- the dosage is the total chlorine added.- the demand is the amount used to oxidize the materials existing in water - The residual = dosage – demandThe total residual chlorine will result from

1-free chlorine such as (HOCl , OCl¯,dissolved Cl2 gas)

2- Combined chlorine compounds residual

-Since the free residual is much more effective in disinfection, all the regulationsrequire a free residual of at least 0.20 mg/l at the farthest tap in the system. TheResidual chlorine in the produced water is typically 2 – 5 mg/l.

The hydrolysis:

When chlorine gas is added to water it rapidly hydrolyzes

Cl₂+H₂O⇔HOCl +HCl (1)

And the the hypochlorous acid will disassociated to hypochlorite ion

As

HOCl⇔ OCl¯ +H⁺ (2)

And the hydraulic acid (HCl) will disassociated also to

HCl⇔ H⁺ +Cl¯ (3)

Equation (1) is rapid forward reaction which produces the strong disinfectant (HOCl), the ion hypochlorite ion is less effective than (HOCl)

The disassociation and equilibrium will depends on

1- Water temperature

2- Water PH

HOCl concentration will be increased in the solution with respect to OCl¯ at high temperature and low PH

the strength of disinfectant will depends on the HOC concentration

Contact time:

The contact time is a measure of effectiveness of chlorine disinfection against Bacteria, Viruses and Protozoa

The minimum time for free residual chlorine of 0.2mg/l is about 30 minute @ water temperature>10⁰C

This will let the contact time (CT) = 6

Where the CT=residual chlorine concentration x time before use in minutes

Required CT values are dependent on pH, residual concentration, temperature, and the Disinfectant usedDisinfection CT values shall be calculated daily, using either the maximum hourly flow and the Disinfectant residual at the same time or by using the lowest CT value if it is calculated more frequently, Actual CT values are then compared to required CT values, Results shall be reported as a reduction Ratio, along with the appropriate pH, temperature, and disinfectant residual. The reduction Ratio must be greater than 1.0 to be acceptable.Normal and maintenance dosage:

Normal operation free chlorine residual is ≥ 0.2mg/l at end of distribution network and max allowable 4mg/l

Reservoir maintenance at water full use 10PPM for 24 hours or (50PPM for 6 hours)

But for empty reservoir cleaning use about 200PPM solution as spraying

Lab tests:The Break-point chlorination is where the chlorine demand has been satisfied, and any additional chlorine will be considered free chlorine.And since free residual appears only after the breakpoint, so we need to decide the breakpoint dosage and the required dosage = breakpoint dosage + free residual In Lab sample is taken from the raw water to determine the breakpoint

The test will depend on

1- Water temperature (as temperature decreases, more dosage is needed)

2- Water PH as PH increases more dosage is needed

3- Water turbidity (more turbidity needs more dosage)

4- Water chemical compounds presents such as (iron, manganese, ammonia compounds, etc)

5-types of Bacteria, Cholera to be disinfected

6- Contact time needed