INTERPRETING NEGATIVE VIRUS RESULTS FROM HIGHLY TREATED WATER

Highly treated waters often contain viruses at concentration levels to o low to detect. This paper describes a method for drawing inferences about the concentration of viruses in treated water when very few or n one are detected in collected samples. The approach uses a volume-vary ing version of the negative binomial distribution to construct one-sid ed confidence intervals about the mean virus concentration. Informatio n on the presence of viruses prior to treatment (i.e., when densities are high enough to measure) is used to estimate parameters for the mod el. Samples are provided based on measurements from three southern Cal ifornia wastewater-treatment plants. Based on 93,679 L of tertiary tre ated sewage effluent, confidence intervals for virus concentrations fo r Pomona, Calif., ranged from 0 to 2.68.10(-5) plaque forming units (P FU) per liter at the 50% confidence level to 0 to 5.83.10(-4) PFU/L at the 95% confidence level. These values are considerably above the ris k-based public health goal that is being used to regulate viruses in d rinking water (2.22.10(-7) PFU/L). Direct measurements to demonstrate treatment to drinking water quality are therefore impracticable.