This paper reviews the research some of the work done over the last twenty years on the thermal pre-treatment 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. Pretreatment 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 on Norway. The results of spreading and crop trials for Cambi digested cake are shown. It is concluded that the Cambi process gives a very stackable and stable product that is a safe fertiliser for the future.
Limitations on Dewatering.
It is well known from observations that activated sludge is very difficult to digest and dewater and has an adverse affect on digestion and dewatering when combined with primary sludge. A lot of work has been done on understanding the nature of water in sludge 1,2,3. It is generally assumed that there are different physical states of water. The aqueous phase is generally described as free water and bound water. The bound water needs a higher energy to be removed and some cannot be removed at all.
From a combination of size distribution analysis and electron microscopy Jorand et al 4 has suggested a model of floc structure which is made up of three basic elements (fig 1). The first level is composed of bacteria of average size 2.5 μ m. They are embedded in a gel like matrix of exoploymers, forming microcolonies of average size 13 μ m.. The microcolonies aggregate together to form microflocs with an average size of 125 μ m. Snidaro et al 5 showed that the microcolonies retained their integrity under 15 bar filtration and that microcolonies appear to be a waterproof structure. They concluded that these waterproof units within the sludge seem to be one of the main limiting factors in water removal efficiency. 'Reaching higher dry solids content can only be done with thermal process or drying beds .....or changing sludge structure.'