ARMY TM 5-813-3AIR FORCE AFM 88-10, Vol 3

WATER SUPPLY, WATER TREATMENT

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D E P A R T M E N T S O F T H E A R M Y A N D T H E A I R F O R C E16 SEPTEMBER 1985

TM 5-813-3/AFM 88-10, Vol 3

REPRODUCTION AUTHORIZATION/RESTRICTIONSThis manual has been prepared by or for the Government and, except to the extent indicated below, is public property and not subject to copyright.Copyrighted material included in the manual has been used with the knowledge and permission of the proprietorsand is acknowledged as such at point of use. Anyone wishing to make further use of any copyrighted material, by it-self and apart from this text, should seek necessary permission directly from the proprietors.Reprints or republications of this manual should include a credit substantially as follows: Joint Departments ofthe Army and Air Force, USA, Technical Manual TM 5-813-3/AFM 88-10, Volume 3, Water Supply, Water Treat-ment.If the reprint or republication includes copyrighted material, the credit should also state: Anyone wishing tomake further use of copyrighted material, by itself and apart from this text, should seek necessary permissiondirectly from the proprietors.

CHAPTER 1. GENERALPurpose and scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Water treatment projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Water quality criteria and standards. . . . . . . . . . . . . . . . . . . . . . . . .

2. WATER TREATMENT PROCESSESProcess selection factor .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Preliminary treatment.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Aeration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Coagulation and flocculation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Sedimentation basins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Filtration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Disinfection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Fluoride adjustment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Taste-and-odor control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Softening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Iron and manganese control. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Corrosion and scale control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Special processes. . . . . . . . . . . . . . . . . . . . , . , . . . . . . . , . . . .

3.WATER TREATMENT SYSTEMSGeneral design criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Plant siting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Process selection and design. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Reliability, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Operating considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Plant capacity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4.MEASUREMENT AND CONTROLlMeasurement of process variables . . . . . . . . . . . . . . . . . . . . . . . . . . .Control, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Design of instruments and controls . . . . . . . . . . . . . . . . . . . . . . . . . .

5.WATER TREATMENT CHEMICALSChemical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Chemical standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Chemical handling and storage. . . . . . . . . . . . . . . . . . . . . . . . . . . . .Chemical application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6.WATER TREATMENT PLANT WASTESQuantities and characteristics of waste . . . . . . . . . . . . . . . . . . . . . . .Waste management. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

APPENDIX A WATER QUALITY CRITERIA AND STANDARDS . . . . . . . . . . . .

1-11-21-3

2-12-22-32-42-52-62-72-82-92-102-112-122-13

3-13-23-33-43-53-6

4-14-24-3

5-15-25-35-4

6-16-2. . . . .

B DESIGN EXAMPLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .C LABORATORIES AND LABORATORY ANALYSES. . . . . . . . . . . . . . . . . . .D METRIC CONVERSION FACTORS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .PREFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. BIBLIO

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TM 5-813-3/AFM 88-10, Vol 3

CHAPTER 1

GENERAL

1-1. Purpose and scope.

This manual, intended for planners and design engi-neers, presents information on water quality stand-ards and design criteria for water treatment processes.This manual also establishes criteria to be followed indetermining the necessity for and the extent of treat-ment, and on procedures applicable to the planning ofwater treatment projects. This manual is applicable toall elements of the Army and Air Force responsible forthe planning and design of military construction,

1-2. Water treatment proiects.

State health department, State water resource, andU.S. Environmental Protection Agency personnel, asappropriate, should be consulted in the early stages ofproject planning regarding supply sources and asso-ciated water treatment needs. In addition to the usualtreatment that may be required to insure delivery ofpotable water, consideration will be given to the needfor special treatment to protect pipelines, water heat,-

ers, plumbing fixtures, and other equipment againstscaling, corrosion, and staining, Because of the widelyvarying conditions and the many types of water, it isnot possible to establish criteria for all cases of specialwater treatment. Treatment for prevention of scalingand corrosion may not be entirely effective; and inmany cases a decision as to the necessity of specialtreatment cannot be reached prior to actual operatingexperiences. In general, special treatment will be pro-vided only in cases where a study of water analysesand experience with the water definitely show thatthere will be severe corrosion of the water system orthat severe scaling of hot-water heaters, storage tanks,and other parts of the plumbing system will occur.Marginal cases will be deferred and treatment pro-vided only after operating experience determinestreatment to be necessary.

1-3. Water quality criteria and standards.

Information on current criteria and standards for rawand potable water are presented in appendix A.

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CHAPTER 2

WATER TREATMENT PROCESS

2-1. Process selection factors.

The design of treatment facilities will be determinedby feasibility studies, considering all engineering, eco-nomic, energy and environmental factors. All legiti-mate alternatives will be identified and evaluated bylife cycle cost analyses. Additionally, energy use be-tween candidate processes will be considered. For thepurpose of energy consumption, only the energy pur-chased or procurred will be included in the usage eval-uation. All treatment process systems will be com-pared with a basic treatment process system, which isthat treatment process system accomplishing the re-.quired treatment at the lowest first cost. Pilot or labo-ratory analysis will be used in conjunction with pub-lished design data of similar existing plants to assurethe optimal treatment. It is the responsibility of thedesigner to insure that the selected water treatmentplant process complies with Federal EnvironmentalAgency, State or local regulations, whichever is morestringent.

2-2. Preliminary treatment.

Surface waters contain fish and debris which can clogor damage pumps, clog pipes and cause problems inwater treatment. Streams can contain high concentra-tions of suspended sediment. Preliminary treatmentprocesses are employed for removal of debris and partof the sediment load.

a. Screens.(1) Coarse screens or racks. Coarse screens, often

termed bar screens or racks, must be provided to inter-cept large, suspended or floating material. Suchscreens or racks are made of l/2-inch to 3/4-inch metalbars spaced to provide 1- to 3-inch openings.

(2) Fine screens. Surface waters require screens orstrainers for removal of material too small to be inter-cepted by the coarse rack, These may be basket-type,in-line strainers, manually or hydraulically cleaned bybackwashing, or of the traveling type, which arecleaned by water jets. Fine-screen, clear openingsshould be approximately 3/8 inch. The velocity of thewater in the screen openings should be less than 2 feetper second at maximum design flow through thescreen and minimum screen submergence.

(3) Ice clogging. In northern areas screens maybe.clogged by frazil or anchor ice. Exposure of racks orscreens to cold air favors ice formation on submerged

parts and should be avoided to the maximum practi-cable extent. Steam or electric heating, diffusion aera-tion and flow reversal have been used to overcome iceproblems.

(4) Disposal of screenings. Project planning mustinclude provision for the disposal of debris removed bycoarse and fine screens.

b. Flow measurement. Water treatment processes,e.g., chemical application, are related to the rate offlow of raw water, Therefore, it is essential that accu-rate flow-rate measurement equipment is provided.Pressure differential producers of the Venturi type arecommonly used for measurement of flow in pressureconduits. An alternative selection for pressure con-duits is a magnetic flow meter if the minimum velocitythrough the meter will be 5 feet per second or more. AParshall flume can be used for metering in open chan-nels. Flow signals from the metering device selectedshould be transmitted to the treatment plant controlcenter.

c. Flow division. While not a treatment process,flow division (flow splitting) is an important treatmentplant feature that must be considered at an early stageof design. To insure continuity of operation during ma-jor maintenance, plants are frequently designed withparallel, identical, chemical mixing and sedimentationfacilities. No rigid rules can be given for the extent ofduplication required because a multiplicity of factorsinfluence the decision. Normally, aerators are not pro-vided in duplicate. Presedimentation basins may notrequire duplication if maintenance can be scheduledduring periods of relatively low raw water sedimentload or if the following plant units can tolerate a tem-porary sediment overload. If it is determined that pre-sedimentation at all times is essential for reliable plantoperation, then the flow division should be madeahead of the presedimentation basins by means ofldentical splitting weirs arranged so that flow overeither weir may be stopped when necessary, Duringnormal operation, the weirs would accomplish a pre-cise equal division of raw water, regardless of flowrate, to parallel subsequent units; rapid-mix, slow-mixand sedimentation. The water would then be combinedand distributed to the filters. If presedimentationunits are not provided, then the flow is commonly splitahead of the rapid-mix units. If a single treatmenttrain is to be provided initially with the expectation ofadding parallel units in the future, then the flow-split-

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ting facilities should be provided as part of the originaldesign, with provision for Mocking flow over the weirwhich is to serve future units.

d. Sand traps. Sand traps are not normally requiredat surface water treatment plants. Their principal ap-plication is for the removal of fine sand from well wa-ter, The presence of sand in well water is usually a signof improper well construction or development. If sandpumping cannot be stopped by reworking the well, thesand must be removed. Otherwise, it will create seri-ous problems in the distribution system by cloggingservice pipes, meters, and plumbing. Centrifugal sandseparators are an effective means of sand removal.These cyclone-separator devices are available assem-bled from manufacturers and require no power otherthan that supplied by the flowing water. They operateunder system pressure; therefore, repumping is notnecessary. Water from the well pump enters tangen-tially into the upper section of the cone and centrifugalforce moves the sand particles to the wall of the cone.They then pass downwater into the outlet chamber.Sand is periodically drained to waste from this cham-ber through a valve that can be manually or automat-ically operated. The clarified water is discharged fromthe top of the cone, These units are available in diam-eters of 6, 12, 18, 24, and 30 inches. providing a capac-ity range from 15 to 4500 gallons per minute (gpm)and are suitable for operation up to 150 pounds persquare inch (psi). Pressure drop through the unitranges from 3 to 25 psi, depending on unit size andflow rate. These separators will remove up to 99 per-cent of plus 150 mesh sand and about 90 percent ofplus 200 mesh. The units are rubber lined for protec-tion against sand erosion.

e. Plain sedimentation. Plain sedimentation, alsotermed presedimentation is accomplished withoutthe use of coagulating chemicals. Whether plain sedi-mentation is essential is a judgment decision influ-enced by the experience of plants treating water fromthe same source. Water derived from lakes or im-pounding reservoirs rarely requires presedimentationtreatment. On the other hand, water obtained fromnotably sediment-laden streams, such as those foundin parts of the Middle West, requires presedimenta-tion facilities for removal of gross sediment load priorto additional treatment. Presedimentation treatmentshould receive serious consideration for water ob-tained from rivers whose turbidity value frequentlyexceeds 1,000 units. Turbidity values of well over10,000 units have been observed at times on some cen-tral U.S. rivers.

