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11 Waste Water Disinfection. Waste Water and Sludge Chlorination for Various Purposes
11 Waste Water Disinfection. Waste Water and Sludge Chlorination for Various Purposes
Chlorine, as chlorinated lime, was used t p deodorize and disinfect waste waters for the first time in London in 1854. In the United States of America, chlorine gas was used for the first time only in 1887 for the same purpose, while liquid chlorine only since 1914. In the U.S.A., about 30% of all municipal treatment plants have chlorination facilities, the percentage being much higher for large treatment plants. Today, chlorine is meci for waste water disinfection as well as for a aeries of technological purposes in the treatment process (odour control, reduction of BOD percentage, etc.).
11.1 Waste, Water Disinfection It must be ment,ioned that. disiizfection is not synonymous with sterilization. When all living organisms in water are destroyed, then the latter is sterile. Sterile water is required for drug manufacturing, researches, etc. However, not even potable water must be sterile. Waste waters are usually disinfected after their mechanical and biological treatment, the disinfection being applied immediately upstream of the discharge section of the waste waters in the stream. Sometimes, disinfection is used for raw waste waters as well. From the health control view-point, the disinfection,process is one of the most important taking place in the treatment plant. The pathogenic agents of many human diseases, present in domestic waste waters, contain bacterial and viral agents from the intestinal, urogenital and respiratory systems. In the past, disinfection efficiency was determined by measuring the removal of bacterial indicators using the most probable number analysis. In recent years, improved sampling and analytical methods have enabled the recovery of many species of enteric and respiratory viruses from waste waters. Since the effects of disinfection do not appear equally on viruses and bacteria, bacterial indicator tests do not accurately reflect the presence or absence of all pathogens in treated
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Waste Water Disinfection
wast.e waters. Also, since infectious hepatitis is known to be water-borne, there is concern that other viral agents remaining in waste waters may transmit diseases through the water route [ Z ] . 11.1.1 Types of Disinfectants and Disinfection Methods
To be of practical service, disinfectants used for waste waters, such as drinking waters, must possess the following properties : they must destroy the kinds and numbers of pathogens that may be in municipal water or waste waters and must do so within a practicable period of time, over the expected range of water temperature, and p,ossible fluctuations in composition, concentration, and condition of the waters or wastewaters t o be treated ; they must be neither toxic to man and ,his domestic animals nor unpalatable or otherwise objectionable in the required concentrations ; they must be dispensable at reasonable cost and safe and easy to store, transport, handle and apply ;their strength or concentration in the treated water must be easily, quickly and preferably automatically determinable ; they must persist within disinfected water in sufficient concentration t o provide reasonable residual protection against its possible re-contamination before use. Today, the disinfection methods and disinfectants used are somewhat limited in number, chemical methods being the most widely-spread and chlorine as liyuid-gas the most used disinfectant. Ch~micaZdisinfection uses as disinfectants the following chemicals : halogens, chlorine, bromine and iodine as well as a number of chlorine compounds on the one hand, and ozone and potassium permanganate on the other. Chlorine, in its various forms i R easily handled and has a reasonable cost [140]. Bromine and iodine are used on a limited scale for the disinfection of swimming-pool waters. Oxonp and potassium permanganate are relatively expensive [248]. Metal, ions, for example, silver and copper ions can be employed as disinfectants but their action is only partial, i.e. copper ions are strongly algicidal but only weakly bactericidal ; strong acids and alkalis (pH < 3 and pH > 11)kill all pathogenic bacteria but, unfortunately, at the same time damage the constructions and equipment with which waste waters come into contact. Cationic detergents are strongly destructive, anionic detergents only weakly so. Disinfection by heat is a safe and commended practice where drinking water is conserned, but it is an inadequate method where waste waters are involved. Disinfection by light as a result of sun rays and artificially produced ultraviolet light can be employed only in exceptional circumstances and for small amounts of waste waters (i.e. waste waters from hospitals), Gamma radiation has been investigated recently as a method of waste water treatment and disinfection but it is used today oaly for sterilizing surgical instruments.
