Disenfiction - KFUPM

Disenfiction - KFUPM

DISINFECTION 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). content

methods of disinfection factors affecting disinfection various disinfectants Chlorine

Chemistry Unit design Ozone Ultraviolet Methods Of Disinfection And Disinfectant Agents Used Chemical Agents

Physical Agents 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. 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/IIm Time Of Contact And Intensity Of Disinfectant T = k/IIm 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] For [n] experiments

Time (t) is called contact time Intensity (I) is called lethal dose Example :time & intensity 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/IL 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.

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. 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

. 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 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/INo) = - 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. 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) Physical Agents ultraviolet light (UV) electron beam gamma-ray irradiation Sonification heat. 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. 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. 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. Example : UV 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. CLO2 (Strong Oxidant) does not form trihalomethanes that are disinfection byproducts 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. CIO is a gas and its contact with light causes it to 2 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. 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/IIOOO m3 of treated water at a power consumption of 10 to 20 kW/Ikg of ozone. complete destruction is accomplished with a residual of 0.3 mg/IL of ozone in 3 min. in distilled water 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 -60e. 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

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.

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/IL of ozone has been found to be effective. Example : Ozone 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 9.4C, 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.

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 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. 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. 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 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:

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

The concentration of 1.0 gmmole/IL of chloride is 35,500 mg/IL. 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/IL; but, 0.1 gmmol/IL 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) 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 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 Expression of chlorine disinfection concentration To unify the concentration, it will be expressed in term of molecular chlorine Cl2 Portents reaction are: & from these reaction Reaction Mediated by

Sunlight Aqueous chlorine is not stable in presence of sunlight which contains UV, this radiation provides energy that derive chemical reaction for breaking up the molecule of hypochlorous acid. Electrons released will reduce chlorine atom in HOCl to chloride.

Disinfectant should be stored in opaque container otherwise the chlorine gas will be converted to hypochloric acid, and the hypochlorites will be converted into corresponding salt. Reaction with Inorganic The major substances that interfere the disinfection process and can be present in water are

Ferrous manganous Nitrites hydrogen sulfide. Reaction with Ammonia & Optimum pH Range for Chloramines Formation

Effluents from sewage treatment plants can contain significant amounts of ammonia that when disinfected, instead of finding free chlorine, substitution products of ammonia called chloramines are found. In addition, in water treatment plants, ammonia are often purposely added to chlorine. This, again, also forms the chloramines. Chloramines are disinfectants like chlorine, but they are slow reacting, and it is this slow-reacting property that is the reason why ammonia is used.

The purpose is to provide residual disinfectant in the distribution system. In other words, the formation of chloramines assures that when the water arrives at the tap of the consumer, a certain amount of disinfectant still exists. The formation of chloramines is a stepwise reaction sequence. When ammonia and chlorine are injected into the water that is to be disinfected, the following reactions occur, one after the other in a stepwise manner. Reactions with pH

Reaction (1) indicates that at the time when one mole of HOCI is added to one mole of NH3, the conversion into monochloramine is essentially complete. In view of the relationship of HOCI and OCL as a function of pH, however, this statement is not exactly correct. From previous discussions, at pH 7.5, hypochlorous acid and the hypochlorite ion exist in equal mole concentrations, but beyond pH 7.5, the hypochlorite ion predominates. OCl does not directly react with NH3 to form the monochloramine, but must first hydrolyze to produce the HOCI before Reaction proceeds. Thus, when the pH is above 7.5, addition of one mole of HOCI to one mole of ammonia does not guarantee complete conversion into NH2Cl. At these pH values, the one mole of HOCI added

becomes lesser, because of the predominance of the hypochlorite ion. HOC1, however, exists at practically 100 concentrations at pH's below 7.0; hence, at this range, a mole for mole addition would essentially guarantee the aforementioned conversion into monochloramine. Same for reaction (2) & (3) ; at pH >7.5, the conversion are not complete

More details about forming N gas Reactions with Organic Nitrogen Chlorine reacts with organic amines to form organic

chloramines. Examples of the organic amines are those with the groups NH2, -NH-, and -N =. Parallel to its reaction with ammonia, HOCI also reacts with organic amines to form organic monochloramines and organic dichloramines by the chloride atom simply attaching to the nitrogen atom in the organic molecule. For example, methyl amine reacts with HOCI as follows As in the conversion of monochloramine to dichloramine, monochloromethyl amine converts to dichloromethyl amine in the second step reaction as follows:

Other nitrogen-containing organic compounds are the amides which contain the group OCNH2 and -CNH-. The ammonia and organic amine molecules have basic properties. They react readily with HOCl, which is acidic. The organic amides, on the hand, are less basic than the amines are; thus, they do not react as readily to form organic chloramides with hypochlorous acid.

