Oxidation and Disinfection

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MODULE 4

DISINFECTION AND OXIDATIONMicrobiologically polluted water has been associated with the transmission of infectiousdiseases such as gastroenteritis, amoebiasis, giardiasis, salmonellosis, dysentry, cholera,typhoid fever and hepatitis A (Craun, 1986) for over a 100 years. Snow, in 1854, was thefirst to demonstrate the relationship between contaminated water and disease. Koch andPasteur developed the theory of disease resulting from germs and Koch demonstrated thatbacteria could be killed by chlorine in 1881. Frohlich (1886) disinfected water with ozone,but Schnbein may have experimented with ozonation in water treatment as early as 1840.The first water disinfection plants were built in Europe, in the late 19th century, some basedon ozone, some on chlorine, no clear evidence to which came first.

Infectious diseases occur as the result of interactions between pathogenic (disease-producing)

microorganisms and the host (Pelczar et al., 1986). Several types of microorganisms arepathogenic. Typhoid, cholera and gastroenteritis are bacterial diseases, which are commonlywaterborne. Similarly, viral diseases such as hepatitis, parasitic worms such as Schistasoma(bilharzia) and some tape worms, together with protozoan diseases such as amoebicdysentery, are waterborne. In the production of potable water, all water-borne organisms butespecially water-borne pathogens are of concern. The majority of these pathogens affect thegastro-intestinal tract and can be bacteria, viruses, protozoa and sometimes fungi (Carlson,1991). For a historical and present day etiology of some of these pathogens, the reader isreferred to White (1992).

Some of the most common waterborne diseases prevented by disinfection are shown in Table

4.1 (adapted from USAID, 1992).

Table 4.1 : Some common water-borne diseases prevented by disinfection

Bacterial Viral Parasitic

Typhoid feverPara-typhoidChildhood bacterial diarrhoeaCholera

Legionnaires disease

HepatitisRotavirus diarrhoea

AmoebiasisGiardiasisCryptosporidiasis

Protozoa have lately been identified as the gravest concern in disinfection. They form cyststhat are more resistant to disinfection than bacterial indicators (Rubin et at, 1983).Cryptosporidium and Giardia lamblia are the most common/problematic protozoa in drinkingwater. In the United States the inactivation of the protozoan Giardia (along with intestinalviruses) has been used as a yardstick for measuring the efficacy of disinfection (Teefy andSinger, 1990).

The mechanism of inactivation of microorganisms has not been established conclusively, butindicators are that chemical changes effected in the enzyme reactions of the cell render the

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organism incapable of ingestion and reproduction. It has been suggested that ozone attackscells directly by oxidation of the cell wall.

Water supplies in the First World are usually disinfected. The widespread adoption ofdisinfection was a major factor in reducing waterborne diseases, and this has been interpreted

as the major single factor in increasing average human life expectancy. Effluents can bedisinfected prior to discharge from treatment plants to receiving waters, to reduce theconcentration of pathogens. This practice is not universally adopted as disinfectant residualsmay be environmentally detrimental and because of financial restrictions.

This disinfectant demand of a water is usually governed by substances other than micro-organisms themselves. Certain dissolved substances are easily oxidized by disinfectants andtheir oxidation competes with the disinfection reaction. Sufficient disinfectant has to bedosed to oxidize these substances and to inactivate pathogens completely. Modern thinkingon water treatment sees oxidation as a separate unit process, independent from disinfection.Oxidation by-products play a significant role in subsequent treatment. Failure to provide for

this can detrimentally affect water distribution, however, proper design of the whole processinvolving oxidation with ozone in particular, can actually benefit the quality of the product.

4.1 REQUIREMENTS OF A DISINFECTANT

Disinfection is the elimination or inactivation of harmful organisms. It should bedistinguished from sterilization, which is the destruction of all life forms. Economicconsiderations preclude such drastic procedures.

A disinfectant must be able to destroy particular pathogens at the concentrations likely to

occur, and it should be effective in the normal range of environmental conditions.Disinfectants which require extremes of temperature or pH, or which are effective only forwaters with a very low turbidity, are unsuitable for large-scale use.

While the disinfectant should destroy pathogens, it must not be toxic to man or other higheranimals, such as fish, in receiving water. Ideally, some residual disinfecting capability shouldbe provided for a water supply to provide protection against re-infection while the water is ina distribution system. The residual, which passes to the consumer, should not be unpalatableor significantly alter the taste.

A disinfectant should be safe and easy to handle, both during storage and during addition.

The availability of simple or automatic analytical procedures ensures a reliable and consistentdosing system, which can be accurately controlled. The other major consideration is that ofcost, which is particularly important for municipal plants where large volumes must bedisinfected continuously.

These factors severely restrict the number of reliable disinfectants. The requirements ofeffectiveness in destroying pathogens, safety of handling, and non-toxicity to man in normaluse, present a major problem. For example, the addition of certain toxic metals can provideeffective destruction of pathogens but the residual toxicity is harmful to humans. Chlorine,the most common disinfectant is a dangerous chemical and requires rigorous safetyprecautions. The other disinfectants that have found large-scale use are ozone and chlorine

dioxide. Others, such as heat, ultraviolet irradiation, ultrasonic vibration, ultra-filtration,silver, bromide and iodine, only find limited application.

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Status of disinfection practices

In a recent survey (Disinfection Committee, 1992) of the disinfection practices in the USA itwas found that most water utilities were making concerted efforts towards reducing

halogenated by-products in potable water. Chlorine reacts with organic compounds presentin the water to form halogenated disinfection by-products (DBPs), most notablytrihalomethanes (THM), which may adversely affect human health. The most significantchange by utilities was to alter the point of application of chlorine, the dosage of chlorineused, and the addition of ammonia. It should be noted that although chloramines arerecommended as a primary disinfectant (Longley and Roberts, 1982) and for controllingbacterial regrowth in distribution systems (Neden et al., 1992), these substances could affectkidney dialysis patients and aquarium fish (White, 1992).

In a world-wide survey on disinfection practices (Hiisvirta, 1993) it was concluded that:

Water-borne epidemic outbreaks still occur today - even in developed countries. Outbreaks in developed countries were due to poor disinfection due to fear of chemical by-product formation.

Chlorination is still the dominant disinfection method throughout the world, with the use of Chlorine dioxide

as a substitute for chlorine decreasing rather than increasing.

Kinetics of Disinfection

The ability of a reagent to destroy pathogens is related to the concentration of the disinfectantand the contact time between the pathogens and disinfectant. One general relationship isknown as Chick's Law (Equation 4.1), which assumes that the inactivation of micro-organisms is controlled by processes of diffusion, resulting in a first-order rate of decrease.This relationship applies for a given concentration of disinfectant.

dN = -kN [4.1]dt

where N is the number of variable micro organisms of one type at time t,and k is a constant (dimension : t-1).

Integrating for N = No at t=0 gives

loge N = -kt [4.2]No

ThereforeN = e-kt [4.3]No

The rate of inactivation depends upon the number of microorganisms which were presentoriginally. If the micro-organisms all possess the same resistance, the kills follow anexponential pattern (Equation 4.3). A complete kill is not feasible according to this model.The efficiency of disinfection reported in terms of the ratio of microorganisms inactivated tothe original number of present, such as 99 % (2 logs) or 99.99 % (4 logs).

The definition of "life" is somewhat vague in microorganisms and "inactivation" is preferred to "kill".

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Chick's Law is not followed by all disinfectants for all micro-organisms. The rate ofinactivation depends upon such factors as the penetration of the cell wall, the time topenetrate vital centers. Each species of microorganism therefore will have a differentsensitivity to each disinfectant. This is accounted for by manipulating k or t in equation 4.3to linearalize plots of N/No. The simplest form of such equations is Watsons Law, which

enables us to quantify the effect of concentration for a given fraction of inactivation overwhich we have linearized:

Cn t = constant [4.4]

where C is the concentration of disinfectant; t is the contact time between disinfectant andmicroorganism to achieve the desired inactivation; and the exponent, n is a constant for onesystem of disinfectant and microorganism at one inactivation rate.

A value of n>1 implies greater dependence on disinfectant concentration, e.g. ozone. Forn


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The wide range of pathogens and their extremely low number preclude routine testing forparticular pathogens. Instead, the requirements for disinfection refer to indicator organisms,usually coliform bacteria or Escherichia coli (see Module 1). Coliform bacteria are the mostcommon intestinal flora and if they have all been eliminated, the disinfection process isconsidered complete. The absence of fecal coliform bacteria, especially E. coli, in at least

three samples of 100mL is acceptable as an indication that drinking water has been properlydisinfected. It is not necessary to inactivate all bacteria and total plate counts of up to 100bacteria per mL are acceptable for drinking water. Many pathogens are much more resistantto disinfectants than coliforms, leading to some doubt over suitable indicator organisms.

4.2 CHLORINE REACTIONS DURING WATER TREATMENT

Under normal conditions, chlorine is a yellow-green, corrosive gas with a density 2.5 timesthat of air. It can be liquefied under a relatively small pressure, 370 kPa. It is very soluble inwater and is a potent disinfectant even at low concentrations. It is normally dosed into a

small sidestream on large plants. It forms hypochlorous acid in water solution, which is theprimary disinfectant. On smaller plants, compounds which release hypochlorous acid, aredosed.

