A COMPARISON OF HETEROTROPOHIC PLATE COUNT ORGANISMS IN FULL-SCALE PREOZONATED AND PRECHLORINATED FILTERS

Stephen Booth1 David Kraska1 Courtney Acker1 Blaise Brazos2 Franz Nestlerode3 Scott Rovanpera4 1. Carollo Engineers, Walnut Creek, CA 2. Missouri Water Resources Research Center, Department of Civil and Environmental Engineering, University of Missouri-Columbia, Columbia, MO 3. City of Vallejo, Vallejo, CA 4. City of Benicia, Benicia, CA Abstract In this study various methods of measuring heterotropohic plate counts were evaluated as a method to determine the extent of biofilm sloughing in full-scale drinking water GAC/sand filters. HPCs were analyzed in the following three ways: standard HPC method; following filtration through a 3 µm filter; and following 60 seconds of sonication. Analyses were conducted at the Fleming Hill WTP, which has ozonation, and at the Benicia WTP, which pre-chlorinates the raw water. All three methods of HPC analysis gave similar results at various stages in the treatment process at both plants and during filter ripening. This indicated a lack of particle associated microorganisms at both plants. Based on the HPC analyses, the effluents of the GAC/sand filters at these plants were readily disinfected by free chlorine.

Introduction Ozonation followed by biologically active filters (BAFs) is a process configuration that provides many benefits, including the removal of biodegradable organics, disinfection by-product precursor removal, and distribution system re-growth mitigation. In fact, many utilities have considered eliminating pre-chlorination and operating their filters biologically as a means of meeting the United States Environmental Protection Agency (USEPA) Stage 1 and 2 Disinfectants and Disinfection By-Products rules. However, certain aspects of the use ozone and BAFs have not been fully characterized. For example, the extent to which elevated numbers of microbial cells or clumps of sloughed biofilm are released from BAFs has not been well documented. If significant quantities of microbes are released, the impact of these organisms on downstream disinfection processes also needs to be investigated. Two drinking water treatment plants in the San Francisco Bay Area were selected for this study. The Fleming Hill WTP, which has preliminary (before flash mix) and intermediate (settled water) ozonation and biologically active filters. The Benicia WTP is a conventionally operated plant that pre-chlorinates the raw water. This study compares various methods for measuring heterotropohic plate counts (HPCs) to determine the extent of biofilm sloughing in full-scale drinking water GAC/sand filters. HPCs were analyzed in the following three ways: 1. Standard HPC method 2. Following filtration through a 3 µm filter 3. Following 60 seconds of sonication The purpose of the 3 µm filtration was to remove particle associated microorganisms. The difference in the HPC values determined with and without filtration allowed the quantities of particle associated and planktonic organisms to be determined. It was hypothesized that the filters at Fleming Hill, which were preceded by ozonation, would contain a greater amount of biomass and hence biofilm sloughing was considered more likely. The feasibility of sonication as means of breaking up clumps of bacterial cells and sloughed biofilm was tested. It was speculated that sonicated samples that initially contained biofilm clumps would have higher HPC results than unsonicated samples.

Raw Water Sources The two primary water sources for Fleming Hill and Benicia are Barker Slough (State Water Project Water, [SWPW]) and Putah South Canal (Solon Project Water, [SPW]). Though the Benecia WTP has the capability of treating SPW, it most often treats SWPW. At the time of this study, the Benicia WTP was treating SWPW. The FHWTP, on the other hand, was treating a blend of SWPW and SPW. The raw water quality of the SWPW from Barker Slough varies seasonally, with high turbidity spikes during winter runoff events, occasional algal blooms in the spring and summer, and moderately high coliform counts throughout the year. Because Barker Slough is interconnected with several channels that drain a largely agricultural watershed, the organic content of the water periodically has elevated concentrations of soil humics, anthropogenic contamination, and microoganisms. The SPW originates in the Putah Creek watershed which is controlled, such that regulated contamination from anthropogenic sources is minimized. Because of this control, SPW is of high quality compared to other sources originating in the Sacramento-San Joaquin Rivers Delta, and is generally lower in turbidity, organic content, and taste and odor.

