Quantification of the exchangeable calcium in activated sludge flocs and its implication to sludge settleability

In this work the exchangeable Ca2+ in activated sludge flocs is quantified next to the Ca2+ present in the floc structure as enmeshed solid precipitates. For this, the Ca2+ concentration was analyzed in the ashes of sludge sampled during 1 year from an industrial wastewater treatment plant exhibiting considerable variations over time of the inorganic fraction between 10% and 70%. From these data it was deduced that the exchangeable Ca2+ concentration in the flocs amounts to 0.7 meq Ca2+/g volatile (organic) suspended solids. This finding was confirmed independently by three additional tests whereby the exchangeable Ca2+ was quantified to be in the range from 0.5 to 1.0 meq Ca2+/g volatile suspended solids by calculating the higher Ca2+ release from the sludge flocs in case they are exposed to a combination of Al3+ and a low pH (releasing both Ca2+ pools from the floc, i.e., exchangeable Ca2+ and enmeshed Ca2+ solids), compared with an exposure to a low pH only (releasing solely the Ca2+ solids from the floc). The quantification of exchangeable Ca2+ in activated sludge flocs is highly relevant because of the major role of Ca2+ as binding agent in the sludge floc formation, which is of prime importance for the good settleability and dewaterability of activated sludge in wastewater treatment plants. The sludge settling behavior over an extended period of 7 years at the mentioned industrial wastewater treatment plant in terms of the sludge volume index (SVI) is interpreted as function of the relative contribution of the two Ca2+ pools on the flocs. From this analysis, it is clear that both exchangeable Ca2+ and precipitated Ca2+ solids fractions play an important role in the settling properties of the sludge flocs, with an increasing importance of the exchangeable fraction at lower inorganic fractions.

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► The exchangeable calcium fraction of sludge was quantified to be 0.7 (±0.1) meq/g MLVSS.
► The enmeshed calcium solids fraction amounts to 40 meq/g MLVSS.
► Sludge settleability is interpreted in terms of both calcium pools.