[HWTS] W03 - Chemical Disinfection | Waste Management Engineering

chlorine is the most widely used chemical disinfectant globally
- has been used for over a century
- beginning with UK and then US
- 98% of water treatment in US
second most common treatment after boiling
- 5.6% of households
chlorine is a strong oxidant
- devastates cell walls, DNA, enzymes and organic compounds in general
destruction mechanism not fully understood
- especially for viruses and protozoa
not effective against Cryptosporidium

when a chlorine source is added to water, it becomes hypochlorous acid (HOCl)
HOCl loses a proton at higher pH and becomes hypochlorite or OCl
- with a negative charge
these two species together are called “Free Chlorine”
- at pH 7.54, the two species are present in equal amounts
- at higher pH, hypochlorite dominates
- at lower pH, hypochlorous acid is the main species
hypochlorous acid is the stronger disinfectant of the two
- water should have pH of less than 8 when chlorinated for disinfection
- to ensure reasonable amount of hypochlorous acid

chlorine is highly reactive, reacts with the chemicals naturally present in water
- the dose has to meet the chlorine demand of these reactions and
- the amount necessary to neutralize the pathogens
chlorine demand may be caused by
- organic carbon (naturally present in surface water)
- iron (ground water)
- manganese (ground water)
- ammonia (ground water)
target residual concentration: at least 0.5 mg/L of chlorine in water
- may require a dosage of 1-5 mg/L with 2mg/L fairly typical

reduces the effectiveness of chlorine both by shielding pathogens
turbidity creates chlorine demand and wastes chlorine by reacting it away
- standard doses are doubled when turbidity is present
ideally, the right dose is determined by trial and error for the desired residual

chlorine doesn’t pose any health risks
concentrated forms need careful handling however
WHO lists a guideline value of 5 mg/L
- this is a conservative guideline
- already results in unacceptable tastes and odors at this level

US Centers for Disease Control and Pan American Health Organization
- chlorine disinfection
- safe storage
- behavior change communication
has been taken up latin america and parts of africa
- involves production of dilute sodium hypochlorite solution (0.75% free chlorine)
hypochlorite form is more stable than hypochlorous acid
- are typically stabilized at pH 11 or greater
for storage, household bleach pH is around 12
- it is slightly irritating to the skin
bottle is designed such that one capful is good for local use containers
- one capful/20L water
- dose of at least 1.9 mg/L
- double if water is turbid

NaDCC
- sodium dichloroisocyanurate
- about 60% free chlorine
- used in swimming pools - reservoir chlorine
- guideline value of 40 mg/L
- doesn’t pose a health risk
consistent free chlorine level
- less taste and odor
- because of the reservoir effect
NaDCC tablets = effervescent tablets
- long shelf life
- 10 L: 1 X 33 mg tablet if clean, 2 if turbid
- 20 L: 1 X 67 mg tablet if clean, 2 if turbid
easy-to-use

dose = Concentration * time (Ct)
- min-mg/L
recommended 0.5 mg/L, 30 min
- Ct = 15 min-mg/L
- equivalent to 1 mg/15 mins
HWTS min concentration: 1.9 mg/L
- Ct = 56 min-mg/L
Ct_99: Ct value required for 99% pathogen kill
- 99% kill = 2 Log Reduction value

free chlorine + ammonia = chloramines
- monochloramine
- dichloramine
- trichloramine
strong odor: the swimming pool odor
less efficient disinfectant

residual chlorine reacts with organic carbon dissolved in water
- if high levels of carbon present
- leads to hazardous by-products
organic carbon
- trihalomethanes (e.g. chloroform)
- haloacetic acids
- carcinogenic in lab animals at high concentrations
guideline values
- 1 in 100000 excess
justification is it is a much smaller risk compared to the major water borne diseases