Ozone and Color Removal
Many
wastewater and textile processors are gradually substituting chlorine with
ozone. Ozone is a powerful oxidizing mean and safer in use in comparison with
other oxidizing means.
Wastewater
& Dye color removal
Water
is shown colored when visible radiation is absorbed from dissolved materials, or
when light is reflected on suspended solids. These two sources of color are the
base for the distinction between the pseudo and true color. The pseudo color is
due to absorption as well as light reflection. The true color depends
exclusively from the kind and quantity of the dissolved substances. Particles with a size of 400-800 nm, that means within the wavelength of visible light, are responsible for
light reflection. It is possible with filtering (membrane 0.45 μm)
the phenomenon of reflection to be eliminated. It must also be noted that the
difference between the pseudo and true color is related to water’s turbidity.
The
units Pt–Co (USA), or mg Pt-Co / l (Europe) are defined as color measurement
units. These units are considered equivalent.
The acceptable limits of color values for the disposal of treated wastewater
ranges from 50-100 units Pt-Co, depending on the nature of the receiver (river,
sea, lake etc).
True
color is created by the presence of compounds that absorb visible light in wavelengths of 400-800 nm, or from compounds that
fluoresce in the 200-400 nm spectrum. These are compounds of poly-aromatic
structure, substituted aromatic structure, polyenia, concentrated
hetero-circular molecules or perplex ions. It should be noted that π
bonds
absorb into the UV (‹200nm ) spectrum and the existence of conjugate bonds (polyenia)
is necessary for the absorption in visible light spectrum. Most compounds
responsible for color
creation contain one or more aromatic rings and start absorbing color at 250 nm.
The
synthetic color carriers come mainly from industrial
plants as dye-houses, clothing industries with washing-machines, food and
beverage industries, slaughterhouses etc.
Wastewater
is processed with ozone after its exit from
the chemical or/and biological treatment plant and the usual dosage varies from
50-150 mg/l, according to the wastewater origin, its temperature, and the degree
of its previous process.
Ozone-wastewater
contact system:
The
contact system consists of a three-chamber tank, height of 4.5-5 meters with inside
splits that guide the wastewater to a vertical labyrinthine flow. Ozone is
supplied to the tanks through diffusers made
of a special porous material of high resistance. These diffusers have the
ability to create multi-numbered and
very thin ozone bubbles, with a diameter of 220μm.
With their appropriate geometric installation in the bottom of the contact tank,
better distribution but also increase in the liquid-gas contact surface to its
maximum, is achieved.
The
diffuser is used due to the high rate of transport (70%) and its trivial energy
consumption. In a tank of three-chambers, diffusers are installed in depth of 5
meters and succeed a transport rate more than 75%. The wastewater must have a
hydraulic retention time greater than 45 minutes.
Color
Removal Quality:
The
quality of the ozone treatment effluent in terms of color removal, depends on:
The
best results concerning color removal are achieved if the wastewater has been
previously treated in order to lower the values of the other characteristics so
that the ozone oxidizing effect is “consumed” only or at least at a maximum
proportion in color removal. Additionally the temperature must be below 30-deg C
in order to achieve the best physical conditions for its solubility.
The
above remark certainly concerns the practical usage of ozone technology in
wastewater treatment, as it
indicates that the increase of the
ozone dosage could give good
results even in unprocessed
wastewater as long as it has been efficiently cooled.
Wastewater
color removal requires an ozone dosage which in most cases fluctuates from 50 to
100 mg/l, for color reduction of 85-92%. This dosage succeeds simultaneously a
COD reduction about 40%, while
small increases of BOD
in the area of 3-7% have been noticed.
The
ozone treatment installation represent a significant construction and purchase
cost. On the other hand a conventional treatment scheme using chemical
coagulants for color removal, has high operational costs (cost of the coagulants
themselves and cost for the produced sludge management requirements). In general
and for the same effluent quality, the investment of an ozone installation can
be paid off in 3-5 years, depending on the size and other specific details of
each case.
Written
by: Konstantinos J. Delimpasis - Chemical
Engineer
