Microstructure and leaching characteristics of sludge treated with low alkalinity additives

Abstract

The prospect of reusing sludge treated by solidification/stabilization prompts this technique and makes it attractive. However, the reuse of solidified/stabilized sludge must deal with the increasing concern of environmental impact in terms of heavy metals leaching, alkaline migration control and acid neutralization. This paper presents the results of an experimental study to investigate the suitability of a new low alkalinity additive (mainly gypsum and polymer). Assessment of two types of treated sludge was conducted through unconfined compression strength (UC) test, microstructure analysis (X-ray diffraction, scanning electron microscopy), pH test, acid neutralization capacity (ANC) test and chemical analysis for Pb leaching. Results of UC test and microstructural analysis show a solidification effect on the treated sludge with the development of new components, mainly hydrated gypsum, and to a lesser extent, hydrated cement products. pH variation measured during 28 days indicates the effectiveness of treatment by a low alkalinity additive. pH values were found to be lower than 9 and influenced by the type of sludge and curing conditions (with and without air contact). ANC was found to be different for raw sludge and was enhanced by the treatment especially in the case of sludge treated with the highest amount of additives. ANC appears to involve dissolution of Ca, Mg, Al, Na and Si containing components. Although an increase of Pb release was observed with the increase of acid addition due to matrix dissolution and pH drop, measured concentrations were seriously reduced after treatment by the additive in treated sludge.