Particles in Filter Effluent: The Roles of Deposition and Detachment

turbidity was monitored continuously (Model 2100N Turbidimeter, Hach Co., Loveland, CO). ...... Yeseul Kim , Seong-Nam Nam , Seolhwa Jeong , Jeill...
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Environ. Sci. Technol. 2004, 38, 6132-6138

Particles in Filter Effluent: The Roles of Deposition and Detachment JAKYUM KIM† AND JOHN E. TOBIASON* Department of Civil and Environmental Engineering, Marston Hall, University of Massachusetts, Amherst, Massachusetts 01003

Particles in the effluent of granular media filters can be classified as influent particles that were never removed or as particles that detached after prior deposition. To determine the effects of particle size, filter media depth and filter run duration on the relative fraction of each class, laboratory experiments were performed using suspensions of four sizes of polystyrene particles (0.2, 1.2, 2.5, and 4.0 µm diameters) that were destabilized with 0.04 M calcium chloride and continuously supplied to filters after flocculation. To investigate particle attachment alone, three sizes (1.4, 4.0, and 9 µm) of fluorescent microspheres (FM) were periodically pulse injected immediately ahead of the filter media. Detachment was assessed as the difference between net removal (particle counts) and deposition (FM counts). FM deposition followed theory, while results show that particle detachment was significant from an early phase of filtration (100 minutes). The detached fraction of effluent particles increased with particle size (1 to 12 µm range) and filter depth. These model system results suggest that detachment plays a significant role in the origin of filter effluent particles in full-scale water treatment systems.

Introduction In drinking water treatment, granular media filters are primarily used to remove particles, with special concern for pathogenic microorganisms, such as the chemical disinfection resistant protozoa Giardia cysts and Cryptosporidium oocysts. Filter effluent quality is typically monitored and regulated based on turbidity, a surrogate measure for the presence of particles, as it is not feasible to appropriately measure actual removals, or filter effluent concentrations, of a range of pathogens that may or may not be present in the raw water at any given time. Over the last 15 to 20 years there has been a significant increase in the use of particle counting for monitoring filter effluent quality. A light blockage instrument that detects individual particles of 2 micrometer (µm) or greater equivalent diameter is typically used for combined or individual filter effluents. Because of the U.S. EPA regulatory focus on achieving specified removals of pathogens and the ability of particle counters to determine number concentrations of particles in the size range of protozoan cysts and oocysts (3-15 µm), there has been a temptation and desire to use particle counting to determine quantitative removal capabilities by comparing particle concentrations in raw and filtered waters. However, careful reasoning and some research * Corresponding author phone: (413)545-5397; fax: (413)545-2202; e-mail: [email protected]. † Present address: Water Resources Research Institute, KOWACO, Taejon, South Korea. 6132

