Filtration - Public Water Systems - page 403

are significantly altered. There is typically a dramatic increase in the assimilable (bio-
degradable) organic compounds (AOC), which is undesirable because it can lead to
bacterial regrowth in the distribution system. Also, several ozone by-products, such as
formaldehyde and acetaldehyde, have been identified as potential human health con-
cerns and may be regulated in the future. Many of these ozonation by-products can
be readily removed by biologically active filters. By-products, such as aldehydes, car-
boxylic acids, aldo acids, and keto acids, are all relatively easily biodegradable, and
removals in excess of 75 percent are normally achieved by biological filtration. 51
Biologically active filters have been used in Western Europe for many years. Com-
mon European practice involves two-stage filtration with the biological filters located
downstream of the main filtration process designed for particulate removal. The trend
in the United States has been to use a single filter that integrates particulate removal
and biological treatment in a single unit. When considering the retrofitting of the
biological step into an existing plant, it is often not practical to add another filtration
step. Experience has shown that it is possible in a single filter to achieve the needed
biological treatment without sacrificing particulate removal effectiveness. 52 This sec-

tion summarizes the key design considerations in single-stage filtration.

Filter Media The most commonly used media for single-stage filtration are granular
activated carbon (GAC)-sand, anthracite-sand, and monomedia beds of GAC, anthra-
cite, or sand. GAC offers some potential advantages in that it offers a macroporous
structure and irregular surface for bacterial attachment sites that are more resistant to
shear stress. In addition, GAC can remove some contaminants by adsorption, at least
until its adsorptive capacity is exhausted. GAC can adsorb some slowly degradable
components that can be degraded by the attached bacteria. The micropores of GAC
are too small (1-100 nm) to allow bacterial growth within the micropores. As a result,
the specific surface area available for biomass attachment might actually be higher in
a sand filter than in a GAC filter, because the effective size of the sand is usually
smaller than that of the GAC. Whether or not the potential advantages of GAC offset
the higher cost must be determined on a case-by-case basis. In several cases, anthracite
and GAC filters have been reported to provide similar, average BOM removals. 52 GAC
is reported to provide better aldehyde removal at colder water temperatures and provide
faster reestablishment of BOM removal after periods when the filters have been out
of service. New York City conducted extensive comparative tests of GAC and anthra-
cite for biological filtration following ozone and dissolved air flotation treatment of its
Croton Lake Supply. 53 Water temperatures varied from 2 Cto22 C with raw-water
total organic carbon (TOC) values of 2-3 mg / L. Both the GAC and anthracite filter
media were tested at filter loading rates of 8-20 gpm / sf with depths of 72-96 inches.
After months of tests, it was concluded that the differences in finished water quality
between the GAC and anthracite were not significant enough to justify the higher cost
of the GAC media. Both media produced excellent turbidity and particle removals,
with finished water turbidities typically in the 0.05 range and particle counts ( 2 m)
of less than 20 / ml. Biologically degradable organic carbon (BDOC) and biostability
tests of the finished water showed no significant difference between the GAC and
anthracite media. An anthracite media of 1.1 mm size, depth of 72 inches, and filtration
rate of 12 gpm / sf was selected for the full-scale Croton Lake biological filtration
facility.

Contact Time Several studies have shown that contact time affects the removal of
BOM within biological filters. Contact time is usually expressed in terms of empty