The water supply industry is faced with three phenomena of great importance: the aggressiveness,
corrosion, and incrustation of water distribution systems (WDS), which are primarily due to the
low alkalinity of water sources and the addition of chemicals used in water treatment processes,
which require pH adjustments during the last stage of the treatment process before going into the
WDS. This article presents the results of using Ca(OH)
When treating water to make it potable, the processes of coagulation and disinfection reduce alkalinity and pH (WHO, 2011), which makes it necessary to equilibrate the chemical balance of water at the end of the treatment to reduce its corrosive capacity and ensure that no encrustations are formed, which can cause reduction in the flow section (Barid, 2004; Casey, 2009). These situations have economic, hydraulic, aesthetic, and public health repercussions, which include water leaks, increased pumping costs, and deterioration of water quality in WDSs (Volk et al., 2000; Gray, 2008).
For the pH adjustment process, there are several alkalising agents in use, which include strong base
neutralisation (hydrated lime (
One technique to evaluate the pH adjustment process is to use stabilisation indices, where the most
widely used are those that indicate water saturation, such as the
By applying stabilisation indices, this study evaluated the influence of alkalising agents (hydrated lime, sodium hydroxide, sodium bicarbonate and sodium carbonate) on controlling corrosion in water treated from the Cauca River, which supplies a high percentage of the water for the city of Cali, Colombia.
The study was conducted in the Puerto Mallarino water treatment plant (WTP) in the city of Cali, which treats raw water from the Cauca River. The following are the stages developed in this study: evaluate the effects of adding chemicals used in water treatment processes on the water's pH and total alkalinity and evaluate the pH adjustment using different alkalising agents.
Historical daily information was gathered and processed during the year 2012, which is available
from EMCALI EICE ESP, regarding the raw water turbidity from the Cauca River and its relationship
and effect on the dosage of the chemical products used during water treatment processes (coagulant,
adsorbent, disinfectant, and alkalising agent) and on the total alkalinity
(
By means of a bibliographical review, a comparative analysis was conducted on the alkalising agents with the most widespread application, such as hydrated lime, sodium hydroxide, sodium bicarbonate, sodium carbonate, and calcium carbonate. The primary characteristics of each alkalising agent were grouped into three categories: technical aspects, occupational health, and economics, leading to a selection of the alkalising agents to be evaluated at the laboratory scale with treated water from the Cauca River.
For the pH adjustment, hydrated lime (
Tests were conducted using jar test equipment in duplicates and with chlorinated water from the
Puerto Mallarino WTP. The sample volume was 2
Table 1 shows the variables monitored to measure the pH adjustment process
through the stabilisation indices presented in Tables 2 and 3, which were
classified in the following way: tendency towards precipitation of
Figure 1 shows the turbidity behaviour (UNT) of the raw water and its effect on the chemical doses
and total alkalinity (
With respect to the raw water of the Cauca River, it can be seen in Fig. 1a that the turbidity
exhibits high variability that is primarily associated with phenomena, such as solid suspensions
(Rodríguez et al., 2010), dragging of domestic and industrial discharges, agricultural and
mining runoff, and sediments (Pérez-Vidal et al., 2014). This variability has a direct effect on
the dosage of chemicals (Fig. 1b), particularly for the coagulant (
The addition of chemicals, both for coagulation and for disinfection, generates the consumption of
total alkalinity (Fig. 1c) and reduces the pH (Fig. 1d), which can be particularly observed for
clarified water given that before this stage, the processes of primary disinfection (chlorination)
and coagulation have occurred. These values remain in the filtered water and require adjustment
after disinfection; in the case of the water studied, the pH adjustment process is performed using
Ca(O
Table 4 shows the primary characteristics of the most widely used alkalising agents during the pH
adjustment process, grouped by technical, health, and economic aspects. It can be seen that strong
bases (
With respect to
NaOH is easy to handle because its high solubility achieves good mixtures, avoids encrustations or increased turbidity in the final water, and is easy to acquire nationally. In addition, it has a high percentage of the active ingredient as a raw material, which is expensive (Sallanko et al., 2013). In terms of occupational health, NFPA 704 classifies it as very dangerous because it is highly corrosive and may cause severe burns, which requires special care for storage, specifically the need to use high-density polyethylene materials and precautions in electric power transmission lines due to the possibility of generating flammable hydrogen (GQ, 2006; Kim et al., 2008; Vargas et al., 2009).
Although the analysed products have the potential of being used in pH stabilisation processes, their
selection depends on the different factors mentioned previously and on aspects such as the
characteristics of the raw water, type of treatment processes previously used, availability at the
site, etc. For this reason,
Figure 2 shows the behaviour of the alkalising agents evaluated with respect to total alkalinity and final pH of the treated water from the Cauca River.
