Deacidification

In water chemistry, deacidification describes the removal of aggressive carbon dioxide from water. The setting of the lime-carbonic acid balance is important for drinking and service water . Water with a higher carbon dioxide content than the equilibrium is corrosive for some materials. For example, unprotected systems made of iron are corroded. Natural waters are often not in the lime-carbonic acid balance. When mixing water, a mixed water problem often occurs due to aggressive carbon dioxide. Treatment of this aggressive water is necessary. Whether or how much aggressive carbon dioxide a water contains can be found in tables in specialist books or calculated using Tillmans' formula .

In general, the neutralization of free acids in solutions is a deacidification, but only the special case of removing aggressive carbon dioxide from water is dealt with here.

Note : Since the terms carbon dioxide (CO ₂ ) and carbonic acid mean largely the same in water chemistry, only the term carbonic acid is used in this article for CO ₂ .

Many different physical and chemical treatment processes have been developed to reduce the content of free carbon dioxide in water.

Physical procedures

calculation

With physical deacidification , also known as mechanical deacidification , desorption of the dissolved carbon dioxide is achieved through rain, aeration, atomization and trickling via cascade and corrugated web systems or packings or a combination of these devices. It is advantageous that this deacidification only removes the aggressive carbon dioxide. The hardness and salinity of the water does not change.

The desorption processes when reducing the carbon dioxide content are mathematically recorded as follows:

{\ displaystyle {\ C_ {t} -C_ {S}} = ({C_ {0} -C_ {s}}) \ cdot {10 ^ {-} {^ {k} {^ {\ frac {O} {V}} {^ {t}}}}}}

or

{\ displaystyle \ log {\ frac {\ (C_ {t} -C_ {S})} {(C_ {0} -C_ {S})}} = - k {\ frac {O} {V}} t }

In addition, a general diffusion constant K _L a is used in order to avoid problems with phase interfaces. Since the initial content of free carbon dioxide is often very high in relation to the equilibrium content, this is defined as follows:

{\ displaystyle \ log {\ frac {\ (C_ {t} -C_ {S})} {(C_ {0} -C_ {S})}} = K_ {L} a \ cdot t}

or in a simplified form

{\ displaystyle \ log {\ frac {\ (C_ {0})} {(C_ {t})}} = K_ {L} a \ cdot t}

The elements of the formulas mean:

C _S = theoretical carbon dioxide content in the water in equilibrium with the carbon dioxide content in the air in mg / l
C ₀ = carbonic acid content of the starting water in mg / l
C _t = carbonic acid content of the water at the time (t) in mg / l
t = ventilation time in (h)
S / V = ratio of surface area of the water surface to the water volume
k = constant in (cm / h)
K _L a general diffusion constant (h ⁻¹ )

For the dimensioning of the degassing system, the greatest possible K _L a value is aimed for, as the system then becomes correspondingly smaller.

Devices for distributing and reducing the carbonation of water

The distribution and degassing systems listed below each have different advantages and disadvantages. In some cases they differ considerably in terms of space, external energy and investment costs. There are also clear differences in the degree of efficiency that can be achieved. When choosing a system, the desired final values for the treated water, the necessary energy requirements and the system costs are decisive. Depending on whether the deacidified water is used as drinking or process water, there are different requirements for the system to be selected. For example, the goal for drinking and many process water is to produce water in a lime-carbonic acid balance .
If partial or full desalination is then carried out, the lime-carbonic acid balance is irrelevant, as the aim here is to keep the residual carbonic acid content as low as possible.

