Abstract Chemical and physical characterization methods were used to analyze ferric, alum, and lime water treatment residual solids (WTRSs) in order to describe why phosphate or arsenate adsorption occurred on the WTRSs, and why ferric WTRSs were the stronger adsorbent for both phosphate and arsenate. In total, five WTRSs, two ferric, two alum, and one lime, were analyzed. Elemental analysis of the WTRSs showed lime residuals contained the greatest molar amount of the primary element (7.04mol Ca/kg solid), followed by the ferric residuals (4.86–4.96mol Fe/kg solid) whereas alum residuals contained the least amount of primary element as compared to the ferric or alum residual solids (3.62–4.67mol Al/kg solid). Mercury porosimetry identified more small pores (<0.006μm) in a ferric WTRSs when compared to an alum WTRSs, indicating that a more detailed pore structure allowing for intraparticle phosphate or arsenate diffusion might be present in the ferric solid. Similarly, SEM images at 1000 times magnification showed a porous surface in both ferric WTRSs, whereas the alum WTRSs showed a smooth surface at the same magnification. Several general equations to describe phosphate or arsenate adsorption on WTRSs were provided.