The reaction between chlorine and water

The addition of chlorine gas (Cl2) to pure water gives rise to the following equilibrium state:

Cl2 + H2O HOCl + H+ + Cl- [4.5]

This equilibrium state may be represented in its mass action form by:

{[HOCl] [H+] [Cl-] }/ [Cl2] = K1 [4.6]

The equilibrium constant (K1) has a value of 4.0 x 10-4 at 20C.

The hypochlorous acid itself ionizes as below.HOCl H+ + OCl- [4.7]

The equilibrium state may be represented in its mass action form by:

{[H+] [OCl-]} / [HOCl] = K2 (3.3 x 10 -8 at 20C) [4.8]

The distribution of the chlorine species according to Reaction 4.7 is shown in Figure 4.1.Below pH 2, chlorine is present in the dissolved molecular (Cl 2) form; at a pH of between 3and 7.5, the main species are hypochlorous acid (HOCl) and the chloride ion (Cl -); and at apH of > 7.5, the predominant species are the hypochlorous and chloride ions (OCl- and Cl-).

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Figure 4.1 Hypochlorous acid (HOCl) and hypochlorite ion (OCl-) species distributionas a function of pH (25 C)

The reaction between hypochlorous acid and ammonia

Extensive laboratory investigations are largely responsible for the present knowledge on thechemistry of chlorine-ammonia systems in water. Several reactions between ammonia andhypochlorous acid or the hypochlorite ion may take place, usually simultaneously, the relativeproportion of the products formed being governed by the pH of the water. The overall effect

is the oxidation of ammonia to chloramines, and even further to nitrogen. The removal ofammonia through oxidation by chlorine is referred to as breakpoint chlorination. Duringoptimum removal by breakpoint chlorination, the ammonia is oxidized to nitrogen gas asfollows:

Cl2 + H2O HOCl + H+ + Cl- [4.5]NH3 + HOCl NH2Cl + H2O [4.9]

2NH2Cl + HOCl N2 + 3H+ + H2O + 3Cl- [4.10]

3Cl2 + 2NH3 N2 + 6H+

+ 6Cl-

(overall reaction) [4.11]

The general reaction scheme can be represented as in Figure 4.2 in which the chlorineresidual is plotted against the total chlorine dose.

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Figure 4.2 Progress of the reaction between ammonia and chlorine in water

For the region A-B, the chlorine added reacts rapidly with any reducing agents in the sample.This reduces the chlorine to chloride (Cl-), which is not a disinfectant. The residual chlorineis low and, for a small chlorine dose, there will be little or no disinfection. The addition ofmore chlorine is represented by region B-C. The chlorine has completely oxidized thereducing agents and has formed monochloramine with ammonia.

Monochloramine and the minor by-product dichloramine are much less powerfuldisinfectants than is free chlorine. If they are present in high concentrations or if a longreaction time is permitted, these chloramines will inhibit pathogens. The chloramines aredisinfectants and are detectable as a chlorine residual. Monochloramine and dichloramine arereferred to as combined available chlorine. In the region (B-C) the addition of chlorineproduces an approximately proportional increase of combined chlorine residual.

The chloramines are oxidized to nitrogen in the region C-D. Stoichiometrically, the oxidationof ammonia through monochloramine to nitrogen gas corresponds with a chlorine toammonia nitrogen mass ratio of 7.6 to 1. The presence of other substances exerting a

chlorine demand will, however, necessitate higher chlorine dosages in order to achievebreakpoint chlorination.

Undesirable side reactions, which are a function of pH may also occur as shown below.

low pH: NH2Cl + HOCl NHCl2 + H2O (dichloramine) [4.12]

low pH: NHCl2 + HOCl NCl3 + H2O (trichloramine) [4.13]

high pH: NH2Cl + 3OCl- NO3- + 4Cl- + 2H+ [4.14]

Further addition of chlorine at a pH lower than about 6 produces di- and trichloramine (4.12and 4.13), while nitrate and nitrous oxide are produced at a higher pH than 8.Nitrogen, nitrates and nitrous oxide do not disinfect and the concentration of chloramines is

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lowered during these oxidation reactions. Thus, in the region C-D, the addition of furtherchlorine reduces the available chlorine and, hence, the ability of the solution to disinfect.

On further chlorine addition these reactions are complete (point D) and most of the ammoniawill be removed. Any subsequent addition of chlorine will remain as free available chlorine

(HOCl) and will act as a strong chlorine residual (DE). Point D is referred to as thebreakpoint. Chlorination is commonly carried out beyond the breakpoint, requiring a dosageof 8 to 10 times the NH3-N concentration.

The relative concentrations of free and combined chlorine residual can be monitored bysimple calorimetric or instrumental means. The methods rely upon the greater reactivity ofthe free chlorine (HOCl) which rapidly gives a reading, while the combined residual (NH2Cl)and NHCl2) reacts relatively slowly. For example, the reagent diethyl-p-phenylene-diaminesulphate (DPD) gives a red color by reaction with free chlorine. The addition of varyingamounts of potassium iodide induces the production of color by monochloramine anddichloramine. A complexing agent is incorporated with DPD sulfate to prevent interference

by trace metals. The reagent is available in tablet form; No 1 DPD will determine freeavailable chlorine, No 2 DPD the free chlorine plus monochloramine, and No 3 DPD the totalchlorine.

Chlorination Terminology

Free chlorine or available free chlorine

Free chlorine may be present as elemental chlorine (Cl 2), hypochlorous acid (HOCl), thehypochlorous ion (OCl-), or as hypochlorous acid together with either of the other forms,depending on the pH of the solution. The free chlorine species distribution is shown in

Figure 4.1. The (available) free chloride residual is the amount of free chlorine available forfurther disinfection after breakpoint has been achieved.

Combined chlorine residualThe combined (available) chlorine residual is the sum of all chlorinated nitrogenous productsdistinguishable as by analytical methods. The relevant species in water containing organicnitrogenous forms are monochloramine, dichloramine and trichloramine (more correctly:nitrogen trichloride).

Total chlorine residualThe total chlorine residual refers to the sum of the combined and free chlorine residuals.

Chlorine demandThe chlorine demandof a water is the difference between the amount of chlorine appliedto atreated supply and the total chlorine residual. It can also be regarded as the excess chlorineintroduced to produce a free chlorine residual. The chlorine demand of a water varies withthe amount of chlorine applied, pH, time of contact and temperature. Any value given forchlorine demand is worthless unless all conditions are specified. The chlorine demand ofwater increases with higher concentrations of nitrogenous compounds.

From the above it is clear that chlorine is still the most universally used disinfectant for theproduction of potable water. However, the designer should take note of the developments

regarding the potential health hazard of disinfection by-products, and how to minimize oravoid it, as well as the advantages and disadvantages of alternative disinfectants.

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The Environmental Protection Agency (EPA) in the USA has developed the so-called"Surface Water Treatment Rules" (SWTR) for the disinfection of surface water (Teefy andSinger, 1990). The SWTR contains tables of disinfectant-Ct values that have beendemonstrated to achieve the required degree of inactivation of viruses and Giardia cysts. Themethods by which C and t values for a particular application are calculated are very important

since factors like pH, temperature and flow pattern in the contact chamber, each plays a role(Rubin et at, 1983; Teefy and Singer, 1990; White, 1992).

Note: When chlorine gas is used for breakpoint chlorination, alkalinity is consumed.Stoichiometrically, 14,3 mg/L of alkalinity, expressed as CaCO3, will be consumed for each1,0 mg/l (NH3-N) that is oxidized in the chlorination process.

ChloraminationThe efficiency of the various chlorine forms as disinfectants differs, and thus, theconcentration of available chlorine is insufficient to characterize process performance. Theinactivation potency of HOCl : OCl - : NH2Cl : NHCl2 is approximately 1 : 0,0125 : 0,005 :

0,0166 (Haas, 1990). Although the immediate disinfection efficacy of chloramines is lessthan that of chlorine, they have a relatively long half life (approximately 100 h in distilledwater). Because of this property, chloramines are often used in a post-chlorination step toprevent secondary microbial growth in water distribution networks.

4.3 CHLORINATION DESIGN

Chlorination is in essence a chemical process and the laws that govern chemical processes arealso applicable in the design of chlorination systems. According to the Chick-Watson law ofdisinfection, three major parameters govern chemical disinfection, namely the number of

pathogens that must be inactivated, the disinfectant concentration, and the contact time.

There are no established standards or codes of practice in the potable water industry for thedesign of chlorination systems in general and the contact tanks in particular. In mostwaterworks the main function of contact tanks is to give temporary storage to treated water,as a buffer between the steady output of the works and the fluctuating demand of thedistribution system (Water Research Centre, 1989).

The nearest approach to a general standard for disinfection is the WHO Guideline whichrecommends that the treated water should have a free chlorine residual of at least 0.2 to 0.5mg/L after a contact time of 30 minutes (Water Research Centre, 1989). In the USA more

attention has recently been given to the control of contact times since the adoption of the"Surface Water Treatment Rule", where Ct values were provided (Teefy and Singer, 1990;Disinfection Committee, 1992). With Ct values recommended of around 100 mgL-1min forfree chlorine in the SWTR, the WHO Guideline would appear to be inadequate for theelimination of intestinal viruses and Giardia.

