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The goal of the research was to investigate various types of polyaluminum (pACls) chlorides with respect to treating a high high color, low aftalinity water supply. Alum was used as Toc, a reference coagulant and PACIs of various basicities were studied. pACls with and without sulfate were examined. Research objectives included: 1) to evaluate alum and various pACls by conventional clarification (i.e., sedimentation) and by dissolved air flotation (DAF),2) to examine the effect of water temperature on the effectiveness of alum and the p and 3) to Acls, develop a testing procedure for the evaluation and selection of pACls for treating a high TOC, high color, low alkalinity supply. The testing procedure for evaluation and selection of pACls is addressed in the previous section. g.re, key findings are summarized for the other objectives.
RAW WATER QUALITY AND CHARACTERIZATION Ashley a water supply for Pittsfield, MA, was used as the model water. Reservoir, Key raw water quality characteristics of this supply are its low alkalinity (< 30 mg/L CaCO3), slightly acidic pH (6-7), low turbidity (0.65-1.7 NTU), high true color (50-80 pt:co), high roc (6-8 mg/L), high DoC (6-7.4 mglL) and high UV absorbance at 254 nm (0 3i.- t;. it . 23-0 nature of the natural organic matter (NOM) is easily determined through calculation of the SUVA (specific UV absorbance), which is the raw water UV expressed in m-r divided by the raw water DOC concentration in mg/L. SUVA values for this model supply were 3.g to 4.g indicating that the NoM is composed mostly of aquatic humic m atter. This leads to the following important findings that apply to this and simllar waters. c The high humic content (SUVA > 4) and high ToC means that the NoM controls coagulant dosages for effective treatment of these type water s upplies. o { stoichiometric relationship exists between the o ptimum coagulant dosage and raw water TOC or UV. Specific relationships were presented in Table l0 at pH 6.3-6.5, and were generally 0.7-l mg Al per mg Toc or 15-20 mg Ar pe, cm-r of UV. These relationships should yield good estimates of coagulant dosing ranges that can be used for jar tests, pilot studies, and full-scale plant triais. COAGULATION AT pH 6 TO 7 Coagulant Chemistry
The best pH conditions to treat this water type by coagulation are between pH 6 and 7, This pH range takes advantage of the chemical speciation of positively charged Al of alum and the PACIs as well as Al solubility considerations, charge on th. dissolved NOM, and floc charge following coagulation. Water temperature affects speciation and solubility, especially for alum' Alum should be used between pH 6 to 6.5 for warrn water conditions tr"y t 10 "Cj and at pH 6.5 to 7 for cold waters (say < 10 "C). PACI solubility depends on pH,in. typ. of pACl (basicity), and water temperature. Their minimum sotuiitity lies betw;en pH 6 and. l depending on PAC1 type and r.vater temperature (Appendix A). Below pH 6 to 7, they can be quite soluble so charge neutralization (chemical reaction with negatively charged dissolved 3l
NOM) can be effective. Overdosing, however, can lead to poor of treatment particle separation and residual Al problems.
Coagulation at higher pH than 6 to 7 is unfavorable for treating this water type. Higher dosages are needed, and lower removals of NoM (Toc, Doc, uv) would be found.
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