Reduced Lime Feeds: Effects on Operational Costs and Water ... · process known as excess lime...

Reduced Lime Feeds: Effects on

Operational Costs and Water Quality

Christopher S. Jones*[email protected]

Ted P. [email protected]

David B. [email protected]

L.D. [email protected]

Des Moines Water Works2201 George Flagg Parkway

Des Moines, Iowa 50321April 13, 2005

*Correspondence should be directed to this author.

1

Water utilities struggling with high source water calcium and/or magnesium often

turn to lime softening to remove hardness. Raising treatment pH above 9.6 converts soluble

calcium bicarbonate hardness to insoluble calcium carbonate, and further pH increases

beyond 10.6 begin to convert soluble magnesium bicarbonate to insoluble magnesium

hydroxide. Aggressive magnesium removal often requires a treatment pH of 11 or higher, a

process known as excess lime softening. Precipitation of magnesium hydroxide produces

water quality benefits, but can also cause operational problems and increase expenses for a

water utility. This two-year study outlines the transition from excess lime to straight lime

softening, and its effects on operational costs and water quality. Based on the results of this

study, which took place at Des Moines Water Works� (DMWW) two water treatment

facilities, the authors believe excess lime softening may not always be the best choice for a

utility when considering hardness reduction.

LIME SOFTENING DISCUSSION

The lime softening processes of today can be traced to 1841 and the Scottish chemistry

professor Thomas Clark, who discovered that increasing the pH of water through the

addition of lime could reduce water hardness (Powell, 1954, and Nordell, 1961).

Humenick (1977) outlined four types of lime softening:

� Single-stage lime process, for source waters with high calcium and low magnesium

carbonate hardness, and low non-carbonate hardness;

� Excess lime process, for source waters with high calcium and magnesium carbonate

hardness, and low non-carbonate hardness;

� Single-stage lime�soda ash process, for source waters with high calcium and low

magnesium carbonate hardness, and some non-carbonate hardness;

� Excess lime�soda ash process, for source waters with high calcium and magnesium

hardness and some non-carbonate hardness.

2

When the most common form of lime, quicklime (calcium oxide�CaO) is hydrated,

calcium hydroxide is formed in a process known as slaking:

CaO + H2O = Ca(OH)2 (1)

Equation (2) is the crux of the process whereby calcium carbonate hardness is removed:

Ca2+ + 2HCO3- + Ca(OH)2 = 2CaCO3(s) + H2O (2)

If the source water contains high levels of magnesium, �excess� lime softening�raising

the pH beyond 10.6, and sometimes to at least 11�removes the pertinacious magnesium

hardness:

Mg2+ + 2HCO3- + Ca(OH)2 = CaCO3(s) + Mg(OH)2(s) + 2H2O (3)

Benefits of excess lime softening. Precipitation of magnesium hydroxide

(Mg(OH)2) during excess softening has water quality benefits�not the least of which is

removal of natural organic matter (NOM), recently and thoroughly discussed by Roalson,

et. al. in this journal (2003). The positively-charged Mg(OH)2 floc is large and fluffy,

providing a large surface area that enables NOM removal above and beyond that provided

by removal of calcium carbonate (CaCO3) (Liao and Randtke, 1985 and 1986, and

Thompson, et. al., 1997). Working in conjunction with other coagulants, Mg(OH)2 floc

also can assist in lowering filter applied turbidity levels.

The high pH associated with excess softening also provides removal and/or

toxicity for microorganisms not achieved by the single-stage lime or lime/soda ash

processes. Virus removal and/or deactivation is seen at pH 11 (Wolf, et. al., 1974 and Rao,

et. al., 1988), and high pH levels provide alkaline toxicity to bacteria (Brock, et. al., 1994).

Indeed, excess softening provides an additional barrier to a multi-barrier approach toward

pathogen removal.

Finally, excess softening enables removal of magnesium hardness. Conventional

wisdom for many years has been that finished water hardness should not exceed 40 mg/L

magnesium as CaCO3 (Larson, et. al., 1959), the primary consequence of magnesium

hardness being scale buildup in water heaters.

3

Limitations of excess softening. Water quality benefits aside, excess softening

does have consequences for the operation and maintenance of a water treatment facility.

Obviously, higher treatment pHs increase costs for lime�more is needed to create

the caustic alkalinity necessary for Mg(OH)2 precipitation in the treatment basins.

Mg(OH)2 floc is strongly hydrophilic; is gelatinous (in contrast to the crystalline

CaCO3) (Faust, et. al., 1998); and has a positive electromobility value, which is not

conducive to sludge coagulation (Black and Christman, 1961). All these factors make

high-magnesium residuals harder to handle and complicate dewatering.

In plants not designed to settle Mg(OH)2, carryover onto the filters can be a

problem (USEPA, 1999).

