3. Results and Discussion
Water quality at the studied water treatment plant changed during the time of its exploitation. Changes depending on the water chemical composition are shown in Table 1
, as well as other parameters associated with water flow, that is, hydraulic retention time (HRT). According to Awual [2
], water may also react with a material with which it is in contact. If a water attack on the exposed metals can be characterized by its corrosion and influence against other surfaces, we can talk about its aggressiveness. Corrosion and aggressiveness of water are common problems, especially for the industrial production of boiler water [10
Treatment method assessment of the impact on the corrosivity and aggressiveness for the boiler feed water was the aim of laboratory and technical experiments conducted for treated groundwater directed to the chosen industrial plant. The plant prepared technological water in an old and ineffective water treatment station. Stains, rust, and poor damage have appeared inside the water supply system and in the water storage tank. This resulted in a greater amount of sediments and increased water color. The chemical reactions produce ferrous, then ferric hydroxides, forming nodules and tuberculation, which can, in the long term, reduce the pipe cross section and significantly increase the head loss. There was a need to stop such processes, especially for boiler feed water, because of the destruction of boiler equipment touched or washed by such water. Awual [17
] writes that water not sufficiently treated could have been driving the whole industrial production system to fail or be destroyed. That is why it was important to reach a high water purification level, including the calculation of water aggressiveness and corrosivity indicators.
For the assessment of water corrosivity and aggressiveness, analyses were carried out of the water pollution parameters: pH, dissolved oxygen, temperature, conductivity, chlorides, sulphates (VI), total hardness, calcium hardness, free carbon dioxide, carbon dioxide, bicarbonate carbon dioxide, aggressive carbon dioxide, total salinity, total alkalinity, HRT (time of water retention in the system), and sodium. The results are presented in Table 1
The estimation of oxygen concentration in water was the first step in the conducted research. Pre-aeration before modernization of the water treatment plant was realized by inefficient open pipes for many years. Old pipes transported air into an open aeration chamber about 1 m under the water surface and did not allow the increase of water alkalinity and removal of carbon dioxide. It was proved by Naykki and others [17
] that aeration makes it possible to remove dissolved gases in raw water, especially aggressive CO2
. In industrial systems, the most optimal solution for pre-aeration is the use of venturi aerators (at constant water intake) or a linear compressor pumping air regulated by a manometer (at variable water intake during the day). The authors decided to the change primary aeration system. This was the first process at the analysed industrial water treatment plant. For experiments, an aeration system including a static in-line mixer combined with an air compressor was designed. After modernization, the oxygen concentration changed from 0.5 mg/dm3
in raw water to 3.5 mg/dm3
after aeration and 2.6 mg/dm3
after filtration. The oxygen fed into the water during primary aeration oxidizes Fe(II) to Fe(III) and other reduced substances, for example, the sulphate ion (IV) to the sulphate ion (VI). Apart from refreshing the water, removing the smell, and initiating the iron precipitation reaction, the influence of this process on water aggressiveness and corrosivity indexes was not noticed. Therefore, deacidification of water by introducing air is important for the treatment of groundwater.
Concerning scientific literature, the role of dissolved oxygen is not exactly known, because different authors [4
] give contradictory reports. Some of them [23
] explain that dissolved oxygen is the main electron acceptor and has a significant impact on the rate of corrosion, which grows with oxygen concentration. Moreover, Sozański et al. [25
] found that an increase in dissolved oxygen concentration reduces the iron release to water and also reduces water turbidity. On the other hand, a lack of oxygen in water can be conductive for anaerobic processes [26
]. They are seriously unfavourable for natural water and for water quality, because they make water organoleptic features worse [1
]. Next, purification processes in the tested water treatment station including cation and anion exchange also required oxygen control. During anion and cation exchange, there is the possibility to create gases as a side effect of water purification [14
]. Furthermore, CO2
is generated as a result of ion exchange. The analysed raw water contained 91 mg CO2
. After pre-aeration and sand filtration, its concentration dropped to 56 mg CO2
. Free carbon dioxide was present even in deionized water at the level of 6 mg CO2
, when its maximal concentration should be under 2.2 mg CO2
. Secondary aeration may remove all gasses from the water. This was the reason to design extra aeration for experiments. Static in-line aerators combined with an air compressor were used.
