A water system experiences corrosion issues. Which water quality parameter is MOST critical to monitor and adjust to mitigate this problem?

The most critical water quality parameter to monitor and adjust to mitigate corrosion in a water system is pH. Corrosion, in the context of water systems, refers to the deterioration of metal components (pipes, tanks, fittings) due to chemical reactions with the water. This deterioration weakens the system and can lead to leaks, reduced water quality, and eventual failure. The pH of water measures its acidity or alkalinity on a scale of 0 to 14, with 7 being neutral. Values below 7 indicate acidic conditions, and values above 7 indicate alkaline (basic) conditions.

Corrosion is significantly influenced by pH because it affects the electrochemical reactions that cause metal degradation. Most metals are more susceptible to corrosion in acidic environments (low pH). This is because acidic water readily donates hydrogen ions (H+), which accelerate the oxidation of the metal. For example, iron (Fe) corrodes according to the following simplified reaction: Fe → Fe2+ + 2e-. The presence of hydrogen ions speeds up this process. Conversely, higher pH values (alkaline conditions) generally reduce corrosion rates for many common water system materials like steel and copper. This is because alkaline conditions reduce the availability of hydrogen ions.

While other water quality parameters like dissolved oxygen, conductivity (a measure of the water's ability to conduct electricity, indicating the presence of ions), chloride concentration, and water hardness (mineral content) also influence corrosion, pH has the most direct and widespread impact. Dissolved oxygen, for instance, can exacerbate corrosion by acting as an electron acceptor in the electrochemical process. High chloride concentrations can increase the aggressiveness of water, promoting pitting corrosion (localized corrosion). Water hardness, specifically the presence of calcium and magnesium ions, can sometimes form a protective scale on pipe surfaces, reducing corrosion, but this scale can also detach and cause other problems. However, adjusting pH is often the most effective and readily controllable method to shift the water chemistry towards a less corrosive state.

For most water systems using ferrous metals (like steel), maintaining a pH between 6.5 and 8.5 is generally recommended to minimize corrosion. The specific optimal pH range depends on the materials used in the system and the water's specific chemistry. Regular monitoring of pH, along with other relevant parameters, allows operators to make adjustments, such as adding chemicals like lime (calcium hydroxide) to raise pH or carbon dioxide to lower it, to maintain a pH level that minimizes corrosion and extends the lifespan of the water system.