The Dewaterability of Disintegrated Sludge Samples Before and After Anaerobic Digestion

Abstract

Sludge pretreatment systems emerged during the past few decades to reduce the ever-increasing quantities of sludge. A variety of different techniques have been studied for their effectiveness in solubilizing the chemical oxygen demand (COD) and increasing the digestability and decreasing the final quantity of sludge. In this study different pretreatment techniques and their effects on dewaterability were investigated at two stages of treatment. First, dewaterability was measured as capillary suction time (CST) right after the application of pretreatment; second, the dewaterability of pretreated and subsequently digested sludge was measured at the end of the anaerobic digestion period. The pretreatment techniques applied were acid and alkali treatment, ultrasonication, microwave (MW) treatment, and a combination of MW and alkali treatment. For all the methods tested, it was found that CST value deteriorated after pretreatment compared to untreated samples. From all the methods studied, acid treatment influenced the CST the least, whereas alkali treatment influenced it the greatest. Sonication and MW had similar intermediate effects. MW treatment helped improve the high CST values caused by the alkali treatment when the two methods were combined. In the second stage of the study, the pretreated samples were anaerobically digested and the CST values were measured afterwards. This time a completely different line of results was observed; there was almost no difference in CST values following digestion regardless of the method used. This indicated that anaerobic digestion process has such a dominant impact on sludge dewaterability that the significant effects created by different pretreatment techniques prior to digestion were dampened and the CST values were brought down to a narrow range following digestion in which the control and pretreated sludge had almost equal CST values.