ASSESSMENT AND OPTIMIZATION of chemical and physicochemical. Hardness removal processes are very ph-dependent, especially for removal of magnesium

Transcription

1 ASSESSMENT AND OPTIMIZATION of chemical and physicochemical Softening Processes Hardness removal processes are very ph-dependent, especially for removal of magnesium (Mg) and silica (Si). Bench-scale tests were conducted with a groundwater that was supersaturated with carbon dioxide and contained calcium (Ca), Mg, and Si. The purpose of this work was to assess and optimize several softening processes to reduce chemical use (i.e., sludge production) and improve turbidity removal. Optimal dosing of lime and soda ash (OLSA) removed 79% of Ca and Mg hardness and 23% of Si hardness. Iron salt addition during OLSA improved the rate of turbidity removal, had minimal effect on Ca or Si removal, but decreased Mg removal unless the ph was readjusted to offset the ph decline that resulted from iron hydroxide formation. Sodium aluminate addition during OLSA improved the rate of turbidity removal and increased settled sludge viscosity, but did not affect Ca, Mg, or Si removal. In separate semibatch aeration softening tests (without lime or soda ash addition), Ca removal increased as a function of aeration rates; Mg and Si were not removed. The addition of a nuclei seed increased dissolved Ca removal from 6% without the nuclei seed to > 8% in the presence of 3 g/l of nuclei seed. Results indicate that the aeration softening process would remove hardness, decrease chemical consumption, and reduce sludge production. BY PENG-FEI CHAO AND PAUL WESTERHOFF C alcium (Ca) and magnesium (Mg ) are abundant alkaline earth metals that can significantly affect water quality, treatability, sludge production, and the economics of using a water supply for domestic or industrial applications (Batchelor et al, 1991). Ca and Mg are divalent cations present in igneous rock minerals as silicates (e.g., feldspar, olivine), in sedimentary rock as carbonates (e.g., calcite, dolomite), or in sandstone and detrital rock as cement between particles. Weathering of these rock types results in mineral dissolution and solubilization of Ca, Mg, silica (Si), and carbonate species (Hem, 1992). CHAO ET AL PEER-REVIEWED 94:3 JOURNAL AWWA MARCH 22 19

2 FIGURE 1 Ca 2+ log concentration Solubility diagram for calcite and magnesium hydroxide CaCO 3(s) ph Soluble Ca 2+ mg/l as CaCO Mg(OH) 2(s) ph Ca 2+ calcium ions, CaCO 3(s) calcite, Mg(OH) 2(s) magnesium hydroxide CA AND MG EFFECTS AND TREATMENT Most of these dissolved minerals do not have significant health effects. However, precipitation of Ca and Mg can clog potable drinking water distribution system pipes, valves, meters, and faucets and form deposits or scales that reduce heat transfer across domestic hot water systems or industrial cooling systems. The presence of Ca, Mg, and Si also affects treatment selection through precipitation and fouling of air-stripping systems designed for removal of volatile organic compounds or of reverse osmosis, ultrafiltration, and nanofiltration membranes. In addition, their presence can affect removal of acidic organic compounds (e.g., fulvic acids) or reduce selectivity of ionexchange systems for lead, copper, or zinc (Faust & Ali, 1998; AWWARF et al, 1996; Water Quality and Treatment, 199; Snoeyink et al, 1987). Therefore, chemical precipitation, ion exchange, or physical separation processes (e.g., reverse osmosis) are often used to remove Ca and Mg prior to domestic and industrial applications. Chemical softening. Hardness of water typically is defined as the sum of Ca and Mg concentrations expressed in units of milligrams per litre as calcium carbonate (CaCO 3 ). Chemical softening, through the addition of lime and soda ash, is a traditional and cost-effective means of removing hardness (Benefield & Morgan, 199). The chemical softening process involves addition of slaked lime (CaO), forming calcium hydroxide [Ca(OH) 2 ], which raises the ph because of the addition of hydroxide (OH ). The solubility of calcite [CaCO 3(s) ] decreases with increasing ph, reaching a minimum level around ph 1.5 (Figure 1). Soda ash (Na 2 CO 3 ) must be added as a source of carbonate to complex with the initial Ca present in water, plus the Ca added as lime, to form calcite. The solubility of magnesium hydroxide [Mg(OH) 2 (s) ] does not approach an asymptotic minimum solubility (Figure 1), and most chemical softening applications operate in the range of ph for Mg removal. The desired level of Ca and Mg hardness removal and the need to remove Mg dictate the required operating ph level. Si removal can also occur at levels > ph 11 (Faust & Ali, 1998). Silica, as Si(OH) 4, probably does not crystallize but polymerizes in a slow reaction when its solubility limit is exceeded. Dissolved Si can adsorb onto Mg(OH) 2 floc; additional Mg feed, either as an oxide, sulfate, or chloride, can be added to remove Si during softening (Spear & Matson, 1984). Lime and soda ash are typically flash-mixed with the water, allowed time to react (3 6 min) for nucleation (particle formation) and aggregation, and settled, filtered, or both. Chemical softening forms significant quantities of sludge, in part because of the high quantities of lime and soda ash needed to promote hardness removal; this large amount of sludge then poses a disposal factor for water suppliers. Purpose of this research. The purpose of this work was to assess and optimize several softening processes to reduce chemical use (and accompanying sludge production) and improve turbidity removal. Results were compared with chemical softening with lime and soda ash alone. First, the authors studied the addition of different iron (Fe) and aluminate (Al 2 O 4 2 ) metal salts, along with an optimal dosage of Ca(OH) 2 and Na 2 CO 3, to demonstrate the effect of metal salts on chemical softening processes and sludge characteristics. Second, they assessed process performance and viability for a physicochemical process to soften water that involved diffused aeration, without addition of lime and soda ash. EXPERIMENTAL AND ANALYTICAL METHODS Two types of bench-scale softening experimental apparatus were used: jar-test apparatus and semibatch aeration reactors. All experiments were conducted at a water temperature of 2 o C ± 2 o C using water from the Concho Well (CW) and the Paterson Well (PW), two groundwater sources collected from the Coronado Power Generation Station (CPGS) in Saint Johns, Ariz. A blend of 6% CW water and 4% PW water was used in all experiments. The water quality of the two water sources and the blended water is shown in Table 1. The water quality is generally characterized by high ionic composition, predominantly CaCO 3 hardness, and supersaturated dissolved carbon dioxide (CO 2 ) levels. The 6% CW and 4% PW blend of water sources represents the typical makeup water for the CPGS, which currently chemically softens (using lime plus soda ash) most of its makeup water for cooling across copper pipes. Annual water consumption at the full-scale facility averaged approximately 2,642 mil gal (1, ML) of treated water per year. Jar tests. Jar tests were conducted to simulate conventional chemical softening processes by using a six-place 11 MARCH 22 JOURNAL AWWA 94:3 PEER-REVIEWED CHAO ET AL

3 gang stirrer apparatus* and six 1-L glass beakers for reactors. Chemicals were rapidly added during a rapidmix stage (1 rpm) for 2 min, followed by a 1-min slow-mixing aggregation stage (2 rpm) and 2-h sedimentation stage (no mixing). Unless otherwise stated, all jar-test data presented represent samples collected after the 2-h sedimentation stage. Samples were withdrawn approximately 1 in. (25 mm) below the surface with a 5-mL glass syringe, syringe-filtered through a.45-µm nylon membrane, and analyzed for Ca, Mg, and Si. Unfiltered samples were also collected for immediate turbidity analysis. The ph was measured directly in the jars to prevent stripping of dissolved CO 2 during sample handling. All ph measurements were recorded to the nearest ±.1 units. Theoretical lime (TH L = 5 mg/l as CaCO 3 ) and soda ash (TH S = 1 mg/l as CaCO 3 ) dosages were calculated based on initial ph, alkalinity, and carbonic acid concentration for excess lime softening conditions (Benefield & Morgan, 199). Jar TABLE 1 tests were performed at theoretical chemical levels, plus five additional levels in order to select optimal conditions for hardness and Si removal. Ca(OH) 2 was added instead of slaked lime. Three levels of Ca(OH) 2 addition (TH L, TH L +25, and TH L +5 mg/l as CaCO 3 ) and two levels of soda ash addition (TH S and TH S + 5 mg/l as CaCO 3 ) were studied. Stock solutions of ferric chloride (FeCl 3 ) and ferric sulfate [Fe 2 (SO 4 ) 2 ] were prepared daily and dosed over a range of 4 mg/l as Fe. A sodium aluminate (Na 2 Al 2 O 4 ) stock solution was diluted from a 33% active solution and dosed over a range of 1 mg/l. During select experiments, ph readjustment was achieved by adding small amounts of concentrated (5 N) sodium hydroxide (NaOH). Aeration reactor apparatus. Two reactors were fabricated from 2-L glass cylinders and fitted with a sample port near the base (Figure 2). A stainless-steel porous stone diffuser was placed in the bottom of the reactor; compressed air was added to the reactors at rates of, 95, 48, 95, and 1,9 cm 3 /min (, 5.8, 29, 58, and 116 cu in./min), controlled with a flowmeter. Air addition increased the volume (water plus air) in the reactors, which was measured. The differences in reactor volumes at the five gas flow rates were due to the presence of, 2.6, 6.5, 13, and 26 cm 3 (,.2,.4,.8, and 1.6 cu in.) of air in the reactors, respectively. A magnetic stirring Parameter* CW PW Blend ph Turbidity ntu Alkalinity mg/l as CaCO Calcium hardness mg/l as CaCO Magnesium hardness mg/l as CaCO Silica mg/l as SiO Dissolved CO 2 mg/l as CaCO *CaCO 3 calcium