Design considerations

The primary purpose of disinfection of potable water is to reduce the potential health riskassociated with pathogens for the user. Although health risks due to pathogen contaminatedwater is probably the most important, risks due to other water quality parameters (see Water

Quality Criteria elsewhere) should be used as a guide to the degree of treatment required.The criteria shown in Table 3 for indicator organisms should be used as basis for determining

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the required Ct values. Since the time of the Ct factor is a function of the residence timedistribution in the chlorine contact tank, reliable Ct factors may only be calculated once theflow pattern in the contact tank is known (see USEPA 1989). The two main additionalparameters to be taken in account are:

pH: HOCl is more effective than OCl-

(see Figure 4.1).Temperature: disinfection rate is considerably influenced by temperature

A chlorination system for disinfection of potable water consists of four separate subsystems:

(i) chlorine storage and supply;(ii) chlorine solution mixing;(iii) provision of contact time, and(iv) chlorine dosing control.

Chlorine storage and supply

Some important design and safety considerations of chlorine storage and supply are given byQasim (1985):

Chlorine storage and chlorinator equipment must be housed in a separate building. If not,

it should be accessible from an outside door.

Adequate exhaust ventilation at floor level should be provided because chlorine gas is

heavier than air. Fan control and gas masks should be located at room entrance.

The temperature in the chlorine supply area should not be allowed to drop below 10 oC.

Dry chlorine liquid and gas can be handled in wrought-iron, SM steel, Hasteloy, Monel

alloys and appropriate stainless steel (AISI 318-322); wet chlorine gas in glass, silver,hard rubber (Masschelein, 1992) and plastic. Valves and pipe fittings should be speciallydesigned for chlorine use.

Chlorine cylinders in use should be set on platform scales set flush with the floor, and loss

of weight should be used for record keeping of chlorine dosages.

Storage should be provided for at least a 30-day supply.

Chlorine solution mixing

One of the most neglected aspects in disinfection with chlorine is the initial mixing of the

relatively small, high chlorine concentration stream with the bulk of the clarified water. Theeffective initial mixing of the chlorine solution can be accomplished in a number of ways(White, 1992; Harnby et al., 1992): (a) Diffusers; (b) Mechanical mixers; (c) Hydraulicmixers including static mixers, and hydraulic jumps. Mixing intensities, measured as thevelocity gradient, G, should be > 500 s -1 (White, 1992).Contact tanks

The chlorine contact tank must be designed to provide the optimum distribution of residencetime for contact between the disinfectant and the pathogens. The distribution of residencetime may differ appreciably in tanks of different geometrical configuration, although tanksvolumes and flow rates are identical. For disinfection the ideal contact tank should have a

configuration that encourage 'plug-flow' characteristics. The flow characteristics of a contacttank can be determined by tracer tests which have become necessary in the USA where the Ct

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standard has become mandatory (Teefy and Singer, 1990). With the proper use of tracerinformation it is possible to rationalize (Trussel and Chao, 1977) and optimize (Tikhe, 1976)the design of chlorination systems. For some design details of chlorine contact tanks thereader is referred to White (1992).

Chlorine dosing equipment

Chlorine is supplied as a liquefied gas under high pressure in containers varying in size from150 to 2000 lbs. Chlorine can be abstracted directly from the gas phase of the pressurizedcontainer with a chlorinator. The most widely used chlorinators are those using vacuum-feeddevices. In each of these systems, the chlorine injector is the basic component. The injectoris used to create the vacuum that is used to draw the chlorine gas from the storage supplythrough the chlorine regulator, which serves as a metering device, and into the injector. Atthe injector, the chlorine dissolves in the injector water to form hypochlorous acid. From theinjector the hypochlorous acid solution flows to the point where it is to be injected into theclarified water. At liquid chlorine temperatures of 10 oC, about 10 kg chlorine/h can be

evaporated per 2000 lb container (Degrmont, 1991). If higher rates are required, morecontainers can be used in parallel, or special heated evaporators installed. A chlorineconcentration of 2,5 g chlorine/L (or higher) at the injector can be obtained in this way. Fora complete discussion on equipment available, design calculations and physical layout thereader is referred to White (1992).

Properties of chlorine

In its normal state chlorine is a greenish-yellow gas with the following physical constants(Degrmont, 1991):

Density relative to air : 2,491;Relative atomic mass: Cl = 35,45;Specific mass : 3.214 kg/m3 at 0 C, 101 kPa;

At 15 C, 101kPa, 1 kg of chlorine converts to 314 L of chlorine gas and 1L of liquid chlorine corresponds to 456 L of gas.

Chlorine liquefies on cooling and compression at a pressure varying with temperature:1000 kPa at 40 C or 500 kPa at 18 CLiquefaction point (at 100 kPa) -34,1CFreezing point -102 CCritical temperature 144 CCritical pressure 77,1 barsHeat capacity of gas 0,518 kJ/kg from 15 to 100 C at 100kPaHeat capacity of liquid 0,92 kJ/kg

Solubility in water varies with temperature:

Temperature (oC) 0 5 10 20 25 30

Solubility (g/l) 14,8 11,8 9,6 6,7 5,4 4,5

Latent heat of evaporation varies with temperature:

Temperature (C) 0 10 20 30

kJ/kg 249,1 242,0 234,1 226,1

At liquid chlorine temperatures of 10 C, about 10 kg chlorine/h can be evaporated from a 2000 lb container.

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Conventional chlorinators

A conventional chlorinator consists of the following units: an inlet-pressure-reducing valve, arotameter, a metering control orifice, and a vacuum-differential regulating valve. A simple

schematic is shown in Figure 4.5. The driving force for the system comes from the vacuumcreated by the chlorine injector. The chlorine gas comes to the chlorinator and is converted toa constant pressure (usually a mild vacuum) by the influent-pressure-reducing valve. Thechlorine then passes through the rotameter, where the flow rate is measured under conditionsof constant pressure (and consequently constant density), then through a metering or controlorifice. A vacuum differential regulator is mounted across the control orifice so that aconstant pressure differential (vacuum differential) is maintained to stabilize the flow for aparticular setting on the control orifice. The flow through the control orifice can be adjustedby changing the opening on the orifice. The control orifice has a typical range of 20 to 1,while the vacuum-differential regulator has a range of about 10 to1. Thus the overall range ofdevices combined is about 200 to 1. On the other hand, a typical rotameter has a range of

about 20 to 1. Thus, the chlorinator should be selected based on design capacities, and therotameter installed at any particular time should be appropriate for current demands.

Chlorination equipment

Simple and conventional chlorination equipment is shown in Figures 4.3, 4.4 and 4.5.

Mixing and Contact Time

Rapid mixing of chlorine at the point of application is important. Some of the chlorineshould be added with a large amount of dilution water through a full channel or pipe diameter

diffuser submerged to the maximum depth available. Chlorine applied prior to a weir will bestripped by the aeration effect of the hydraulic jump. Chlorine added before a suction pumpor the upstream side of a valve can cause severe corrosion problems to brass unless applied ata sufficient distance upstream to obtain full dissolution.

Chloramine disinfection

The avoidance of THMs has led to renewed interest in selecting chloramine as a disinfectant,because chloramine impedes the formation of THMs. In a survey conducted in 1984 in theUnited States, none of the utilities using chloramine reported any adverse effects onbacteriological quality (Staff, 1985a). A recent survey on the occurrence of disinfection by-

products indicated that the occurrence of almost all of the by-products was significantlyreduced in the systems using chloramine as opposed to chlorine (Bryant et al.., 1992). Theadvantages and disadvantages of using chloramine as a disinfectant are listed in Table 4.2.

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Figure 4.5 Flow diagram for conventional chlorinator

Advantages Disadvantages

Insignificant formation of trihalomethanes((THMs) and other disinfection by-products.

Eliminates certain taste and odor conditionsassociated with chlorine.

More stable residual in water distributionsystem.

Introduction of chloramine is simple andsimilar to that of chlorine.

More stable than chlorine (Kreft et al..,1985).

Not as effective as chlorine in deactivatingbacteria, viruses, and Giardia.

May produce chlorinated phenols, which givestaste to water.

May produce gas poisoning hazards similar tothat of chlorine.

Uncontrolled dosage of ammonia could lead tonitrification problems.

Takes a much longer time than chlorine foreffective disinfection.

Table 4.2 Relative Merits of Chloramine as a Disinfectant (Adapted from Vigneswaran et al., 1987)

For pHs above 8 the OCl- ion is comparable to monochloramine in its disinfecting power.Many of the plants that currently use the chloramine process add chlorine first and maintain afree chlorine residual for some time.

4.4 DISINFECTION BY-PRODUCTS

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Chlorination may have saved millions of lives, but it is being challenged more and morebecause it is now known that chlorine reacts with aquatic organic material present in naturalwater to form trace levels (between 0.01 and 1.0 mg/L) of the trihalomethanes (THMs), agroup of lightweight chlorinated hydrocarbons which are suspected carcinogens. Other

chlorinated organic compounds may also form and their toxicity is being evaluated. Manywater utilities may not be able to adequately disinfect their water supply using chlorinewithout exceeding limits or future lower limits imposed by various authorities. A number ofoptions are available for controlling formation of trihalomethanes and other chlorination by-products, and these are widely discussed in the literature. This module will be limited to adiscussion of chlorination and the use of alternative disinfectants.

