Effectiveness of Secondary Digesters as a Pathogen Controller in Winter

A revision of the Regulations regarding the use of sewage sludge in agriculture has forced a change in emphasis in sludge treatment from stabilisation to achieving pathogen kill. Previous work carried out by United Utilities Service Delivery highlighted temperature as being a key factor in pathogen kill in mesophilic anaerobic digestion and a possible important factor in pathogen destruction in secondary digestion, which is normally carried out in large open tanks where temperature control is not possible. It was found that temperature is a key factor in the rate of pathogen de-activation under isothermal conditions. However, in the full-scale secondary digester trial under winter conditions it was found that there was a significant difference between the surface temperature and the bulk temperature within the sludge; at the surface the temperature decreased more quickly. There was no significant difference between the performance of the 'old'and 'new'design of concrete tank with respect to wall thickness, cooling rates and E. coli reduction rate. The glass-coated steel-tank design showed a marked increase in the cooling rate and, despite the low ambient temperatures and fluctuating sludge temperature, a minimum 1-log reduction in E. coli was achieved.