CHAPTER 10PRECIPITATION

Once water has been taken from its source, where it may have been in a state ofequilibrium, it is often exposed to pumping, aeration, and heating, any of whichmay upset its stability and lead to corrosion or scaling. Whether a particular waterwill tend to corrode metal or form a CaCO3 scale can be roughly predicted by itsstability index, which can be calculated from the solubility product of calciumcarbonate and the concentrations of certain ions in the water. The same principlesused in predicting water stability apply to all precipitation processes.

The precipitation process makes use of the solubility product of a compoundcontaining an ion or radical that is considered detrimental, and that should, there-fore, be removed before the water is put to use. The reduction of calcium ionconcentration by precipitation as calcium carbonate is one example of this.

Adsorption is a process with some similarities to precipitation. The choice ofan adsorbent and the degree of removal that can be achieved may be only roughlydetermined; data for estimating purposes may be found in technical literature,and these are useful as a guide to bench evaluation of the selected process. Anexample of adsorption is removal of silica from water on magnesium hydroxideprecipitate.

Temperature is an important factor in both precipitation and adsorption reac-tions. The solubility product is affected by temperature; knowing the solubilitycharacteristics of the desired precipitate will influence the selection of treatmentequipment. For example, if preheating water to a higher temperature producesimproved results over those expected at ambient temperature, then heat exchang-ers may be justified. Temperature also influences the rates of all chemical reac-tions, and heating may make it possible to select smaller reaction or sedimenta-tion vessels for the process.

One of the fundamental principles of precipitation is that the size of a precip-itate increases if the chemical reaction is encouraged to occur on previously pre-cipitated particles. If a small crystal and a large crystal of the same substance areplaced in a saturated solution of the substance in a beaker, the small crystal willslowly disappear as the larger one grows; if a crystal of salt is introduced into asupersaturated, clear salt solution, it will grow as salt comes out of solution onthe surface of the seed crystal in preference to forming individual crystal nuclei.Because of these reactions, most precipitation processes in water treatment areconducted by introducing precipitating chemicals into the water in the presenceof previously precipitated sludge.

SOFTENING BY PRECIPITA TION

Lime softening, the most widely used precipitation process, serves well to illus-trate the importance of four key variables in precipitation: (1) solubility, (2) par-ticle charge, (3) temperature, and (4) time. Lime softening is the reduction ofhardness by the application of hydrated lime to water to precipitate CaCO3,Mg(OH)2, or both.

At first glance, it may appear paradoxical that lime, a compound of calcium,can be added to water to remove calcium; the explanation is that the hydroxylradical is the reactive component of lime, converting CO2 and HCO3" to CO3

2",causing CaCO3 to precipitate, as shown by reactions (1), (2), and (3).

Ca(OH)2 - Ca2+ + 2OH~ (1)

Ca(OH)2 + 2CO2 — Ca(HCO3)2 (2)

or, in ionic form:

2OH- + 2CO2 - 2HCO3-

Ca(OH)2 + Ca(HCO3), - 2CaCO31 + 2H2O (3)

in ionic form:

2OH- + 2HCO3- - 2CO32- + 2H2O

Other hydroxide compounds (NaOH, KOH) could also be used, but these can-not usually compete against the low cost of lime except in special circumstanceswhere they may be available as by-products.

Most softening reactions are carried out at a pH of about 10. At this pH,CaCO3 usually carries a negative, and Mg(OH)2 a positive, charge. If these chargesare not neutralized, colloidal hardness may resist flocculation and carry over intothe effluent. Cationic coagulants may be needed when the bulk of the precipitateis negatively charged CaGO3, as in partial lime softening. Sodium aluminate isfrequently used as an anionic coagulant when low magnesium residuals[positively charged Mg(OH)2] are needed in complete lime softening. The coag-ulant may be supplemented by an anionic or a nonionic flocculant.

The water chemist distinguishes between cold process lime softening, usuallycarried out in the range of 40 to 9O0F (4 to 320C), and hot process, at 215 to 23O0F(102 to 11O0C). Results achievable at intermediate temperatures are often of inter-est. Obtaining data in the intermediate range may require bench testing, as veryfew plants have operated in this range. At about 12O0F (490C), silica removal,which is negligible in cold process softening at temperatures close to freezing,increases. At about 140 to 16O0F (60 to 710C), lime softening of sewage, which israther incomplete cold, begins to approach hot process results. These phenomenaillustrate the importance of laboratory testing under actual operating conditionsto obtain reliable data before a plant is designed and built.

