Drinking Water Treatment Overview Filtration and Disinfection

1
Drinking Water Treatment OverviewFiltration and
Disinfection

• April 16th, 2009
• Yousry Hamdy, M.Sc., P.Eng
• Nicolás Peleato, EIT

2
Table of Contents

• Introduction
• Filtration
• Cartridge filters
• Chemical assisted filtration
• Slow sand filters
• Diatomaceous earth
• Activated carbon
• Membrane filters
• Disinfection
• Chlorine
• Chloramines
• Ultraviolet Radiation

3
IntroductionPurpose of Treatment

• The objective is to provide safe drinking water
  to consumers that meet the Provincial Drinking
  Water Quality Standards (O.Reg. 169/03)
• Treatment of source water is aimed at
• Achieving (O.Reg 170)
• 4 log removal for viruses,
• 3 log removal for Giardia lamblia,
• 2 log removal of Cryptosporidium
• Reduction of harmful chemical compounds to meet
  the Standards

4
IntroductionTreatment Elements

• Treatment can generally be broken up into three
  aspects
• Filtration
• Physically removes contaminants, physical
  impurities, micro-organisms
• Primary Disinfection
• Inactivates micro-organisms
• Secondary Disinfection
• Ensures continual disinfection and prevents the
  re-growth of micro-organisms during distribution

5
FiltrationCartridge Filters
Crypt. Credit 2-log Giardia Credit 2-log Virus
Credit 0

• Small in-line filters which physically block
  particulates and contaminants
• Typical 5 micron pore size (can be less)
• Can be constructed from several materials,
• Ceramics
• Wool, paper, woven fibers
• Carbon
• Depending on the filter material they are either
  disposable or can be washed and reused

6
FiltrationCartridge Filters
Crypt. Credit 2-log Giardia Credit 2-log Virus
Credit 0

• Some also contain ion exchange resins for
  reduction of aqueous ions (iron, lead, copper,
  etc.)

Cartridge
Housing
7
FiltrationCartridge Filters
Crypt. Credit 2-log Giardia Credit 2
log Virus Credit 0

• To claim removal credits,
• Raw water must have turbidity less than 5 NTU,
  colour less than 5 TCU
• Use filter elements and housing certified for
  surrogate particle removal evaluation in
  accordance with testing procedures and
  manufacturing quality control specified in
  ANSI/NSF Standard 53 or equivalent

8
FiltrationCartridge Filters
Crypt. Credit 2-log Giardia Credit 2
log Virus Credit 0

• To claim removal credits (contd),
• Ensure that differential pressures across the
  filter medium do not exceed manufacturers rating
  and materials coming in contact with water
  conform to ANSI/NSF Standard 61.
• Turbidity of treated water should be at least
  monitored daily (continuous recommended)
• Monitoring results should indicate turbidity less
  than 0.2 NTU for 95 of measurements in a month

8
9
FiltrationCartridge Filters
Crypt. Credit 2-log Giardia Credit 2
log Virus Credit 0

• Advantages
• Easy to operate and maintain
• Issues
• Only suitable for low turbidity sources
• Cartridges can foul quickly and need to be
  replaced

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FiltrationChemical Assisted
Crypt. Credit 2.0-log Giardia Credit
2.5-log Virus Credit 2.0-log

• The physical filtration process is preceded by
  flocculation/coagulation where a chemical allows
  for suspended particles to aggregate and form a
  larger particle
• Process is initiated by the addition of a
  flocculant,
• Aluminum sulfate (Alum)
• Ferric sulfate
• Various polymers
• Others

11
FiltrationChemical Assisted
Crypt. Credit 2.0-log Giardia Credit
2.5-log Virus Credit 2.0-log

• Four steps to the process
• Charges on suspended particles are neutralized to
  allow them to aggregate
• Rapid mixing to eliminate concentration gradients

Coagulation
Sedimentation
Filtration
Flocculation
Untreated Water
Addition of flocculant/coagulant

• Particles aggregate into flocs (aided by the
  chemical)
• Gentle mixing as to promote floc growth without
  breaking them

• Flocs are allowed to settle by gravity. The
  process can be sped up by using inclined settling
  plates.