They consume chlorine, however, so organic amides as well as organic amines are important in chloramination. Although the organic chloramides and organic chlorarnines have some disinfecting power, they are not as potent as the ammonia chloramines; thus, their formation is not beneficial. Breakpoint Reactions (1) before A

Figure shows the status of chlorine residual as a function of chlorine dosage. From zero chlorine applied at the beginning to point A, the applied chlorine is immediately consumed. This consumption is caused by reducing species such as Fe2+, Mn2+, H2S, and NO.

no chlorine residual is produced before point A (2) between A-B organic amines and their decomposition products such as ammonia may be present. In addition, ammonia may be purposely added for chloramine

formation to produce chlorine residuals in distribution systems. Also, other organic substances such as organic arnides may be present as well. chloroorganic compounds and organic chloramines are formed. Ammonia will be converted to monochlorarnine at this (3) beyond B breakpoint

the chloro-organic compounds and organic chlorarnines break down. the monochlorarnine starts to convert to the dichloramine, but, at the same time, it also decomposes into the nitrogen gas. the dichloramine converts to the trichloramine, the conversion being complete at the lowest point indicated by "breakpoint In addition, nitrates will also be formed from the dichloramine before reaching the breakpoint. As shown by the downward swing of the curve, the reactions that occur between point B and the

breakpoint are all breakdown reactions. Substances that have been formed before reaching point B are destroyed in this range of dosage of chlorine. These breakdown reactions have been collectively called breakpoint reactions.

The breakpoint reactions only break down the decomposable fractions of the respective substances. All the nondecomposables will remain after the breakpoint. This will include, among other nondecomposables, the residual organic chloramines, residual chloro-organic compounds, and residual ammonia chloramines. any amount of chlorine applied beyond the breakpoint will appear as free chlorine residual.

Important knowledge is gained from this "chlorine residual versus applied chlorine" curve. We have learned that all the ammonia chloramines practically disappear at the breakpoint. We have also learned that the organic chlorarnines are not good disinfectants. Therefore, as far as providing residual disinfectant in the distribution system is concerned, chlorination up to the breakpoint should not be practiced. The practice of chlorinating up to and beyond the breakpoint is

called superchlorination. Superchlorination ensures complete disinfection; however, it will only leave free chlorine residuals in the distribution system, which can simply disappear very quickly. If superchlorination is to be practiced to ensure complete disinfection and it is also desired to have long-lasting chlorine residuals, then ammonia should be added after superchlorination to bring back the chlorine dosage to the point of maximum monochloramine formation.

Reactions with phenols Chlorine reacts readily with phenol and organic compounds containing the phenol group by substituting the hydrogen atom in the phenol ring with the chlorine atom. These chloride substitution products are extremely odorous. Figure shows the threshold odor as a function of pH and the concentration of chlorine dosage

chlorination at acidic conditions would produce very bad odors compared to chlorination at high pH values. This is very unfortunate, because HOCI predominates at the lower pH range, which is the effective range of disinfection. increasing the chlorine dosage produces the worst nightmare for odor production.

Following Figure shows the reaction scheme for the breakdown of phenol to odorless low molecular weight decomposition products using HOCl. The threshold odor concentrations of the various chloride substituted phenolic compounds are also indicated in brackets. Note that the worst offenders are 2monochlorophenol and 2, 4-dichlorophenol, which have an odor threshold of 2.0 pg/IL. In order to effect these breakdown reactions,

superchlorination would be necessary, which would also mean that the odor had increased before it disappeared. Formation of trihalomethanes Reaction with organic compounds such as humic acids produce undesirable byproducts. These known as Disinfection byproduct DBPs. Such as, chloroform. See figure.

Chloroform formation enhanced at high pH. So disinfection at low pH will prevent chloroform formation and this is better as HOCl will be dominant. Acid Generation Whether or not acid will be produced depends upon the form of chlorine disinfectant used. Using chlorine gas will definitely

produce hydrochloric acid. Sodium hypochlorite and calcium hypochlorite will not produce any acid; on the contrary, it can result in the production of alkalinity. Superchlorination using HOCI will definitely produce acids. a mole of hydrochloric acid is produced per mole of chlorine gas that reacts. Chlorination uses up the disinfectant, so this reaction would be driven to the right and any mole of chlorine gas added will be consumed. Thus, if a mmol/IL of the gas is dosed, this will produce a mmol/IL of HCl. This is equivalent to one mgeq of the

acid, which must also be equivalent to a mgeq of alkalinity. The analytical equivalent mass of alkalinity in terms of CaCO 3 is 50 mg CaCO3 per mgeq. Thus, the mmol/IL of hydrochloric acid produced will need 50 mg/IL of alkalinity expressed as CaCO 3 for its neutralization. Or, simply, one mmol of hydrochloric acid requires 50 mg of alkalinity expressed as CaCO3 for its neutralization. Available chlorine

