DISINFECTION

1. DISINFECTION

2. Definition Disinfection is a unit process involving reactions that render pathogenic organisms harmless companion unit process is sterilization which is killing of all organisms (not practiced in water and waste water treatment).

3. content • methods of disinfection • factors affecting disinfection • various disinfectants • Chlorine • Chemistry • Unit design • Ozone • Ultraviolet

4. Methods Of Disinfection And Disinfectant Agents Used Chemical Agents Physical Agents

5. Factors Affecting Disinfection disinfectants are affected by the following • time of contact between disinfectant and the microorganism • the intensity of the disinfectant • age of the microorganism • nature of the suspending liquid • temperature.

6. Time Of Contact And Intensity Of Disinfectant • It is a universal fact that the time needed to kill a given percentage of microorganisms decreases as the intensity of the disinfectant increases, and the time needed to kill the same percentage of microorganisms increases as the intensity of the disinfectant decreases, therefore T  1/Im

7. Time Of Contact And Intensity Of Disinfectant T = k/Im Known as Universal Law of Disinfection K (proportionality constant) can be determined experimentally Taking ( ln ) of both sides Ln t = Ln k – m Ln I Straight line equation with y-intercept [Ln k] and slope [m]

8. For [n] experiments • Time (t) is called contact time • Intensity (I) is called lethal dose

9. Example :time & intensity

13. Age of Microorganism • Young bacteria can be easily killed, while old ones are resistant. • As bacterium ages, a polysaccharide sheath is developed around the cell wall. • Using 2 mg/L chlorine for bacteria culture about 10 days old, it takes 30 min contact time to produce the same reduction as for young culture of about one day old dosed with one minute of contact time.

14. Nature Of The Suspending Fluid • For example, extraneous materials such ferrous, manganous, hydrogen sulfide, and nitrates react with applied chlorine before the chlorine can do its job of disinfecting. Also, the turbidities of the water reduces disinfectant effectiveness by shielding the microorganism. Hence, for most effective kills, the fluid should be free of turbidities.

15. Effect Of Temperature • The variation of the contact time to effect a given percentage kill with respect to temperature can therefore be modeled by means of the Van't Hoff equation

16. KT1 and KT2 are the equilibrium constants at temperatures T1 and T2, respectively. H0298 the standard enthalpy change of the reaction and R is the universal gas constant. If KT1 is replaced by contact time tT1 at temperature Tl and KT2 is replaced by contact time tT2 at temperature T2, the resulting equation would show that as the temperature increases, the contact time to kill the same percentage of microorganisms also increases. Of course, this is not true. Thus, the replacement should be the other way around. Doing this is the same as interchanging the places in the difference term between T1 and T2 inside the exp function. Thus, doing the interchanging .

19. Other Disinfection Formulas • The literature reveals other disinfection formulas. These include Chick's law for contact time, modifications of Chick's law, and relationship between concentration of disinfectant and concentration of microorganisms reduced in a given percentage kill. Chick's law and its modification called the Chick-Watson model, however, are not useful formulas, because they do not incorporate either the concentration of the disinfectant that is needed to kill the microorganisms or the incorporation of the concentration is incorrect. • The relationship of the concentration of disinfectant and the concentration of the microorganisms is also not a useful formula, since it does not incorporate the contact time required to kill the microorganisms. It must be noted that for a formula to be useful, it must incorporate both the concentration (intensity) of the disinfectant and the contact time corresponding to this concentration effecting a given percentage kill. For these reasons, these other disinfection formulas are not discussed in this book. • The Chick-Watson model needs to be addressed further. Watson explicitly expressed the constant k in Chick's law in terms of the concentration of disinfectant C as Cn , where  is an activation constant and n is another constant termed the constant of dilution. Chick's Law, thus, became Ln(N/No) = - Cn where N is the concentration of microorganisms and t is time. Note that C is a function of time. When this equation was integrated, however, it was assumed constant, thus producing the famous Chick-Watson model, where No is the initial concentration of microorganisms. Because the concentration C was assumed constant with time during integration, this equation is incorrect and, therefore, not used in this book.

20. Disinfection happens as a result of • damage to the cell wall : cell lysis and death. • alteration of cell permeability : causes the membrane to lose selectivity to substances and allow important nutrients such as phosphorus and nitrogen to escape the cell. (phenolic compound) • alteration of the protoplasm : alteration of the structure and producing a lethal effect on the microorganism. ( Heat & Acids and Alkali ) • inhibition of enzymatic activities : cause the rearrangement of the structure of enzymes. ( chlorine)

21. Physical Agents ultraviolet light (UV) electron beam gamma-ray irradiation Sonification heat.

22. Physical Disinfection • Gamma rays are emitted from radioisotopes, such as cobalt-60, because of their penetrating power, have been used to disinfect water and wastewater. • electron beam uses an electron generator. A beam of these electrons is then directed into a flowing water or wastewater ,to be disinfected. For the method to be effective, the liquid must flow in thin layers. Disadvantage: production of intermediates and free radicals as the beam hits the water. • sonification high-frequency ultrasonic sound waves are produced by a vibrating-disk generator. These waves rattle microorganisms and break them into small pieces.