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show this to be an improper approach as the origin of filter effluent particles is not necessarily the raw water because most filter influent particles, especially of cyst or oocyst size, are the result of particle precipitation and flocculation due to coagulant addition (or softening if practiced). Furthermore, filter effluent particles can be classified as influent particles that were never removed or as particles that detached after being part of the deposited material in the filter pores. Thus, comparison of filter influent and filter effluent particle counts is not likely to yield accurate quantitative assessment of pathogen removal capability. Improved knowledge of the origin of filter effluent particles should provide a more rational basis for improved filter design and operation for pathogen control. The net removal of particles by granular media filtration is the result of attachment, detachment, and reattachment of detached particles in deeper layers of the filter. Particles are removed by deposition on filter media surfaces and on previously deposited particles (1, 2). Some general deposit configurations are spherical caps on the top of grains (3-5) and tubular structures within the pores (6-8). If the superficial filtration rate (flow per unit surface area) remains constant, interstitial velocities within pores will increase as particles accumulate, causing decreased single collector efficiency and increased hydrodynamic drag forces on deposited particles. Particles may detach when hydrodynamic forces exceed net attachment forces (9-13). Detachment has been related to the amount of particles deposited within the pores of the original filter media (14, 15) and the resulting hydraulic gradient (13, 16, 17). Particle detachment can be examined by comparing turbidity or particle counting data for filter influents with corresponding data for filter effluent and by direct observation (18-20). Several studies have shown that the size range of particles most detachable from the filter is 3-7 µm in diameter and noted that smaller particles could form flocs on the media and detach as larger flocs (13, 21-23). A possible concern is that the most readily detached particles are in the size range of Cryptosporidium oocysts. However, the degree of net removal of oocyst size particles, in general, does not correlate well quantitatively with the removal of Cryptosporidium and Giardia by filters. Typically a 1 to 2 log removal of particle counts greater than 2 µm in size is found for granular media filters in water treatment, but the magnitude of protozoa removal can be 4 to 6 log (24, 25), with similar findings for turbidity versus pathogens (26, 27). Recently some work has demonstrated good correlation between removals of Cryptosporidium and similar sized polystyrene microspheres by granular media filtration (27, 28), suggesting the utility of microspheres as surrogates for protozoan pathogens in research. As detachment is related to deposited particles, it is rational to assume that particles in the filter effluent are composed of different fractions of detached particles and influent particles that were never removed. Studies are needed to determine the nature and origin of particles in filter effluents. The primary objective of this study was to determine the relative fractions of detached versus never removed particles in the effluent of granular media filters. The effects of particle size, filter media depth, and filter run duration were evaluated. The focus of this paper is on results for the steady-state filtration and breakthrough phases in granular media filtration as used following coagulation and flocculation (direct filtration). 10.1021/es0352698 CCC: $27.50

 2004 American Chemical Society Published on Web 10/12/2004

Experimental Design and Methods Direct filtration experiments were conducted with aqueous suspensions of polystyrene microspheres. The filtration system, suspensions, analytical methods, and methods of data analysis are described below. Filtration System. A laboratory-scale direct filtration apparatus (Figure A, Supporting Information) was used to assess particle deposition and detachment for a wellcontrolled model system. Raw water suspensions of a specified mixture of polystyrene particles in 5 × 10-5 M NaHCO3 were destabilized by injecting CaCl2 (dose to suspension of 0.04 molar (M), pH∼7) prior to a static mixer and then flocculated for 20 min in a single stage continuous flow vessel (center shaft, multiple propeller type mixing blades, cylindrical shape). Flocculated suspension was supplied by gravity flow directly to the filter bed at a loading rate of 7.3 m/h. Organic matter was not included in the raw water, and a nonprecipitating coagulant was used in order to have a relatively ideal or model system. A 20 cm long, 25 mm internal diameter glass chromatography column (Kontes Corporate Headquarters, Vineland, NJ) with a flow adapter was used to create the filter beds. Three different depths, shallow (17 mm), baseline (34 mm) and deep (68 mm), of silica sand filter media with a mean size of 0.52 mm (U.S. sieve size -30, +35) were utilized. The column to media diameter ratio of 48 is considered sufficient for minimizing wall effects on the flow through the media (29). Filter influent turbidity was monitored until a filter run was started, and then filter effluent turbidity was monitored continuously (Model 2100N Turbidimeter, Hach Co., Loveland, CO). Particle concentrations (1 to 300 µm) in the filter influent and effluent were determined by analysis of grab samples using a light blockage particle counter (Model WGS 260 with LB1010 sensor, Met One, Inc., Grants Pass, Oregon) in accordance with Standard Method 2560 (30). Differentiation between never removed and detached particles in the filter effluent was made by injecting pulses of fluorescent microsphere (FM) “marker” particles immediately upstream of the filter column using a syringe pump (Harvard Apparatus, South Natick, MA) at several different times during a filter run. The FMs were injected such that aggregation prior to the filter bed was minimized (i.e., addition point within a few millimeters of media surface, use of lowest concentration to allow evaluation of removal). After injection, the filter effluent was sampled and FM concentrations were determined using epifluorescence microscopy in accordance with Standard Method 9216 (30). Vacuum filtration of filter effluent sample through poretics PCTE black membrane filters of 25 mm diameter and 0.2 µm pore size (Osmonics Inc., Minnetonka, MN) was used to capture the particles. The results reported in this paper were collected during a series of nine filter runs, each of 36 h duration, with three replicate runs conducted for each of the three filter media depths (17, 34, and 68 mm). The experiments were designed with shallow media depths and relatively high influent particle loading to be able to observe particle detachment and significant impacts of media depth within a reasonable amount of time. Prior work shows that most particles in granular media filtration are removed in the top few centimeters of the filter (15, 22, 23). Characterization of Particles. Raw suspensions with 5 mg/L of a mixture of four sizes (0.2, 1.2, 2.5, and 4.0 µm diameters at mass concentrations of 1.0, 1.3, 0.8, and 1.9 mg/L each, respectively) of polystyrene particles (Interfacial Dynamics Corporation, Portland, Oregon, U.S.A.) in 5 × 10-5 M NaHCO3 were used to simulate a particle size distribution (PSD) that might occur in natural raw water. Physical and