Lastly, precipitation of Mg(OH)2 can come at a cost to finished water total

hardness. Following Mg(OH)2 precipitation, sufficient CO2 must be added to neutralize

the caustic alkalinity. This occurs between pH 10.0 and 10.5. The equations associated

with this step are:

Ca2+ + 2OH- + CO2 = CaCO3(s) + H2O (4)

Mg2+ + 2OH- + CO2 = Mg2+ + CO32- + H2O (5)

In most circumstances, additional CO2 must then be added to further stabilize the water.

The carbonate ions formed in reactions (4) and (5) redissolve as bicarbonate, as shown in

the following reactions:

CaCO3(s) + H2O + CO2 = Ca2+ + 2HCO3- (6)

Mg2+ + CO32- + CO2 + H2O = Mg2+ + 2HCO3

- (7)

Depending on source water characteristics, calcium ion concentrations in treated water

begin to increase once the lime dose reaches a certain level (about 200 mg/L in the

Roalson study). A portion of these calcium ions are converted to soluble bicarbonate

during recarbonation, potentially increasing calcium hardness of the finished water to

4

levels higher than if the softening pH was held below 10.6. Figure 1 shows the

relationship between lime dose, softening pH, and finished water total hardness at one of

DMWW�s treatment facilities.

The data points represent monthly averages during a calendar year. One can see that

treatment pH levels of 10.7 or greater usually require limes dosages greater than 200

mg/L. One can also see from Figure 1 the relationship of finished water hardness versus

softening pH at the same plant, and that there is little benefit to finished water total

hardness with increasing softening pH beyond 10.2. Essentially, during excess softening

some magnesium hardness is removed by displacing it with calcium hardness.

5

Figure 1: Lime Dose vs Treatment pH vs.Finished Water Hardness

10

10.2

10.4

10.6

10.8

11

11.2

100 150 200 250 300

mg/L CaO

pH

140

142

144

146

148

150

152

154

156

mg/

L to

tal h

ardn

ess

as

CaC

O3

Softening pH Finished HardnessLinear (Softening pH) Linear (Finished Hardness)

BACKGROUND ON DMWW

Water treatment facilities. DMWW�s Fleur Drive Treatment Plant is located near

the confluence of the Des Moines and Raccoon Rivers, and DMWW operates a water intake

on each river. The utility also owns and maintains a 3-mile (5 km)-long infiltration gallery

that runs parallel to the Raccoon River, 30 feet (9m) underground in the alluvial sands and

gravels. Water drawn from the infiltration gallery benefits from bank-side filtration and has a

lower turbidity and TOC, but usually is chemically harder than the river water.

The Fleur Drive plant (Figure 2) uses enhanced coagulation and lime softening to treat

an average of 40 million gallons of water per day (MGD) (151 million liters per day�MLD).

River water undergoes pretreatment in two underground basins where ferric chloride is added

as a coagulant and a slurry of powdered activated carbon is added for taste and odor control

and organics removal. Treated river water from the presedimentation step, water from the

infiltration gallery, alum, and additional ferric chloride are combined in a static mixing

chamber prior to four underground, conventional-type, continuous lime softeners where a

lime slurry is added and hardness settles and is removed. Each river can contain substantial

non-carbonate hardness during the winter months, and soda ash can be fed to the mixing

chamber if necessary. Additional treatment includes recarbonation to a pH of 9.2 to 9.7,

polyphosphate injection, gravity sand filtration, ion exchange nitrate removal during high

nitrate episodes in the source water, fluoridation, and disinfection with free chlorine using

liquid hypochlorite.

6

DMWW�s treatment plant at Maffitt Reservoir (Figure 3) lies 10 miles (17 km)

west of the Fleur Drive plant. Here water is drawn from five radial collector wells and

one horizontal well, all heavily influenced by the Raccoon River. The utility also

periodically obtains water from a 200-acre (81-hectare) reservoir and a 65-acre (26-

hectare) water-filled gravel pit on the site. The Maffitt Plant was built in 2000 and is an

enhanced coagulation and softening facility using ferric chloride, lime, and occasionally

soda ash. Softening and sedimentation take place in two up-flow clarifiers, and following

recarbonation and addition of polyphosphate, the water undergoes gravity filtration

through sand/anthracite filters. Fluoride is then added and disinfection is accomplished

with free chlorine using chlorine gas. Treatment capacity is 25 MGD (95 MLD).

7

Treatment history. For most of the past 60 years, DMWW practiced excess lime

softening at the Fleur Plant. This same strategy was implemented during the first three

years of the Maffitt Plant�s existence. Source water magnesium hardness at both plants is

fairly high�often exceeding 100 mg/L and 1/3 of total hardness. Most texts recommend

excess softening for this level of magnesium hardness.

