The Role of the Schmutzdecke in Pathogen Removal in Slow Sand and Riverbank Filtration

Transcription

1 The Role of the Schmutzdecke in Pathogen Removal in Slow Sand and Riverbank Filtration Michael Unger Master s Thesis Defense University of New Hampshire Gregg Hall Room 110 November 10, 2006

2 Acknowledgements Advisor: Dr. Robin Collins Committee: Dr. Nancy Kinner, Dr. Aaron Margolin EPA WTTAC Water Treatment Technology Assistance Center at UNH WTTAC Staff Robert Moody, Nancy Whitehouse Wes East and Staff of UNH/Durham WTP 2

3 Slow Sand Filtration (SSF) Effluent Flow Control Structure Headspace Vent Raw Water Supernatant Water Adjustable weir Overflow Weir Sand Media Schmutzdecke Filter Drain & Backfill Support gravel Drain Tile Control Valve 3

4 The Schmutzdecke German: Schmutz = dirt; Decke = covering Slow Sand Filtration Design Handbook: Layer of material on top of the SSF that causes headloss disproportionate to its thickness Deposited and synthesized (AWWARF 1991) 2 Regions Biomat (slime) Biologically active sand 4

5 Full-Scale Filter Schmutzdecke Cleaning 5

6 Pilot Filter Schmutzdecke 6

7 Biomat (Slime) ~1-2 cm Biologically Active Zone ~10 cm Source: Robin Collins 7

8 Almost all biomass located in schmutzdecke Source: Page (1997) 8

9 Ripening Turbidity removal increases with filter runtime Source: Collins et al (1989) 9

10 Riverbank Filtration (RBF) 10

11 Horizontal Collector Well Pumping Tower Background Well laterals River River laterals 11

12 Vertical Well PUMP Background well River River Screen Collector well 12

13 Total Aerobic Spore Concentrations in the River and Riverbank Filtrates Figure 7.9 Total Aerobic Spore Concentrations in the River and the Riverbank Filtrates Total Spore Counts (Colonies Total Spore Counts / (Colonies/ ml) River n=18 W1 n=2 W2 n= 2 W3 n=2 L4 n= Riverbank (Aquifer) Depth (feet) Riverbank (Aquifer) Filtration Depth (ft.) Most spore removal occurs near the riverbed Source: Wang et al. (2002) 13

14 Advantages of SSF and RBF SSF Inexpensive O&M Requires fewer and less skilled operators Requires fewer / no chemicals Produces almost no sludge RBF Multibarrier approach to water treatment Mitigates shock loads 14

15 Microbial Removal Processes Physical / Chemical Straining Adsorption (Transport and Attachment) Biological Predation Inactivation / Death due to presence of exotoxins released by antagonistic organisms Biologically mediated adsorption? 15

16 Objectives 1. Rank various media characteristics and operational conditions wrt E. coli removal. 2. Assess effect of sudden removal of schmutzdecke and filter s ability to recover at varying temperatures. 3. Assess role of schmutzdecke biofilm characteristics (extracellular polysaccharides [EPS], bioactivity, toxicity) on E. coli reduction. 4. Estimate the potential influence of protistan predation. 16

17 E. coli Methodology: Challenge Organism Coliform bacterium Well understood Easily and reproducibly quantified with IDEXX Quanti-Tray MPN method Wells positive for E. coli are yellow and fluoresce under UV Source: IDEXX Corporation 17

18 Methodology: Filter Media Status Biomass: Phospholipids (Findlay et al. 1989; Wang and Summers 1993, Klevens 1995, Page 1997, Mercier 1998) Biological Activity: CO 2 Respiration EPS (Extracellular Polymeric Substances): Carbohydrates (Dubois 1956) Protein (Lowry 1951) 18

19 Methodology: 4.8 cm Inner Diameter Columns Influent Tubing From Pump Effluent Sample Ports Filter Columns Raw Water Constant Flow Tank 19

20 Methodology: 4.8 cm Inner Diameter Columns 20

21 Objective 1 Screening Various Filtration Variables Objective: Rank relative importance of biological filtration characteristics and operational parameters on microbe removal. 4 Parameters at 2 Levels with Duplication 1. Empty Bed Contact Time: 15 min, 60 min 2. Hydraulic Loading Rate: 0.2 m/h, 0.6 m/h 3. Filter Media Size (d 10 ): 0.3 mm, 1.2 mm 4. Extent of Biological Ripening: ripened, virgin 21

