Introduction Post Aerobic Digestion (PAD) is a recently developed advanced digestion process, where an aerobic digestion step with a five to seven day solids retention time (SRT) is designed to follow anaerobic digestion (schematically shown in Figure 1). The PAD reactor directly treats the effluent solids flow from the anaerobic digester, as either a mainstream or slip-stream process. A cyclic aeration schedule can be utilized for both nitrification and denitrification with ammonium oxidizing bacteria (AOB) and heterotrophic bacteria, respectively. Denitrification is driven by the oxidation of slowly oxidized compounds as well as the degradation of products previously identified as unbiodegradable (Friedrich, Takacs, & Tranckner, 2016). The most significant reason for implementing PAD is the reduction of nitrogen recycled back to the liquids stream without a need to add supplemental carbon. Other benefits include volatile solids reduction, odor reduction, struvite stabilization, and possible dewatering improvements.
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