AWWA WQTC63968

A method was developed to monitor precision for heterotrophic plate count (HPC) using both blind duplicates and lab replicates as part of a project looking at pilot drinking water distribution systems. Precision control charts were used to monitor for changes in assay variability with time just as they are used for chemical assays. In adapting these control charts for the HPC assay, it was determined that only plate counts ¿¿¿¿¿¿¿ 30 cfu per plate could be used for Quality Assurance (QA) purposes. In addition, four dilutions were used for all known Quality Control (QC) samples to insure counts usable for QC purposes would be obtained. The method worked well for HPC but resulted in a 50% increase in the required labor for a given number of samples when blind duplicates and lab replicates were run in parallel with the samples. The methodology developed for HPC can be adapted to other biological assays for precision (for example, assimilable organic carbon - AOC). Includes reference, tables, figures.