1324 WATER TREATMENT
form of monochloramine. Beyond point 3, all chlorine
added remains in solution. This phenomenon is known as
break-point chlorination, and in order to ensure satisfac-
tory disinfecting properties chlorination must go beyond
the break-point 3 on the curve.
In certain instances, such as when phenols are present
in the water, small concentrations of free available chlorine
will combine with the phenols, forming chlorphenols which
produce a distinctive taste and odor at very, very small con-
centrations. When this occurs, chlorine dioxide is frequently
used as a disinfecting agent as this does not react with the
phenols but in fact destroys them.
Other substances which are used for disinfecting pur-
poses are the other halogens such as bromine and iodine,
although these are not commonly used in water supplies.
Occasionally they have been used in swimming pools for
similar purposes.
Ozone is a particularly effective disinfectant, but it has
certain disadvantages in that it must be manufactured on the
site, using fairly sizeable and expensive capital equipment.
No residual can be maintained due to the instability of the
substance, and the methods of detection are rather imperfect.
However to compensate for this, the disinfecting properties
of ozone are considerably greater than chlorine.VOC REMOVALTreatment of Volatile Organic Compounds Found
in Groundwater SourcesWith the advent of technological advances in testing of water
supplies and concerns regarding possible contamination
of groundwater sources, many water supply systems have
focused on the treatment of Volatile Organic Compounds
(VOC’s). VOC’s are man-made chemicals, some of which
have been shown to be carcinogenic. VOC’s are generally
found in industrialized settings where substances such as
cleaning fluids, degreasers or solvents have been disposed
of improperly.
The treatment of VOC’s utilizing conventional water
treatment techniques involving flocculation, sedimentation
and filtration are relatively ineffective at reducing VOC con-
centrations. VOC’s may be treated by either packed tower
aeration (air stripping) or granular activated carbon (GAC)
absorption. Details of the two treatment techniques are as
follows:Packed Tower Aeration (Air Stripping)Aeration is the process where air and water are brought into
contact for the purposes of transferring volatile substances
from water to air. This process is commonly referred to as
air stripping. Air stripping basically involves the transfer
of dissolved gas molecules from the liquid phase to the gas
phase. There are two major factors which determine theremoval efficiency of various volatile compounds by air
stripping; 1) the ratio of concentration of VOC’s in the gas-
eous phase to the concentration of VOC’s in the aqueous
phase at equilibrium, and 2) the rate at which equilibrium
is obtained.
Numerous types of aeration devices have been used
where air stripping can occur. Some of these alternatives
involve diffused aeration, spray aeration and water fall
aeration.
In packed towers or stripping towers, water flows down-
ward by gravity and air is forced upward. The tower is filled
with various forms of packing material which serves to
continuously disturb the liquid flow, creating and improving
the air-to-water interface. Packed towers typically have void
volumes in excess of 90 percent which allows for a large
liquid-air interface and minimizes the pressure drop through
the column, an operating cost consideration. Packed towers,
which are currently in service, have provided VOC remov-
als in the 95–99.9 percent range. A schematic of a typical
airstripping facility is shown on Figure A.
There are three major design factors controlling the mass
transfer of VOC’s from water to air.
Packing depth —is the primary factor influencing removal
efficiency. Increasing the packing depth will increase the
removal efficiency of the tower.
Tower diameter —controls the liquid loading rate as
measured in gallons per minute per square foot, (GPM/
sq. ft.). The lower the liquid loading rate, the greater the
removal efficiency due to the increased air-to-water interac-
tion zone.
Air-to-water ratio —is the most influential parameter
with respect to removal efficiency. Generally, the removal
efficiency increases as the air-to-water ratio is increased.
In stripping towers, packing materials are used to provide
high void volumes together with high surface area. The water
flows downward by gravity and air is forced upward. The raw,
untreated water is evenly distributed on the top of the pack-
ing with either spray or distribution trays and the air is forced
through the tower by either blowers or induced draft fans.
Many options exist for packings involving a variety of
shapes and materials. Packings are available in plastic, metal
and ceramic. Plastics are best suited for water treatment
because of their durability and low cost.
Since the mass transfer of VOC’s is basically accom-
plished by passing significant quantities of air through a
fixed quantity of water, the air-to-water ratio can be varied
by either, i) increasing the diameter of the column, or
ii) increasing the air blower capacity. Hence, an optimum
balance of tower diameter and blower size must be evaluated.
Given a specific water loading rate and a packing selection,
the air-to-water ratio determines the height of the stripping
tower required to provide the specified removals.
Various liquid loading rates are evaluated to optimize
the tower diameter versus air pressure drops. Once the tower
diameter is determined, a cost analysis comparing capital
and operating cost is determined. A matrix of air-to-water
ratios and depth of packing is then developed to determine
the optimum design.C023_006_r03.indd 1324C023_006_r03.indd 1324 11/18/2005 1:32:36 PM11/18/2005 1:32:36 PM