[WATER] Competitive exclusion as a means to reduce E. coli regrowth in digested sludge

The recycling of sludge to agriculture is regarded as the Best Practical Environmental Option in Europe. However, there are many challenges with the practice with concern about the risk of pathogen transfer to farm produce being a major issue. In recent years, it has been reported that numbers of Escherichia coli (E. coli) increase significantly following centrifugation. This is known as the E. coli regrowth phenomenon and it is of serious concern because E. coli is used as an indicator of the microbiological quality of sludge-derived products and compliance failure of the quality standards would result in higher disposal costs and loss of consumer confidence in the industry. It has been proposed that the apparent sudden increase in E. coli numbers in sludge subject to centrifugal dewatering is caused by reactivation of viable but nonculturable cells. Subsequent rapid growth of E. coli in the first few days of cake storage following dewatering is likely to be caused by the combination of a ready supply of nutrients and a lack of microbial competition for those nutrients. The hypothesis to be tested in this research is that the regrowth of E coli in stored sludge can be reduced by the application of the competitive exclusion principle; i.e. by competition between E. coli and fast-growing non-pathogenic bacteria that will be introduced to the dewatered sludge. The principle of competitive exclusion has been used successfully as a means of infection control for many years in the food industry. In this research we aim to develop inocula of differing microbial community composition and evaluate their ability to suppress the growth of E. coli in digested sludge. Objective 1: Test the competitive exclusion concept To test the basic premise of the research, a controlled and replicated bench-scale experiment will be designed in which sludge samples are sterilised and then re-inoculated with E. coli and competing microorganisms. In this experiment the E. coli will be challenged with inocula which present differing degrees of competition for available resources. Where the E. coli are presented with no or limited competition we would expect an increase in population. Where E.coli have to compete with a diverse and well adapted community for available resources we would expect population growth to be slower or absent. Objective 2: Optimise the competitive exclusion product Once the competitive exclusion concept has been successfully tested a competitive exclusion product will be optimised. We will investigate: (i) What is the best source of the challenge inoculum - for example from soils or from digested cake itself. (ii) How big does the competing population need to be? Is a seed population sufficient to compete with E. coli? Or is it necessary for the competitive exclusion product to reach a certain size before it is capable of competing? (iii) How important are key characteristics of microbial community structure of the competitive exclusion product. Measurements of the structure of the microbial community will be made by means of phospholipid fatty acid (PLFA) phenotypic profiling. Objective 3: Scale-up Once proven on bench scale, the culture work will be scaled up using a pilot-scale fermenter. Should the results of this pilot-scale work be promising there will be the opportunity to trial the process at full scale at a United Utilities facility.