During the past decade, the mindset at DMWW was that precipitation of

Mg(OH)2 was necessary to minimize NOM in treated water, and thus prevent the

formation of disinfection byproducts. Even though source water TOC is not unusually

high (rivers average 5 mg/L; ground water, < 2 mg/L), the motivation to reap the benefits

of Mg(OH)2 floc was high because DMWW continues to use free chlorine for both

primary and residual disinfection, rather than ozone or chloramines�techniques that can

reduce DBP formation. The visual stimulus of large, beautiful �snowflake� Mg(OH)2

floc in the Maffitt clarifiers powerfully reinforced the notion that the excess softening

treatment scheme was the correct one.

In recent years, however, staff began to gradually consider the possibility of

reduced treatment pH levels. Increased costs for chemicals certainly was a factor, as well

as issues related to residuals. Pilot tests in the laboratory showed that reducing lime feeds

would not dramatically affect finished water total hardness. But, it is well-known in the

water industry that the customer frequently equates consistency with quality. An

operational change of this magnitude is approached with some trepidation�will

customers notice a change in the water, and if so, will their perceptions be good?

Furthermore, staff was concerned about what sort of consequences the change would

have in terms of regulated water quality, i.e., corrosivity, and disinfection by-product

levels and TOC removal as required by the disinfection byproduct rule (DBPR).

8

In March of 2003, the decision was made to reduce lime feed rates, at least

temporarily, at the Maffitt Plant, and assess the impact on operational costs and finished

water quality. This facility was selected first because the plant produces a more consistent

finished water, is more efficient, and operational control of the plant and water quality is

easier compared to the Fleur Drive Plant. After it became apparent that conversion from

excess lime to straight lime softening was producing desirable results, staff began to operate

the Fleur Plant in this manner in September, 2003.

Throughout the remainder of the article, the authors will refer to Year 1 and Year 2.

Year 1 is the last 12 months of excess softening; Year 2 is the first 12 months of the trial

where treatment pH was reduced to levels associated with regular lime softening.

IMPACT ON OPERATIONAL COSTS

Lime. Excepting energy and labor, quicklime is the single largest expense for

DMWW, with annual purchases potentially exceeding $1 million. Lime is purchased from

two separate vendors, depending on availability and quality. Average CaO levels are

comparable for the two products.

Lime slakers at Fleur Drive produce a continuous supply of lime slurry. Grit and

unslaked lime are removed from the slakers periodically by water operators. The slakers at

the Maffitt Plant operate in a batch-processing fashion. This, coupled with the fact that the

Maffitt Plant is operated remotely, limits the use of lime to one vendor whose product

contains a very low amount of insolubles; i.e. grit accumulating in the slakers doesn�t pose

a problem for this unmanned facility. During the course of this study, the approximate price

DMWW paid for lime varied from $72 to $83 per ton ($65.30 to $75.28 per metric ton).

Rather than target a specific lime dose, DMWW operators vary lime feed as they

attempt to maintain softening basin pH at a specific level. During Year 1 at Fleur Drive, the

target pH averaged 11.02, which required an average lime dose of 240 mg/L as CaO. Year

9

1 at Maffitt saw an average softening basin pH of 10.91 produced by a lime dose averaging

250 mg/L.

At the beginning of Year 2, a conscious effort was made to reduce treatment pH

levels to a range varying from a low of 10.1 in the summer to a high of 11.0 in the winter.

Staff was given latitude to adjust lime feed rates to compensate for changes in water quality

such as high turbidity, hardness, TOC, etc. Year 2 treatment pH averaged 10.71 at Fleur and

10.43 at Maffitt. It goes without saying that this resulted in a dramatic reduction in lime

used at both plants, as shown in Figure 4.

Not only was less lime used, it was used more efficiently. One can imagine a lime

efficiency coefficient (LEC) of:

LEC = Weight of hardness removed/Weight of lime used (8)

By this measure, the lime was used nearly 40% more efficiently during Year 2 at the

Maffitt Plant, while only marginally so at Fleur Drive, also depicted in Figure 4. This

10

Figure 4: Lime Use

2020

1660

1939

1380

0.8080 0.8188

0.74

1.05

0

500

1000

1500

2000

2500

Year 1 Fleur Year 2 Fleur Year 1 Maffitt Year 2 Maffitt

Poun

ds L

ime

per M

illio

n G

allo

ns

0.6

0.7

0.7

0.8

0.8

0.9

0.9

1.0

1.0

1.1

1.1

Wei

ght o

f Har

dnes

s R

emov

ed p

er

Wei

ght o

f Lim

e U

sed

Pounds Lime per Million Gallons Wt. Hardness Removed/per Wt. Lime Used

obviously is a reflection of the different treatment pHs at the two plants during Year 2:

10.71 at Fleur vs. 10.43 at Maffitt. This is also reflected in the total hardness of the

finished water at the two plants. Finished water total hardness increased 6% at Fleur from

Year 1 to Year 2, but decreased 4% at Maffitt. Because the Fleur Drive plant relies more

on high-turbidity surface water, operators were much less comfortable with the reduced

lime regimen. As Year 2 progressed, however, their confidence level grew and the plant

reverted to high treatment pHs much less frequently. This is reflected in the lime

efficiency of the last three months of Year 2 at Fleur Drive: 0.95, much higher than the

average for the entire year which was 0.82. As a result of the reduced treatment pH, Year

2 costs for lime were reduced $155,800 at Fleur and $91,700 at Maffitt.

