Automated Chlorination Systems Advantages and … 1 - Romano.pdf · The cost of chlorine...
Transcript of Automated Chlorination Systems Advantages and … 1 - Romano.pdf · The cost of chlorine...
Automated Chlorination
Systems – Advantages and
Disadvantages
Jones & Henry Engineers, Ltd.
Overview
Disinfection Basics
Non-Automated (Manual) Systems
Automated Systems
A Few Case Studies
Conclusions
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Overview
Chlorine Disinfection is the final component of the
treatment process, and is directly related to (2)
primary NPDES permit limits:
Chlorine Residual and Fecal Coliform
The challenge is to add enough chlorine to properly
disinfect the final effluent while not “overfeeding”.
Many continuous discharge WWTP’s manually monitor
and adjust chlorination/de-chlorination feed rates
based on experience and standard procedures.
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Disinfection Basics
Human exposure to wastewater discharged
into the environment has steadily increased in
the last 30 years with the rise in population
and the greater demand for water resources
for recreation and other purposes.
There is no perfect disinfectant. However,
there are certain characteristics to look for
when choosing the most suitable disinfectant:
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Disinfection Basics
Ability to penetrate and destroy infectious
agents under normal operating conditions;
Lack of characteristics that could be harmful to
people and the environment;
Safe and easy handling, shipping, and storage;
Absence of toxic residuals, such as cancer-
causing compounds, after disinfection; and
Affordable capital and operation and
maintenance (O&M) costs.
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Disinfection Basics
Chlorine is the most widely used wastewater
disinfectant in the U.S., and it kills most
bacteria, viruses, and other microorganisms
that cause disease.
Chlorine is introduced to wastewater in the form
of gas, hypochlorites, and other compounds.
The different forms of chlorine used at
wastewater treatment plants are gaseous
chlorine, sodium hypochlorite solution, calcium
hypochlorite, and bromium chloride.7
Disinfection Basics
Wastewater and chlorine are first mixed
completely in less than 1 second and then enter
a baffled contact chamber to allow time for
disinfection to occur.
The effluent is then discharged to the receiving
water.
Chlorine residuals can persist in treated
wastewater for many hours.
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Disinfection Basics
To minimize the effect on aquatic life and the
environment, most states require that
chlorinated wastewater be dechlorinated.
Dechlorination is the process of reducing the
chlorine residual prior to discharge.
Some commonly used dechlorinating
chemicals are sulfur dioxide, sodium bisulfite,
sodium metabisulfite, and activated carbon.
The following figure is a flowchart of the
chlorination process using liquid chlorine and
de-chlorination using sulfur dioxide. 9
TYP. CHLORINE DISINFECTION
Jones & Henry Engineers, Ltd.
Disinfection Basics
What determines the performance of chlorine
disinfection?
The effectiveness of chlorination depends on
the dose, the chlorine demand of the
wastewater, the chlorine residual, and the
amount of time the wastewater is in contact with
the chlorine,
And the fecal coliform count in the wastewater,
and other wastewater characteristics.
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Disinfection Basics
The required degree of disinfection can be
achieved by varying the dose and the contact
time for any chlorine disinfection system.
The chlorine dose usually ranges from 4 to 20
milligrams per liter (mg/L).
For optimum performance, the chlorination
system must be designed so that the
wastewater flows turbulently in a plug flow
fashion throughout the contact chamber,
ensuring complete mixing.12
Disinfection Basics
This mixing allows the chlorine to have
maximum contact with the wastewater and
ensures that there are no dead areas (unused
portions) of the tank.
What is the cost of chlorine disinfection?
The cost of chlorine disinfection systems
depends on the manufacturer, the site, the
capacity of the plant, and the characteristics of
the wastewater to be disinfected.
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Disinfection Basics
For an average dry weather flow of 1 MGD, an
estimated O&M cost of $50,000 per year, (a
chlorine dose of 5 to 20 mg/L was used from a
1-ton gas cylinder). The annual O&M costs
include power consumption, cleaning supplies,
equipment repairs, and personnel costs.
Generally, the total cost of chlorination will
increase by about 30 to 50% when adding the
dechlorination step. In addition, hypochlorite
compounds are more expensive than Cl2 gas.
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Disinfection Basics
The chlorine dose required depends on two
considerations: the chlorine demand and the
desired chlorine residual.
Dose = Demand, mg/L + Residual, mg/L
The chlorine demand is the amount used in
reacting with various components of the water
such as harmful organisms and other organic
and inorganic substances. When the chlorine
demand has been satisfied, these reactions stop.
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Disinfection Basics
To convert from mg/L to lbs/day, or vice versa,
the following equation is used:
(mg/L Cl2) (MGD flow) (8.34 lbs/gal) = lbs/day Cl2
Determine the chlorinator setting (lbs/day)
needed to treat a flow of 3 MGD with a chlorine
dose of 4 mg/L.