23. Ultraviolet Light (UV) • Water, air, and foodstuff can be disinfected using UV. • radiation destroys bacteria, bacterial spores, molds, mold spores, viruses, and other microorganisms. • Radiation at a wavelength of around 254 nm penetrates the cell wall and is absorbed by the cell materials including DNA and RNA stopping cell replication or causing death. • The use of UV radiation for disinfection dates back to the 1900s in disinfecting water supplies. • The low-pressure mercury arc lamp is the principal means of producing ultraviolet light at a wavelength of 253.7 nm which is within the optimum range for germicidal effect. • however, that the optimum range for germicidal effect is within the UV-B (<320 nm) a dangerous range for causing skin cancer.

25. Unit Operation In UV Disinfection • Generation of UV Radiation • Inserting into or above the flowing water or wastewater to be disinfected • Contact time is very short being in range of seconds to few minutes • Intensity is normally expressed as milliwatts per square centimeter or projected area • To be effective, the “sheet” of flowing liquid should be thin so that radiation can penetrate • Lamb bulbs are typically 0.75 m to 1.5 m in length and 15-20 mm in diameter.

26. Example : UV

29. Chemical Agents widely used chemical agent is chlorine. Other chemical gents are ozone, CI02, the halogens bromine and iodine and romaine chloride, the metals copper and silver, KMn04, phenol, alcohols, soaps and detergents, quaternary ammonium salts, hydrogen peroxide, and various alkalis and acids.

30. CLO2 (Strong Oxidant) • does not form trihalomethanes that are disinfection by-products and suspected to be carcinogens. • effective in destroying phenolic compounds that often cause severe taste and odor problems • Although its principal application as been in wastewater disinfection, chlorine dioxide has been used in potable water treatment for oxidizing manganese and iron and for the removal of taste and odor. Disadvantages: • Similar to the use of chlorine, it produces measurable :sidual disinfectants. • CIO2 is a gas and its contact with light causes it to photooxtize, however. Thus, it must be generated on-site. • its probable conversion to chlorate, a substance toxic to humans, makes its use for potable water treatment questionable.

31. Ozone • very strong oxidizer and has been found to be superior to chlorine in inactivating resistant strains of bacteria and viruses. • very unstable • half-life of only 20 to 30 min in distilled water. therefore generated on site before use. • Typical dosage is 1.0 to 5.3 kg/IOOO m3 of treated water at a power consumption of 10 to 20 kW/kg of ozone. • complete destruction is accomplished with a residual of 0.3 mg/L of ozone in 3 min. in distilled water

32. Ozone Production first refrigerating air to below the dew point to remove atmospheric humidity. The dehumidified air is then passed through desiccants such as silica gel and activated silica to dry to -40 to -60°e. The dried and dehumidified air is then introduced between two electrically and oppositely charged plates or through tubes where an inner core and the inner side of the tube serve as the oppositely charged plates. Passage through these plates converts the oxygen in the air into ozone according to the following reaction 3 O2 2 O3

33. demand in ozonation is also first exerted before the actual disinfection process take place. This immediate ozone demand is due to ferrous, monogamous, nitrites, and hydrogen sulfide. The immediate demand reaction with ozone are as follows: these reactions must be satisfied first before the actual act of disinfecting commences. a mole of ozone grabs 2 moles of electrons, making it a strong oxidizer. molecular O2 has been produced from the decomposition of ozone. This is one of the advantages in the use of ozone; the effluent is saturated with dissolved oxygen.

36. Unit Operations In Ozonation • In its simplest form, the unit operations of ozonation involve the production of ozone and the mechanics of dissolving and mixing the ozone in the water or wastewater. • the contact time and dosage being best determined by a pilot plant study. But, a contact time of 20 min is not unreasonable, and a residual of 0.4 mg/L of ozone has been found to be effective.

37. Example : Ozone

38. Chlorine • The first use of chlorine as a disinfectant in America was in New Jersey in the year 1908 (Leal, 1909). At that time George A. Johnson and John L Leal chlorinated the water supply of Jersey City, NJ. • The principal compounds of chlorine that are used in water and wastewater treatment are the molecular chlorine (CI2), calcium hypochlorite [Ca(OCL)2] and sodium hypochlorite [NaOCl]. Sodium hypochlorite is ordinary bleach. Chlorine is a pale-green gas, which turns into a yellow-green liquid when pressurized. Both the aqueous and liquid chlorine react with water to form hydrated chlorine. Below 9.4°C, liquid chlorine forms the compound Cl2. 8H2O. • Chlorine gas is supplied from liquid chlorine that is shipped in pressurized steel cylinders ranging in size from 45 kg and 68 kg to one tonne containers. It is also shipped in multiunit tank cars that can contain fifteen I-tonne containers and tank cars having capacities of 15, 27, and 50 tonnes.