(1) Plain sedimentation basins. Plain sedimenta-tion or presedimentation basins may be square, circu-lar, or rectangular and are invariably equipped withsludge removal mechanisms.

(2) Design criteria. Detention time should be ap-

proximately 3 hours. Basin depth should be in the ap-proximate range of 10 to 15 feet, corresponding to up-flow rates of 600 to 900 gallons per day (gpd) persquare foot for a detention period of 3 hours. Short-cir-cuiting can be minimized by careful attention to de-sign of inlet and outlet arrangements. Weir loadingrates should not exceed approximately 20,000 gpd perfoot. Where presedimentation treatment is contin-uously required, duplicate basins should be provided.Basin bypasses and overflows should also be included.

2-3. Aeration.

The term aeration refers to the processes in whichwater is brought into contact with air for the purposeof transferring volatile substances to or from water.These volatile substances include oxygen, carbon diox-ide, hydrogen sulfide, methane and volatile organiccompounds responsible for tastes and odor. Aeration isfrequently employed at plants treating ground waterfor iron and manganese removal.

a. Purpose of aeration. The principle objectives ofaeration are:

(1) Addition of oxygen to ground water for theoxidation of iron and manganese. Ground waters arenormally devoid of dissolved oxygen. The oxygen add-ed by aeration oxidizes dissolved iron and manganeseto insoluble forms which can then be removed by sedi-mentation and filtration.

(2) Partial removal of carbon dioxide to reduce thecost of water softening by precipitation with lime andto increase pH.

(3) Reduction of the concentration of taste-and- -

odor producing substances, such as hydrogen sulfidesand volatile organic compounds.

(4) Removal of volatile organic compounds whichare suspected carcinogens, (see para 2-13b.).

b. Types of aerators. Three types of aerators arecommonly employed. These are: waterfall aeratorsexemplified by spray nozzle, cascade, and multiple-tray units; diffusion or bubble aerators which involvepassage of bubbles of compressed air through the wa-ter; and mechanical aerators employing motor-drivenimpellers alone or in combination with air injection de-vices. Of the three types, waterfall aerators, employ-ing multiply trays, are the most frequently used in wa-ter treatment. The efficiency of multiple-tray aeratorscan be increased by the use of enclosures and blowersto provide counterflow ventilation.

c. Design criteria.(1) Multiple-tray, tower aerators.

(a) Multiple-tray aerators. Multiple-tray aera-tors are constructed of a series of trays, usually threeto nine, with perforated, slot or mesh bottoms. The wa-ter first enters a distributor tray and then falls fromtray to tray, finally entering a collection basin at thebase. The vertical opening between trays usually

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ranges from 12 inches to 30 inches. Good distributionof the water over the entire area of each tray is essen-tial. Perforated distributors should be designed to pro-vide a small amount of head, approximately 2 incheson all holes, in order to insure uniform flow. In aera-tors with no provision for forced ventilation, the traysare usually filled with 2- to 6-inch media, such as coke,stone, or ceramic balls to improve water distributionand gas transfer and to take advantage of the catalyticoxidation effect of manganese oxide deposits in themedia. The water loading on aerator trays should be inthe range of 10 to 20 gpm per square foot. Good, nat-ural ventilation is a requirement for high efficiency.For multiple tray aerators designed for natural venti-lation, the following empirical equation can be used toestimate carbon dioxide (CO2) removal:

trayn number of trays, including distribution trayk = 0.11 to 0.16 depending on temperature, tur-

bulence, ventilation, etc.Where icing is a problem and the aerator must behoused, artificial ventilation by fans or blowers is nec-essary. An enclosed induced- or positive-draft aeratorrequires approximately 3.5 to 6 standard cubic feet ofventilating air per gallon of water aerated. Thus, foran enclosed aerator operating at a rate of 1.5 milliongallons per day (mgd), air requirements will be in therange of 3600 to 6200 standard cubic feet of air perminute. Positive-draft aerators employing the higherair-flow rates exhibit the highest efficiency for the ad-dition and removal of dissolved gases and oxidation ofiron, manganese, and sulfide. Power requirements fora natural draft, multiple-tray aerator having an overallheight of 10 feet will be approximately 1.7 kilowattsper mgd of aeration capacity. Power demands forforced draft units will be greater.

(b) Counter-current packed column aeration. Acounter-current parked column aerator tower is simi-lar to operation to counter-current multiple tray aera-tors, but are particularly efficient at the removal ofvolatile organic compounds (VOCs) through air-strip-ping. Packed column aerators consist typically of along thin tower filled with either a random dumpedmedia (Rasching rings, Ber) saddles, Pall rings) or cor-rugated sheet media, held by a packing support plate.Water is pumped to the top of the tower over a distri-bution plate and allowed to fall through the media. Airis blown up through the tower by a fan counter to thefalling water. Redistributor plates are used through-out the column to prevent channeling of the water orair stream. Efficiency of the tower is dependent on theextent of contact between the air and water. Detaileddesign can be found in various chemical engineering

literatures and handbooks or AWWA, EPA publica-tions.

(2) Diffusion aerators. Compressed air is injectedinto the water as it flows through a rectangular basin.A variety of air injection devices may be employed in-cluding perforated pipes, porous plates or tubes andvarious patented sparger devices. Basin size is deter-mined by desired detention time, which commonlyranges from 10 to 30 minutes. Tank depth is usuallyfrom 10 to 15 feet. Air requirements, supplied by acompressor, generally range from 0.1 to 0.2 standardcubic foot per gallon of water aerated. Major advan-tages of a diffusion aeration system include practicallyno head loss and freedom from cold-weather operatingproblems. An additional advantage is that a diffusionaerator may also be used to provide chemical mixing.Power requirements are those associated with air com-pression and range from 1.0 to 2.0 kilowatts per mgdof aerator capacity. Aeration efficiency in terms of ad-dition of oxygen or removal of carbon dioxide is gen-erally similar to that provided by multiple-tray aera-tors employing natural ventilation.

(3) Mechanical aerators. Mechanical aerators typi-cally consist of an open impellar operating on the wa-ters surface. Basin size is determined by detentiontime required. Basin depth can vary from 5 to 17 feetwith the average depth being 10 feet. Major advan-tages of mechanical aerators are practically no headloss and the ability to provide mixing. Mechanicalaerators are generally not as efficient as aeration tow-ers or diffused aerators and longer detention times arerequired.

d. Criteria for installation of aerators. Aeration is agas transfer process which is not needed at all watertreatment plants. A decision as to whether to aerate ornot requires assessment of the economic and waterquality benefits achieved by its use.

(1) Addition of oxygen. Aeration processes arecommonly used in adding oxygen to groundwaters andto oxidize iron, manganese, hydrogen sulfide and to alimited extent, organic matter. Groundwaters areusually deficient in oxygen and acration is an effectivemeans of adding it. Oxygen addition is normally re-quired if iron and manganese removal is a treatmentobjective. Aeration will also help oxidize hydrogen sul-fide and some organic matter.

(2) Partial removal of volatile substances. Aera-tion is a useful method of removing volatile substancesfrom water. Groundwaters while being deficient inoxygen can contain objectionable levels of carbon diox-ide. An efficient aerator will result in near saturationwith oxygen and about 90 percent reduction of the car-bon dioxide content of groundwater. At lime-soda wa-ter softening plants, any carbon dioxide dissolved inthe water at the point of lime application will consumelime without accompanying softening. For high (>50

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mg/L) carbon dioxide concentrations, as encounteredin some groundwaters, aeration for its removal is prob-ably justified. For concentrations on the order of 10mg/L, or less, aeration is probably not economicallyvalid. Before deciding to aerate for carbon dioxide re-moval, the cost of purchasing, maintaining and operat-ing the aerator should be compared to the value of thelime saved. At softening plants, each mg/L of carbondioxide removed will effect a saving of about 1.3 mg/Lquicklime (95 percent calcium oxide). It will also re-duce the quantity of softening sludge produced propor-tionately.

