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Slow Sand Filtration

Slow Sand Filtration

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Slow Sand Filtration

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  1. Slow Sand Filtration • The Slow Sand Filter Mystery • Major Events in Slow Sand Filtration History • Research at Cornell • Particle Removal Mechanisms • Search for the Mystery Compound • SSF research by CEE 453

  2. Slow Sand Filtration • An old technology that is poorly understood • Particle removal improves with time! • Until recently no one knew how particles were removed by slow sand filters • The mystery is not yet solved • Potential for new useful knowledge

  3. Slow Sand Filter Schematic A. Valve for raw water inlet and regulation of filtration rate B. Valve for draining unfiltered water C. Valve for back-filling the filter bed with clean water D. Valve for draining filter bed and outlet chamber E. Valve for delivering treated water to waste F. Valve for delivering treated water to the clear-water reservoir A Filter Cake B Sand F E D Gravel Underdrains C

  4. Slow Sand Filtration:A Brief History • 1790 - SSF used in Lancashire, England to provide clean water for textile industry • 1829 - SSF used to filter municipal water (London) • 1850: John Snow established the link between drinking water (from a contaminated well) and Cholera • 1885- SSF shown to remove bacteria • 1892 - Cholera outbreak in Hamburg, Altoona saved by slow sand filters • 1980s - Giardia shown to be removed by SSF • 1990s - Cryptosporidium not always removed by SSF

  5. Bioengineering in the 1800's • In 1885 Percy F. Frankland used the recently devised 'gelatin process' of Robert Koch to enumerate bacteria in raw and filtered water • Frankland showed that filtration reduced the average bacteria concentration from Thames water 97.9% “It is most remarkable, perhaps, that these hygienically satisfactory results have been obtained without any knowledge on the part of those who construct these filters, as to the conditions necessary for the attainment of such results.” (Percy F. Frankland)

  6. 1892 Cholera outbreak in Hamburg, Germany Hamburg • Large outbreak of Cholera in Hamburg • 17,000 cases; 8,600 deaths • Very few cases in neighborhoods served by Altoona's filtered water supply • Hamburg's sewers were upstream from Altoona's intake! Altoona Altoona's water intake Hamburg's sewer and filter beds outfalls Elbe River Hamburg's water intake

  7. The Challenge of the 1990's: Cryptosporidiosis • Milwaukee (March 1 to April 10 1993): an estimated 370,000 city residents suffered from diarrhea, nausea, and stomach cramps caused by Cryptosporidiosis • Evidence suggests that a substantial proportion of non-outbreak-related diarrheal illness may be associated with consumption of water that meets all current water quality standards • Slow sand filters shown to remove less than 50% of Cryptosporidium oocysts at an operating plant in British Columbia

  8. In Search of the Secret in the 1990's • How do slow sand filters remove particles including bacteria, Giardia cysts, and Cryptosporidium oocysts from water? • Why don’t SSF always remove Cryptosporidium oocysts? • Is it a biological or a physical/chemical mechanism? • Would it be possible to improve the performance of slow sand filters if we understood the mechanism?

  9. by medium Straining (fluid and gravitational by forces) previously removed particles Physical-Chemical to medium Attachment (electrochemical Particle to previously forces) Removal removed Mechanisms particles Attachment to biofilms Biological Suspension feeders Capture by predators Grazers Particle Removal Mechanisms

  10. Slow Sand Filtration Research Apparatus Manometer/surge tube Cayuga Lake water (99% or 99.5% of the flow) Manifold/valve block Peristaltic pumps Sampling Chamber Auxiliary feeds (each 0.5% of the flow) Sampling tube Lower to collect sample To waste 1 liter E. coli feed 1 liter sodium azide Filter cell with 18 cm of medium

  11. Quiescent Cayuga Lake water 1 Sodium azide (3 mM) Control 0.1 0.05 0 2 4 6 8 10 Time (days) Biological and Physical/Chemical Filter Ripening Continuously mixed Cayuga Lake water 1 Fraction of influent E. coli remaining in the effluent 0.1 0.05 0 1 2 3 4 5 Time (days)

  12. Biological Poison Fraction of influent E. coli remaining in the effluent

  13. Effluent Mystery Particles Effluent particle count (Dnumber/µl/Dparticle diameter)

  14. Chrysophyte long flagellum used for locomotion and to provide feeding current short flagellum 1 µm stalk used to attach to substrate (not actually seen in present study)

  15. Chrysophyte Culture

  16. Chrysophyte Inoculum Mechanisms

  17. Particle Removal by Size

  18. Biological Mechanisms • The biological activity of microorganisms being removed in the filter column was not significant • The biological activity of the filter biopopulation was only significant for removal of particles smaller than 2 µm. • Biofilms were expected to increase removal of particles larger than 2 µm as well by increasing the attachment efficiency. The lack of biologically enhanced removal of particles larger than 2 µm suggested that “sticky” biofilms did not contribute significantly to particle removal.

  19. Biological Mechanisms • The immediate and reversible response of slow sand filters to sodium azide was consistent with bacterivory and inconsistent with particle removal by biofilms. • Biologically mediated mechanisms together with physical-chemical mechanisms accounted for removal of particles smaller than about 2 µm in diameter. In this research bacterivory was the only significant biologically mediated particle removal mechanism. Mechanisms

  20. Filter with Few Particles in Influent Day 5

  21. Filters with Many Particles in Influent Day 5

  22. Physical-Chemical Particle Removal Mechanisms • Physical-chemical particle removal mechanisms are significant in slow sand filters. • Physical-chemical particle removal efficiency was greatest when particles previously had been retained by the filter (within the bed or in the filter cake) and was considered to be caused by attachment of particles to retained particles. • Further work is necessary to determine what types of particles are most effective for filter ripening. Mechanisms

  23. Sludge from Bolton PointEureka! CEE 453 1997

  24. ? Sludge from Bolton Point = Alum(oops) CEE 453 1998 C/Co Time (minutes)

  25. Research project 2000 • Successfully extracted a coagulant from Cayuga Lake Seston using 1.0 N HCl • The CLSE fed filters removed up to 99.9999% of the influent coliforms. • Analysis of the CLSE • Nonvolatile solids was 44% of the TSS • Volatile solids was 56% of the TSS • Aluminum was dominant metal

  26. CLSE Experiment 2001 • Groups of 4 • Assemble filter apparatus • Measure head loss, flow rate, turbidity • Coat filter with CLSE • Observe _______________ • Challenge filter with kaolin • Observe ________and _______ • Control? increased head loss head loss turbidity

  27. Raw Water Apparatus