Autothermal thermophilic aerobic digestion (ATAD) Part I: Review of origins, design, and process operation

Increased legislative constraints have fuelled an interest in developing sustainable and economical methods for sludge digestion. Autothermal thermophilic aerobic digestion (ATAD) is a robust process that produces Class A biosolids from a wide range of organic sludge (e.g., animal waste, sewage sludge, food processing waste etc.). The advantages of this technology include good biomass biodegradation, pasteurization and process stability. Thermophilic temperatures result from the metabolic heat released by microorganisms during digestion. Efficient aeration and mixing are needed in addition to adequate reactor insulation to maintain thermophilic temperatures. Significant advances have been made in the optimization and adaptation of ATAD technology since it was first introduced in the early 1970s. Continuing innovation and advancement of the process is reflected in the number of patents for "next" generation technologies. Despite the apparent benefits of this process, ATAD is still not well understood. This article seeks to establish the existing state-of-the-art for the ATAD process. Information from a wide range of sources is presented to provide an insight into the key issues, discuss some of the advantages and perceived disadvantages, and list some of its operating limitations.