• Water is then typically passed through sand bed
  filters to remove any remaining flocs and
  provides further filtration
• Filtration process is generally considered rapid,
  gt 4 m/h velocity

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FiltrationChemical Assisted
Crypt. Credit 2.0-log Giardia Credit
2.5-log Virus Credit 2.0-log

• Advantages
• Highly effective filtration process
• Flow rate through filters is high
• Disadvantages
• High operational duty between adjusting the
  flocculation/coagulation processes and filter
  backwash
• Large equipment footprint

13
FiltrationChemical Assisted
Crypt. Credit 2.0-log Giardia Credit
2.5-log Virus Credit 2.0-log

• To be effective,
• Chemical coagulant/flocculant has to be used at
  all times and dosage must be adjusted in response
  to raw water quality fluctuations
• Sand filters must be backwashed regularly
• Filtrate turbidity must be continuously monitored
• Filtered water should be less than 0.3 NTU for
  95 of the measurements each month

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FiltrationDirect Filtration
Crypt. Credit 2.0-log Giardia Credit
2.0-log Virus Credit 1.0-log

• The sedimentation step of chemically assisted
  filtration can be skipped and is then considered
  direct filtration
• Only used when turbidity is less than 20 NTU
• Must meet criteria for chemical assisted
  filtration to claim credits.

15
FiltrationDirect Filtration
Crypt. Credit 2.0-log Giardia Credit
2.0-log Virus Credit 1.0-log

• Advantages
• Effective filtration process
• Flow rate through filters is reasonably high
• Need for a sedimentation tank (or time) is
  eliminated
• Disadvantages
• High operational duty between adjusting the
  flocculation/coagulation processes and filter
  backwash
• Only suitable for low turbidity waters which are
  seasonally consistent

16
FiltrationSlow Sand
Crypt. Credit 2.0-log Giardia Credit
2.0-log Virus Credit 2.0-log

• The slow sand filtration is both a physical and
  biological process
• Slow sand filters differ from rapid sand filters,
• Velocities of 0.1 0.4 m/s
• Microbial growth is promoted at the top of the
  filter
• Backwashing procedures cannot be carried out
• Cleaning is typically performed by removing and
  discarding the top layer of sand
• Several weeks are needed to allow for the
  microbial layer to form (before filter can be
  used)

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FiltrationSlow Sand
Crypt. Credit 2.0-log Giardia Credit
2.0-log Virus Credit 2.0-log

• To be effective,
• An active biological layer must be maintained
• The filter must be cleaned regularly
• Filtrate turbidity must be monitored
• Filtered water should be less than 1 NTU for 95
  of the measurements in a month

18
FiltrationSlow Sand
Crypt. Credit 2.0-log Giardia Credit
2.0-log Virus Credit 2.0-log

• Advantages
• Low cost
• Simple operation, reliable
• Does not require extensive control
• Issues
• Not suitable for high turbidity waters
• Filter surface needs regular maintenance
• Large equipment footprint due to low flow rate

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FiltrationDiatomaceous Earth
Crypt. Credit 2.0-log Giardia Credit
2.0-log Virus Credit 1.0-log

• Skeletal remains of small single celled organisms
  (diatoms)
• Used as filter media
• A thin layer of DE is formed on a septum and
  water is passed through
• DE is well suited for small systems because,
• Low cost
• Chemical coagulation/flocculation is not needed

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FiltrationDiatomaceous Earth
Crypt. Credit 2.0-log Giardia Credit
2.0-log Virus Credit 1.0-log
21
FiltrationDiatomaceous Earth
Crypt. Credit 2.0-log Giardia Credit
2.0-log Virus Credit 1.0-log

• To be effective,
• A minimum thickness of the pre-coat must be
  maintained (32mm)
• Discharge or recycle water involved in the
  pre-coat process
• Continuously monitor filtrate turbidity
• Filtered water must be less than 1.0 NTU for 95
  of the measurements in a month

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FiltrationDiatomaceous Earth
Crypt. Credit 2.0-log Giardia Credit
2.0-log Virus Credit 1.0-log

• Advantages
• Effective filtration process
• Simple to operate
• Low initial capital cost
• Issues
• Only suitable for source waters of low turbidity
  and bacterial counts (turbidity lt 20 NTU)
• Potentially difficult to maintain pre-coat

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FiltrationMembrane Filtration
Crypt. Credit 3.0-log Giardia Credit - Virus
Credit 2-3 log

• Involves passing water through a thin synthetic
  semi-permeable membrane
• Many different types generally categorized by
  their selectivity (size of retained particles)
• Micro-filtration gt 100 nm
• Ultra-filtration gt 10 nm
• Nano-filtration gt 1 nm
• Reverse Osmosis lt 1 nm
• Can require high pressures to achieve reasonable
  flow rates (depends on type of membrane)

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FiltrationMembrane Filtration
Crypt. Credit 3.0-log Giardia Credit - Virus
Credit 2-3 log