(3) Reduction of hydrogen sulfide. Aeration is alsoused for removing hydrogen sulfide from well water. Itmay be sufficient in itself if the hydrogen sulfide con-centration is not more than about 1.0 or 2,0 mg/L.Otherwise, it maybe used in conjunction with chlorineto oxidize the hydrogen sulfide not removed by aera-tion.

(4) Reduction of Volatile Organic Compounds(VOCs). Recent studies have shown that aeration canbe successfully employed to reduce volatile organiccompounds (VOCs) such as total Trihalomethane(TTHM) concentration in chlorinated water to meetcurrent US EPA regulations limiting TTHM concen-trations. Aeration by diffused air or multiple-trayaerators can reduce TTHM concentration at low cost,with cost increasing with higher concentrations of Tri-halomethane (THM). Counter-current packed toweraeration is most efficient in achieving mass transfer ofVOC.

e. Aeration summary. Where icing is a problem andthe aerator must be housed, artificial ventilation byfans or blowers is necessary. An enclosed induced- orpositive-draft aerator requires approximately 3.5 to 6standard cubic feet of ventilating air per gallon of wa-ter aerated. Thus, for an enclosed aerator operating ata rate of 1.5 mgd, air requirements will be in the rangeof 3600-6200 standard cubic feet of air per minute.Positive-draft aerators employing the higher air-flowrates exhibit the highest efficiency for the additionand removal of dissolved gases and oxidation of iron,manganese, and sulfide. Counter-current packed col-umn aeration is particularly efficient to remove vola-tile organic compounds, Requirements for a naturaldraft, multiple-tray aerator having an overall height of10 feet will be approximately 1,7 kilowatts per mgd ofaeration capacity, Power demands for forced draftunits will be greater. In general, aeration is worthy ofconsideration in connection with the treatment ofgroundwater supplies in conjunction with lime soften-ing and for the removal of some VOCs. Surface watersusually exhibit low concentrations of carbon dioxide,no hydrogen sulfide and fairly high dissolved oxygen.As a consequence, aeration is not required for the re-moval or addition of these gases. However, surfaces

waters contain higher levels of THM precursors thangroundwaters and therefore a need for aeration mayarise to reduce TTHM following chlorination. Waterhigh in the bromine-containing THMs are difficult totreat by aeration and other methods of removal shouldbe used, such as coagulation and flocculation or con-tact with granular activated carbon.

2-4. Coagulation and flocculation.

Coagulation and flocculation processes are defined asfollows: Coagulation means a reduction in the forceswhich tend to keep suspended particles apart. Thejoining together of small particles into larger, settle-able and filterable particles is flocculation. Thus,coagulation precedes flocculation and the two process-es must be considered conjunctively.

a. Purposes of coagulation and flocculation. Rawwater supplies especially surface water supplies, oftencontain a wide range of suspended matter, includingsuspended minerals, clay, silt, organic debris andmicroscopic organisms ranging in size from about0.001 to 1.0 micrometer. Small particles in this sizerange are often referred to as colloidal particles.Larger particles, such as sand and silt, readily settleout of water during plain sedimentation, but the set-tling rate of colloidal particles is so low that removalof colloidal particles by plain sedimentation is notpracticable. Chemical coagulation and flocculationprocesses are required to aggregate these smaller par-ticles to form larger particles which will readily settlein sedimentation basins. The coagulation-flocculationprocesses are accomplished step-wise by short-timerapid mixing to disperse the chemical coagulant fol-lowed by a longer period of slow mixing (flocculation)to promote particle growth.

b. Chemical coagulant. The most frequently usedchemical coagulant is aluminum sulfate (Al2(SO 4)3 14H 2O). This aluminum coagulant is alsocalled alum or filter alum, and dissociates in waterfor form S04 =, Al

3+ ions and various aluminum hy-droxide complexes. Other aluminum compounds

Magnesium hydroxide (Mg(OH)2), is also an effectivecoagulant, Organic polyelectrolyte compounds, ap-plied in low dosages alone or in combination with themetal coagulant, are also employed, Polyelectrolytesare high-molecular-weight polymers that dissociate inwater to give large highly charged ions, The polyelec-trolytes and dissociated ions destabilize the colloidsand promote their settling, These polymers can be clas-sified an anionic, cationic or nonionic according totheir dissociated polymeric ions being negatively

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charged, positively charged or both negatively andpositively charged,

c. Coagulation for Removal of TrihalomethanePrecursors. Recent US EPA regulations limit allow-able TTHM concentrations in finished potable water(see para 2-13). To help meet the current maximumcontaminant level (MCL) of 0.10 mg/L for TTHM,trivalent metal ion coagulant, such as aluminum sul-fate or ferrous sulfate, and a variety of organic poly-electrolytes have been used to remove THM precursorsbefore chlorination. Naturally-occurring THM precur-sors, such as humic and fulvic compounds, are onlypartially removed by coagulation and filtration. Forcoagulation with alum, a pH of between 5 and 6 is theoptimum for the removal of fulvic and humic acid com-pounds. Ferrous sulfate exhibits an optimum pH forremoving organic compounds of between 3 and 5. Ful-vic acids require twice the dosages of alum needed forhumic acids, The addition of anionic polymers at dosesfrom 1 to 10 mg/L can also provide some removal ofhumic compounds. The efficiency of removal dependsupon the type and concentration of organic compoundspresent in the water supply, pH, coagulant dose, andsolids-liquid separation step. Optimum precursor re-moval can only be estimated using laboratory simula-tion techniques, such as simple jar testing, followed bysettling or removal of precipitated colloids with mem-brane filters. This procedure can provide the informa-tion necessary to determine the optimum conditionsfor the removal of trihalomethane precursor com-pounds. Monitoring of the removal of organic precur-sor compounds by coagulation and filtration can be fa-cilitated by the measurement of total organic carbon.

d. Design criteria for mixing. Criteria for rapid- andslow-mix processes have been developed on the basis ofdetention time, power input, velocity gradient (G) andthe product (Gt) of velocity gradient and detentiontime. The values of G and Gt are computed from:

Gt = product of G and t, a dimensionless numberwhere

P = the power dissipated in the water (ft-lb/see)u =

V = volume of mixing basin (cubic feet)t = mixer detention time (seconds)

e. Rapid mixing. For rapid-mix units, detention pe-riods usually range from 10 to 30 seconds with in-stalled mixer power approximately 0,25 to 1.0 hp permgd. Power and detention time should be matched sothat values of G will be in the approximaterange: 500-1000 see-1. A wire-to-water efficiency of80 percent, a water temperature of 50 F, a power in-put of 1.0 hp per mgd and a detention time of 10 sec-

onds, yield a G value of about 1000 see-l and a Gtvalue of 10,000. Similarly, a 30-second detention timegives a G value of about 600 and a Gt value of 18,000.Long detention period for rapid-mix basins should beavoided because of higher power requirements and in-ferior coagulation results. The rapid-mix basin shouldbe designed to minimize short circuiting.

f. Slow mix, For slow-mix (flocculating) units, de-tention periods should range from 30 minutes to 60minutes, with installed mixer power of approximately0.1 to 3.5 hp per mgd. G values in the range of 20 see -1

to 100 see-l are commonly employed, CorrespondingGt values will, therefore, be in the range of 36,000 to360,000. Tapered, slow mixing with G decreasingfrom a maximum of about 90 see-l down to 50 see-1

and then to 30 sec -1 can be used and will generally pro-duce some improvement in flocculation. Somewhathigher G values, up to 200 see -1, are employed in somewater softening plants. For normal flocculation, usingalum or iron salts, the maximum peripheral speed ofthe mixing units should not exceed about 2.0 fps andprovision should be made for speed variation. To con-trol short circuiting, two to three compartments areusually provided. Compartmentation can be achievedby the use of baffles. Turbulence following flocculationmust be avoided, Conduits carrying flocculated waterto sedimentation basins should be designed to providevelocities of not less than 0.5 fps and not more than1.5 fps, Weirs produce considerable turbulence andshould not be used immediately following flocculation.

2-5. Sedimentation basins.

Sedimentation follows flocculation, The most commontypes of sedimentation basins in general use are shownin figures 2-1 and 2-2. A recent innovation in clari-fiers is a helicai-flow solids contact reactor, consistingof a above ground steel conical basin as shown in fig-ure 2-3. However, these above ground basins require ahigh head and additional pumps may be required. Aminimum of two basins should be provided to allowone unit to be out of service for repair or maintenance.The design must include arrangements that permit useof a single basin when necessary.

a. Design criteria. The design of a sedimentationtank is based on the criterion as listed in table 2-1,The sedimentation basins should have adequate capac-ity to handle peak flow conditions and to prevent ex-cessive deteriorated effluent water qualities. Theabove design data represent common conditions,higher overflow rates may be used at lime softeningplants and at some plants employing upflow clarifica-tion units as indicated in the tables of Water Treat-ment Plant Design by ASCE, AWWA, CSSE (see appE). Unusual conditions may dictate deviation fromthese general criteria. Detention time in the range of 8to 12 hours, or more provided in several stages, maybe

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b. Flocculation-sedimentation basins. Units of thistype, usually circular, combine the functions of floc-culation, sedimentation and sludge removal, Floccula-tion is accomplished in a circular center well, Sedimen-tation occurs in the annular space between the floc-culation section and the perimeter effluent weir. De-sign criteria are generally similar to those applicable

to separate units.c. Suspended solids contact basins, Basins of this

type combine rapid-mixing, flocculation, sedimenta-tion, and sludge removal in a single unit. Coagulationand flocculation take place in the presence of a slurryof previously formed precipitates which are cycledback to the mixing and reaction zone. Upflow rates atthe point of slurry separation should not exceed about1.0 gpm per square foot for units used as clarifiers fol-lowing coagulation and approximately 1.5-1.75 gpmper square foot for units used in conjunction with limesoftening.