PARTIAL LIME SOFTENING

The most prevalent precipitation process in water treatment is the reduction ofcalcium hardness by its precipitation as CaCO3. Since the alkalinity of mostwaters is in the bicarbonate form, and because there is usually CO2 present, the

precipitation of calcium carbonate requires the conversion of CO2 and bicarbon-ate to carbonate, as shown by reactions (1), (2), and (3).

As was pointed out previously, CaCO3 precipitation is not as simple a reactionas it appears to be because of the formation of ion pairs and because of interfer-ences, such as the stabilization of colloidal calcium carbonate preventing its pre-cipitation in the normal reaction time. Because of this, the theoretical solubilitycurve is of limited value in predicting the actual results that might be achieved ina precipitation reaction such as partial lime softening. For estimating purposes,the solubility as shown by Figure 10.1 is commonly used in the water treatmentindustry. This assumes the solubility of calcium carbonate at ambient river wateror well water temperatures to be about 35 mg/L as CaCO3. (Obviously, if thewater supply is already less than 35 mg/L in calcium hardness, lime softening forcalcium reduction is of no value.) It also assumes about 60 min detention in thereaction tank.

In predicting the results of partial lime softening, there are two cases to beconsidered, one in which the calcium hardness of the raw water exceeds its alka-linity and the other in which the alkalinity exceeds the calcium hardness.

Carbonate,mg/l as CaCO3

FIG. 10.1 The data used for estimating calcium carbonateresiduals in conventional lime-softening plants with 60- to 90-min detention.

Where the calcium exceeds the alkalinity (Figure 10.2), the results of treatmentare calculated by establishing first that there has been no change in any anionsexcept alkalinity, which is converted to 35 mg/L CO3, as CaCO3. In the cationportion of the analysis, the magnesium is shown as being reduced by about 10%.The sodium remains unchanged, and the calcium is then calculated by difference.Figure 10.2 shows an analysis of a raw water having a calcium hardness exceedingthe alkalinity, with results after partial lime softening, following this method of

Cold water at 50-7O0F(10-2O0C)

Hot water at 22O0FItOS0C)

Ca

lciu

m,m

g/l

as C

aCO

s

FIG. 10.2 Partial cold lime softening results where Ca > M alkalinity in the raw water analysis.

calculation. The approximation of pH is taken from alkalinity relationships dis-cussed in an earlier chapter.

Where the alkalinity exceeds the calcium (as shown in Figure 10.3), the cationsection of the analysis is calculated first: the calcium is shown as 35 mg/L; themagnesium is reduced by 10%, and the sodium is unchanged. In the anion sectionof the analysis, except for alkalinity, the remaining anions are unchanged. Thetotal alkalinity is then calculated by difference. Of the total alkalinity, at least 35mg/L must be present as carbonate. The balance may be bicarbonate, or as muchof this may be converted to carbonate as wished, depending on the desired sta-bility index and pH of the system. Figure 10.3 shows a raw water having an alka-linity in excess of calcium hardness and shows the results of treatment, with sev-eral examples of varying degrees of conversion of bicarbonate to carbonate.

Identification of Analyses Tabulated Below:

A. Raw water @ 60°F D.

B. Estimated results E.

C. F.

ConstituentCalciumMagnesiumSodium

Total Electrolyte

Bicarbonate

CarbonateHydroxylSulfateChloride

Nitrate

M AIk.

P AIk.

Carbon Dioxide

PH

SilicaIron

TurbidityTDSColor

AsCaCOj

CaCO3

CaCO,

CaCO3

Fe

A

1256520

210

100OO6050

100O10

7.3

100.2

230

B

656020

145

O35O6050

351810

10.0

10Nil

165

C D E F

FIG. 10.3 Partial cold lime softening results where M alkalinity > Ca in the raw water analysis.

With experience in using such calculations and in reviewing actual plantresults, the water treatment engineer can improve accuracy in prediction of resultsof partial lime softening. For example, it is apparent that if the calcium is greatlyin excess of alkalinity in the raw water, the shape of the solubility product curveis such that the carbonate would be lowered below 35 mg/L. Carbonate alkalini-ties as low as 20 mg/L have been observed in partial lime softening of well waterat 550F (130C) having an excess calcium of more than 200 mg/L.

The reason for showing a magnesium reduction despite the fact that this maynot be one of the goals of the treatment program is that it is impossible to instan-taneously mix a slurry of lime into a large body of water; at the point of limeintroduction the water is massively overtreated with lime, and it is inevitable thatsome magnesium will precipitate because of this. The 10% reduction is arbitrary

Identification of Analyses Tabulated Below:

A Raw water @ 60°F D.