• Removal capability depend on the manufacturer and
  type of membrane (must be certified)
• To be effective,
• The membrane must be backwashed or cleaned
  regularly
• Monitor membrane integrity (through particle
  count, pressure decay measurements, etc.)
• Continuously monitor filtrate turbidity
• Filtered water must be less than 0.1 NTU (99 of
  the time)

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FiltrationMembrane Filtration
Crypt. Credit 3.0-log Giardia Credit - Virus
Credit 2-3 log

• Advantages
• Small equipment footprint
• Automated operation
• Issues
• Membrane fouling can be a significant problem
  depending on the quality of source water

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FiltrationActivated Carbon
Crypt. Credit - Giardia Credit - Virus
Credit -

• A type of filter media that can be used
  (partially or fully) in bed filters, cartridge
  filters, or as a powder
• Processed carbon which results in a structure
  with very high surface area
• 1 g of activated carbon has a surface area of
  apprx. 500 m2
• Adsorbs particulates, metal ions, micro-organisms
  
• Must be recharged or replaced periodically

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FiltrationActivated Carbon
Crypt. Credit - Giardia Credit - Virus
Credit -

• Sometimes added in powdered format (PAC) during
  seasonal taste and odour events.
• PAC adsorbs contaminants and can then be filtered

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FiltrationActivated Carbon
Crypt. Credit - Giardia Credit - Virus
Credit -

• Advantages
• Powerful non-selective adsorbent
• Can be recycled and reused
• Issues
• Must be recharged periodically
• More expensive than typical medias

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Summary
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Disinfection Chlorine

• Most common method of disinfecting water
• Can be applied by using,
• Chlorine gas
• Sodium hypochlorite
• Calcium hypochlorite
• Electrochemical process
• Disinfection is caused by free chlorine

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Disinfection Chlorine

• Inactivates a broad range of pathogens through
  oxidation
• Can be considered both a primary and secondary
  disinfectant since free chlorine persists for
  extended periods of time
• The extent of disinfection is determined by the
  concentration of residual chlorine and the amount
  of time it is in contact with the water.
• Contact chambers are used to increase contact
  time
• Log credits calculated through the CT
  disinfection concept

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Disinfection Chlorine

• CT disinfection concept
• CT concentration (mg/L) x contact time
  (minutes)
• Disinfectant residual concentration is measured
  at the end of a treatment step
• CT requirement table example (virus inactivation)

33
Disinfection Chlorine

• Chlorine is effective at removing almost all
  pathogens and providing secondary disinfection
• Chlorine (gas)
• Cheapest based on available chlorine
• Very dangerous gas and must be handled with care
• Sodium hypochlorite (liquid solution)
• More expensive
• Easier to handle than gas (but still corrosive)

34
Disinfection Chlorine

• Calcium hypochlorite (solid)
• Very stable and can be stored for a year or more
• Corrosive
• Reaction between calcium hypochlorite and organic
  materials can generate enough heat to start a
  fire
• Readily absorbs moisture and releases chlorine
  gas

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Disinfection Chloramination

• Combination of chlorine and ammonia
• Can form several species of chloramines however
  monochloramine is desirable
• Promoted using mass ratio of 4.51
  (chlorineammonia)
• Not suitable for primary disinfection
• More persistent and less reactive than free
  chlorine,
• Better for large distribution systems
• Reduces number of disinfection by-products formed
  in distribution system

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Disinfection Chloramination

• Advantages
• Results in fewer DBPs formed in distribution
  system
• More persistent residual (good for distribution
  systems with high retention times)
• Issues
• Weak disinfectant
• Disagreeable taste and odour

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Disinfection Ultraviolet Radiation

• Application of UV radiation provides primary
  disinfection by inactivating the reproductive
  abilities of micro-organisms
• Provides no residual (no secondary disinfectant)
• Disinfection depends on the intensity of the
  radiation, transmittance, and contact time
• Dose is calculated at a wavelength of 254nm
• Minimum of 40 mJ/cm2 (flux)

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Disinfection Ultraviolet Radiation

• UV systems must be certified by the NSF, EPA, or
  equivalent organizations
• Systems must have an alarm and auto-shut off
  feature
• If dose falls below set-point
• If turbidity exceeds manufacturer standards
  causing low transmittance of UV

39
Disinfection Ultraviolet Radiation
39
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Disinfection Ultraviolet Radiation

• Advantages
• Produces no known toxic residuals
• Requires very short contact times
• Easy to operate and maintain
• UV lamps only need to be replaced every one or
  two years
• Effective disinfectant
• Issues
• Source water must have high transmittance at 254
  nm