Application of Surrogate Measures to Imporve Treatment Plant Perfomances OBJECTIVE: Evaluation of the use of microbial parameters for improving water treatment plant performance has been undertaken in this study. This study yielded information regarding the effectiveness of (1) using surrogate organisms (total and fecal coliform, E. coli, heterotrophic plate count bacteria, aerobic spore-forming bacteria [Bacillus], anaerobic spore-forming bacteria [Clostridium], and two bacterial viruses-somatic and male-specific coliphage) as indicators of Giardia and Cryptosporidium in source water and filtered water, and (2) using surrogates as treatment optimization tools to assess treatment plant effectiveness. BACKGROUND: The challenge associated with producing the best possible drinking water quality-even at well-operated water treatment plants-is associated with the difficulties in monitoring for microbial pathogenic organisms such as Giardia, Cryptosporidium, and enteric viruses. The need for measures that water utility operators could use to determine how well the plants are optimized for the removal of pathogens is paramount if these plants are to achieve maximum treatment effectiveness. HIGHLIGHTS: Although no "ideal" surrogate for predicting occurrence of the target pathogens could be identified, the study found that aerobic spores and particle counts can be used as surrogate measures to evaluate or improve treatment plant performance. Examples and suggestions for selecting surrogates are presented in the report, and tips on when to use removal calculations versus absolute treatment values are also provided. APPROACH: The approach used to meet the project objective consisted of four tasks: Selection of treatment plants representing different geographic regions, treating different quality surface water, and employing different treatment processes and treatment chemicals. Identification of candidate pathogen surrogates that represent target pathogens in removal through treatment. Advanced characterization of source and treated water quality to develop relationships between removal of conventional water quality parameters (turbidity and particle counts), microbial pathogens, and the potential pathogen surrogates. Selection of the candidate pathogen surrogates based on their performance in characterization of plant performance, analytical simplicity, turnaround time, and costs. Twenty-four utilities participating in the study provided historical records, on-line monitoring data, and routine water quality analyses. A comprehensive matrix was developed to relate water quality and process variables for the participating plants and possible surrogates to the target pathogens.