Growth potential: a new method for estimation of the risk of E.coli regrowth in digested sludge cake

Sprigings, A. and Le, M.S., United Utilities Group PLC, UK



Currently a significant challenge facing sludge undertakers is the resurgence of E. coli in digested cake. The re-appearance of the bacterium after the centrifugation of digested sludge has caused concerns among operators because E. coli is commonly used as an indicator of the health risks of the treated sludge. Whilst current guidelines, the Safe Sludge Matrix, stipulates a log reduction and maximum count of E. coli and Salmonella (1/g DS) of treated sludge, they do not account for the re-growth of E. coli following de-watering, possibly due to the increased nutrient levels as the result of a high shear operation. Clearly, for more effective management of the pathogen issue, there is a need to determine the risk of E. coli re-growth.

Growth Potential (GP) has been developed as a novel approach to risk quantification. It takes into consideration the initial population of E. coli in the treated sludge, the available nutrients and the cake storage condition. It simplifies E.coli re-growth monitoring negating the need for protracted, intensive sampling regimes. The data produced will allow operators to determine whether a dewatered product will remain compliant with the HACCP and SSM standards, demonstrate the effectiveness of new unit operations and process changes on E.coli re-growth and inform management if certain sites are at risk and point the way to the effective remedial action (GP reduction vs. cost analysis).

Key words:
biosolids; centrifuge; digestion; E. coli; re-growth; sewage sludge; growth potential


UK sludge regulations

The European Union is responsible for producing 9.4 million tonnes of sewage sludge (biosolids) per annum. Just over 50% of this is treated to suitable standards for use in agriculture. Regarded as the best practicable environmental option (BPEO), agricultural recycling is not only economical when compared to other disposal routes such as incineration; it is also the most environmentally sustainable. Indeed, the benefits of utilising biosolids as a soil improver include the recycling of nutrients such as N, P and K and improved soil structure and water retention (Wallace et al., 2009). However, despite its environmental credentials, agricultural disposal comes under pressure from consumer groups and the public due to faecal aversion, objection to odour and fears regarding the contamination of grazing land and food crops (Le, MS, 2007). This has in turn led to strict regulation of the practice.

In the UK, biosolids bound for agriculture must conform to standards stipulated in the Safe Sludge Matrix (SSM) and the hazard analysis critical control point (HACCP) approach (Barrel et al., 2003). Both standards use the indicator species E.coli to demonstrate the presence of pathogenic bacteria and their removal. The use of indicator species is attractive because it reduces the cost and complexity of analysing biosolids for individual pathogens. E.coli was chosen as an indicator species because they are initially abundant in raw sewage sludge and there is a strong correlation between the presence / absence of the indicator and faecally derived pathogens. It is part of a wider group of organisms known as faecal coliforms, which are used in EPA methods to indicate faecal contamination. The enumeration of E. coli by either Membrane Filtration (MF) or the Most Probable Number (MPN) technique is relatively rapid and cost effective. It is important to note that the environmental strains of E.coli cultured during these tests are utilised as indicator species only and are not in themselves pathogenic. Using E.coli and Salmonella, the SSM defines two classes of biosolids- ‘conventionally treated’ (Class B) and ‘enhanced treated’ (Class A) which have different land application protocols based on the perceived health risks. Conventionally treated biosolids require a 2 log (99%) removal of E.coli and a maximum allowable concentration of 100,000 E.coli / g DS. To achieve enhanced treated status, tests must demonstrate that a 6 log (99.9999%) reduction in E.coli has been attained with a maximum count of <1000 E.coli / g DS and the absence of Salmonella in the final product. To ensure these standards are met the HACCP approach identifies critical control points (CCP) throughout the sludge treatment process which are monitored and controlled to ensure the required pathogen reduction has been achieved. Samples which fail the standards are known as permitted concentrate value (PCV) failures.

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