Coagulants. Fearing the loss of Mg(OH)2 in the treatment basins would have a

deleterious effect on finished water quality, water operators elevated alum feed rates at

the Fleur Drive plant. Its use increased 52% during Year 2 at an increased cost of $56,076,

tempering somewhat the reduction in costs associated with reduced lime purchases.

Typical alum doses in Year 1 were 10-15 mg/L, while Year 2 doses were in the 15-20

mg/L range. During exceptional water quality events in the rivers, such as ice-breakup in

early spring that results in very high turbidity, alum doses could approach 30 mg/L.

Alum is not fed at the Maffitt Plant. The primary water source for Maffitt is low-

organic shallow groundwater, and the coagulant that is fed there�ferric chloride�was

maintained at relatively constant dosages throughout the 2-year study period. Operators

did not attempt to increase doses of ferric chloride at Maffitt during Year 2. Previous

experiences showed that over-feeding ferric chloride resulted in a fine, heavy, dense floc

that would eventually plug the clarifier discharge lines. This type of floc also compacts

into the clarifiers, making it difficult to get it into the mixing zone.

11

Carbon Dioxide. During Year 2, water operators maintained a caustic alkalinity

(2 x phenolphthalein alkalinity minus the total alkalinity) of zero in the treatment basins.

This contrasts with Year 1 levels as high 30 mg/L during the winter months. This change

of course reduced the amount of magnesium removal, but also greatly diminished the

amount of carbon dioxide needed to stabilize the water, previously depicted in reactions

(4) and (5). Proper stabilization prevents deposition of calcium carbonate on the filter

media and in distribution piping. Carbon dioxide use is shown in Figure 5. Costs associated

with reduced CO2 purchases saved the utility $1.34 per MG ($5.07 per ML) at Fleur and

$2.82 per MG ($10.67 per ML) at Maffitt, for a total savings of close to $27,000.

Residuals. Residuals at the Fleur Drive Plant are dewatered first by settling, and

then by a plate and frame filter press (Innocenti, 1988). DMWW contracts with an outside

firm to haul the residuals from the plant.

12

Figure 5: Pounds Carbon Dioxide per Million Gallons

200

124

227

100

0

50

100

150

200

250


Poun

ds C

O2

Hauling invoices for Year 1 and Year 2 were evaluated to assess the amount of

sludge produced at Fleur Drive. The amount of sludge produced per million gallons did

decrease during Year 2. Initial estimates indicated the tons of sludge decreased from

44.07 to 41.51 per MG (151 to 142.5 metric tons per ML), a decrease of 5.8%. This

decrease was not as dramatic as what might have been expected, in lieu of the fact that

carbonate hardness removed per MG (and ML) decreased 15.8%, and lime used per MG

(and per ML) decreased 17.8%. Year 1 sludge contained greater amounts of Mg(OH)2

and likely contained quite a bit more water, so one might think that the weight of the

sludge produced during Year 2 would have dropped by a bigger percentage than either

the carbonate hardness removed or the lime used per MG (and per ML). Differences in

non-carbonate hardness and coagulant use do not explain this problem. Further

investigation revealed that during Year 1, incomplete control of the Fleur Drive

dewatering facility resulted in a significant loss of lime sludge to a lagoon near the plant.

The lagoon is actively used to store residuals from the presedimentation basin.

Nonetheless, the total savings realized for reduced hauling charges during Year 2 was

$30,351, and could have been as high as $87,000 if accurate accounting of sludge

production for Year 1 was known.

Lagoons are used to process residuals at the Maffitt Plant. Sludge is actively

introduced to one lagoon while another is drying. Decant water from the active lagoon

is recycled back into the plant. To date, no residuals have been removed from either

lagoon, and no accurate record exists of the amount of sludge introduced to them.

Anecdotal evidence and observations suggest, however, that the amount of sludge

produced during Year 2 at Maffitt was substantially less than during Year 1. For example,

sludge was ejected from the clarifiers at an average interval of 1.5 hours during Year 1

and 9.4 hours during Year 2, a dramatic difference and one that indicates substantially

13

less sludge production. The ejection time is relative to the volume of sludge blown off the

clarifier; the density of sludge likely changed from Year 1 to Year 2, so a direct weight

relationship cannot be made. Still, it stands to reason that in the long term, a cost savings

related to sludge hauling will also be seen for this facility.

Summary of costs. Making the transition from excess lime to straight lime

softening had obvious financial benefits for the utility, at least during the two years

evaluated. The total annual savings realized from this operational change was nearly

$250,000. More accurate accounting of sludge production at both plants likely would

increase this figure substantially, maybe as much as $100,000 per year.