B Estimated results with complete conversion to CO3

C. Estimated results with partial conversion to CO..«__« -* "•.._.

ConstituentCalciumMagnesiumSodium

Total Electrolyte

Bicarbonate

CarbonateHydroxylSu I fateChloride

Nitrate

M AIk.

P AIk.

Carbon Dioxide

PH

SilicaIron

TurbidityTDSColor

As

CaCO3

CaCO3

CaCO3

CaCO3

Fe

A

100

5075

225

125O

— *T-50

125OO

8.3

80.3

255

B354575

155

O55

— ̂CT-

50

5528O

10.0

8Nil

185

C D E F354575

155

2035O5050

5518O

9.8

8Nil

185

Bicarbonate alkalinity of raw water, mg/1 as CaCOs

FIG. 10.4 Cold process lime softening.

and empirical, based on a ratio of Ca:Mg of 2:1; if this ratio is reduced, magne-sium reduction increases.

Mg2+ + Ca(OH)2 - Mg(OH)2 J + Ca2+ (4)

In this process, the lime requirement is based on the CO2, alkalinity convertedto carbonate, and magnesium reduction. A simplified chart for determining limerequirement is shown in Figure 10.4.

COMPLETE LIME SOFTENING

Sometimes the residual calcium hardness after partial lime softening may still behigher than the municipality or industrial operation requires. If so, additional cal-cium reduction is achieved by adding soda ash. The reaction is as follows:

Ca2+ H- Na2CO3 — CaCO3 I + 2Na+ (5)

As calcium is precipitated as CaCO3, it is replaced by the sodium from the sodaash. Using the same example as shown earlier for partial lime softening, Figure10.2, the further reduction of calcium hardness by soda ash addition in severalstages is shown in Figure 10.5. The amount of soda ash required is simply cal-culated on the basis of the additional calcium hardness reduction wanted. Thisaddition can continue until the calcium reaches a level of about 35 mg/L; beyondthis, excess soda ash has only a partial, rather than a direct, effect in reducingcalcium further. Figure 10.5 shows the results of excess soda ash addition on cal-cium hardness, alkalinity, and dissolved solids.

Req

uire

d lim

e [9

3%

Ca(

OH

) 2],

lbs/

10

00

gals

FIG. 10.5 Cold lime-soda softening for calcium reduction.

If reduction of magnesium is desired along with calcium reduction, additionallime must be added beyond that required for partial softening to react with all ofthe magnesium and to provide an excess hydroxide alkalinity. The procedure forestimating results again depends on which of two categories the water falls into:in the first, the total hardness exceeds total alkalinity, and in the second thereverse is true.

Where total hardness exceeds alkalinity, the anions are calculated first. Thereis no change in anions other than alkalinity. Since magnesium reduction isdesired, an excess hydroxide alkalinity of 20 mg/L is usually selected. The car-bonate alkalinity is shown as 35 mg/L, and there is no bicarbonate alkalinity. Inthe cation section, the sodium is unchanged and the magnesium is reduced to 20mg/L. The calcium is then determined by difference. If this calcium level is still

Identification of Analyses Tabulated Below:

A. Raw water @ 60°F p. "C" after an additional 30 mq/1 soda ash*

BEstimated results with lime E

C11B" after addition of 30 mg/1 soda ash* F

ConstituentCalciumMagnesiumSodium

Total Electrolyte

BicarbonateCarbonateHydroxy!SulfateChlorideNitrate

M AIk.

P AIk.

Carbon Dioxide

PH

SilicaIron

TurbidityTDSColor

As

CaCO3

CaCO3

CaCO.

CaCO3

Fe

A

1256520

210

100OO6050

100O10

7.3

100.2

230

B

656020

145

O35O6050

3518O

10

10Nil

165

C D E F

35 2060 6050 80

145 160

O O35 50O O60 6050 50

35 5018 25O O

10 10

10 10Nil Nil

165 180

*Expressed as CaCO3 = 32 ng/1 Na2<)O3

FIG. 10.6 Complete cold lime-soda softening where raw water hardness exceeds alkalinity.The 20% reduction in silica can be improved by longer detention, higher solids in the recircu-lation zone, temperature, or a combination of these.

higher than desired, soda ash is added to reduce it, and each increment of calciumreduction results in an incremental sodium increase.

The lime is again calculated as that required for CO2, bicarbonate conversion,and magnesium reduction plus an excess of 20 mg/L. The soda ash dosage is cal-culated by the desired calcium reduction in a stepwise fashion. Figure 10.6 showscomplete cold process lime soda softening of a well water where the hardnessexceeds the alkalinity.

