Introduction To Water Treatment

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Introduction To Water Treatment

  1. 1. Introduction to Water Treatment Peter Hillis Principal Process Engineer United Utilities
  2. 2. Why do we treat water? Protect Public Health Provide Safe Drinking Water Waterborne disease • Cryptosporidium • E.Coli • Cholera • Giardia • Polio • Typhoid • Malaria • Botulism • Hepatitis • Legionellosis
  3. 3. “ waterutilities should ensure that the design and operation of treatment plants is optimised in a cost effective way for particle removal, taking into account the level of the risk at each plant” recommendation 22 of second Badenoch Report
  4. 4. Infectious diseases caused by pathogenic bacteria, viruses, and protozoa or parasites are the most common and widespread health risk associated with drinking water. WHO Guidelines for Drinking Water Quality
  5. 5. Cases of cryptosporidiosis 1993 – Milwaukee – 403,000 ill with 10% hospitalised 1995 – Torbay, Devon – 575 cases of illness 1996 – Cranbook British Columbia – 2,000 cases of illness 1996 – Kelam, British Columbia – 15,0000 cases of illness 2000 – Clitheroe, Lancashire – 46 cases of illness 2001 – North Battlefield, Saskatchewan – 7,100 cases of illness 2005 – Anglesey – 200 cases of illness – 61,000 people on a boil water advice 2008 – Anglian Water – 250,000 customers on boil water for over 1 month
  6. 6. How do we treat water? Holistic approach Start at the source Protection of catchment Prevention of contamination Intake protection Monitoring
  7. 7. Water Treatment Processes Inlet works Fish screens Band screens - mm Microstrainers - micron
  8. 8. Coagulation - Definitions Coagulation: destabilisation of particles / formation of microflocs Coagulants: chemicals which destabilise particles and colloids Flocculation: aggregation of destabilisation particles/ microflocs into larger aggregates (Flocs) Flocculants: Chemicals which assist in the formation of large aggregates.
  9. 9. Definitions flocculants stable destabilised aggregated collisions coagulants Rapid mix Flocculation Static Mixing <1 secs 3 – 40 mins
  10. 10. Individual Flocs
  11. 11. Separation Processes Removal of floc material Physical processes • Clarification – Settlement processes – Settlement Tanks, Lamella’s, Actilfo, Dissolved Air Flotation • Filtration – Rapid Gravity Sand Filtration, Direct Filtration, Microfiltration (Membranes)
  12. 12. Direct Filtration – Watchgate WTW Reception Chamber Lime 56 Rapid Gravity Filters Alum 4 streams of Poly Raw Water 3 Microflocculators Blending 3 Lamella Clarifiers 2 Supernatant Sumps 2 Dirty Backwash Balancing Tanks Chlorine 3 Sludge Thickeners 3 Sludge Holding Tanks Contact Tank Sludge to Foul 3 Sludge Sewer Thickener 2 Thickened Lime Feed Tanks Supernatant 3 Filter Presses Sumps To Haweswater Aqueduct
  13. 13. Lamella Clarifier
  14. 14. Raw Clarified/Filtered
  15. 15. Membranes
  16. 16. Membrane Microfiltration Feed water Membrane surface Permeate
  17. 17. Secondary Processes Removal of inorganic contaminants • Manganese • Iron • Aluminium • Arsenic • Micropollutants
  18. 18. Disinfection Process of removing harmful organisms from water supply Starts at the beginning of the process – catchment Continues through removal of particles Final Disinfection • Chlorine • UV • Ozone • Chlorine Dioxide • Chloramination
  19. 19. Chlorine 1 0.9 0.8 - 0.7 HOCl OCl Fraction (molar) 0.6 5oC 15oC 0.5 35oC 5oC 0.4 15oC 35oC 0.3 0.2 0.1 0 5 6 7 8 9 10 pH pH 7 85% HOCl pH 8 35% HOCL
  20. 20. Chlorine Contact Time CT Principle Concentration (mg/l of Chlorine) * Time (mins) WHO guidelines – 30 mg.min/l i.e. 1 mg/l of chlorine for 30 minutes UU Standards – Effective CT Principle Effective Disinfectant Effective time 15 mg.min/l surface water 5 mg.min/l secure groundwater
  21. 21. Defining Contact Tank Performance t90 τ t50 t0 = time to initial detection of tracer Tracer concentration t10 = time for 10% of tracer to pass tp tp = time to maximum tracer concentration t50 = time for 50% or tracer to pass τ = hydraulic retention time t90 = time for 90% of tracer to pass t10 t0 Time
  22. 22. Tank performance based on tx Baffling t1/τ τ t5/τ τ t10/τ τ Poor 0.14 0.27 0.37 Average 0.30 0.44 0.52 Superior 0.55 0.66 0.73
  23. 23. Contact tank chamber profile r B C r/w influences extents of w dead zones at A, B and C A Corner fillets C to reduce dead zones Perforated C baffles with low r/w ratio A
  24. 24. UV Disinfection Physical Process Light energy is absorbed by DNA of organisms Inhibits replication Organism that cannot replicate cannot infect
  25. 25. Cryptosporidium Inactivation
  26. 26. UV Disinfection
  27. 27. UV Disinfection
  28. 28. Irradiation Field
  29. 29. UV Plant – Clay Lane
  30. 30. Summary Primary objective to protect public health Water Treatment starts at source Multi-barrier approach Source to tap approach

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