Reducing the numbers of pathogenic bacteria is one of the chief objectives when treating
sewage sludge for land applications. There is currently some uncertainty as to exactly when
and where wastewater operators should monitor for E. coli. If the Safe Sludge Matrix
eventually gets on the statute book and the end product is clearly defined as the material
leaving the site, this would have serious implications for wastewater operators. Whether the
E. coli increase after centrifugal dewatering is due to reactivation or re-colonization, it can be
expected that digested cake will frequently have a higher E. coli content than digested liquid.
Operators should consider how their processes are affected by such increase and consider
potential mitigation strategies.
Real time quantitative PCR (RT-qPCR) protocol for E. coli enumeration provides an effective
tool for E. coli investigation. It can provide an alternative method for studying the changes in
E. coli number across a sludge digestion process. It also offers a means to detect and study
any possible reactivation potential. In this study a qPCR protocol has been developed for the
detection and quantification of E. coli from sludge samples. Although relatively quick and easy
once optimised (around 7 hours total turnover time) the need for skilled staff and the costs
involved in equipping a lab for molecular work means that RT-qPCR is not necessarily
suitable for routine monitoring at present.
Further work is required to understand the mechanisms involved in the changes of E. coli
population numbers in the sludge digestion process, particularly as a result of dewatering of
the digested sludge by a centrifuge. Molecular methods can potentially provide a wealth of
information to aid the rational design of novel treatment processes. In order to maximise this
potential it will be necessary to close the gap between fundamental research and industrial
application. Closer collaboration between microbiologists, chemists and engineers and the
application of research to specific industrial problems would be one way to achieve this.
E. coli; Regrowth; Reactivation; Pathogens; qPCR
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