Autothermal thermophilic aerobic digestion (ATAD) - Part II: Review of research and full-scale operating experiences

Autothermal thermophilic aerobic digestion (ATAD) is an exothermic process where sludge is subjected to temperatures greater than 55°C for at least 4 hours, over 6-10 days. Organic solids are degraded and the heat released during the microbial degradation is used to bring the process temperature within the thermophilic range. It produces a biologically stable product, achieving a reduction in biomass, while using smaller digesters, compared to mesophilic aerobic and anaerobic digestion. There are no regulatory requirements in North America and Europe for the reduction of the volume of total solids in sludge processing. However, a reduction in the volume of material for final disposal has cost benefits. By virtue of the residual mass, volume reductions are easily made through dewatering or dehydrating steps following ATAD. Despite the apparent advantages of ATAD, limited information on the process is available in the literature. Concerns still exist about documented cases of odour issues, problems with sludge dewaterability, foaming, excess use of polymers and high-energy consumption. This article presents some relevant bench-scale and pilot ATAD study data, with appropriate discussion. It also assembles information from a range of sources and provides an insight into actual application and experiences with full-scale ATAD.