At one time, batch lime-soda softening was a common process in treatingwater for steam locomotives. Hardness levels considerably below those shown inFigure 10.6 were achieved because of the long detention time. Batch softeners are

Identification of Analyses Tabulated Below:

A Raw well water @ 55°F D

Q After lime treatment E

C. After lime-soda treatment F.

Constituent

CalciumMagnesiumSodium

Total Electrolyte

Bicarbonate

CarbonateHydroxyl

SulfateChloride

Nitrate

M AIk.

P AIk.

Carbon Dioxide

PH

SilicaIron

TurbidityTDSColor

AsCaCO3

CaCO3

CaCO,

CaCO3

Fe

A175100

25

300

200OO6040

200

O20

7.3

150.5

350

B110

2025

155

O35206040

55

38O

10.6

12Nil

205

C D E F

2020

130

170

O50206040

7045O

10.6

12Nil

220

still in use in some steel mills today producing final hardness of less than 20mg/L.

When the alkalinity exceeds the hardness, the cation section is calculated first,and soda ash is never used. The sodium is shown unchanged, the calcium isreduced to 35 mg/L, and the magnesium reduced to 20 mg/L. The anions otherthan alkalinity remain unchanged, the hydroxide alkalinity is shown as 20 mg/L,and the balance is carbonate. If this carbonate alkalinity is too high, it can bereduced by the addition of gypsum, CaSO4, each increment of gypsum producingan equivalent reduction of alkalinity, according to the following reaction:

CO32- + CaSO4 -* CaCO31 + SO4

2~ (6)

In all of these cold process reactions, it is extremely important that previouslyprecipitated sludge be returned to the reaction chamber for mixing with raw waterand treatment chemicals. A typical design of a cold process lime softening precip-itation unit is shown in Figure 10.7. Another design, shown in Figure 10.8(#), isa unit in which the reactions occur on a bed of calcium carbonate granules. Thisis more compact than the first design, but it requires sophisticated chemical feedequipment so that the chemical feed rate is instantaneously adjusted to changesin water flow rate, since there is no provision for recirculation within the reactionvessel itself. The detention time is only about 12 to 15 min. The precipitation ofCaCO3 from the lime reaction occurs directly on the granules, but neitherMg(OH)2, Fe(OH)3, nor raw water suspended solids are trapped, so these remainin the effluent. This type unit has had a long history of use in Europe on hard wellwaters. It is sometimes used as the first stage of treatment, with the effluent goingto a conventional sludge-contact softener for final treatment, including removalof suspended solids.

In all of these softening systems, a check of the Langelier index shows that thetreated water is supersaturated with respect to CaCO3. After-precipitation willoccur, hastened if the water is heated, unless it is further treated or stabilized. Thecommonly used treatment is pH reduction, addition of a polyphosphate or anequivalent sequestering agent, or both. Recarbonation is usually the preferredmethod of pH reduction, especially in municipal softening plants, although acidis a common substitute. Recarbonation, however, can often do more harm thangood if the softened water contains OH alkalinity, because this will be convertedto carbonate in reacting with CO2, further supersaturating the water with CaCO3.

FIG. 10.7 Typical sludge-blanket cold process lime softener with high-rate sludge recircula-tion. (Courtesy oflnfilco Degremont Incorporated.)

Hood

EffluentClarified water

OrificeChemical

Return Flow Zone

Primary Mixing ft Reaction ZoneConcentrator

Sample cock

Slurry PoolInfluent

Draft Tubes

WalkwayChemicalImpeller Drive

Secondary Mixing andReaction Zone

Sludge Discharge

Blow-off and drain

FIG. 10.8 (a) Granular bed lime softener producing a concentrated, pelletlike sludge. (Courtesy of thePermutit Company.) (b) Plant installation. (Courtesy of the Permutit Company.)

Softenedwater outlet

Chemical inlets

Raw waterinlets

Raw waterinlet

Chemicalinlets

Blowdown

FIG. 10.8 (c) Left, typical lime softener sludge; right, pellets produced by granular bed soft-ener. (Courtesy of the Permutit Company.)

This often results in filter plugging, scaling of piping, deposits in low-flow areas,or all three. (See Chapter 13, Neutralization.)

MORE COMPLETE SOFTENING A T HIGHERTEMPERATURES

Hot process softening is somewhat different in that the solubility of calcium andmagnesium are both lower at elevated temperatures, and the rate of reaction isconsiderably increased. The CaCO3 precipitates as calcite in the cold process andaragonite in the hot process. The precipitates settle much faster in hot water,

Identification of Analyses Tabulated Below:

A. Raw well water @ 55 F D.

B. After treatment* E

C. F.

FIG. 10.9 Hot process lime-soda softening at 22O0F.

