Process simulation has been successfully used in the chemical and oil industries since the early sixties for optimizing integrated processes and evaluating waste minimization options. The same benefits can be derived from the application of that technology to water purification and wastewater treatment processes. To address the need for efficient design, evaluation, and operation of integrated water purification, wastewater treatment and recycling processes, engineers at Intelligen in collaboration with researchers at Lucent Technologies have expanded the scope of an existing process simulator. This presentation will focus on the challenges for modeling water purification, wastewater treatment, and recycling processes. The results of an industrial case study pertaining to semiconductor fabrication facilities will be discussed. Opportunities for potential benefits in other industries will be presented as well.