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DESIGN ASPECTS OF WATER TREATMENT

DESIGN ASPECTS OF WATER TREATMENT. Bob Clement Environmental Engineer EPA Region 8. SLOW SAND FILTRATION(SS).

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DESIGN ASPECTS OF WATER TREATMENT

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  1. DESIGN ASPECTS OF WATER TREATMENT Bob Clement Environmental Engineer EPA Region 8

  2. SLOW SAND FILTRATION(SS) • An alternate BAT for complying with the SWTR is SS. SS is a biological process that requires sufficient natural organic matter (NOM) to provide a nutrient supply to the biological mat.

  3. SLOW SAND FILTRATION (SS) • SS requires influent water that does not exceed the following parameters: • Turbidity of less than 10 NTU. • Color of less than 30 units. • Algae of less than 5 mg per cubic meter of chlorophyll A.

  4. SLOW SAND FILTRATION (SS) • SS is 50 to 100 times slower than normal filtration. • SS requires smaller sand particles (smaller pore spaces), effective size 0.25 to 0.35 mm, with a uniformity coefficient of 2 to 3. • Start-up of a SS may take as long as 6 months to develop the initial biological mat.

  5. SLOW SAND FILTRATION (SS) • SS filters perform poorly for 1 to 2 days after filter cleaning, called the “ripening period.” The ripening period is the time required by the filter after a cleaning to become a functioning biological filter. This poor water quality requires a filter- to-waste cycle.

  6. SLOW SAND FILTRATION (SS) • Because of the length of time required for cleaning and ripening, redundant SS filters are needed. • The biggest enemy to a biological mat is the lack of moisture. Therefore, a SS filter must always be submerged.

  7. SLOW SAND FILTRATION (SS) • Initial headloss is about 0.2 feet, maximum headloss should be no more than 5 feet to avoid air binding and uneven flow of water through the filter medium. • SS filters should be enclosed in a building so that they can be cleaned in the winter months and avoid ice buildup.

  8. SLOW SAND FILTRATION (SS) • The housing should also be light free to eliminate algae growth. Regardless of the type of filtration technology used, design should consider ways to minimize algae growth (e. g., sed basins housed with no outside light).

  9. SLOW SAND FILTRATION (SS) • The normal length of time between cleanings is 20 to 90 days. Cleaning involves scraping manually 1 to 2 inches and discarding the sand. Another method of cleaning is called harrowing and uses a very low backwash rate while manually turning the media. New sand should be added when sand depth approaches 24 inches, approximately every 10 years.

  10. SLOW SAND FILTRATION (SS) • No chemical pretreatment is done for SS. SS has been successfully used in South America treating waters with greater than 1000 NTUs when roughing filters are used. • Capital costs may be higher, but operational costs are lower.

  11. DIATOMACEOUS EARTH (DE) FILTRATION • DE is composed of siliceous skeletons of microscopic plants called diatoms. Their skeletons are irregular in shape therefore particles interlace and overlay in a random strawpile pattern which makes it very effective for Giardia and crypto removal.

  12. DIATOMACEOUS EARTH (DE) FILTRATION • Difficulty in maintaining a perfect film of DE of at least 0.3 cm (1/8 in) thick has discouraged widespread use of DE except in waters with low turbidity and low bacteria counts. • The minimum amount of filter precoat should be 0.2 lb/ft2 and the minimum thickness of precoat should be 0.5 to enhance cyst removal.

  13. DIATOMACEOUS EARTH (DE) FILTRATION • The use of a alum (1 to 2% by weight) or cationic polymer (1 mg per gram of DE) to coat the body feed improves removal of viruses, bacteria and turbidity but not necessarily Giardia.

  14. DIATOMACEOUS EARTH (DE) FILTRATION • Continuous body feed is required because the filter cake is subject to cracking. Also, if there is no body feed there will be a rapid increase in headloss due to buildup on the surface.

