Demonstrating Virus Treatment Efficiencies for Biosolids

Effective treatment of digested biosolids (sewage sludges), by heat or other means, is likely to reduce virus concentrations to below economically realistic detection limits. This paper describes how such data can be interpreted using a sample size varying version of the negative binomial distribution, derived as a Poisson stopped-sum of the logarithmic distribution. A dispersion parameter is determined from samples taken prior to treatment and then used to formulate one-sided upper confidence limits for the mean density of viruses after treatment. To illustrate the approach, a continuously mixed heat-treatment system with a mean retention time of 30 min is considered. Viruses were not detected after treatment at either 60° or 70°C. Confidence limits for mean virus densities after treatment ranged from 25.2 plaque forming units per g dry weight for treatment at 60°C with 95 confidence (41.3 g assayed) to 2.8 plaque forming units per g dry weight for treatment at 70°C with only 50 confidence (96.1 g assayed). Since viruses were not recovered after heat treatment at either temperature, this range of confidence limits was due to the desired statistical confidence level and the biosolids mass collected and measured rather than to treatment temperature.