TABLE 1. Properties of Polystyrene Particles Used in This Studya IDC polystyrene background particles mean diameter (µm) std. dev. (µm) solids concentration of stock (%) surface charge density (µC/cm2)

0.2 0.01 8.2 0.6

1.2 0.029 8.2 5.2

2.5 0.11 8.3 7.7

4.0 0.14 8.2 5.5

IDC fluorescent microspheres mean diameter (µm) std. dev. (µm) solids concentration of stock (%) surface charge density (µC/cm2) a

1.4 0.049 2.0 4.8

4.0 0.029 1.9 5.5

8.7 0.089 1.7 2.0

For all particles, specific gravity ) 1.055 g/cm3.

surface characteristics of the particles are shown in Table 1. The particles have surface sulfate functional groups resulting in a negative surface charge in water, and a density of 1.055 gm/cm3, similar to that of microorganisms and precipitated amorphous Al or Fe hydroxide particles. The pH, zeta potential (Zeta Meter System 3.0, Staunton, Virginia, U.S.A.), and temperature of the raw suspension were in the ranges 6.9 to 7.2, -30 to -45 mV, and 20 to 22 °C, respectively. Pulses of fluorescent microspheres (FMs), or marker particles, were periodically injected immediately ahead of the filter bed to evaluate deposition without detachment. FMs were used to be able to differentiate between marker particles and background particles in the filter effluent. The FMs had sulfate surface functional groups (Interfacial Dynamics Corporation, Portland, Oregon), and the fluorescent dye color was yellow green with excitation and emission wavelengths of 490 and 515 nm, respectively. Three sizes of FMs were used: 1.4, 4.0, and 8.7 (nominally 9) µm diameters (Table 1). The 1.4 µm (small) size was chosen to investigate critical size particles which have the theoretical minimum clean bed removal. The 4.0 (intermediate) and 9 µm (large) sizes were chosen to investigate the removal of larger, protozoan oocyst and cyst size particles. A Cryptosporidium parvum oocyst is a biological colloid with an almost spherical shape, a diameter of 3 to 7 µm, and a geometric mean density of 1.045 g/cm3 (31), similar to the FM density of 1.055 g/cm3. The zeta potential of oocysts as determined by electrophoretic mobility has been found to be negative at -15 mV to - 30 mV in most natural pH conditions (32-34). The zeta potential of the FMs used in this research was approximately -25 to -40 mV at pH 6.9-7.2, which is similar to Cryptosporidium oocysts; thus, fluorescent microspheres might be good surrogates for cysts and oocysts (27, 28, 35). For comparison and analysis of the removal of overall particle counts and FMs, particle count size ranges that represent the 1.4, 4.0, and 9.0 µm FMs were 1-2 µm, 3-5 µm, and 6-12 µm, respectively. The important consideration in the selection of a coagulant for this study was to maintain the same type of surface interactions for all destabilized particles in the experimental system and to not precipitate new particles. The fluorescent microspheres needed to be destabilized instantly when injected into the filter influent. Calcium chloride was chosen as the main coagulant because of its fast and controllable destabilizing ability. A coagulant dose 0.04 M CaCl2 was selected based on jar tests and short-term filter ripening tests (36). Determination of Detached versus “Never Removed” Effluent Particles. Determination of the fraction of the total filter effluent particles that were “never removed influent VOL. 38, NO. 22, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Average fraction of influent turbidity remaining in effluent for three filter media depths.