Since the turn of this century, chlorination was (and still is) the universal disinfectant used forthe disinfection of potable water (White, 1992). The dominant role of chlorination hasseriously been challenged since the discovery of the presence of organo-halogenatedcompounds in chlorinated drinking water (Rook, 1974; Bellar et al., 1974). The best known

of these products are the trihalomethanes (THMs) of which chloroform (CHCl 3) has beenfound to be an animal carcinogen (Pieterse, 1988). This has lead to the abandonment of theuse of chlorine in some industrialized countries (like the Netherlands) with a correspondingincrease in the use of alternative disinfection methods and chemicals (Wondergem and VanDijk-Looijaard, 1991; Kruithofet al., 1992). In the Netherlands, microbiological regrowth inthe reticulation systems is being controlled by the elimination of growth-promotingsubstances in the water. According to Van der Kooij (1992) a low AOC (assimilable organiccarbon) content can be obtained, resulting in the chances for regrowth being low.

None of these alternative methods are, however, entirely devoid of problems. Many studieshave shown that potential hazardous chemical by-products are formed with virtually every

type of disinfectant method or chemical used (Jacangelo et al., 1989; Lykins et al., 1990;Lykins et al., 1992). After a comprehensive review on the potential health risks of THMs,Pieterse (1988) concluded that the removal of microbiological contamination remains themost important consideration in ensuring the safety of potable water. Chlorination should notbe phased out in preference to other methods until the potential dangers of alternativedisinfectants are fully evaluated.

A large amount of research has been done on the possible formation, types, and prevention ofdisinfection by-products (Stevens et al., 1976; Jolley et al., 1977; Krasner et al., 1989;Duke et al., 1980):

_ Formation: Disinfection by-products are formed due to the action of the disinfectionchemical on naturally occurring organic matter, such as fulvates and humates thatconstitute natural organic color in surface waters.

_ Types: The most common disinfection by-products include: trihalo-methanes,haloacetonitriles, haloketones, haloacetic acids, chlorophenols, aldehydes (and manymore). The latest scare is bromate formation when ozone is used (Kruithof and Meijers,1994). This view has now been superseded by the realization that chlorination alsoresults in the formation of bromate.

_ Prevention: There are generally four approaches in minimizing or preventing the

formation of especially THMs:

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(i) Remove the THM formation potential (THMFP) by improving organic removal. In some cases itmay be justified to lower the pH for better coagulation of organics. Pre-ozonation can usuallylower the THMFP, but there are exceptions.

(ii) Control the concentration and contact time of free chlorine. This may be achieved by altering thepoint of chlorine application - usually to after clarification. In the case of pre-chlorination, theaction of free chlorine can be ended by ammonification.

(iii) The use of alternative disinfectants such as chloramines, chlorine dioxide and ozone which do notform or form little THM's, and

(iv) Removal of THMFP as well as THMs by activated carbon. From the various methodsinvestigated this method was the least cost effective.

(v) Bromate formation can be averted at lower pH values (


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ClO2 10 1.5 0.6 0.1HOCl 20 1.0+ 0.05 0.01OCl- 0.2 0.02 0.0005 0.0005NH2Cl 0.1 0.005 0.001 0.02

*(mg/L).min, assuming that n is equal to 1

Table 4.4 Specific Lethality* of Alternative Disinfectants

4.5 CHLORINE DIOXIDE

Chlorine dioxide is a useful alternative among disinfectants. Chlorine dioxide is effective indestroying phenols, yet it does not form trihalomethanes in significant amounts. Chlorinedioxide's disinfectant properties are not adversely affected by a higher pH, as those of freechlorine residue are. Consequently, chlorine dioxide is a useful disinfectant at higher pHs. Inwestern Europe, use of chlorine dioxide is popular, particularly in Belgium, Germany, Franceand Switzerland in regions where potable water is produced from polluted rivers. In theselocations, chlorine dioxide is used for disinfection, often as an adjunct to ozonation.

Chlorine dioxide can be reduced by two alternative pathways in aqueous solution, as shown below.

ClO2

ClO2-

Cl-+ 2O2-

+e-

+5e-

The redox potential of chlorine dioxide in being reduced to ClO 2- is 1.15 V. Reduced to Cl-,its potential is 1.9 V. By comparison, the redox potential of Cl when reduced to Cl- is 1.4 V.At the pHs normally encountered in water utility practice, chlorine dioxide is most oftenreduced to the chlorite ion (ClO2-); hence, both of its redox potential and the redoxequivalents are lower for chlorine dioxide than for chlorine. On the other hand, chlorinedioxide is a good disinfectant, and because it is selective, it does not participate in a numberof undesirable side reactions as chlorine does. For example, chlorine dioxide does not reactwith organic compounds to form trihalomethanes, nor does it react with ammonia to formchloramines. As a result, chlorine dioxide residuals are sometimes purported to last longer

than free chlorine residuals under the same circumstances. One of chlorine dioxide'sprincipal advantages as an oxidant is that it effectively removes phenols, a continuing odorproblem in certain water supplies. Chlorine dioxide does not dissociate or disproportionate aschlorine does at normal drinking water pHs. Like chlorine, chlorine dioxide exerts a demandwhen it is first added to a water supply, which must be overcome if a persistent residual is tobe maintained. Again like chlorine, chlorine dioxide is photosensitive (light sensitive).

The principal reason for the increasing interest in chlorine dioxide in the United States is thefact that it does not form trihalomethanes. On the other hand, the organic by-products ofchlorine dioxide are not yet well understood; it may have other undesirable reaction products.Information presently available indicates that the reaction products would include aldehydes,

carboxylic acids, and ketones. Few chlorinated by-products are known, although some arelikely. The principal inorganic by-products of chlorine dioxide reactions within water

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treatment are chlorite (ClO2), chloride (Cl-), and chlorate (ClO3-), in the order listed. Bothchlorate and chlorite, particularly the chlorite ion, have been implicated in the formation ofmethaemoglobin. Consequently, most European countries limit the level of chlorine dioxidewhich can be used, and the EPA has considered doing so in the United States as well. Thecurrent EPA recommendation is that the sum of chlorine dioxide, chlorite, and chlorate in the

distribution system be less than 1.0 mg/L.

Generation of chlorine dioxide

All chlorine dioxide for drinking water treatment is generated from sodium chlorite. Mostgeneration techniques use the oxidative process, in which chlorine (either as a gas or insolution) is mixed with a sodium chlorite solution. The stoichiometry of this reaction is(written for molecular chlorine):

2NaClO2 + Cl2 2ClO2 + 2NaCl (4.15)

In addition to the desired formation of chlorine dioxide, chlorate ion may be formed in thegeneration system as an undesired by-product in a competing reaction:

NaClO2 + Cl2 + OH- NaClO3 + HCl + Cl- (4.16)

The goal in generating chlorine dioxide from chlorine and sodium chlorite is to maximize thechlorine dioxide yield, defined as the molar ratio of chlorine dioxide produced to thetheoretical maximum. The term 'conversion' is also used when referring to chlorine dioxidegeneration reactions; this is the molar ratio of the amount of chlorine dioxide formed to theamount of sodium chlorite fed to the system. For Reaction 4.15, yield and conversion willhave the same value. For other reactions that produce chlorine dioxide, such as the

hydrochloric acid-sodium chlorite reaction, yield and conversion will have different values:

5NaCl2 + 4HCl 4ClO2 + 5NaCl + 2H2O (4.17)

For Reaction 4.17, maximum yield is 100 percent; maximum conversion, 80 percent.

Studies of the mechanism and kinetics of the chlorine-sodium chlorite reactions have shownthat conditions favoring the formation of chloride dioxide are those in which the reactants arepresent in high concentrations and the chloride is present as hypochlorous acid or molecularchlorine (Cl2). Two methods for the generation of chlorine dioxide from chlorine and sodiumchlorite are commercially available. They are the aqueous chlorine-sodium chlorite system

and the gas chloride-sodium chlorite system.

Aqueous chlorine-sodium chlorite system

The earliest systems produced chlorine dioxide by simply pumping a sodium solution into achlorine solution, followed by a short reaction time. Acceptable yields were achieved byfeeding 200 to 300 percent more chlorine than the stoichiometric requirement according toreaction 4.15. The chlorine dioxide solution from a generator of this type contains highlevels of chlorine in addition to the chlorine dioxide. A side reaction that occurs in the

chlorine dioxide solution under these conditions is:

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2ClO2 + HOCl + H2O 2ClO3- + 2H+ + HCl (4.18)

The discovery that potentially toxic chlorinated organic compounds are generated by thereaction of chlorine and naturally occurring humic substances in water supplies, plus theinterest in chlorine dioxide as a replacement for some chlorination practices, led to a search

for generation methods that would produce a chlorine free chlorine dioxide. One of the mostcommon methods for chlorine dioxide generation currently in use, that strives to meet thisrequirement, is the pH adjusted method.

The pH-adjusted system utilizes hydrochloric acid fed into the chlorine solution before thereaction with the sodium chlorite. The acid shifts the chlorine hydrolysis equilibrium to theleft, favoring molecular chlorine formation:

Cl2 + H2O HOCl + HCl (4.5)

The acid feed must be carefully controlled so that the pH of the chlorine dioxide solution can

be maintained between 2 and 3. Higher pH values result in decreased yields. At a lower pH,however, Reaction 4.17 becomes significant, again reducing yield because of the maximumconversion of only 80 percent from this reaction. Yields of more than 90 percent have beenreported from the pH-adjusted system, with approximately 7 percent excess (unreacted)chlorine remaining in the solution.

