Solids retention time (SRT), so-called sludge age, is one of the design parameters in a continuously activated sludge process. Pan (2003) investigated the role of SRT in aerobic granulation in SBR, and found that the sludge age of 20 days or longer is favorable for the formation and maintenance of stable aerobic granules with good settleability and activity (Fig. 5.6). As noted by Liu and Tay (2002), in more than 100 years of
research history of the activated sludge process, aerobic granulation has never been reported in activated sludge processes operated in an extremely wide range of SRT, from several hours to hundreds of days. It seems reasonable to think that SRT would not be a triggering or inducing force for aerobic granulation.
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