2-6. Fi l tration.

Filtration of water is defined as the separation of col-loidal and larger particles from water by passagethrough a porous medium, usually sand, granular coal,

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on a 50-mesh (U.S. Series) sieve. Approximately 100percent by weight, of the sand should pass the16-mesh sieve and 90 to 100 percent be retained on a50-mesh sieve. Filter sand should be clean silica sandhaving a specific gravity of not less than 2.5. Thehydrochloric acid volubility of the sand should be lessthan 5 percent.

(b) Anthracite. Anthracite is an alternative me-dium consisting of hard anthracite coal particles. Theeffective size commonly ranges from about 0.45 mm to0.6 mm with a uniformity coefficient not to exceed1.7. The hardness should not be less than 2.7 on theMoh scale and the specific gravity not below 1.4. Also,the anthracite should be visibly free of clay, shale, anddirt.

(c) Multimedia. Multimedia filters employ twoor three layers of media of different size and specificgravity. A common arrangement, the dual mediafilter, is 20 inches of anthracite overlaying a sandlayer of approximately 8 to 12 inches. The anthracitelayer has size range of about 0.8 to 2.0 mm; the sandlayer, about 0.4 to 1.0 mm. Tri-media filters employ an18-inch anthracite layer, an 8-inch sand layer, and anunderlying 4-inch layer of garnet or ilmenite having asize range of 0.2 to 0.4 mm. Garnet has a specific grav-it y of about 4, and ilmenite about 4.5.

(3) Filter gravel and underdrains, The filter mediais commonly supported by a 10- to 18-inch layer ofcoarse sand and graded gravel. The gravel depth mayrange from 6 inches to 24 inches, depending on the fil-ter underdrain system chosen. The gravel should con-sist of hard, rounded stones having a specific gravityof at least 2.5 and an acid volubility of less than 5 per-cent. A 3- to 4-inch transition layer of coarse (torpedo)sand, having a size range of about 1.2 to 2.4 mm, isplaced on top of the filter gravel. Gravel size usuallyranges from about 0.1 inch to about 2.5 inches. Filterunderdrains may be constructed of perforated pipegrids or various proprietary underdrain systems. Avariety of the latter are available. Design details forpipe underdrains are given in numerous texts andhandbooks. Manufacturers will furnish design and in-stallation criteria for proprietary systems.

(4) Sand, anthracite, gravel specifications. De-tailed specifications for filter sand, anthracite andgravel are contained in AWWA B100.

(5) Number of filters. Not less than two filtersshould be installed regardless of plant size. For largeplants, rough guidance as to the number of filters to beprovided may be obtained from:

N = number of filter unitsQ = design capacity in mgd

Thus, a 9 mgd plant would require eight filters.(6) Size of filter units. The maximum filter size is

related to wash water flow rate and distribution. Nor-

mally, individual filters sizes do not exceed about 2100square feet corresponding to a capacity of about 6 mgdat a flow rate of 2.0 gpm per square foot. A unit of thissize would require a maximum backwash water rate of about 60 mgd, which is excessive. Consequently, itshould be divided into two parts of equal size arrangedfor separate backwashing. Total filter depth should beat least 9 feet.

(7) Filter backwash. Backwash facilities should becapable of expanding the filter media 50 percent. Thiswill require wash rates in the range of 10 to 20 gpmper square foot. Backwash water can be supplied by abackwash pump or from elevated storage provided spe-cifically for this purpose. Filter down-time duringwash periods commonly average 10 to 20 minutes in-cluding a 5- to 15-minute wash period. For a 15-minutebackwash of a single unit, at maximum rate, the washwater volume will be 300 gallons per square foot of fil-tration area in that unit. In addition to backwashing,auxiliary scour is commonly provided. This aids incleaning the filter and is commonly accomplished byrotary or fixed surface-wash equipment located nearthe top of the bed. It is operated for a time periodequal to that of the backwash, Water pressures of40-100 psi are required for surface-wash operation ata rate of 0.5 gpm per square foot. Air scour may alsobe employed but is not generally used. If an independ-ent washwater storage tank is used, it must refill be-tween washes. Tank capacity should be at least 1.5times the volume required for a single wash.

(8) Wash water troughs. Wash water troughsequalize the flow of wash water and provide a conduitfor removal of used water. Two or more troughs areusually provided. The elevation of the trough bottomsshould be above that of the expended bed. The clearhorizontal distance between troughs should not exceed5 to 6 feet, and the top of the troughs not more than 30inches above the top of the bed.

(9) Filter piping and equipment. Essential filtercontrol valves, etc., are shown schematically in figure2-4. Each filter should be equipped with a rate-of-flowcontroller plus associated equipment for automatic fil-ter water-level control. The latter senses the waterlevel in the main influent conduit and transmits a sig-nal to the flow controllers. The controllers, in responseto this signal, adjust filtration rates to match the in-flow from the sedimentation basins. Thus, within prac-tical limits, total filter outflow always equals total in-flow and the filter water level remains virtually con-stant. A device that will sense maximum permissibleclearwell level should also be provided. This should bearranged so that at maximum allowable clearwellwater level, a shut-off signal will be transmitted to allfilter controllers and also to an audible alarm. Otherdesigns, not involving rate controllers, such as in- -

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have been developed and may be employed at the dis-cretion of the designer. In general, each filter musthave five operating valves: influent, wash water,drain, surface wash, and filter-to-waste. It is empha-sized that the filter-to-waste piping must not be direct-ly connected to a plant drain or sewer. An effluentsampling tap must be provided for each filter. Valvescan be manually, electrically, hydraulically, or pneu-matically operated. Butterfly type valves are recom-mended for filter service. Design velocities commonlyemployed for major filter conduits are as follows:

The effluent conduit must be trapped to present back-flow of air and provide a seal for the rate controllers.The filter pipe gallery should have ample room andgood drainage, ventilation, and lighting. Dehumidifi-cation equipment for the gallery should receive carefulconsideration. Filters should be covered by a super-structure except under favorable climatic conditions.Drainage from the operating floor into the filtershould be prevented by a curb. Access to the entire bedshould be provided by a walkway at operating floorlevel around the filter. Filters may be manually orautomatically controlled from local or remote loca-tions. Facilities permitting local, manual control arerecommended irrespective of other control features.Used backwash water should be discharged to a washwater recovery basin or to a waste disposal facility.Regulatory agencies generally view filter wash wateras a pollutant and forbid its direct discharge to thenatural drainage.

(10) Essential instrumentation. Minimum essen-tial instrumentation for each filter will be provided asfollows: rate=of-flow indicator; loss-of-head indicator;effluent turbidity indicator; wash water rate-of-flowindicating and totalizing meter. If a wash water stor-age tank is provided, it must be equipped with a water-level indicator. While not absolutely required, a tur-bidity indicator on the main filter influent is desirable.

b. Diatomite filters. Filtration is accomplished by alayer of diatomaceous earth supported by a filter ele-ment termed a septum, This layer of diatomaceousearth is about l/8-inch thick at the beginning of filtra-tion and must be maintained during filtration by aconstant feed of diatomaceous earth (body feed) to theinfluent water. At the conclusion of a filter run, thelayer of diatomaceous earth will have increased inthickness to about 1/2 inch. Filtration rates generallyvary from 0.5 to 2.0 gpm per square foot. The princi-pal use of diatomite filters has been for swimming pool

waters, but some have been installed for the treatmentof potable water.

c. Pressure filters. Pressure filters are similar inconstruction and operating characteristics to rapid sand filters. However, in a pressure filter the media,gravel bed, and underdrains are enclosed in a steelshell. There are a variety of new pressure filters in usetoday. The most common of these are the conventionaldownflow filter, the high-rate downflow filter and theup flow filter. An advantage of any pressure filter isthat any pressure in waterlines leading to the filter isnot lost, as in the case of gravity filters, but can beused for distribution of the filter effluent. Between 3and 10 feet of pressure head are lost through the filter.The primary disadvantage of a pressure filter is that,due to the filter being enclosed in a steel shell, accessto the filter bed for normal observation and mainte-nance is restricted. Also, the steel shells require care-ful periodic maintenance to prevent both internal andexternal corrosion. The use of pressure filters is not ad-vantageous in most systems. However, if the pressurerequirements and conditions in a particular system aresuch that repumping of filtered water can be elim-inated, cost savings will be realized,

(1) Conventional downflow filters. Conventionaldownflow pressure filters consist of a bed of granularmedia or multi-media and are good in removing sus-pended solids comprised of floe. The advantages overgravity filters include lower installation cost andadaptability y to different piping systems. Hydraulicloadings range from 1 to 4 gpm/sq. ft.