carbonate, SiO 2 silicon dioxide, CO 2 carbon dioxide CW Concho Well water PW Paterson Well water 6% CW plus 4% PW (blended total dissolved solids = 1,137 mg/l) FIGURE 2 Gas flowmeter Typical composition of water used during study Schematic of aeration reactor for aeration softening process Sampling port Magnetic stirrer Mixer ph meter 2-L reactor Diffusing stone Compressed air gas supply Mixer bar in the bottom of the reactor provided continuous mixing. Reactors were operated in a semibatch mode: fixed water volume (no flow) and continuous gas addition. During selected experiments, a nuclei seed was added to the aeration reactors. The seed was obtained by drying (15 o C for 24 h) settled sludge collected from the full-scale chemical softening plant located at the CPGS facility. Unfiltered samples were collected from the sampling port for turbidity whereas syringe-filtered samples (.45-µm nylon membrane) and the filtrate analyzed for Ca, Mg, and Si. To prevent stripping of dissolved CO 2 during sample handling (see Figure 2), ph was continuously measured at a depth of 3 in. (76 mm) below the water surface in the jars and reported to the nearest ±.1 units. Standard errors were calculated by averaging the difference between two samples collected at the same time from duplicate aeration experiments. The standard error for ph and Ca remaining was 3% and 14%, respectively. Analytical methods. Atomic adsorption spectroscopy** was used to measure Ca, Mg, and Si, in accordance with *Phipps & Bird, Richmond, Va. Gelman, Ann Arbor, Mich. Fisher Scientific, Springfield, N.J. Cole Palmer Inc., Niles, Ill. **Model 311, Perkin Elmer, Norwalk, Conn. CHAO ET AL PEER-REVIEWED 94:3 JOURNAL AWWA MARCH

4 TABLE 2 Optimal lime and soda ash dosages Parameter* Jar 1 Jar 2 Jar 3 Jar 4 Jar 5 Jar 6 Ca(OH) 2 dose mg/l Na 2 CO 3 dose mg/l ph during softening Turbidity ntu [Ca 2+ ] remaining mg/l as CaCO [Mg 2+ ] remaining mg/l as CaCO Total hardness removal % [Si 4+ ] remaining mg/l as SiO *Ca(OH) 2 calcium hydroxide, Na 2 CO 3 soda ash, [Ca 2+ ] calcium ions, CaCO 3 calcium carbonate, [Mg 2+ ] magnesium ions, [Si 4+ ] silica ions, SiO 2 silicon dioxide Jar 4 had the best removal. Dissolved concentrations remaining after sedimentation period in jar test methods 3111B and 3111D (Standard Methods, 1995). Hardness was reported as the sum of Ca and Mg. Alkalinity and carbonic acid were measured by colorimetric titration* according to method 232. The ph was measured following method 45H+ and using a glass electrode and ph meter. Turbidity measurements were made with a nephelometric turbidimeter and based on method 213. Sludge viscosity was measured with a viscometer and was based on ASTM D (ASTM, 1994). RESULTS AND DISCUSSION Chemical softening. Lime and soda ash addition alone. Table 2 shows the results of the jar tests conducted using several dosages of lime and soda ash. Increasing lime FIGURE 3 ph Effect of reduced theoretical lime and soda ash dosages in the presence of 1 mg/l iron as ferric sulfate on ph and dissolved Ca, Mg, and Si removal Ca Mg Si ph Fraction of Theoretical Lime and Soda Ash Dosage Ca calcium, Mg magnesium, Si silica Dissolved Ca, Mg, or Si Removed After Sedimentation % dosages led to higher ph levels and improved Mg removal. Chemical conditions in jar 4 (TH L + 25 mg/l and TH S + 5 mg/l) achieved the greatest Ca and Mg hardness removal (79%) and Si removal (48%). This optimal dosage was selected for use in iron and aluminate addition experiments. Iron and aluminate addition during chemical softening. Chemical softening (lime and soda ash) plus an iron salt addition decreased Mg hardness removal but improved the rate of turbidity removal. Table 3 shows the effect of FeCl 3 dosing as well as the optimal lime and soda ash dosing (OLSA). Addition of 5 mg/l as iron decreased Mg removal from 48% to only 25%, and higher iron dosages achieved only 2 3% Mg removal. In contrast, iron addition had no statistical effect on Ca or Si removal until a high dosage (4 mg/l) was added; iron dosages relative to Si content in excess of 1:1 may be required for Si removal (Faust & Ali, 1998). In separate experiments using Fe 2 (SO 4 ) 2, no statistical difference was observed from the results using FeCl 3. Figure 3 shows results from separate iron experiments (1 mg/l Fe) conducted in the presence of lime and soda ash dosages ranging from 5 to 1% of the theoretical dosage. Lime and soda ash dosages less than theoretical levels for maximum Ca hardness removal resulted in ph levels < 1.5. Consequently, Ca removal efficiency decreased slightly. Decreased Mg and Si removal efficiencies were observed for lime and soda ash dosages below theoretical levels because of lower ph levels. Mg removal is very ph-dependent. Less than 1% of the initial Mg or Si was removed at ph levels < Changes in Mg and Si removal efficiency roughly paralleled each other and supported the theory that Si is removed by adsorption onto Fe 2 (SO 4 ) 2 flocs. For economic reasons, *Model 169 Digital Titrator, Hach Co., Loveland, Colo. ph meter 34, Corning, Acton, Mass. Model 21A, Hach Co., Loveland, Colo. Viscometer Model HAT, Brookfield, Middleboro, Mass. 112 MARCH 22 JOURNAL AWWA 94:3 PEER-REVIEWED CHAO ET AL

