DESIGN ASPECTS OF WATER TREATMENT

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DESIGN ASPECTS OF WATER TREATMENT

• Bob Clement
• Environmental Engineer
• EPA Region 8

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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.

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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.

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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.

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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.

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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.

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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.

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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).

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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 gt 1 NTU).

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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 lt1 to 3 logs of removal.
• Cartridge filters show 3.51 to 3.68 logs of
  removal. Better removal due to pleats.

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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.

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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.

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BAG AND CARTRIDGE FILTRATION

• Monitoring of filter integrity may be needed.
• States to decide on what type of integrity tests
  may be needed.

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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.

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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)

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MEMBRANES

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

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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.

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MEMBRANES

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

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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.

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MEMBRANES

• Filtration Spectrum Overhead

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MEMBRANES

• Ultrafiltration Rejection Mechanisms Overhead

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MEMBRANES

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

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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MICROFILTRATION (MF)

• A 500 micron screen is usually the only
  pretreatment needed.
• Higher levels of pretreatment are needed towards
  RO.

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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.

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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 lt 1 mg/l.
• Turbidity 500 NTU to 0.08 - 0.05 NTU.

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MICROFILTRATION (MF)

• Removal levels for MF (continued)
• Silt density index (SDI) over 5 to lt 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.

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MICROFILTRATION (MF)

• Removal levels for MF (continued)
• Microorganisms 105 colony forming units (cfu)/ml
  to lt 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.

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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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MICROFILTRATION (MF)

• Removal levels for MF (continued)
• The mechanism of removal appears to be related to
  three factors physical sieving/adsorption, cake
  layer formation and changes in the fouling state
  of the membrane.
• The highest log removal was attributable to
  fouling. The remaining virus removal, to 4 log
  removal/inactivation, is achieved through
  disinfection.

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MEMBRANES

• SEM Overheads

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MEMBRANES

• SEM Overheads

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MICROFILTRATION (MF)

• MF is a low pressure membrane (20-35 psi). High
  pressure RO membranes can require pressures of
  greater than 300 psi.
• Recovery for MF is 90. Recovery decreases
  towards RO and the waste streams increase
  significantly towards RO.
• Range of flow 0.6 to 22 MGD.

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MICROFILTRATION (MF)

• For the Town of Winchester a 1.0 MGD MF plant was
  estimated to cost 1.5 M. If financed at 8
  interest over a 20-year period, the annual dept
  would be 152,820. Therefore, the capital costs
  were 0.42 capital per 1000 gallons. The
  operating costs were 0.165 operation per 1000
  gallons and included power for pumps and
  compressors, chlorine, membrane replacement
  (22,500 per year) and cleaning chemicals (4000
  per year).

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MICROFILTRATION (MF)

• Membrane life for MF is 3-5 years.
• Backwash volume for MF is 6 for low turbidity
  up to 12 for high turbidity. Gas backwash is
  very efficient in removing foulants.

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MICROFILTRATION (MF) MAINTENANCE

• Cleaning is usually done with a 2 mixture of a
  caustic detergent every 30 days and takes less
  than 3 hours to complete.
• The cleaning solution is recovered and reused.

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MICROFILTRATION (MF) MAINTENANCE

• A citric acid cleaning following the caustic has
  been found to be effective in cleaning membranes
  with high hardness and/or iron.
• If no pretreatment chemicals are used, the spent
  cleaning fluid is the only waste stream requiring
  special attention.

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MICROFILTRATION (MF)

• Automatic membrane integrity tests are based on
  the principle that air pressure must overcome the
  capillary resistance before an intact membrane
  leaks. An integral module will exhibit little,
  if any, decay over the test period.

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MICROFILTRATION (MF)

• The hollow tube configuration is the most widely
  used format for membrane construction due to its
  bi-directional strength which makes backwashing
  possible.
• The hollow tube maximizes the available
  filtration surface area within the smallest
  physical area. Materials of construction for
  membranes can be polymeric or ceramic.

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MICROFILTRATION (MF)

• There must be redundancy of units in case one of
  the units fails, or is being cleaning, or is
  undergoing membrane replacement.
• One company has installed 44 MF systems
  nationwide.

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MICROFILTRATION (MF)

• The largest MF facility is the 5 MGD plant at San
  Jose California. It is an unmanned plant.
• The installation is successful but is
  mechanically complex with 100 automatic valves
  and more than 7,000 connections that require
  o-rings to achieve a tight water seal.