(2) High-rate downflow filters. High-rate down-flow filters have filtration rates of 10-20 gpm/sq. ft.The higher downflow velocities require coarser mediawhich allow suspended solids to penetrate deeper intothe medium. As a result, more solids can be stored inthe filter bed before backwashing is required, Manyunits exhibit a 1-4 lbs/sq. ft. solids-loading capacity,The higher filtration rates also allow smaller or fewerfilters to be used over conventional filters. However,the high solids-loading capacity of this filter requireshigher backwashing flow rates and hence larger back-washing water storage tanks.

(3) Upflow filters. Upflow multi-media filtersallow filtration of high solids-loaded liquids in concen-tration up to 1,000 mg/L. The advantage of upflowmulti-media filters is that the coarser material at theinlet collects the heavier particles, while the finer ma-terial collects the smaller particles, thus efficiency ofthe filter is increased.

(4) Upflow continuous backwash sand filters. Up-flow continuous backwash sand filters continuouslyclean the filter medial by recycling the sand internallythrough an air lift pipe and sand washer. The regen-erated sand is then redistributed to the top of the sand

bed. Once the sand migrates down to the bottom of the

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bed it is again airlifted and repeats the cycle. Upflowcontinuous backwash sand filters require no backwashvalves, storage tanks, or backwash pumps, thereforetheir operation is greatly simplified.

2-7. Disinfection.

Disinfection involves destruction or inactivation oforganisms which may be objectionable from the stand-point of either health or esthetics. Inasmuch as thehealth of water consumers is of principal concern tothose responsible for supplying water, design of facili-ties for disinfection must necessarily be carefullyexecuted.

a. Chlorination. The application of chlorine to wateris the preferred method of disinfecting water suppliesat military installations.

(1) Definitions. Terms frequently used in connec-tion with chlorination practice are defined as follows:

(a) Chlorine demand. The difference betweenthe concentration of chlorine added to the water andthe concentration of chlorine remaining at the end of aspecified contact period. Chlorine demand varies withthe concentration of chlorine applied, time of contact,temperature, and water quality.

(b) Chlorine residual. The total concentration ofchlorine remaining in the water at the end of a speci-fied contact period,

(c) Combined available residual chlorine. Anychlorine in water which has combined with nitrogen.The most common source of nitrogen is ammonia, andcompounds formed by the reactions between chlorineand ammonia are known as chloramines. The disinfect-ing power of combined available chlorine is about 25 to100 times less than that of free available chlorine.

(d) Free available residual chlorine. That part ofthe chlorine residual which has not combined withnitrogen.

(2) Chlorination practice.(a) Combined residual chlorination, Combined

residual chlorination entails the application of suffi-cient quantities of chlorine and ammonia, if ammoniais not present in the raw water, to produce the desiredamount of combined available chlorine (chloramine) ina water. If enough ammonia is present in raw water toform a combined chlorine residual, only chlorine needbe added to the water. Combined residual chlorinationis generally used only when maintaining an adequatefree chlorine residual in the distribution system isdifficult or when objectionably high levels of TTHMswould be formed as a result of free residual chlorina-tion. Due consideration of other TTHM controlalternatives should be made before using chloramines,(see para 2-13).

(b) Breakpoint chlorination. If a water contains-- ammonia or certain nitrogenous organic matter which

reacts with chlorine, the addition of chlorine causes

the formation of chloramines until the ratio of ele-mental chlorine to ammonia compounds is about 5 to1. Further addition of chlorine results in the oxidationof chloramines to gaseous nitrogen and nitrogenoxides, which decreases the quantity of chloraminespresent. After all of the chloramines have been oxi-dized, additional chlorine added to the water formsonly free available chlorine. The point at which all ofthe chloramines have been oxidized and only free chlo-rine is formed is called the breakpoint . If no am-monia is present in the water, there will be no break-point. The chlorine required to reach the breakpoint isusually about 10 times the ammonia nitrogen contentof the water. However, in certain waters, because ofthe presence of other chlorine consuming substances,as much as 25 times the ammonia nitrogen concentra-tion may be required. Enough chlorine should be addedpast the breakpoint to ensure an adequate freechlorine residual.

(c) Marginal chlorination. Marginal chlorinationinvolves the application of chlorine to produce a de-sired level of total chlorine residual regardless of therelative concentrations of free or combined chlorinepresent. In marginal chlorination the initial chlorinedemand has been satisfied but some oxidizable sub-stances remain.

(d) Chlorine dosages. Figure 2-4 provides mini-mum cysticidal and bactericidal free chlorine residualsand minimum bactericidal combined chlorine residualsfor various pH and temperature levels. Since water-borne bacteria are the major concern at fixed installa-tions, minimum bactericidal levels will be maintainedin treated water in all parts of the distribution systemunder constant circulation. Even at lower pH levels,free chlorine residuals should not fall below 0.2 mg/Land combined chlorine residuals should not fall below2.0 mg/L. If marginal chlorination is practiced, thetotal chlorine residual must not be less than 2.0 mg/l.Whenever epidemological evidence indicates an out-break of a nonbacterial waterborne disease such asamebiasis, infectious hepatitis, or schistosomiasis inthe area of a fixed military installation, cysticidal freechlorine residuals shall be maintained in the watersupply. Further guidance on disinfection requirementsmay be obtained from the Surgeon Generals office.Air Force policy on minimum chlorine levels is estab-lished in AFR 161-44.

(3) Other effects of chlorination. In addition tothe disinfection achieved with chlorination, otherbeneficial effects should be noted. Since the oxidizingpower of chlorine is high, in the presence of free chlo-rine, hydrogen sulfide is oxidized, nitrites are oxidizedto nitrates, and soluble iron and manganese are oxi-dized to their insoluble oxides. Free chlorine also re-acts with naturally occurring taste, odor and color-producing organic substances to form chloro-organic

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compounds, e.g., trihalomethanes (see para 2- 13.b.).The US EPA, after much discussion over costs/benefits, has chosen a maximum contaminant level for

serving above 10,000 persons and has indicated a

water treatment industry to avoid costly modificationsto existing plants. To reach the US EPAs future maxi-mum contaminant level for TTHMs, more significantchanges in disinfection practices will be required.

(4) Application of chlorine. Chlorine may be ap-plied to water of two forms: As gaseous elementalchlorine or as hypochlorite salts. Gaseous elementalchlorine shall be used for water disinfection at all fixedinstallations. The cost of hypochlorite salts is prohibi-tive in all plants larger than 0.5 mgd. For remote sites

at fixed installations, some well sources require 5 gpmor less. These sources with small demands can usehypochlorite for disinfection.

(a) Point of application. Chlorine may be ap-plied to water in a variety of locations in the watertreatment plant, storage facilities, or distribution sys-tem. It is absolutely essential that the chlorine appliedto the water be quickly and thoroughly mixed with thewater undergoing treatment. If required, special chlo-rine mixing facilities should be provided. In conven-tional water treatment plants, chlorine may be applied

.. prior to any other treatment process (prechlorination),following one or more of the unit treatment process(postchlorination), and again in the more distantpoints of the distribution system (dechlorination).

1 Prechlorination., Prechlorination has oftenbeen used so the water would maintain a chlorineresidual for the entire treatment period, thus length-ening the contact time. The coagulation, flocculation,and filtration processes were thought to be improvedby prechlorination of the water, and nuisance algaegrowths in settling basins were reduced. In prechlo-rination, the chlorine was usually injected into the rawwater at or near the raw water intake. Prechlorinationwas the most accepted practice of disinfection in thepast. However, since many surface waters containTHM precursors that will combine with the free chlo-rine during prechlorination and form potentially car-cinogenic THMs, such as chloroform, the point ofapplication has been shifted further down the treat-ment process to take advantage of precursor removalduring treatment.

2 Postchlorination. Postchlorination general-ly involves the application of chlorine immediatelyafter filtration and ahead of the clear well. The designand construction of water treatment plants for mili-tary installations will include the necessary provisionsfor changing the locations of chlorine applications as

may later be desirable for improving treatment or dis-infection processes.

3 Dechlorination. Dechlorination is the prac-tice of adding chlorine to water in the distribution sys-tem to maintain a minimum chlorine residual through-out the system.

(b) Chlorination equipment. Hypochlorite saltsmust be applied to the water in solution form. Hypo-chlorite solutions are pumped by a diaphragm pumpthrough an injection system into the water to be chlo-rinated. If elemental chlorine is used for disinfection,it shall be injected by solution-type chlorinators. Sincechlorine solutions are acidic, many components of achlorination system must be constructed of corrosionresistant materials such as glass, silver, rubber, orplastics. Maintaining the chlorination apparatus in atrouble-free state is essential, Key spare parts and re-pair kits for chlorination systems must be kept onhand. Critical components of the chlorination systemshall be installed in duplicate.

(c) Automatic control. If automatic chlorinationcontrol is utilized, the chlorine feed rate should be con-trolled primarily by the rate of flow of water, with asignal from a downstream residual chlorine analyzerused to trim the feed rate. Provision for manual con-trol during emergency situations must be included.

(5) Superchlorination and dechlorination. Super-chlorination may be necessary if there are large vari-ations in chlorine demand or if available contact timeis brief. Water which has been superchlorinated gen-erally requires dechlorination before discharge to thedistribution system. Dechlorination may be achievedthrough the application of sulfur dioxide, sodium bi-sulfite, or sodium sulfite, or by passing the waterthrough granular activated carbon filters. The de-chlorination process (and subsequent dechlorination, ifnecessary) shall be controlled so that the free residualchlorine remaining in the water is at least 0.2 mg/L.Careful monitoring must be practiced to assure thatpotentially harmful levels of TTHMs are not exceeded.A summary of TTHM regulations are presented intable 2-2.