Below is a table with the more frequently used technical devices for reducing CO ₂ . The values given for K _L a / Wh · m³ roughly indicate the degassing effect, taking into account the energy requirements of the various devices. The higher this value, the better the degassing effect with low energy consumption:

contraption	General Information	K _L a / Wh · m³
Irrigation / rain	Simple degassing effect low energy consumption low desorption for CO ₂	- ~ 0.3
Perforated bottom distribution	easy water distribution mainly used for water distribution inexpensive	- ~ 0.3
Air injection	simple degassing device only used for low CO ₂ degradation inexpensive, but with energy requirements for the air supply	- ~ 0.6
Baffle plate	good water distribution mainly used as a water distributor for downstream systems Observe energy requirements for water distribution	- ~ 2.0
Atomization	Formation of fine droplets, thus large surface good mass transfer water / air often used as a water distributor for downstream systems Energy requirements for water distribution	- ~ 2.0
Cascade distributor	better water distribution than perforated bottom more expensive construction	- ~ 1.5–5 depending on the construction
Corrugated column	large surface with good air contact Degassing effect selectable via column height the reverse chimney effect ensures good air supply without energy consumption	- ~ 6-8 depending on the height of the column
Random packing	large surface Degassing effect selectable via the dumping height Energy requirement for air supply	- ~ 3–6 depending on the height and type of bed

In addition to the type of device , other parameters such as temperature and pH value of the water are also important for reducing the carbonic acid content. Warmer water can be treated with significantly less effort than cold water. A lower pH value has the same effect.

Irrigation, rain or air injection are used when only a slight reduction in the carbon dioxide content is required. In addition, however, the content of dissolved oxygen is also increased. This technique is therefore often used for the treatment of drinking water from well water. Drinking water should both contain sufficient oxygen and be largely free of aggressive carbon dioxide . Only such water forms a lime-rust protective layer on the surfaces of ferrous materials .

Particularly good reductions in the carbon dioxide content can be achieved with combinations such as atomization and packing or perforated floor and corrugated sheet distribution. The latter combination, which was developed in the early 1980s, is preferred for treating large amounts of water. Energy requirements and system costs are low and advantageous with this combination.

In addition to the systems listed above, there are other special devices. For the ventilation of inland lakes, for example, a combination of injector with feed pump and riser pipe was developed in the mid-1980s. However, this was less aimed at reducing the carbon dioxide than at introducing oxygen. However, this system can also be used in large tanks and tanks with the aim of reducing the carbonic acid content.

Chemical process

The lime-carbonic acid balance can be achieved by increasing the content of calcium ions or by chemical bonding of the carbonic acid.

When adding calcium salts - either as calcium chloride (CaCl ₂ ) or as calcium sulphate (CaSO ₄ ) - the equilibrium is only adjusted. As the calcium content of a solution increases, so does the content of the associated carbon dioxide for the carbonate hardness . However, this method is only suitable for setting the balance in very soft water and low levels of aggressive carbon dioxide. In the case of harder and highly aggressive water, equilibrium water would only be achieved through high levels of hardness.

With direct chemical deacidification, the aggressive carbon dioxide is set either by adding alkaline solutions or by contact with alkaline substances. Only this deacidification process is discussed in more detail below. The following solutions or materials are used for this in technology:

Deacidification with calcium hydroxide

With the addition of lime water (rarely used) or milk of lime , the aggressive carbonic acid is converted into carbonate hardness . The reaction equation is as follows:

{\ displaystyle \ mathrm {2CO_ {2} + Ca (OH) _ {2} \ longrightarrow Ca (HCO_ {3}) _ {2}}}

aggressive carbonic acid (CO ₂ ) and calcium hydroxide react to calcium hydrogen carbonate (carbonate hardness)

0.6 ° dH carbonate hardness is formed per 10 mg / l carbon dioxide . The low price of the calcium compound used - unslaked lime (CaO) or white lime (Ca (OH) ₂ ) - is an advantage. The disadvantage is that the technical lime contains insoluble components, so that the deacidified water is cloudy without downstream filtering.

These disadvantages are avoided when using clear lime water. However, since calcium hydroxide is only sparingly soluble - 1.7 g / l at 20 ° C - large volumes must be used. Furthermore, the non-dissolvable portion must be separated off during the production of the lime water. The higher investment costs therefore limit the use of this process.