Description of Water Treatment Plants This section provides a general description of the WTPs at which sampling was conducted. Both treatment plants chosen for this study have coagulation, flocculation, sedimentation, filtration, and disinfection unit processes. Fleming Hill Water Treatment Plant Pre-ozonation at Fleming occurs in an ozone contactor with a residence time 6 minutes. Ozone is added to the settled water in the intermediate ozone contactors. The intermediate ozone contactors each contain six cells. Ozone is transferred in the first three cells. The last three cells provide contact time. The water flows counter current to the ozone gas in the first and third cells. There are 16 filters at the plant, each with a surface area of approximately 400 square feet. The filters contain 30 inches of GAC over 10 inches of silica sand supported by 10 inches of size-graded gravel. The effective sizes of the GAC and sand are 0.85-1.05 millimeters (mm) and 0.45-0.50 mm, respectively. The backwash water contained free chlorine residual. In additional the filters are equipped with a surface wash system with agitators that are located at the GAC/sand interface. Table 1 presents a summary of the operating conditions at Fleming Hill during this study.

TABLE 1 SUMMARY OF FLEMING HILL WTP OPERATING CONDITIONS (MARCH TO JUNE, 2000)

Production Rate (MGD) Flash Mix Alum Dose (mg/L) Coagulant Aid Dose (mg/L) Total Ozone Dose (mg/L)1 Settled Water pH Filtration: Loading Rate (gpm/sf) EBCT (min) 1

Minimum

Maximum

13 12.2 3.0 1.3 7.0

26 67.0 12.0 4.1 7.9

2.1 7.3

3.4 12

Sum of preliminary and intermediate ozone doses.

Benicia Water Treatment Plant There are four dual-media filters at Benicia, which are operated at a constant flow. Filter media consists of a 2 inch gravel base supporting a 6 inch layer of sand and a 30 inch layer of GAC. The GAC has an effective size of approximately 1.0 mm with a maximum uniformity coefficient of 2.1. The filters are backwashed with plant finished water, which contains a chlorine residual and a surface wash is also used during backwashing. Table 2 presents a summary of the operating conditions at Benicia during this study.

TABLE 2 SUMMARY OF BENICIA WTP OPERATING CONDITIONS (MARCH TO JUNE, 2000)

Production Rate (MGD) Flash Mix Alum Dose (mg/L) Coagulant Aid Dose (mg/L) Settled Water pH Filtration: Loading Rate (gpm/sf) EBCT (min)

Minimum 3.4 30 0.42 6.3

Maximum 9.6 117 0.70 7.2

0.97 7.2

3.1 23

Materials And Methods Samples for dissolved organic carbon analysis were first filtered through a 0.45 µm filter, which had been thoroughly rinsed with ultrapure water. The total organic carbon (TOC) concentration was then determined on the filtered water using a Sievers 800 Total Carbon Analyzer, according to Method 5310C of

Standard Methods for the Examination of Water and Wastewater (Eaton et al., 1995). Each reported value was the average of 4 determinations. UV absorbance at 254 nm was measured using a Hach DR/4000u spectrophotometer. Samples for heterotrophic plate count (HPC) were enumerated by pour plate on plate count agar, containing tryptone, glucose, and yeast extract agar, at 35 ºC for 96 hours, according to method 9215 of Standard Methods for the Examination of Water and Wastewater (Eaton et al., 1995). Prior to HPC analysis some samples were prefiltered in order to remove particle associated microorganisms. Neutron track-etched polycarbonate filters, which have been shown to function as screens which quantitatively separate and size fractions of particles from water samples (Sheldon, 1972), were used for this purpose. In the field of aquatic microbiology, 3 µm is the consensus filter size utilized for the separation of unattached planktonic bacteria from particle-associated bacteria (Iriberri et al., 1987). In order to test the feasibility of sonication as means of breaking up clumps of bacterial cells and sloughed biofilm, some samples were sonicated using an Artek – Sonic 300 Dismembrator, prior to HPC analysis. A 30 mL sample was sonicated for 60 seconds in a 50 mL sterile conical centrifuge tube using the intermediate tip at full power. RESULTS AND DISCUSSION Sampling and analysis at Fleming Hill and Benicia were conducted from March to June, 2000. Filter effluent samples were generally collected from the same filter at each plant during each sampling event. Upon occasion the specified filter was not in service during the time of sample collection, and in these cases samples were collected from another filter. The authors did not expect significant performance differences among filters at a given plant. The results of the analyses conducted are discussed in the following sections. The raw water quality for the study period is summarized below for Fleming Hill (Table 3) and Benicia (Table 4). In general, water quality was similar at both plants, as expected.

