Characteristics of four alkaline biosolids produced from sewage sludge

Abstract

Four alkaline biosolids were produced from sewage sludge using lime, lime:sodium silicate, cement and cement:sodium silicate. X-ray diffractometry (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDAX), single chemical extraction and sequential extraction were used to determine the characteristics of alkaline biosolids and untreated sewage sludge. The results of extraction tests revealed that the percentages of organically-bound metals in untreated sewage sludge were in the order of Cu Pb Cr Zn. The percentages of exchangeable metals in four alkaline biosolids were also in this order, implying that the higher the affinity of heavy metals to organics in untreated sludge, the more unstable the heavy metals were in alkaline biosolids. This effect was attributed to the irreversible dissolution of organically-bound metals at very high pH during processing and air drying of sludge. Sodium silicate addition promoted alkaline biosolids drying due to the formation of silica:silicate gels which held free water in sludges. The alkaline biosolids had relatively high CaCO3 equivalencies, ranging from 32.6 to 42.1% and may serve as potential liming amendments for acid soils. The CaCO3 peaks (29.46° 2u) in XRD graphs for alkaline biosolids indicate the occurrence of carbonation, especially for lime:sodium silicate- and cement:sodium silicate-treated biosolids. The porousness of lime:sodium silicate-treated biosolids as shown in the SEM micrograph corresponds to its smaller density (0.58 g:ml), compared to lime-treated biosolids. The micrograph of cement:sodium silicate-treated biosolids shows the absence of needle-like ettringite which commonly appears in Portland cement hydration. © 1997 Elsevier Science B.V.