5 many softening plants operate at ph levels below ph 1.5 when only Ca hardness removal is necessary. Operation at ph 8.5 versus ph 1.5 would represent approximately an order of magnitude difference in the Ca 2+ solubility (Figure 1) but could still represent a high removal efficiency for hard waters (> 25 mg/l as CaCO 3 ). Decreased Mg removal resulted from the consumption of hydroxide and subsequent depression in ph, attributable to the formation of iron hydroxide: Fe OH Fe(OH) 3(S) (1) FIGURE Turbidity ntu 6 4 Effect of ferric chloride addition (in the presence of an optimal lime and soda ash dosage) on turbidity removal throughout a jar-test experiment mg/l 5 mg/l 1 mg/l 15 mg/l 2 mg/l 4 mg/l The effect of increased iron dosage on ph after sedimentation is shown in Figure 3. Addition of an iron salt decreased the solution ph to below ph 11., thus leading to higher dissolved Mg levels (i.e., lower percentage removed). Another jar-test experiment was conducted with FeCl 3 (1 mg/l); however the ph was readjusted with NaOH to 11.1 during the entire jar-test period (Table 3). The ph readjustment had a negligible effect on Ca removal compared with lime and soda ash addition alone; however, Mg removal was 81%. With ph readjustment, the Mg removal was greater than the OLSA treatment without iron (47% Mg removal) or with 1 mg/l Fe without ph readjustment (28% Mg removal). These results demonstrate the strong dependence of ph and Mg removal on iron salts addition. These results suggest that without ph readjustment, iron addition was not beneficial for Mg removal. With ph readjustment during iron addition, Mg removal improved. Mg removal was higher than lime and soda ash addition TABLE 3 without iron addition, probably because of improved turbidity removal. The effect of iron addition (no ph readjustment) on turbidity levels during jar-testing is shown in Figure 4. Low iron dosages, in the presence of an OLSA dosage, initially resulted in higher turbidity levels than OLSA addition without iron. However, the turbidity particles quickly settled and resulted in lower turbidity levels than without iron. At higher iron doses (> 15 mg/l), low turbidity levels were rapidly achieved. The results indicated that iron addition improved the settleability of particulate matter that contained Ca, Mg, and Si. Iron addition plus ph readjustment also led to low turbidity Jar-test Reaction Time min Effect of FeCl 3 and ph readjustment in jar tests (in the presence of an optimal lime and soda ash dosage) FeCl 3 dose mg/l as Fe Parameter* ph [Ca 2+ ] remaining mg/l as CaCO 3 [Mg 2+ ] remaining mg/l as CaCO 3 Total hardness removal % [Si 4+ ] remaining mg/l as SiO 2 Turbidity at 15 min ntu Turbidity at 12 min ntu *[Ca 2+ ] calcium ions, CaCO 3 calcium carbonate, [Mg 2+ ] magnesium ions, [Si 4+ ] silica ions, SiO 2 silicon dioxide FeCl 3 ferric chloride ph was readjusted with sodium hydroxide after iron addition. Dissolved concentrations remaining after 2-h sedimentation period in jar test levels (Table 3). Improved or more rapid sludge-settling characteristics would reduce the settling time necessary to achieve a specific turbidity level, thus reducing the size of clarifiers that follow chemical softening processes. The addition of an aluminate salt (Na 2 Al 2 O 4 ) had different effects than iron salt addition (Table 4). Addition of 1 mg/l Na 2 Al 2 O 4 slightly affected Ca and Mg removal but had little effect on overall hardness removal. Addition of Na 2 Al 2 O 4 also had little effect on the turbidity after sedimentation. After 15 min, however, turbidity levels decreased upon increased aluminate addition (Table 4). The experiments at 5 and 1 mg/l Na 2 Al 2 O 4 were CHAO ET AL PEER-REVIEWED 94:3 JOURNAL AWWA MARCH

6 FIGURE 5 Measured ph Change in ph during aeration softening experiments as a function of mixing and aeration rates No mixing Mixing 95 cc/min 48 cc/min 95 cc/min 1,9 cc/min ,2 1,5 Aeration Time min repeated at a higher ph (11.). Improved removal of hardness and Si were observed, compared with experiments at ph 1.4 (Table 4). Although Na 2 Al 2 O 4 addition improves turbidity removal and does not affect ph, control of ph is still a necessity for high removal efficiencies for Ca, Mg, and Si. Field observations at the CPGS with the use of Na 2 Al 2 O 4 had indicated a propensity for the sludge to adhere on sludge collection equipment and potentially cause serious equipment failure. To investigate this and to understand the improved settleablity of turbidity upon Na 2 Al 2 O 4 addition, the authors measured the viscosity of settled sludge. Viscosity was measured on settled, decanted sludge after large-scale jar-testing (4 L) upon Na 2 Al 2 O 4 addition in the