FIGURE 1. Comparison of tracer and FM breakthrough following pulse inputs. FM data collected at breakthrough phase of filtration for a deep (68 mm) filter. particles” versus the fraction of effluent particles that were detached from the filter deposit was made by conducting the periodic pulse injections of fluorescent microspheres. FMs in the filter effluent following the pulse input were considered to be influent particles that were never removed (i.e., no detachment), while overall particle counts included both detached and “never removed” particles. Thus, the difference between these measures, for various size ranges, represents the level of detached particles in that size range. The number of detached particles in the filter effluent was calculated by subtracting the concentration of “never removed” particles from the total particle counts for a specific size range. The fraction of effluent particles that were detached is the ratio of the number of particles detached to the total particle counts for the specific size range. Tracer studies were performed to assess the hydraulic characteristics of the filter columns and to determine the injection time and the sampling period for FM injections. Sodium nitrate (5 × 10-3 M or 5 × 10-4 M NaNO3) was used as the tracer and was injected using the same syringe pump and tubing as for the FM injections. Nitrate concentration in grab samples was measured by UV absorbance at a wavelength of 208 nm.

removed. The results also support the concept that detached particles may be aggregates, with detachment occurring significantly later in time than the initial deposition of a particle that is part of the aggregate. Thus it is argued that the use of the pulse injection of discrete (nonaggregated) microspheres is a reasonable method to differentiate direct deposition from net removal that also includes detachment impacts.

Results and Discussion

Overall Particle Removal - Bed Depth and Filter Run Time Effects. Results for the average fraction of influent turbidity remaining (influent turbidity of 7 to 8 NTU) for the three bed depths as a function of filter run time (FRT) are shown in Figure 2. The results show typical stages of a granular media filter run: clean bed removal, ripening (increasing removal with time), pseudo-steady-state performance, and breakthrough (increasing effluent turbidity with time). As expected there is greater removal with greater bed depth and evidence of breakthrough appears in less run time for shallower depth. A striking feature of the results is the effect of media depth on eliminating the highly variable and catastrophic nature of breakthrough at shallower depths; a depth of only 68 mm showed no evidence of significant turbidity breakthrough behavior for direct filtration after 36 h, while significant breakthrough events began after 12 h at the 17 mm bed depth. These results show the importance of bed depth in both increasing removal of influent particles and in providing opportunities for detached particles to be removed. Turbidity measurements, however, do not provide information about the size distribution of particles nor their origin (never removed from filter influent versus detached).

Assessment of FM Injection Method. FM counts were made for samples collected every 30 s for a period of 8 min 30 s immediately after the start of an FM injection that was performed during the breakthrough phase of a preliminary filter run with a 68 mm bed depth. As shown in Figure 1, the FM breakthrough curve essentially follows the trend of a nitrate tracer curve obtained using the same methods. FMs were not detected in the filter effluent until 60 s after the FM injection. The FM counts in samples collected 360 s and longer after the injection had less than 0.1% of the total number of injected FM, although the FMs showed slightly more tailing than the tracer. Two FM count samples were collected prior to and well after the FM injection to detect detached FMs which might result from the injections. Results confirmed that there were no significant detached FMs in either sample. The relative similarity of the tracer and FM curves in Figure 1 and the associated FM sampling results indicate that the FM injection method (pulse) measures direct deposition of the influent FM particles without detachment, and as such, filter effluent FMs represent influent particles that were never

