Fig. 2. Scanning electron microscope (SEM) image (upper left) and elemental (P, S, Ca, Fe, and Al) dot maps of alum-stabilized biosolids. The color intensity (white to black, or yellow to red, pink, blue, and black in the colored figures published online) reflects elemental concentration from high to low. The particles of (a) Ca-P; (b) Al-P; (c) gypsum; and (d) a probable residual CaCO₃ are shown on the SEM image.