22 Objective 1 Acclimator for Ripening Sand Raw Water In Raw Water Constant Flow Tank 22

23 Objective 1 Screening Experiment Results E. coli Log Removal Column # Most Important Results Columns 5 and 8 best performance: 60 min EBCT, small sand Column 1 worst performance: 15 min EBCT, large sand, 2 inches sand depth Scraping invariably reduces performance Ripened Scraped 23

24 Objective 1 Analysis of Variance Ripened Filters Sum of Source DF Squares F Ratio Prob > F % Contribution EBCT ** 39.4 Size ** 35.8 HLR HLR*EBCT Error * = Significant at 90% Confidence Level ** = Significant at 95% Confidence Level 24

25 Objective 1 Analysis of Variance Scraped Filters Source DF Sum of Squares F Ratio Prob > F % Contribution EBCT ** 63.5 HLR * 8.0 Size * 6.7 Size*EBCT Block Size*HLR Error * = Significant at 90% Confidence Level ** = Significant at 95% Confidence Level 1 Block term accounts for differing conditions (raw water, influent E. coli concentration) between two challenges. 25

26 Objective 2 Green Fluorescent Protein (GFP) Labeled E. coli Goals Differentiate between challenge E. coli and natural E. coli GFP-labeled E. coli Investigate distribution of adsorbed E. coli in filter Monitor for detachment 26

27 Objective 2 GFP E. coli Results Distribution of GFP Challenge E. coli Recovered from Two Sand Filters Filter A Filter B 30 1e+4 1e+5 1e+6 GFP Recovered / gram dry weight sand Most Important Results Majority of recovered E. coli in top 7 cm Concentration of E. coli constant in top 7 cm Almost 10 5 GFP E. coli / gdw present even at bottom of 22.5 cm test filter 27

28 Simulation of Schmutzdecke Objective 2 Removal and Effect on Treatment Performance Objectives: Determine removal efficiency after cleaning or scouring schmutzdecke. Determine ability of schmutzdecke to recover from cleaning or scouring under various conditions Temperature Ripening Time Depth of Scour 28

29 9 Sand Raw water 29

30 Objective 2 Challenge After 2 Weeks Ripening in Series Log Removal of Biomass, Top 2.5 cm Challenge After 2 Weeks Ripening in Parallel Log Removal of Biomass, Top 2.5 cm Total Coliforms (nmol PO 4 / gdw) Total Coliforms (nmol PO 4 / gdw) Col A ( cm) ± ±4 Col B ( cm) ± ±2 Col C ( cm) 0.2 9± ±2 Col D ( cm) ± ±2 Full Train (90 cm total)

31 Objective 2 Effect of Temperature on Schmutzdecke Recovery C E lii Effl MPN/ Biomass (as Phospholipids), nmol PO 4 / gdw and CO2 Respiration, mg as Carbon / gdw sand / h Respiration (24 0 C Columns) Biomass (24 0 C Columns) Respiration (8 0 C Columns) Biomass (8 0 C Columns) 31

32 Objective 2 Effect of Temperature on Schmutzdecke Recovery Warm columns recovered more effectively than cold columns Correlation between higher respiration in schmutzdecke and better removal No similar correlation for biomass 32

33 Objective 3 Correlating Biofilm Characteristics to E. coli Removal E. coli Log Removal E. coli Log Removal EPS (μg as Carbon / gdw) Phospholipids (nmol PO4 / gdw) 33

34 Objective 3 Correlating Biofilm Characteristics to E. coli Removal E li Respiration (mg CO 2 as C / gdw/ h) y = x r 2 =

35 Objective 3 Toxicity Influent Effluent Difference E. coli Concentration Col1 Col2 Col3 Col4 Col5 Col6 Col7 Control RO Most Important Result No toxic effects in any microbe-free Schmutzdecke Extract Column Green diamonds = 95% Confidence Intervals All confidence intervals overlap control 35

36 Objective 4 Influence of Protistan Predation Flagellate Abundance: 6 x 10 6 ± 2 x 10 6 per cm 2 in top 5mm Potential E. coli Uptake Rate: 5 x 10 6 ± 2 x 10 6 per cm 2 / hour Actual Removal Rate: 1.3 x 10 2 bacteria / cm 2 / hour 4 orders of magnitude difference Assumptions E. coli used to spike influent water are only bacteria being consumed by flagellates. All flagellates are feeding continuously. Flagellates occupy schmutzdecke uniformly in space. 36