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(6) Safety precautions for chlorination. TheAWWA manual Safety Practice for Water Utilitiescontains safety recommendations regarding the use ofchlorine. These recommendations shall be followed atall military water treatment facilities. Further discus-sion on safe operation of chlorination facilities forArmy installations are contained in TB MED 576, ap-pendix L.

b. Alternate Disinfectants. If the use of chlorine asa disinfectant causes unacceptably large concentra-

tions of chlorinated organic compounds, and if allother methods for reducing TTHMs have been ex-hausted, such as moving the point of chlorination,aeration, and special coagulant (as shown in table 2-3 for chloroform which is the main constituent ofTTHMs in many cases) and if an alternate raw watersource, such as a ground water source, is not available,an alternative disinfectant must be considered. Anyalternate disinfectant system installed as the primarymeans of water disinfection shall have chlorination fa-cilities available and operative for stand-by use. Fivealternative disinfectants are discussed below; ozone,chlorine dioxide, chloramines, ultraviolet (UV) radi-ation, and UV and Ozone combined. While chlorine isthe least costly disinfectant, considering dosage andenergy consumption basis. However alternate disin-fectants are not significantly more expensive.

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Table 2-3: Effectiveness of Various Unit Processesfor Reducing Chloroform Formation Potential (Cent d)

(1) Ozone. Ozone is an extremely powerful disin-fectant that has been used in Europe either as a soledisinfectant, or in conjunction with postchlorinationto impart a persistent chlorine residual in the waterdistribution system. United States potable waterplants have in the past used ozone to control taste andodor. Today ozonation is being increasingly used as aprimary disinfectant prior to rapid mixing, floccula-tion and filtration. Ozonation does not produce THMs..It is reduced to oxygen and does not leave any residualdisinfectant. Hence, the need for postchlorination.Ozone is generated electrically, as needed using theelectric discharge gap (corona) technique. Air oroxygen stream, a cooling water stream and alternatingelectric current are required. Efficient cooling is essen-tial to reduce thermal decomposition of ozone. Bubblediffusers appear to be the most economic ozone con-tractors available.

(2) Chlorine Dioxide, Chlorine dioxide is a highlyeffective disinfectant producing minimal THMs in thepresence of their precursors. Chlorine dioxide uses inthe United States have been limited to taste and odorcontrol although it has been used elsewhere as a pri-mary disinfectant and is presently receiving more at-tention in the United States. The common method ofchlorine dioxide production is to react chlorine gasfrom a conventional chlorinator with a sodium chloritesolution. Following the mixing of the chlorine and so-dium chlorite streams and prior to introduction intothe main stream the mixed stream is passed through apacked column contactor to maximize chlorine dioxideproduction. A major disadvantage of chlorine dioxideis the formation of chlorate and chlorite which are po-tentially toxic.

-. (3) Chloramines, The use of chloramines as a dis-infectant fell into disuse after the introduction of

breakpoint chlorination. To achieve the same disinfec-tion ability of chlorine, 10 to 15 times the amount ofchloramines are needed or longer contact time is re-quired. More chloramines are needed if high concen-trations of organic material are found in the influentwater, Chloramines are easy to generate, feed, andproduce a persistant residual that will remain throughthe water distribution system. Chloramines may beproduced by introducing ammonia to the water streamprior to the addition of free chlorine. This process canbe optimized for minimum THM production and maxi-mum disinfection. Recently however there has beensome concern over chloramine toxicity.

(4) Ultraviolet Radiation. Ultraviolet (UV) radi-ation has undergone development, but has not beenused on a large scale for drinking water supply disin-fection. Most of its uses include product or processwater disinfection where high purity, sterile water isneeded. UV radiation has been used to disinfect drink-ing water at remotely located hotels and on cruiseships. Few large scale water processing plants use UVdisinfection, although its application is feasible. UVdisinfection does not leave a disinfectant residual andshould be accompanied by postchlorination. Ultra-violet irradiation is also effective in oxidizing organiccompounds in water, Water turbidity will inhibit theeffectiveness of UV disinfection.

(5) UV and Ozone, Recently there has been someexperimentation in a combined UV and ozone con-tactor. Results from these tests show promise. How-ever, there is no known water treatment plant oper-ating with this method of disinfection.

2-8. Fluoride adjustment.

a. Health effects. An excessive fluoride concentra-tion will damage the teeth of children using the waterfor extended periods. On the other hand, moderateconcentrations, 0.7- 1.2 mg/L, are beneficial to chil-drens teeth. Most natural waters contain less than theoptimum concentration of fluoride. Upward adjust-ment of the fluoride concentration can be achieved byapplication of a measured amount of a fluoride chem-ical to the water. For installations where it is desirableand feasible to add fluoride, control limits andoptimum concentrations are as follows:

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b. Fluoridation chemicals. Chemicals most fre-quently used for fluoridation are: Sodium silicofluo-

of chemical will depend principally on delivered costand availability.

(1) Sodium fluoride. This chemical is commercial-ly available as a white crystalline powder having apurity of 95 to 98 percent. (Sometimes it is artificiallycolored nile blue.) Volubility is approximately 4 per-cent at 770 F. The pH of a saturated solution is 6.6.The 100 percent pure material contains 45.25 percentfluoride. It is available in 100-pound bags, 125 to 400pound drums, and bulk.

(2) Sodium silicofluoride. This compound is com-mercially available as a white powder with a purity of98 to 99 percent. Volubility is only about 0.76 percentat 770 F. The pH of a saturated solution is 3.5. The100 percent material contains 60.7 percent fluoride. Itis available in 100 pound bags, 125 to 400 pounddrums, and bulk.

(3) Fluosilicic acid. This chemical is commerciallyavailable as a liquid containing 22 to 30 percent byweight of fluosilicic acid. It is sold in 13 gallon car-boys, 55 gallon drums, and in bulk. The 100 percentpure acid contains 79.2 percent fluoride. The pH of a 1percent solution is 1.2, and the use of fluosilicic acid asa fluoridation agent in a water of low alkalinity willsignificantly reduce the pH of the water. It should notbe used for fluoride adjustment of waters of this typeunless pH adjustment is also provided.

c. Point of application. It is essential that all waterpass the point of injection of the fluoridation chemicaland that the flow rate past this point be known withreasonable accuracy. At a water treatment plant, thepreferred application point is usually the combined ef-fluent of all filters. The fluoride chemical can be fed atan earlier stage of treatment, for example, the com-bined filter influent, but part of the fluoride appliedwill be removed by the filtration process. Coagulationand lime softening will also remove a small amount ofthe applied fluoride. A larger dose is required to offsettreatment process losses. If ground water is the supplysource, the fluoride chemical should be injected intothe discharge pipe of the well pump. Where the supplyis from several wells, each pumping independently tothe distribution system, it will be necessary to providean injection point at each well. If flow past the injec-tion point is variable, automatic equipment that willfeed fluoride chemical at a rate proportional to flow isa requirement.

d. Fluoride feeders. Volumetric or gravimetric dryfeeders equipped with dissolvers are suitable forsodium fluoride or sodium silicofluoride. Feedersshould be equipped with weighing devices that will ac-curately measure the weight of chemical fed each day

and the feed equipment should be designed to mini-mize the possibility of free flow (flooding) of chemicalthrough the feeder. Normally, the feed machinessupply hopper should hold no more than 100 to 200pounds of chemical. Large extension hoppers holding - -

much greater quantities of dry fluoride chemical in-crease the danger of flooding and overfeeding and arenot recommended for most installations. Solutions ofsodium silicofluoride are acidic and corrosion-resistantdissolvers and solution piping must be provided wherethis chemical is employed. If fluosilicic acid is used, itcan be applied by means of a small metering pump intoan open channel or a pressure pipe. Storage tanks,feeders, and piping for fluosilicic acid must be made ofcorrosion-resistant material. The acid is slightly vola-tile and the feed system should be enclosed. If not en-closed, special exhaust ventilation should be providedto protect personnel from fluoride fumes.

e. Fluoride removal. Fluoride removal can be accom-plished by passage of the water through beds of acti-vated alumina, bone char, or tricalcium phosphate.When the capacity of the bed to remove fluoride is ex-hausted, it can be regenerated by treatment with acaustic soda solution followed by rinsing and acid neu-tralization of the residual caustic soda. Other methodsof fluoride removal include electrodialysis, reverse os-mosis and ion exchange. Some fluoride reduction canbe obtained by water softening using excess lime treat-ment. Fluoride reduction by this method is associatedwith magnesium precipitation and the extent of fluo-ride removal is a function of the amount of magnesiumprecipitated from the water. All removal processesproduce liquid wastes and suitable provision must bemade for their disposal. Guidance as to the fluoride re-moval process to be employed can be obtained fromlaboratory studies of process effectiveness and fluo-ride removal capacity, using samples of the water thatis to be treated.

2-9. Taste and odor control.

