Grantee Research Project Results
1999 Progress Report: Mechanistic-Based Disinfection and Disinfection Byproduct Models
EPA Grant Number: R826831Title: Mechanistic-Based Disinfection and Disinfection Byproduct Models
Investigators: Westerhoff, Paul , Amy, Gary , Reckhow, David A. , Chowdhury, Zaid
Institution: Arizona State University , Malcolm Pirnie , University of Colorado at Boulder , University of Massachusetts - Boston
EPA Project Officer: Hahn, Intaek
Project Period: December 15, 1998 through December 14, 2001
Project Period Covered by this Report: December 15, 1998 through December 14, 1999
Project Amount: $339,583
RFA: Drinking Water (1998) RFA Text | Recipients Lists
Research Category: Drinking Water , Water
Objective:
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 to facilitate the evaluation of DBP control alternatives. Accurate predictive models for DBPs can facilitate the evaluation of treatment alternatives for disinfection and DBPs. For this reason, the U.S. Environmental Protection Agency (EPA) has developed a water treatment plant simulation model (Harrington, et al., 1992) that incorporates the current state of knowledge for predicting DBP formation based on the water quality entering a treatment plant, chemical dosages applied at various locations within the treatment process, and the detention times in these processes. This model is used for conducting regulatory impact assessments in support of developing DBP regulations. However, the current DBP modeling approach is empirical-based, rather than mechanistic-based.
The goal of the proposed research is to develop and calibrate an accurate kinetic-based mechanistic model for several chlorinated DBPs of interest. The model will predict DBPs (e.g., four trihalomethane [THM] species [THM4] and nine haloacetic acid [HAA] species [HAA9]) as a function of dissolved organic carbon (DOC), disinfectant level (type and dosage), reaction time, temperature, pH, and bromide concentrations. The project has the following specific objectives:
- Compile existing databases on DBP formation experiments into a single Unified Database. Some data from the compiled database will be used to develop and/or verify mechanistic DBP prediction equations. Data deficiencies will be identified.
- Conduct controlled batch-scale experiments with raw/untreated water targeted at transform- ing the amount (mg/L) and chemical structure of natural organic matter (NOM)/DBP pre- cursors. Transformations in NOM properties will be quantified.
- Develop and calibrate numerical models for predicting the behavior of disinfectants (free-chlorine) and the formation of DBPs (THMs and HAAs). Controlled experiments will be performed to assess inorganic reactions, disinfectant decay, DBP formation, and DBP stability. Model parameters for DBP formation will be statistically compared against NOM properties.
- 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.
Progress Summary:
Progress on the project is excellent and no major obstacles have been encountered. This section contains a summary of the preliminary findings and their significance.
Objective 1?Several large database files have been compiled and represent our Unified Database. The Unified Database includes the following types of information: (1) water identification categories (source ID, type of treatment, and date); (2) water quality data (DOC, UVA, pH, temperature, ammonia, and bromide levels); (3) chlorination conditions (dose, reaction time, residual); and (4) by-product formation (individual and total THM and HAAs). The Unified Database contains more than 2,500 chlorination laboratory experiments and more than 500 sets of data from full-scale treatment plants, representing work in the United States, Canada, and New Zealand. An electronic file (Microsoft Excel) will be included with the final project report.