IMPACT ON WATER QUALITY

Any sort of operational change of this magnitude causes concern about finished

water quality. These concerns relate to compliance with regulations, water safety,

hardness, corrosivity, organics removal, and perhaps aesthetic parameters. As mentioned

earlier, water customers place a premium on consistency.

From the beginning of Year 2, water quality was monitored very closely by water

operators, laboratory staff, and management. If filter effluent turbidities began trending

upward, lime feeds were temporarily increased to overcome poor water quality episodes

in the source water.

With the lower treatment basin pH, there was little doubt that the formation of

Mg(OH)2 had been reduced. Initially, staff were apprehensive about the effect this would

have on turbidity and organics removal. But, since the laboratory was closely monitoring

the situation, and since a return to excess softening was easily accomplished, if necessary,

with no equipment modifications, worries quickly subsided.

Finished Water Hardness. As previously discussed, excess lime softening

increases calcium ion solubility, and as shown in Equations (4) through (7), stabilization

14

of water that has undergone excess lime softening can subsequently increase calcium and

total hardness of the finished water. Because of this, DMWW staff surmised that

converting from excess lime to straight lime softening would not significantly increase

finished water hardness, and might actually decrease it. For the most part this was true,

as shown in Figure 6. During Year 2, total hardness increased 6% at Fleur Drive, but

decreased 4% at Maffitt. Source water variability could account for the difference

between the plants, but the increased use of alum at Fleur Drive, which consumes

alkalinity, may also have been a factor in the total hardness increase at that plant. The two

plants are obviously configured differently, with Fleur using conventional-type softeners

while upflow clarifiers are used at Maffitt, and this also may account for some of the

difference.

Figure 6 shows the conversion from excess lime to straight lime softening

essentially replaced calcium hardness in the finished water with magnesium hardness.

This too was expected, although the amount of finished water magnesium hardness

during Year 2 was troubling. As mentioned previously, it has long been thought that

15

Figure 6: Finished Hardness as CaCO3

144 153 144 138

49 58 56 54

95 95 88 84

49

78

45

7895

7599

60

020406080

100120140160180

Year 1Fleur

Year 2Fleur

Year 1Maffitt

Year 2Maffitt

mg/

L

Total

Carbonate

Non Carbonate

Magnesium

Calcium

magnesium hardness should be less than 40 mg/L in the finished water to prevent

excessive Mg(OH)2 scale in water heaters and hot water tanks. During Year 2,

magnesium hardness averaged 78 mg/L in water from both plants. A close inspection of

other water quality data and Larson�s 1959 paper, however, alleviated some of the fears

associated with the increased magnesium hardness. Larson proposed a formula whereby

one could calculate a Magnesium Index, a relative numerical guide to potential

problems related to magnesium hydroxide in the distribution system:

MI = 2 pH + log Mg + 0.02t - 21.2 (9)

Where MI is the Magnesium Index, t is temperature (oF) and Mg is mg/L magnesium

hardness as CaCO3. The lower the MI, the fewer problems a utility can expect due to

magnesium hardness in finished water. During Year 1, the last year of excess lime

softening, MI averaged 0.93 at Fleur and 1.01 at Maffitt. Year 2, the first year of straight

lime softening, saw the MI increase only slightly at Fleur to 0.97, but drop 32% to 0.69

at Maffitt. The reason�lower finished water pH during Year 2 evidently counteracted

increases in magnesium hardness. Figure 7 shows finished water pH levels for both

plants during the 2-year study.

16

Figure 7: Finished pH

9.67

9.59

9.68

9.41

9.259.309.359.409.459.509.559.609.659.709.75


Some magnesium hardness removal did occur during Year 2 (about 40% at both

plants), as shown in Figure 8. During Year 2, magnesium removal was 30% less at Fleur,

35% less at Maffitt. It�s usually thought that treatment pH must be raised to 10.5 or

higher to remove significant quantities of magnesium in the form of Mg(OH)2 (AWWA,

1990), and treatment pH levels frequently exceeded 10.5 at both plants (Year 2 softening

pH averaged 10.71 at Fleur, 10.43 at Maffitt). Based strictly on the annual average pH

values and the Ksp of Mg(OH)2, the authors expected magnesium removal to suffer

more than it actually did. Along these lines, Figure 9 (Benefield, et. al., 1982) shows the

relationship between total soluble magnesium, pH, and carbonic species concentration.

17

Figure 8: Percent Magnesium Removal

55.4

39

63.1

40.9

0

10

20

30

40

50

60

70


% R

emov

al

Using the Ksp and Figure 9 as a guide, Mg(OH)2 should have been four times

more soluble during Year 2 at Fleur, and nine times more soluble at Maffitt when

compared to Year 1. This presents the question of why magnesium removal decreased

by only 30-35% during Year 2. A possible hypothesis is that very high pH levels (>11)

occur where lime first mixes with incoming water. In the Fleur Drive softening basins,

it is difficult to measure a pH difference between the zone where lime mixes with water

and the zone where settling occurs, because the entire process takes place underground.