presence of OLSA. The settled sludge viscosity increased from 17.7 centipoise with just lime and soda ash addition to 19. centipoise with 5 mg/l Na 2 Al 2 O 4 addition in the presence of OLSA dosing and 25.2 centipoise with 1 mg/l Na 2 Al 2 O 4 addition in the presence of OLSA dosing. Thus, the addition of 1 mg/l Na 2 Al 2 O 4 increased viscosity by 42%. Therefore, increased viscosity of the turbidity particles was probably responsible for the observed increased rate of turbidity removal (Table 4). The tradeoff for the improved settleability may be the propensity for the sludge to adhere on mechanical sludge collection equipment. Viscosity was a simple measurement that may provide guidance for assessing sludge characteristics during softening. Both iron and aluminate salts improved the rate of turbidity removal. However, both chemicals would require additional feed facilities and would increase the overall sludge production. Without ph readjustment, iron addition decreased the overall hardness removal, whereas Na 2 Al 2 O 4 addition slightly improved Ca and Mg hardness and Si removal. NaOH or Ca(OH) 2 could be added for ph readjustment in full-scale systems. Aeration softening without lime or soda ash. Aeration effects on solution ph. The authors also investigated a physicochemical process to soften water that does not involve lime and soda ash addition; termed aeration softening, the process was examined as a means to reduce chemical usage and sludge production. TABLE 4 Effect of Na 2 Al 2 O 4 in jar tests in the presence of an optimal lime and soda ash dosage Na 2 Al 2 O 4 dose mg/l Parameter* ph [Ca 2+ ] remaining mg/l as CaCO [Mg 2+ ] remaining mg/l as CaCO Total hardness removal % [Si 4+ ] remaining mg/l as SiO Turbidity at 15 min ntu Turbidity at 12 min ntu *[Ca 2+ ] calcium ions, CaCO 3 calcium carbonate, [Mg 2+ ] magnesium ions, [Si 4+ ] silica ions, SiO 2 silicon dioxide Na 2 Al 2 O 4 sodium aluminate ph was readjusted with sodium hydroxide after Na 2 Al 2 O 4 addition. Dissolved concentrations remaining after 2-h sedimentation period in jar test 114 MARCH 22 JOURNAL AWWA 94:3 PEER-REVIEWED CHAO ET AL

7 Figure 5 shows the effects of several mixing and aeration rates on ph for experiments conducted in the aeration reactor. Without mixing, the solution ph in an open reactor gradually increased from ph 7. to 7.5 over 24 h. Mixing, provided via a magnetic stirrer, resulted in a gradual ph increase to approximately ph 8.2. However, the addition of pressurized air raised the ph rapidly to above ph 8.5, and higher aeration rates increased the rate and extent of the ph change. Aeration rates of 48 cc/min and higher developed a maximum ph level that occurred after 3 6 min of aeration. Continued aeration beyond the maximum ph level resulted in a period characterized by a ph dip. This observation was consistent and reproducible. The change in ph was caused by the removal of dissolved CO 2 (Holm & Schock, 1998; Trussell, 1998; La- Motta, 1995) and is discussed later relevant to aeration softening. At any given time, the dominant form of carbonic acid (H 2 CO 3 *) is dissolved CO 2. The equilibrium relationship between dissolved CO 2 and gaseous CO 2 is set by the Henry s law relationship (K H = for CO 2 at 2 o C). Carbonic acid is in equilibrium with bicarbonate (HCO 3 ), which affects the ph levels. Assuming that calcite does not precipitate and remove dissolved inorganic carbon, then the final ph (ph f ) can be approximated by the equilibrium-based relationship given as Eq 2 (Lytle et al, 1998) [CO 2 ] f [CO2 ] i ph f = ph i log (2) in which ph i is the initial solution ph level and [CO 2 ] f and [CO 2 ] i are the final and initial dissolved CO 2 concentrations, respectively. Removal of dissolved CO 2 increases the final ph level. The kinetic driving force for stripping dissolved CO 2 can be represented by the following expression derived from two-film theory: FIGURE 6 Ca Concentration mg/l as CaCO d[c O2] = k dt L a [CO 2 ] [CO 2 ] eqm (3) 5 Change in dissolved Ca during aeration softening experiments as a function of mixing and aeration rates No mixing Mixing 95 cc/min 48 cc/min 95 cc/min 1,9 cc/min ,2 1,5 Aeration Time min Ca calcium, CaCO 3 calcium carbonate FIGURE 7 Ca Concentration mg/l as CaCO Effect of nuclei seed addition on the change in dissolved Ca during aeration softening experiments No seed 1 g/l 2 g/l 3 g/l ,2 1,5 Aeration Time min Ca calcium, CaCO 3 calcium carbonate, 95 cc/min aeration rate in which the dissolved CO 2 concentration ([CO 2 ]) is a function of the local mass transfer coefficient, k L a, and the dissolved CO 2 concentration if it is in equilibrium ([CO 2 ] eqm ) with the gas phase. The value of k L a is pri- CHAO ET AL PEER-REVIEWED 94:3 JOURNAL AWWA MARCH