According to Fig. 2, strong bases increase total alkalinity values and pH with low doses, whereas
salts require higher doses, which agrees with the results reported by De Sousa et al. (2010).
However, strong bases do not provide the minimum alkalinity required for maintaining a buffer
capacity in the WDS (approximately 40
In terms of final pH, the only alkalising agent that did not achieve the minimum pH
(pH
Figure 3 shows the behaviour of the alkalising agents evaluated through the indices that exhibit the
tendency to precipitate calcium carbonate (LSI, DFI, and CCPP) and those that are used depending on
the type of piping (AI, LKI, and RSI). It can be seen that all alkalising agents, except for
With respect to the indices that apply to specific types of piping materials in the WDS, such as AI
for asbestos cement piping, i.e., AC (31.9 % of the city of Cali's WDSs is made of AC and is
also the oldest), it can be seen that all the alkalising agents, with the exception of
For the LKI index applied to metallic piping (11.5 % of the city of Cali's WDSs), only
Regarding the use of products containing sodium (
Table 5 shows certain international guidelines and rules in addition to the Colombian code for the
operation conditions in terms of doses, pH, and total alkalinity in accordance with the optimum
results of pH stabilisation indices, LSI, DFI, CCPP, and AI, for the alkalising agents evaluated,
with the exception of
The results in Table 5 show that in terms of application dose,
As the main variable in the stabilisation process, pH must be controlled to reduce corrosion in the WDS to a bare minimum, which avoids possible water contamination due to metal dissolution and adverse effects in appearance and flavour (Tam and Elefsiniotis, 2009; Wilczak et al., 2010; WHO, 2011).
One aspect of these results that should be highlighted is that the current Colombian code does not
allow for adequate conditions to be obtained to remove the calcium carbonate film in Cali's WDSs,
which is primarily due to the pH limitation because getting good results requires the pH to exceed
the pHs, which is between 8.7–9.0, a range that is at the upper bound of Resolution
2115/2007. However, the fact that other countries, such as Brazil, the European Union, and the
United Kingdom, allow maximum values of pH
The value of alkalinity at the end of the stabilisation process for treated water from the Cauca
River should be at a minimum range, between 27.3–36.7
The results make it advisable to continue evaluating the pH adjustment process to create
combinations of alkalising agents that provide adequate values of total alkalinity and pH and to
evaluate other alternatives for process control and optimisation, such as increasing alkalinity from
the supply source; using other types of coagulant; and mixing alkalising agents or strong bases with
the addition of
Alkalising agents, such as hydrated lime, sodium hydroxide, and sodium carbonate, create water conditions that have a tendency to precipitate a calcium carbonate film in the WDS of the city of Cali; however, their selection depends on the characteristics of the raw water and on technical, economic, and occupational health aspects.
The application dosage recommended for generating water conditions with the tendency to precipitate
a calcium carbonate film and non-aggressive conditions for asbestos cement piping in the WDS of the
city of Santiago de Cali, according to the LSI, DFI, CCPP, and AI indices, are between 6 and
10
Application doses greater than 200
Application of the stabilisation indices shows their great potential for monitoring the pH
adjustment process and better predicting the behaviour of water in WDSs; however, because there is
no index that is universally applicable and others depend on the type of piping material of the WDS,
it is necessary to use at least one index that provides information regarding the tendency for
The required pH values that guarantee adequate pH adjustment are higher than those indicated in the
Colombian code for drinkable water because post-treatment pH should be equal or higher than the
saturation pH (pH
The authors thank the Universidad del Valle for financing the Internal Proposal Project “pH stabilisation in treated water from the Cauca River” in the framework in which this study was conducted.
Test variables.
Indices with the greatest applicability in controlling the pH stabilisation process:
tendency towards precipitation
Source: Schock (2002), Varó Galváñ et al. (2004), APHA et al. (2005), Imran et al. (2005), Trujillo et al. (2008), De Sousa et al. (2010), De Moel et al. (2013), Bueno-Zabala et al. (2014).
Indices with the greatest applicability in controlling the pH stabilisation process: type of piping.
Source: Schock (2002), Varó Galváñ et al. (2004), APHA et al. (2005), Imran et al. (2005), Trujillo et al. (2008), De Sousa et al. (2010), De Moel et al. (2013), Bueno-Zabala et al. (2014).
Technical, occupational health, and economic aspects of the alkalising agents used in pH adjustment.
Operation conditions and results of pH stabilisation with different alkalising agents.
Turbidity behaviour of the Cauca River, chemical doses, and their effect on alkalinity and pH during the water treatment processes. Historical daily data for the year 2012.
Effect of different alkalising agents on total alkalinity and pH for the evaluated doses.
LSI, DFI, CCPP, AI, LKI, and RSI indices for hydrated lime, sodium hydroxide, sodium bicarbonate, and sodium carbonate.