CHAPTER 7 (Part 2) Water Processing

Transcription

1 CHAPTER 7 (Part 2) Water Processing

2 7 10 SYNTHETIC ORGANIC CHEMICALS Dissolved SOCs rarely adsorb to metal hydroxides formed by coagulant chemicals and to polymers Coagulation/sedimentation/filtration provides limited removal

3 7 10 SYNTHETIC ORGANIC CHEMICALS Alternatives that may improve treatment: Changing coagulants or polymers Adjustment of ph Application of powdered activated carbon (PAC) More effective SOC removal can be achieved by filtration through granular activated carbon (GAC)

4 7 10 SYNTHETIC ORGANIC CHEMICALS Groundwaters may be contaminated by agricultural pesticides or industrial volatile organic chemicals The two processes for removal from groundwater are stripping by aeration and GAC adsorption Aeration may not be able to reduce the concentrations to drinking-water standards Due to low allowable maximum contaminant levels Stable in groundwaters, VOCs are rarely found in surface waters

5 7 11 FLUORIDATION Excessive fluoride levels increase occurrence/severity of dental fluorosis (mottling of teeth) Optimum levels occur at 0.6 to 1.2 mg/l Maximum reduction in caries with no significant mottling Controlled fluoridation in treatment to bring the natural content to optimum levels gives the same benefits Over half the U.S. population consumes water with near optimum fluoride content In communities that add a chemical to provide fluoride ion

6 7 11 FLUORIDATION The three most commonly used fluoride compounds in water treatment are sodium fluoride, sodium silicofluoride, and fluorosilicic acid

7 7 11 FLUORIDATION Fluoridation system selection is based on: Size and type of water facility Chemical availability and cost Type of operating personnel available Fluoride solution feed must be paced to water flow Injected at a point where all water being treated passes If no single common point exists, separate feeding installations are required for each water facility

8 7 11 FLUORIDATION When possible fluoride should be added after filtration To avoid losses as a result of reactions with other chemicals Fluoride injection points should be as far as possible from any chemical that contains calcium To minimize loss by precipitation

9 7 12 CHLORINATION Chlorine gas is the predominant disinfectant of water With choloramine second Chlorine is: A heavier-than-air, greenish-yellow-colored, toxic gas A strong oxidizer reacts with most elements/compounds Extremely corrosive when moist A respiratory irritant that can cause serious injury

10 7 12 CHLORINATION Chlorine combines with water to form hypochlorous acid, which, in turn, can ionize to the hypochlorite ion Below ph 7, the bulk of the HOCl remains un-ionized, while above ph 8 the majority is in the form of OCI Hypochlorites added to water yield the hypochlorite ion directly

11 7 12 CHLORINATION Chlorine existing in water as hypochlorous acid and hypochlorite ion is defined as free available chlorine Chlorine readily reacts with ammonia in water to form chloramines, as follows: Reaction products formed depend on ph, temperature, time, and initial chlorine-to-ammonia ratio

12 7 12 CHLORINATION Free available residual chlorine is residual chlorine in a water as hypochlorous acid or hypochlorite ion Combined available residual chlorine exists in chemical combination with ammonia (chloramines) or organic nitrogen compounds Chlorine demand is the difference between the amount added to a water and the quantity of free and combined available chlorine remaining at the end of a specified period

13 Disinfection Chlorination is used to disinfect drinking water to destroy microorganisms that cause diseases The rate of disinfection depends on concentration and form of available chlorine residual, time of contact, ph, temperature, and other factors A combined chlorine residual is often established in treated water entering the distribution system To maintain a protective residual and control bacterial growths in the distribution piping network

14 Oxidation Hydrogen sulfide present in groundwater can be rapidly converted to the sulfate ion using chlorine

15 Distribution System Chlorination After installation, a new main should be: Pressure tested Flushed to remove all dirt and foreign matter Disinfected

16 Distribution System Chlorination The continuous-feed method supplies water to the main with a chlorine concentration of at least 50 mg/l The chlorinated water should remain in the pipe for a minimum of 24 hours All valves and hydrants along the main are operated to ensure disinfection

