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5.18 CHAPTER FIVE

As water flows downward through the packing, it tends to migrate to the column wall,
and redistributors are installed at intervals to support the packing material and redirect the
water back toward the center of the column. Four commonly used distributor styles are
the orifice plate and the trough (see Figure 5.9), orifice headers, and spray nozzles.

Packing. Packing materials are designed to simultaneously provide a low pressure drop
for air passing through the column and maximum air-water contact area. Packing pieces
for "dumped packing" are randomly dumped into the column. They are available in var-
ious shapes of ceramic, stainless steel, and plastic materials. Plastic is most commonly
used in water treatment applications because it is durable, lightweight, and cost-effective.
Some common shapes are illustrated in Figure 5.10.
Fixed packing comes in prefabricated sheets mounted at intervals inside the column.
Although the initial cost is higher for this type of packing, manufacturers claim higher
transfer efficiency.

Liquid Distribution. After the packing material, the distribution of the water over the
packing material is the next most important design consideration for a packed column.
Proper liquid distribution typically determines the successful operation of the packed col-
umn. Inadequate liquid distribution can be caused by improper design of the distribution
tray or plate, or insufficient sealing between the distribution plate and the walls of the
column. The result can be short-circuiting of the incoming water down the sides of the
column and consequently reduced contact between the air and water. The distribution plate
must be designed to provide even distribution of the water under all flow conditions. Some
packed columns have been designed using spray nozzles for liquid distribution to over-
come this potential problem. Also, wall wipers or redistribution rings have been used to
reduce the effects of short-circuiting.

Air Blower. Airflow is provided at the base of the column by a centrifugal blower driven
by an electric motor. Small towers are designed with blowers requiring a motor as small
as 5 hp (3,700 W), but much larger blowers are required for larger units and when a high
air-to-water ratio is required. Care must be taken in providing screens and locating the air
inlet to prevent insects and airborne contaminants from being blown into the column. In
some instances, clogging of the air inlet screens has caused a reduction in the air-to-
water ratio and a resultant reduction in the treatment efficiency. Screens must be checked
regularly or designed with pressure drop indicators to sense a drop in airflow through the
intake.

Site Considerations. In general, water temperature in the column stays close to the tem-
perature of the influent water. It has been demonstrated in several installations that there
is no danger of freezing even under air temperatures well below freezing. However, in
very. cold conditions, this assumption should be confirmed. In cold climates, blowers and
pumping equipment are usually housed for protection from ice and snow. Housing this
equipment also provides increased security, reduced noise, and reduced maintenance.
Site considerations include zoning restrictions, height restrictions, and noise restric-
tions. There are instances where residents have opposed installation of a column in a res-
idential neighborhood because of potential noise, visual impact of the relatively tall tower,
and the perceived inhalation danger of the off-gas. Where there is public opposition to in-
stallation of a column, other means of treatment may have to be considered, or the source
water may have to be piped to a more remote location for treatment.