8 FIGURE 8 Ca Removal Rate mg/l Ca min Nuclei Seed Dose g/l Ca calcium Linear regression through calculate Ca removal rates during the first 5 min of aeration softening as a function of nuclei seed addition y =.42x R 2 =.997 marily a function of the aeration equipment. Integration of Eq 3 from [CO 2 ] i at time zero to the dissolved CO 2 concentration at some time t ([CO 2 ] t ) yields Eq 4 [CO 2 ] t = [CO 2 ] i + [CO 2 ] eqm [CO 2 ] i 1 e (k La) t (4) in which [CO 2 ] t is the CO 2 concentration at time, t. Assuming that calcite does not precipitate and that alkalinity (the acid-neutralizing capacity of water) does not change, then kinetic changes in ph level can be predicted by Eqs 2 and 4. The authors hypothesized that during the initial period of aeration (marked by a rapid rise in ph) CO 2 removal controlled the observed ph changes. Furthermore, the ph dip occurred because of the formation of soluble or solid forms of CaCO 3 (Eqs 5 and 6) that could liberate protons to depress the ph: CaCO 3 o (aq) + H + Ca 2+ + HCO 3 K = (5) CaCO 3(s) + H + Ca 2+ + HCO 3 K sp = (6) Ca removal during aeration. Figure 6 shows the effects of aeration on Ca removal. Without mixing or aeration, dissolved Ca levels decreased by < 1% after 24 h in the aeration reactor open to the atmosphere. Providing mixing alone resulted in approximately a 15% removal of dissolved Ca. At the lowest aeration level of 95 cc/min plus mixing, Ca concentrations gradually declined over 24 h and resulted in an overall reduction of approximately 35%. However, higher aeration rates resulted in a rapid decrease of Ca concentrations (i.e., increased Ca removal), followed by a period of negligible incremental Ca removal. The three highest aeration rates resulted in roughly a 6% decrease in Ca levels. Aeration rates of 95 and 1,9 cc/min were not statistically different at a 95% confidence level, based on an analysis of variance twotailed analysis. The process responsible for Ca removal is based on the precipitation (Eq 6) of calcite [CaCO 3(s) ]. As solution ph increased during aeration from ph 7. to levels between 8.5 and 9. (Figure 5) the solubility of calcite was reduced; reference Figure 1. The saturation index (SI) can be calculated to represent the potential for a solid to precipitate based on Eq 7 (Pisigan & Singley, 1985; Rossum & Merrill, 1983) + } { HCO SI = log {Ca2 3 } K { H+ } (7) sp in which K sp is the solubility product for calcite. An SI value > 1 would indicate supersaturation and the potential, given sufficient time, for precipitation to occur. Thus, during aeration as ph increased (i.e., decreased proton activity), the SI value would rise and indicate supersaturated conditions. Prior to any treatment, the SI value for the water used in this study was.5. The calculated SI after aeration would be > 2., based on the initial bicarbonate alkalinity and observed ph between 8.5 and 9. (Figure 5). Supersaturation created a driving force for precipitation of calcite. Mg and Si were not removed during aeration, because the SI for corresponding solid phases did not exceed 1. at the ph conditions present during aeration. Nuclei seed addition. The addition of nuclei seed particles at the beginning of aeration increased the rate and extent of Ca removal (Figure 7); results shown were averaged from duplicate experiments. Nuclei seed dosages of 1, 2, and 3 g/l increased the Ca removal after 24 h of aeration (95 cc/min) from approximately 6% to 7, 8, and 85%, respectively. Similar ph changes during aeration were observed with and without seed dosing (not shown); ph levels reached within the first 5 min of aeration. No ph spike or ph dip was observed in the presence of the nuclei seed. During the first 5 min of aeration, the addition of 1, 2, and 3 g/l seed increased the Ca removal from 4% without seed addition to 16, 45, and 6%, respectively. Because the nuclei seed used in this work was obtained from a full-scale chemical softening plant operating near ph 11., the nuclei seed contained Ca, Mg, and Si. Dissolution experiments were carried out at ph 8; dissolved Mg and Si were released. As a result, during aeration experiments with the nuclei seed, although Ca was removed, Mg and Si levels increased by roughly 2% each. 116 MARCH 22 JOURNAL AWWA 94:3 PEER-REVIEWED CHAO ET AL

9 TABLE 5 Conceptual model for aeration softening Equation Process Description Governing Relationship Number Step 1: Aeration strips dissolved CO 2 * from solution [CO 2 ] t = [CO 2 ] i + {[CO 2 ] eqm [CO 2 ] i } {1 e (k La) t } Eq 4 CO2] f Step 2: Solution ph increases with removal of CO 2 ph f = ph i log [ [ CO2] Eq 2 i Step 3: Calcite becomes oversaturated (saturation index > 1) Step 4: Calcite precipitation is enhanced by the presence of nuclei seed crystal surface area SI = log {Ca 2+ } { HCO3 } Ksp { H+ } Eq 7 d[c a ] = ks dt ] [Ca 2+ ] * 2 Eq 8 Step 5: Aeration process promotes particle particle interaction Orthokinetic and differential settling flocculation and aggregation (Stumm, 1992) Step 6: Calcite removal Sedimentation or filtration *CO 2 carbon dioxide The rate of dissolved Ca removal (mg/l as CaCO 3 per min) during the first 5 min of the experiment increased linearly with nuclei seed dosage (Figure 8). The rate of removal in subsequent time intervals was only 1 2 mg/l as CaCO 3 per min. Precipitation kinetics are dependent on the available crystal surface area for nucleation to occur. Ca removal in CaCO 3 systems has been found to correspond to the rate law given as Eq 8 (Stumm & Morgan, 1981; Nancollas & Reddy, 1975): 2+] d [C a = ks ] [Ca dt 2+ ] * 2 (8) [Ca2+ in which k is a rate constant, S is the surface area available for precipitation (mg/l of a certain particle size), [Ca 2+ ] is the actual dissolved Ca concentration, and [Ca 2+ ] * is the theoretical dissolved Ca concentration in equilibrium with calcite. The slope of a linear regression through the data points in Figure 8 yields a value of.42 mg Ca per gs-min and represents the lefthand side of Eq 8 divided by S [g/l]. The value of k was computed by using the actual dissolved Ca concentration ([Ca 2+ ]) from the experiments of 164 mg/l CaCO 3 (1.63 mm as Ca) shortly after seed addition, plus a calculate [Ca 2+ ] * value of.16 mm based on the observed ph of 8.13 at 5 min into the experiments. The value of k was then calculated by substituting these values into Eq 8, which yielded a k value of.46 L 2 mg TABLE 6 S 1 mole Ca 1 min 1. The k value from the current study was significantly lower than the value of 5.76 L 2 mg S 1 mole Ca 1 min 1 found by an earlier study (Nancollas & Reddy, 1975) for a different solution and crystal surface. Aeration model. Table 5 shows the conceptual model for aeration softening. Equations 2, 4, 7, and 8 were used to simulate the experiment with nuclei seed (2 g S/L) addition (Figure 9). The model equations were capable of predicting the general trend of ph change and Ca removal. The value for k L a was varied between.5 and.1 min 1 (simulation presented with.2 min 1 ); otherwise initial conditions and the calculated k value were used. Increasing the k L a value increased the rate of change of the ph but rapidly reached a threshold at ph 9.1. Increasing the ph rate of change (i.e., increasing k L a) resulted in a more rapid Ca removal (not shown). The overestimation of the ph could be attributable to neglecting inor- Summary of removal efficiencies for different advanced softening processes Removal Range % Treatment Process* Process ph Calcium Magnesium Silica OLSA OLSA + iron 1 mg/l Fe mg/l Fe + ph readjustment OLSA + 5 mg/l Na 2 Al 2 O Aeration (95 cc/min) ~ ~ Aeration (95 cc/min) + 3 g/l seed *OLSA optimal lime and soda ash, Fe iron, Na 2 Al 2 O 4 sodium aluminate Magnesium (Mg) and silica (Si) concentrations increased because of dissolution of the nuclei seed containing Mg and Si precipitates in the sludge from the full-scale system at Coronado Power Generation Station, which was used as the seed source. CHAO ET AL PEER-REVIEWED 94:3 JOURNAL AWWA MARCH

10 FIGURE 9 Ca Concentration mg/l ph , 1,5 Reaction Time min A B Measured and predicted Ca removal (A) and change in ph (B) for aeration experiment and equations in Table 5 Measured Ca removal Predicted Ca removal Measured change in ph Predicted change in ph , 1,5 Reaction Time min Ca calcium. Initial conditions ph = 7.5, Ca = 264 mg/l calcium carbonate (CaCO 3 /L), alkalinity = 32 mg/l, ionic strength =.28 M, 2 g/l seed. Table 5 equations k =.46 L 2 mg 1 mole 1 min 1, k L a =.2 min 1 ganic Ca and carbonate complexes. If ph were accurately predicted, then the model would more closely simulate the observed Ca removal. IMPLICATIONS Chemical softening is very effective for hardness removal, but it requires significant chemical use and generates large amounts of settled sludge. Aeration softening may be a viable alternative process that does not require chemical addition and thus would reduce sludge production. Several potential configurations for a process could be developed, but only a simple two-compartment schematic is discussed here. The first compartment would contain aeration equipment (e.g., diffusers and mechanical mixers) to strip CO 2, providing time for precipitation and aggregation. Sludge would settle in the second chamber. A portion of the settled sludge would be recirculated to the beginning of the process as a source of nuclei seed. If removal of Mg, Si, or both was required, a two-stage process could be implemented, employing aeration softening as a first stage followed by chemical softening with NaOH in the second stage. Water chemistry factors could affect the performance of an aeration softening system. First, aeration must be capable of raising the ph to a point at which calcite is supersaturated. Therefore, the system would be wellsuited for waters with high dissolved CO 2 levels (e.g., groundwaters). Second, sufficient carbonate must be present or added as Na 2 CO 3 for calcite formation; waters with predominantly CaCO 3 hardness would not require Na 2 CO 3 addition. Third, the presence of high Mg hardness may require additional treatment. Finally, aeration may provide alternative benefits