the flotation clarifier’s bottom. The gas flow rate is about 1% of influent liquid
flow rate. The attachment of gas bubbles to the impurities can be a result of
physical entrapment, electrochemical attraction, surface adsorption, and/or gas
stripping. The specific gravity of the bubble-impurity agglomerate is less than
one, resulting in buoyancy or non-selective flotation (i.e., Save-All).
Electroflotation It is process involving the generation of hydrogen and oxygen
bubbles in a dilute electrolytic aqueous solution by passing a direct current
between two electrodes: (a) anode and (b) cathode. Anode reaction generates
oxygen bubbles and hydrogen ions, while cathode reaction generates hydrogen
bubbles and hydroxide ions. Either aluminum or steel sacrificial electrodes can be
employed for generating the gas bubbles as well as coagulants at the same time.
Non-sacrificial electrodes are employed for generating the gas bubbles only and
can be made of titanium (as the carrier material) and lead dioxide (as the coating
material). Electrical power is supplied to the electrodes at a low voltage potential
of 5–20 VDC by means of a transformer rectifier. Small bubbles in the range of
20–50 m microns are produced under laminar hydraulic flow conditions feasible
for flotation separation of fragile flocs from water in a small system. The floats on
the water surface are the impurities/pollutants removed from water. The clarified
water is discharged from the flotation clarifier’s bottom. There can be unexpected
advantages and disadvantages when electroflotation is employed. For instance,
chlorine bubbles may be generated as a water disinfectant if the water contains
significant amount of chloride ions. Certain unexpected gas bubbles may be
generated and may be undesirable.
Filtration It is usually a granular medial filtration process which involves the
passage of wastewater or water through a bed of filter media with resulting
deposition of suspended solids. Eventually the pressure drop across the bed
becomes excessive or the ability of the bed to remove suspended solids is
impaired. Cleaning is then necessary to restore operating head and effluent
quality. The time in service between cleanings is termed the filter run time or
run length. The head loss at which filtration is interrupted for cleaning is called
the terminal head loss, and this head loss is maximized by the judicious choice of
media sizes. Dual media filtration involves the use of both sand and anthracite as
filter media, with anthracite being placed on top of the sand. Gravity filters
operate by either using the available head from the previous treatment unit or
by pumping to a flow split box after which the wastewater flows by gravity to the
filter cells. Pressure filters utilize pumping to increase the available head. A filter
unit generally consists of a containing vessel; the filter media; structures to
support the media; distribution and collection devices for filter influent, effluent,
and backwash water flows; supplemental cleaning devices; and necessary controls for flows, water levels, and backwash sequencing. Backwash sequences can
include air scour or surface wash steps. Backwash water can be stored separately
or in chambers that are integral parts of the filter unit. Backwash water can be
pumped through the unit or can be supplied through gravity head tanks.
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D. Kaya and Y.-T. Hung
flow rate. The attachment of gas bubbles to the impurities can be a result of
physical entrapment, electrochemical attraction, surface adsorption, and/or gas
stripping. The specific gravity of the bubble-impurity agglomerate is less than
one, resulting in buoyancy or non-selective flotation (i.e., Save-All).
Electroflotation It is process involving the generation of hydrogen and oxygen
bubbles in a dilute electrolytic aqueous solution by passing a direct current
between two electrodes: (a) anode and (b) cathode. Anode reaction generates
oxygen bubbles and hydrogen ions, while cathode reaction generates hydrogen
bubbles and hydroxide ions. Either aluminum or steel sacrificial electrodes can be
employed for generating the gas bubbles as well as coagulants at the same time.
Non-sacrificial electrodes are employed for generating the gas bubbles only and
can be made of titanium (as the carrier material) and lead dioxide (as the coating
material). Electrical power is supplied to the electrodes at a low voltage potential
of 5–20 VDC by means of a transformer rectifier. Small bubbles in the range of
20–50 m microns are produced under laminar hydraulic flow conditions feasible
for flotation separation of fragile flocs from water in a small system. The floats on
the water surface are the impurities/pollutants removed from water. The clarified
water is discharged from the flotation clarifier’s bottom. There can be unexpected
advantages and disadvantages when electroflotation is employed. For instance,
chlorine bubbles may be generated as a water disinfectant if the water contains
significant amount of chloride ions. Certain unexpected gas bubbles may be
generated and may be undesirable.
Filtration It is usually a granular medial filtration process which involves the
passage of wastewater or water through a bed of filter media with resulting
deposition of suspended solids. Eventually the pressure drop across the bed
becomes excessive or the ability of the bed to remove suspended solids is
impaired. Cleaning is then necessary to restore operating head and effluent
quality. The time in service between cleanings is termed the filter run time or
run length. The head loss at which filtration is interrupted for cleaning is called
the terminal head loss, and this head loss is maximized by the judicious choice of
media sizes. Dual media filtration involves the use of both sand and anthracite as
filter media, with anthracite being placed on top of the sand. Gravity filters
operate by either using the available head from the previous treatment unit or
by pumping to a flow split box after which the wastewater flows by gravity to the
filter cells. Pressure filters utilize pumping to increase the available head. A filter
unit generally consists of a containing vessel; the filter media; structures to
support the media; distribution and collection devices for filter influent, effluent,
and backwash water flows; supplemental cleaning devices; and necessary controls for flows, water levels, and backwash sequencing. Backwash sequences can
include air scour or surface wash steps. Backwash water can be stored separately
or in chambers that are integral parts of the filter unit. Backwash water can be
pumped through the unit or can be supplied through gravity head tanks.
370
D. Kaya and Y.-T. Hung