ConstituentCalciumMagnesiumSodium

Total Electrolyte

BicarbonateCarbonateHydroxylSulfateChloride

Nitrate

M AIk.

P AIk.

Carbon Dioxide

PH

SilicaIron

TurbidityTDSColor

AsCaCO3

CaCO3

CaCO,

CaCO3

Fe

A175100

25

300

200OO6040

200O20

7.3

150.5

350

B15

2133

150

O40106040

I4025O

10.3

1-2O

185

C D E F

* Not corrected for about 15% steam dilution

whose density and viscosity are appreciably less than cold water's. There are twobasic hot process systems, lime-soda and lime-zeolite.

The lime-soda process is similar to the cold process system, but the results aredifferent, as shown in Figure 10.9. The amount of lime required is less becauseusually none is needed for CO2 reaction, since most CO2 is eliminated by heatingthe water and spraying it into the reaction vessel. Some high CO2 well waters aredifficult to degas, and may require excess lime for the residual CO2. Silica is appre-ciably reduced by adsorption on the magnesium hydroxide precipitate, and theresidual hydroxide alkalinity needed for magnesium precipitation is also consid-erably reduced.

There are two basic designs of hot process softeners applicable either to thelime-soda process or the lime-zeolite process. The first (Figure 10.10) is a sludgeblanket design, particularly effective where silica reduction is important, butsomewhat sensitive to load fluctuations. The second (Figure 10.11) is called adownflow design, and depends on a recirculation pump entirely for providingsludge contact; it is generally not quite as effective for silica reduction as thesludge blanket unit, but is less susceptible to upset caused by load fluctuations.

With the lime-zeolite design, complete hardness removal is achieved and loweralkalinity levels can be produced because soda ash is not required for reducingcalcium hardness. Figure 10.12 shows the results of treating a water high in excesshardness. If the water has an alkalinity in excess of hardness, alkalinity reductioncan be achieved either by acid feed ahead of the softener or by the addition ofgypsum to the reaction zone. Figure 10.13 illustrates a complete lime-zeolitesystem.

SILICA REMOVAL

Although silica can be removed by adsorption on iron floe in the coagulation pro-cess, efficiency is low. For that reason, where silica removal is required, it is usu-ally done in a hot or warm process system. The silica is adsorbed on magnesiumhydroxide precipitate, which is either formed from the lime softening reaction oradded by using dolomitic lime as the water-softening reagent. Typically, theequipment used is similar to the hot process softeners shown in Figures 10.10 and10.11. Since the process can be made to operate effectively at temperatures as lowas 12O0F (490C), these systems can be modified to maintain this temperaturewithin rather close limits, which is necessary to avoid thermal currents that wouldupset the sedimentation process. The results anticipated by precipitation of mag-nesium hydroxide for hot process adsorption of silica are shown by Figure 10.14.

Rules of thumb can often be misleading, and the frequent statement that silicaremoval is poor in cold process softening is in that category; it assumes that (1)in winter, the water will be close to 320F (O0C); (2) the detention time in the soft-ener is only about 60 min; and (3) the precipitation of magnesium is only in therange of 20 to 30%.

Under proper conditions, silica removal can be quite effective in the cold pro-cess, as shown in the treatment of a 7O0F (210C) well water in Mississippi with asoftener having effective solids recirculation and substantial magnesium precipi-tation (Figure 10.15). Silica residuals are shown for three conditions of operation.In some plants having high CO2 concentrations in the raw water, silica removalis enhanced by installing a magnesium dissolving basin ahead of the softener. A

FIG. 10.10 Sludge blanket design of hot process softener with accessory equip-ment. (Courtesy ofCochrane Division, the Crane Company.)

FIG. 10.11 Downflow design of hot process softener with control devices and provisionfor sludge recirculation. (Courtesy ofCochrane Division, the Crane Company.)

FIG. 10.12 Hot process lime-zeolite treatment at 22O0F.

portion of the sludge from the softener is recycled to this mixing basin and theCO2 selectively redissolves magnesium from the sludge, increasing the magne-sium subsequently reprecipitated in the softener to improve silica adsorption.

HEA VY METALS REMOVAL

Heavy metals are usually removed from water by precipitation, although ionexchange and adsorption are also used. Iron is a typical example of a heavy metalrequiring removal, because it is a common constituent of well water and must be

Identification of Analyses Tabulated Below:

A Raw well water Q 55°F D

B After lime treatment* E

C. "B" after zeolite treatment* F

Constituent

CalciumMagnesiumSodium

Total Electrolyte

Bicarbonate

CarbonateHydroxyl

SulfateChloride

Nitrate

M AIk.