This isn�t the case at Maffitt. In the Maffitt upflow clarifiers, the slurry pool (settled

floc slurry) and incoming water is entrained into the center column of the clarifier and

the slaked-lime slurry is fed onto the surface of this up-flowing mixture. The pH at this

lime-water interface can be measured. It is very unstable, but is typically higher, as

much as 0.6 pH units higher, than the water in the larger body of the clarifier.

Furthermore, a saturated calcium hydroxide solution is known to have a pH of 12.454

(Durst, 1975), so local areas and microzones of very high (>11.5) pH likely exist, even

when the average basin pH is relatively low (10-10.5). This supports an idea that much

18

Figure 9: Soluble Magnesium vs. pH

-7-6-5-4-3-2-1012

8 9 10 11 12

pH

log

[Mg]

tota

l

C(T) = 0.00001M C(T)=0.0001M C(T)=0.001M

of the Mg(OH)2 formation occurs in this small mixing area of the basins and clarifiers.

This also explains why Fleur and Maffitt had similar magnesium removal during Year

2, even though the pH at Maffitt was substantially less (10.43 vs. 10.71) than Fleur. This

lime-water mixing zone at Maffitt is more compact than is the flocculation zone at Fleur.

The authors speculate that this facilitates the formation of a high-pH �hot zone� where

Mg(OH)2 formation can occur, even under low lime-feed conditions. There is also

evidence that alum (fed at Fleur Drive, but not at Maffitt) can assist with magnesium

removal, especially at low temperatures (Larson), but this was not observed in jar tests

at the DMWW laboratory.

Prior to the transition from excess lime to straight lime softening, anecdotal

evidence showed water heater scaling to not be a problem in the areas served by

DMWW. Also, informal studies comparing water heater life in Des Moines to cities

served by other utilities indicated DMWW water was no more prone to leave scale in

water heaters than other area utilities� water. Based on the lower finished water pH and

the steady-to-decreasing Magnesium Index, DMWW does not expect this to change

with continued application of straight lime softening.

Corrosivity. During the last round lead/copper monitoring in 2002, DMWW

analyzed 52 samples, none of which had detectable levels of lead. Copper was detected

in a handful of samples; all tested well below the Action Level for copper. Iowa

Department of Natural Resources, the drinking water enforcement body for Iowa,

requires DMWW to maintain a Calcium Carbonate Precipitation Potential (CCPP) at a

long-term average of 7.5 or greater. This was easily achievable under the excess lime

softening regimen, and continued to be after the conversion to lime softening. Figure 10

compares CCPP levels in the water from both plants during Years 1 and 2.

19

Water operators at DMWW adjust CO2 feed at both plants by targeting a specific

pH just downstream from the point of CO2 injection. This pH has remained at 9.7 for

many years, including both years of this study. Nonetheless, Figure 8 showed that the

finished water pH decreased during Year 2 in the water from both plants. This can be

explained by returning to equations (6) and (7), and the diminished need for stabilization.

The conversion to lime softening reduced the amount of CO2 needed for these reactions,

leaving more available for the formation of carbonic acid, and hence further reductions in

finished pH during Year 2. Observations at DMWW show that pH reduction continues for

some time after CO2 injection; some likely is still occurring after filtration in the

clearwell. At any rate, this reduced finished water pH during Year 2 obviously reduced

CCPP levels, and likely for the better. CCPP levels were higher than they needed to be

during excess softening, and rethinking the lime feed also provided the motivation to

rethink what the optimum CCPP values were for the finished water.

NOM and disinfection byproducts (DBPs). The biggest water quality concern

related to the reduced treatment pH was removal of NOM. As mentioned earlier, it is well

established that Mg(OH)2 floc is an excellent coagulant that removes NOM left

20

Figure 10: Calcium Carbonate Precipitation Potential

15.7

11.1

18.9

13.1

0.0

5.0

10.0

15.0

20.0


CC

PP

mg/

L

untouched by the precipitation of CaCO3. Since DMWW continues to use free chlorine as

its only primary and residual disinfectant, increases in finished water TOC and the

potential for increased DBP levels were a concern. Historically, DBP levels had not been

a problem for DMWW, and even though source water TOC is not unusually high, staff

knew there were potential consequences to losing the benefits of Mg(OH)2.

TOC removal at the Maffitt Plant did decrease, as shown by Figure 11. Figure 11

also shows actual finished water TOC levels. Interestingly, TOC removal rates at the Fleur

Plant were unaffected. This likely is explained by the increased alum feed at the Fleur

Plant during Year 2. The Maffitt Plant does not have alum feed capabilities, but ferric

chloride is fed as a coagulant. As explained earlier, over-feeding ferric chloride at the

Maffitt Plant has negative consequences; thus increases in the feed rate were avoided.

