This book describes theory and approach for a comprehensive and applied physicochemical model for wastewater treatment. These are reactions which occur without a biological mediator, and are critical to both the biology of wastewater treatment, and stand-alone chemical treatment units. The book includes description of acid-base theory, solution ion pairing and non-ideality, participation with other phases, and chemical oxidation/reduction. Full implementation details are provided, including in a plant wide modelling context, and with respect to required extensions to biological models to describe more complex aspects of the iron-sulfur-phosphorous system, which requires all components of the model to be properly described.