Miller, K. J.1, Harris, J.2, Pawlett, M.2, Ritz, K.2, Tyrrel, S. 2 and Le, M.S. 1, 1United Utilities Group PLC, UK, 2Cranfield University(free)
Dewatering sludge cake by centrifugation often leads to a significant increase in E. coli immediately after the operation and quite substantially within a few days thereafter. It is hypothesised that the primary mechanism of this increase is through the release of nutrients during centrifugation, which are subsequently available to support such re-growth. This paper discusses the development of a microbial community comprised of competitor micro-organisms (probiotic culture) that when introduced into the residual sludge solids will rapidly reduce nutrient concentrations, thereby preventing E. coli re-colonisation of the cake. Initial screening of five sources, which included enzymic hydrolysis treated sludge, fresh and aged cake, a soil and Bacillus thuringiensis, was conducted to evaluate the ability of micro-organisms from each source to utilise sludge cake as a growth substrate. Following identification of a potentially suitable culture derived from the enzymic hydrolysate source, its competition potential was assessed in laboratory-scale experiments. E. coli re-growth in the presence of the probiotic culture was typically 100-fold less than in the control in a liquid sludge cake-extract medium. However where the medium had not had the solids removed by centrifugation, E. coli suppression was not observed. Phospholipid fatty acid (PLFA) analysis demonstrated that the resultant probiotic phenotypes were sensitive to the culturing conditions, offering the potential to generate prescribed phenotypic structures which are suppressive to E. coli re-growth. With further development it is likely that the present approach could be implemented on an industrial scale to suppress E. coli re-growth in digested sludge cake.
Keywords: Biosolids, centrifugation, E. coli re-growth, probiotic culture, sewage sludge
Introduction: In order to facilitate its transportation and storage requirements, digested sludge often undergoes dewatering, typically by centrifugation. This practice typically leads to a significant population increase in E. coli, a microbial indicator species used to demonstrate the presence of pathogenic bacteria. This increase has been found to occur immediately after centrifugation and quite substantially within a few days thereafter (Higgins et al., 2008). It is accompanied by a noticeable increase in odour, which has created considerable alarm among observers since it points to an increased pathogen risk. The mechanism suggested for the rise in E. coli numbers is re-growth due to nutrients being made available by centrifugal actions (Baddeley and Le, 2009).
It is hypothesised that by introducing a population of fast-growing, non-pathogenic microorganisms (i.e. a probiotic culture) into residual sludge solids it should be possible to limit the quantity of nutrients made available to E. coli, thereby reducing the opportunity for E. coli to colonise the cake. This research focused on identifying such a culture. A range of potential sources of probiotic culture was screened to evaluate their suitability. Phospholipid fatty acid (PLFA) analysis of the culture deemed most promising allowed an assessment of its microbial phenotypic composition. A series of experiments was then conducted using this culture to assess its ability to suppress E. coli re-growth in digested sludge cake.