Objective 2?Raw/untreated water from two sources?Colorado River Water from the Central Arizona Project (CAP) and Lake Houston water (LHW)?have been collected (~ 120 gallons) and subjected to several parallel bench-scale treatment processes. These processes included: (1) no treatment; (2) continuous-flow ozonation to achieve a CT ~ 1 mg-min/L; (3) simulated enhanced alum coagulation (10 mg alum/mg total organic carbon [TOC]); (4) simulated enhanced softening (pH 11.0 with lime and soda ash addition); (5) activated carbon adsorption to remove ~ 50 percent of the DOC; and (6) low-pressure ultrafiltration. Water chemistry and NOM characteristics were measured before and after each treatment. NOM characterization included: DOC, UV adsorption at multiple wavelengths, 3-D fluorescence, percentage DOC as hydrophobic/hydrophilic/ultrahydrophilic, and molecular weight by size exclusion chromatography. Chlorination, plus TTHM and HAA9 by-product formation, under the following conditions and an orthogonal experimental matrix were assessed for the CAP water: pH 5.5, 7.5, and 9.0; ambient (80 mg/L), +100, and +500 mg/L bromide; temperatures of 2, 18, and 25oC. A reduced experimental matrix currently is being conducted on the LHW water sample. A third water source also will be treated and chlorinated, and probably represent a different geographic location (e.g., Harwoods Mill, VA).
Objective 3?Several modeling hypotheses were proposed as a basis for a mechanistic-based model for disinfectant decay and DBP formation. The central modeling hypothesis is that a two-site reaction mechanism can be used to predict disinfectant decay in the presence of NOM. It assumes that NOM contains both slow and fast disinfectant-reacting and DBP-forming sites. NOM site densities and concentrations are related to the concentration, size, structure, and functionality of NOM. A series of distribution functions, based upon the predicted ratios of free-bromine to free-chlorine, will be used to estimate each of the four trihalomethane species (TTHM) and each of the nine haloacetic acid species (HAA9). Computer optimization codes have been developed using Scientist to model the series of differential equations. Fitted rate constants have been obtained for the CAP experiments, and will be repeated for LHW and one other source. In addition, selected data from the Unified Database also are being parameterized in an attempt to capture a more diverse selection of waters. However, a critical factor for the model is short-term DBP formation (< 2 hours), and many of the data in the Unified Database represent timeframes of several hours to days. This was a significant deficiency in the database; therefore, a focus of our experiments was on short-term DBP formation.
Objective 4?We are in the first steps of developing a window-based computer interface program. Malcolm Pirnie, Inc., recently has completed updates to the EPA Water Treatment Plant Simulation Model, and aspects of that type of model will serve as the base structure for our advanced disinfectant and DBP model. A goal of this project is to develop a set of kinetic models for predicting the behavior of disinfectants and DBPs in water treatment systems and to incorporate these kinetic models into a computer program that can be used by researchers and utilities to predict disinfectant and DBP levels within their system. The model will include the following six basic modules: (1) user-interface module; (2) disinfectant consumption reaction module; (3) inorganic reaction module; (4) organic DBP formation reaction module, (5) DBP stability reaction module; and (6) numerical solution module. The user will be able to use either default model parameters, site-specific parameters, or estimated parameters based upon NOM characterization. In addition to the focal two-site reaction mechanistic model, other modeling forms also will be used to fit our data (power function models, chlorine consumption models, delta UVA models).
Future Activities:
Experimental work will continue over the next 9 months. Chlorination and DBP formation experiments will be completed for the LHW source. A third water source also will be subjected to the five treatment processes outlined above, and along with the untreated water sample subjected to subsequent chlorination and DBP analysis. Model parameterization will continue on the Unified Database and data collected from ongoing laboratory experiments. Work on the user-interface model will be initiated.Journal Articles:
No journal articles submitted with this report: View all 23 publications for this projectSupplemental Keywords:
drinking water treatment, environmental chemistry, oxidation, drinking water, DBP, disinfection by-product, modeling., RFA, Scientific Discipline, Water, Applied Math & Statistics, Environmental Chemistry, Mathematics, Analytical Chemistry, Drinking Water, monitoring, chlorine decay, oxidation, unified database, chemical byproducts, disinfection byproducts (DPBs), treatment, chlorine-based disinfection, chloramines, DBP risk management, water quality, drinking water contaminants, drinking water system, mechanistic-based modelsRelevant Websites:
http://www.eas.asu.edu/~civil/
http://www.ecs.umass.edu/cee/
http://civil.colorado.edu/
http://www.malcolmpirnie.com/Index.html
Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.