Automated Chlorination

Systems – Advantages and

Disadvantages

Jones & Henry Engineers, Ltd.

Presenters

Paul Romano, P.E.

Principal, Kalamazoo Office Director

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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.

In Conclusion

An automated chlorination

system saves operator time

and plant chemical overhead.

Adds an extended, full-time

element of control throughout

plant operations.

Helps better meet government

effluent regulations.

Thank you for listening!

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