Coagulation and Flocculation - Public Water Systems - page 297

MCLs for DBPs (e.g., 80 g / L for TTHM and 60 g / L for HAA5). To comply with
this more stringent regulation, many utilities began relying on using coagulants to
remove DBP precursors, mainly NOM. 67 Usually a higher coagulant dosage is required
to achieve TOC removal than what is required for optimal turbidity removal. 68 With
higher coagulant dosages applied, the formation of large-heavy flocs can be easily
achieved and therefore the operating parameters (such as coagulant dosage and pH)
are usually optimized for organic removal rather than for particulate removal. This
type of TOC-emphasized coagulation is referred to as enhanced coagulation (EC).
TOC removal by coagulation increases with decreased pH and increased coagulant
doses. Although both aluminum- and iron-based coagulants can remove significant
amounts of TOC, ferric coagulants seem to have slightly higher sorption capacity for
NOM than alum under the same condition. For waters that contain high TOC, coag-
ulation at lower pH is usually required to achieve desirable TOC removal efficiency.
Ferric coagulants are usually used (at pH as low as 3.8) under such circumstances,
since the solubility of aluminum becomes significant at pH below 5.5. The pH of
treated water should be adjusted back to 7.5 8.2 for corrosion control. When water
coagulated at lower pH (e.g., pH 4.0) is readjusted to neutral pH after sedimentation,
some reflocculation may occur and cause increased turbidity. This post-flocculation
may be due to the precipitation of soluble iron or to the recoagulation of pinpoint
ferric hydroxide flocs that do not settle at lower pH (due to high positive charge on
the particle surfaces). These colloidal ferric particles will likely be recoagulated when
the pH of settled water is adjusted to neutral, since the positive particle charge reduces
at neutral pH, thereby reducing the electrostatic repulsion among particles.
Using the coagulation process for the removal of other trace contaminants (such as
arsenic) may be accomplished at conventional coagulant dosages that are used for
turbidity removal, since the concentrations of these contaminants are usually very low
(in the ppb range). However, powdered activated carbon can also be added to the
conventional coagulation process to remove trace organic contaminants, such as taste
and odor compounds (e.g., geosmine and MIB) and / or pesticides. The addition of
PAC will also assist flocculation and sedimentation processes through similar ballasted
sedimentation mechanisms.
REFERENCES
1. Edzwald, J. K., Tobiason, J. E., Amato, T., and Maggi, L. J., ''Integrating High-rate DAF
Technology into Plant Design,'' JAWWA 91:12:41, 1999.
2. Best, G., Mourato, D., and Singh, M., ''Application of Immersed Ultrafiltration Membranes
for Color and TOC Removal,'' Proceedings, AWWA Annual Conference, Chicago, IL, 1999.
3. Stumm, W., ''Chemical Interactions in Particle Separation,'' in Chemistry of Wastewater Tech-
nology (A. J. Rubin, ed.), Ann Arbor Science Publishers / Technomic Publishing Co., Inc.,
Lancaster, PA, 1978.
4. Black, A. P., and Hannah, S. A., ''Electrophoretic Studies of Turbidity Removal by Coagu-
lation with Aluminum Sulfate,'' JAWWA, Vol. 53, No. 4, p. 438, 1961.
5. Packham, R. F., ''Some Studies of the Coagulation of Dispersed Clays with Hydrolyzing
Salts,'' J. Coll. Science, Vol. 20, 1965.
6. Rook, J. J., ''Haloforms in Drinking Water,'' JAWWA, Vol. 68, No. 3, p. 168, 1976.
7. Black, A. P., and Willems, D. G., ''Electrophoretic Studies of Coagulation for Removal of
Organic Color,'' JAWWA, Vol. 53, No. 5, p. 589, 1961.