Two large scale projects have been successfully brought on line utilising the thermal pretreatment to ensure production of a Class A biosolid product. In Reading (UK), the sludge is pasteurised prior to mesophilic anaerobic digestion while in Dublin (Ireland), the sludge is both pasteurised and hydrolysed prior to digestion.
Sludge is pasteurised by holding a batch of sludge at 70°C (158°C) for greater than 30 minutes. Sludge is pasteurised and hydrolysed thermally by holding a batch at 165°C (329°F) for 24 minutes at 6 bar pressure. The combination of the high temperature during the cycle and the pressure drop at the end of the cycle breaks open cells making the sludge more readily biodegradable.
Sludge pasteurisation was chosen for Reading to achieve a Class A product and to minimise sludge storage (holding) times on a plant in a sensitive area. We expected improved volatile destruction and dewatering but to date have seen no evidence of either of these.
Thermal Hydrolysis (TH) was chosen for Dublin to minimise the amount of sludge that had to be dried and to reduce the size of the anaerobic digesters.
The purpose of this paper is to compare the two approaches. Both systems use batch reactors to treat the sludge and both systems control the digester temperature.