  15. DIATOMACEOUS EARTH (DE) FILTRATION • Interruptions of flow cause the filter cake to fall off the septum, therefore, precoating should be done any time there are operating interruptions to reduce the potential for passage of pathogens.

  16. DIATOMACEOUS EARTH (DE) FILTRATION • Body feed rates must be adjusted for effective turbidity removal. Filter runs range from 2 to 4 days depending on the rate of body feed and DE media size.

  17. DIATOMACEOUS EARTH (DE) FILTRATION • An EPA study showed greater than 3.0 log removal for Giardia for all grades of DE. Whereas the percent reduction in TC bacteria, HPC, and turbidity were strongly influenced by the grades of DE used.

  18. DIATOMACEOUS EARTH (DE) FILTRATION • For example the coarsest grades of DE will remove 95 percent of cyst size particles, 20-30 percent of coliform bacteria, 40-70 percent of HPC and 12-16 percent of the turbidity.

  19. DIATOMACEOUS EARTH (DE) FILTRATION • The use of the finest grades of DE or alum coating on the coarse grades will increase the effectiveness of the process to 3 logs bacteria removal and 98 percent removal for turbidity.

  20. DIATOMACEOUS EARTH (DE) FILTRATION • Systems in Wyoming have shown as high as six logs of microorganism removal, whereas others have shown negative log removal for particles which might be the media passing the septum.

  21. OTHER FILTRATION TECHNOLOGIES • These include cartridge, bag, membranes, and other types of filters. • You must be able to prove to the state that they will meet state regulatory requirements. These may include studies on performance for turbidity removal, Giardia, crypto and virus removal through pilot studies.

  22. BAG AND CARTRIDGE FILTRATION • Units are compact. • Operates by physically straining the water -- to 1.0 micron. • Made of a variety of material compositions depending on manufacturer. • Pilot testing necessary.

  23. BAG AND CARTRIDGE FILTRATION • Depending on the raw water quality different levels of pretreatment are needed: • Sand or multimedia filters. • Pre-bag or cart. of 10 microns or larger. • Final bag or cart. of 2 microns or less. • Minimal pretreatment for GWUDISW.

  24. BAG AND CARTRIDGE FILTRATION • Units can accommodate flows up to 50 gpm. • As the turbidity inc the life of the filters dec (e.g., bags will last only a few hours with turbidity > 1 NTU).

  25. BAG AND CARTRIDGE FILTRATION • Both filters have been shown to remove at least 2.0 logs of Giardia Lamblia but for crypto: • Bags show mixed results <1 to 3 logs of removal. • Cartridge filters show 3.51 to 3.68 logs of removal. Better removal due to pleats.

  26. BAG AND CARTRIDGE FILTRATION • In an MS-2 Bacteriophage challenge study no virus removal was achieved. Therefore, there must be enough disinfection contact time to exceed 4.0 logs of inactivation of viruses for both filters.

  27. BAG AND CARTRIDGE FILTRATION • Factors causing variability in performance: • The seal between the housing and filters is subject to leaks especially when different manufacturers housings and filters are used. • Products use nominal pore size (average) rather than absolute pore size. 2 um or less absolute should be used.

  28. BAG AND CARTRIDGE FILTRATION • Monitoring of filter integrity may be needed. • States to decide on what type of integrity tests may be needed.

  29. BAG AND CARTRIDGE FILTRATION • For a conventional or direct filtration plant that is on the borderline of compliance installing bag/cart filtration takes the pressure off by increasing the turbidity level to 1 NTU and increases public health protection by applying two physical removal technologies in series. Check with State Drinking Water programs.

  30. MEMBRANES • Many investigations in the last decade have shown that membrane filtration are very powerful treatment processes. Membranes have been utilized commercially for over 25 years. There are four membrane technology groups: • Reverse Osmosis (RO) • Nanofiltration (NF) • Ultrafiltration (UF) • Microfiltration (MF)

  31. MEMBRANES • Reverse Osmosis (RO) used for desalination and specific inorganic contaminant removal. Excludes atoms and molecules < 0.001 microns--the ionic range.

