Thermal hydrolysis of raw sludge prior to mesophilic digestion is a process that has been successfully used for more than a decade. In this period it has proved to be a reliable tool for the waste water plant designer. When used as a pre treatment of mesophilic digestion, it provides increased volatile solids reduction, higher gas yields, better biosolids dewaterability and reduces the required mesophilic digestion volume.When initially deployed, the technology was very much a "Black Box" and limited information was available to allow optimum integration of the technology within the overall design of a waste water plant. This paper outlines how improved knowledge of sludge rheology, a better understanding of the steam requirements of thermal hydrolysis over the treatment cycle and a better understanding of the heat balance round the thermal hydrolysis process has allowed better integration of thermal hydrolysis within the overall works design. The paper illustrates how this knowledge has been used in the design of a 121,000 tonnes/yr thermal hydrolysis plant for United Utilities at Davyhulme, Manchester in the UK.