TABLE 3 SUMMARY OF FLEMING HILL WTP RAW WATER QUALITY (MARCH TO JUNE, 2000) Parameter Turbidity (NTU) pH Alkalinity (mg/L) Temperature (°C) TOC (mg/L) UV-254 (cm-1) SUVA (L/mg·m)

Minimum 8 7.3 113 13 1.9 0.03 1.6

Maximum 54 8.4 150 22 14.1 0.49 3.4

TABLE 4 SUMMARY OF BENICIA WTP RAW WATER QUALITY (MARCH TO JUNE, 2000) Parameter Turbidity (NTU) pH Alkalinity (mg/L) Temperature (°C) TOC (mg/L) UV-254 (cm-1) SUVA (L/mg·m)

Minimum 4.8 7.3 98 13 2.3 0.05 2.0

Maximum 81 8.2 158 22 11.3 0.38 3.5

Dissolved Organic Carbon Removal Plant profiles of DOC for Fleming Hill and Benicia are presented in Figures 1 and 2, respectively. Similar trends were observed at each plant, although raw water DOCs tended to be higher at Benicia during March. DOC removals through the coagulation, flocculation, and settling processes were relatively high at each plant (average of 40%). Finished water DOCs were similar throughout the study period. Average DOC removals for the filters at Fleming Hill and Benicia are summarized in Figure 3. The error bars in Figure 3 represent one standard deviation about the mean. The GAC in the filters at each plant has been in service for a number of years, and therefore DOC removals were due to biological activity, and not adsorption. Higher removals were observed at Benicia during March, compared to Fleming Hill. Essentially identical removals

were observed during the period from April to June. The results from April to June are in close agreement with data reported for the summer of 1999 (Booth et al., 1999). The data collected from April to June for Benicia were generally in agreement with the study by Booth et al. (1999), and the variability in these results was also similar.

DOC (mg/L)

16 14

Raw

12

Settled Filtered

10

Finished

8 6 4 2 0 24-Feb

15-Mar

4-Apr

24-Apr

14-May

3-Jun

23-Jun

Figure 1: Plant Profile of DOC at Fleming Hill.

DOC (mg/L)

16

Raw

14

Settled

12

Filtered

10

Finished

8 6 4 2 0 24-Feb 15-Mar

4-Apr

24-Apr 14-May

3-Jun

23-Jun

Figure 2: Plant Profile of DOC at Benicia

13-Jul

Percent DOC Removal

60 Fleming Hill

50

Benicia

40 30 20 10 0 March

April to June

Figure 3: DOC Removal in Filters at Fleming Hill and Benicia

Heterotrophic Plate Counts HPCs were analyzed in the following three ways, as discussed above: - Standard HPC method - Following filtration through a 3 µm filter - Following 60 seconds of sonication HPC data collected in June, 2000 at Fleming Hill and Benicia are presented in Figures 4 and 5, respectively. Trends in these data were generally similar for both plants. Raw water HPCs tended to be higher at Benicia compared to Fleming Hill, although settled water values tended to be lower at Benicia. The lower settled water HPCs at Benicia were likely due to the chlorination of the water prior to flash mixing. Filtered water HPCs were higher at Benicia compared to Fleming Hill. The finished water HPCs never exceeded 1 CFU/mL at either plant. This result is in agreement with the findings of other investigators who have shown that the overall inactivation of HPCs were not statistically different between biologically and non-biologically treated waters (Pernitsky et al., 1995).