P AIk.

Carbon Dioxide

pH

SilicaIron

TurbidityTDSColor

AsCaCO.,

CaCO3

CaCO.

CaCO.,

Fe

A

175100

25

300

200OO6040

200O20

7.3

150,5

350

B

982

25

125

O15106040

2518O

10.3

1-2O

I 160

C D E F

Nil

Nil

125

125

O15106040

2518O

10.3

1-2O

160

* Not corrected for approximately 15% s;team dilution

Chemical feed Hot process sof tener F i l ters Zeol i te sof teners Regenerating equipment

FIG. 10.13 Line diagram of a typical hot lime-zeolite system. (Courtesy ofCochrane Division, the CraneCompany.)

Timer

Meter

Raw waterinlet

Inlet control valveVent Direct contact

gas concentratorSteam inlet

Over f low

Treated water outlet

Wash water from f i l te rs

Wash water to f i l t e rs

Back pressure v a l v e ,

-Hot displacement water

•Brine measuring tank

Salt storagetank

Toboilers

To 'waste

Brinepump

Boosterpump

Automaticdesludgmq(opt ional )

Sludgerecirculationpump

To waste

Sludge

Sludgeb low-o f fva lve

Washpump

Magnesium precipitated, mg/l as CaCO3 + dolomitic

lime added, mg/1

FIG. 10.14 Silica reduction in hot process softening. Results areaffected by time, temperature, sludge density, the amount of Mgprecipitated from ionic form versus paniculate MgO added, andinterferences from organic matter (color). The high silica levels(100 to 150 mg/L) found in arid regions (e.g., northern Mexico andthe southwestern United States) are removed much more effec-tively by magnesium precipitation (almost a stoichiometric reac-tion) than are the lower concentrations typical of many lakes andrivers (5 to 20 mg/L). As this chart indicates, it is surprising to findthat it is as difficult to reduce SiO2 from 10 mg/L to a residual of 1mg/L as to reduce it from an initial level of 100 mg/L down to 10mg/L.

removed from potable supplies. It is also frequently found in wastewaters requir-ing treatment before discharge.

The hydroxides of heavy metals are usually insoluble, so lime is commonlyused for precipitating them. However, sometimes the carbonates, phosphates, orsulfides are less soluble than the hydroxides, so precipitation in these forms mustalso be considered. There are probably rituations where economics justify partialprecipitation with lime or soda ash to the solubility level of the hydroxide or car-bonate followed by a secondary treatment with phosphate or sulfide for reductionto the specified limits (Figure 10.16). So, the choice of the reactant is the firstconsideration in the precipitation of heavy metals. Since solubility is affected bytemperature, this becomes a second consideration.

A third important factor in precipitation of heavy metals is the valence stateof the metal in the water. For example, ferrous iron is considerably more solublethan ferric iron. Because of this, treatment of the water with an oxidizing agentsuch as chlorine or potassium permanganate to convert ferrous iron to the ferricstate is an essential part of the iron removal process. Another example is chro-mium, whose hexavalent form, chromate, CrO4

2" (Cr6+ + O48"), is considerably

Appr

oxim

ate

rem

oval

of

SiO

a, %

INJ A L C O CH E M IC AL COMPANY

Form 402-PC Printed in U.S.A. WG 3/72

FIG. 10.15 Partial cold lime softening in slurry-type solids contact unit with silica removalwith magnesium precipitation.

WATER TREATMENT PROCESS DATA

FIGURE 10.15

File: Partial Cold Lime Softening in Slurry-Type Date: February 16, 1983solids contact unit with silica removal withmagnesium precipitation

Identification of Analyses Tabulated Below:

A Water from comb, wells Q

g Accelator effluent - lime g

softened @ 65°F F.

ConstituentCalciumMagnesiumSodium

(Hardness )

Total Electrolyte

BicarbonateCarbonateHydroxylSulfateChlorideNitrate

M AIk.

P AIk.

Carbon Dioxide

pH

SilicaIron

TurbidityTDSColor

As

CaCO3

CaCO3

CaCO3

CaCO3

Fe

A256168114

(424)

538

366OO

Nil

172

O

366O

7.0

28

Nil

B

70

32122

(102)

224

O16

36Nil

172

O

5242

10.6

7

Nil

C D E F

FIG. 10.16 Comparison of sulfide solubilities of certain cations to theirhydroxides and carbonates.

A final aspect of heavy metals precipitation is the possible formation of com-plex ions, which are common when dealing with wastewaters containing ammo-nia, fluoride, or cyanide along with the heavy metals. For example, iron may becomplexed as the ferrocyanide ion, which is rather soluble, and will remain insolution unless the complex can be broken by chemical treatment.

