The relationship between raw water characteristics and corresponding minimum
effective alum doses (MEADs) was investigated. To test the effects of raw water
characteristics on minimum effective chemical conditions for coagulation and, in
some cases, subsequent filtration, the concentrations of colloidal particles and natural
organic matter (NOM) in model raw waters were systematically varied. Jar tests
were performed using waters with varying colloidal and NOM concentrations and,
for each combination of colloidal and NOM concentrations, the minimum effective
coagulant dose was observed for the removal of settled and filtered turbidity and
DOC. Monodisperse silica particles (129 nm in diameter) and NOM from the Great
Dismal Swamp in Virginia were used in preparing the waters to be tested. The
minimum effective alum dose for the coagulation of low silica waters without NOM
decreased as silica concentration increased, whereas it increased proportionally with
increasing silica concentration for high silica waters. At low silica waters, contact
opportunity for floc formation is limited by low solid volume and removal is
achieved via a sweep flocculation mechanism. Increasing silica concentration within
this range provides additional floc volume thereby reducing the alum dose required to
induce sweep flocculation. Once the requirement for effective flocculation is met by
sufficient silica concentration, the minimum effective alum dose increases
stoichiometrically with increasing silica. Removal of silica in the presence of NOM
showed two distinct results. First, at low silica concentration, the presence of low
NOM (0.75 mg/L) lowered the minimum effective alum dose dramatically, possibly
by promoting the precipitation of Al and/or Al-NOM solids. Jar tests conducted with
the addition of sulfate suggest that the presence of multivalent anionic moieties, such
as those in NOM, accelerate floc formation as aluminum hydroxide precipitate. The
presence of simple anions (such as chloride) had much less effect. Second, the
minimum effective alum dose showed a strong linear stoichiometric relationship with
NOM concentration at all silica concentrations, with NOM dominating the alum
demand at low silica concentrations. The minimum effective alum doses for waters
high in both NOM and silica particles were somewhat additive; they increased both
as silica and NOM increased. In summary, NOM controlled the alum demand, from
reducing alum dosages for waters low in both DOC and turbidity to requiring
stoichiometric increase at high DOC concentrations. Includes 10 references, tables, figures.
| Edition : | Vol. - No. |
| File Size : | 1
file
, 400 KB |
| Note : | This product is unavailable in Ukraine, Russia, Belarus |
| Number of Pages : | 13 |
| Published : | 06/17/2004 |