  32. MEMBRANES • Nanofiltration (NF) used for softening and natural organic matter removal (best technology for meeting the DBP rule). Excludes molecules greater than 0.001 microns in size--multivalent ion range.

  33. MEMBRANES • Ultrafiltration (UF) used for organic and protein removal. Excludes molecules greater than 0.005 microns in size--molecular weight cutoff ~10,000.

  34. MEMBRANES • Microfiltration (MF) used for particles, suspended solids, bacteria and cyst removal. Excludes particles and molecules greater than 0.2 microns--the macro molecular range.

  35. MEMBRANES • Filtration Spectrum Overhead

  36. MEMBRANES • Ultrafiltration Rejection Mechanisms Overhead

  37. MEMBRANES • Conventional filtration can remove particles down to 1.0 micron--the micro and macro particule range.

  38. MICROFILTRATION (MF) • MF is a physical separation (sieving) process and removes all particles greater than 0.2 microns (1 x 10-6 meters). Excludes molecules greater in size than 200,000 molecular weight cutoff.

  39. MICROFILTRATION (MF) • MF is easy to operate and produces greater than 6 logs of removal for protozoans. • With Programmable Logic Controllers they can be left unattended with only periodic monitoring and data logging.

  40. MICROFILTRATION (MF) • The advantage is that filter quality is achieved irrespective of changes in turbidity, microorganism burden, algae blooms, pH, temperature, or operator interaction. • Conventional treatment is cumbersome and is operator intensive compared to microfiltration.

  41. MICROFILTRATION (MF) • Membrane systems lose operational performance such as increasing pressure differentials across the membrane and shortening of the cleaning frequency, instead of compromising finished water quality.

  42. MICROFILTRATION (MF) • The biggest concern is failure of the membrane since it is a single barrier, whereas filtration is multi-barrier. Consider bag filtration as a backup barrier for a failed membrane.

  43. MICROFILTRATION (MF) • MF is compact, the building and area needed for installation is small. • MF reduces the dosage of chlorine needed due to the reductions of microorganisms and chlorine demand.

  44. MICROFILTRATION (MF) • MF with a molecular weight cutoff of 200 can remove DBP precursors greater than 90%. • MF can achieve a 10% reduction of Disinfection Byproduct (DBP) Precursors. • MF used in conjuncture with coagulants can obtain DBP removals similar to a conventional plant.

  45. MICROFILTRATION (MF) • A 500 micron screen is usually the only pretreatment needed. • Higher levels of pretreatment are needed towards RO.

  46. MICROFILTRATION (MF) • For RO and NF systems to operate economically, suspended solids, microorganisms, and colloids have to be removed before these technologies can effectively remove dissolved contaminants.

  47. MICROFILTRATION (MF) • Removal levels for microfiltration: • Acceptable range of raw water pH 2-14. • pH adjustments are not required for scaling control, since MF does not remove uncomplexed dissolved ions. • Suspended solids 200 mg/l to < 1 mg/l. • Turbidity 500 NTU to 0.08 - 0.05 NTU.

  48. MICROFILTRATION (MF) • Removal levels for MF (continued) • Silt density index (SDI) over 5 to < 1.0. An SDI of less than 1.0 means that the fouling rate potential is low. MF is recognized as the most appropriate technology for pretreatment for RO. Fouling susceptibly increases towards RO.

  49. MICROFILTRATION (MF) • Removal levels for MF (continued) • Microorganisms 105 colony forming units (cfu)/ml to < 1 cfu/ml. Bacteria are typically greater than 0.2 microns in size. This includes algae removal. • Crypto & Giardia 106 cysts/100ml to none detected. Size exclusion is the major mechanism of removal, and is an absolute barrier as long as the membrane is intact.

  50. MICROFILTRATION (MF) • Removal levels for MF (continued) • Viruses 103 plaque forming unit (pfu)/100ml to 101 pfu/100ml. • Viruses are usually smaller in size than 0.2 microns (MS2 phage is 0.027 microns).

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