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Facilities in the chemical and refining industries will, at some point, need to calculate emissions from wastewater systems, whether for calculating potential to emit for permitting or actual emissions for emissions reporting requirements. If your facility is dusting off that old Water9 report from 2005, then perhaps now is the time to determine if a switch to Toxchem, a wastewater emissions modeling software developed by Hydromantis, is right for you.

Many simplified air emission estimation methods, like those for batch chemical operations, assume ideal mixing in the liquid and vapor phases, achieving of near instant vapor liquid equilibrium, and negligible resistance to chemical movement in the liquid phase. These assumptions quickly break down for wastewater systems, where dilute chemical concentrations and interaction of chemical species with water, solids, and each other have a significant effect on chemical emissions. To account for this, programs like Toxchem and EPA's Water9 estimate the effect of these interactions by accounting for the following mechanism in unit operations contained in wastewater systems.

  • Gas/liquid mass transfer (volatilization)
  • Liquid/liquid mass transfer (molecular movement in the liquid, partitioning of water-oil)
  • Solid-liquid mass transfer (adsorption onto solid surface)
  • Biotransformation (biodegradation of chemical compounds)
  • Abiotic transformation (user-defined chemical reactions)

Toxchem was originally developed in the early 1990s to overcome the limitations of Water8 software from EPA. Today, Toxchem continues to provide significant advantages to EPA's Water9 program, potentially offsetting the cost from transitioning from EPA's free Water9 program to a proprietary model. In addition to providing a user friendly interface and helpful tools, such as sensitivity analysis, back-solving functions, and Microsoft Excel- based spreadsheet reports, Toxchem also provides more refined data and calculation methods compared with Water9, including the following:

  • An updated database of critically - reviewed chemical properties, with the option to revert to Water9 compound properties for conducting comparative models.
  • Modeling of partial dissociation of chemicals based on pH, providing a more accurate estimation of chemical concentration available for vaporization at the liquid-vapor interface.
  • Enhanced modeling of aeration systems using fractional saturation, which more accurately estimates concentrations at the vapor-liquid interface by incorporating resistance a chemical faces moving through the bulk liquid.
  • Option to use Monod Kinetics for biological growth. Water9 uses 1st order kinetics, which are more appropriate for municipal systems with low substrate levels that feed the bugs, while Monod Kinetics provide more accurate growth model for industrial processes with higher substrate levels.

If you have a need to evaluate wastewater emissions for your facility and would like to see how Toxchem can fit into your plans, please contact your local Trinity office or David Dempsey of Trinity's Chemical Sector Services Group at (630) 495-1470.


Note: This article was originally published by Trinity Consultants on September 30, 2015.