Shifts in E. Coli and Total Microbial Community As a Function of the Sludge Treatment Process.

Sewage sludge is a valuable fertiliser and recycling to land is the most viable reuse option. However, sludge can contain enteric pathogens which may pose a threat to human health. Steps are taken to reduce pathogen risk, including digestion, composting, and pasteurisation. Sludge is then dewatered by centrifugation; however pathogen regrowth is often observed following this process. Regrowth may be due to contamination within the centrifuge, reactivation of viable but non-culturable (VBNC) organisms, or increased niche space and nutrient availability within the sludge post-treatment. Knowledge of  the population dynamics within sludge before and after treatment is required to fully understand mechanisms surrounding regrowth phenomenon. An experiment was designed to look at shifts in E. coli and total microbial community composition as a function of the sludge treatment process. Sludge was collected from 2 sites managed by United Utilities (UK) comprising conventional and enhanced treatment. Samples were taken at 3 stages of the treatment process, including raw, digestate, and dewatered sludge cake. Samples were screened for the presence of E. coli by extracting in saline solution and plating onto Sorbitol MacConkey agar. A colony phenotype characteristic of E. coli growth was observed in 280 isolates. The number of presumptive E. coli isolates was reduced through a series of biochemical tests including lactose fermentation, citrate utilisation and Indole production. 16s rRNA genes were amplified from the extracted DNA using bacterial and universal primers, and were subsequently sequenced to confirm the presence of E. coli. Further work will be carried out on these isolates to determine phylo-group distribution and growth characteristics in soil and sludge. The sludge microbial community structure will be further investigated, in an effort to link the presence/absence of specific species or total diversity of the sample to the emergence of particular E. coli strains.