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When designing an
ozone water treatment system for residential, commercial customers or
small Community systems, use a questionnaire to facilitate the information-gathering
process. A diligently completed questionnaire with good water quality
information will serve as the basis of your treatment system design and
guide to specifying the correct equipment.
It is recommended that
you have a water analysis performed by a certified laboratory. Be sure to
have an analysis done to determine levels of each contaminant you will be
treating (all oxidizable components) in the water. If possible get the results
from several analyses performed over a period of time to determine if there
are seasonal variations in the water quality.
If you suspect high levels
of organics there are two other tests that will help with system design:
chemical oxygen demand (COD) and biochemical oxygen demand (BOD). The COD
and BOD results will bear on calculating total ozone demand and treatment
system design.
Some water quality information
is best gathered on-site because the test must be performed immediately
upon taking a water sample for the results to be of any value to the treatment
design. For example, the level of hydrogen sulfide (H2S) and
water temperature. Less sensitive to time, but more relevant if monitored
immediately, is oxidation/reduction potential (ORP).
For residential potable
water treatment systems it is recommended that at minimum you have the ability
to test for the following: iron, manganese, hydrogen sulfide, tannins, temperature,
pH, ozone, hardness, alkalinity, total dissolved solids, nitrites and nitrates.
Determining the ozone
demand
Most ozone generators
designed for residential water treatment are sized by the grams of ozone
produced per hour of operation. To choose the right-sized ozone generator
for each job, determine the potential work ozone can do. This work can
also be thought of as the ozone demand of the water.
Determining the ozone
demand of the water requires an accurate water analysis and a little math
using the demand each contaminant places on ozone, or the amount of ozone
required to oxidize the contaminant. The amount of ozone needed for oxidation
is known as the demand on ozone or the required dosage (see table 2).
Other variables, such
as water temperature, organically bound compounds or seasonal variations
bear on the exact ozone treatment system design. It would be sensible to
figure a slightly higher (perhaps 20 percent) ozone demand and increase
contact time to factor in unknowns.
It is sometimes recommended
that ozone residual after appropriate contact time is all that is needed
to treat surface waters. However, it is better that ozonation be a two-step
process when treating surface waters with bacteria, viruses or cysts and
multiple other oxidizable contaminants.
In step 1, ozonation
and filtration will oxidize contaminants in the water (remove iron, manganese,
hydrogen sulfide, etc.). In step 2, ozone will again be injected and an
ozone residual will be maintained to insure disinfection. A possible alternative
to the second point of ozone injection could include the use of filter systems
approved for the removal of Giardia or ultraviolet radiation after
filtration.
Determine equipment
and chemical needs
Each component of
the ozone treatment system must be properly sized to work together and
produce treated water at the required rate of flow:
· Ozone injection:
Two common methods of injecting ozone into the water are available: using
a pump to force ozone through a diffuser and venturi injection. Venturi
injection is preferable because it is very efficient and requires no moving
parts.
Water pressure at the
inlet of the venturi injector must be higher than the outlet pressure during
the entire pump cycle. This difference is known as the pressure differential.
The pressure differential required for each treatment system is determined
by two variables:
Variable 1. Required
rate of injection, stated as liters per minute (l/m) or the equivalent in
standard cubic feet per hour (SCFH). The required rate for ozone generators
can be obtained from equipment manufacturers. Request a chart showing various
rates of ozone production at various injection rates. Proper use of this
information will enable a certain amount of customization for each application.
Variable 2. The
selected venturi injector. Use the injector charts to select an injector
that meets or exceeds the required injection rate set by the manufacturer
at the required water flow rate.
· Ozone contact and
off-gas. After injection, ozone must have time to do its work, known
as contact time: the time ozone has to oxidize and disinfect or the time
the water is allowed to hold the disinfectant. For ozone, contact time is
measured from the point of injection to the filter. Depending upon the circumstances
of each installation, there may still be ozone in the water after filtration,
but for purposes of calculating contact time the filter marks the end.
Suppose ozone is injected
into a 25-gallon properly designed contact vessel followed by a filter and
the flow rate is 5 gpm, the contact time in this situation would be 25 gallons
divided by 5 gallons per minute resulting in five minutes contact time.
The contact time needed
varies with the matter to be oxidized. Time required for oxidation ranges
from almost instantly to 10 minutes or more. A general rule of thumb is
no less than 4 minutes contact time with a measurable residual of 0.1 milligrams
per liter (mg/L) ozone and more contact time is better. Don’t take a good
thing too far, because ozone is so fast reacting and has such a short life
once generated it is possible to have too much contact time in which case
you wouldn’t have a measurable residual.
The following are
two of several reasons more contact time is better:
1. Contaminants
floc or precipitate at varying rates and with varying degrees of density,
giving more time will often make a treatment system a success by providing
time for material to fully precipitate, which in turn enhances filterability.
2. Ozone must
contact the contaminant to oxidize or disinfect; more contact time increases
the odds that all water will be subjected to oxidation.
If using ozone to disinfect,
there are guidelines set by the Environmental Protection Agency (EPA). The
EPA has proposed the use of a "CT value" to assure the attainment of primary
disinfection at a minimum cost. The CT value is the numerical product of
the concentration of residual disinfectant (C), in mg/L, multiplied by the
time (T), in minutes, during which the residual is present: C (mg/L) x T
(minutes) = CT value (mg/L-minutes). Thus, units for CT-values are expressed
as mg/L-min.
CT tables show the required
CT value at a given temperature and pH. To be more precise and meet regulatory
requirements, refer to the approved CT tables used by local regulating authorities.
Most agencies use the EPA-recommended CT values.
All gases inducted through
the venturi must be properly disposed of or "off-gassed." At a bare minimum,
this is accomplished by simply venting the contact tank to the outside.
It may be required to route the gases through an ozone destruct unit to
insure no ozone gases are being emitted from the treatment system.
Gabe Ergler is an
applications specialist for O3 Water Systems, LLC, in Snohomish, WA. |