12000 Standard HPC

HPC (CFU/mL)

10000

3 um Filtered 8000

Sonicated

6000 4000 2000 0 Raw

Settled

Filtered

Finished

Figure 4: Plant Profile of HPCs at Fleming Hill

12000 Standard HPC

HPC (CFU/ml)

10000

3 um Filtered Sonicated

8000 6000 4000 2000 0 Raw

Settled

Filtered

Finished

Figure 5: Plant Profile of HPCs at Benicia

Results for all three methods were essentially identical at both plants. This indicated that the 3 µm filtration did not remove an appreciable quantity of particle associated microorganisms. Similarly the sonication procedure did not break up clumps of biofilm or detach microorganisms from particles. Thus, it is reasonable to conclude that the concentration of particle associated microorganisms was negligible at both plants, and that little biofilm detachment occurred during normal filter operation..

The authors hypothesized that the release of sloughed biofilm would be greater immediately following a backwash than later in the filter run. In order to test this possibility the same three HPC analyses were performed at 15 minutes, 30 minutes, and 60 minutes after backwashing (Figures 6 and 7). All three methods gave similar results, again indicating the lack of biofilm clumps and particle associated microorganisms in the filter effluents. HPC values generally increased to a maximum at 30 minutes after backwashing at both plants. After 60 minutes of filter run time the HPC values had not yet decreased to the values observed prior to backwashing.

3000 Standard HPC HPC (CFU/mL)

2500

3 um Filtered

2000

Sonicated

1500 1000 500 0 Before Backwash

15 min

30 min

60 min

Time After Backwashing

Figure 6: HPCs in Filter Effluent at Fleming Hill

3000

Standard HPC 3 um Filtered

HPC (CFU/mL)

2500

Sonicated

2000 1500 1000 500 0 Before Backwash

30 min

60 min

Time After Backwash

Figure 7: HPCs in Filter Effluent at Benicia

Conclusions HPCs analyzed in the standard manner, following filtration through a 3 µm filter, and following 60 seconds of sonication gave similar results at both plants. This indicated a lack of particle associated microorganisms and biofilm clumps throughout both treatment processes. Although HPCs tended to increase in the subsequent filter run following a back wash, all three HPC analyses gave similar results. This indicated that insignificant quantities of particle associated microorganisms and biofilm clumps were released during the filter ripening period. The time required for HPC values to decrease to the level observed prior to backwashing was greater than 60 minutes at both plants, although the HPCs reached their highest value at about 30 minutes into the subsequent filter run. Based on the HPC analyses, the effluents of the GAC/sand filters at these plants were readily disinfected by free chlorine. Key Words Ozone, Heterotrophic Plate Counts, Drinking Water Biofiltration References Booth., S.D.J., Kraska, D., Hadler, K., Neslterode, F., Rovanpera, S., and Kerstiens, J., “A Comparison of Particle Removal Efficiency in Biological and Conventional Filters”, CD-ROM Proc. AWWA Water Quality Technology Conf. (1999). Eaton, A.D., L.S. Clesceri, and A.E. Greenberg (Eds.), Standard Methods for the Examination of Water and Wastewater, 19th Edition. (Washington, D.C.: American Public Health Association, 1995). Iriberri, J., Unanue, M., and Barcina, I.,”Seasonal Variation in Population Density and Hetrotrophic Activity of Attached and Free-Living Bacteria in Coastal Waters”, Appl. Environ. Microbiol. 53:2308-2314 (1987). Pernitsky, D.J., Finch, G.R., and Huck, P.M., “Disinfection Kinetics of Heterotrophic Plate Count Bacteria in Biologically Treated Potable Water”, Wat. Res. 29 (5):1235-1241. Sheldon, R.W., “Size Separation of Marine Seston by Membrane and Glass Fiber Filters”, Limnol. Oceanogr. 17:494-498 (1972).

Acknowledgement The authors gratefully acknowledge the assistance of the City of Vallejo, the staff of the Fleming Hill WTP, the City of Benicia, and the Benicia WTP staff.