After water aeration, it is necessary to check whether only carbon dioxide, which is bound in the form of bicarbonates or hydrocarbonates, remains in the water. If aggressive CO2
is also present, it must be chemically removed [26
]. The conducted study indicated that bicarbonates were never present in the water at any treatment stage. The carried out tests showed the presence of hydrocarbonates in the water. It is total alkalinity, which is the parameter calculated based on HCO3
in the water. First, processes of water treatment at the station did not remove it from the boiler feed water. The greatest amount was in the raw water—over 200 mg/dm3
. With primary aeration, filtration, and cation exchange, we can see a slow reduction from 185 to 155 mg/dm3
. Anions exchange remove it efficiently from the water to 25 mg/dm3
. Next, processes did not change its concertation.
Nevertheless, industrial boiler feed water cannot include a higher concentration of oxygen or carbon dioxide, which may lead to boiler damage. This was the reason to design the last water treatment process—extra deaeration. For experiments, a thermal pressure deaerator was used. Deaeration removes all the gases dissolved in the water up to trace amounts. The solubility of gases in water is determined by its temperature. As the temperature increases, the partial pressure of the gases dissolved in the water decreases. At the boiling point of the water, these pressures drop to zero. At this and higher temperatures, the theoretical solubility of a gas in water also drops to zero. The conducted research confirmed this theory. Oxygen and free carbon dioxide concentrations dropped to 0.0 mg/dm3 after degassing.
The other water parameters mentioned in the Table 1
were used to calculate the aggressiveness and corrosion indicators: the Saturation Index (LSI), the Ryznar Stability Index (RI), the Larsoni-Skold Index (LI), the Singley Index (SI) and the color increase. The results of calculations are presented in the Table 2
and Table 3
as well as in the Figure 1
, Figure 2
, Figure 3
, Figure 4
and Figure 5
Corrosion may be defined as the destruction of a metal, usually by chemical or electrochemical reaction. The water aggressiveness is defined as its propensity to attack calcium-containing materials. At the water treatment station, it is water hardness. The treated water at the studied station had negative corrosive and aggressive features. The calculations of indicators resulting from the conducted research interchangeably proved that raw water and treated water have revealed aggressive features.
Lee and others [26
] list the water parameters that determine its corrosive properties. These include pH, alkalinity, chlorides, sulphates (VI) and other inorganic ions, water temperature, dissolved oxygen concentration, and natural organic matter. Such parameters include in its formula the Singley Index (SI) presented in Table 2
and Figure 1
. The reached results showed that water going through the first treatment processes is corrosive. This property was noticed after primary aeration and sand filtration. The SI for pre-aeration oscillated was about 0.93, which was close to the corrosion level. The sand filtration process and ion (cation and anion) exchange helped the SI decrease to the level of 0.81. However, the water, despite advanced purification processes, was not safe for a high-temperature boiler. Limits of water corrosivity and aggressiveness are presented in Table 3
, and show that stable water becomes by SI between 1 and 1.2, and neither corrosive nor aggressive water is for SI < 0.8. The new designed process, extra aeration, reduced water corrosion. The SI achieves a value of about 0.75 and comes to a stable state. The last process, degassing, improved the effect. It made water neither corrosive nor aggressive and allowed obtaining SI = 0.67.
