Drinking Water Treatment Overview Filtration and Disinfection

Transcription

1 Drinking Water Treatment Overview Filtration and Disinfection April 16 th, 2009 Yousry Hamdy, M.Sc., P.Eng Nicolás s Peleato, EIT 1

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 2

3 Introduction Purpose of Treatment The objective is to provide safe drinking water to consumers that meet the Provincial Drinking Water Quality Standards (O.Reg( 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 3

4 Introduction Treatment Elements Treatment can generally be broken up into three aspects Filtration Physically removes contaminants, physical impurities, micro-organisms organisms Primary Disinfection Inactivates micro-organisms organisms Secondary Disinfection Ensures continual disinfection and prevents the re-growth of micro-organisms organisms during distribution 4

5 Cartridge 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 5

6 Cartridge 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 6

7 Cartridge 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; 7

8 Cartridge Filters Crypt. Credit 2-log Giardia Credit 2+ log Virus Credit 0 To claim removal credits (cont d), Ensure that differential pressures across the filter medium do not exceed manufacturer s 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 Cartridge 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 9

10 Chemical Assisted Crypt. Credit Giardia Credit Virus Credit 2.5-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 10

11 Chemical Assisted Four steps to the process Crypt. Credit Giardia Credit Virus Credit 2.5-log Coagulation Flocculation Sedimentation Filtration Untreated Water Addition of flocculant/coagulant Charges Particles Flocs Water are is then on aggregate allowed suspended typically to into settle passed particles flocs by (aided gravity. through are by neutralized The sand the process chemical) bed to filters allow can to be them to Gentle sped remove aggregate up mixing any by using remaining as to inclined promote flocs settling and floc provides growth plates. without further breaking filtration Rapid them Filtration mixing process to eliminate is generally concentration considered gradients rapid, > 4 m/h velocity 11

12 Chemical Assisted Crypt. Credit Giardia Credit Virus Credit 2.5-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 12

13 Chemical Assisted Crypt. Credit Giardia Credit Virus Credit 2.5-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 13

14 Direct Filtration Crypt. Credit Giardia Credit 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. 14

15 Direct Filtration Crypt. Credit Giardia Credit 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 15

16 Slow Sand Crypt. Credit Giardia Credit Virus Credit The slow sand filtration is both a physical and biological process Slow sand filters differ from rapid sand filters, Velocities of 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) 16

17 Slow Sand Crypt. Credit Giardia Credit Virus Credit 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 17

18 Slow Sand Crypt. Credit Giardia Credit Virus Credit 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 18

19 Diatomaceous Earth Crypt. Credit Giardia Credit 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 19

20 Diatomaceous Earth Crypt. Credit Giardia Credit Virus Credit 1.0-log 20

21 Diatomaceous Earth Crypt. Credit Giardia Credit 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 21

22 Diatomaceous Earth Crypt. Credit Giardia Credit 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 < 20 NTU) Potentially difficult to maintain pre-coat 22

23 Membrane Filtration Involves passing water through a thin synthetic semi-permeable membrane Many different types generally categorized by their selectivity (size of retained particles) Micro-filtration Ultra-filtration Nano-filtration Reverse Osmosis Crypt. Credit Giardia Credit - Virus Credit > 100 nm > 10 nm > 1 nm < 1 nm 3.0-log+ 2-3 log Can require high pressures to achieve reasonable flow rates (depends on type of membrane) 23

24 Membrane 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) 24

25 Membrane 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 25

26 Activated 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 m 2 Adsorbs particulates, metal ions, micro-organisms organisms Must be recharged or replaced periodically 26

27 Activated 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 27

28 Activated 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 28

29 Summary Technology Chemical assisted filtration Log Removal Credit Giardia Viruses Crypto Direct filtration Slow Sand filtration DE filtration Membrane Cartridge filters

30 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 30

31 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 31

32 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) Water Temperature CT values for various log credits 2 log (ph = 6-9) 3 log log

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) 33

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 34

35 Disinfection Chloramination Combination of chlorine and ammonia Can form several species of chloramines; however monochloramine is desirable Promoted using mass ratio of 4.5:1 (chlorine:ammonia( chlorine:ammonia) 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 35

36 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 36

37 Disinfection Ultraviolet Radiation Application of UV radiation provides primary disinfection by inactivating the reproductive abilities of micro-organisms 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/cm 2 (flux) 37

38 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 38

39 Disinfection Ultraviolet Radiation Pathogen E coli O157:H7 Cryptosporidium Giardia Adenovirus (type 15) Dose (mj/cm 2 ) required for various log inactivation credits 1 <2 <2 < <2 <2 ~ <2 < <

40 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 40