Dissolved Air Flotation

DAF system proves effective in membrane pretreatment
It has been known for hundreds of years that membranes can be used for solids removal and concentration in potable water treatment schemes. In recent years, new low-pressure membranes have greatly enhanced effluent quality to meet increasingly strict regulations. In addition, water treatment professionals have discovered that the performance of these membranes can be improved when a pretreatment system is utilized to reduce the contaminants that cause fouling.

A new dissolved air flotation (DAF) system, the Leopold Clari-DAF system, has proven to be an effective membrane pretreatment system, providing a multi-barrier approach to the treatment scheme, increased membrane flux rate, reduced membrane cleanings and prolonged membrane life. In addition to improving membrane performance, the Leopold Clari-DAF system is cost-effective, and its footprint is much smaller
than conventional settling basins.

Need for membrane pretreatment

As the use of membrane treatment systems becomes more widespread, there is increasing agreement among consultants, engineers and membrane technology providers alike that membrane pretreatment is necessary to help ensure reliable and troublefree membrane filter performance.

According to Aqueous Solutions, Inc., “All membrane systems, specifically RO, require proper pretreatment to ensure reliable and trouble-free operation.” In November 2003, Black & Veatch stated:

“When membranes were initially
used to treat surface water in the mid-
1990s, a simple membrane filtration
approach with minimal pretreatment
was generally applied. Pretreatment
typically consisted solely of a protective
backwashable screen with a rating
in the 200- to 500-micron range to
prevent comparatively large particles
from plugging the membrane unit.

In many cases, however, simple
membrane filtration is inadequate to
fully meet the requirements of the
Safe Drinking Water Act because many
water supplies contain dissolved contaminants
that exceed regulatory levels.
Although they offer many benefits,
MF/UF systems alone do not effectively
control dissolved contaminants.

Treatment process designers today
are likely to combine membranes with
a variety of pretreatment technologies
to address a broad range of water quality
problems. In addition to expanding
plant treatment capabilities, these pretreatment
processes have been found
to optimize membrane performance
and reduce capital and whole-life
costs,” according to Balck & Veatch.

An effective pretreatment choice

The Leopold Clari-DAF system, which
utilizes a high-rate dissolved air flotation
clarification process, provides one of the
best membrane pretreatment alternatives.
The process is designed with the
concept that it is easier to float lowdensity
solids than to settle them.

Color, organics, soluble metals or
colloidal solids that are removed by
adding inorganic chemicals to form
aluminum or iron hydroxides have low
density and require excess chemicals
and mixing time to create particles
large enough to settle.

Normally, solids hundreds of
microns in size are required to settle,
while particles tens of microns in size
can be floated. It is also recognized
and well documented that algae in
the source water that will foul membranes
is best removed by flotation.

In the Clari-DAF system process,
raw water particles are flocculated and
separated out of the water by floating
them to the surface, rather than settling
them to the bottom of a basin.
The process introduces micro-sized air
bubbles through diffusers at the bottom
of the contactor where it mixes
with the coagulated solids and floats
the floc. The air bubbles are produced
by recycling a portion of the effluent
through a tank where air is introduced
and the water saturated, and then
reduced to ambient pressure in the
reaction zone of the DAF cell, thus
creating the pressurized flow.

The floated sludge is removed from
the top of the basin by mechanical or
hydraulic means, while laterals from
the bottom of the basin remove the
clarified water. The Clari-DAF system
provides a small footprint with its typical
flocculation mixing time of 10 to
12 minutes and a loading rate of 12 to
16 gpm/ft2 based on collection area.

The benefits of providing Clari-
DAF systems for pretreatment are: a
multi-barrier approach to the treatment
scheme; increased membrane
flux rate; reduced membrane cleanings;
and prolonged membrane life.

Better performance

Recently, Black & Veatch reported
on their investigation of DAF for
membrane pretreatment versus other
pretreatment methods for a project
with the South San Joaquin Irrigation
District in California:

“On a case-by-case basis, alternative
clarification methods, such as plate settlers,
dissolved air flotation and ballasted
flocculation, may be more costeffective
than conventional sedimentation.
A new 40-mgd UF membrane filtration
plant was designed for the
South San Joaquin Irrigation District
and partner cities of Manteca, Lathrop,
Escalon, and Tracy, Calif.

Due to the colloidal nature of the
turbidity-causing suspended particles
present in the source water—which
resist settling without significant
polymer addition—the project team
evaluated DAF as well as conventional
coagulation-sedimentation with
a plate settler in a three-month
pilot study.

The pilot study revealed that the
application of DAF for pretreatment
provided superior turbidity reduction,
resulting in turbidity of 0.5 NTU compared
with 1.5 NTU using conventional
coagulation-sedimentation pretreatment.
In addition, UF performance
was more stable with longer
intervals between cleanings following
DAF pretreatment. UF following DAF
was selected for full-scale treatment,”
according to Balck & Veatch.

A second example pilot study was
conducted in September 2002 at a
water utility in Rhode Island on a
water source with turbidity <2 NTU,
true color <20 Pt-Co, total iron levels
from 0.10 to 0.30 mg/L and total
manganese levels from 0.02 to 0.20
mg/L. The data obtained by the pilot
test at this site demonstrated optimum
performance, as shown in Table 1.

The testing indicated very little
organics fouling of the membranes
following the Clari-DAF system, thus
prolonging the membrane life. Table
2 provides a 20-year capital cost
analysis based on a design flow of 8
mgd; a membrane replacement of
five years for the process without pretreatment
with a Clari-DAF system;
and seven years for the process with
pretreatment with a Clari-DAF system.

The total savings is $600,000
(16%) using the Clari-DAF system for
membrane pretreatment. With the
longer time between backwashes
and chemically enhanced backwashes,
the membranes will have
less fatigue due to cleanings, which
will result in longer times between
membrane replacements (reducing
capital cost) and lower chemical
cost (reducing operational cost).

Increased flux rates

The third pilot study, completed
for a California utility, compared
conventional settling with plate settlers
and the Clari-DAF system. The
pilot study established the highest
flux rate that could be achieved
before the transmembrane pressure
accelerated. The optimum flux rate
for the plate settlers was 45 gfd, while
optimum flux rate for the Clari-DAF
system was 60 gfd. The Clari-DAF system
will allow the membrane system
to operate at a 33% higher flux rate,
resulting in fewer membrane modules
required for the treatment process.

In addition to the effluent water
quality that will improve membrane
performance, the Clari-DAF system
produces a high sludge solids content
to reduce the solids handling
portion of the utilities operation
(both dewatering and disposing
cost). The operation of the Clari-DAF
system can occur without the use of
polymers or fine ballasted materials
that can carry over in competing
clarification processes and foul the
membranes. It is a simple system to
operate and typically can be started
up and optimized within 45 minutes.
The footprint compared to conventional
settling basins will be one-sixth
to one-tenth the size.

The Clari-DAF system high rate
flotation process provides the most
cost-effective membrane pretreatment
process. It effectively removes
potential fouling contaminants from
the raw water source, does not use
potentially fouling flocculating materials,
and provides the highest solids
content of sludge removed from the
clarification process—all with one of
the smallest footprints for a clarification
treatment process.

James E. Farmerie is product manager for
the F.B. Leopold Co., Inc. He can be reached
at 724/453-2120 or by e-mail at
jfarmerie@fbleopold.com.

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