This study investigated the regrowth of total coliform, Salmonella, and Clostridium perfringens in anaerobically digested sludge after centrifuge dewatering in the presence and absence of daylight. Sludge cake and centrate samples were collected from a treatment plant, and half of the samples was stored in daylight and the other half was stored in dark for three weeks. The bacteria levels in the cake and centrate samples were measured periodically throughout the storage period, and all three bacteria showed substantial regrowth. Presence of daylight increased the regrowth of Salmonella both in sludge cake and centrate, and increased the regrowth of total coliform in centrate. Salmonella exhibited the highest regrowth rate in cake among the three bacteria tested both in the presence and absence of light. Daylight did not appear to have a significant impact on the regrowth of Clostridium perfringens in cake and centrate, and on the regrowth of total coliform in cake. This might, however, be caused by the masking effect of the higher initial numbers of these bacteria in the samples. There is need for more research to thoroughly understand the effect of daylight on the regrowth of sludge bacteria.

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