Another modification that produces high yields of chlorine dioxide, with minimal amounts ofchlorine remaining in the chlorine dioxide solution, requires that the chlorine solution usedfor generation have a chlorine concentration greater than 4g/L.

Since this concentration of chlorine in solution is near the upper operating limit of

commercial chlorine ejectors, and these ejectors operate at constant water flow rates, the yieldof this method of generation is dependent on the production rate, with lower production ratesresulting in lower yields. This type of generator is normally operated on an intermittent basisto maintain high yield when less-than-maximum production capacity is required. Chlorinedioxide solutions in the 6- to 10-g/L concentration range are prepared and immediatelydiluted to about 1g/L for storage and subsequent use as needed.

A schematic of the aqueous chlorine-sodium chlorite system is shown in Figure 4.6.

Figure 4.6 Aqueous chlorine - sodium chlorite system with optional acid feedGas-chlorine chlorite system

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The most recent development in chlorine dioxide generator technology is a patented systemthat reacts gas chlorine with a concentrated sodium chlorite solution under vacuum. Thechlorine dioxide produced is removed form the reaction chamber by a gas ejector, which isvery similar to the common chlorine gas vacuum feed system.

This generation technique produces chlorine dioxide solutions with yields in excess of 95percent. The chlorine dioxide solution concentration is 200 to 1000 mg/L and contains lessthan 5 percent excess chlorine, which is defined as the amount of unreacted chlorineremaining in the chlorine dioxide generator effluent. The system is operated on a continuousbasis, and achieves a high yield over the entire production range (see Fig 4.7).

Figure 4.7 Gas chlorine-sodium, chlorite system schematic

4.6 OZONE DISINFECTION

The first use of ozone in the treatment of water coincided with the first use of chlorine for

disinfection, until recently ozonation was a common practice in only a small number ofcountries, notably France and Switzerland. It has become much more popular recently and isnow in use in about 3000 drinking water and waste water treatment plants all over the world.

Ozone is a highly reactive gas formed from oxygen by electrical discharges. Its mostdistinguishing characteristic is a very pungent odor. In fact, the word "ozone" is derived froma Greek word which means "to smell". The use of this gas in water treatment requires anunderstanding of its physical and chemical behavior. The physical chemistry of ozone isimportant because a number of complex factors affect its solubility, reactivity, auto-decomposition, and stability.

Ozone is a strong oxidant and an excellent disinfectant. It finds wide application in watertreatment and increasingly in wastewater treatment. Ozonation can play an important role in

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improved water treatment and help to achieve a better, safer water quality. Ozone is usuallyused as more than just a disinfectant. Its excellent properties in aiding coagulation,promoting the removal of organic and inorganic contaminants and removal of taste and odorshave made in a popular choice in modern water treatment practice. It is used as an earlytreatment procedure to enhance coagulation and to improve the biodegradability of organic

constituents for subsequent removal during biological filtration. Whereas the older drinkingwater plants were using ozone towards the end of the process for disinfection, the moveworldwide has been towards using ozonation as a pretreatment. With few exceptions,chlorination is used as a final disinfection stage. Ozone is an excellent disinfectant on its ownin tertiary wastewater treatment.

Caboolture Shire uses ozonation in a water reclamation plant of 10ML/d. Studies for theimplementation of ozonation have been conducted for the Gold Coast City Council and theArmidale City Council. The author had involvement in all three and also with plants inAfrica, ie the Windhoek Water Reclamation Plant, Western Transvaal Regional WaterCompany and a large pilot scale for disinfection of mine service water for GoldFields.

This chapter describes the process of ozonation and its applications in some detail. Thechemistry of ozone is described, the equipment required for ozonation and some of theapplications are discussed.

Ozone production in nature

Ozone is formed by the interaction of energy with oxygen molecules to form oxygen radicalswhich then combine with oxygen molecules as shown in the following reactions;

O2 + Energy O + O (4.15)

O + O2 O3 (4.16)

These reactions occur naturally in the Earth's ozone layer under the influence of sunlight, andartificially using electrical discharge, ultraviolet light and electrolysis.

Oxidation of inorganic compounds

Ozone will oxidize most metals, such as Mn(II), Mn(IV), Fe(II), Co(II), cyanide to cyanate orto carbon dioxide and nitrogen, sulfides, sulfur dioxide, bromide and nitrite. At pH > 8, ozonewill oxidize ammonia.

Example: A simple oxidation reaction for ozone is:

O3 + NO2- NO3

- + O2

Oxidation of organic compounds

In theory ozone can oxidize organic pollutants to carbon dioxide and water. In practice thisdoes not happen, as it would take too long. The oxidation reactions usually attack C-Cdouble bonds or aromatic groups in humic molecules, and aliphatic side chains or fatty acidsby hydrogen atom abstraction. It will also oxidize complexed metal ions.

Care needs to be taken when using ozone for reversible oxidations, for example the removal

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of color from dye waste. If the treated waste comes into contact with a reducing environmentthe molecule may convert to its previous form. This is not important for irreversible reactions.

Oxidation capability

Ozone, O3, is a triatomic allotrope of oxygen. It is a powerful oxidant with an Eo value of2.07 V. Ozone also produces hydroxyl radicals which have an Eo value of 2.80 V. The

oxidation potentials of various compounds is shown in Table 4.5, from which it can be seenthat ozone and hydroxyl radicals are the most powerful oxidants.

Table 4.5 Oxidation potentials of various commonly used aqueous oxidants

Compound Eo (V)

FOH radical

3.202.80

O3 2.07

H2O2 1.77

MnO4- 1.68

HOCl 1.63

Cl2(aq) 1.40

Cl2(g) 1.36

Cr2O72- 1.33

HCrO4- 1.20

ClO2(g) 1.15

Corrosivity of ozone

Being a strong oxidant, ozone can be aggressive to many metals and polymers, both as a gasand in solution. Mild steel is resistant only when ozone concentrations in water are below 0.2mg/l and only under turbulent conditions. Ozone resistant paints are available but are nottotally satisfactory.

Stainless steels, concrete and ceramics (304 & 316) are totally resistant to oxidation by ozone.

Seals need to be manufactured from Viton, Teflon or ozone resistant rubbers.

The decomposition of ozone

Ozone rapidly degrades to oxygen by a series of reactions involving a range of intermediatehydroxyl radicals which are utilized in some oxidation reactions. The overall reaction is:

2O3 3O2 (4.17)

This reaction occurs very rapidly, and approximately follows first order kinetics. In "pure"water ozone typically has a half life of approximately 20 minutes. The half life is affected bythe pH of the water, reductants and carbonate ion concentration and is typically between 10seconds and 2 minutes in surface waters.

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Transfer of ozone from the gas phase to the aqueous phase

Ozone is normally produced in air or oxygen. In order to use ozone in the treatment of waterit is necessary to dissolve it in water. The solubility of ozone in water depends on the gasphase concentration, water temperature, water pressure and diffusion.

The quantity of ozone, which can be transferred into water, is limited by the equilibriumbetween the amount of ozone in the gas phase and the amount dissolved in water. Table 4.6shows equilibrium concentrations of ozone dissolved in water for various gas phaseconcentrations.

Table 4.6 Ozone solubility in water at equilibrium, mg/L

Ozone concentration in gas phase

Temperature oC 0.001 % 0.1% 1% 1.5% 2% 3%

15 0.006 0.60 6.04 9.05 12.06 18.10

25 0.004 0.35 3.53 5.29 7.05 10.5830 0.003 .027 2.70 4.04 5.39 8.09

The values in Table 4.6 represent theoretical maximums at ambient pressure. The solubilityof ozone follows a linear relationship with concentration and pressure according to HenrysLaw,PiH = Ci, where Pi is the partial (fractional) pressure of ozone in the gas (air or oxygen) andCi the concentration of ozone in the water. The value ofHwould normally be expressed as aliquid phase concentration per (partial) pressure of the ozone in the gas phase.

It should be appreciated that during the transfer of ozone from the gas phase to water that the

gas phase concentration decreases. If, for instance, 90% of the ozone has been transferred,the gas phase concentration has been decreased to 10% of its initial value. Consequently theequilibrium water phase concentration will be reduced by approximately 90%.

Diffusion

Values in Table 4.6 represent maximum concentrations of ozone reached at equilibrium afteran indefinite period of time of contact. The transfer is, however, affected by various diffusionrate phenomena such as; the net area of contact between the gas and water, the degree of gassaturation of the water, boundary layer and mixing conditions. These characteristics areconsequently influenced by the selection of the equipment used for ozone transfer.

A general relationship for gas diffusion is Fick's Law, which can be expressed as:

N c u Ddc

dbA A o AB

A

= (4.18)

NA is the diffusion flux, CA is the concentration of component A, uo is the velocity which

components A and B travel past each other, DAB is is the diffusivity of component A in

mixture with component B, dCA/db is the concentration gradient between component A and

component B.