17 Distribution System Chlorination In the slug method, a continuous flow is fed to the main with a chlorine concentration of at least 300 mg/l Flow rate is such that the column of chlorinated water contacts the interior surfaces for at least 3 hr Used principally for large-diameter mains where the continuous-feed technique is impractical As the slug passes other connections, the valves are operated to ensure disinfection of appurtenances

18 Distribution System Chlorination The tablet method is least satisfactory, since it precludes preliminary flushing Commonly used for small-diameter mains Calcium hypochlorite tablets are placed in each section of the pipe, hydrants, etc., during construction The main is slowly filled with water to dissolve the tablets without flushing them to one end of the pipe

19 Distribution System Chlorination Following disinfection, the chlorinated water should be flushed to waste by using potable water Microbiological tests should be conducted before placing the main in service After repair, disinfection includes swabbing new pipe sections & fittings with a 5% hypochlorite solution Where conditions permit, a repaired section should be isolated and disinfected by procedures for new mains Tanks & reservoirs should be disinfected before being placed into service or following inspection & cleaning

20 Distribution System Chlorination After well construction/repair of wells, disinfection is done using a mg/l free chlorine residual A new well should be operated until the water is practically free of turbidity A quantity of chlorine solution equal to at least twice the volume of water in the well is added Pump cylinder & drop pipe are washed with a strong chlorine solution After a minimum of 12 hr, the chlorinated water is pumped to waste

21 Equipment and Feeders Design and operation of all facilities should minimize hazards associated with connecting, emptying, and disconnecting containers Liquid chlorine is shipped in pressurized steel cylinders A characteristic odor provides warning of leaks Leaks are detected by holding a cloth swab saturated with strong ammonia water near the suspected area It reacts with ammonia to form dense white fumes

22 Equipment and Feeders The most essential unit in a chlorine gas feeder is the variable orifice to control cylinder flow rate (Fig. 7 18) To solve problems of conveyance & direct introduction of chlorine gas, an injector is used for solution feed Feeders are controlled by flow, chlorine residual or both Combined automatic flow & residual control maintains a preset chlorine residual in the water (Fig. 7 19) Positive-displacement diaphragm pumps are used for metering hypochlorite solutions (Fig. 7 20)

23 Chlorine Dioxide Chlorine dioxide (ClO 2 ) is applied for disinfection or taste and odor control It is produced in the treatment plant by mixing sodium chlorite and chlorine in controlled proportions The greatest disadvantage is potential formation of chlorate and chlorite residuals toxic chemicals

24 7 13 DISINFECTION BY-PRODUCTS Chlorination of water with humic organic substances from natural sources produces toxic by-products. The principal kinds are volatile hydrophobic compounds, referred to as trihalomethanes (THMs) Chloroform; Bromodi-chloromethane; Haloacetic acids

25 Control of Disinfection By-Products Breakpoint chlorination as an initial step in processing contributes to formation of THMs & HAA5 Sustained high free chlorine residual increases the concentrations of by-products with time Applying chlorine to later stages in treatment of surface waters is the easiest method for reducing formation Alternatives that reduce by-product formation are chloramines, chlorine dioxide, and ozone

26 Control of Disinfection By-Products Chloramines are weak disinfectants, and their use increases risk of pathogens in treated drinking water They can be used as a secondary disinfectant to establish a combined residual Ozone is an effective disinfectant that forms no known products that could be detrimental to human health Disadvantages are high cost and lack of residual Chlorine dioxide has limited application as a water disinfectant, but is used for taste and odor control Produces a persistent residual without chloramines

27 Control of Disinfection By-Products By-products are difficult to remove from water The preferred approach to control by-products is prevention rather than removal

28 Disinfection/Disinfection By-Products Rule Conflicting requirements of disinfection & reducing the adverse effects of by-products led to the Stage 1 Disinfection/Disinfection By-Products (D/DBP) rule

29 7 14 OZONE As a strong oxidizing gas, ozone (O 3 ) is an effective disinfectant and oxidant of taste & odor compounds Reaction is rapid in inactivating microorganisms, oxidizing iron, manganese, sulfide, and nitrite Reaction is slower in oxidizing organic substances Pesticides, volatile organic chemicals & other organic compounds Ozone decomposes in water to produce oxygen and free hydroxyl radicals

