Getting the Bugs Out of Digestion and Dewatering with Thermal Hydrolysis

This paper reviews some of the work done over the last twenty years on the thermal pretreatment of sludge and particularly activated sludge. It is demonstrated that the optimum time temperature for thermal pre-treatment is 170°C for half an hour. Pre-treatment at 70°C has little or no effect on digestibility and dewaterability. The practical application of this approach in the Cambi process is demonstrated in terms of digestibility and dewaterability for the HIAS project in Norway, which has been operating 5 years. Results from 2000 for the Naestved project in Denmark demonstrate a marked improvement in the digestion of WAS. The results of land application and crop trials for hydrolysed digested cake are shown. It is concluded that the thermal hydrolysis process gives a very stackable and stable product that is a safe fertiliser for the future. This has led to the rapid adoption of the technology in Europe in the last 3 years in projects from 5 to 100mgd.