Study of removals of humic acid by orthokinetic flocculation and filtration

During disinfection, humic substances undergo oxidation and substitution reactions with chlorine and form trihalomethanes (THMs). Thus, humic substances, which mainly consist of humic and fulvic acids, are considered THM precursors. THMs are carcinogens, and a maximum contaminant level of 100 $\mu$g/l of such compounds in drinking waters was established by the Environmental Protection Agency (EPA). Many studies were conducted to examine the removal of humic substances by alum. Alum hydrolysis products undergo adsorption and charge neutralization with the functional groups of humic molecules (mainly phenolic and carboxyl functional groups). Evidence of surface complexation between alum hydrolysis products and such functional groups has been provided. Reported data also indicate that optimum alum coagulation occurs in the pH range of 5-6, and that the large molecular weight (MW) fractions are better removed by alum than the small MW fractions. However, the mechanism of alum coagulation of humic substances is not conclusively established. A pilot-plant of orthokinetic flocculation followed by filtration was studied to examine the removal mechanism of THM formation potential (THMFP) by alum, using humic acid as the THM precursor. Orthokinetic flocculation and clarification are accomplished in an up-flow filter of buoyant plastic medium. Alum was dosed to a rapid mix tank prior to the flocculation process. The THMFP test is complex. The correlations between THMFP and color, UV absorbance, and bromoform formation potential (BFP) were evaluated. Only BFP can be used as a surrogate measurement for THMFP with a correlation factor of 0.944. Five variables: alum treatment mechanism (adsorption and sweep), flowrate, humic acid concentration, addition of kaolin, and addition of powdered activated carbon (PAC) were studied using two-level fractional factorial design. The design gives unconfounded main effects and second-order interactions. The following main effects and second-order interactions were found significant on the removal of THMFP: (1) alum treatment, PAC addition, humic acid concentration, and flowrate main effects, and (2) flowrate/PAC, humic acid concentration/PAC, alum treatment/PAC, alum treatment/flowrate, and humic acid concentration/kaolin second-order interactions. Evaluation of the main effects and interactions indicated that higher removal of THMFP can be achieved using adsorption alum treatment, PAC, and low flowrate. The effect of kaolin was inconclusive. Temperature between 4-15$\sp\circ$C did not affect THMFP removal. The higher THMFP removal during adsorption alum coagulation resulted from the removal of small MW fractions of humic acid. The removal of these fractions is believed due to chelation interactions with aluminum hydroxide polymers which are present under optimum conditions of alum adsorption coagulation. Rapid mixing and orthokinetic flocculation implemented during this work provided adequate physical transport for the chelation interactions to occur