Environmental Engineering

Lecture 7

Dr. Hasan Hamouda

DisinfectionAs practiced in water treatment, disinfection refers

to operations aimed at killing or rendering harmless, pathogenic micro-organisms.

The other treatment procedures like coagulation and filtration should remove > 90 per cent of bacteria and viruses.

A good disinfectant should:Be toxic to micro-organisms at concentrations well

below the toxic threshold to humansHave a fast rate of killBe persistent enough to prevent regrowth of

organisms in the distribution systems

DisinfectionThe rate of destruction of micro-organisms is often

postulated as a first-order chemical reaction

DisinfectionThe following factors can result in low efficiency of

disinfection:

Turbidity Resistant organisms (Giardia) High amount of organic material Deposits of iron and manganese Oxidizable compounds

The most commonly used disinfectants are: Chlorine dioxide Ozone UV radiation Chlorination

Disinfection Chlorine Dioxide

Yellowish or yellow-green or reddish gas that liquefies at approximately l0oC.

Its application does not cause any deterioration of taste and odor.

Its disinfection efficiency is largely pH dependent.

The formulation of tri-halogen methane (THM) can be neglected (although other chlorinated compounds can also be formed).

It does not react with ammonia.

The solution in water is not stable and degrades, especially when exposed to light therefore chlorine dioxide must be produced on site before application.

Chlorine dioxide forms toxic inorganic compounds (Chlorate ClO3-), may provoke methanoglobinemia in babies (like nitrates N03 -) if concentrations in drinking water exceed the value of 0.1 mg/L.

Disinfection Chlorine Dioxide

Disinfection Ozone

Ozone is a bluish gas with an unpleasant smell.

It is one of the most powerful oxidizing agents

It can be produced in a high-strength electrical field from oxygen in pure form or from ionization of clean dry air

Ozone is chemically unstable it must be produced on site and used immediately

Substantial amounts of energy (10 to 20 kWh per kg ozone) are required to split the stable oxygen bond to form ozone.

Disinfection Ozone

The typical dosages ranging from 1 to 5 g/m3.Therefore, costs of ozonation are 2 to 3 times

higher than the costs for chlorinationeffective in killing virusesImprovement of odor and tasteTransformation of almost non-degradable

substances into easily degradable onesLargely pH independentNo remaining residuals - (Re-growth of micro-

organisms within the water supply system due to the production of more easily degradable substances)

Disinfection UV RadiationUV light is a promising method of disinfection.

Although it leaves no residuals, effective in disabling both bacteria and viruses. The most effective band for disinfection is in

the shorter range of 250 to 280 ηm. This is the range where the UV light is

absorbed by the DNA of the micro-organisms which then lead to a change in the genetic material so that they are no longer able to multiply.

Disinfection UV RadiationLight of this wavelength range can be generated with low-pressure mercury vapor lampsThe properties of UV radiation as a disinfectant include:

Necessity of having clear water(turbidity free) and thin sheets of waterNo residualNo odor, or taste problemsNo chemicals added

Disinfection Chlorination During the past 50 years, it has been the most widely used

procedure for the treatment of water However, the application of high doses of chlorine run the

risk of developing large amounts of potentially carcinogenic by-products

chlorine is a yellow-greenish gas showing high toxicity to humans and animals

Chlorine can be liquefied at room temperature

HOCl= H+ + OCl-

Disinfection Chlorination

HOCl molecule is the most effective compound for the disinfecting process.

Its efficiency is considered to be 80 times as high as that of the hypochlorite ion (OCl-).

Disinfection ChlorinationMost of the chlorine feed systems in use are

gas-to-solution systems, implemented only for indirect chlorination.

Indirect chlorination means the preparation of a chlorine solution from Cl2 gas and water on site, which then serves as the disinfectant. Instead of adding Cl2 gas to the water

Choosing hypochlorites is for safety reason because Chlorine gas is very toxic and its handling requires extreme care.

Disinfection Chlorination

A residual of at least 0.2 mg/L must be detectable after the disinfection step. The maximum level must not exceed 0.5 mg/LThe by-products of organics oxidized by chlorine are often:

Trihalogen methanesChlorinated phenolsHalogenated methanes and ethanesHalogenated hydrocarbons Chlorinated aldehydes

ADVANCED WATER TREATMENT PROCESSESThe purposes of advanced water treatment

processes are: To take a water treated by standard processes and to

improve it to an exceptionally high quality as often required by particular industries, e.g. beverage, pharmaceutical

To treat a water containing specific chemical or microbiological contaminants to an acceptable standard, e.g. the removal of iron and manganese, the removal of algae, the removal of specific organics

ADVANCED WATER TREATMENT PROCESSES

Examples of advanced water treatment processes are: Water softening (hardness removal) Iron and manganese removal Ion exchange reaction (IX)Adsorption of organicsMembrane processes including reverse osmosis

(RO) Electro-dialysis Reversal (EDR)