Reduced Lime Feeds: Effects on Operational Costs and Water Quality Christopher S. Jones*
[email protected]
Ted P. Corrigan
[email protected]
David B. Graham
[email protected]
L.D. McMullen
[email protected]
Des Moines Water Works 2201 George Flagg Parkway Des Moines, Iowa 50321 April 13, 2005
*Correspondence should be directed to this author.
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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.
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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 Benefits of excess lime softening.
(3)
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 Mg2+ + 2OH- + CO2 = Mg2+ + CO32- + H2O
(4) (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+ + 2HCO3Mg2+ + CO32- + CO2 + H2O = Mg2+ + 2HCO3-
(6) (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
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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.
Figure 1: Lime Dose vs Treatment pH vs. Finished Water Hardness
154
11
152
pH
10.8
150
10.6
148 146
10.4
144 10.2 10 100
mg/L total hardness as CaCO3
156
11.2
142 150
200
250
140 300
m g/L CaO Softening pH Linear (Softening pH)
Finished Hardness Linear (Finished Hardness)
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
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.
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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 (26hectare) 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).
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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).
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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
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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)
Figure 4: Lime Use 1.05 2020
1.1
1939
2000
1.0
1660
1.0 1380
1500 0.8080
1.1
0.8188
0.9 0.9
1000
0.8
0.74
0.8
500
0.7 0.7
0
0.6 Year 1 Fleur
Year 2 Fleur
Pounds Lime per Million Gallons
Year 1 Maffitt
Weight of Hardness Removed per Weight of Lime Used
Pounds Lime per Million Gallons
2500
Year 2 Maffitt
Wt. Hardness Removed/per Wt. Lime Used
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
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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 loworganic 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.
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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.
Figure 5: Pounds Carbon Dioxide per Million Gallons 250
Pounds CO2
200
227 200
150
124 100
100 50 0 Year 1 Fleur
Year 2 Fleur
Year 1 Maffitt
Year 2 Maffitt
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.
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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
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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
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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.
mg/L
Figure 6: Finished Hardness as CaCO3 180 160 140 120 100 80 60 40 20 0
153
144 95 95 49 49
95 7875 58
144
138 88
56
99
45
8478 60 54
Total Carbonate Non Carbonate Magnesium Calcium
Year 1 Fleur
Year 2 Fleur
Year 1 Maffitt
Year 2 Maffitt
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
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.
Figure 7: Finished pH 9.75 9.70 9.65 9.60 9.55 9.50 9.45 9.40 9.35 9.30 9.25
9.68
9.67 9.59
9.41
Year 1 Fleur
Year 2 Fleur
Year 1 Maffitt
16
Year 2 Maffitt
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.
Figure 8: Percent Magnesium Removal 70
% Removal
60
63.1 55.4
50
40.9
39
40 30 20 10 0 Year 1 Fleur
Year 2 Fleur
17
Year 1 Maffitt
Year 2 Maffitt
log [Mg] total
Figure 9: Soluble Magnesium vs. pH 2 1 0 -1 -2 -3 -4 -5 -6 -7 8
9
10
11
12
pH C(T) = 0.00001M
C(T)=0.0001M
C(T)=0.001M
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
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.
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Figure 10: Calcium Carbonate Precipitation Potential 18.9
20.0
CCPP mg/L
15.7 15.0
13.1 11.1
10.0 5.0 0.0 Year 1 Fleur
Year 2 Fleur
Year 1 Maffitt
Year 2 Maffitt
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
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.
Figure 11: Total Organic Carbon 48.95
49.5
1.80
1.68
1.66
Percent Removal
45 40
1.60
38.4
1.46
1.50
1.39
35
1.70
1.40 1.30
30 23.9
25
1.20 1.10
20
1.00 Year 1 Fleur
Year 2 Fleur
Year 1 Maffitt
% Removal
21
TOC
Year 2 Maffitt
Finished Water TOC mg/L
50
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.
Figure 12: Total Trihalomethanes 50
TTHM ug/L
40
40
44 36 29
30 20 10 0 Year 1 Fleur
Year 2 Fleur
22
Year 1 Maffitt
Year 2 Maffitt
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.
Figure 13: Average Daily Maximum Turbidity 0.07
0.07 0.06
NTU
0.05
0.04
0.04
0.04
Year 1 Maffitt
Year 2 Maffitt
0.04 0.03 0.02 0.01 0.00 Year 1 Fleur
Year 2 Fleur
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
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counts for the two plants. Total coliform numbers roughly quadrupled under the reducedlime 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
Average Daily Counts/100 ml
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31.6
30 25 20 15 10
9.9
8.8
7.3
5 0 Fleur Year 1
Fleur Year 2
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Maffitt Year 1
Maffitt Year 2
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
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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.
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REFERENCES American Water Works Association, 1990. Water Quality and Treatment, a Handbook of Community Water Supplies, 4th Edition. Frederick W. Pontius, editor. McGraw-Hill, Inc., New York, NY. American Water Works Association Research Foundation, 1994. The Removal and Disinfection 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 and Wastewater, 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 of Microorangisms, 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 the Solids-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 Organic Contaminants by Lime Softening. Water Res., 20:1:27. Nordell, E., 1961. Water Treatment, Van Nostrand Reinhold Company, New York, NY Thompson, J.D., et. al, 1997. Enhanced Softening: Factors Influencing DBP Precursor Removal. Jour. AWWA, 89:6:94. Powell, S.T., 1954. Water Conditioning for Industry, McGraw-Hill Book Company, New York, NY.
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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.: National Lime 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 Enhanced Precipitative 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 Inactivation during Tertiary Treatment. Jour. AWWA, 66:9:526.
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