Treatment Processes for Potable Water

Treatment Processes for Potable Water

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

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

General Characteristics of Water Sources Groundwater Constant composition High mineral content Low turbidity Little or no odor May be bacteriologically safe No dissolved oxygen High hardness H 2 S, Fe, Mn Possible chemical toxicity Surface Water Variable composition Low mineral content High turbidity Color Microorganisms present Dissolved oxygen Low hardness Tastes and odors Possible chemical toxicity

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

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

Sedimentation Frequently done using circular clarifiers

Clarifier

Sedimentation Can also use dissolved air floatation

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

Slow Sand Filtration Low Loading Rate 340 3,400 gal/ft 2 /d Cake / Sludge Layer adds to efficiency Let cake build until it hampers flow Used more in Europe

Rapid Sand Filtration Higher Application Rates 3400-26,000 gal/ft 2 /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.

Normal Flow Backwash

Aeration Strips dissolved gases (H 2 S, CH 4, etc.) Oxidizes and precipitates metals (Fe, Mn) Performed prior to sedimentation and filtration since precipitates may be formed

Oxygen Transfer Oxygen transfer into water during batch aeration is described by Cs Ct K Lat e C C s 0 C s = DO concentration at saturation (mg/l) C t = DO concentration at time, t (mg/l) C 0 = initial DO concentration (mg/l) K L a = volumetric oxygen transfer coefficient (time -1 ) independent of temperature

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) Oxidants

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

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

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

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.

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

Outbreaks of Waterborne Disease in the U.S. No. No. No. Year Incidents Cases Deaths 2007 20 2,456 0 2008 16 1,672 3

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

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

Calcium Hypochlorite Commonly used for disinfection in swimming pools, but sometimes used in small treatment plants Somewhat more expensive than gaseous chlorine Similar to Cl 2

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

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

Ozone Fairly Common in Europe Benefits No Taste/Odors Reverts back to O 2 quickly No THMs Drawbacks No Residual Disinfection Must be produced on site Expensive (energy intensive)

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

Disinfectant Requirement Chick-Watson relationship can be used to determine amount of disinfectant needed: N Ct e N 0 N = organism concentration at time t (count/ml) N 0 = initial organism concentration (count/ml) λ = specific lethality coefficient (L min/mg) C = disinfectant concentration (mg/l) t = contact time (min)

Disinfectant Requirement Normally interested in % reduction of organisms: % reduction 1 N N 0 100