Because of these important aspects in the precipitation of heavy metals, thereis no way to predict the best solution of a specific problem without undergoing aseries of bench tests to evaluate the alternatives available. The removal of ironfrom water is an interesting example of this.

more soluble than the trivalent form, Cr3+. In this case, the chromate (in whichCr is present as Cr6+) must be reduced, usually with SO2 at a low pH, for removalof chromium (as Cr3+) by a precipitation process.

A fourth aspect of the precipitation process is the zeta potential of the initialheavy metal colloidal precipitate. In many plants where heavy metals are beingremoved, one of the principal problems in reaching the desired effluent limits isthe colloidal state of the precipitated materials—they have not been properly neu-tralized, coagulated, and flocculated.

Con

cent

ratio

n, m

illim

oies

/lit

er

FIG. 10.17 Schematic of iron-removal filter plant. Closed pressure aeratormay be used in place of the open tower shown. Where an overhead storage tankis used for town or plant supply this can be used to supply backwash water.

Manganese-treated zeolite is often used as the filtration medium in filters ofthis type. Where this is done, permanganate may be fed to the water ahead ofthese filters, and lime may be fed for pH correction as well. Those who haveworked with this process claim that the oxides of manganese produced by thisreaction are catalytic to the oxidation of iron within the filter bed. A permanga-nate-treated water supply using manganese zeolite as the filter medium is illus-trated by Figure 10.18.

In the precipitation of iron from water, the first step is the oxidation to theferric condition, as illustrated by the following reaction:

2Fe2+ + Cl20 - 2Fe3+ + 2CT (7)

Sometimes air can be used successfully to oxidize iron, but most frequently chlo-rine or potassium permanganate is required. The chlorine may be applied as chlo-rine gas or a calcium hypochlorite. Unless there is past experience with a specificwater supply, each of these oxidation reagents must be evaluated to select the bestprocess.

The pH of the water must be adjusted to an optimum value, determined notonly by the solubility of the precipitate, but also by its charge. It may be necessaryto determine the zeta potential in selecting the optimum pH for treatment. Thereare distinctive differences in the chemical used for pH adjustment also. For exam-ple, lime is usually more effective than caustic soda at the same pH, and this mayvery well be attributed to the charge on the particles and charge neutralization.

Once the iron has been oxidized and precipitated, the volume of sludge pro-duced must be examined to decide whether the treated water can then be clarifiedby direct filtration or will require treatment through a sedimentation tank priorto filtration. Generally where the iron is less than 5 mg/L, the oxidized water canbe fed directly to a filter. This is usually a mixed media filter provided with airscour devices so that the bed can be kept clean. A typical iron removal plant ofthis type used in municipal service is illustrated by Figure 10.17.

Overheadstoragetank

To plant

F i l t e r s

Lime andHTH feed

Aerator

Well water

FIG. 10.18 Manganese zeolite filter used for removal of iron from well water supplies.

One of the complications of the iron removal process is the fact that some iron-bearing waters also contain sulfide. This adds to the demand for chlorine or otheroxidizing agents.

MISCELLANEOUS PROCESSES

Using lime softening or iron removal type equipment, precipitation processes areoften used to remove manganese from raw water supplies, phosphate frommunicipal sewage, fluorides from industrial wastes, and copper from metallurgicalwastes. Each system is unique and requires bench testing to determine the bestreagent for the process, optimum pH, proper temperature conditions, and thenature of the sludge expected, which will influence the selection of solids/liquidsseparation devices.

Sulfates are present at high concentrations in a variety of industrial wastes. Ifdischarged into cement sewers, concentrations in excess of about 500 mg/L sulfatemay deteriorate the cement, so reduction of sulfate to below this level may berequired to protect the sewer system. Precipitation with lime to yield CaSO4 issometimes effective, but because the solubility OfCaSO4 is on the order of 1800mg/L, the residual sulfate may be too high. Precipitation as a Ca:Al:SO4:OH com-plex is effective in reducing the sulfate well below 500 mg/L. Sodium aluminateand lime, combining with some of the calcium ions already present in the waste-water, are effective agents to be used in this process at a pH over 9.5 to 10.0.

Sulfates can be removed by anion exchange (Chapter 12); when the exchangeris regenerated with salt, the by-product is a concentrated sodium sulfate solution,which must then be processed by precipitation for disposal. However, the precip-itation treatment of the smaller volume OfNa2SO4 brine on a batch basis may beeasier than treatment of the lower sulfate concentration of the raw wastewater ona continuous basis.