From this equation the diffusion flux for ozone is increased as:

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the relative velocity between the two phases decreases, the concentration of ozone in the gas phase increases

Figure 4.8 shows the effect of diffusion phenomena on the transfer of ozone from gas to water

Time (min)

Ozoneconcentrationinwa

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 2.5 5 7.5 10 12.5 15 17.5 20 22.5 25

Decreasing: Bubble diameter

Improved: Mixing

Figure 4.8 Typical relationship between time of contact and ozone concentration

reached during batch ozonation

Developing an ozone residual

For disinfection a residual of ozone needs to be obtained. Due to its high oxidation potential,ozone reacts rapidly with a range of contaminants in water. The oxidation reactions are sofast that they are difficult to measure. These reactions consume ozone and are referred to asozone demand.

Until the ozone demand has been met, the residual of ozone will be consumed rapidly withconsequently poor disinfection. It is impossible to predict the ozone demand of a particularwater so experimental work must be done in order to determine the dose required to obtain asufficient residual. This can be done in a laboratory or by pilot scale testing.

Ozone demand is determined by applying ozonated air (or oxygen) to a sample of water. Theozone concentration of the ozonated air, the air leaving the water and the ozone concentrationof the water are measured. The ozone dosage can be calculated from a mass balance of theozone applied and the ozone in the exit gas. The ozone concentration measured in the wateris the residual. The difference between ozone dosage and the residual measured in the wateris the ozone demand of the water. These destinations of ozone are shown in Figure 4.9.

Testing is essential to the design an ozonation system to ensure the applied dose is adequateto meet the ozone demand of the water and to avoid unnecessarily over-design. Many waterswill have low ozone demands, whereas it possible that other waters have ozone demands sohigh as to make it uneconomic to use ozonation. Figure 4.10 shows a typical relationshipbetween the amount of ozone applied to a water sample and the residual that becomesavailable for disinfection.

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Ozone in Exit Gas Measured

Water

Ozone Residual Measured

Ozone Demand Inferred: Applied - Exhaust - Residual

Ozonated Air Applied Measured

Figure 4.9 Ozone pathways in water treatment

OZONE DOSAGE (mg/l)

OZONERESIDUAL(mg/l)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8

Figure 4.10 Typical relationship between ozone dosage and the residual in water

As a sample of water moves through an ozone contactor, the residual slowly builds up in thewater sample. Once the water leaves an ozone contacting unit this residual breaks downaccording the half life decomposition discussed above. Figure 4.11 shows how a disinfectionresidual builds up and breaks down with time in a typical contacting system.

TIME (min)

OZONERESIDUA

L(m

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 1 2 3 4 5 6 7 8 9 10

Ozone dose B

Ozone dose A

Figure 4.11. Typical build-up of ozone residual in a contacting chamber and the

subsequent breakdown when leaving the chamber

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As mentioned, the ozone residual is short lived. Ozone cannot be used, therefore, for residualdisinfection of water distribution systems. Disinfectants, which are typically used forestablishing a residual, are chlorine, chloramines and chlorine dioxide.

Ct Values

Ct values are used by the United States Environmental Protection Agencies' (USEPA)Surface Water Treatment Rules (SWTR) to protect the public from contracting waterbornedisease from drinking water. Instead of setting maximum contaminant levels, Ct values areused to emphasize water treatment techniques as the condition for compliance to regulation.Ct values can be calculated for disinfection methods, as well as other processes in potablewater treatment such as coagulation and filtration. C is the concentration of the disinfectantresidual and t is the time that the water is in contact with the disinfectant. The product of Cand t is the Ct value.

Ct can be calculated by the integration of a concentration-time curve like the one shown in

Figure 4.11. The area under the dose B curve represents the Ct value for that specific dose.This means that for every type of water the Ct values for ozone need to be determinedexperimentally. This is different from chlorination where the chlorine product's stabilityensures a relatively constant relationship between concentration and time.

Ct values are useful for making estimates of the size requirements of a disinfection system,and for comparisons between disinfectants. Tables 4.7 and 4.8 show the Ct valuesrecommended by the USEPA for 99.9% inactivation of Giardia lamblia and 99.99%inactivation of enteric viruses, for various disinfectants. From these tables it can be seen thatozone is much faster acting than other disinfectants.

TemperatureDisinfectant pH 10o C 15o C 20o C 25o C

free chlorine 2 mg/l 7 124 83 62 41

8 182 122 91 61

ozone 6-9 1.4 0.95 0.72 0.48

chlorine dioxide 6-9 23 19 15 11

preformed chloraminesa 6-9 1850 1500 1100 750

Extracted from: Von Huben, 1991

Table 4.7. Ct values for inactivation by 99.9% ofGiardia lamblia and 99.99% of enteric

viruses with various disinfectants

Temperature

% inactivation pH 10o C 15o C 20o C 25o C

90 % 6-9 0.48 0.32 0.24 0.16

99 % 6-9 0.95 0.63 0.48 0.32

99.9 % 6-9 1.4 0.95 0.72 0.48

Extracted from: Von Huben, 1991

Table 4.8 Ct values for different levels of inactivation ofGiardia lamblia with O3

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4.7 DISINFECTION BYPRODUCTS (DBPs)

Ozone forms fewer DBPs than other disinfectants, except chlorine dioxide. This is one of themajor considerations leading to the use of ozone as a disinfectant. Concern about THMs hasbeen one of the major reasons for increasing use of ozone in disinfection. All disinfectants

can leave by-products in water, some of which are shown in Table 4.9.

Disinfectant HOCl NH2Cl ClO2 O3

Byproduct

Chloroform (THM) X (X)

Bromodichloromethane (THM) X (X)

Chlorodibromomethane (THM) X (X)

Bromoform (THM) X (X) X

Dichloroacetic acid X (X)

Trichloroacetic acid X (X)

2,4,6-Trichlorophenol X (X)

Formaldehyde X (X) X

Other aldehydes & Ketones (X) X

Bromate X X

Cyanogen chloride X

Chlorite/chlorate X

Table 4.9 Some significant disinfection by-products of the common disinfectants# Chloramine by-products depend on the method of chloramination. When chlorine is added prior to ammonia, chloramines will have thesame by-products as chlorine. When chlorine and ammonia are mixed prior to dosing, chloramine has very few by-products, mainlycyanogen chloride which only forms when conditions are suitable.

Environmental toxicity of disinfectants

Ozone has very limited residual toxicity for distribution systems or the environment. Due torapid decomposition most ozone will have left the system within an hour, having no furtherimpact on the environment or humans.

Hypochlorite, chlorine dioxide and chloramine, the most commonly used disinfectants andoxidizing agents, all maintain a residual in water. Free chlorine and monochloramine arelethal to insects and crustaceans at levels of as low as 0.03 mg/L. They are lethal to some fishat levels of 0.014 mg/l. Sub-lethal effects such as avoidance behavior and reproductive

failure occur at 0.002 - 0.005 mg/L in fish.

BromoformOzone will form bromoform, bromoacetones and bromoacetonitriles. This has been of majorconcern with ozonation. Reactions are as shown below:

O3 + Br O2 + OBr (4.19)

HOBr H+ + OBr- (4.20)

HOBr + DOC TOBr (4.21)

Where DOC is dissolved organic carbon and TOBr total brominated organic compounds.

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Chlorination will also oxidize bromide to hypobromous acid leading to the same result asshown below.

HOCl + Br- HOBr + Cl- (4.22)

Bromate

Bromate formation has recently been perceived as a major problem with ozonation. Bromatewill also form during chlorination. The formation of bromate is described in the followingreactions:

For ozone:O3 + Br

- O2 + OBr- (4.23)

2O3 + OBr- 2O2 + BrO3

- (4.24)

For chlorine:

HOCl + Br- HOBr + Cl- (4.25)2OBr- BrO2

- + Br- (4.26)

OBr-+ BrO2- BrO3

- + Br- (4.27)

Bromate formation can be avoided by adjusting conditions, in several ways:

Below a pH of 6.5 all hypobromous (OBr-) species are present as hypobromous acid (HOBr),

which cannot be oxidized. Bromate cannot form, therefore.

Below a pH of 6 bromide and bromate will rapidly react with each other to formhypobromous acid, reducing the amount of bromate present in water.

In the presence of ammonia, monobromamine will form which is rapidly oxidized by ozone:

OBr- + NH3 OH- + NH2Br (4.28)

2NH2Br + O3 2Br- + N2 + O2 + H2O + 2H

+ (4.29)

Reaction 4.28 is favored over reaction 4.24 resulting in a cycling of brominated products inReactions 4.28, 4.29 and 4.23 causing bromine to become unavailable for the formation ofbromate.

4.8 OZONATION EQUIPMENT

The preceding discussion shows several important characteristics of ozone. Ozone has twomain applications in water and wastewater treatment: oxidation and disinfection.

For both oxidation and disinfection:

1. Ozone must be generated on-site due to its instability

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2. Ozone contact with water must be carefully designed to overcome poor solubility and to provide theconditions for diffusion from the gas phase into the liquid phase

For disinfection:

3. Applying enough ozone to satisfy the ozone demand and leave an adequate residual concentration to providea driving force for disinfection.

A final important consideration is that ozone at ground level is a lung irritant and animportant greenhouse gas and in general should not be released to the atmosphere.

Figure 4.12 is a flow chart showing the elements typically required for a practical ozonationsystem when considering the reactions and chemistry of ozone.