The Larson–Skold Index (LI) indicates that higher salinity, higher temperature, and a longer retention time significantly increase corrosion. The LI is presented in Table 2
and Figure 2
. Sarin [26
] describes the operating parameters affecting corrosion and writes that the intensity of corrosion is also influenced by other factors related to the water station exploitation and the real state of the water supply system. The following are mentioned in particular: the flow rate of water, the retention time of the water in the system, the condition of the installation, its age, and pipes’ diameter. Only bicarbonate ions have inhibiting properties. That is why corrosion may be prevented by water alkalinity increase [6
]. In this study, the Larson–Skold Index calculations proved that water treatment processes after modernization did not have any influence on water corrosion. Purified water in every individual purification process was neither corrosive nor aggressive. Analysis of the corrosivity indexes, that is, LI and SI, indicated some divergences. Calculations of the Larson–Skold Index showed that the every process of water treatment at the studied station did not influence the change of water corrosivity. Water was not corrosive. However, the Singley Index, which additionally takes into account the alkalinity parameter and water retention time in its formula, showed the opposite situation.
Apart from water corrosivity, its aggressiveness was also assessed. Water, depending on its composition and other flow-related parameters, may change during its exploitation and react with the material in contact. For the assessment of water aggressiveness, parameters such as the saturation index, the Langelier Saturation Index (LSI), and Ryznar Stability Index (RI) are used. However, these indexes do not characterize the corrosive properties of water and cannot be used to assess the water corrosivity [21
]. These indexes determine the state of carbonate–calcium balance of the water. They point to the chemical stability of water and its ability to form and/or dissolve protective layers, that is, precipitation/dissolution of CaCO3
sediments. The state of calcium–carbonate balance is determined by the water temperature, total salt content, calcium ions bicarbonate concentration, and water pH [13
]. Also, bicarbonates with total water hardness (calcium and magnesium) are corrosion inhibiting ions. The Larson–Skold Index shows that an increase in water alkalinity can compensate for the aggressive action of chloride and sulphate ions (VI). Higher alkalinity is associated with water storage and usually higher pH. Sarin and his team [27
] showed a positive influence of alkalinity increase on the decrease of iron compounds release to water from the installation. This was confirmed by the authors of this paper, considering the reached results of water parameters after anion exchange. Chloride and sulphate ions (VI) have the greatest influence on corrosion. Their high concentration in the transported water affected its aggressiveness [3
The Langelier Saturation Index (LSI) calculation results are presented in Table 2
and Figure 3
According to LSI values in the Table 3
, water is stable and does not have aggressive properties if LSI >0. The reached results proved that most of the water treatment processes used at the studied boiler feed water station are inefficient in water aggressiveness removal. The LSI changed from −0.34 for raw water to −0.12 for pre-aerated water. It was close to the stability level when LSI = 0. However, the next processes allow LSI to go down up to −0.23 for quartz sand filtration and −0.83 after cation and anion exchange. Additional aeration neutralized this negative result. The LSI increased from the lowest level up to 0, which made water stable. Extra deaeration sustained this value and kept water from being aggressive.
The calculation formula of LSI is the simplest one among other indicators, and considers only water pH [15
]. For most natural waters, the water pH should not affect the aggressiveness rate. In the pH range of 4.5–9.0, the corrosion rate is almost constant. However, the pH value may determine the type of corrosion. According to Schock and Lytle [15
], homogeneous corrosion occurs at pH <6.5, whereas pitting corrosion is observed in the pH range between 6.5 and 8.0. The pH range of the water flowing through the studied treatment station ranged from 7 to 7.5 for the first treatment processes, that is, pre-aeration and sand filtration. Cation exchange reduced its value to 6.58, but anion exchange increased it to over 8.0. Additional aeration and degassing kept the pH value at this level. Slightly alkalic water stopped corrosion, which faced in values of the Langelier Saturation Index [9
Ryznar Stability Index (RI) calculation formula is based on the same parameters as those of LSI, that is, the pH of water. The RI values in Table 3
confirm that water is stable when RI hesitates between 6 and 7, and it is not aggressive when RI < 5. The reached results of RI are presented in Table 2
and Figure 4
. The calculation showed that experimental water in the whole purification system was aggressive. Higher values were noticed for raw water when RI was 8.24. Pre-aeration and sand filtration allowed it to decrease slightly to 7.23 and 7.56. The next processes of ion exchange increased its value to the top point of RI = 9.03. Additional aeration closed it to the level of stability and RI was about 7. The last process, deaeration, allowed RI to decrease and reach the not-corrosive-standard of about 5.0.
