Adjustments to the carbon dioxide dosage in recarbonation should be based on both the Langelier saturation index and results of coupon testing.

• A. Total Hardness Tests, Jar Tests
• B. Alkalinity Tests, Coupon Testing
• C. Langelier Saturation Index, Coupon Testing
• D. Langelier Saturation Index, Jar Tests

Answer: (C)

The key idea is that setting CO2 dosage for recarbonation should combine a predictive chemical indicator with real-world performance data. The Langelier Saturation Index estimates whether water is likely to dissolve or precipitate calcium carbonate based on pH, temperature, calcium hardness, and alkalinity. When you adjust CO2, you change pH and alkalinity, which shifts the LSI, helping you avoid both corrosive water and unwanted scaling.

Coupon testing adds the actual, measured corrosion response of the system materials to the mix. While LSI gives a theoretical sense of saturation, coupon tests show how the water will interact with metals in practice, accounting for factors like flow, deposits, and material specifics. Using both ensures the CO2 dose leads to a favorable, predictable balance in the water chemistry and confirms that the chosen dose effectively controls corrosion in the real system.

Other options focus on tests that don’t directly address carbonate saturation and corrosion performance in recarbonation. Alkalinity tests alone don’t predict saturation behavior; jar tests are about coagulation chemistry rather than carbonation and corrosion. Therefore, pairing the Langelier Saturation Index with coupon testing is the most comprehensive approach.