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Waste W'ater Disinfectwn. Waste W a t e r and Sludge Chlorination
From review of the various disinfection methods mentioned above, the cost of the substances used, their handling and use, it follows that, generally, it is only chlorine and its compounds that can be economically used for waste water disinfection. 11.1.2
Theory of Chemical Disinfection
Green and Stumpf [ 7 8 ] have shown that in the disinfection process, chlorine and its compounds react with enzymes (Chapter 4) that are essential t o the metabolic process of living cells. Cells die when these key substances are inactivated. Enzyme destruction also remains the primary lethal mechanism of disinfectants when a radical process, such as heat, coagulates cell c o n t p t s . Because enzymes are generated within t8he cell plasma, chemical disinfection theoretically proceeds in two steps : the first, that of penetrating the cell wall, and the second, reaction with the cell enzymes. Factors governing chemical disinfection technology fall essentially into several categories, as follows : T h e nature of the organisms to be destroyed and their concentratioti, distribution and condition in the water to be disinfected : non-spore forming bacteria are less resistant to disinfectants than spore-forming bacteria ; among the enteric bacteria, Esch.coZi appears to be somewhat more resistant tthan the pathogenic bacteria; a number of enteric viruses, however, are measurably more resistant to chlorination than Esch. coli (i.e. Foliomyelitis virus Type 1).The virus of infectious hepatitis appears to be an especially hardy organism. The disinfectant reacts more actively if organisms are distributed uniformly through the water and in motion. Stirring and mixing of waste waters will do this. T h e nature, distribution and concentration of the disinfecting substance and its reaction products in the water to be disinfected. To provide maximum efficiency in the disinfection process, disinfectants must be uniformly distributed in water, and their concentration must correspond to the resistance of the organisms to be destroyed. Besides the already mentioned advantages displayed by chlorine and its compounds, it is worth mentioning that in water, another series of compounds with disinfecting qualities are generated. T h e nature and condition of the water to be disinfected. Suspended matter may shelter embedded organisms against chemical disinfection as well as against destructive light rays. Organic matters in large amounts lead to an increase of the required chlorine dose, because chlorine is consumed for organic matter oxidation. T h e higher the temperature of waste waters, the quicker are aquatic organisms destroyed. T h e time of contact. The longer the time, the greater is the opportunity for destruction. However, a minimal time of displacement is the governing factor for disinfection.
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Waste Water Disinfection
From an examination of these categories, it follows that the three factors that can be adjusted to maximize disinfection efficiency are: the nature and concentration of the disinfectant ; the degree of stirring to which water is subjected ; and the time of contact between organisms and disinfectant. Certain changes in the quality of waste waters can be made to generate favourable conditions for disinfection, for instance, changing water pH or temperature. However, they require additional treatment and adequate equipment respectively which raise the cost of the disinfection process. 11.1.3
Kinetics of Chemical Disinfection
Under ideal conditions all cells of a single species of organism are discrete units equally susceptible t o a single sort of disinfectant ; the disinfectant and cells are uniformly dispersed in the water; the disinfectant stays substantially unchanged in chemical composition and substantially constant in concentration throughout the period of contact ; the water contajns no interfering substances. Under such conditions the rate of disinfection is a function of the following variables : the time of contact ; the concentration of the disinfectant; and the temperature of water. Tiwhe of contact - under the above-mentioned ideal conditions follows Chick’s law of disinfection [ S O ] . This states that y, the number of organisms destroyed in unit time, is proportional to N , the number of organisms remaining, or : d,
__ =
dt
k . ( N o- y ) ;
(11-11,
where : k
- coefficient of proportionality (rate of killing), or the rate constant with dimension ( t - 1 ) j N o - initial number of organisms. By integration between the limits y = 0 at t = 0, and y = y, at t = t : (11-2).
or : N- exp (-
NO
k.t).
