. Aldehydes, AOC, and biomass concentrations were measured by a private laboratory. 8 The aldehydes were measured using modified method 6252 (Standard Methods, 995) and included formaldehyde, acetaldehyde, glyoxal, and methylglyoxal. AOC concentrations were measured using method 927 (Standard Methods, 995). AOC values are reported as micrograms per litre as carbon (C) based on the total of the P7 strain and the NOX strain. The biomass density measurement was not a standard method but was developed by the laboratory for this project. The method, which was adapted from LeChevallier and McFeters (99), included the following steps:. Thirty grams of drained GAC was mixed with 27 ml of sterile deionized water. 2. The sample was placed in a sonicator 9 and mixed manually every 3 s. 3. Sample aliquots were removed at 6, 7, 8, 9, and min after sonication began. Quality of Cape Fear River water as received at the Sweeney Water Treatment Plant during the study period (October 2 ) Parameter Average Minimum Maximum Total organic carbon mg/l Dissolved organic carbon mg/l Filtered UV 254 absorbance cm Specific ultraviolet absorbance L/(mg-m) Color Pt Co Alkalinity mg/l as CaCO ph Turbidity ntu Temperature o C 2 28 UV 254 ultraviolet absorbance measured at 254-nm wavelength 96 JANUARY 25 JOURNAL AWWA 97: PEER-REVIEWED NAJM ET AL

4 4. The aliquots were spread-plated onto R2A agar plates in duplicate by diluting the aliquots with sterile buffered water to obtain about 3 to 3 colonies per plate (Standard Method SM925C, 995). 5. The plates were incubated at 22 o C for at least four days and then counted. 6. Duplicate results were averaged, and the sampling time with the highest average count was used for reporting. TESTING RESULTS AND DISCUSSION Table 3 lists the values of key water quality parameters measured in raw Cape Fear River water as received at the plant. The water contains relatively high concentrations of TOC and color, with low alkalinity and ph. Depending on the local runoff conditions, the turbidity in Cape Fear River water ranged from a low of 3.5 ntu to a high of 73 ntu on the days when samples were collected. The water is generally warm, with an average temperature of 2 o C (68 o F). The lowest temperature recorded during the study was o C (52 o F), and the highest temperature was 28 o C (82 o F). Figure 2 shows a monthly timeline of the concentrations of DOC in the raw water, filter influent water, and the effluent waters from the three filters. Because of the high alum dose used at the plant (3 to 75 mg/l) and the low ph of coagulation (about 5.8), DOC removal with chemical clarification during the months of monitoring ranged from 46 to 68%, with an average of 55%. The average DOC concentration in the effluent of the control filter was 2. mg/l, and the average DOC concentration in the effluent of each of the bituminous and lignite filters was.9 mg/l. Using a paired t-test, the investigators determined that the difference between the DOC concentrations in the effluent of the two test filters was not statistically significant at the 95% confidence level (p =.67). Figure 3 shows a plot of the calculated DOC removal achieved across the three filters over time. The DOC removal ranged from about.2 mg/l in to about.2 mg/l in. This was likely caused by the significant decrease in the empty-bed contact time (EBCT) through the filters between (5 6 min) and (about min). This is discussed later in the article. A paired t-test was conducted between the DOC removal measured across the control filter and that measured across each of the test filters, as well as between the DOC removals across the two test filters. The p values were.28 between the control filter and the lignite filter,.32 between the control filter and the bituminous filter, and.67 between the lignite filter and the bituminous filter. These values suggest that there was no statistical difference in the DOC removal among all three filters at the 95% confidence level. This analysis also suggests that the reduction in the DOC concentration resulted mostly from biological activity and not adsorption because the control filter had certainly lost its adsorptive capacity by the time the test was conducted. FIGURE 2 Dissolved organic carbon concentration profile across the treatment plant DOC Concentration mg/l Raw water Control filter effluent FIGURE 3 Dissolved organic carbon removal measured across the three filters DOC Removed mg/l Figure 4 shows a timeline profile of the turbidity measured in the raw water, filter influent, and the effluent waters from all three test filters. Although conventional clarification maintained the filter influent turbidity just below. ntu, the effluent turbidity from all three test filters was around. Control filter effluent NAJM ET AL PEER-REVIEWED 97: JOURNAL AWWA JANUARY 25 97

