Statistical Evaluation of Pathogen Inactivation for a Conventional Low‐Cost Technology Class A Biosolids Process

Statistical methods were developed for analyzing the results of a study of pathogen densities for sludge samples taken over the four stages of the solids processing trains (SPTs) operating at the Stickney and Calumet Water Reclamation Plants of the Metropolitan Water Reclamation District of Greater Chicago, Illinois (District). These methods also apply to pathogen density studies for other biosolids processes. Analysis of covariance models were used to estimate expected pathogen densities for individual solids processing stages. Cross‐validation was used to select appropriate analysis of covariance models. Nonparametric methods were used to estimate distributions for pathogen density reductions between solids processing stages and to assess the effect of hypothetical surges and expansions in initial stage pathogen densities on final stage pathogen densities. These statistical analyses demonstrate that the District's SPTs achieve target reductions in enteric virus and viable helminth ova densities with high probabilities. Furthermore, the District's SPTs would still meet U. S. Environmental Protection Agency Class A restrictions for these pathogens with high probabilities, even if the initial stage pathogen densities observed in the study undergo extreme hypothetical surges or extreme hypothetical uniform expansions, that is, exceptionally large isolated bursts of pathogens or exceptionally large sustained increases in pathogens in the feed to the SPTs.