30 7 14 OZONE Chlorine must be added to ozone treated water to establish a protective residual And control bacterial growth in distribution piping Because of the rapid decay of stored ozone gas, it must be generated on the treatment plant site

31 7 14 OZONE Ozone is application is usually for a combination of disinfection plus additional beneficial reasons Taste, odor, or color control Oxidation of humic organic substances that react with chlorine to form trihalomethanes and haloacetic acids Destabilization of colloids for improved flocculation at reduced coagulant dosage

32 7 14 OZONE An ozonation system (Fig. 7 21) consists of: Air preparation or oxygen feed Electrical power supply Ozone generation Ozone contacting Ozone contactor exhaust gas destruction

33 7 15 DISINFECTION OF POTABLE WATER The EPA has established maximum goals of zero levels for Giardia lamblia, Cryptosporidium, enteric viruses, and Legionella for public water supplies The three categories of water supplies are: Surface water open to the atmosphere And subject to surface runoff Groundwater under the direct influence of surface water Groundwater

34 7 15 DISINFECTION OF POTABLE WATER Disinfection of surface waters requires coagulation and granular-media filtration followed by chlorination Groundwater not under influence of surface water from properly constructed wells can be disinfected by chlorination, if necessary Natural disinfection occurs by: Filtration through surface soils and the subsurface aquifer by straining and trapping of microorganisms Natural attenuation of pathogens in the adverse underground environment

35 Concept of the C tproduct Inactivation of a microorganism is a function of concentration of the disinfectant and time of contact Other functions are: Kind of disinfectant Temperature & ph Viability of the microoganisms and turbidity The C t product is expressed in units of milligrams per liter times minutes [(mg/l) min]

36 Concept of the C tproduct Each disinfectant has distinctive characteristics that result in different values for the same microorganism and same conditions Turbidity removal by coagulation and filtration is essential for optimum disinfection action Estimated C t values for different inactivation rates can be calculated assuming first-order kinetics

37 Concept of the C tproduct Effectiveness from strongest to weakest: ozone, free chlorine, chlorine dioxide, and chloramine

38 Determination of Actual C t in Water Treatment The C t for a system is the summation of the calculated values for tanks, reservoirs, and piping Transporting chlorinated water before it arrives at the first customer C is the free disinfectant residual measured at the end of each chlorination segment in mg/l t is the calculated contact time of the segment in minutes The hydraulic character of a tank or reservoir is defined by the residence time distribution of individual particles of water in the discharge

39 Determination of Actual C t in Water Treatment Due to shortcircuiting and back mixing, particle travel times through a tank vary From less than to greater than theoretical detention time of volume divided by rate of inflow A residence time-distribution curve is given by a step tracer input (Fig. 7 22) Common tracer ions are chloride and fluoride They are nontoxic and approved for potable water use

40 Surface-Water Disinfection The EPA requires: 99.9% removal and/or disinfection of G. lamblia cysts 99% removal of Cryptosporidium species oocysts removal and/or inactivation of enteric viruses Surface waters are all open waters receiving runoff and groundwaters subject to contamination by direct influence of nearby surface waters The EPA requires chemical coagulation and granular media filtration followed by disinfection (Fig. 7 23)

41 Surface-Water Disinfection Chlorine residual in the distribution network must be detectable in at least 95% of samples tested monthly Filtration of all surface waters is the EPA objective Use of unfiltered water is permitted under very stringent conditions A comprehensive watershed control program, a water-quality monitoring schedule, and yearly on-site inspection is required

42 7 16 GROUNDWATER TREATMENT A groundwater may require processing to remove toxic contaminants or to improve aesthetic quality (Fig. 7 25) Calcium, manganese, iron & magnesium are common In agricultural regions, groundwater may contain nitrate from infiltration of fertilizer and pesticides Synthetic chemicals from improper disposal of industrial wastewaters Arsenic, radionuclides, and other inorganic chemicals may originate from natural geological formations