Mechanistic-based Disinfectant
and Disinfectant By-Product Models
USEPA Grant # R 826831-01-0
Final Report June 2002
Investigators:
Paul Westerhoff
Department of Civil and Environmental Engineering
Arizona State University Tempe, AZ 85287
David Reckhow
Department of Civil and Environmental Engineering
University of Massachusetts Amherst, MA 01003
Gary Amy
Department of Civil, Environmental, and Architectural Engineering
University of Colorado Boulder, CO 80309
Zaid Chowdhury
Malcolm Pirnie Inc. Phoenix, AZ 88008
The water industry faces new challenges in understanding and controlling disinfection by-product (DBP) formation as health concerns demonstrate a need for more stringent regulatory DBP requirements. Mechanistic tools for understanding and predicting the rate and extent of DBP formation are required in order to facilitate the evaluation of DBP control alternatives.
The goal of this project was to develop and calibrate an accurate kinetic-based model for several chlorinated DBPs of interest. The model predicts DBPs (e.g., four THM species (THM4) and nine HAA species (HAA9)) as a function of DOC, disinfectant level (type and dosage), reaction time, temperature, pH, and bromide concentrations. The project had the following specific objectives:
| Compile existing databases on DBP formation experiments into a single Unified Database. Some data from the compiled database were used to develop and/or verify mechanistic DBP prediction equations. Data deficiencies were identified. |
| Develop and calibrate numerical models for predicting the behavior of disinfectants (free-chlorine) and the formation of DBPs (THMs and HAAs) and improve DBP prediction accuracy over existing empirical DBP models through consideration of formation mechanisms and DBP stability. Controlled experiments were performed to assess inorganic reactions, disinfectant decay, DBP formation, and DBP stability. Additional laboratory experiments were performed to augment the Unified Database, and overcome data deficiencies. |
| Develop an easy-to-use computer model capable of predicting DBP formation, through a combination of mechanistic subroutines, as a function of disinfectant decay and water quality conditions. |
Existing databases were found to have several deficiencies limiting their usefulness for calibrating mechanistic models. These include lack of short-term kinetic chlorine consumption and/or THM plus HAA species formation. Bench-scale experiments were therefore conducted to fill the datagaps. The purpose of the bench-scale experiments was to obtain a wide range of NOM material from natural waters that simulated a range of treatment (NOM removal processes: coagulation, softening, ozonation, activated carbon sorption, and ultrafiltration) and disinfection (chlorine dose) conditions representative of full-scale water treatment facilities.
Based on the literature reports of mechanisms involved in chlorination process and in light of previous observations during chlorination of natural waters, a comprehensive mechanistic model was developed. The mechanistic model simulates DBP formation as a set of reactions between chlorine, bromine, and natural organic matter (NOM) as represented by three classes of reactive-sites: instantaneous, fast, and slow reacting NOM sites. Mechanistic models were parameterized for kinetic constants, and NOM reactive site concentrations optimized to fit observed data. It was concluded that chlorine decay and DBP formation were correlated, and less than 5% of the DOC concentration was present as chlorine reactive and DBP precursor material. The mechanistic models were coded into the existing USEPA Water Treatment Plant Simulation model (Version 2.1).
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