Figure 3 shows the average particle counts remaining in the filter effluent throughout 2160 min of FRT for the three filter depths for the three particle count size ranges. Filter influent particle counts changed by less than 10% during the filter run. For the small (1-2 µm) particles (Figure 3a), the effluent concentrations show little fluctuation and consistently decrease with increasing bed depth as expected. All bed depths showed ripening and a stable operation period, with earlier and more evidence of breakthrough as bed depth decreased. Results for the intermediate size (3-5 µm) particles (Figure 3b) show expected effects of bed depth and ripening, but there were more fluctuations in the effluent particle counts and earlier signs of breakthrough (at the 17 and 34 mm depths) as compared to the small particles. The deepest bed showed the most stable effluent quality and had the lowest particle counts among the three depths. Results for the larger size (6-12 µm) particles (Figure 3c) have some similar trends, but show more fluctuations and less net removal (sometimes no net removal) than for the two smaller particle size ranges. The 68 mm bed depth had the lowest and most stable effluent particle counts.

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FIGURE 4. Effect of FM particle size on deposition as a function of filter run time for each bed depth: (a) shallow (17 mm), (b) baseline (34 mm), (c) deep (68 mm). FIGURE 3. Particle counts for three filter depths for each size range as a function of filter run time: (a) 1-2 µm, (b) 3-4 µm, (c) 6-12 µm. The particle count results in Figure 3 show decreasing fluctuations, or instability, in effluent quality as bed depth increases, consistent with the turbidity results. The results also show that fluctuations and instability increase with increasing particle size, consistent with increasing hydrodynamic shear as particle size increases (12) and other theoretical and experimental results (11, 22, 23). Figure 3b,c shows evidence of detachment of intermediate and large particles for the entire filter run for the shallow and baseline depth beds and that the degree of detachment of the large particles was greater than that of the intermediate particles. The results also show that the 68 mm bed depth prevented detached particles from escaping out of the filter and that the major role of the deeper depth may be to remove detached particles, rather than removing influent particles that have not yet been removed. Influent Particle Removal based on Fluorescent Microsphere Injections. The effects of particle size, filter bed depth, and filter run time on FM removal are summarized in Figure 4. The measured log removals of FMs increased with increasing particle size and bed depth as predicted by filtration theory (1). Initial increases in FM removals with FRT were due to ripening of the filter, with the extent and duration of ripening increasing with increasing filter bed depth. The results also generally show a maximum and subsequent decrease in removal as FRT increased, and the change in removal during the filter run was greater as particle size increases. The observed trends of FM removal as the filter run proceeded result from the competing phenomena of increased deposition due to increased specific deposit (ripening) and decreased deposition due to increased interstitial velocities caused by the reduced available pore space due to the same increasing deposit. The relative importance of these

