Biogranules developed in the upflow anaerobic sludge blanket (UASB) reactors were analysed by Xray spectrometry for the local concentration of individual inorganic elements and by X-ray dot mapping for the pictorial distribution of these elements in the cross-section of the biogranules. This information is of significance for the understanding of the sludge granulation mechanism and the role of inorganic elements. WTRODUCTION Among the high-rate anaerobic reactors developed in recent years, the upflow anaerobic sludge blanket (UASB) reactor (Lettinga, et al., 1992; Fang aud Chui 1993;Fang, et al., 1995) has probably received most commercial interests, especially in Europe and, more recently, in Asia. In a UASB reactor, bacteria aggregate to form biogranules which have high activity as well as superb settleability.
However the granulation mechanisms, particularly the effect of inorganic elements have not been fully understood. Lettinga et al. (1980) reported that high levels of Ca", Mg'+, and BaZC in wastewater bad a positive effect on the granulation of anaerobic sludge, probably because their presence enhanced the mechanical strength and the settleability of the granules. Guiot et al. (1988), on the other band, found that the presence of Ca" did not enhance sludge granulation, but the presence of other trace metals 7 such as Fe2+ , Ni", Co '+ and Mn2+, did. Dohing et al. (1985) found about FeS contributed to 30% of the ash content of UASB granules, and, thus, hypothesized that FeS might also play a role in the granulation.