(4 mg/L Cl2) (3 MGD flow) (8.34 lbs/gal) = 100
lbs/day Cl2
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Disinfection Basics
What should the chlorinator setting be (lbs/day)
to treat a flow of 2.46 MGD if the chlorine
demand is 3.1 mg/L and a chlorine residual of
0.8 mg/L is desired?
Chlorine Dose = 3.1 mg/L + 0.8 mg/L = 3.9 mg/L
Then calculate the chlorine dosage (feed rate) in
lbs/day:
(3.9 mg/L)(2.46 MGD)(8.34 lbs/gal) = 80 lbs/day
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Non-Automated (Manual) System
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Non-Automated System
Constant (Cavity) Feed
Manual Adjustments
Based on Test Data
Non-Automated (Manual) System
PRO’s
Cheaper System to Implement
Less Parts to Maintain
CON’s
Personnel Required to Make Adjustments
Higher Possibility of Manual Error
Potentially Wasted Chemical Cost
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Automated System
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Automated System
Variable Feed Optimization
Automatic Adjustments Based on
Sensed Data
Upstream & Downstream Sensors
PLC
Automated Changeover
Regulator
Automated System
Upstream Sensors
Measures Flow Rates
Determines Wastewater Characteristics
Downstream Sensors
Located Below Chlorine Induction,
De-chlorinator Induction, and Prior to Plant
Effluent
Measures Chlorine and Dechlorinator
residuals
Measures Dissolved Oxygen Levels
Automated System
Pro’s
Chemical Adjustments Can Be Made
Continuously
Balancing Results in Less Chemical Usage
Feed Variablity means Versatility
Less Chance for Discharge Violations
Con’s
More Cost to Implement
More Parts to Maintain
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Automated System
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The goal is to provide real-time adjustments,
24-7 that reduce chemical overage costs,
labor overhead, and effluent regulation
deficiencies.
A Few Case Studies
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Designed to treat 5 MGD
Treats an average of
650,000 gpd
During its first year of
operation, the plant
experienced serious
difficulties maintaining
sufficient chlorine residual in
its final effluent
Ex. WWTP #1
A Few Case Studies
Causes
The plant was stripping ammonia out of the waste stream prior to chlorination, hindering the ability to achieve breakpoint chlorination in the chlorine contact chamber.
Chlorine dosage control at the plant was being performed through flowpacing, and this often proves problematic because chlorine profile and chlorine demand are constantly changing in a continuous flow system such as a wastewater treatment plant.
A Few Case Studies
Solution
Feeding a steady rate of nonhazardous ammonia to flows just prior to chlorine injection
Installed a Strantrol® 890 automated, demand-based chlorination control system programmed with an operator-determined HRR setpoint that corresponds to the chlorination value required to meet the plant’s discharge requirements.
A High Resolution Redox® (HRR) sensor that monitors chlorine activity and demand was installed in the flow approximately 10 minutes downstream from chlorine injection.
A Few Case Studies
Results
The controller compensates for changes in lag
time between the chemical injection point and the
sensor location.
“Our controller has required no maintenance and,
after almost three years, it still has its original
sensor,” assistant wastewater supervisor for the
community.
A Few Case Studies
Peak flow of 20 MGD
Utilizes V-10K chlorinators and
sulfonators to feed gaseous
chlorine and sulfur dioxide
Under flowpacing/residual
measurement, the plant had great
difficulty maintaining consistent
feed and control
Ex. WWTP #2
A Few Case Studies
Solution
Installed a Strantrol® 890 automated, demand-based
chlorination and dechlorination control system from
USFilter’s ChemFeed & Disinfection Group
Utilizing High Resolution Redox® (HRR) technology,
the Strantrol controller provides automatic, demand-
based control of chlorination and dechlorination
One of the controller’s probes, located approximately
10 minutes downstream from chlorine injection,
monitors for chlorine activity and demand. A second
probe, monitoring dechlorination, is located just
before effluent discharge.
A Few Case Studies
Results
The system monitors both the oxidant and
reductant demand in the water and automatically
modulates the amount of chlorine and sulfur
dioxide required to meet chlorine residual and
fecal coliform limits in the plant effluent.
The controller compensates for changes in lag
time between the chemical injection point and the
sensor location.
In water disinfection applications, the ORP value
of the solution is more meaningful than mg/L
measurements of free residual or total chlorine.
A Few Case Studies
Results
The molecular form of free chlorine in water is
HOCl, or hypochlorous acid, a strong, fast-acting
oxidizer. As the pH increases, the HOCl converts
to its ionic form, OCl (the hypochlorite ion), which
is a weaker, slower acting oxidizer.
To maintain free chlorine in its most active form,
solution pH should be maintained between 7.4
and 7.6. An increase to a pH of 8.0 will convert 80
percent of the free chlorine to the hypochlorous
ion form.