Understanding Water Treatment and the Corrosion Control Process
Corrosion can cause drinking water to be unsightly, unpalatable or even unsafe.
Drinking water sources are subject to contamination and require appropriate treatment to remove disease-causing agents. Public drinking water systems often use various methods of water treatment to provide safe drinking water for their communities. The most common type of conventional water treatment includes a multi-phased process such as pretreatment, coagulation, flocculation and corrosion control. In this article, we'll cover water treatment as it relates to corrosion control.
The pretreatment program blocks the introduction of pollutants, which can cause damage to equipment and interference with the wastewater treatment process. Pre-chlorination and oxidation are the most common types of pretreatment.
The purpose of pre-chlorination is to kill disease-causing organisms, control taste and odor issues, and reduce algae growth. However pre-chlorination may not always possible because some water contains a specific chemical that can react with chlorine to produce trihalomethanes, which are carcinogenic.
Manganese or iron can cause aesthetically adverse color in water, and create taste and odor problems. Water rich in iron and manganese will also irreparably stain laundry. Oxidizers such such permanganate, chlorine, aeration, or ion-exchange resin can be employed to reduce iron and manganese levels.
After pretreatment, the coagulation process begins. Coagulation is the reaction between the alkalinity of the treated water and the coagulant. Coagulants are introduced to cause very fine particles to come together and form larger particles. Coagulation destabilizes the negatively charged particles in natural water by adding positively charged chemical compounds. The attraction of oppositely charged particles results in the formation of floc.
This process requires water with pH in the range of 5.4 to 7.4. This pH range is typically controlled by the addition of sodium hydroxide (NaOH) and lime. Chemical coagulants are added and flash mixed with a mechanical mixer. Rapid or flash mixing is paramount to the process. Alum (Al2(SO4)3-14H2O), which is the common name for aluminum sulfate and ferric chloride (FeCl3), are common coagulants. Polymers are then routinely added as an aid to the main coagulant.
Flocculation is a process of forming insoluble particles, called floc, by colliding the charged particle in the raw water. In a sedimentation basin, this is achieved by gentle stirring using flocculators. In the clarifier, this is achieved by means of hydraulic pulsation. As a result, the gathered floc becomes easier to remove in the next process.
Corrosion Control in Water Treatment
Water is corrosive by nature because it constantly seeks reactions that result in the release of one of its hydrogen atoms. As a countermeasure, corrosion inhibitors are added to prolong the service life of piping throughout a water distribution system.
Corrosion is a complex series of reactions between the water and metal surfaces (metal piping) and materials in which the water is stored or transported. The corrosion process is an oxidation/reduction reaction that results in refined or processed metal being changed to their more stable state. With respect to the corrosion potential of drinking water, the primary concerns include the potential presence of toxic metals , such as aluminum, copper, nickel, lead and zinc; deterioration and damage to the household plumbing; and aesthetic problems such as: stained laundry, bitter taste, and greenish-blue stains around basins and drains.
The main source of health concern is the potential for the presence of high levels of lead and copper or other metals in the water. The primary source of the lead includes the use of lead pipes, lead lined tanks, and use of 50/50 lead/tin solder. Because of the concern with lead, the EPA banned the use of high lead solders in 1986.
Calcium-based compounds, soda and sodium polyphosphates can be added to reduce the water’s corrosiveness. The choice depends on the chemical analysis of the finished water. The corrosion inhibitor is added after filtration to avoid fouling.
Assessing the Impact of Corrosion Control Measures
Unlike chemical contaminants that can be present in the source water, metal contamination generally results from the corrosion of the distribution system and residential plumbing. Consequently, the type of material used in the distribution and interior plumbing systems are often similar to the type of metals found at the consumer’s tap. The leaching of lead, copper and zinc from plumbing material was one of the most significant sources of these metals in drinking water.
One study evaluated the effectiveness of different corrosion control strategies with different target pH ranges at each source, as shown in Table 1. To evaluate the effectiveness of these corrosion control measures, several types of metal concentrations in the drinking water were measured and compared with the metal concentration in the water at the source.
Corrosion Control Plan
Table 1: Current corrosion control plans at the different water sources
The Effect of Treated Source Water pH on Metal Concentration in Drinking water
Lead, copper and zinc concentrations at the tap were compared with the efficacy of the different corrosion control measures currently used. All measured metal levels in drinking water were in compliance with public health guidelines.
The results suggest that increasing the pH of the water from a pH of 6 to a pH of 7 or 8 did not affect the amount of corrosion control obtained with respect to lead. This was also supported by other studies that water with low alkalinity needs to have a pH of 8.4 or above before significant reduction of lead in the tap was observed.
For all hot water samples, lead concentrations were below the detection limit, suggesting that the contribution of lead made by the hot water tank or hot water pipes was minimal. The lead concentrations in the cold samples were below the detection limit, indicating that corrosion of lead in the drinking water distribution system was not significant. It was also possible that there were few sources of lead in the distribution system
Statistically, there were no significant differences among the zinc concentrations in the calculated first-litre flush from the water source A, B, and C. However, samples from D water source, with soda ash treatment and target pH of 8.5, had lower zinc concentrations than the other three sample areas, and this difference was statistically significant.
The zinc concentrations in cold samples, from all four source areas, were below the detection limit, indicating that corrosion of zinc in the distribution system was not significant. It was also possible that there were few sources of zinc in the distribution system. The hot samples had zinc concentrations that were similar to the zinc concentrations in the water, leaving the treatment facility. This suggests that either the source water treatment had no effect on the amount of zinc entering the drinking water or there was no source of zinc in the distribution system.
Unlike the other results, copper concentrations were influenced by the source water treatment. Water samples taken from the distribution area A (without neither chemical added nor targeted pH) had the highest copper concentration, while water samples taken from distribution system D with chemical added and targeted pH, had the lowest copper concentrations.
These observed differences correspond to different pH and alkalinity levels in the treated waters, both of which have been shown to significantly impact the rate of copper corrosion. This suggests that targeting higher pH and alkalinity levels will result in lower copper corrosion rates.
Lead and copper entering drinking water from household plumbing materials such as pipes, lead solder and taps containing brass or bronze, can almost always be controlled by changing specific water quality parameters. The water quality parameters that have the greatest effect on lead and copper corrosion are pH, alkalinity or dissolved inorganic carbonate (DIC) concentration, and orthophosphate concentration (if used). Other parameters, such as temperature, buffer intensity, total dissolved solids (TDS) concentration, the concentrations of dissolved oxygen and other gases, and chlorine residual can also be important. There are many other factors, such as NOM i.e non organic matter concentration, and the chloride-to sulfate mass ratio that may result in corrosion.
Analysis of the metal concentrations in samples taken at the tap suggests that the different pH and alkalinity levels of the various treated waters can impact the observed concentrations of some metals in the drinking water at the tap. No trend was seen with respect to lead concentrations in samples collected from all four treated source waters. However, there was a significant difference in the amount of copper and zinc in the samples from the four treated source waters. The highest copper and zinc concentrations were found in samples from the most aggressive water type, while the lowest copper and zinc concentrations were found in samples from the least aggressive water type.