As written above, the newly designed processes at the studied water treatment station, additional aeration and degassing, do not fully remove its aggressive properties, as confirmed by Soldatov and others [30
]. However, on the basis of the conducted studies, a clear positive effect of pre-aeration and extra aeration was observed. The values of the Langelier Saturation Index and the Ryznar Stability Index were closer to stability than with the advanced purification process—ion exchange. The figures show that the ion exchange with cation exchange columns affected the water aggressiveness by its significant increase. The usage aeration clearly neutralizes this negative character.
] and Awual [11
] write that water conductivity below 1000 µS/cm indicates weaker corrosion properties. It is indirectly related to the Langelier Saturation Index and Ryznar Stability Index. Apart from sulphates (VI) and chlorides, these water properties are also influenced by calcium and magnesium ions and bicarbonates forming the alkalinity of water [25
]. The decrease in the concentration of these ions in water was observed after weak base exchange columns when a decrease in aggressiveness and corrosivity indices was calculated. On the other hand, the decrease in hardness and calcium ion concentration after cation exchange columns resulted in an additional increase in the LSI and RI values.
An important indicator of water corrosivity and aggressiveness is the change of water colour ΔB
. It is calculated with a developed formula that includes many cations and anions. This parameter depends on water temperature, dissolved oxygen, sodium, sulphates (VI), chlorides, water alkalinity, and water retention time in the system. The ΔB
analyses are presented in Table 1
and Figure 5
. The higher water colour was noticed for primary aeration and reached the value close to 5.0 mg Pt/dm3
. An increase in comparison with raw water when ΔB
= 3.1 may be caused by reduced substances oxidation. Sand filtration started its decrease from 2.1 to 0.0 after extra aeration and degassing.
Included in the ΔB
formula, water temperature has a significant influence on the speed of chemical reaction and the intensity of corrosion. Ranjbar [29
] noted that with higher temperature, the rate of diffusion of oxygen to the metal surface is higher, while its solubility decreases. Temperature influences many processes and parameters important for the corrosion rate, as well as the composition and properties of formed sediments. The following should be mentioned in particular: salt and oxygen solubility, viscosity, diffusion, reaction enthalpy, oxidation rate, and biological activity. The corrosion rate and water colour change depend on seasonal changes in water temperature [6
]. It also has a direct effect on the Langelier Saturation Index, the Ryznar Stability Index, and the Larson–Skold Index. It can be compared to the conducted study at the boiler feed water station. The obtained results proved that a sudden increase in the Larson–Skold Index was noticed when the temperature in the extra deaerator was increased. The aggressiveness indexes, LSI and RI, where an increase in temperature caused a decrease in their value, show an opposite relationship.
Loewenthal with others [4
] and Cocorilo et al. [33
] state that the ΔB
formula clearly indicates that the corrosive properties of water increase with the content of sulphate (VI) and chloride ions, and decrease with the water alkalinity. At the studied water treatment station, the concentration of sulphates in water ranged from 176 mg/dm3
for raw water and pre-aeration to 18 mg/dm3
for anion exchange and the next processes. Similarly, chloride concentration decreased from 80 mg/dm3
for raw water, pre-aeration. Filtration and cation exchange to 12 mg/dm3
for anion exchange, extra aeration, and degassing. Consequently, an increase in water alkalinity may compensate for the undesirable corrosive effect of chloride and sulphate ions (VI). Total alkalinity for examined water increased from about 3 mval/dm3
to 5.5 mval/dm3
in the purified water.