5 FIGURE 4 Turbidity profile across the treatment plant FIGURE 5 Color profile across the treatment plant Turbidity ntu. Color Value Pt Co units. Raw water Control filter. Raw water Control filter. FIGURE 6 Filtration rate and resulting empty bed contact time values FIGURE 7 Run times for the three test filters Filtration Rate gpm/sq ft (mm/s) 4. (2.7) 3.5 (2.4) 3. (2.) 2.5 (.7) 2. (.3).5 (.). (.68).5 (.34). Filtration rate control filter Filtration rate bituminous filter Filtration rate lignite filter EBCT control filter EBCT bituminous filter EBCT lignite filter EBCT min Run Time h Control filter Bituminous filter Lignite filter EBCT empty bed contact time ntu throughout the monitoring program. Examining the profiles in Figure 4 shows that the three filters performed equally well in turbidity removal throughout the testing period. A paired t-test showed that the difference in the turbidity values between the lignite and bituminous filters was not significant at the 95% confidence level (p =.327). Figure 5 shows a plot of the color value measured in the raw water, the filter influent, and the effluent of the three filters (the color value in the filter influent water was not measured during the initial three months of the study). The upstream processes of ozonation and coagulation/sedimentation reduced the color from about 5 to 98 JANUARY 25 JOURNAL AWWA 97: PEER-REVIEWED NAJM ET AL

6 about 5.5 platinum cobalt (Pt Co) units. The color was further reduced from 5.5 to about 2. Pt Co through the filters. Similar to the other water quality parameters, there was no significant difference in the effluent color value at the 95% confidence level between the bituminous filter and the lignite filter (p =.645), between the lignite filter and the control filter (p =.447), and between the bituminous filter and the control filter (p =.). Figure 6 shows plots of the filtration rate and EBCT in the three test filters during the monitoring program. The EBCT values reported are for the entire media depth (GAC and sand). During the first half of the study, the filtration rate was quite low (about.34 mm/s [.5 gpm/sq ft]), which resulted in significantly high EBCT values (5 to 6 min). This was caused by two factors. First, the demand in the distribution system was low during the fall and winter seasons. Second, a high-rate filtration study was being conducted at the north plant at the same time this study was being conducted at the south plant. The high-rate study required the plant staff to maintain a high flow through the north plant, which resulted in a period of low flow through the south plant. During the second half of the study, however, the filtration rate was gradually increased until it was at its maximum design value of.7 mm/s (2.5 gpm/sq ft) in June and. This resulted in an EBCT value of about min in each filter. It is noted in Figure 6 that the three test filters were operated under the same filtration conditions. Figure 7 shows a plot of the filter run times for all three test filters during the study. For most of the runs, all filters were terminated based on the maximum run time criteria (about h) and not because of turbidity breakthrough or head-loss limitations. Operational issues resulted in differences in filter run times in November and. The study demonstrated, however, that the three filters could be operated to the same run times with no operational limitations. AOC removal across the filters is shown in Figure 8. The filter-influent AOC concentration ranged from a low of 526 µg/l as C in to a high of,9 µg/l as C in. The AOC concentrations in the effluent of the three test filters were virtually identical, averaging about 2 µg/l as C. Throughout the full monitoring period, all three filters achieved approximately the same AOC removal (about 75%). A paired t-test showed that the difference in the AOC values between the lignite and bituminous filter effluents was not significant at the 