The strength of a chlorine disinfectant is measured in terms of available chlorine. Available chlorine is defined as the ratio of the mass of chlorine to the mass of the disinfectant that has the same unit of oxidizing power as chlorine. The unit of disinfecting power of chlorine may be found as follows in terms of one mole of electrons:

the unit of oxidizing power of CI2 is C12/I2 = 35.5 Consider another chlorine disinfcctant such as NaOCI. To find its available chlorine. its unit of disinfecting power must also, first, be determined. the unit of disinfecting power of NaOCI is NaOC1/I2 = 37.24.

the available chlorine of NaOCI is the ratio of the mass of chlorine to the mass of NaOCI that has the same unit of oxidizing power as chlorine. or available chlorine of NaOCl = 35.5/I37.24 = 0.95 or 95%. In other words, NaOCl is 95% effective compared with chlorine.

Design of Chlorination Unit Operation Important parameters to be considered Chlorine Feeder

Dosage Control Chlorine Injection Initial Mixing Contact time & Chlorine Dosage Maintenance & self-cleaning velocity Chlorine Feeder Chlorine Gas Feeder

Hypochlorite Solution Feeder Chlorine feeder using Gas Feeder Chlorine feeder using tonne container Chlorine feeder using chlorine cylinder

Hypoclorinator paced by mainline meter Dosage Control There are five ways of providing dosage control. (1) manual control. This is the simplest and involves the operator adjusting the now rate of chlorine to match requirements. The chlorine residual is checked at

intervals of time such as 15 min and dosage adjusted accordingly. The residual desired may be in the vicinity of 0.5 mg/IL This method of control is obviously used in small facilities. (2) program control. The program control is a selected set pattern of dosage that must have already been determined to effect the desired disinfection. Program control is the cheapest way to attain automatic control. (3) flow-portioned control. Flow-proportioned control proportions dosage according to the flow rate of the water to be disinfected. The rate of suction of the solution is proportioned to the reading of the meter. Flow-proportioned control is also called flow-paced control. (4) residual-proportioned control. Residual proportioned control proportions dosage according to the amount of chlorine residual desired. This system

requires an automatic residual chlorine analyzer at the effluent and a signal transmitter. The signal is sent to a controller that then changes valve settings for proper dosage. (5) combination of the flow proportioned and residual-proportioned control. In this setup, the two signals coming from the flow meter and the residual chlorine analyzer are transmitted to a controller that calculates the resulting valve setting according to these signals' input.

Dosage control type 5 Chlorine Injection & Initial Mixing To ensure complete disinfection chlorine should be mixed at point of application

Rapid mixing should be instituted at the point of application so that the disinfectant can immediately act on microorganism rather than wasting time reacting with intervening substances Contact time and chlorine dosage

The two most important parameters used in the design of chlorine contact tanks is the contact time and dosage of chlorine. Maintenance of self-cleaning velocity In the case of sewage treatment plants. some solids would have escaped settling. In the case of water treatment plants,

however, the effluent should be very clear with no danger of solids depositing on the chlorine contact tank. As in any design of open channels, the velocity through the cross section should be self-cleaning. We have seen this requirement in design of sewers. The design of chlorine contact tanks is no exception. Self-cleaning velocities of 2.0 to 4.5 m/Imin have been mentioned in the literature. Dechlorination

Effluents from sewage treatment plants are not allowed to contain residual chlorine in excess of tolerable values as determined by water quality standards. Thus, chlorinated

effluents should be dechlorinated. Sulfur dioxide, sodium sulfite, sodium metabisulfite, and activated carbon have been used for dechlorination. Because sulfur dioxide, sodium sulfite, and sodium metabisulfite contain sulfur, we will call them sulfur dechlorinating agents. Dechlorination is an oxidation-reduction reaction. Sulfur Dechlorination Agents HOCL represents the residual chlorine

Using SO2 Using Na2SO4 Using Na2S2O5

. Activated Carbon Dechlorination Carbon is a reduced agent. So chlorine will be reduced to chloride, and carbon will be oxidized to carbon dioxide

Effect of Dechlorination Effluents on DO of Receiving Stream Sulfur dechlorination agents should be controlled and effluent minimized because of its effect in DO of water. The reaction of these residual on oxygen are:

This will reduce DO and increase BOD, COD. And also production of H+ ions may decrease the pH Thanks Q&A

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