phenomena vary with the magnitude of the specific deposit and thus vary with bed depth and filter run time under the same chemical conditions. Initially, the improved removal due to increasing deposit was more important than decreased pore volume, but eventually the deposition rate decreased. The large FMs showed the greatest removals and a somewhat different profile with FRT as compared to the smaller FMs. The fluctuations in large FM removal in the shallow filter as shown in Figure 4a are possibly due to the dynamics of detachment of background particles which could significantly change the pore space remaining for the shallow bed. Due to hydrodynamic forces, the larger FMs may be more sensitive to these changes than the intermediate and small FMs. Effects of Filter Run Time and Particle Size on Detachment. The concentration of detached particles in the filter effluent is considered in this study to be the difference between measured total filter effluent particle counts and estimates of the concentration of influent particles that were never removed based on the FM pulse addition experiments. Results from the three filter runs for each media depth were averaged to show trends in particle detachment. Detailed results for each experiment are described elsewhere (36). Figure 5 shows the measured particle count concentration and the estimated concentration of influent particles that were never removed for the filter effluents as a function of filter run time for the three particle size ranges for the baseline depth filter. The percent change in the filter influent small particle concentration (1-2 µm) was 1% over the 2140 min filter run. The average concentration of small particles in the filter effluent (Figure 5a) decreased until 1440 min of FRT, was stable from 1440 to 1800 min of FRT, and then increased significantly at 2140 min of FRT. The average concentration of never removed particles decreased rapidly, reached a minimum at 960 min of FRT, and then had a similar trend as the total effluent particle concentration. Thus, the average concentration of detached particles decreased slightly with filter run time until 1800 min of FRT and then increased significantly at 2140 min of FRT. The detached fraction for VOL. 38, NO. 22, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. Concentrations of measured filter effluent total particle counts and calculated particles never removed for the baseline depth filter as a function of filter run time and particle size: (a) 1-2 µm, (b) 3-5 µm, (c) 6-12 µm. the small particles was relatively low and stayed at approximately 25% of the total effluent particle concentration except for the last sample. At the end of the filter run, a considerable amount of particles detached and contributed more to breakthrough than the increase in particles never removed. The results for the intermediate size particles (3-5 µm) in Figure 5b show some similar and some different trends as compared to the smaller particles. Ripening and then decreased removal were observed for both particle counts (net removal) and the FM particles (deposition only), resulting in a minimum concentration of detached particles at 960 min of FRT. In contrast to the small particle results, throughout the filter run the majority of the filter effluent intermediate size particles were detached particles. Deposition of influent particles and net removal followed similar trends, yet net removal decreased more significantly than direct deposition after 960 min of FRT, thus detachment increased with FRT. Data analysis shows that based on deposition alone, the fraction of influent particles in the effluent would have been 0.02 to 0.15, while the net fraction remaining was 0.13 to 0.36, again showing the significance of detachment. At the very end of the run (2140 min), the decrease in performance (net removal) was mostly due to increased detachment as compared to decreased attachment (79% versus 21%). This may be due to an avalanche type of particle detachment, while, simultaneously, incoming particles were deposited only somewhat less efficiently. Results for the large particles (6-12 µm) for the baseline depth filter (Figure 5c) are shown using a log-scale abscissa due to the very large difference in total effluent particle counts and estimates of detached particles. To an even greater extent than for the intermediate size particles, filter effluent large 6136

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FIGURE 6. Average fraction of detached filter effluent particles for the three size ranges for three filter depths: (a) shallow (17 mm), (b) baseline (34 mm), (c) deep (68 mm). particles were almost entirely detached particles based on the very high removals of injected FMs during these experiments. As noted earlier, although removals were high, the larger particles exhibited a greater sensitivity than smaller particles to decreased deposition as the specific deposit increased, with a maximum in direct deposition at 600 min of FRT. At 2140 min of FRT, detachment of the large particles caused a significant breakthrough event, as observed for the small and the intermediate particles, with 97% of the increased large particle concentration due to detachment. Figure 5 results indicate that the most vulnerable particle size range for detachment is 6-12 µm, which is somewhat different from the findings of Moran et al. (22) that 3-7 µm particles were most readily detached. A more detailed analysis of the full particle count data (36, Figure B in the Supporting Information) shows that this difference arises in part because of the specific size ranges (absolute value and range) used in the present analysis and the present use of a method to distinguish between net removal and direct deposition. Detailed data from the present work suggest that the 4-7 µm size range had the greatest detachment (i.e., within the 6-12 µm size range the 6-7 µm particles had the greatest detachment), consistent with the earlier study (22). Summary of Detachment. Results similar to those in Figure 5 were analyzed for the shallow (17 mm) and deep (68 mm) bed experiments (36). A summary of the fraction of detached particles in the filter effluent for all particle sizes for the three filter depths is presented in Figure 6. For the shallow depth filter (Figure 6a), the detached fraction for the small particles was less than 0.2 except for the sample at 2140 min FRT. The detached fraction for the intermediate particles was less than 0.4 and was less than one-half the detached fraction for the large particles, which was 0.9 for

