Abstract
Several chemical reactions controlling the alkalinity of solidified/stabilized sludge using a low alkalinity additive (mainly composed of gypsum and slag) are investigated experimentally. The influence of curing conditions (open air or sealed) on the alkali leaching characteristics of the treated sludge was evaluated from the viewpoint of carbonate uptake and leaching properties of cations (Ca, K, Na, Mg, Al and Si). Modified batch leaching tests were conducted for the treated sludge cured in both open air and sealed conditions to characterize the alkali and cation leaching behaviour. Mineralogical characterization was performed through X-ray diffraction (XRD) analysis, and the Calcimeter test traced the variation of carbonate amount in the treated sludge under curing. In addition, the type of hydration products in treated sludge was also estimated by the acid neutralization capacity (ANC) tests. Experimental results showed that open air curing has an effect to decrease the pH of solutions from batch leaching tests compared to sealed curing, since the sludge cured in open air contained more carbonate and less hydroxide than those cured in sealed condition. Promoting the carbonation of Ca(OH) 2 by contact with carbon dioxide could reduce the alkalinity of treated sludge. In addition, the amount of magnesium carbonates (e.g. MgCO 3 ) and the fraction of Mg ions in treated sludge could affect the pH of leached water.
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