Most taste and odors in surface water are caused bylow concentrations of organic substances derived fromdecomposing vegetation, microscopic organisms, sew-age and industrial waste pollution, etc. Treatment fortaste and odor removal involves destruction of theodorous substance by chemical oxidation or its re-moval by aeration or adsorption or activated carbon.

a. Chemical oxidation. Chemical oxidizing agentswhich have been found effective and which can beused in the treatment of potable water are chlorine,chlorine dioxide, potassium permanganate, and ozone.No single chemical is completely effective under alloperating conditions.

b. Aeration. Aeration is helpful in eliminating odorcaused by hydrogen sulfide, but is ineffective in signif-

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icantly reducing odor associated with dissolvedorganics.

c. Absorption. Powdered activated carbon is com-monly used for removal of tastes, odor and color by ad-sorption. The carbon can be applied to the water at anypoint in the treatment plant prior to filtration, but it isusually advisable to apply it early in the treatmentprocess to prolong contact. For maximum effective-ness, carbon should be applied well ahead of chlorine,and preferably in advance of lime softening, The in-fluent to a presedimentation basin is normally an ef-fective carbon application point. Powdered carbon dos-ages usually range from 5 to 10 mg/L, but as much as50 mg/L may be required. The use of powdered acti-vated carbon adds more suspended solids and increasesthe amount of sludge, thereby creating a sludge dis-posal problem. Powder activated carbon is marginallyeffective in reducing TTHMs. Granular activatedcarbon (GAG) has also been used for taste and odor re-moval. It has been employed as a separate treatmentstep in the form of carbon columns and as a substitutefor sand in the filtration process. Used in this way, thegranular carbon serves in a dual capacity as a filtrationmedium and for taste and odor removal. Granular acti-vated carbon is also excellent at reducing TTHMs.Granular activated carbon must be reactivated on aregular basis to keep its absorptive abilities. Becauseof the cost of reactivation of GAC, other methods oftaste-and-odor control and reduction of TTHMs shouldbe considered. Aeration is generally more cost-effective than GAC contractors.

2-10. Softening.

Whether water softening is provided will depend en-tirely on the type of project and the uses to be made ofthe water. Two general types of processes are used forsoftening: The lime-soda ash process and the cationion exchange or zeolite process.

a. Applications.(1) Permanent posts or bases. Softening of the en-

tire supply for a permanent post or base may be con-sidered if the hardness exceeds 200 mg/l, with hard-ness expressed as equivalent CaCO3. Softening of apost water supply to a total hardness of less than 100mg/L is not required, however, softening to less thanthis amount is justified for the special purposes andservices given in paragraphs (3), (4), (5), and(6) below.

(2) Nonpermanent bases. For Army temporaryconstruction and for Air Force bases not in the perma-nent category, the entire supply will not be softenedunless the total hardness exceeds 300 mg/L. However,when a treatment plant is constructed for the removalof turbidity or iron, the plant may also be designed toaccomplish partial softening.

(3) Laundries. Water for laundries shall have ahardness of 50 mg/l or less. Installation of cation ion

exchange water softeners to reduce the hardness tozero is recommended.

(4) Boiler water. Boiler water for power plantsand heating plants may require softening, but satisfac-tory results can often be obtained by application ofcorrosion and scale inhibitors. Depending on the pres-sure at which the boiler is to operate, partial water-de-mineralization may also be necessary, See paragraph2-13a. for additional information on demineralization.

(5) Dining facilities. The installation of softenersfor small dining facilities, latrines and bathhouses isnot recommended. However, water softeners to reducehardness to 50 mg/L maybe justified for large centraldining facilities to protect equipment and to insuresatisfactory washing of dishes. Each such instance willbe justified separately.

(6) Hospitals. When the water supplied to a hospi-tal has a hardness of 170 mg/L or more, the water willbe softened to approximately 50 mg/L. Where criticalequipment requires water having a hardness of lessthan 50 mg/L, as special study will be made to deter-mine the most feasible means of obtaining water of thenecessary hardness. Zero hardness water may be pipedfrom the main softener or maybe supplied from smallindividual softeners, whichever is the more feasible.The sodium content of the treated water must be takeninto account when selecting a softening method forhospitals.

b. Lime-soda ash process,(1) Softening chemicals and reactions. The princi-

pal chemicals used to effect softening are lime, either

to be softened and react with the calcium carbonateand magnesium in the ater to form insoluble com-pounds of calcium carbonate and magnesium hydrox- .ide. If quicklime is used, it is usually converted to aslurry of hydrated lime by slaking with water prior toapplication. The chemistry of the process can be illus-trated by the following equations:

All of the above reactions can be accomplished in asingle stage of treatment. Lime and soda ash can beadded at the same point and will react with each other;

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however, the net effect will be as illustrated by reac-tions 2 through 7.

(2) Chemical requirements.(a) Lime. A reasonably accurate estimate of lime

requirements for softening can be computed from a

linity and, magnesium. Requirements of quicklime orhydrated lime can be computed as follows:

hardness of the raw water and to establish the amountof noncarbonated hardness to be left in the finished wa-ter. The latter is termed residual noncarbonated hard-ness. Inasmuch as most commercial soda ash is 990/0+

purity of this chemical.lbs soda ash per million gallons = [8.34] [NCH-R]

where NCH = mg/L of noncarbonated hardness R = mg/L of residual noncarbonated hardness (The term [NCH-R] is the mg/L of noncarbonated

hardness removed)(3) Characteristics of lime-softened water. The

carbonate hardness of the water, after application andreaction of the softening chemicals plus sedimentationand filtration, should be approximately 50 mg/L. Thetotal hardness will consist of the carbonate hardness,50 mg/L, plus the residual noncarbonated hardness thatwas intentionally allowed to remain in the water. It isnot advisable to reduce the carbonate hardness to thelowest possible value because such water will be corro-

sive. In lime softened wasters, it is desirable that themagnesium hardness be reduced to 40 mg/L or less.The residual calcium hardness should be approximate-ly 50 mg/L and the alkalinity also about 50 mg/L. Some ground water supplies contain no noncarbonatedhardness. For such waters, lime treatment alone willsuffice for softening.

(4) Sludge production. The lime-soda ash soften-ing process produces chemical sludge composed princi-pally of calcium carbonate and magnesium hydroxide.As withdrawn from sedimentation basins equipped formechanical sludge removal, the proportion of dry sol-ids in the sludge will generally fall within the range of2 to 10 percent. The weight of dry solids produced bysoftening reactions will average approximately 2.5times the weight of commercial quicklime used. Forhydrated lime, softening solids produced will be rough-ly twice the weight of commercial hydrated lime em-ployed. Fairly accurate values of total solids produc-tion at an operating plant can be developed utilizing amass balance which takes into consideration the sus-pended solids in the raw water, the quantity of dis-solved calcium and magnesium in the raw and finishedwater, the quantity and purity of lime applied, thequantity of coagulant used, and the stoichiometry ofthe softening and coagulation reactions. Means of dis-posal of waste solids from softening plants must re-ceive careful consideration at an early stage of treat-ment plant design. See chapter 6.

(5) Lime-caustic soda process. An alternative soft-ening process, sometimes used, is the lime-caustic sodaprocess. The process is worth consideration when con-siderable reduction in noncarbonated hardness is re-quired. Application of the process involves substitu-tion of caustic soda (sodium hydroxide) for soda ashand part of the lime. The remaining lime reacts withcarbonate hardness constituents as previously indi-cated. The caustic soda also reacts with carbonate hardness as follows:

will reduce the noncarbonated hardness as previouslyindicated. All of the reaction products are chemicallyidentical to those obtained by the use of lime and sodaash. The amount of caustic soda required can be calcu-lated from the theoretical quantities of pure lime andsoda ash required. Less calcium carbonate sludge isformed with the lime-caustic soda process. This may bean advantage if softening sludge disposal is a problem.For water softening purposes, caustic soda should bepurchased as a 50 percent solution containing 6.38pounds of pure NaOH per gallon. A 50 percent solutionmust be stored at temperatures above about 600 F. toprevent freezing. As a storage alternative, the 50 per- cent solution may be diluted to 25 to 30 percent

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strength which has a freezing point in the approximate

viewed as a hazardous substance, capable of causingserious burns. Personnel responsible for handling andfeeding the chemical must understand its potentiallydangerous nature, know what precautions should betaken and be supplied with appropriate protectiveclothing, safety showers, etc.

(6) Recarbonation. Recarbonation involves the in-troduction of carbon dioxide and/or bicarbonate ioninto softened water for the purpose of neutralizing ex-cess hydroxide alkalinity and relieving calcium carbon-ate and magnesium hydroxide supersaturation. Car-bon dioxide should either be purchased as liquefiedcarbon dioxide, which must be stored at the plant in arefrigerated storage tank, or generated at the watertreatment plant by the combustion of coke, oil, or gas.Recarbonation can also be achieved by utilizing carbondioxide and bicarbonate available in the raw water.This is the split treatment process.

(a) Chemical reactions. The following reactionsillustrate the chemistry of the recarbonation process:

Neutralization of excess lime.