Operators consciously increased the alum dose at Fleur Drive in anticipation of losing

Mg(OH)2 as a coagulant, and this strategy apparently worked.

21

Figure 11: Total Organic Carbon

48.95 49.5

38.4

23.9

1.46

1.68

1.39

1.66

20

25

30

35

40

45

50


Perc

ent R

emov

al

1.00

1.10

1.20

1.30

1.40

1.50

1.60

1.70

1.80

Fini

shed

Wat

er T

OC

mg/

L

% Removal TOC

Total Trihalomethane (TTHM) levels (Figure 12) in the distribution systems

served by the two plants did increase slightly during Year 2, more so in the system served

by Maffitt. This stands to reason based on the reduced TOC removal rate at Maffitt. Even

with the slight TTHM increases, measured values remain far below the EPA�s maximum

contaminant level of 80 ug/L. It bears mentioning that the lower finished water pH seen

during Year 2 likely helped in preventing larger increases in TTHM concentrations, as it

is known that higher pH favors their formation (Roalson, et. al, 2003).

It seems apparent from this data that a large, surface water utility can indeed

remain in compliance with DBP regulations while disinfecting with free chlorine, and

without the additional coagulation provided by excess softening-produced Mg(OH)2.

22

Figure 12: Total Trihalomethanes

4044

29

36

0

10

20

30

40

50


TTH

M u

g/L

Turbidity. DMWW water operators have long felt that Mg(OH)2 helps minimize

filter applied turbidity levels. Meeting turbidity requirements has never been difficult at

either plant, and the conversion from excess lime to lime softening did little to change

that. Finished water turbidity did increase at Fleur Drive, as shown in Figure 13, but never

approached the 0.3 NTU level of concern. Since Fleur Drive relies much more heavily on

surface water, the observation that turbidity increases were more pronounced at this plant

was expected.

Removal/deactivation of microorganisms. It�s been known for decades that lime

softening has biocidal properties. A 1913 paper reported that lime softening and filtration

induced at least a 3-log removal of Bacillus coli (Riehl, 1962). It has been reported that

operating at a pH range of 10.6 to 10.9 reduced coliforms by 99%; operating at a pH of

10.2 resulted in an 83.5% reduction in coliforms (AWWA, 1994). DMWW�s experiences

during the two year study also supports the concept of high pH�high toxicity. Water in

the DMWW Fleur Drive treatment basins was substantially less toxic to microorganisms

while operating in the reduced-pH scenario. Figure 14 shows filter applied coliform

23

Figure 13: Average Daily Maximum Turbidity

0.04

0.07

0.04 0.04

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07


NTU

counts for the two plants. Total coliform numbers roughly quadrupled under the reduced-

lime regimen at Fleur, where high-bacteria river water is regularly used. Numbers at Maffitt,

where the water source is usually limited to groundwater, actually decreased slightly, and

are probably related more to the frequency of surface water use than they are to the lime

feed rate. There also seemed to be more anomalous events when coliform counts were been

noticeably higher in filter applied water than the long-term norm. This often occurred when

treatment pH levels were below 10.5 for long periods of time. Figure 15 shows the number

of days that total coliform exceeded 10 colonies/100ml and E. coli was detected in the filter

applied water at Fleur Drive. One can see that there were significant increases for both

during Year 2. That said, there were no coliforms isolated in the finished water leaving either

plant, and only one sample in the distribution system (from over 1000 tested) tested positive

during Year 2. All chlorine concentration x time (CT) requirements were easily met during

Year 2, and thus reduced alkaline toxicity in the treatment basins has not been a large

concern. Obviously, DMWW�s continued ability to use free chlorine as the primary and

residual disinfectant helps maintain the relative ease with which CT requirements are met.

The lower finished water pH has helped in this regard also by increasing the effectiveness

of the disinfectant and shortening the time requirement for necessary CT.

Figure 14: Filter Applied Coliform Bacteria

8.8

31.6

9.97.3

0

5

10

15

20

25

30

35

Fleur Year 1 Fleur Year 2 Maffitt Year 1 Maffitt Year 2

Ave

rage

Dai

ly C

ount

s/10

0 m

l

24

Customer perceptions. Customer complaints did not significantly increase during

Year 2, and it is safe to say virtually no residential customers noticed differences in water

hardness between Year 1 and 2. One industrial user did immediately notice a difference in

the water at the beginning of Year 2. This particular customer had been reducing hardness

at the point of use through lime softening, and noticed his process was no longer reducing

total hardness to levels seen during Year 1. After consulting with DMWW staff, the

customer made process adjustments (namely, raising treatment pH) to precipitate

Mg(OH)2, and no further complaints were heard. It should be noted that the Des Moines

Area is not highly-industrialized; utilities with many industrial customers that have

specialized water quality requirements may indeed experience more concerns or questions

than did DMWW regarding a change of this sort.

