SM
Conditioning Water Chemically
TECHNIFAX®
In treating water with chemicals, the water is also conditioned in other ways: bicarbonate alkalinity is removed, turbidity is reduced, and, under proper conditions, some oxygen, carbon dioxide, and silica is removed. The extent of this water conditioning depends on a number of factors: water temperature, type and amount of chemicals used, and equipment used in the softening process. Water softening reverses the conditions under which water dissolves hardness and holds it in solution. How hardness gets into water is, therefore, the key to how it can be removed.
HOW WATER DISSOLVES HARDNESS Natural waters pick up carbon dioxide from the air, from microorganisms, and from decaying vegetation. The carbon dioxide helps water dissolve calcium and magnesium out of alkaline “rock” by making the water slightly acid: H2O + CO2 ↔ water carbon dioxide
H2CO3 carbonic acid
(1)
In addition, carbon dioxide helps keep hardness in solution. It reacts with dissolved calcium and magnesium carbonates, making them more soluble. CaCO3 + CO2 + H2O → Ca(HCO3)2 calcium calcium carbonate bicarbonate
(2)
MgCO3 + CO2 + H2O → Mg(HCO3)2 magnesium magnesium carbonate bicarbonate
(3)
The bicarbonates formed are more than a hundred times more soluble than the carbonates.
SOFTENING PRINCIPLES For soluble calcium hardness to be removed, then, the following two reactions must take place:
2. Conversion of calcium bicarbonate to insoluble calcium carbonate Adding enough hydroxide to raise the pH to a certain level will force both of these conversions to take place. For magnesium hardness to be removed, free hydroxide must be present. This free hydroxide reacts with magnesium to form insoluble magnesium hydroxide. Due to economic considerations, the form of hydroxide most often used to raise the pH in softening is lime (Ca(OH)2). Figure 1 shows the relative concentration of the various carbonate species at different pH's. As seen in Figure 1, the reduction of carbon dioxide continues until the pH reaches 8.2–8.4, at which point the concentration of carbon dioxide drops to near zero. Above this pH, a portion of the bicarbonate ion converts to carbonate. Also, at pH's above 8.2–8.4, free hydroxide (OH–) begins to convert bicarbonate alkalinity to carbonate alkalinity. Significant amounts of free hydroxide do not appear until pH exceeds 9.8–10.0. Also above this pH, the relative concentration of bicarbonate (HCO3–) drops to low levels with carbonate (CO3–2) concentration predominating. As
1. Conversion of carbon dioxide to bicarbonate alkalinity
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TF-16
Calcium and magnesium salts are called water hardness because they react with soap and make water hard to wash with. Softening is the process of reducing or removing these hardness minerals. This can be done with softening chemicals (lime, soda ash, caustic, phosphate, etc.), which react with dissolved hardness, causing it to come out of solution. The precipitated hardness is then settled out or filtered from the water.
It is this carbonate that reacts with calcium to produce calcium carbonate precipitate. According to Figure 1, the conversion of bicarbonate to carbonate continues until pH 10.2 when all the bicarbonate is converted. As more carbonate is produced and the pH approaches 10.2, more calcium can precipitate. Above pH 10.2, free hydroxide persists, and can precipitate with magnesium. It is significant that precipitation occurs only when the required alkalinity species is present. Calcium does not precipitate as calcium carbonate below pH 8.2, and magnesium is not removed in appreciable amounts below pH 10.2. However, in practice, a small percentage of the magnesium will precipitate during any lime softening operation because of localized high pH in the vicinity of the lime feed point.
Figure 1 — Alkalinity species as a function of pH
pH increases in lime softening, the following interactions between alkalinity and hardness take place: 1. At typical raw water pH's of 5.5–8.0, carbon dioxide coexists with bicarbonate alkalinity. The hydroxide from lime reacts with carbon dioxide to form bicarbonate: CO2 + OH– hydroxide
↔
HCO3–
(4)
HCO3– + OH– → CO3–2 + H2O carbonate
(5)
Ca(OH)2 + MgSO4 → lime magnesium sulfate Mg(OH)2↓ + CaSO4 magnesium calcium hydroxide sulfate
(9)
Additional chemicals are needed to supplement lime in removing both carbonate and noncarbonate hardness.
Soda Ash Reactions Soda ash (sodium carbonate) converts hardness salts to the carbonate form, making them less soluble, as shown in the following reactions: CaSO4 → calcium sulfate CaCO3↓ + Na2SO4 calcium sodium carbonate sulfate
Na2CO3 + soda ash
Lime Reactions Lime reacts with CO2, calcium bicarbonate, and magnesium bicarbonate, as shown in the following reactions: Ca(OH)2 + 2CO2 ↔ Ca(HCO3)2
(6)
Ca(OH)2 + Ca(HCO3)2 ↔ 2CaCO3↓ + 2H2O
(7)
Mg(HCO3)2 + Ca(OH)2 ↔ Mg(OH)2↓ + CaCO3
(8)
bicarbonate
2. When the pH is raised to 8.2, all of the carbon has reacted to form bicarbonate alkalinity. Above pH 8.2, additional hydroxide reacts with bicarbonate to form carbonate:
Lime is not effective in precipitating hardness that is not associated with alkalinity (noncarbonate hardness). While it does precipitate magnesium sulfate, for example, the reaction adds calcium hardness to the solution:
Remember that for these reactions to take place, the pH requirements explained earlier must be met.
(10)
Soda ash is therefore used for precipitation and removal of noncarbonate hardness.