for treating waters with reduced metals [e.g., Fe(II) or Mn(II)] because the process would also serve as an oxidation process. Previous consideration of the use of aeration in combination with softening focused solely on the ability of aeration to rapidly remove carbonic acid, thus reducing the lime dosage necessary to achieve operating ph levels ( ) (ASCE & AWWA, 199). The aeration process discussed here takes advantage of CaCO 3 chemistry and allows for additional processes (e.g., precipitation onto nuclei) to occur in a single reactor. CONCLUSIONS This study evaluated removal processes for chemical softening with the addition of metal salts and aeration softening (no lime or soda ash addition). Removal efficiencies were very ph-dependent. Table 6 summarizes results from the different softening techniques investigated. Aeration softening has the potential to produce less sludge than chemical softening (i.e., lime, soda ash, metal salts) because aeration softening does not add additional Ca. However, aeration softening may only be applicable to groundwaters supersaturated with dissolved CO 2 and of predominantly carbonate hardness. Therefore, for systems not meeting these conditions, optimization of metal salt addition and ph conditions can maximize Ca and Mg removal. In most cases shown in Table 6, ph was the master variable that controlled Ca and Mg removal efficiencies. At equivalent ph levels, the addition of metal salts during chemical softening or nuclei seed during aeration softening simply affected the kinetics of dissolved species and turbidity removal rate. Metal addition experiments supported the following conclusions. Addition of metal salts improved the rate of turbidity removal through formation of new particles and increased solids viscosity. 118 MARCH 22 JOURNAL AWWA 94:3 PEER-REVIEWED CHAO ET AL

11 Addition of iron salts formed iron hydroxide solids, resulting in consumption of hydroxide that lowered the solution ph. Consequently, when iron salts are used during Mg removal, extra lime and soda ash may be required to maintain the ph above 11., compared with the dosage without iron salts. Without ph readjustment, iron salt addition decreased Mg removal. No performance differences were observed between FeCl 3 and Fe 2 (SO 4 ) 2. Addition of Na 2 Al 2 O 4 had a minimal effect on Ca and Mg removal. Si removal tended to parallel Mg removal. Mg and Si removal were very ph-dependent. Aeration softening experimentation led to the following conclusions. Aeration of a water containing predominantly carbonate hardness resulted in Ca removal via calcite precipitation. Aeration stripped dissolved CO 2 from the groundwater, resulting in a rise of ph and consequentially supersaturation of calcite. Higher aeration rates (i.e., higher k L a values) increased the rate of change in ph and dissolved Ca levels. Adding nuclei seed particles increased the rate of dissolved Ca removal. Mg and Si were not removed during aeration. Stripping of dissolved CO 2 was the primary process responsible for Ca removal. Stripping raised the ph, resulting in supersaturation of calcite. Aeration stripping would be most appropriate for waters supersaturated with CO 2 (e.g., groundwaters) and containing predominantly CaCO 3 hardness. Na 2 CO 3 could be added for waters with noncarbonate hardness. Settled sludge could be used as nuclei seed in continuous-flow applications. ACKNOWLEDGMENT The authors thank Don Goldstrohm for providing technical input. This work was funded by Salt River Project (Phoenix, Ariz.) as part of a collaborative research program with Arizona State University (Tempe). ABOUT THE AUTHORS: Peng-Fei Chao is a doctoral student in the Department of Civil and Environmental Engineering at Arizona State University (ASU) at Tempe. He has an MS degree in civil and environmental engineering from ASU and a BS degree in chemistry from Chinese Cultural University, Taipei, Taiwan. Chao s recent research focuses on disinfection by-product formation during the ozonation process. Paul Westerhoff* is an associate professor in the Department of Civil and Environmental Engineering, Engineering Center G252, POB 536, Tempe, AZ , <p.westerhoff@asu.edu>. *To whom correspondence should be addressed If you have a comment about this article, please contact us at <journal@awwa.org>. REFERENCES ASCE (American Society of Civil Engineers) & AWWA, 199. Water Treatment Plant Design. McGraw-Hill, New York. AWWARF (AWWA Research Foundation); Lyonnaise des Eaux; & Water Research Commission of South Africa, Water Treatment: Membrane Processes. McGraw-Hill, New York. 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Aquatic Chemistry An Introduction Emphasizing Chemical Equilibria in Natural Waters. Wiley-Interscience, New York. Trussell, R.R., Spreadsheet Water Conditioning. Jour. AWWA, 9:6:7. Water Quality and Treatment, 199 (4th ed.). (F.W. Pontius, editor). McGraw-Hill, New York. CHAO ET AL PEER-REVIEWED 94:3 JOURNAL AWWA MARCH