SUMMARY

The conventional wisdom in the water industry is that customers frequently equate

consistency with quality. Changes in water quality, even those for the better, are often

noticed and perceived as a negative by some customers, at least for the period of time

necessary for acclimatization to the new water. For this reason, utilities may be reluctant

to tinker with a process known to produce good, or even adequate, water quality. The

authors of this study, however, believe much can be gained by evaluating the details of

even a tried-and-true process like lime softening.

There is little doubt that precipitation of Mg(OH)2 during excess lime softening

has potential water quality benefits; namely removal of NOM and magnesium hardness.

It also can have financial consequences for a utility: increased costs related to chemical

purchases and residuals handling, as this study shows. Straight lime softening at treatment

pH levels less than 10.5 may produce high quality water at a substantially lower cost than

excess softening. There likely are also hidden cost savings associated with conversion

25

from excess lime to lime softening, specifically maintenance and labor costs as well as

power, that result whenever a process is made more efficient and waste is reduced.

Utilities may want to assess how practical, and indeed how relevant, the 40 mg/L

magnesium hardness level is for their water. Other finished water quality characteristics,

such as CCPP and pH, may make higher levels of magnesium hardness tolerable for the

great majority of water customers.

Some water producers may also want to closely evaluate whether or not to follow

the trend toward free chlorine alternatives. Results of this study show that a large, surface

water utility can use free chlorine as the primary and residual disinfectant and still produce

DBP-compliant water without the coagulation benefits provided by excess lime softening.

26

REFERENCESAmerican Water Works Association, 1990. Water Quality and Treatment, a Handbook ofCommunity Water Supplies, 4th Edition. Frederick W. Pontius, editor. McGraw-Hill, Inc.,New York, NY.

American Water Works Association Research Foundation, 1994. The Removal andDisinfection Efficiency of Lime Softening Processes for Giardia and Viruses, Logsdon, et.al., preparers. AWWA, Denver, CO.

Benefield, L.D., Judkins, J.F., Weand, B.L., 1982. Process Chemistry for Water andWastewater, p. 124, 292. Prentice Hall, Englewood Cliffs, NJ.

Black, A.P. and Christman, R.F. Jour. AWWA, 1961, 53:737.

Brock, T.D., Madigan, M.T., Martinko, J.M., and Parker, J., 1994. Biology ofMicroorangisms, 7th Edition. Prentice Hall, Englewood Cliffs, NJ.

Durst, R.A., 1975. Standard Reference Materials: Standardization of pH Measurements.NBS Spec. Publ. 260-53, National Bureau of Standards, Washington, D.C.

Faust, S.D. and Aly, O.M., 1998. Chemistry of Water Treatment. Ann Arbor Press,Chelsea, MI, p. 329.

Humenick, M.J., 1977. Water and Wastewater Treatment, Marcel Dekker, New York, NY.

Innocenti, P., 1988. Techniques for Handling Water Treatment Sludge. Opflow, 14:2:1.

Larson, T.E., Lane, R.W., Neff, C.H., 1959. Stabilization of Magnesium Hydroxide in theSolids-Contact Process. Jour. AWWA, 51:12:1551.

Liao, M.Y. and Randkte, S.J., 1985. Removing Fulvic Acid by Lime Softening. Jour.AWWA, 77:8:78.

Liao, M.Y. and Randkte, S.J., 1986. Prediciting the removal of Soluble OrganicContaminants by Lime Softening. Water Res., 20:1:27.

Nordell, E., 1961. Water Treatment, Van Nostrand Reinhold Company, New York, NYThompson, J.D., et. al, 1997. Enhanced Softening: Factors Influencing DBP PrecursorRemoval. Jour. AWWA, 89:6:94.

Powell, S.T., 1954. Water Conditioning for Industry, McGraw-Hill Book Company, NewYork, NY.

27

Rao, V.C., Symons, J.M., Ling, A., Wang, P., Metcalf, T.G., Hoff, J.C., and Melnick, J.L.,1988. Removal of Hepatitis A Virus and Rotavirus in Drinking Water Treatment Processes.Jour. AWWA, 80:2:59.

Riehl, M.L., 1962. Water Supply and Treatment, 9th edition. Washington, D.C.: NationalLime Association.

Roalson, S.R., Kweon, J., Lawler, D.F., and Speitel Jr., G.E., 2003. Enhanced Softening:Effects of Lime Dose and Chemical Additions. Jour. AWWA, 95:11:97.

U.S. Environmental Protection Agency, 1999. Enhanced Coagulation and EnhancedPrecipitative Softening Guidance Manual (EPA 815-R-99-012), Washington, D.C.

Wolf, H.W., Safferman, R.S., Mixson, A.R., and Stringer, C.E., 1974. Virus Inactivationduring Tertiary Treatment. Jour. AWWA, 66:9:526.

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