95% confidence level (p =.474). The total aldehydes in the filter influent water ranged from a low of 56 µg/l to a high of 52 µg/l (Figure 9). The concentrations of total aldehydes measured in the effluent of the three test filters were similar and ranged from a low of 2 µg/l to a high of 36 µg/l. On the basis of the results from individual months, all three filters equally removed an average of 75% of the total aldehydes present in the water. A statistical analysis was conducted using a paired FIGURE 8 Assimilable organic carbon removal across the three test filters AOC Concentration µg/l as C,4,2, Control filter effluent t-test, and it determined that the difference in aldehyde removal between the lignite and bituminous filters was insignificant at the 95% confidence level (p =.692). Figure shows a plot of the log of the biomass density measured on the surface of the GAC samples collected from the three test filters in, January 22,, and. The values plotted AOC assimilable organic carbon, C carbon FIGURE 9 Removal of total aldehydes across the three test filters Aldehydes Concentration µg/l Control filter effluent NAJM ET AL PEER-REVIEWED 97: JOURNAL AWWA JANUARY 25 99

7 FIGURE Biomass density on the surface of the granular activated carbon in the three test filters FIGURE Geometric mean heterotrophic plate count values across the treatment process train, Log Biomass cfu/g Control filter Bituminous filter Lignite filter November January 2 22 April 22 July 22 Geometric Mean HPC cfu/ml,, Raw Water Preozone Effluent Settled Water Intermediate Ozone Effluent Filter Influent Control Filter Effluent Bituminous Filter Effluent Lignite Filter Effluent HPC heterotrophic plate count represent the geometric means of the three samples collected from each filter, and the error bars shown represent the logs of the maximum and minimum values among each set of triplicate samples. The results show that seven months after the installation of the new GAC (i.e., by ), the biomass densities on the new GACs were still lower than that on the old(er) GAC (by about..5 logs). Nevertheless, the biomass densities on the new GACs were still quite high (± 6 cfu/g), which resulted in the high removals of AOC and aldehydes shown in Figures 8 and 9. After one year of exposure to the influent water, the biomass densities on the new GAC media were virtually identical to that on the old(er) GAC. Figure shows a plot of the TABLE 4 geometric mean HPC bacterial levels measured across the treatment train during the monitoring period. The monitoring results show that Surface wash the lack of a disinfectant residual in the flocculation and sedimentation Low-rate wash basins, along with the expected increase in AOC concentration after preozonation, resulted in a significant increase in HPC bacterial High-rate wash counts across those processes. The intermediate ozonation process Low-rate wash decreased those counts by about two orders of magnitude. The filter influent samples contained significantly lower HPC levels than the intermediate ozone effluent samples. This is likely attributed to the carryover of some ozone residual after the intermediate ozone contactor. Nevertheless, as a result of the biological activity in the three test filters, the HPC counts in the filtered waters from the bituminous and lignite filters contained about 25 cfu/ml; those collected from the control filter contained about 65 cfu/ml. FILTER BACKWASHING The Sweeney WTP staff uses a standard procedure, summarized in Table 4, for backwashing the plant fil- Filter backwashing procedures used in this study Parameter Control Filter Bituminous Filter Lignite Filter Duration min Rate gpm/sq ft (mm/s) 9.7 (6.6) 9.7 (6.6) 5.8 (3.9) Duration min Rate gpm/sq ft (mm/s) 3.9 (9.4) 3.9 (9.4).6 (7.2) Duration min Rate gpm/sq ft (mm/s) 9.7 (6.6) 9.7 (6.6) 5.8 (3.9) Duration min JANUARY 25 JOURNAL AWWA 97: PEER-REVIEWED NAJM ET AL

