Tetra Tech (RTW) Model for Water Chemistry, Process and Corrosion Control performs useful calculations for water treatment processes. The software is an updated version of RTW Model for Corrosion Control and Process Chemistry.

Tetra Tech program:
• Calculates the Langelier Saturation Index and the Ryznar Stability Index, two useful corrosion indexes to determine the corrosive nature of an influent or treated water.
• Calculates effect of pH changes due to chemical addition on Cl₂ residual and TTHM concentrations.
• Predicts conditions under which a water system will precipitate Cu₂(OH)2CO₃, CuO, Cu(OH)₂, Fe(OH)₂, Fe(CO₃), Fe(OH)₃, and Al(OH)₃ (fresh and aged).
• Calculates parameters such as water pH, alkalinity, CCPP , and DIC.
• Calculates the effect of more than 20 common water treatment chemicals, with the ability to add a chemical to the list if the relevant chemical properties are known.
• Measures the effect of increased pH on contact time requirements for chlorine gas and hypochlorite disinfection.
• Calculates the percentage change in total trihalomethane concentration due to pH changes and chlorine gas or hypochlorite dose.

More features:
• Blending Applications determines the characteristics of blended water from two separate sources while accounting for the equilibrium of the carbonate buffering system. 
• Coagulant Dosage Calculations 
• Oxidant Dosage Calculations
• Sodium Hypochlorite Calculations

Includes tutorial:
• Determine relevant species and reaction in water for chemicals.
• Calculate alkalinity, acidity, calcium, chloride, and sulfate contribution of a chemical.