INFLUENT

OZONE GENERATOR AIR/OXYGEN PREPARATION AIR

OZONE

CONTACT

CHAMBER

OZONE DESTRUCTOR VENT

OTHER OXYGEN APPLICATIONS

EFFLUENT

Alternative paths for enriched oxygen recycle

Figure 4.12 Typical layout of an ozone system

Selecting an ozone generation system

Ozone can be generated in air or oxygen. The concentrations achievable with oxygen arehigher than in air. Making a choice between using air or oxygen for the production of ozonerequires a full cost benefit analysis. Table 4.10 shows the approximate concentrations ofozone produced in air and oxygen with technology currently on the market.

Ozone generation Gas utilized

Air Oxygen

UV irradiation 0.001 to 1.0% Not typically used

Corona discharge 2 to 4% 6 to 12%

Table 4.10 Concentrations of ozone produced in air or oxygen using current technology

Oxygen is not typically used as a source of gas for UV systems due to the expense of oxygenpreparation equipment and the small concentration of ozone produced.

Generating Ozone

There are three systems used to generate ozone: corona discharge, UV and electrolysis.

Corona discharge

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A corona discharge is created when a high voltage is applied across two electrodes separatedby a dielectric and a discharge gap. Electrons flowing thought the discharge gap are used tobombard oxygen atoms in the feed gas, causing some of the oxygen molecules to break intoatoms which then combine with molecular oxygen to form ozone.

The electrodes are usually arranged as two concentric reaction cylinders around each otherwith the discharge gap between them. The outer electrode is usually jacketed in coolingwater. The dielectric is usually made of glass or ceramic and together with a metallic coatingthis makes the inner electrode. The outer electrode is usually stainless steel.

Up to 20kV is required to ensure an adequate corona discharge rate. Frequencies exceeding50 Hz improve the efficiency. Most ozone generators are equipped with solid state mediumfrequency converters to produce frequencies around 500 Hz ensuring higher ozoneconcentrations, higher production and to reduce power consumption.

Gas preparation for corona discharge

Adequate air preparation is essential in a corona discharge system. Comparatively wet gasentering the discharge system can cause substantial problems. It must not be assumed that ahigh purity oxygen preparation system will provide an adequately low dew point for ozonegeneration.

The gas used must have a dew point of approximately -60oC, and should be relatively free ofhydrocarbons. Water can cause several problems in a corona unit. Ozone production in a gas

with dew point of -20oC is almost half that produced at a dew point of -60oC. A moresignificant problem arises when nitrogen gas enters the corona discharge unit with moist air.Oxides of nitrogen are produced by reaction with ozone, which will then react with water

vapor to produce nitric acid. This will damage components of the generator and downstreamequipment.

In large systems cooling followed by desiccation is used to lower the dew point. Desiccantsare usually either alumina or silica, which are thermally regenerated. Regeneration is usuallydone on a daily basis to allow the desiccant to cool following regeneration.

In smaller systems air can be compressed and passed over activated alumina where the higherpressure favors the adsorption of water. The alumina is regenerated by releasing the pressureand purging it with low pressure air. Two columns of alumina can be used with constantswitching between pressurized feed air and purge air for regeneration of the alumina on a

continuous basis every few minutes.

Generation of ozone with UV

Ozone can be produced using ultra violet irradiation. This is a much less expensive systemthan corona discharge, although lower concentrations of ozone are formed. Air is passedthrough a quartz sleeve with UV light shone through the quartz into the air. Quartz is used asit is transparent to UV whereas glass and plastic absorb UV.

Production of ozone with UV has the advantage of not requiring air to be dried. UV units arealso easy to control as the rate of ozone production is linearly related to the power applied to

the lamp. Low production of ozone limits this application to small systems It is however a farcheaper system as it requires no air preparation and the UV equipment is less expensive than

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corona discharge.

Low production of ozone in UV is due to the wavelengths of light generated by low pressuremercury lamps which are used as the UV light source. They produce light at wavelengths of187 nm and 254 nm. 187 nm light causes reactions which lead to the production of ozone and

the 254 nm light causes photo-dissociation of ozone as shown in equation 4.30 below. Thenet result is that only a small amount of ozone is produced.

O3 + h O2 + O (h = 254 nm) (4.30)

Future developments in UV lamps may lead to improved efficiency.

Electrolysis

The electrolysis process consists of electrodes placed in water dosed with an acidicelectrolyte. Ozone is produced at the anode by the oxidation of water and oxygen is reducedat the cathode to produce water. To date practical problems related to power consumption

have limited this technology. It is possible in the near future that this process will develop tothe stage of being economically feasible.

Ozone Transfer into Solution

Several technologies are available for dissolving ozone in water. As mentioned earlier, ozoneis not very soluble and has a very short half-life in water, so particular attention should bepaid to selecting a contacting system that will obtain the most cost-effective results. Ozonecan be contacted with water in bubble columns, spray columns, deep U-tube, packedcolumns, mechanical mixers and venturi tubes. Table 4.11 shows the efficiency of three ofthese technologies

Efficiency

55m deep U-tube > 95%

Bubble column ~ 85%

Static mixer < 75%

Table 4.11 A comparison of the efficiency of three types of ozone contacting devices(Van Leeuwen and van der Westhuizen, 1992)

Bubble columns

Bubble columns are the most commonly used ozone contact unit (Figure 4.13). The ozonatedair or oxygen is introduced at the bottom of a cylindrical column or baffled chamber, which isusually 5 to 8 m deep. The ozone is diffused through a porous diffuser. Water is introduced atthe top at rates of up to 120 m/h to create a countercurrent flow against the bubbles, whichrise at a rate of around 180 m/hr.

Coalescence of bubbles limit the amount of gas that can be introduced and irregularities inwater flow patterns are often the cause of under-performance of this type of equipment.

Packed columns

Packed columns are similar to bubble columns except that they are packed with ceramic

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beads. These beads increase the turbulence and surface area of the water, improving thediffusion of ozone into water. Little or no back mixing of the water is possible, but gas phaseshort-circuiting can occur.

Spray columns

In spray columns water is introduced as a spray through nozzles at the top of a reactor andozonated feed gas is introduced at the bottom of the reactor. In these units short residencetimes limit the dosage and ozone transfer efficiency.

Figure 4.13 A typical bubble column for use in small systems(Van Leeuwen and van der Westhuizen, 1992)

Mechanical mixing devices

Mechanical mixing devices are best used for the re-contacting of spent gases from a previousozonation step as they have an ozone transfer efficiency of less than 75%. Two types aremainly used; surface aerators, which require totally enclosed chambers, and self-aspiratingsubmerged aerators.

Deep U-Tube

A deep U-Tube is shown in Figure 4.14. Ozonated feed gas and water are mixed and passedco-currently down a tube at 360 m/hr which is fast enough to prevent the rise of air bubbles,which rise at approx. 180 m/hr. The mixture is taken to depths greater than 15m, so that the

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pressure rises to several hundred kPa increasing the solubility of ozone in water. The mixturethen rises to the surface again through the annulus of the concentrically arranged outer tube.A 55m deep U-tube has an ozone transfer efficiency of greater than 95%. Exhaust air isseparated from the water stream by a small unit on top of the U-tube. The only pumpingeffort required for this system is that necessary to overcome normal pipe friction losses,

making this an economical method of obtaining high ozone transfer efficiencies provided thatreaction rates are high.

Figure 4.14 A typical deep U-tube (Van Leeuwen and van der Westhuizen, 1992)

Venturi systems

Venturi systems consist of a constricted pipe, which has an air inlet perpendicular to thedirection of flow of the water. The ozonated air is drawn into the water by the pressuredifferential created in the pipe constriction, the turbulence of the water is then relied upon tocreate an intimate contact between the water and the gas phase. It may also have a staticmixer such as vanes and flow dividers downstream of the inlet to improve the ozone transfer.

Venturi systems are not very efficient because they rely on a single stage completely mixedsystem where the exhaust gas concentration will be in equilibrium with the residual ozoneconcentration in the water. They are however, cheap and small and can be quite effective inwater with a high demand as a pretreatment.

Off-gas Treatment

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Ozone should generally be destroyed before exhaust gases are released to the atmosphere.In some cases away from populated areas where the off-gas can be adequately diluted notreatment may be necessary. It should be realized that ground level ozone is a greenhousegas, but that it also decomposes rapidly into oxygen. Ozone can also cause minor lung and

mucous irritation if it does not dissipate rapidly.

In most cases catalytic or thermal destruction of ozone is required. Off-gas can also be re-utilized for other applications, which will reduce the volume of ozone requiring destruction.One method of this is to send the spent gas to a water pretreatment stage for mechanicalmixing before the water is treated in the main ozone contact system.

Instrumentation and Control

Requirements for instrumentation and control can vary from almost zero for simpleapplications such as UV generated ozone for use in home spas to totally automatic control in

large plants where adequately trained personnel is not available. Measurements can be donein batch or continuous samples, control can be fully automated with a range of automaticoverrides and shutdowns. Selection of instrumentation and control systems is by necessity adecision, which is individual to each plant.

Instrumentation and control systems can increase the cost of a plant by betweenapproximately 50 and 100% over the cost of the basic equipment.