39. Handling Chlorine Gas the following points are important to consider: • Chlorine gas is very poisonous and corrosive. Therefore, adequate ventilation should be provided. In the construction of the ventilation system, the capturing hood vents should be placed at floor level, because the gas is heavier than air. • The storage area for chlorine should be walled off from the rest of the plant. There should be appropriate signs posted in front of the door and back of the building. Gas masks should be provided at all doors and exits should be provided with clearly visible signs

40. Chlorine solutions are very corrosive and should therefore be transported in plastic pipes. • The use of calcium hypochlorite or sodium hypochlorite as opposed to chlorine gas should be carefully considered when using chlorination in plants located near residential areas. Accidental release of the gas could endanger the community. Normally, small plants that usually lack well trained personnel, should not use gaseous chlorine for disinfection. • Calcium hypochlorite can oxidize other materials, Sodium hypochlorite is also affected by heat and light. Thus, both should be stored in a cool dry place and in corrosion-resistant containers. • High-test calcium hypochlorite, HTH, contains about 70% chlorine. • Sodium hypochlorite can contain 5 to 15% available chlorine.

41. Chlorine Chemistry ( content ) • hydrolysis and optimum pH range of chlorination • expression of chlorine disinfectant concentration • reaction mediated by sunlight • reactions with inorganic • reactions with ammonia • reactions with organic nitrogen • breakpoint reaction • reactions with phenols • formation of trihalomethanes • acid generation • available chlorine.

42. Chlorine Vs Hypochlotites All the chlorine disinfectants reduce to the chloride ion (Cl-) when they oxidize other substances, which must, of course, be reducing substances. The chlorine starts with an oxidation state of zero and ends up with a -1; it only needs one reduction step. One the other hand, the hypochlorites start with oxidation states of + 1 and end up with also a -1; thus, they need two reduction steps. Because the chlorine atom only needs one reduction step, while the hypochlorites need two, the chlorine atom is a stronger oxidizer than the hypochlorites. As a stronger oxidizer, it is also a stronger disinfectant

43. Hydrolysis & Optimum pH Range of Chlorination • in the form of liquefied chlorine. The liquid must then be evaporated into a gas. As the gas, CI2(g) is applied into the water or wastewater, it dissolves into aqueous chlorine, CI2(aq) as follows: • CI2(aq) then hydrolyzes, one of the chlorine atoms being oxidized to + 1 and the other reduced to -1. This reaction is called disproportionation. The reaction is as follows:

44. Note that hydrochloric acid is formed. This is a characteristic in the use of the chlorine gas as a disinfectant. The water becomes acidic. Also the chlorine molecule is a much stronger oxidizer than the hypochlorite ion and, hence, a stronger disinfectant. If the water is intentionally made acidic, the reaction will be driven to the left, producing more of the chlorine molecule. This condition will then produce more disinfecting power. HOCI further reacts to produce the dissociation reaction:

45. Distribution of CI2(aq) and HOCl Taking log and rearrangement pKH is the negative logarithm to the base 10 of KH

47. The concentration of 1.0 gmmole/L of chloride is 35,500 mg/L. This will never be encountered in the normal treatment of water and wastewater. Disregarding this entry in the table, the concentration of CI2(aq) is already practically nonexistent at around pH 4.0 and above. ill fact, it is even practically nonexistent at pH's less than 4, except when the pH is close to zero and chloride concentration of 0.1 gmmol/L; but, 0.1 gmmol/L is equal to 3,500 mglL, which is already a very high chloride concentration and will not be encountered in the treatment of water and wastewater. • Practically, then, for conditions encountered in practice, at pH's greater than 4.0, [HOCI] predominates over CI2(aq)

48. Distribution of HOCl and OCl- • Note that from the previous result, HOCI predominates over CI2(aq) above pH 4.0, CI2(aq) being practically zero. Thus, above this pH, the distribution of the chlorine disinfectant species will simply be for HOCI and OCl- • Taking log. And rearrangement

50. This table shows that HOCI predominates over OCl- at pH's less than 7.5. Also considering Table 17.3, we make the conclusion that for all practical purposes, HOCI predominates over all chlorine disinfectant species in all pH up to less than 7.5, the concentration of HOCl and OCl- are equal and above that pH OCl- predominant over all chlorine disinfectant. But HOCL is 80 – 100% more effective than OCl so the optimum range will be up to pH 7.0. and beyond this range OCL predominant and disinfection become less effective. The three species Cl, HOCL, OCL are called free chlorine