Deacidification with sodium hydroxide

If instead of milk of lime an aqueous solution of sodium hydroxide (caustic soda) is used to set the aggressive carbonic acid, the reaction equation is:

{\ displaystyle \ mathrm {CO_ {2} + NaOH \ longrightarrow NaHCO_ {3}}}

Carbonic acid reacts with sodium hydroxide solution (NaOH) to form sodium hydrogen carbonate

As can be seen, instead of carbonate hardness, the chem. Formed compound sodium hydrogen carbonate . The hardness content of the water does not change. Another advantage of this type of deacidification is that the final water is not cloudy and filtering is not necessary. The significantly higher costs for the caustic soda are a disadvantage. In addition, local excessive alkalinization must be avoided at all costs during the addition in order to prevent the precipitation of calcium carbonate. A quick and intensive mixture of water and diluted lye is therefore necessary. Otherwise there is a risk of local decarbonisation . Once calcium carbonate has formed, it is difficult to convert it back into dissolved calcium hydrogen carbonate.

Deacidification with calcium carbonate

The chemical setting of carbonic acid takes place with finely broken limestone according to the following equation:

{\ displaystyle \ mathrm {CO_ {2} + CaCO_ {3} + H_ {2} O \ longrightarrow Ca (HCO_ {3}) _ {2}}}

aggressive carbonic acid reacts with limestone (CaCO ₃ ) to form calcium hydrogen carbonate

The advantage is that there can be no overreaction. Excessive setting is not possible even with new filter fillings. Fine crystalline calcium carbonate from Jurassic or Devonian deposits or porous material from mussel or coral limestone deposits is used as the raw material . The disadvantage of this type of deacidification is the long exposure time required for the aggressive carbonic acid to bind to equilibrium. This means that large fill quantities and thus large filter systems are required. The application is therefore limited to systems for smaller amounts of water.

Calcium carbonate artificially produced from calcium hydroxide , called hydro-calcite (CaCO ₃ ), is faster to react . However, due to the manufacturing process, this material contains some free calcium oxide (CaO) or calcium hydroxide (Ca (OH) ₂ ). In the case of new filter media, over-alkalization occurs initially for some time when the system is at a standstill. When the system is operated again, pure water with too low a carbonic acid content is released for a short time.

Deacidification with dolomitic filter material

Many deacidification systems use dolomitic filter materials. The reason for this is the much faster adjustment of the carbonic acid equilibrium when using magnesium-containing materials. Due to the shorter reaction times, the required filter quantities and thus also the expenditure on equipment are significantly lower. The chemical reactions are comparable to those of the calcium-containing filter materials. However, both calcium and magnesium hydrogen carbonate are formed. With half-burnt dolomite , also called magno, the equation is:

{\ displaystyle \ mathrm {CaCO_ {3} \ cdot MgO + 3CO_ {2} + 2H_ {2} O \ longrightarrow Ca (HCO_ {3}) _ {2} + Mg (HCO_ {3}) _ {2}} }

Magno binds carbon dioxide in the water with the formation of calcium and magnesium carbonate hardness

Further details are given under Magno .

Since granulated dolomite requires significantly longer reaction times than Magno, the latter is mainly used for deacidification. The disadvantage is that Magno tends to overreact during the filter run-in period. More carbonic acid is bound than necessary for equilibrium. With regard to the lime-carbonic acid balance, the water is located in the area that separates calcite and thus tends to precipitate carbonate. The reason for this behavior with fresh filter material is the preferred and particularly rapid reaction of the magnesium oxide in the Magno. This overreaction does not occur until the magnesium oxide content in the mass has been reduced .

In order to avoid such operating conditions, either blending with raw water or lowering the pH by adding acid is necessary. These overreactions occur more intensely in the event of a standstill and subsequent operation.

Summary of the chem. Procedure

The listed procedures are briefly summarized in the following table.