Lime-Soda Reactions When lime and soda ash are added to the water, they react to form caustic soda. Na2CO3 + Ca(OH)2 → soda ash lime 2NaOH + CaCO3↓ caustic soda
(11)
remove hardness precipitation, sodium aluminate, which is alkaline, releases caustic soda as it dissolves in water:
(Caustic, as such, is not usually added to the water alone because it is difficult to handle and usually too expensive for softening reactions.) The caustic produced is an effective softening chemical: 2NaOH + caustic
MgSO4 → magnesium sulfate Mg(OH)2↓ + Na2SO4 magnesium sodium hydroxide sulfate
Na2Al2O4 + 4H2O → sodium water aluminate 2Al(OH)3 + 2NaOH aluminum caustic hydroxide soda
(12)
In general, lime and soda ash produce hardness compounds in their least soluble form. While dissolved hardness can be reduced to low levels, it is not completely eliminated by this process. Good control of softening chemical dosages is needed for best results since excess lime can actually increase hardness by contributing calcium to the water.
Figure 2 — Hot process lime softening (based on OH excess of 10 ppm)
CALCULATING LIME-SODA DOSAGES Since lime reacts with carbon dioxide, calcium bicarbonate, and magnesium, lime dosages are based on the amounts of each of these. A slight excess is usually figured in the dosage to give a hydroxide residual. Lime dosages for hot process softening can be determined as shown in Figure 2. For cold process softening, the diagonal lines represent “P” alkalinity plus the magnesium to be removed. Soda ash reacts with calcium noncarbonate hardness (calcium hardness minus the alkalinity) as shown in Reaction 10. Soda ash requirements are therefore based simply on the amount of noncarbonate hardness in the water (Figure 3).
Figure 3 — Soda ash requirements for complete softening
Once initial dosages are established, some adjustment must be made routinely (based on control tests) to take care of variations in water composition.
SOFTENING AIDS After the softening chemicals have reacted, the precipitated hardness must be effectively removed. Sodium aluminate aids in the removal process, reacting with the precipitated hardness to form larger particles that can be removed more effectively. It is especially effective in magnesium removal. In addition to helping
(13)
As illustrated in Reaction 12, caustic is an effective softening chemical. Using sodium aluminate, then, will result in a reduction of lime and/or soda ash required for softening. Along with sodium aluminate, two other types of chemicals can be used to improve softening: organic coagulants and flocculants. Coagulants function to neutralize any surface charge or suspended particles that may be present in a softener. Flocculants, which consist of long molecular chains and branches, are then added to bridge these neutralized particles into larger floc that is more easily filtered. As a result, settling rates are improved, and filter run length is maximized. Unlike sodium aluminate, coagulants and flocculants do not release caustic soda. Jar tests are the best way to choose the initial dosage and combination of coagulants, flocculants, and sodium aluminate.
HOT VERSUS COLD PROCESS SOFTENING Most hardness salts are less soluble in hot water than in cold. In addition, the softening reactions are speeded up as water is heated. Figure 4 shows the relative effectiveness of hot and cold softening processes and how sodium aluminate aids softening.
As the silica content of water gets lower, it becomes progressively harder to reduce. In cold process softening, silica is rarely reduced below 5 ppm. Hot process softening, however, can reduce silica to as low as 1 ppm when chemical conditions and control are good.
SOFTENING EQUIPMENT
Figure 4 — How sodium aluminate improves softening
Hot process softening also provides side benefits. Heating the water drives off the dissolved gases, reducing the oxygen and carbon dioxide in the water. This reduces lime requirements. Better silica removal is also accomplished in hot process softening.
SILICA REMOVAL Silica is removed in softening processes by being adsorbed on floc that is present in the water. In cold process softening, sodium aluminate is most effective in reducing silica. Sodium aluminate neutralizes the magnesium hydroxide precipitates, which in turn flocculate to form larger floc that more readily adsorbs or entraps silica particles in the water. For reduction of silica in hot process softeners, the addition of magnesium oxide is most often the best method. In colder water, adding magnesium oxide only increases total hardness without significantly reducing silica.
The cold softening process is either a batch or a continuous operation. Batch-type softening simply involves adding chemicals to water in a tank, allowing the sludge to settle, and drawing softened water off the top. Continuous softening of unheated water is done in equipment very similar to that used for clarification. Flow rates through the equipment are usually higher in softening than in clarifying, however, since the hardness precipitates are heavier and settle faster than natural turbidity. These are two basic designs of hot process, lime-soda softeners: the upflow (sludge blanket) type and the downflow (up-take funnel) type. These designs are illustrated in Figure 5. In both types, raw water enters the top and is sprayed with steam to heat it to within 5–10° of steam temperature. The hot water releases most of its “free” carbon dioxide, and dissolved oxygen is reduced to about 0.3–0.5 ppm. These gases are discharged through vents. Chemicals are injected into the softener above the water level to get good distribution. As the chemical reactions occur, the precipitates accumulate as sludge.
Figure 5 — Equipment used in hot process softening
In an upflow softener, the treated water goes down through a funnel, or “downcomer.” The discharge of the lower end of the funnel is in a blanket of sludge. The treated water must pass up through this sludge, which coagulates and filters the precipitated hardness. In the downflow softener, the water flows downward and is drawn off through an inverted funnel. This design is somewhat less sensitive to rapid changes in flow rates.
CONCLUSION The main advantage of lime-soda softening is that in reducing hardness, alkalinity, turbidity, and silica can also be reduced. In hot process softening, oxygen and carbon dioxide are also reduced. The main limitation of lime-soda softening is that while hardness is reduced it is not completely removed. For complete hardness removal, phosphate or ion exchange softening “polishes” the lime treated water.
