Evolution of TPAD Systems in the US

Temperature Phased Anaerobic Digestion (TPAD) systems evolved in Germany starting around 1990 for wastewater sludge processing and then in the US within the 1990s and more recently. This paper reviews that history and presents some of the more significant TPAD projects, including those that have involved Class A biosolids configurations. Sludge heating to the thermophilic temperature range (typically low-50s up to 60 C range) is required, as well as a cooling step to bring the temperature down for the mesophilic phase (typically 35 to 40 C). Recent and advanced research on the process continues to show advantages of increased volatile solids reduction and greater gas production within total TPAD system SRTs, compared with similar conventional mesophilic digestion SRTs. Another key benefit/advantage of the process (over conventional mesophilic digestion) is to improve the stability of the digested material, and therefore the dewatered product stability/odor level.