8 ters. Durations of the various components of the backwashing cycle were maintained the same for all three filters. However, because lignite GAC is slightly less dense than bituminous GAC (dry bulk density of 37 kg/m 3 [23 lb/cu ft] for lignite GAC versus 45 kg/m 3 [28 lb/cu ft] for bituminous GAC), and because the lignite GAC size used was smaller than the bituminous GAC, the backwash water velocity required to adequately agitate lignite GAC is lower than that required for bituminous GAC. In this study, the lignite filter was backwashed at a lower backwash rate than the control filter and the bituminous filter (both of which contained bituminous GAC). The low high low backwash rates for the lignite filter were mm/s ( gpm/sq ft), compared with mm/s ( gpm/sq ft) for the control filter and the bituminous filter. As discussed earlier in this article, the lower backwash velocity used for the lignite filter did not compromise its operation and effectiveness for either turbidity or BOM removal. No attempt was made in this study to determine whether the control filter and the bituminous filter could have also been backwashed at the lower rate without compromising their performances. At the lower backwash rate, each lignite filter backwash cycle used 33 m 3 (8,7 gal) of water less than that used by either the control filter or the bituminous filter. In addition, the city would incur lower pumping energy costs because of the lower backwash flow rates. SUMMARY AND CONCLUSIONS This full-scale operations and monitoring study compared the performance of lignite GAC to that of bituminous GAC for biofiltration at the Sweeney WTP in Wilmington, N.C. Over a -month period, water quality and operational parameters were monitored across the plant. AOC and aldehydes removals across the filters were monitored, and biomass concentration on the surface of the GAC was measured four times during the study. The results showed that lignite GAC and bituminous GAC performed equally well for turbidity removal and biofiltration. One possible advantage of lignite GAC is its lower density, which requires a lower backwash rate for effective backwashing. This translates into less backwash water usage and lower energy consumption. The lower density also results in fewer pounds of GAC required to fill the required filter volume, which translates into lower costs. For example, when the plant staff decides to replace the GAC in the 2 filters, it will require 275 m 3 (9,72 cu ft) of GAC. This translates into 2 tons of lignite GAC (at 37 kg/m 3 [23 lb/cu ft]), but 36 tons of bituminous GAC (at 45 kg/m 3 [28 lb/cu ft]), a difference of approximately 8%. ACKNOWLEDGMENT This study was funded by NORIT Americas and the city of Wilmington, N.C. The authors thank the members of the Sweeney WTP s operations and laboratory staff for their hard work in collecting and analyzing the operational and water quality samples during the study. ABOUT THE AUTHORS Issam Najm (to whom correspondence should be addressed) is president and founder of Water Quality & Treatment Solutions, 8 Eddleston Dr., Northridge, CA 9326; (88) ; issam.najm@wqts.com. Najm is a member of AWWA, the International Water Association, and the American Chemical Society. He holds a BS degree in civil engineering from the American University of Beirut, Lebanon, and MS and PhD degrees in environmental engineering from the University of Illinois at Urbana- Champaign. In 99, AWWA presented Najm with its Best Doctoral Dissertation Award, and in 999, the University of Illinois Alumni Association awarded Najm its Young Civil Engineer Achievement Award. With 4 years of experience in evaluating and optimizing water treatment technologies, Najm is a registered professional engineer in California and also serves as an instructor of environmental engineering at the University of California, Los Angeles. Michael Kennedy is the plant supervisor of the Sweeney WTP, and William Naylor is a senior applications engineer with NORIT Americas. FOOTNOTES Superpulsator, Infilco Degremont, Richmond, Va. 2 NORIT GAC 83, NORIT Americas, Marshall, Texas 3 HYDRODARCO 3, NORIT Americas, Marshall, Texas 4 Dohrmann DC-8, Dohrmann, Santa Clara, Calif. 5 Pall Corp., Ann Arbor, Mich. 6 DR/4U, Hach Co., Loveland, Colo. 7 ATI Orion, Boston, Mass. 8MWH Laboratories, Monrovia, Calif. 9 Model 82, Branson, Danbury, Conn. REFERENCES If you have a comment about this article, please contact us at journal@awwa.org. LeChevallier, M., & McFeters, G., 99. Drinking Water Microbiology. Progress and Recent Developments. (G. McFeters, editor). Springer-Verlag, New York. Standard Methods for the Examination of Water and Wastewater, 995 (9th ed.). APHA, AWWA, and WEF, Washington. Standard Methods for the Examination of Water and Wastewater, 998 (2th ed.). APHA, AWWA, and WEF, Washington. NAJM ET AL PEER-REVIEWED 97: JOURNAL AWWA JANUARY 25