Sources and ecology of Escherichia coli in pulp and paper mill biosolids

Abstract

Pulp and paper mills generate biosolids as a by-product of their effluent treatment systems. These biosolids show excellent potential as a soil conditioner. However, the detection of high levels of E. coli (10 2 to 105 CFU/gdw), in the absence of any significant fecal loading, has caused concern among effluent treatment system operators, land applicators and regulators. This research examines the sources and ecology of E. coli strains comprising the population of E. coli in the biosolids of an eastern Ontario pulp and paper mill by applying the molecular microbial source-tracking tool repetitive sequence-based polymerase chain reaction. Confirmed E. coli were successfully isolated from 2 forested sites, treated mill process feed water, storm effluent, mill fibres, wood chips, primary and secondary clarifier effluents and sludges, and biosolids. While laboratory isolates could be accurately distinguished from forest and industrial isolates, cluster and jackknife analyses were incapable of differentiating reliably among isolates from the forest, on-site inputs, effluent treatment system and biosolids (average rates of correct classification values ranged from 43.9% to 72.9%). No input could be excluded as a source of E. coli to the effluent treatment system. Many fingerprint types were unique to the primary and secondary clarifiers; however, only a subset of the fingerprint types recovered from the primary and secondary clarifiers were detected in the biosolids. This suggests that the E. coli strains found in mill biosolids were those able to survive the hot, desiccating conditions of dewatering. Highly similar fingerprint types (> 80% similarity) were recovered repeatedly over an 8-month period from the effluent treatment system (primary and secondary clarifiers) and biosolids. These results are consistent with growth within the effluent treatment system and biosolids, rather than fecal loading. Over a 3-year period the pulp and paper mill effluent treatment system underwent large-scale changes, including a 50% reduction in effluent volume upon the elimination of on-site pulping activities. New fingerprint types were detected in the biosolids following the restructuring, and the Shannon index of diversity increased. Disruption of established operating conditions could have opened niches, allowing new strains to colonize the mill effluent treatment system successfully.