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AGSM 337/BAEN 465 Potable Water Treatment Page 1 of 5 Overview Purpose is to provide safe, aesthetically pleasing water Water must be free of hazardous chemicals and pathogenic microorganisms Water must have a pleasant taste, odor, and color Systems vary depending on source of water, generally less treatment is required for ground water than for surface water Surface water treatment systems may include: source, pumping, coagulation and flocculation, sedimentation, filtration, adsorption, disinfection, storage, distribution Ground water treatment systems may include: source, pumping, aeration, filtration, disinfection, storage, distribution Coagulation Helps settling of smaller particles (colloids) Most naturally occurring particles in water carry a negative charge (particularly clays) which keep them from joining together and aid in their suspension in water Coagulants are added to water to reduce this net negative surface charge After charge is reduced, particles are more likely to come close together When they are close, van der Waals forces pull them together Coagulants containing divalent or trivalent cations may also form precipitates which further aid the trapping of particles Precipitating coagulants include ferric chloride, alum (aluminum sulfate), ferric sulfate, and lime Organic polymers – long chain organic chemicals – are also available for use as coagulants The long chain polymer molecules attach to several colloidal particles and thus facilitate settling Flocculation Gentle mixing aimed at forcing small particles to collide and stick together Conducted during or after the addition of a coagulant Mixing is intentionally gentle – anything stronger might break “flocs” apart Flocculation of small particles increases the settling velocity (recall Stoke’s Law, v d2) dramatically AGSM 337/BAEN 465 Potable Water Treatment Page 2 of 5 Sedimentation Removes solids – those that settle individually and those brought together during the coagulation and flocculation process Typically done in tanks referred to as clarifiers Usually involves flocculant settling (recall 4 types of settling from lectures on sedimentation) but sometimes may include hindered settling near bottoms of tanks Solids (sludge) on the bottom of the tank are pushed to the tank center by gravity or a scraper and are removed Filtration Removes solids that are too small to be removed in a timely fashion by sedimentation Water is passed through a clean porous medium (sand or anthracite coal) Solids are removed by straining, settling within the medium pores, and adhesion to medium particles (aided by addition of coagulants and reduction of surface charge) Slow sand filtration Uses a low loading rate (340-3400 gal/ft2/d) Relies on the accumulation of a sludge layer on the surface of the filter for efficient operation, water passing through a newly cleaned filter is often wasted When sludge buildup hinders flow, the filter is stopped and the sludge layer is removed from the top of the medium Widely used in Europe, but not so much in the US Rapid sand filtration Uses higher application rates (3400 - 26000 gal/ft2/d) Water is ponded above the surface of the medium to drive flow through the filter Filter usually contains a mixture of media and is graded by size with larger, less dense particles near the surface and smaller, denser particles below Filters are cleaned by “backwashing” or fluidizing the medium by forcing flow up from beneath – trapped particles are washed away and discarded The different densities of the mixed media, e.g., coal and sand, allow the mixture to return to the original graded structure following backwash operations (sand, even though smaller, will settle faster because of higher density) Widely used in the US AGSM 337/BAEN 465 Potable Water Treatment Page 3 of 5 Taste and Odor Control Gas Stripping (Aeration) Removes dissolved gasses (hydrogen sulfide, methane) and helps oxidation and precipitation of reduced metals (iron and manganese) Sometimes a stronger oxidant (chlorine or ozone) may be used to oxidize metals if concentrations are high – removed for taste and color control (iron causes brown stains and manganese gray) Since aeration may produce precipitates, it is performed prior to sedimentation and filtration Tray aerators pass water downward through a series of screened trays Diffused aeration relies on air bubbled up from the bottom of the aerator A batch process for transfer of oxygen into water is described as follows: Cs Ct e K Lat Cs C0 Where Cs = saturated DO in water, mg/L (depends on temperature and solute concentration) Ct = DO at time t, mg/L C0 = initial DO, mg/L KLa = volumetric oxygen transfer coefficient, 1/time t = time Example Water is aerated at a temperature of 20 ºC. The initial DO is 5.6 mg/L. What is the DO after 10 minutes if the oxygen transfer coefficient is 0.04 1/min? Determine Cs = 9.1 mg/L Cs Ct 9.1 Ct e K Lat e 0.04 (10) Cs C0 9.1 5.6 Solve for Ct = 6.75 mg/L AGSM 337/BAEN 465 Potable Water Treatment Page 4 of 5 Carbon Adsorption Water is passed through columns containing granular activated carbon (GAC) which are similar to home water filter systems, but much larger Removes trace organic compounds and some other compounds which do not dissolve readily in water (lead, other heavy metals) Dissolved constituents adsorb to (adhere to the surface of) the GAC Not really effective on contaminants which readily dissolve in water (salts) May be combined with filtration where one of the media used is GAC Disinfection Destruction or killing of pathogenic organisms Intent for public water supply is to kill all pathogens currently present and leave a residual for treatment of any organisms introduced in the distribution system An ideal disinfectants should quickly kill (or deactivate) existing organisms, provide a residual, be inexpensive, not create harmful byproducts, and be safe for the environment Chlorine Gas Effective and inexpensive, widely use in the US Some taste and odor from residual Leads to formation of trihalomethanes (THMs), which are carcinogenic, when it comes into contact with organic compounds Calcium Hypochlorite Commonly used for disinfection in swimming pools, but sometimes used in small treatment plants Somewhat more expensive than gaseous chlorine Works like chlorine gas and has similar benefits and problems Chlorine Dioxide Increasing in popularity because it forms fewer THMs Some problems – must be produced on-site, explosive at elevated temperatures or when exposed to light or organics, contains chlorine gas as an impurity leading to THM formation AGSM 337/BAEN 465 Potable Water Treatment Page 5 of 5 Chloramines Formed from chlorine and ammonia Fewer problems with THMs Not as effective as chlorine gas, so higher concentrations must be used Ozone Fairly common in Europe Benefits – no associated taste or odor (reverts back to O2 quickly), no THMs Drawbacks – no residual disinfectant, must be produced on-site, expensive (requires a lot of energy to produce) Anticipated to have increased use in the US in the future, especially if chlorine compounds are banned May be used with low amounts of chlorine to provide residual Chick-Watson Relationship Can be used to determine the contact time necessary to achieve a desired level of pathogen inactivation N e Ct N0 Where N = concentration of microorganisms at time t, count/mL N0 = initial concentration of microorganisms, count/mL λ = coefficient of specific lethality, L·min/mg C = disinfectant concentration, mg/L t = contact time, min Note, normally you will want a % reduction in microorganism population N/N0 is the fraction remaining, so 1-N/N0 would be the fraction reduction Example Determine the time required to obtain a 99% inactivation of the Polio virus using hypochlorite. The specific lethality is 0.5 L·min/mg and the hypochlorite concentration is 0.5 mg/L. The desired reduction rate is 99% or 0.99. The fraction remaining, N/No, would be 1-0.99 = 0.01 N 0.01 e ( 0.5)( 0.5)t No t = 18.42 min
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