The above reactions are accompanied by importantchanges in the pH of the softened water, and the pHvalue is used as a recarbonation control parameter. Re-carbonation can be practiced in a single-stage or two-stage configuration. If recarbonation is accomplishedin two stages, the first stage is devoted to neutraliza-tion of most of the excess lime. This involves conver-sion of excess lime to calcium carbonate and a pHchange from about 11 to approximately the 9.5-10range. Following the first stage of recarbonation, thewater must be flocculated and settled to remove excesscalcium carbonate. Coagulant such as silica, starch,polymer or ferric sulfate may be employed to assist incoagulation and settling of the calcium carbonate par-ticles. The second stage of recarbonation, usually justahead of filtration, serves principally as a trim stagein which final pH adjustments are made, as necessary.Guidance as to the correct pH can be obtained throughcalculation of the saturation index (see para 2-12c).For softened waters of low alkalinity, a plus index isgenerally advisable. Carbon dioxide added in the sec-ond stage converts carbonates to bicarbonates. If onlya single stage of recarbonation is employed, the carbon

---dioxide feed must be adjusted so that the previouslydescribed reactions take place to the extent necessaryat the single point of recarbonation. Single stage recar-

TM 5-813-3/AFM 88-10, Vol 3

The magnesium hardness of the finished water can beestimated from the following:

MgH = magnesium hardness of finished water inmg/L

MgS = magnesium hardness of the first stage sof-tened water in mg/L

MgR = magnesium hardness of the raw water inmg/L

P = /o bypass water(8) Incidental benefits of lime softening.

(a) Disinfection. Excess lime provides excellentbactericidal treatment, especially at pH values above10.5. Lime treatment, while not a substitute for chlori-nation, is an effective supplement,

(b) Reduction of dissolved solids. Removal ofcarbonate hardness by lime treatment results in reduc-tion in the total dissolved solids content of the water.All reaction products of lime softening are relativelyinsoluble. The lime added to the water, as well as thecarbonate hardness constituents in the water, arelargely precipitated.

(c) Iron and manganese removal. Lime softeningis also highly effective as a means of iron and manga-

nese removal. The high pH achieved insured essential-ly complete precipitation of any iron and manganesepresent in the raw water.

(d) Clarification. Lime softening provides excel-lent coagulation and clarification as a result of the pre-cipitation of magnesium hydroxide plus a largeamount of calcium carbonate.

(9) Softening plant design. The equipment, bas-ins, and filters required for lime, lime-soda ash, lime-caustic, or split treatment softening are generallysimilar to the facilities used in conventional coagula-tion-filtration plants. Two stages of treatment areusually advisable. The design of a lime-soda ash orsimilar softening plant is a complex and difficult taskrequiring the services of engineers experienced in proj-ects of this kind. Their assistance should be sought inearly stages of project planning.

(a) Mixing equipment, One problem encoun-tered at softening plants is vibration of rapid mixingdevices due to nonuniform deposits of calcium carbon-ate scale._ Frequent cleaning of the mixer may be re-quired. The frequency of such cleaning can be reducedby recirculation of previously precipitated calcium car-bonate sludge from the settling basin to the rapid-mixchamber. Parshall flumes can serve as mixing devices.

(b) Flocculation and clarification. Each separatestage of flocculation and clarification should have a to-tal detention time at design flow of about 2.5 hours, 30minutes for flocculation and 2 hours for clarification.Average depths of both flocculation and clarificationunits should be 8 to 15 feet. The overflow rate in clari-fiers at design flow should be about 0.75 gpm persquare foot.

(c) Sludge removal and recirculation. First-stagesettling basins shall have mechanical sludge removalequipment. Such equipment is also desirable in the sec-ond-stage basins which follow recarbonation. Sludgerecirculation is generally desirable except during oc-currences of severe taste and odor problems. Recyclingof a portion of the settled sludge, which is high in cal-cium carbonate, to the rapid-mix chamber is effectivein promoting the softening reactions, especially car-bonate precipitation. Where_ presedimentation is em-ployed, recycling sludge to the presedimentation basininfluent will enhance the performance of the presedi-mentation basin.

(d) Solids contact units. Solids contact type ba-sins may be used at many softening plants, particular-ly those treating ground water, These basins providethe functions of mixing, sludge recirculation, sedimen-tation and sludge collection in a simple compact unit.Basins of this type, if properly sized, will provide ef-fective softening and clarification treatment. Overallbasin depths of 10 to 15 feet should be used, and theunit should be designed so that the softening slurry isrecirculated through the center chamber at a rate of

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flow 3 to 5 times as great as the rate of flow throughthe entire unit. The upflow rate at the slurry separa-tion level in the clarification zone should not exceedapproximately 1.5 gpm per square foot.

(e) Chemical application and storage. Lime feed-ers and slakers are key items of equipment at a soften-ing plant and must be selected on the basis of reliabil-ity. Another important item requiring careful consid-eration by the designer is chemical storage. Dependingon the size of the plant, bulk or bag unloading and stor-age for lime and soda ash must be provided. Storageequivalent to at least 30 days average use shall be pro-vided. Caustic soda, if used, will generally be pur-chased as a 50 percent solution and appropriately sizedstorage tanks must be provided for this chemical.

(f) Sludge disposal. A disadvantage of any limesoftening process is the production of a large mass ofsludge of high water content. Provision for its disposalin an environmentally acceptable manner must bemade and this problem must be carefully considered inconnection with softening plant location and design.

c. Cation exchange softening. Hardness is causedprincipally by the cations calcium and magnesium, andcation exchange softening is accomplished by exchang-ing these ions for a cation, usually sodium, which doesnot contribute to hardness. This exchange is achievedby passage of the water through the bed of a granular

change water softeners at fixed military installationshall use polystyrene resins as the softening media.Such resins must have a hardness exchange capacity ofat least 25,000 grains of hardness per cubic foot of res-in.

(2) Regeneration of ion exchange softeners. Theregeneration process generally involves three steps: (1)backwashing, (2) application of regeneration solutions,and (3) rinsing.

(a) Back washing. The purposes of water soften-er backwashing are generally the same as the purposesof filter backwashing. Any turbidity particles filteredout of the water during softening are removed by thebackwashing process. For polystyrene resin media, bedexpansions of from 50 to 100 percent are normally re-quired, which involves backflow rates of 4 to 10 gal-lons per minute per square foot of bed area. Backwashperiods generally range from 2 to 5 minutes. Ion ex-change water softeners which operate upflow ratherthan downflow will not require backwashing, but thewater to be softened must be virtually free of suspend-ed matter.

(b) Application of salt brine. After the unit hasbeen backwashes, a salt solution is applied to the me-dium in order to regenerate its softening capabilities.

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brines should be 10 to 15 percent solu-tions of salt. The more salt used in the regeneration ofa softener, the more complete the regeneration will be,and the greater the exchange capacity of the regen-erated medium will be. The costs of the extra salt re-quired to obtain the added exchange capacity must beweighed against the advantages of the higher ex-change capacity in order to determine which salt dos-age to use. Salt consumption commonly ranges fromabout 0.3- to 0.5-pound of salt per 1,000 grains ofhardness removed. The contact time of the brine withthe softening medium also has a direct effect on theexchange capacity of the regenerated medium. Con-tact times of 20 to 35 minutes will generally be used.

(c) Rinsing. After regeneration, the brine mustbe rinsed from the unit before softening is resumed.Disposal of backwash water, spent regenerant, andrinse water must be carefully considered.

(3) Ion exchange water softeners. Although mostion exchange softeners at military installations will bedownflow pressure softeners, softening can also beachieved upflow. Larger ion exchange softening facili-ties are often operated upflow in order to avoid the ne-cessity of backwashing. In general, ion exchange soft-eners are of two types; open gravity softeners andpressure softeners.

(a) Open gravity softeners. Open gravity soften-ers are constructed in much the same manner as rapidsand filters, and the modes of operation are very simi-lar. However, the ion exchange medium used in opengravity softeners is much lighter than the sand used infilters, so backwash rates for open gravity softenersmay also be operated upflow, but the softener will notachieve any filtering effects so the influent water mustbe virtually free of suspended matter.

(b) Pressure softeners. A polystyrene resin me-dium used for pressure softening shall have a mini-mum bed depth of 24 inches and physical propertiesapproximately the same as the following:

rate throughto 8 gpm per

square foot but must not exceed 10 gpm per squarefoot under the most severe loadings. Severe reductionsin exchange capacity are experienced if the softeneroperates at rates of flow in excess of 10 gpm per cubicfoot for sustained periods of time. With upflow soften-ing, the rate of flow should be adjusted to maintain abed expansion of from 40 to 60 percent. The degree ofbed expansion is a function of both the flow rate and

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the temperature of the influent water, so the flow ratemust be decreased as water temperature decreases if aconstant bed expansion is to be maintained,

(4) Blending, An ion exchange softener operatingproperly will produce a water having a hardness ap-proaching zero. Inasmuch as it is not generally eco-nomical nor desirable to soften all water to this lowhardness level, provisions, for blending the softenedwater with the unsoftened water are desirable.

(5) Other factors affecting ion exchange soften-ing,

(a) Turbidity, Turbidity particles present in thewater influent to the softener are deposited on thesoftening medium and may cause losses of exchangecapacity and excessive head losses through the soften-er. If turbidity levels are excessive, the particles mustbe removed from the water prior to softening or spe-cial backwashing procedures must be implemented.

(b) Bacterial slimes. Unless proper disinfectionis practiced, bacterial slimes can form in the softeningmedium and cause excessive head losses