Storage of Ozone

Due to its instability ozone cannot be effectively stored or bottled. This is a particular

disadvantage in shock dosing situations where the ozone generator has to have sufficientcapacity for the highest dose. This leaves huge idle capacity during lower dosing periods.The only prospect that exists for the storage of ozone is a Japanese patent for liquefaction.

Ozone is a liquid at -50oC though it is unstable and explosive in this state.

4.9 POTABLE WATER TREATMENT APPLICATIONS

In potable water treatment ozone is primarily used as an oxidant to improve the removal ofundesirable products. Ozone is also used as a disinfectant but usually another disinfectant issubsequently required to establish a stable residual for distribution systems. Chlorine,

chloramine or chlorine dioxide, are typically used.

Pre-ozonation

Pre-ozonation, that is ozonation ahead of the whole treatment procedure, has been shown tohave a range of benefits when is used in drinking water treatment, including:

Control of algae

Removal of cyanobacterial toxins

Removal of taste, odors and color

Oxidation of iron and manganese

Microflocculation Partial oxidation and volatilization of organics

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Longer filter and activated carbon run lengths

Reduction of DBPs, particularly THM

To enhance biological activated carbon performance

The benefits of using ozone in disinfection are:

Less residual disinfectant is required

Reduced formation of DBPs, particularly THM

Short residence times for ozone contact compared with chlorine

High levels of viral and bacterial inactivation.

Figure 4.15 shows the locations in potable water treatment at which ozone is typicallyapplied.

Ozone

Pre-ozonation Disinfection

G

Raw Water Screening Clarification Filtration A Distribution

C

Residual

Disinfection

Figure 4.15 Typical locations of ozone application in modern potable water treatment

Residual disinfection would be chlorine, chloramines or chlorine dioxide. The philosophyabout this in some countries, particularly the Netherlands and Germany, is to have no residualdisinfectant. It is argued that with proper pretreatment, particularly ozonation followed by abiological treatment step on granular activated carbon and/or deepbed filtration, thebiodegradable material has been removed to such an extent that there is little opportunity forbacterial regrowth during distribution.

Benefits of pre-ozonation

Reduction of THM formation potential

Trihalomethane (THM) and other halogenated organic formation has become a majorproblem as consumers demand a safer water supply. The precursors of THM are organicmolecules present in all water supplies. When the organic molecules are oxidized in thepresence of chlorine halogenated organics and THMs form.

Ozone oxidizes these precursors, reducing the potential for the formation of THMs and other

halogenated organics. This has become a very important reason for the use of ozone inpotable water supply.

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Iron(II) and manganese(II) removal

Iron and manganese can impart undesirable color to water, causing aesthetic problems forwater suppliers. Iron and manganese form in impoundments when anoxic conditions occur.

During seasonal turnover these cations become mixed into throughout the reservoir, andeventually to the consumers taps.

Ozone acts to oxidize these contaminants to less soluble oxides as shown in the reactionsbelow.

2Fe2+ + O3 + 2H+ 2Fe3+ + O2 + H2O

Fe3+ + 3OH- Fe(OH)3(s)

overall reaction:

2Fe2+ + O3 + 2H+ + 6OH- 2Fe(OH)3 + O2 + H2O

Mn2+ + 2O3 MnO2(s) + 2O2

There can be difficulty with overdosing of ozone leading to the formation of permanganateresulting in "pink water" as shown below.

MnO2 + 2O3 MnO4- + 2O2

Removal of taste and odorGeosmin and 2-methylisoborneol (2-MIB) which are produced by microbial activity ineutrophic waters are responsible for many of the earthy, musty odors found in some drinkingwater supplies. They can have a detrimental effect on the aesthetics of water at thresholdconcentrations of 4 ng/l for geosmin and 8.5 ng/l for 2-MIB.

Ozone on its own or followed by GAC treatment can be very effective in the removal oftastes and odors caused by these organisms. Ozone probably acts by causing the formation ofhydroxyl radicals, which then oxidize these substances.

Control of algaeOzone is effective in the destruction of algae in water treatment systems. The algae are anuisance organism in water treatment plants, tending to impair the efficiency of treatment.

Removal of cyanobacterial toxinsToxic cyanobacteria in regions, which have had blooms of these organisms in their watersupply, pose a significant health risk.

Break-point chlorination is typically used as a pre-oxidant in water treatment for thedestruction of algal toxins. The use of chlorine in a pre-oxidation step has the disadvantage

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of causing high THM and other chlorinated byproducts.Ozone has been found to be particularly effective in destroying the peptide hepatotoxinMycrocystin-LR which is produced by Microcystis cyanobacteria which frequent eutrophicwaters. It also apparently destroys cylindrospermopsin, a toxin from Cylindrospermopsis.Color removal

Color removal is achieved by oxidation of the responsible organic molecules. The color ismainly caused by light absorption on conjugated bonds. These bonds are easily oxidized byozone resulting in the disappearance of color.

MicroflocculationPre-ozonation has a micro-flocculant effect at levels of approximately 0.5 to 2 mg/L. Itassists in the removal of particles in the 1 to 60 m size range. The microflocculation iscaused by particle destabilization and is dependant on the carbonate content of the water andthe organic content.

Improved filter run lengths, and reduced coagulation demand are the benefits of

microflocculation. Reduction in the required doses of coagulant and flocculant has beenobserved in Queensland coastal waters. A further subsequent benefit of microflocculation isa reduced production of sludge, reducing the load on clarifiers and potentially opening thedoor to direct filtration.

Improved biodegradabilityThe oxidation of organics in water by ozone will convert many non-biodegradable organicmolecules into a more biodegradable form. If followed by some type of biological processthis will result in a decrease in the total organic carbon (TOC) of the water.

Care must be taken to remove the now biodegradable organics. Ozone biodegradation will

result in an increase in BOD without having changed the TOC of the water. If provision forbiodegradation (e.g. biological activated carbon, or biofiltration), ozonation is a benefit to theremoval of organics. Without this provision ozonation can be a liability with respect topromoting biological regrowth within the distribution system.

4.10 OTHER APPLICATIONS OF OZONATION IN THE WATER FIELD

The most important applications of ozone in wastewater treatment are disinfection and sludgebulking control. Tertiary treatment of wastewater is closely related and similar to potablewater treatment. Other uses of ozone include cooling water treatment, swimming pool

disinfection, advanced oxidation, biological growth enhancement and industrial applications.

Wastewater disinfection

Wastewater discharges in many countries including Australia have a maximum dischargestandard for fecal coliform or heterotrophic plate count bacteria (HPC). In disinfection ofwastewater where discharge is direct to surface water residual disinfectants should be viewedas contaminants due to their environmental toxicity. Ozone forms no residual, making it idealfor wastewater disinfection particularly where the water is to be released to the environment.Systems for wastewater disinfection are similar to those for potable water disinfection.

Care should be exercised when using ozone in wastewater treatment as BOD5 can beincreased. This can create problems with achieving discharge requirements unless taken into

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account during the design of wastewater treatment systems.

Sludge bulking control

Filamentous bacteria cause foaming and sludge bulking which occur in most activated sludge

plants around the world, severely affecting solids separation.

Ozone is used to destroy the exposed ends of filamentous bacteria in flocs which assists insettlement. The advantage of ozone over disinfectants with a residual is that the ozone willdestroy filamentous bacteria projecting from the floc without penetrating which woulddestroy all bacteria within the floc. High doses of residual disinfectants such as chlorine canpenetrate the floc destroying bacteria inside the floc.

There are three locations at which ozone can be applied to reduce bulking problems. Themost effective location is within the activated sludge reactor itself as close to the effluent exitpoint as possible. Ozone can also be applied between the reactor and the settler or in the

sludge return lines, though this is the least effective application point. Figure 4.16 shows anozonation system that incorporates sludge bulking control.

The Daspoort Sewage Treatment plant, in the heart of the capital Pretoria, employs threeparallel nutrient removal activated sludge plants, each of 13ML/d capacity. An ozonationsystem, capable of dosing 2kg/h of ozone into one of the plants, was tested in an effort toameliorate sludge bulking. Ozone was introduced through a number of porous diffusorsdirectly into the final aerobic stage. Although bulking was reduced to some extent, it did notmake a huge difference as compared with chlorine, which was also trialed. It did effectivelycombat foaming and it did not interfere with the sensitive nitrifiers and phosphate removal aschlorine did (Saayman, Schutte and van Leeuwen, 1996).

Ozone recycle for bulking control

Waste Activated sludge Ozone Discharge

Water Disinfection

Ozone

Sludge recycle

Sludge disposal

Figure 4.16 Ozone use for sludge bulking control

Water Reclamation

The aim of water reclamation is to reuse water from treated sewage effluent for anotherpurpose. Some applications such as irrigation or industrial reuse require a single disinfectantto ensure that human contact with the water is safe from pathogenic contamination, whereas

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uses such as garden watering and drinking water require multiple barriers to infection.

The benefit of ozone in multiple barrier water reuse systems is that ozone works to disinfectwater by a considerably different route to chlorine disinfectants. Changing conditions thatmake one of the disinfectants less effective will often not affect the efficiency of the other.

Different disinfectants have slightly different effectiveness on particular organisms. Thus anorganism that is resistant to chlorine disinfection is often susceptible to ozone disinfection.Finally if one system breaks the other is able to continue to form a barrier to pathogens.

The first use of ozone in the treatment of water coincided with the first use of chlorin

Oxidation and Disinfection

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