•Reactiontime '

Raw waterheader

Chlorinator Permanganatefeeder

.0.7-0.8 mmanthracite

0.3-0.35 mmgreensandzeolite

Air-metering tube

Air inlet

Treated water outlet and backwash inletTo waste-backwashand rinse

Fluorides can be precipitated by lime at a pH of about 10 to a residual in therange of 10 to 20 mg/L F. Alum or aluminate used with the lime produces a flu-oride complex, and the residual F, as such, is negligible by the usual tests for thefluoride ion. However, the complex fluoride can be detected if the sample is evap-orated to dryness and analyzed. Reduction to low fluoride levels requires a secondstage of treatment—adsorption by activated alumina, calcium phosphate, or mag-nesium hydroxide.

A modification of the conventional lime-soda softening process has beenapplied to treatment of cooling tower blowdown for its recovery (see Chapter 38).

TABLE 10.1 Side-Stream Treatment of Cooling Water;Two-Stage Softening in Slurry-Type Units

Circulating TreatedVariable water water

Calcium, mg/L as CaCO3 811 129Magnesium, mg/L as CaCO3 480 15Silica, mg/L as SiO2 137 7

Source: Abstracted from Grobmyer, Edge, and Hancock. Amer-ican Power Conference, April 1983.

In some cases, the blowdown is treated directly for reduction of calcium, mag-nesium, and silica; in other cases, it is more advantageous to combine the blow-down with raw water makeup and then process the blend. A complicating factoris the presence of dispersants in most concentrated cooling waters, deliberatelyadded to inhibit scale and deposit formation, and these have an inhibiting effecton some of the softening reactions. Data from a unique two-stage pilot plant treat-ing a side stream from a cooling water cycle are shown in Table 10.1. In thisprocess, lime is added to pH of about 10.8 in the first stage, followed by recar-bonation and soda ash addition to the second stage.

BENCH AND PILOT PLANT TESTING

Each special application, as in these several illustrations, must be bench tested—and often carried through to pilot plant evaluations—because so many wastewa-ters contain interfering or inhibiting additives.

The test procedures usually involve a modification of the jar test where theremay be provision for heating if high temperature is required for minimum solu-bility. In doing these tests, the residuals are usually determined both on the super-natant sample and also on a filtrate after the supernatant has passed through a0.45-/um membrane filter. The volume of sludge produced is measured with anImhoff cone to help establish the type of solids/liquids separation equipmentrequired. If the jar test does not produce good results, more sophisticated equip-ment, such as a zeta meter, may be needed to more precisely determine the natureof the particle being removed.

In all of these processes, accurate and reliable chemical feeding is essential. The

FIG. 10.19 Automatic low-level hardness analyzer. (Cour-tesy Hack Chemical Company.)

metal after treatment. This imposes special limitations on the plant, often requir-ing that the treated effluent be held temporarily in an inspection basin. The oper-ator analyzes the final discharge and the precipitator effluents, and if specificationsare not met, he must correct the treatment before the plant effluent is dischargedfrom the retention basin to a receiving stream or sewer.

SUGGESTED READING

AWWA: Water Quality and Treatment, McGraw-Hill, New York, 1971.Babber, N. R.: "Sodium Bicarbonate Helps Metal Plant Meet Federal Standards," Industrial

Wastes, January-February 1978.

pH may be critical, so the chemicals used for pH control must be proportionedto the flow of water and be corrected by a signal showing deviations from thecontrol pH setting.

Although there are instruments available for recording effluent hardness (Fig-ure 10.19), where a precipitation process is selected for removal of heavy metalsthere may not be an instrument available for a direct measurement of the heavy

Christoe, J. R.: "Removal of Sulfate from Industrial Wastewaters," /. Water Pollut. ControlFed., 48(12) 2807 (December 1976).

Grobmyer, W. P., Edge, H. D., Jr., and Hancock, Fred: "Water Supply Pilot Testing Key toDesign Optimization," Proceedings of the American Power Conference, April 1983.

Nancollas, G. H., and Reddy, M. M.: "Crystal Growth Kinetics of Minerals Encountered inWater Treatment Processes," in Aqueous-Environmental Chemistry of Metals (Rubin, J.,ed.), Ann Arbor Science, Ann Arbor, Mich., 1976.

Neil, R. E., and O'Connell, R. T.: "A New Approach to Softening Plant Sludge Reduction,"Proceedings American Water Works Association, June 1976, Paper 5897-676-3C.

"Treating Lead and Fluoride Wastes," Environ. Sd Tech. 6(4): 321 (April 1972).Water Treatment Handbook, Infilco-Degremont, Inc., Halsted Press, New York, distribu-tors, 1979.