Gravity settling and centrifugation increase the acid buffer capacity of activated sludge

Buffer capacity is a critical parameter in sludge management of domestic wastewater treatment plants that determines acid/base usage. It is here shown that gravity settling or centrifugation significantly increased the buffer capacity in the supernatant of the sludge. The sludge thickening considerably elevated the total alkalinity of the sludge from 16.0 to 31.5 mgCaCO₃ taking pH 5.0 for example with the sludge concentration times increasing from 2 to 20 times, while insignificantly affected the total acidity (initial 335.3 vs 240.2 mgCaCO₃ at concentration times of 10 considering pH increased to 11.0). These findings indicate that the inherent buffer in sludge can be released during sludge thickening and the primary component of this buffer is alkalinity. The released buffer may be correlated to a negative surface charge inside sludge flocs, as it consumed base in titration. The increased buffer capacity in supernatant could be due to the buffer released from bound water to free water, and in particular, from the release of interstitial water, an important part of the bound water. Further mechanism analysis suggested that the increased buffer capacity in thickened sludge could relate to extracellular polymeric substances, for which more studies are needed. Overall, this study for the first time reports that sludge thickening can change the buffer capacity of sludge, affecting the efficiency and acid/base usage of sludge treatment.