3.1.1. Flocculation region
Samples of flocculation experiments were classified into four categories: (a) clear liquid samples with no perceivable hydrolysis; (b) turbid samples with obvious hydrolysis but no visible flocs; (c) supernatant samples with RT > 5 NTU because of unsatisfactory flocculation; and (d) supernatant samples with RT < 5 NTU resulted from good flocculation. The data of good flocculation (category d) defined the flocculation region of Ti(SO4)2 in Fig. 1. The flocculation region of aluminum salts summarized by Amirtharajah and Mills (1982) was also drawn (shaded area) for comparison, using arithmetic scales for the vertical concentration axis. Fig. 1 illustrated that at almost all pH values titanium sulfate hydrolyzed readily when dosages exceeded 0.17 mmol/L, and pH 4–6 was the most effective flocculation range. Good flocculation could even be effected when dosages exceeded 0.03–0.04 mmol/L at about pH 5. The data showed clearly that Ti(SO4)2 flocculated in a narrower and more acidic pH environment than aluminum hydroxides. The flocculation region of titanium sulfate coincided with the 5.3–5.7 pH range where enhanced coagulation (Amirtharajah and O’Melia, 1999 and USEPA, 1999) was usually practiced to remove natural organic materials (NOM). It was suggested that Ti(SO4)2 might be effective for NOM removal.