Treatment Processes for Potable Water

1. Treatment Processes for Potable Water

2. Drinking Water • Potable water is water that is free of chemicals, microorganisms, and other contaminants and is safe to drink • Drinking water also should be palatable, i.e., free of taste and odor

3. Drinking Water • Important characteristics: • physical • appearance, temperature, taste, odor • chemical • concentrations of chemical constituents • microbiological • elimination of pathogens • radiological

4. General Characteristics of Water Sources

5. Surface Water Treatment Screen Sedimentation Basin Disinfection Rapid Sand Filtration Rapid Mix (disperse coagulant) Pump Flocculation Basin (slow mix) Sludge Storage

6. Water Treatment • Screen • Used to prevent debris from entering system and damaging equipment • Coagulation • Usually done using trivalent cations (ferric or aluminum) • Interaction with negatively charged particles (clays, etc.) • Rapid mixing brings added chemical and colloidal particles together • Slow mixing in flocculation tank aids in formation of flocs

7. Sedimentation • Frequently done using circular clarifiers

8. Clarifier

9. Sedimentation • Can also use dissolved air floatation

10. Filtration • Removes solids too small for sedimentation • Water passed through clean porous media • Sand • Anthracite coal • Solids removed by • Straining • Settling within media pores • Adhesion to media

11. Slow Sand Filtration • Low Loading Rate • 340-3400 gal/ft2/d • Cake/Sludge Layer adds to efficiency • Let cake build until it hampers flow • Used more in Europe

12. Rapid Sand Filtration • Higher Application Rates • 3400-26,000gal/ft2/d • Water ponded above media surface to drive flow through • Clean by backwashing (reversal of flow) • Media characteristics (density variation) allow reuse • Widely used in U.S.

13. Normal Flow Backwash

14. Aeration • Strips dissolved gases (H2S, CH4, etc.) • Oxidizes and precipitates metals (Fe, Mn) • Performed prior to sedimentation and filtration since precipitates may be formed

15. Oxygen Transfer • Oxygen transfer into water during batch aeration is described by • Cs = DO concentration at saturation (mg/L) • Ct = DO concentration at time, t (mg/L) • C0 = initial DO concentration (mg/L) • KLa = volumetric oxygen transfer coefficient (time-1)

16. Oxidants • Sometimes a stronger oxidant (chlorine or ozone) may be used to oxidize metals if concentrations are high and need to be removed for taste and color control (iron causes brown stains and manganese gray)

17. Carbon Adsorption • Water is passed through columns containing granular activated carbon (GAC)

18. Carbon Adsorption • Dissolved constituents adsorb to (adhere to the surface of) the GAC • Not effective for contaminants that readily dissolve in water (salts) • May be combined with filtration where one of the media used is GAC

19. Carbon Adsorption • Removes trace organic compounds and some other compounds which do not dissolve readily in water (lead, other heavy metals)

20. Carbon Adsorption • Activated carbon must be regenerated periodically as surfaces become saturated with compounds being adsorbed. • Carbon is removed and sent to incinerator for regeneration.

21. Disinfection • Definition – destruction or total kill of pathogenic organisms • Purpose • kill all pathogens currently present • leave residual agent to kill pathogens that enter the water later in the distribution system

22. Outbreaks of Waterborne Disease in the U.S.

23. Ideal Disinfectants • Should quickly deactivate existing organisms • Provide residual active agent • Inexpensive • No harmful byproducts • Safe for the environment

24. Chlorine Gas • Effective • Inexpensive • Widely used in the US • Some taste and odor from residual • Leads to formation of trihalomethanes when it comes in contact with organic compounds: THMs are carcinogenic

25. Calcium Hypochlorite • Commonly used for disinfection in swimming pools, but sometimes used in small treatment plants • Somewhat more expensive than gaseous chlorine • Similar to Cl2

26. Chlorine Dioxide • Increasing popularity – forms fewer THMs • Problems • Must be produced on site • Explosive at higher temperatures when exposed to light, organics • Contains Cl gas as an impurity – leads to THMs

27. Chloramines • Formed from chlorine and ammonia • Fewer problems with THMs • Not as effective as free chlorine – higher concentrations must be used

28. Ozone • Fairly Common in Europe • Benefits • No Taste/Odors • Reverts back to O2 quickly • No THMs • Drawbacks • No Residual Disinfection • Must be produced on site • Expensive (energy intensive)

29. Ozone • Anticipated to have increased use in US, especially if chlorine products are banned • May be used with low amounts of chlorine to provide residual treatment

30. Disinfectant Requirement • Chick-Watson relationship can be used to determine amount of disinfectant needed: • N = organism concentration at time t (count/mL) • N0 = initial organism concentration (count/mL) • λ = specific lethality coefficient (L·min/mg) • C = disinfectant concentration (mg/L) • t = contact time (min)

31. Disinfectant Requirement • Normally interested in % reduction of organisms: