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Compact accelerated precipitation softening (CAPS) as pretreatment for membrane desalination II. Lime softening with concomitant removal of silica and heavy metals
DESALINATION ELSEVIER
Desalination 113 (1997) 73-84
Compact accelerated precipitation softening (CAPS) as pretreatment for membrane desalination II. Lime softening with concomitant removal of silica and heavy metals Alexandra Masarwaa*, Dan MeyersteinaYb,Naphthali Daltrophe”, Ora KedemC “Chemistry Department, Ben-Gurion University of the Negev, Beer-Sheva, Israel Far +972 (7) 647-2943 bThe College of Judea and Samaria, Ariel, Israel ‘The Institutes for Applied Research, Ben-Gurion Universi@ of the Negev, Beer-Sheva, Israel
Received 2 April 1997; accepted 3 1 July 1997
Abstract In a previous paper the CAPS softening process is suggested as a pretreatment step for RO: in addition to removal of calcium it is an effective filtration and may thus replace part of the customary pretreatment. In the present study the removal of silica and heavy metals in addition to water softening was investigated. Precipitation with both sodium hydroxide and with lime was studied. With lime precipitation in CAPS, it was found that magnesium is necessary for silica removal, if no other additives such as Al and Zn are used. With conventional methods of precipitation, the well-known reagents for silica removal, aluminium salts, lead to very slow precipitation or difficult filtration. In contrast, aluminium chloride used as an additive in CAPS can remove a major part of the silica without interfering with filtration, if the precipitation conditions are chosen correctly; optimal conditions were in keeping with the regime of softening plants. It was found that zinc chloride, which is environmentally more acceptable than the alurninium salt, could also be used for silica removal, but in larger amounts than aluminium chloride. With these two salts, a substantial fraction of the silica can be removed, but the precipitation of silica results in decreased calcium removal. To achieve simultaneous softening and silica removal, it is necessary to add carbonate. Heavy metals are coprecipitated with the calcium carbonate. Keywords: Water softening; Silica; Heavy metals; Clogging
*Corresponding author.
001 l-9164/97/$09.50 Q PZZ 001 l-9164(97)001
1997 Elsevier Science B.V. All rights reserved 16-1
74
A. Masama et al. /Desalination I13 (1997) 73-84
1. Introduction In the preceding paper [l] a compact lime softening process (CAPS) was described. It was suggested that CAPS be used as a pretreatment to membrane desalination, which would decrease both calcium sulfate scaling and fouling. The aim of this pretreatment is an increased maximal recovery ratio. In addition to hardness, scaling may also be caused by silica and magnesium hydroxide. While the precipitation of Ca and Mg salts has been studied intensively, the formation of silica precipitates [2-51 is less well understood. Similarly, reverse osmosis (RO) technologies concentrate on the prevention of Ca and Mg scales, while the formation of Si scales was investigated in a smaller number of studies [6-91. The importance of silica scale formation and its prevention in RO systems have recently been described in detail [lo]. In the present study the effects of a number of additives that are expected to accomplish Si removal in addition to a reduction in Ca and Mg concentrations during lime softening were investigated. The effects of Mg2’, A13+,and Zn2+ salts on the removal of silica from synthetic tap water were studied. Since these substances are also potential coagulants, their addition might facilitate a reduction of turbidity through flocculation. Integration of these ideas into the CAPS process was evaluated.
2. Experimental 2. I. Preparation
of synthetic tap water
It should be noted that although the pH of the water was initially adjusted to a value that corresponds to a negative or only slightly positive saturation index (SI) [l], the concentration of Ca was reduced over a period of several weeks. The precipitation of CaCO, should per se reduce pH, but in fact a rise in pH was observed. This change in pH was apparently due to a loss of CO,, which
caused the precipitation. The concentration of CO, in the water was larger than its solubility at about pH 7.0 under atmospheric conditions. In light of these considerations, the water was used as fast as possible after its preparation, since it was difficult to obtain water of a stable composition. During the course of the experimental work, it became obvious that removal of Si and Mg by lime softening hardly depended on the Ca or carbonate concentration of the water, whereas the efficiency of Ca removal was strongly influenced by the carbonate content of the water. Two batches of tap water (Table 1) that differed only slightly from one another were made up in close agreement with a representative analysis of Beer-Sheva (Israel) tap water. These samples, which were used for our experiments, were made up as follows: Batch a: This water was used in the NaOH softening experiments. The synthetic water was made by adding salts to deionized water that already contained a considerable amount of SiO, (about 5ppm). After addition of 10 ppm Si, the total silica concentration was thus rather high. Although no scaling was visible, the Ca content of this batch decreased appreciably over a period of one month (pH 7.4). Batch b: Lime softening experiments were performed with this type of water. This batch was basically the same water as batch a, but it differed in that it was prepared with distilled and deionized water and the pH was adjusted to 7.6. The water was used within a week for the softening experiments so that carbonate and calcium reduction were kept to a minimum. ??
??
2.2. Analytical procedures The standard solutions were prepared as follows: Si: Titrisol (Merck) (SiCl, in 14% NaOH) 1000 ppm Si (= 2 140 ppm SiO,)
??
A. Masarwa et al. /Desalination
113 (I 997) 73-84
75
Table 1 Composition of synthetic tap water, representative of Beer-Sheva, Israel, tap water MW
Salt
MgSO, (70.0%) b CaCI,*2H,O KHCO, NaHCO, NaCl HCOj clso;Si (1000 ppm)
120.37 147.01 100.11 84.01 58.44
g/101”
2.32 2.42 0.05 3.36 1.75
mM’
1.35 1.65 0.05 4.0 3.0 4.05 6.3 1.35 0.36
mg/l as CaCO,
135 165 200 150 315 135
Ion
ppm, mg/l Batch a
Batch b
30 38”-66 2 92 69 247 223 130 15
34 57
8
Mg*+ Ca2+ K+ Na’. Na+ HCO; Cl- (+HCI) so;Si
aAmount used for preparation of the water. bPercentage according to analysis of 50 ppm standards made up of these salts. ‘After about 1 month.
9 Mg: MgS04 dried (Frutarom) (ca. 70%), 700 ppm (2.477 g/500 ml); Ca: CaC12.2 H20 (Merck) (91.3%), 913 ppm (1.834 g/500 ml); Lime solution: a suspension of 50 mM lime (Ksp =6.5 Y 1O-6 - 18.7 mM soluble).
??
??
??
??
2.3. Analytical
methods
All spectrophotometric determinations were performed employing a Hewlett Packard 8452A diode array spectrophotometer. Si was analyzed using the heteropolyblue spectrophotometric determination with Spectroquant8 (Merck). The proposed procedure recommends a 5-min waiting period for the reduction of molybdosilicic acid (step 3), but kinetic experiments showed that a longer period was necessary for the blue color to develop and reach its maximum absorption, so the procedure was slightly modified to allow for a minimum 15-min waiting period. Ca and Mg were determined using a modified spectrophotometric determination with Eriochrome Blue SE.1 1 The following conditions were used:
Determination of Ca at pH 14, at 640 nm, using a calibration curve of 2-50 ppm Ca (quadratic with offset): 1.5 ml of sample (or say 0.5 ml of sample + 1.0 ml of water), 1 .O ml of 0.13% dye solution, and 2.5 ml of 8% NaOH. Determination of Mg (+Ca) at pH 9 at 548 nm: 1.5 ml of sample (or say 0.5 ml sample + 1.0 ml water), 2.0 ml water, 0.5 ml dye, 1.O ml 5x concentrated pH 9 buffer solution [i.e., 5x (0.05 M H,BO,, 0.05 M KCl, 0.022 M NaOH)].
The buffer system of pH 9 was used because the water samples contain a carbonate buffer system and the absorbance of the dye is very pH dependent. A pH of about 13 (2.5 ml of 0.8% NaOW5 ml) recommended in the literature [ 141 was found barely suitable for the analysis of mixtures containing more Ca than Mg (only 0.4 mM, about 15 ppm of Ca and Mg combined can be determined under those conditions). In our modification the concentration of the dye is reduced. At the reduced dye concentration and at pH 9, the formation of the complex (Mg-dye) or (Ca-dye) is observed at 520-550 nm
76
A. Masarwa et al. /Desalination
and the decrease of the free dye concentration above 600 nm. Apparently, the Mg-dye complex has the higher formation constant at this pH, so that in a mixture of Ca and Mg the contribution of the Ca-complex absorbance is only additive for very small Mg concentrations (l-3 ppm Mg). The effect of Ca can be ignored for Mg concentrations >5 ppm. The following corrections can be applied at 548 nm (A=absorption units): l-2 ppm Mg, 8-33 ppm Ca: 0.011 Alppm Ca: 3-5 ppm Mg: 0.0035 Alppm Ca; 5-8 ppm Mg: 0.0012 Alppm Ca. Zn does not affect the Ca analysis, but the Mg analysis is not accurate when more than about 3 ppm of Zn remain in the samples, which apparently occurs only in samples filtered in the absence of a CaCO, cake at the lower lime concentrations. The influence of Zn manifests itself in an additional absorbance at the otherwise isosbestic point of the Mg-dye determination (578 nm), so that the amount of Zn can be determined at 578 nm. However, the effect of the Zn-dye complex at 548 nm (max. of the Mg/dye complex) is not simply additive, so that a deconvolution is difficult and inaccurate. Other cations were measured by ICP/OES on an Optima 3000 Perkin Elmer instrument.
I1 3 (1997) 73-84
pJ;n=A [Jrl(l+fl)]
- 0.3 I (for 1~0.5)
where I is the ionic strength. At 20°C pK, =8.45; 0.506
pK2 = 10.38; pK,-pK,
= 1.93; A=
Synth. tap water: 1=0.0175 Therefore, 5pjin = 0.27; and pHs= 1.93 + 2.78 + 2.39 + 0.27 = 7.37
The calculations given above show that tap water at pH >7.4 is oversaturated (positive SI) and thus has a tendency to precipitate CaCO,. Below these pH limits (negative SI), the water is corrosive. By preparing synthetic water and adjusting its pH we had to find a careful balance between a pH at which CO, evolution could be kept to a minimum and at which the water was only slightly oversaturated. 3.2. Hardness/lime dosage calculations [12]
3. Hardness and Saturation Index The following SI and hardness calculations are applicable to the synthetic water studied. 3.1. Saturation Index [I 21 SI=pH-
pH,
pHs =pK, - pK, +p[Ca2’] +p[HCO;] + 5pfi where pHs is the equilibrium p, K2 the second dissociation constant of carbonic acid, KS the solubility constant of calcium carbonate, andfi the activity coefficients. Under the conditions studied here:
The amount of lime necessary to achieve lime softening alone is presented in Table 2. Carbon dioxide and carbonate hardness (calcium and magnesium bicarbonate) are precipitated by lime. To remove Mg and Ca non-carbonate hardness (calcium and magnesium sulfates, chlorides or silicates), as well addition of soda ash is necessary. To remove Mg as Mg(OH), the pH of the solution has to be raised to >10.5, thus an excess of lime is required. However, it is known that about 10% of Mg is carried down as Mg(OH), during CaCO, precipitation. Thus, lime softening without the addition of soda ash removes only the carbonate hardness of the water. An equivalent point for the removal of
A. Masanva et al. /Desalination Table 2 Hardness and lime dosage necessary to achieve softening Hardness, mg.0 as CaCO,
Total hardness CaCO, hardness aCa non-carbonate hardness MgCO, hardness aMg non-carbonate hardness H2C03
HCOj
Lime dosage
mg/las CaCO,
mM
300 165 0
165 0
1.65 0
23.5 111.5
47 111.5
0.47 1.11
30
0.3
30 188
“Requires the addition of soda ash (1 equiv.) to affect a net change in hardness level. the Ca carbonate hardness alone (no Mg removal)
can be calculated, adding the amounts of lime required to neutralize H,CO, and to remove the Ca carbonate hardness. Thus, about 2 mM of lime are needed for reduction of Ca in the tap water to the practical limit, i.e., about 30-50 mg/l as CaCO, = 12-20 ppm Ca.
4. Results and discussion It is well known that silica can be removed from water by suitable additives, mainly aluminium salts [2-51. The precipitates formed are flocky, difficult to filter, and slow to settle. Preliminary experiments confirmed the removal of silica in our procedures and showed that efficacy of removal depends on the conditions of precipitation. We tested zinc salts as an alternative to aluminium salts for the precipitation of silica, concomitant with lime softening. Zinc should behave similarly to aluminium (amphoteric hydroxide), and its addition to water may be less restricted, being environmentally less The following drinking water dangerous. standards apply:
I I3 (I 997) 73-84
us:
preferable limit, 5 ppm desirable limit, virtually absent
Israel:
recommended max. level, 5 ppm maximum allowance, 15 ppm
77
Four types of precipitation/filtration were tried: Lime solutions and additives were added to the water, and the resulting suspensions were allowed to stand for different times with intermittent shaking and then filtered either through filter paper (“regular”) or through a CaCO, cake (types 1 and 2). Alternatively, a fine precipitate of CaCO, was added to the water. Lime and additives were added to the resulting slurry, and this was filtered after a short retention time either through filter paper or through a cake (types 3 and 4). In all cases precipitated CaCO, (Riedel de Haen or Merck) was used. Best results were obtained for precipitation on slurry followed by cake filtration (type 4). It should be noted that these conditions simulate the CAPS process carried out in plants, which is described in detail for a single filter tube in our previous paper [I]. During the course of the experiments, it was noticed that the Si content of the solutions that had been filtered through a freshly washed cake decreased dramatically, sometimes accompanied by an increase in pH. Therefore, to obtain reproducible results which can be translated into an industrial process, the cake has to be washed with at least a 50-ml portion of Si-containing water before each series of experiments. [The results indicated that a 30 g cake is able to “absorb” only a small amount of Si (about 3-5 ppm of SV50 ml sample, i.e., about 0.1 mg of Si)]. One of the major advantages of CAPS over conventional lime softening is the filterability of the CaCO, precipitate. Since it seemed likely that the customary additives used for removal of silica during CAPS might spoil this essential feature we set out to study the influence on filterability of the different conditions of precipitation.
78
A. Masanva et al. /Desalination
4.1. Filtration rates during synthetic tap water
lime softening
of
I13 (1997) 73-84
For regular filtration the increase in filtration time, dt, was approximately 600s for Al and 230s for Zn, as compared with 53 s without additives (Table 3). The effect of the additives on the filtering through the cake is much less severe. Retention of the reaction mixture for 3 min or more before filtration improves filterability for both regular and cake filtration, but the reduction of dt is more pronounced for the cake. Table 3 shows that dt caused by the addition of aluminium chloride is drastically reduced in cake filtration following retention of 3 min. The addition of calcium carbonate particles to the hard water before softening changed the picture completely. When the slurry was retained for 3 min, following the addition of lime and zincor aluminium chloride, the filtration time observed for each of the consecutive samples was 25*5 s. This is similar to filtration of water through the cake. In other words, clogging is eliminated by precipitation on a carbonate slurry and retention for a few minutes. The regime followed here simulates the continuous CAPS process: precipitation on slurry, retention for a few minutes, and subsequent cake filtration.
Upon addition of the lime suspension to the hard water, with or without additives, a precipitate starts to form immediately. Table 3 shows examples of filtration times for these precipitates under different conditions. A series of samples with O&4.5mM of lime (six samples per series) were filtered successively in order of concentration. The most significant quantity for comparison of the experiments is the difference, dt, between the time of filtration of the first and the last samples of each series, since this parameter best indicates the extent to which the formed precipitate clogs the filter. It is obvious that filtration of the precipitate formed causes clogging of the filter (medium paper or the cake), since the filtration time increases and exceeds the time of filtering through a 20g layer of CaC03, although only small amounts of precipitate are added to the cake. The effect is quite pronounced even with only small quantities: a series of six samples is only 300 ml of water. The addition of Zn or Al increases the filtration time drastically when regular filtration is used, the effect being more pronounced for Al.
Table 3 Comparison of filtration times (s) for 50 ml solution samples with different additives (Add.) and retention times (t,,,) mM Lime
Filtration time, s Add. none t ret<*o s
Add. 27 ppm Zn trct
Add. 27 ppm Zn t Ter.5-l h
Add. 30 ppm Zn t ret 3 min
Add. none tret 3 min
Reg.
Cakea
Reg.
Cake a
Reg.
Cake a
Reg.
Cake
Reg.
Cake
Reg.
Cake
0 0.5 1.0 1.9 2.9 4.5
10 * * * * 120
24 * * * * 45
40 48 60 70 80 110
<:lO 10 33 70 105 140
80 80 85 100 90 100
22 22 25 27 26 28
~10 12 25 38 45 63
18 23 23 22 23 22
< 10 21 140 350 510 600
21 27 30 35 38 40
dt
110
21
290
70
130
10-20
230
6
53
Insig.
600
20
-.
Add. 20 ppm Al tret 3 min
Reg., one filter paper used for series 04.5 mM lime. Cake, new or reconditioned (washed and top layer removed) CaCO, cake (20 g/IO cm 0) used for each series, except: a samples from three series all filtered consecutively through the same cake. *. time not measured.
A. Masanva et al. /Desalination 113 (1997) 73-84 Precipitation on a slurry of CaCO, not only improved filtering properties but also appreciably enhanced the ability to remove Ca, Mg and Zn. Moreover, additional filtration through the cake further improved this removal of cations. No Zn residue was measurable (I 1 ppm) when precipitation took place on the slurry and/or by filtration through a cake, whereas in “regular” filtered samples up to 20ppm of Zn (for 30ppm Zn addition) remained in the treated water samples for low lime concentrations. A 0.5% slurry seems to be as effective as a more concentrated slurry with regard to Si, Ca and Mg removal and reduction of filtration time. No measurable difference could be detected on softening tap water with 3 mM lime, when slurries of 0.5, 1.O, 2.0, 3.0 or 5.0% were used. Since it was advantageous to work with a slurry, we only report here the results of the slurry experiments in detail. Generally, a 3 wt% slurry was used, and a maximum of 3 min retention time was allowed. It was realized during the course of this work that Ca softening by the lime process was reduced appreciably when the water did not contain enough carbonate. Consequently, most experiments were conducted with freshly prepared water to avoid Ca and carbonate reduction. Since it was thought that carbonate reduction was the likely cause of the impairment of Ca softening when Si was removed by Zn or Al, the addition of NaHCO, (instead of say soda ash) was also investigated for these samples. 4.2. Softening by precipitation and cake$ltration
on CaCO, slurry
The following series of experiments which constitute a representative example of the effect of lime softening in a simulation of the CAPS process are described in greater detail. The results are summarized in Figs. l-3 and Tables 4 and 5. Precipitation was carried out on a slurry of 3% CaCO,, added as 10.5 g of solid in a 350ml water sample. The additives Zn and Al, and finally lime
79
as 50 mM suspension, were added to 50ml samples which were filtered after a retention time of 3 min through a 20g cake. If an old cake was used, the top layer was removed before each series of experiments, and the cake was washed with distilled water and then 50ml of tap water containing 3 wt% CaCO,. All solutions were clear after filtration. 4.2. I, No additives As can be seen from Figs. 1 and 2, the experimental equivalence point of Ca removal lies around 2 mM of lime, as calculated. Only the practically soluble portion of CaC03 seems to remain in solution (< 10 ppm) at this point. Above 3 mM of lime, Ca is reintroduced to the solution by the excess addition of Ca(OH),. Si and Mg are removed significantly only at higher lime concentrations (>3 mM, pH> 10). The beginning of Mg removal is in accordance with the fact that a pH higher than 10 has to be achieved to remove Mg as Mg(OH),. Mg is necessary for the removal of silica, indicating the formation of magnesium silicate and/or coprecipitation/adsorption. Moreover, the profile of Si removal follows that of Mg removal. It is, known in fact, that MgO can effectively remove Si at pH2 10 by adsorption [4]. Table 4 shows that for slurry concentrations 0.5% to 5% any changes of ion concentrations with the concentration of slurry are within experimental error. Mere contact with CaCO, in slurry and cake, without the addition of lime, causes precipitation from the supersaturated solutions, change of pH, and possibly some coprecipitation. 4.2.2. Addition of an aluminium salt, with and without carbonate supplement In the simulation of the CAPS process, 10 or 30 ppm Al as AlCl, were added with or without a carbonate supplement (Fig. 1). The pH of the treated water remained appreciably lower with Al addition. Si was very efficiently removed by the addition of Al: up to 85% removal with about
80
A. Masanva
et al. / Desalination
1 I3 (199 7) 73-84
12 a) 11 0 I cl
0
0
0
10
??
@I 9
0
8
I’
?I
.
0 :
4
I 1
.
I 2
A
0
A A
0
0
yulvalenca A point
’
I 3
I 4
u
\4
0 5
0
I 2
1
. lime,
mM
80
A
a
I 3
.
I 4
60
A
equivalcncr?
A
pOl”t
0
0
,
0
1
5
d)
A
?? 0
20
A .
mM
d
A
0 :.
42-
equ1valcnce pol”t 6
lime,
440J
is
t
0
k
b)
Cl
A
0
E % _
A
??
5”
A lo-
0
I.,
,
2
3 lime,
zo-
,
4
0 ,
5
‘a
equivalence pJl”l
0 0
mM
I 1
*
4 I 2
.
lime,
0
I 3
*
I 4
’ 5
mM
Fig. 1. Lime softening of tap water on a CaCO, slurry: effect of aluminium addition on pH (a), Si (b), Ca (c) and Mg (d) for water containing 35 ppm Mg, 0; 10 ppm AI, 0; 10 ppm Al + 2 mM carbonate, 0; 30 ppm Al, A; 30 ppm Al + 2 mM carbonate, A.
Table 4 Influence of slurry concentration time)
on removal of Si, Ca and Mg (filtration over cake, 20 g/IO cm 0, after 3 min retention
[Lime], mM
[Slurry], %
PH
[Sil, rvm
PI, wm
Wk17wm
Comments
0
0
0
0
0.5
0
1.0 2.0 3.0 5.0 0.5 1.0 2.0 3.0 5.0
7.78 7.22 6.76 7.08 7.16 7.05 7.06 6.16 6.39 6.41 6.05 6.22
79.9 77.6 64.8 72.1 71.6 65.8 68.7 24.6 24.6 23.8 25.0 23.9
45.8 42.4 39.5 41.9 41.9 41.1 41.4 34.1 35.3 34.8 33.1 33.3
Blank Blank
0
7.85 7.85 8.06 8.07 8.07 8.08 8.06 10.45 10.46 10.39 10.50 10.44
0 0
0
2.8 2.8 2.8 2.8 2.8
A. Masarwa et al. /Desalination
I13 (1997) 73-84
81
b)
a)
lime,
lime, mM
mM
40 Ca in water
4 50
d)
A
40
30 -hfl: equivalence
0.
E ::
0
A
sE
8
zo-
0
0
0
4
1
A
0
r”
0
a
0
-r
0
T
2
1
lime,
3
4
5 lime, mM
mM
Fig. 2. Lime softening of tap water on a CaCO, slurry: effect of zinc addition on pH (a), Si (b), Ca (c) and Mg (d) for water containing Mg, Cl; 10 ppm Zn, 0; 10 ppm Zn + 2 mM carbonate, 0; 30 ppm Zn, A; 30 ppm Zn + 2 mM carbonate, A.
Table 5 Removal of metals during CAPS Lime, mM Blankb Blank over cakeb 0
0.5 1.0 1.9 2.8 4.5 Limits (drinking)
WI, wm
Md, mm
[Cal7mm
[Mnl,wm”
[F4 wma
7.90 8.10
7.31 6.87
42.8 41.5
72.7 65.5
0.60 0.11
1.06 0.00
8.10 8.19 8.30 8.70 9.53 10.77
7.18 7.25 7.10 6.50 5.30 3.90
43.3 42.5 41.1 39.0 350 12.1
66.8 58.8 47.3 30.4 31.0 61.7
0.13 0.11 0.07 0.03 0.01 0.00
0.00 0.00
0.05
0.3
PH
0.01 0.04 0.07 0.05
82
A. Masanva et al. /Desalination
3 mM lime and 30 ppm Al, starting at low lime concentrations. However, the removal of Si took place at the expense of the Ca softening. Approximately 90% of Ca removal was achieved without any additives, -70% with 10 ppm Al, and only -35% with 30 ppm Al. Ca removal was improved with added carbonate, while the Si reduction was decreased slightly. A good compromise for softening and concomitant Si removal, using only small amounts of chemicals, seemed to be the addition of about 10 ppm of Al to 2-3 mM of lime. Si was reduced by more than 50%, while the Ca hardness was reduced to 10 to 20 ppm. Samples were also analyzed for residual Al. Under most conditions, less than 0.2 ppm of Al generally remained after the lime softening (CAPS simulation) for Al concentrations of 1O-30ppm and lime concentrations of 0 to 5 mM. Maximum residues of 0.20.5 ppm Al were observed for about 3 mM lime. 4.2.3. Addition of zinc salt, with and without carbonate supplement When 10 or 30 ppm of Zn as ZnCl, were added, with or without carbonate, in simulations of the CAPS process, the following results were obtained (Fig. 2). The pH of the treated water remained lower the more Zn is added. Removal of Si during the lime process was significant at lime concentrations below the equivalence point for softening of Ca carbonate hardness and was higher with increasing Zn concentrations, while the Ca reduction decreased as the Zn content rose. When additional hydrogen carbonate (2 mM HCO;) is added to the water, Ca removal could be restored to a value similar to that in samples without Zn addition around the equivalence point, and was improved appreciably above the equivalence point. However, Si removal with added carbonate requires rather high Zn concentration. Depending on the amount of Si removal
113 (1997) 73-84
required and the cost of the additives the following options are available: (a) a small amount of Zn addition alone, e.g., 10 ppm Zn and 2-3 mM of lime reduce Si by 20-25% with only a small decrease in Ca removal; or (b) a higher amount of Zn in combination with NaHCO,, e.g., 30 ppm Zn, 2 mM NaHCO, and 2-3 mM lime reduce Si by approximately 30-35%, with satisfactory softening.
4.3. Precipitation by NaOH on a CaCO, slurry CAPS softening can be carried out with NaOH or lime, or if necessary with lime/soda ash. The choice of reagent(s) will be dictated by the composition of the water, requirements for the extent of softening, TDS, etc. It is therefore necessary to test whether the removal of silica during the softening is also feasible when NaOH is used as the base. Precipitation by NaOH on a slurry of 3% CaCO,, added as 1.5 g of solid/50ml water sample, was examined allowing 2min retention time. Silica removal was followed (a) with no additive, (b) with addition of 10 ppm Al, and (c) with addition of 10 ppm Zn. Fig. 3 sums up the NaOH slurry experiments with and without the various additives. Changes in pH, Ca, Mg and Si are illustrated. The relationship between Ca removal and pH is similar to that found in the companion study [I ] though the composition of the water and hence the results are not quite identical. By the use of excess NaOH, Ca could be removed very efficiently to undetectable limits. Mg was removed efficiently only by the addition of more than 5 mM NaOH in the presence or absence of Zn. Si was not removed significantly in the absence of additives. Upon addition of Al, 10 ppm, silica was reduced to about 50%. Excess base had to be used to neutralize the aluminium chloride, while the pH remained below 9. Zn was not effective as an additive.
A. Masarwa et al. / Desalination 1I3 (1997) 73-84 11
al
n
10
0
0
9
??
0 0
0
A
A
b)
A
0
Ti
83
A A
A
??
A
0 A
A
0 A
a
7
/ 0
2
r
I
,
4
6
1
0
a
2
NaOH, mM
4
6
a
NaOH, mM
40 d) 30 Fa
E B
_ 20
?? 0 A
E”
A 10
0
2
4
A
0 0 A
03 6
a
NaOH, mM
0
2
4
6
i
NaOH, mM
Fig. 3. Softening of tap water with NaOH on CaCO, slurry: effect on pH (a), Si (b), Ca (c) and Mg (d) for water containing 35 ppm Mg, 0; 10 ppm Al (+Mg), A; 10 ppm Zn (+Mg), 0. dv.02 4.4. Removal of heavy ions by lime sopening To test removal of iron and manganese during CAPS, divalent iron and manganese were added to tap water as FeSO,.(NH,)S0,.6H,O and MnC1,.4H20. Precipitation was carried out on a 2% suspension (3-4 min retention time) with various lime additions (OWlSmM). Table 5 shows the measured concentrations of Fe, Mn, Ca, Mg and Si as measured by ICP (soluble portion), before and after treatment. Iron and manganese are usually removed from contaminated water by aeration or chemical oxidation, resulting in formation of the relatively insoluble Fe(OH), and Mn02 and precipitation
(for iron at pH>6 and for Mn at pH>8.5), followed by filtration. Experimental difficulties are encountered with the filtration of manganese unless one uses filtration media coated with manganic oxides, which serve as a catalyst for its oxidation and removal. It is also known that limesoda softening will remove iron and manganese. Our results show that for water containing iron and manganese it is sufficient to simply filter the water over a CaCO, cake to remove iron completely and manganese by about 80%. The CAPS lime process thus facilitates removal of both iron and manganese to well below the allowed limits.
84
A. Masanva et al. /Desalination
5. Conclusions It is possible to remove a major portion of Si during lime softening by the addition of AICI, or ZnC12. Neither Zn nor Al residues above the allowed limits remain after treatment of water by the CAPS process. It is well known that the conventional filtration of the precipitates formed by aluminium salts and silica is very difficult. In contrast, the coprecipitation of silica with AlCI, or ZnCl, in the simulated CAPS process, precipitating on CaCO, slurry and filtering through a cake, does not measurably increase the filtration time. At the same weight concentrations, Al is superior to Zn for removal of Si. Silica removal was achieved at the expense of the Ca removal, but Ca removal could be improved or restored to its original level by added carbonate. Depending on the purpose of softening (e.g., pretreatment for RO, potable water, etc.) complete softening does not always have to be achieved (e.g., if there is no carbonate in water, there will be no CaC03 scaling). Where complete Ca softening is desired together with Si removal, precipitation with aluminium salt can be improved by the addition of soda ash or softening with NaOH (+A1 addition). Addition of about 10 ppm of Al (with 2-3 mM of lime) seems to be a good solution for softening of tap water. Si is removed by more than 50% while the Ca hardness is removed to cl0 to 20 ppm. When Zn or Al are added in the treatment process, Mg is removed at the same lime concentration, with a considerably lower PH. The lower pH with Al or Zn addition is an additional advantage. In the regular lime process, the treated water will usually have a pH of 2 10 and it will be necessary to lower the pH to 8.4- 8.6, usually by recarbonation. Water treated with Al therefore would not have to be acidified after the combined
I13 (1997) 73-84
softening&i removal process for use as softened water. Heavy metals can be coprecipitated and reduced below measurable limits by either the CAPS lime process alone or in combination with coagulation by A13+or Fe3+ salts. Acknowledgment We are indebted to Mr. Barrie Rose of Canada and to Mr. Reuben Kunin of Canada for their generous support of this study.
References PI 0. Kedem, and G. Zalmon. Compact accelerated precipitation (CAPS) as a pretreatment for membrane desalination. 1. Softening by NaOH. Desalination, 113 (1997) 65. VI G.R. Bell et al., Characterization and removal of silica from Webster, South Dakota and Roswell, New Mexico Well Waters (Part 1); US Department of the Interior, Research and Development Progress Report No. 286, 1968. Review, US Department of [31 Silicate Reactions-A the Interior, Research and Development Progress Report No. 307, 1969. 141 H. Harder, Geochimica Cosmochim Acta, 29 (1965) 429. [51 G. Okamoto, T. Okura and K. Goto, Geochim. Cosmochimica Acta, 12 ( 1957) 123. [61 F.H. Butt, F. Rahman and U. Baduruthamal, Desalination, 101 (1995) 219. [71 A.E. Jaffer, Desalination, 96 (1994) 71. PI M. Oinuma, S. Sawada and K. Yabe, Desalination, 98 (1994) 59. [91 E.M. Lohman, Desalination, 96 (1994) 349. UOI D.L. Kronmiller, Desalination, 98 (1994) 401. Pll F.D. Snell, CT. Snell and C.A. Snell, in: Calorimetric Methods of Analysis, Vol. 2A, D. van Nostrand Company, Princeton, NJ, 1972, p. 498. 1121 J.W. Clark, W. Viessman, Jr. and M.J. Hammer, Water Supply and Pollution Control, International Textbook Company, Scranton, Toronto, London, 1971.
Desalination 113 (1997) 73-84
Compact accelerated precipitation softening (CAPS) as pretreatment for membrane desalination II. Lime softening with concomitant removal of silica and heavy metals Alexandra Masarwaa*, Dan MeyersteinaYb,Naphthali Daltrophe”, Ora KedemC “Chemistry Department, Ben-Gurion University of the Negev, Beer-Sheva, Israel Far +972 (7) 647-2943 bThe College of Judea and Samaria, Ariel, Israel ‘The Institutes for Applied Research, Ben-Gurion Universi@ of the Negev, Beer-Sheva, Israel
Received 2 April 1997; accepted 3 1 July 1997
Abstract In a previous paper the CAPS softening process is suggested as a pretreatment step for RO: in addition to removal of calcium it is an effective filtration and may thus replace part of the customary pretreatment. In the present study the removal of silica and heavy metals in addition to water softening was investigated. Precipitation with both sodium hydroxide and with lime was studied. With lime precipitation in CAPS, it was found that magnesium is necessary for silica removal, if no other additives such as Al and Zn are used. With conventional methods of precipitation, the well-known reagents for silica removal, aluminium salts, lead to very slow precipitation or difficult filtration. In contrast, aluminium chloride used as an additive in CAPS can remove a major part of the silica without interfering with filtration, if the precipitation conditions are chosen correctly; optimal conditions were in keeping with the regime of softening plants. It was found that zinc chloride, which is environmentally more acceptable than the alurninium salt, could also be used for silica removal, but in larger amounts than aluminium chloride. With these two salts, a substantial fraction of the silica can be removed, but the precipitation of silica results in decreased calcium removal. To achieve simultaneous softening and silica removal, it is necessary to add carbonate. Heavy metals are coprecipitated with the calcium carbonate. Keywords: Water softening; Silica; Heavy metals; Clogging
*Corresponding author.
001 l-9164/97/$09.50 Q PZZ 001 l-9164(97)001
1997 Elsevier Science B.V. All rights reserved 16-1
74
A. Masama et al. /Desalination I13 (1997) 73-84
1. Introduction In the preceding paper [l] a compact lime softening process (CAPS) was described. It was suggested that CAPS be used as a pretreatment to membrane desalination, which would decrease both calcium sulfate scaling and fouling. The aim of this pretreatment is an increased maximal recovery ratio. In addition to hardness, scaling may also be caused by silica and magnesium hydroxide. While the precipitation of Ca and Mg salts has been studied intensively, the formation of silica precipitates [2-51 is less well understood. Similarly, reverse osmosis (RO) technologies concentrate on the prevention of Ca and Mg scales, while the formation of Si scales was investigated in a smaller number of studies [6-91. The importance of silica scale formation and its prevention in RO systems have recently been described in detail [lo]. In the present study the effects of a number of additives that are expected to accomplish Si removal in addition to a reduction in Ca and Mg concentrations during lime softening were investigated. The effects of Mg2’, A13+,and Zn2+ salts on the removal of silica from synthetic tap water were studied. Since these substances are also potential coagulants, their addition might facilitate a reduction of turbidity through flocculation. Integration of these ideas into the CAPS process was evaluated.
2. Experimental 2. I. Preparation
of synthetic tap water
It should be noted that although the pH of the water was initially adjusted to a value that corresponds to a negative or only slightly positive saturation index (SI) [l], the concentration of Ca was reduced over a period of several weeks. The precipitation of CaCO, should per se reduce pH, but in fact a rise in pH was observed. This change in pH was apparently due to a loss of CO,, which
caused the precipitation. The concentration of CO, in the water was larger than its solubility at about pH 7.0 under atmospheric conditions. In light of these considerations, the water was used as fast as possible after its preparation, since it was difficult to obtain water of a stable composition. During the course of the experimental work, it became obvious that removal of Si and Mg by lime softening hardly depended on the Ca or carbonate concentration of the water, whereas the efficiency of Ca removal was strongly influenced by the carbonate content of the water. Two batches of tap water (Table 1) that differed only slightly from one another were made up in close agreement with a representative analysis of Beer-Sheva (Israel) tap water. These samples, which were used for our experiments, were made up as follows: Batch a: This water was used in the NaOH softening experiments. The synthetic water was made by adding salts to deionized water that already contained a considerable amount of SiO, (about 5ppm). After addition of 10 ppm Si, the total silica concentration was thus rather high. Although no scaling was visible, the Ca content of this batch decreased appreciably over a period of one month (pH 7.4). Batch b: Lime softening experiments were performed with this type of water. This batch was basically the same water as batch a, but it differed in that it was prepared with distilled and deionized water and the pH was adjusted to 7.6. The water was used within a week for the softening experiments so that carbonate and calcium reduction were kept to a minimum. ??
??
2.2. Analytical procedures The standard solutions were prepared as follows: Si: Titrisol (Merck) (SiCl, in 14% NaOH) 1000 ppm Si (= 2 140 ppm SiO,)
??
A. Masarwa et al. /Desalination
113 (I 997) 73-84
75
Table 1 Composition of synthetic tap water, representative of Beer-Sheva, Israel, tap water MW
Salt
MgSO, (70.0%) b CaCI,*2H,O KHCO, NaHCO, NaCl HCOj clso;Si (1000 ppm)
120.37 147.01 100.11 84.01 58.44
g/101”
2.32 2.42 0.05 3.36 1.75
mM’
1.35 1.65 0.05 4.0 3.0 4.05 6.3 1.35 0.36
mg/l as CaCO,
135 165 200 150 315 135
Ion
ppm, mg/l Batch a
Batch b
30 38”-66 2 92 69 247 223 130 15
34 57
8
Mg*+ Ca2+ K+ Na’. Na+ HCO; Cl- (+HCI) so;Si
aAmount used for preparation of the water. bPercentage according to analysis of 50 ppm standards made up of these salts. ‘After about 1 month.
9 Mg: MgS04 dried (Frutarom) (ca. 70%), 700 ppm (2.477 g/500 ml); Ca: CaC12.2 H20 (Merck) (91.3%), 913 ppm (1.834 g/500 ml); Lime solution: a suspension of 50 mM lime (Ksp =6.5 Y 1O-6 - 18.7 mM soluble).
??
??
??
??
2.3. Analytical
methods
All spectrophotometric determinations were performed employing a Hewlett Packard 8452A diode array spectrophotometer. Si was analyzed using the heteropolyblue spectrophotometric determination with Spectroquant8 (Merck). The proposed procedure recommends a 5-min waiting period for the reduction of molybdosilicic acid (step 3), but kinetic experiments showed that a longer period was necessary for the blue color to develop and reach its maximum absorption, so the procedure was slightly modified to allow for a minimum 15-min waiting period. Ca and Mg were determined using a modified spectrophotometric determination with Eriochrome Blue SE.1 1 The following conditions were used:
Determination of Ca at pH 14, at 640 nm, using a calibration curve of 2-50 ppm Ca (quadratic with offset): 1.5 ml of sample (or say 0.5 ml of sample + 1.0 ml of water), 1 .O ml of 0.13% dye solution, and 2.5 ml of 8% NaOH. Determination of Mg (+Ca) at pH 9 at 548 nm: 1.5 ml of sample (or say 0.5 ml sample + 1.0 ml water), 2.0 ml water, 0.5 ml dye, 1.O ml 5x concentrated pH 9 buffer solution [i.e., 5x (0.05 M H,BO,, 0.05 M KCl, 0.022 M NaOH)].
The buffer system of pH 9 was used because the water samples contain a carbonate buffer system and the absorbance of the dye is very pH dependent. A pH of about 13 (2.5 ml of 0.8% NaOW5 ml) recommended in the literature [ 141 was found barely suitable for the analysis of mixtures containing more Ca than Mg (only 0.4 mM, about 15 ppm of Ca and Mg combined can be determined under those conditions). In our modification the concentration of the dye is reduced. At the reduced dye concentration and at pH 9, the formation of the complex (Mg-dye) or (Ca-dye) is observed at 520-550 nm
76
A. Masarwa et al. /Desalination
and the decrease of the free dye concentration above 600 nm. Apparently, the Mg-dye complex has the higher formation constant at this pH, so that in a mixture of Ca and Mg the contribution of the Ca-complex absorbance is only additive for very small Mg concentrations (l-3 ppm Mg). The effect of Ca can be ignored for Mg concentrations >5 ppm. The following corrections can be applied at 548 nm (A=absorption units): l-2 ppm Mg, 8-33 ppm Ca: 0.011 Alppm Ca: 3-5 ppm Mg: 0.0035 Alppm Ca; 5-8 ppm Mg: 0.0012 Alppm Ca. Zn does not affect the Ca analysis, but the Mg analysis is not accurate when more than about 3 ppm of Zn remain in the samples, which apparently occurs only in samples filtered in the absence of a CaCO, cake at the lower lime concentrations. The influence of Zn manifests itself in an additional absorbance at the otherwise isosbestic point of the Mg-dye determination (578 nm), so that the amount of Zn can be determined at 578 nm. However, the effect of the Zn-dye complex at 548 nm (max. of the Mg/dye complex) is not simply additive, so that a deconvolution is difficult and inaccurate. Other cations were measured by ICP/OES on an Optima 3000 Perkin Elmer instrument.
I1 3 (1997) 73-84
pJ;n=A [Jrl(l+fl)]
- 0.3 I (for 1~0.5)
where I is the ionic strength. At 20°C pK, =8.45; 0.506
pK2 = 10.38; pK,-pK,
= 1.93; A=
Synth. tap water: 1=0.0175 Therefore, 5pjin = 0.27; and pHs= 1.93 + 2.78 + 2.39 + 0.27 = 7.37
The calculations given above show that tap water at pH >7.4 is oversaturated (positive SI) and thus has a tendency to precipitate CaCO,. Below these pH limits (negative SI), the water is corrosive. By preparing synthetic water and adjusting its pH we had to find a careful balance between a pH at which CO, evolution could be kept to a minimum and at which the water was only slightly oversaturated. 3.2. Hardness/lime dosage calculations [12]
3. Hardness and Saturation Index The following SI and hardness calculations are applicable to the synthetic water studied. 3.1. Saturation Index [I 21 SI=pH-
pH,
pHs =pK, - pK, +p[Ca2’] +p[HCO;] + 5pfi where pHs is the equilibrium p, K2 the second dissociation constant of carbonic acid, KS the solubility constant of calcium carbonate, andfi the activity coefficients. Under the conditions studied here:
The amount of lime necessary to achieve lime softening alone is presented in Table 2. Carbon dioxide and carbonate hardness (calcium and magnesium bicarbonate) are precipitated by lime. To remove Mg and Ca non-carbonate hardness (calcium and magnesium sulfates, chlorides or silicates), as well addition of soda ash is necessary. To remove Mg as Mg(OH), the pH of the solution has to be raised to >10.5, thus an excess of lime is required. However, it is known that about 10% of Mg is carried down as Mg(OH), during CaCO, precipitation. Thus, lime softening without the addition of soda ash removes only the carbonate hardness of the water. An equivalent point for the removal of
A. Masanva et al. /Desalination Table 2 Hardness and lime dosage necessary to achieve softening Hardness, mg.0 as CaCO,
Total hardness CaCO, hardness aCa non-carbonate hardness MgCO, hardness aMg non-carbonate hardness H2C03
HCOj
Lime dosage
mg/las CaCO,
mM
300 165 0
165 0
1.65 0
23.5 111.5
47 111.5
0.47 1.11
30
0.3
30 188
“Requires the addition of soda ash (1 equiv.) to affect a net change in hardness level. the Ca carbonate hardness alone (no Mg removal)
can be calculated, adding the amounts of lime required to neutralize H,CO, and to remove the Ca carbonate hardness. Thus, about 2 mM of lime are needed for reduction of Ca in the tap water to the practical limit, i.e., about 30-50 mg/l as CaCO, = 12-20 ppm Ca.
4. Results and discussion It is well known that silica can be removed from water by suitable additives, mainly aluminium salts [2-51. The precipitates formed are flocky, difficult to filter, and slow to settle. Preliminary experiments confirmed the removal of silica in our procedures and showed that efficacy of removal depends on the conditions of precipitation. We tested zinc salts as an alternative to aluminium salts for the precipitation of silica, concomitant with lime softening. Zinc should behave similarly to aluminium (amphoteric hydroxide), and its addition to water may be less restricted, being environmentally less The following drinking water dangerous. standards apply:
I I3 (I 997) 73-84
us:
preferable limit, 5 ppm desirable limit, virtually absent
Israel:
recommended max. level, 5 ppm maximum allowance, 15 ppm
77
Four types of precipitation/filtration were tried: Lime solutions and additives were added to the water, and the resulting suspensions were allowed to stand for different times with intermittent shaking and then filtered either through filter paper (“regular”) or through a CaCO, cake (types 1 and 2). Alternatively, a fine precipitate of CaCO, was added to the water. Lime and additives were added to the resulting slurry, and this was filtered after a short retention time either through filter paper or through a cake (types 3 and 4). In all cases precipitated CaCO, (Riedel de Haen or Merck) was used. Best results were obtained for precipitation on slurry followed by cake filtration (type 4). It should be noted that these conditions simulate the CAPS process carried out in plants, which is described in detail for a single filter tube in our previous paper [I]. During the course of the experiments, it was noticed that the Si content of the solutions that had been filtered through a freshly washed cake decreased dramatically, sometimes accompanied by an increase in pH. Therefore, to obtain reproducible results which can be translated into an industrial process, the cake has to be washed with at least a 50-ml portion of Si-containing water before each series of experiments. [The results indicated that a 30 g cake is able to “absorb” only a small amount of Si (about 3-5 ppm of SV50 ml sample, i.e., about 0.1 mg of Si)]. One of the major advantages of CAPS over conventional lime softening is the filterability of the CaCO, precipitate. Since it seemed likely that the customary additives used for removal of silica during CAPS might spoil this essential feature we set out to study the influence on filterability of the different conditions of precipitation.
78
A. Masanva et al. /Desalination
4.1. Filtration rates during synthetic tap water
lime softening
of
I13 (1997) 73-84
For regular filtration the increase in filtration time, dt, was approximately 600s for Al and 230s for Zn, as compared with 53 s without additives (Table 3). The effect of the additives on the filtering through the cake is much less severe. Retention of the reaction mixture for 3 min or more before filtration improves filterability for both regular and cake filtration, but the reduction of dt is more pronounced for the cake. Table 3 shows that dt caused by the addition of aluminium chloride is drastically reduced in cake filtration following retention of 3 min. The addition of calcium carbonate particles to the hard water before softening changed the picture completely. When the slurry was retained for 3 min, following the addition of lime and zincor aluminium chloride, the filtration time observed for each of the consecutive samples was 25*5 s. This is similar to filtration of water through the cake. In other words, clogging is eliminated by precipitation on a carbonate slurry and retention for a few minutes. The regime followed here simulates the continuous CAPS process: precipitation on slurry, retention for a few minutes, and subsequent cake filtration.
Upon addition of the lime suspension to the hard water, with or without additives, a precipitate starts to form immediately. Table 3 shows examples of filtration times for these precipitates under different conditions. A series of samples with O&4.5mM of lime (six samples per series) were filtered successively in order of concentration. The most significant quantity for comparison of the experiments is the difference, dt, between the time of filtration of the first and the last samples of each series, since this parameter best indicates the extent to which the formed precipitate clogs the filter. It is obvious that filtration of the precipitate formed causes clogging of the filter (medium paper or the cake), since the filtration time increases and exceeds the time of filtering through a 20g layer of CaC03, although only small amounts of precipitate are added to the cake. The effect is quite pronounced even with only small quantities: a series of six samples is only 300 ml of water. The addition of Zn or Al increases the filtration time drastically when regular filtration is used, the effect being more pronounced for Al.
Table 3 Comparison of filtration times (s) for 50 ml solution samples with different additives (Add.) and retention times (t,,,) mM Lime
Filtration time, s Add. none t ret<*o s
Add. 27 ppm Zn trct
Add. 27 ppm Zn t Ter.5-l h
Add. 30 ppm Zn t ret 3 min
Add. none tret 3 min
Reg.
Cakea
Reg.
Cake a
Reg.
Cake a
Reg.
Cake
Reg.
Cake
Reg.
Cake
0 0.5 1.0 1.9 2.9 4.5
10 * * * * 120
24 * * * * 45
40 48 60 70 80 110
<:lO 10 33 70 105 140
80 80 85 100 90 100
22 22 25 27 26 28
~10 12 25 38 45 63
18 23 23 22 23 22
< 10 21 140 350 510 600
21 27 30 35 38 40
dt
110
21
290
70
130
10-20
230
6
53
Insig.
600
20
-.
Add. 20 ppm Al tret 3 min
Reg., one filter paper used for series 04.5 mM lime. Cake, new or reconditioned (washed and top layer removed) CaCO, cake (20 g/IO cm 0) used for each series, except: a samples from three series all filtered consecutively through the same cake. *. time not measured.
A. Masanva et al. /Desalination 113 (1997) 73-84 Precipitation on a slurry of CaCO, not only improved filtering properties but also appreciably enhanced the ability to remove Ca, Mg and Zn. Moreover, additional filtration through the cake further improved this removal of cations. No Zn residue was measurable (I 1 ppm) when precipitation took place on the slurry and/or by filtration through a cake, whereas in “regular” filtered samples up to 20ppm of Zn (for 30ppm Zn addition) remained in the treated water samples for low lime concentrations. A 0.5% slurry seems to be as effective as a more concentrated slurry with regard to Si, Ca and Mg removal and reduction of filtration time. No measurable difference could be detected on softening tap water with 3 mM lime, when slurries of 0.5, 1.O, 2.0, 3.0 or 5.0% were used. Since it was advantageous to work with a slurry, we only report here the results of the slurry experiments in detail. Generally, a 3 wt% slurry was used, and a maximum of 3 min retention time was allowed. It was realized during the course of this work that Ca softening by the lime process was reduced appreciably when the water did not contain enough carbonate. Consequently, most experiments were conducted with freshly prepared water to avoid Ca and carbonate reduction. Since it was thought that carbonate reduction was the likely cause of the impairment of Ca softening when Si was removed by Zn or Al, the addition of NaHCO, (instead of say soda ash) was also investigated for these samples. 4.2. Softening by precipitation and cake$ltration
on CaCO, slurry
The following series of experiments which constitute a representative example of the effect of lime softening in a simulation of the CAPS process are described in greater detail. The results are summarized in Figs. l-3 and Tables 4 and 5. Precipitation was carried out on a slurry of 3% CaCO,, added as 10.5 g of solid in a 350ml water sample. The additives Zn and Al, and finally lime
79
as 50 mM suspension, were added to 50ml samples which were filtered after a retention time of 3 min through a 20g cake. If an old cake was used, the top layer was removed before each series of experiments, and the cake was washed with distilled water and then 50ml of tap water containing 3 wt% CaCO,. All solutions were clear after filtration. 4.2. I, No additives As can be seen from Figs. 1 and 2, the experimental equivalence point of Ca removal lies around 2 mM of lime, as calculated. Only the practically soluble portion of CaC03 seems to remain in solution (< 10 ppm) at this point. Above 3 mM of lime, Ca is reintroduced to the solution by the excess addition of Ca(OH),. Si and Mg are removed significantly only at higher lime concentrations (>3 mM, pH> 10). The beginning of Mg removal is in accordance with the fact that a pH higher than 10 has to be achieved to remove Mg as Mg(OH),. Mg is necessary for the removal of silica, indicating the formation of magnesium silicate and/or coprecipitation/adsorption. Moreover, the profile of Si removal follows that of Mg removal. It is, known in fact, that MgO can effectively remove Si at pH2 10 by adsorption [4]. Table 4 shows that for slurry concentrations 0.5% to 5% any changes of ion concentrations with the concentration of slurry are within experimental error. Mere contact with CaCO, in slurry and cake, without the addition of lime, causes precipitation from the supersaturated solutions, change of pH, and possibly some coprecipitation. 4.2.2. Addition of an aluminium salt, with and without carbonate supplement In the simulation of the CAPS process, 10 or 30 ppm Al as AlCl, were added with or without a carbonate supplement (Fig. 1). The pH of the treated water remained appreciably lower with Al addition. Si was very efficiently removed by the addition of Al: up to 85% removal with about
80
A. Masanva
et al. / Desalination
1 I3 (199 7) 73-84
12 a) 11 0 I cl
0
0
0
10
??
@I 9
0
8
I’
?I
.
0 :
4
I 1
.
I 2
A
0
A A
0
0
yulvalenca A point
’
I 3
I 4
u
\4
0 5
0
I 2
1
. lime,
mM
80
A
a
I 3
.
I 4
60
A
equivalcncr?
A
pOl”t
0
0
,
0
1
5
d)
A
?? 0
20
A .
mM
d
A
0 :.
42-
equ1valcnce pol”t 6
lime,
440J
is
t
0
k
b)
Cl
A
0
E % _
A
??
5”
A lo-
0
I.,
,
2
3 lime,
zo-
,
4
0 ,
5
‘a
equivalence pJl”l
0 0
mM
I 1
*
4 I 2
.
lime,
0
I 3
*
I 4
’ 5
mM
Fig. 1. Lime softening of tap water on a CaCO, slurry: effect of aluminium addition on pH (a), Si (b), Ca (c) and Mg (d) for water containing 35 ppm Mg, 0; 10 ppm AI, 0; 10 ppm Al + 2 mM carbonate, 0; 30 ppm Al, A; 30 ppm Al + 2 mM carbonate, A.
Table 4 Influence of slurry concentration time)
on removal of Si, Ca and Mg (filtration over cake, 20 g/IO cm 0, after 3 min retention
[Lime], mM
[Slurry], %
PH
[Sil, rvm
PI, wm
Wk17wm
Comments
0
0
0
0
0.5
0
1.0 2.0 3.0 5.0 0.5 1.0 2.0 3.0 5.0
7.78 7.22 6.76 7.08 7.16 7.05 7.06 6.16 6.39 6.41 6.05 6.22
79.9 77.6 64.8 72.1 71.6 65.8 68.7 24.6 24.6 23.8 25.0 23.9
45.8 42.4 39.5 41.9 41.9 41.1 41.4 34.1 35.3 34.8 33.1 33.3
Blank Blank
0
7.85 7.85 8.06 8.07 8.07 8.08 8.06 10.45 10.46 10.39 10.50 10.44
0 0
0
2.8 2.8 2.8 2.8 2.8
A. Masarwa et al. /Desalination
I13 (1997) 73-84
81
b)
a)
lime,
lime, mM
mM
40 Ca in water
4 50
d)
A
40
30 -hfl: equivalence
0.
E ::
0
A
sE
8
zo-
0
0
0
4
1
A
0
r”
0
a
0
-r
0
T
2
1
lime,
3
4
5 lime, mM
mM
Fig. 2. Lime softening of tap water on a CaCO, slurry: effect of zinc addition on pH (a), Si (b), Ca (c) and Mg (d) for water containing Mg, Cl; 10 ppm Zn, 0; 10 ppm Zn + 2 mM carbonate, 0; 30 ppm Zn, A; 30 ppm Zn + 2 mM carbonate, A.