the first sample and stayed in the range of 0.90 to 0.98 throughout the filter run. As shown in Figure 6b for the baseline depth filter, the detached fraction for small size particles ranged from 0.24 at 100 minutes to 0.42 during the catastrophic breakthrough at 2140 min of FRT. The detached fraction for intermediate particles for the baseline depth filter was greater than for the small particles and relatively constant at 0.75-0.86, while the detached fraction for the large particles was greater than 0.97 for the entire filter run. Detached fraction results for all particle sizes for the deep filter are presented in Figure 6c. The detached fraction for small size particles was 0.24 at 100 minutes of FRT and increased to 0.5 by 1800 min of FRT. The detached fraction for intermediate particles was greater than for the small particles, in the range of 0.76-0.95, while the detached fraction for the large particles was greater than 0.99 for the entire filter run. Overall, the results in Figure 6 show that the degree of particle detachment was greatest for the large particles, followed by the intermediate and small particles, for all three depths. This is consistent with the expected greater effect of hydraulic shear as particle size increases. In general, while the concentration of suspended particles decreased with filter depth, the fraction of those particles which were detached increased with increasing bed depth. This is logical as both the opportunities for deposition of influent particles and the sources for particle detachment increase with media depth. The mass of particle deposit as a function of bed depth (by 17 mm deep sections) was measured at the end of each filter run. Representative results (36) show the expected exponential decrease from top to bottom: 130, 63, 40, and 18 mg/cm for bed sections 0-17, 17-34, 34-51, and 51-68 mm, respectively. Thus, the relative amount of detached suspended particles was not proportional to the total amount of deposit. This suggests a dynamic interaction of deposition and detachment at the surface of the deposit, while simultaneously the net long-term deposit increases over time. The results suggest that a main function of a deep filter is to remove particles that detached from the upper part of the filter as well as to continue to provide removal opportunities for filter influent particles which have never deposited. The relative changes in the detached fraction with filter run time were not as large as the change in the filter effluent particle concentrations. An unexpected result was that significant particle detachment occurred in the sample at 100 minutes of FRT which was at the end of the ripening phase. It might be expected that the contribution of detachment would be very minor at the beginning of the filter run when there is less deposit. Results of experiments focused on only the ripening stage (36) show minimal detachment only at the very beginning of filter runs initiated with clean filter media, with detachment occurring very early in the run. These findings are somewhat consistent with the observed effects of filter media depth where the detached fraction is not proportional to the total amount of deposited particles. The detachment analysis in this work results from the fact that the removal of injected FMs was greater than the net removal of background particles of similar size at the same filter run time at each filter depth. The analysis assumes that the removal of FMs represents only deposition of particles, as opposed to the net removal due to both attachment and detachment (based on particle counts), and that background influent particles of similar size are deposited to the same extent. The assumption that the FM removal is due to deposition only is supported by the parallel tracer results and detailed assessment of the pulse input method (Figure 1).

It thus appears that the results of this work demonstrate the significance of detachment with respect to the origin of most filter effluent particles. This phenomena may help to explain the much greater removal of oocysts than the apparent net removal of similar size particles as observed by several researchers (27, 28), as the net removal would include detachment of cyst sized particles that may be aggregates of smaller size particles in the filter influent. Cysts and oocysts may also be incorporated into much larger aggregates with minimal detachment probability.

Supporting Information Available Schematic diagram of experimental apparatus (Figure A) and fractional removals for a baseline depth filter for each particle counter size channel, three ranges of particle counts, and the three sizes of FM (Figure B). This material is available free of charge via the Internet at http://pubs.acs.org.

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Received for review November 13, 2003. Revised manuscript received August 6, 2004. Accepted August 11, 2004. ES0352698