Procedure	advantages	disadvantage	Material requirement per 10 mg CO ₂
Dosage of calcium hydroxide (as milk of lime)	very short response time low chemical costs low investment costs	Increase in carbonate hardness Turbidity must be removed	8.4 mg Ca (OH) ₂
Dosage of calcium hydroxide (as lime water)	very short response time low chemical costs no turbidity has to be removed	Increase in carbonate hardness higher investment costs	8.4 mg Ca (OH) ₂
Dosing of sodium hydroxide (as a very dilute solution)	very short response time no increase in carbonate hardness no cloudiness low investment costs	Risk of local overreaction high chemical costs	9.1 mg NaOH
Filtering over limestone (CaCO ₃ )	no overreaction possible low material costs	Increase in carbonate hardness long response time high investment costs	22.8 mg CaCO ₃
Filtration through hydro-calcite (CaCO ₃ )	Overreaction is only possible for a short time slightly faster response time slightly lower investment costs	Increase in carbonate hardness slightly higher chemical costs	22.8 mg CaCO ₃
Filtering over dolomite (CaCO ₃ MgCO ₃ )	shorter response time no overreaction possible low material costs favorable investment costs	Increase in carbonate hardness	20.9 mg CaCO ₃ • MgCO ₃
Filtering over half-burnt dolomite (CaCO ₃ MgO)	significantly shorter response time cheap material costs significantly lower investment costs	new masses tend to overreact Increase in carbonate hardness	10.9 mg CaCO ₃ • MgO

Individual evidence

↑ ^a ^b ^c ^d ^e ^f ^g B. Mörgeli, JC Ginocchio; wlb water, air and operation, vol. 22, 1978, issue 4, p. 146, in: Deacidification in water treatment .
^ Dieter Jaeger; gfw water · waste water, vol. 129, 1988, issue 12; in: Tibean - a new type of device for hypolimnic water aeration .
↑ Dieter Stetter, Horst Overath; bbr, Fachtechnik Wasseraufbereitung, Volume 48, 1997, Issue 9, p. 32; in: The Rösrath process - a new high-performance process for deacidifying water with milk of lime.
↑ Martin Söller; bbr, Fachtechnik Wasseraufbereitung, Volume 48, 1997, Issue 5, p. 33; in: Deacidification and hardening with chemically reacting filter materials .
↑ Martin Söller; bbr, Fachtechnik Wasseraufbereitung, vol. 48, 1997, issue 5, p. 34; in: Deacidification and hardening with chemically reacting filter materials .

literature

Ulrich Hässelbarth , gfw Gastechnik, Wasser und Abwasser, 104 Jg. Issue 18, May 3, 1963
WW experience report , WLB, year 1974, issue 11, pp. 615–617
B. Mögeli and JCGinocchi , WLB, vol. 1978, issue 4, pp. 144-148
Heinrich Sontheimer, Paul Spindler, Ulrich Rohmann: Water chemistry for engineers . DVGW research center at the Engler-Bunte-Institute of the University of Karlsruhe 1980, ZfGW-Verlag Frankfurt, ISBN 3-922671-00-4
Dieter Jäger , WLB, born 1988, issue 12, pp. 787-793
Martin Sölter , bbr, Volume 48, Issue 5, 1997, pp. 32-35

[B.Mörgeli146-1] ↑ ^a ^b ^c ^d ^e ^f ^g B. Mörgeli, JC Ginocchio; wlb water, air and operation, vol. 22, 1978, issue 4, p. 146, in: Deacidification in water treatment .

[2] Dieter Jaeger; gfw water · waste water, vol. 129, 1988, issue 12; in: Tibean - a new type of device for hypolimnic water aeration .

[3] Dieter Stetter, Horst Overath; bbr, Fachtechnik Wasseraufbereitung, Volume 48, 1997, Issue 9, p. 32; in: The Rösrath process - a new high-performance process for deacidifying water with milk of lime.

[4] Martin Söller; bbr, Fachtechnik Wasseraufbereitung, Volume 48, 1997, Issue 5, p. 33; in: Deacidification and hardening with chemically reacting filter materials .

[5] Martin Söller; bbr, Fachtechnik Wasseraufbereitung, vol. 48, 1997, issue 5, p. 34; in: Deacidification and hardening with chemically reacting filter materials .