Strength of natural soil flocs

To obtain the strength of flocs against breakup is crucial for controlling flocculation in water treatment and predicting transport of colloidal particles in aqueous environments. Recently, the author reported a method to obtain floc strength from a simple experiment of floc breakup subjected to a laminar converging flow. In this study, this method was applied to natural soil flocs. The flocs were formed by coagulation with 0.5 M NaCl (pH 5.4–5.5, pH 6.6) solutions, 0.1 M CaCl2 (pH 6.4–6.9) solutions, or acidified distilled water with dilute HCl (pH 5.6). Obtained floc strengths were 0.3, 0.7 and 4 nN for Na-, Ca-, and H-coagulated flocs, respectively. Also, floc strength did not change with floc size. These values of floc strengths were 1–3 orders smaller than those of flocs formed with polymer flocculants and/or precipitated ferric or aluminum coagulants.