Lecture 4: Free chlorine and hypochlorite

Prepared by

Husam Al-Najar

The Islamic University of Gaza- Environmental Engineering DepartmentEnvironmental Measurements (EENV 4244)

Introduction

Why we should measure chlorine?

What do we mean by residual chlorine?

What is the source for presence of chlorine in water and wastewater?

Is it mandatory to measure residual chlorine in water and wastewater?

Treated of filtered water is deemed to be fit for consumption only if it is devoid of diseases producing microorganism.

Chlorination is primarily adopted to destroy or deactivate diseases- producing microorganisms in the public water supply.

Chlorine is added to water, some of the chlorine reacts first with organic materials and metals in the water and is not available for disinfection (this is called chlorine demand of the water).

The remaining chlorine concentration after the chlorine demand is accounted for is called total chlorine.

Chlorine is one of the most widely used disinfectants.

It is very applicable and very effective for the deactivation of pathogenic microorganisms.

Chlorine can be easily applied, measures and controlled. It is relatively cheap.

However, we only started using it as a disinfectants on a wider scale in the nineteenth century, after Louis Pasteur discovered that microorganisms spread certain diseases.

Chlorine has played an important role in lengthening the life-expectancy of humans.

Chlorine can be used as a disinfectant of the following forms:

Gaseous form Cl2.

Bleaching solution (NaOH): Bleach consists of chlorine gas dissolved in an

alkali-solution, such as sodium hydroxide (NaOH).Chlorine reacts with sodium hydroxide to sodium hypochlorite (NaOCl).

Bleaching powder Ca(OCl)2: This is produced by directing chlorine through

calcium hydroxide (CaOH). The benefit of bleaching powder is that it is a solid.

Water borne diseases

Total chlorine

Combined chlorine

Reacts with nitrates and unavailable for disinfection

Free chlorine

Available to inactivate diseases causing- organisms

Residual chlorine is usually tested in finished water which is ready to be released into the distribution system, although operators must also ensure that there is adequate residual at the extreme ends of the distribution system.

The chlorination process

• Chlorine readily combines with chemicals dissolved in water, microorganisms, small animals, plant material, tastes, odors, and colors.

• These components "use up" chlorine and comprise the chlorine demand of the treatment system.

• It is important to add sufficient chlorine to the water to meet the chlorine demand and provide residual disinfection.

1. Chlorine demand is the amount of chlorine required to kill bacteria, oxidize iron or other elements in the water.

2. Free available chlorine residual is the amount of chlorine remaining in the water after the chlorine demand has been met.

3. Contact time is the amount of time that the chlorine is present in the water. The combination of chlorine residual and contact time determines the effectiveness of the chlorination treatment.

Combined

FreeBreakpoint

SP 4 SP 1

SP 6

SP 3

SP 7

SP 8

SP 9

SP 2

SP 5

Flow from the source

Water distribution network – sampling locations

Residual Chlorine vs. distance from chlorination source

Source: H. Sadallan and H. Najar, 2013

Average free chlorine vs. water borne diseases (Diarrhoea)

Source: H. Sadallan and H. Najar, 2013

Kinetics of Disinfection

Inactivation is a gradual process involving a series of physicochemical and biochemical steps. Inactivation is described by the equation:

Nt/N0 = e-kt

Where:

N0 = number of microorganisms at time = 0Nt = number of microorganisms at time = tk = a decay constant (1/time)t = time

Lo

g (N

/N )

Time

Shoulder

Ideal, first order

Rapid, initialinactivation

Ta iling o ff

0

t

o

-x

Ideally, inactivation follows first-order kinetics (blue line), but often non-ideal behaviors occur resulting from clumping of cells or multiple hits of critical sites before inactivation

Concentration and Contact Time

Effectiveness of chlorination depends primarily on the concentration used and the time of exposure

Disinfectant effectiveness can be expressed as a C ▪ t value where:

C = disinfectant concentrationt = time required to inactivate a 99% of the population under specific conditions

The lower the C ▪ t, the more effective the disinfectant

In general, resistance to disinfection is in the following order:

vegetative bacteria < enteric viruses < spore-forming bacteria < protozoan cysts

Sampling and Sample handling and preservation

As a general guide, the World Health Organization (WHO, 2006) recommended that one sample per 1000 persons served should be examined each month for water supply system serves up to 100,000 persons

Preservation of the sample is not practical. Because biological activity will continue after sample has been taken, changes may occur during handling and storage.

If analysis is to be carried out within 2 hours of collection, cool storage is not necessary.

If analysis can not be started within 2 hours of sample collection keep the sample at 40C.

Do not allow sample to freeze

Do not open the sample bottle before analysis

Begin analysis within 6 hours of sample collection.

Total residual chlorine measurement methods

N, N diethyl-P- Phenylenediamine DPD

Amperometric titration Iodometric

Environmental significance: active chlorine (free and combined) should be determined at each stage in the treatment process of drinking water and in the water mains in order to guarantee bacteriological impeccable water.

U.S. EPA-approved analytical methods for chlorine