Presentation on theme: "Environmental Engineering Lecture 8. Disinfection As practiced in water treatment, disinfection refers to operations aimed at killing or rendering harmless,"— Presentation transcript:
Disinfection As 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 humans Have a fast rate of kill Be persistent enough to prevent regrowth of organisms in the distribution systems
Disinfection The rate of destruction of micro-organisms is often postulated as a first-order chemical reaction
Disinfection The 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 l0 o C. 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 – 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. The typical dosages ranging from 1 to 5 g/m3. Therefore, costs of ozonation are 2 to 3 times higher than the costs for chlorination effective in killing viruses Improvement of odor and taste Transformation of almost non-degradable substances into easily degradable ones Largely pH independent No remaining residuals - (Re-growth of micro-organisms within the water supply system due to the production of more easily degradable substances)
Disinfection – UV Radiation UV 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. Light of this wavelength range can be generated with low- pressure mercury vapour lamps The properties of UV radiation as a disinfectant include: Necessity of having clear water (turbidity free) and thin sheets of water No residual No odor, or taste problems No 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 – Chlorination Most 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 Cl 2 gas and water on site, which then serves as the disinfectant. Instead of adding Cl 2 gas to the water Choosing hypochlorites is for safety reason because Chlorine gas is very toxic and its handling requires extreme care. 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/L The by-products of organics oxidized by chlorine are often: Trihalogen methanes Chlorinated phenols Halogenated methanes and ethanes Halogenated hydrocarbons Chlorinated aldehydes
ADVANCED WATER TREATMENT PROCESSES The 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 particu1ar 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 organics Membrane processes including reverse osmosis (RO) Electro-dialysis Reversal (EDR)
Water Softening Example The chemical reaction for removal of calcium hardness by lime precipitation softening is given by What dosage of lime with a purity of 78 % CaO is required to combine with 70 mg/l of calcium 1 mole =162 g Lime 1 mole = 56 g
1 mole =162 g We have 283 g Lime 1 mole = 56g ?
Softening by Ion Exchange IX processes are reversible and the direction of the reaction depends on the concentrations and the level of saturation of Na resin. A water softening unit consists of a bed of the medium of about 0.5 to 2 m high with a filtration rate of 4l/s/m2.
Iron and Manganese Removal Fe Reduced water when comes in contact with air, it is aerated easily and the Fe is oxidized after a short time. Mn The pH has to be 9 or 10 before Oxidation by oxygen takes place.