Table 5 Removal of metals during CAPS Lime, mM Blankb Blank over cakeb 0
0.5 1.0 1.9 2.8 4.5 Limits (drinking)
WI, wm
Md, mm
[Cal7mm
[Mnl,wm”
[F4 wma
7.90 8.10
7.31 6.87
42.8 41.5
72.7 65.5
0.60 0.11
1.06 0.00
8.10 8.19 8.30 8.70 9.53 10.77
7.18 7.25 7.10 6.50 5.30 3.90
43.3 42.5 41.1 39.0 350 12.1
66.8 58.8 47.3 30.4 31.0 61.7
0.13 0.11 0.07 0.03 0.01 0.00
0.00 0.00
0.05
0.3
PH
0.01 0.04 0.07 0.05
82
A. Masanva et al. /Desalination
3 mM lime and 30 ppm Al, starting at low lime concentrations. However, the removal of Si took place at the expense of the Ca softening. Approximately 90% of Ca removal was achieved without any additives, -70% with 10 ppm Al, and only -35% with 30 ppm Al. Ca removal was improved with added carbonate, while the Si reduction was decreased slightly. A good compromise for softening and concomitant Si removal, using only small amounts of chemicals, seemed to be the addition of about 10 ppm of Al to 2-3 mM of lime. Si was reduced by more than 50%, while the Ca hardness was reduced to 10 to 20 ppm. Samples were also analyzed for residual Al. Under most conditions, less than 0.2 ppm of Al generally remained after the lime softening (CAPS simulation) for Al concentrations of 1O-30ppm and lime concentrations of 0 to 5 mM. Maximum residues of 0.20.5 ppm Al were observed for about 3 mM lime. 4.2.3. Addition of zinc salt, with and without carbonate supplement When 10 or 30 ppm of Zn as ZnCl, were added, with or without carbonate, in simulations of the CAPS process, the following results were obtained (Fig. 2). The pH of the treated water remained lower the more Zn is added. Removal of Si during the lime process was significant at lime concentrations below the equivalence point for softening of Ca carbonate hardness and was higher with increasing Zn concentrations, while the Ca reduction decreased as the Zn content rose. When additional hydrogen carbonate (2 mM HCO;) is added to the water, Ca removal could be restored to a value similar to that in samples without Zn addition around the equivalence point, and was improved appreciably above the equivalence point. However, Si removal with added carbonate requires rather high Zn concentration. Depending on the amount of Si removal
113 (1997) 73-84
required and the cost of the additives the following options are available: (a) a small amount of Zn addition alone, e.g., 10 ppm Zn and 2-3 mM of lime reduce Si by 20-25% with only a small decrease in Ca removal; or (b) a higher amount of Zn in combination with NaHCO,, e.g., 30 ppm Zn, 2 mM NaHCO, and 2-3 mM lime reduce Si by approximately 30-35%, with satisfactory softening.
4.3. Precipitation by NaOH on a CaCO, slurry CAPS softening can be carried out with NaOH or lime, or if necessary with lime/soda ash. The choice of reagent(s) will be dictated by the composition of the water, requirements for the extent of softening, TDS, etc. It is therefore necessary to test whether the removal of silica during the softening is also feasible when NaOH is used as the base. Precipitation by NaOH on a slurry of 3% CaCO,, added as 1.5 g of solid/50ml water sample, was examined allowing 2min retention time. Silica removal was followed (a) with no additive, (b) with addition of 10 ppm Al, and (c) with addition of 10 ppm Zn. Fig. 3 sums up the NaOH slurry experiments with and without the various additives. Changes in pH, Ca, Mg and Si are illustrated. The relationship between Ca removal and pH is similar to that found in the companion study [I ] though the composition of the water and hence the results are not quite identical. By the use of excess NaOH, Ca could be removed very efficiently to undetectable limits. Mg was removed efficiently only by the addition of more than 5 mM NaOH in the presence or absence of Zn. Si was not removed significantly in the absence of additives. Upon addition of Al, 10 ppm, silica was reduced to about 50%. Excess base had to be used to neutralize the aluminium chloride, while the pH remained below 9. Zn was not effective as an additive.
A. Masarwa et al. / Desalination 1I3 (1997) 73-84 11
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Fig. 3. Softening of tap water with NaOH on CaCO, slurry: effect on pH (a), Si (b), Ca (c) and Mg (d) for water containing 35 ppm Mg, 0; 10 ppm Al (+Mg), A; 10 ppm Zn (+Mg), 0. dv.02 4.4. Removal of heavy ions by lime sopening To test removal of iron and manganese during CAPS, divalent iron and manganese were added to tap water as FeSO,.(NH,)S0,.6H,O and MnC1,.4H20. Precipitation was carried out on a 2% suspension (3-4 min retention time) with various lime additions (OWlSmM). Table 5 shows the measured concentrations of Fe, Mn, Ca, Mg and Si as measured by ICP (soluble portion), before and after treatment. Iron and manganese are usually removed from contaminated water by aeration or chemical oxidation, resulting in formation of the relatively insoluble Fe(OH), and Mn02 and precipitation
(for iron at pH>6 and for Mn at pH>8.5), followed by filtration. Experimental difficulties are encountered with the filtration of manganese unless one uses filtration media coated with manganic oxides, which serve as a catalyst for its oxidation and removal. It is also known that limesoda softening will remove iron and manganese. Our results show that for water containing iron and manganese it is sufficient to simply filter the water over a CaCO, cake to remove iron completely and manganese by about 80%. The CAPS lime process thus facilitates removal of both iron and manganese to well below the allowed limits.
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5. Conclusions It is possible to remove a major portion of Si during lime softening by the addition of AICI, or ZnC12. Neither Zn nor Al residues above the allowed limits remain after treatment of water by the CAPS process. It is well known that the conventional filtration of the precipitates formed by aluminium salts and silica is very difficult. In contrast, the coprecipitation of silica with AlCI, or ZnCl, in the simulated CAPS process, precipitating on CaCO, slurry and filtering through a cake, does not measurably increase the filtration time. At the same weight concentrations, Al is superior to Zn for removal of Si. Silica removal was achieved at the expense of the Ca removal, but Ca removal could be improved or restored to its original level by added carbonate. Depending on the purpose of softening (e.g., pretreatment for RO, potable water, etc.) complete softening does not always have to be achieved (e.g., if there is no carbonate in water, there will be no CaC03 scaling). Where complete Ca softening is desired together with Si removal, precipitation with aluminium salt can be improved by the addition of soda ash or softening with NaOH (+A1 addition). Addition of about 10 ppm of Al (with 2-3 mM of lime) seems to be a good solution for softening of tap water. Si is removed by more than 50% while the Ca hardness is removed to cl0 to 20 ppm. When Zn or Al are added in the treatment process, Mg is removed at the same lime concentration, with a considerably lower PH. The lower pH with Al or Zn addition is an additional advantage. In the regular lime process, the treated water will usually have a pH of 2 10 and it will be necessary to lower the pH to 8.4- 8.6, usually by recarbonation. Water treated with Al therefore would not have to be acidified after the combined
I13 (1997) 73-84
softening&i removal process for use as softened water. Heavy metals can be coprecipitated and reduced below measurable limits by either the CAPS lime process alone or in combination with coagulation by A13+or Fe3+ salts. Acknowledgment We are indebted to Mr. Barrie Rose of Canada and to Mr. Reuben Kunin of Canada for their generous support of this study.
References PI 0. Kedem, and G. Zalmon. Compact accelerated precipitation (CAPS) as a pretreatment for membrane desalination. 1. Softening by NaOH. Desalination, 113 (1997) 65. VI G.R. Bell et al., Characterization and removal of silica from Webster, South Dakota and Roswell, New Mexico Well Waters (Part 1); US Department of the Interior, Research and Development Progress Report No. 286, 1968. Review, US Department of [31 Silicate Reactions-A the Interior, Research and Development Progress Report No. 307, 1969. 141 H. Harder, Geochimica Cosmochim Acta, 29 (1965) 429. [51 G. Okamoto, T. Okura and K. Goto, Geochim. Cosmochimica Acta, 12 ( 1957) 123. [61 F.H. Butt, F. Rahman and U. Baduruthamal, Desalination, 101 (1995) 219. [71 A.E. Jaffer, Desalination, 96 (1994) 71. PI M. Oinuma, S. Sawada and K. Yabe, Desalination, 98 (1994) 59. [91 E.M. Lohman, Desalination, 96 (1994) 349. UOI D.L. Kronmiller, Desalination, 98 (1994) 401. Pll F.D. Snell, CT. Snell and C.A. Snell, in: Calorimetric Methods of Analysis, Vol. 2A, D. van Nostrand Company, Princeton, NJ, 1972, p. 498. 1121 J.W. Clark, W. Viessman, Jr. and M.J. Hammer, Water Supply and Pollution Control, International Textbook Company, Scranton, Toronto, London, 1971.