Civil Engineering Reference
In-Depth Information
Extended filter run times
•
Increased filter application rates
•
Taken together, these benefits may significantly reduce both the capital and operating
costs for treatment systems. In Europe, preozonation has been applied extensively in
conventional coagulation-settling filtration systems, whereas United States applications
have focused primarily on direct filtration applications, due largely to success at the
such large facilities as the Los Angeles Aqueduct Filtration Plant.
60
One full-scale
study of diatomaceous earth filtration found that preozonation significantly extended
filtration runs while meeting filtered water turbidity goals.
61
With respect to water quality objectives, the coagulant aid properties of preozona-
tion have largely been associated with attainment of settled- or filtered-water turbidity
goals.
62
Practice has shown that preozonation can improve coagulation for some waters
but not for others. Also, for waters amenable to this process, seasonal variations can
render preozonation temporarily ineffective.
When the goal is reduction of organic matter, findings are more varied and contro-
versial. Some researchers have found that preozonation may improve TOC removal,
while others have found the opposite effect.
61,63
Work by Edwards and Benjamin
64,65
suggests that preozonation hinders TOC removal at conventional ratios of coagulant
dose to TOC concentrations. This work also indicates that preozonation can increase
the residual concentration of metals present after coagulation when alum and iron salts
are used as the coagulants.
In cases where preozonation is desired for other treatment objectives, potential
detrimental impacts to TOC removal can be offset by increased coagulant dosages,
optimization of pH, use of different coagulants (polymeric metal salts, etc.) or appli-
cation of biologically active filtration downstream of the ozonation step.
The coagulation impacts of ozonation appear complex and varied, and the mech-
anisms are unclear. Based on theoretical considerations and some empirical observa-
tions, a number of explanations have been proposed, including loss of organic coating,
increased aluminum complexation, increased calcium complexation, organic polym-
erization, breakup of iron and manganese complexes, and reactions with algae.
66
An AWWARF study
62
investigating preozonation as a coagulant aid offered the
following observations:
The impacts of preozonation on subsequent coagulation are highly dependent on
the nature of the raw water and its organic and inorganic constituents.
•
Iron, organic matter, and algae are all important in determining ozone's effect on
subsequent coagulation.
•
Efforts to find a link between high calcium concentrations and greater benefits of
preozonation were unsuccessful.
•
In lakes and reservoirs of low to intermediate color, seasonal changes in the effect
of preozonation may occur due to changes in algae and iron.
•
Moderately colored waters containing clay turbidity may have reduced polymer
requirements following ozonation. This effect is most likely to occur with waters
containing high-molecular-weight organics.
•
The presence of high levels of iron or certain species of algae may render a water
more susceptible to the coagulating effects of ozone. The impact of preozonation
on waters high in iron is dependent on both the iron concentration and the chem-
•