37 Objective 4 Protistan Abundance e+5 1e+4 Most Important Results Removal continues to increase with increased filter runtime. Protistan abundance correlates well to removal Ripening Time (days) E. coli log removal Protists / gdw (in top 5 mm) Protists / Gram 37

38 Objective 4 Effect of Draining Protists per Gram Dry Weight / 10^5 Protists 10 5 Protists x 105 / gdw / gdw Drained drained Undrained undrained Most Important Results Draining does not appear to decrease protistan abundance in schmutzdecke immediately Effect of desiccation was not investigated 38

39 Objective 4 GFP E. coli Mass Balance Term Average of 2 Filters (Std. Dev.) Total In 4.13 x (0.02 x ) Total Out 2.27 x 10 9 (0.76 x 10 9 ) Most Important Results Majority of E. coli were not recovered either in effluent or from filter. Possible Explanations 1. Attach to filter apparatus. Total Retained Total Losses 2.35 x 10 9 (0.65 x 10 9 ) 3.67 x (0.16 x ) 2. Die. 3. Consumed by grazers after attaching. 4. Intercepted and consumed by protists in pore water. 39

40 Objective 4 GFP E. coli Time Series 4 sand HLR = 0.6 m/h Filter # Time Range of Effluent Sampling (hours) Total Extractable GFP on Filter Average GFP / ml in Effluent 1 During challenge x E x E x E x E x E+05 40

41 Objective 4 GFP E. coli Time Series 0.0 Most Important Results D th ( ) Col1 Col2 Col3 Col4 Col5 Total # of bacteria on filter did not decrease significantly over time. Bacteria continued to elute for >24 hours e+7 1.0e+8 1.5e+8 2.0e+8 2.5e+8 3.0e+8 GFP E. coli / Gram Dry Weight Sand 41

42 Objective 4 Protist Seeding Filter 1 No Seed 2 High Seed 3 Low Seed Protists in Seed Solution (# / ml) x x 10 4 (top 5 mm) 7e+6 6e+6 5e+6 4e+6 3e+6 2e+6 1e High Seed 1.3 x 10 5 High Seed Low Seed No Seed High Seed 240 ml seed solution added 42

43 Objective 4 Protist Seeding Most Important Results E. coli Log Removal Filter 1 No Seed Filter 2 High Seed Filter 3 Low Seed Protists were successfully extracted from an operating schmutzdecke. Protists were successfully seeded onto sand filters. No significant difference in E. coli removal was observed between seeded and unseeded columns 43

44 Conclusions For the conditions of this study: 1. Filter Characteristics Assessment: EBCT appeared to be more important than HLR, size, or biological ripening Above an EBCT of 30 minutes, E. coli removal did not significantly improve Majority of bacteria removal occurred in top 7 cm, but some challenge organisms penetrated beyond schmutzdecke 44

45 Conclusions 2. Evaluation of Schmutzdecke Removal: Scraping or scouring dramatically reduced bacterial removal efficiency, but filter recovered in < 4 days Water temperature affected extent of recovery Cold filters (8 0 C) achieved a significantly lower ultimate removal efficiency than warm filters (24 0 C) Most initial attachment of E. coli occurred in schmutzdecke. Captured E. coli detached from schmutzdecke after challenge was stopped 45

46 Conclusions 3. Role of Schmutzdecke Biofilm: Strong correlation between bacterial removal and biological activity Assessment of EPS by total carbohydrate and total protein did not provide strong correlation indicative of biologically mediated attachment Inactivation or death caused by toxins in schmutzdecke was not observed 46

47 Conclusions 4. Role of Protists: Protistan predation may play critical role either by grazing to limit detachment or by interception in pore water, but neither mechanism was confirmed. Draining filters did not immediately decrease number of protists in schmutzdecke. Protists can be seeded onto operating filters. 47

48 Recommendations Compare removal of other challenge microorganisms to E. coli. Investigate other biofilm characteristics besides gross measures of biomass and EPS and gather more data to improve correlations. Examine effect of desiccation on protists by allowing filters to sit after draining. Improve experimental methods for examining effect of seeding with protists. 48

49 Thank you for your attention!