(11-3)
However, even under the ideal conditions assumed, certain deviations from Chick’s law are possible: the rate of killing ( k ) rather than being constant may increase or decrease with time. Increase in rate of killing can be explained either as a result of the slow diffusion of chemical disinfectants through the cell wall and a rate of killing accelerating with the aecumulation of disinfectant within the cell, or by the consequence o f
441
Waste Water Disinfection. Waste Water and Sludge Chlorination
a time lag before the disinfectant can reach a lethal number of vital centers in the organism. Decrease in the rate of killing is generally explained by an increased resistance of cells to disinfection action. Concentration of Disinfectant. For changing concentrations of disinfectant, the observed disinfecting efficiency is generally approximated by the relationship : c'
where : c tp
.tp = constant ;
(11 -4)
- concentration of disinfectant ; - the time required to effect a
constant percentage of the organisms killed ; n - coefficient of dilution or, according to van't Hoff, a measure of the order of the reaction. Examples of time-concentration relationships for chlorine as HOCl =6.3 and 99 yokilling of Esch. coli and enteric viruses at 0 to 6°C are : co*8ti-tp for Coxsackie virus A2 ; c0."-tp = 1.2 for poliomyelitis virus 1 ; and co*86.t, = 0.098 for adenovirus 3 ; all in comparison with co.86.tp = 0.24 for Esch. coli [40].When n > 1,the efficiency of the disinfectant decreases rapidly as it is diluted ; when n < 1, time of contact is more important than dosage ; when n = 1, concentration and time are of equal weight and a first-order reaction may be in progress. Equation 11 -4 is empirical ;it is the result of laboratory observations. The concentration of organisms can also be expressed by an empirical equation : c q - N p= constant; (11-5) where : c - concentration of the disinfectant ; N p - concentration of organisms that is reduced by a given percentage in a given time ; - coefficient of disinfectant 8trength. q Temperature of Disin-feetion. I f the rate of disinfection is determined by the rate of diffusion of the disinfectant through the cell wall or by the rate of the reaction with an enzyme, temperature effects usually conformably to the ven't Hoff - Arrhenius relationship : (I1 -6)
where : !C2, TI- two absolute temperatures (in degrees Kelvin) for which the rates are to be compared ; t l , t , - the times required for equal percentages of destruction with fixed concentrations of disinfectant ; - activation energy (normally in calories) and a constant E characteristic of the reaction ; - the gas, constant (1.99 cal per deg C , for example). R 4 42
Waste Water Disinfection
For T,- T , = 10, tho useful ratio t&, called Q,,, is related to E approximately as follows at normal water temperatures : log Ql0 11.1.4
= log (t,/t,) =
E/39,000.
(11-7)
Disinfection by Chlorinc and Chlorine Compounds
4'hloriue is available as a liquid, A gas (chlorine) or as chlorine compounds {chlorinated lime, calcium hppochlorite, etc.). Liquid or gaseous chlorine is the preferred form for waste water treatment, because of its lower cost as compared to chlorine compounds. Elemental chlorine is a poisonous yellow-green gas at ordinary temperaturc and prc::+\me. Other properties of chlorine are : atomic weight (('1) 35.46 ; molecular weight (Cl,) 70.914 ; specific weight 3.12 g/l a t 15°C and 760 mm Hg ; liquefying point - 31.1"C' ; freezing point - 102°C ; the solubility of chlorine in water 1-aries with temperature : 1 litre of water will dissolve 6 g of chlorine a t 25°C ; 7.3 g of chlorine at 20°C ; 10 g of chlorine at lO"C, a n d 14.6 g at 0°C. The odour of chlorine in air is detected a t a concentration of 3.5 ppm in volume ; it becomes extremely irritating at concentrations of 40 to 60 ppni ; breathing of chlorine gas with concentrations of 40 to 60 ppm for 3 0 to 40 inin is dangerous ; at concentrations of 1000 ppm it becomes lethal. ZI.2.4.l Liquid or gaseous chlorine. Chlorine is generally purchased as liquid ch7orine or chlorine gnu which has been liquefied by compression. Liquid chlorine is not inflammable, it is clear, amber in color and, about 1.J t irnes heavier than water. C'hlorine gas has a highly irritating Bnd penetrating odour and is about 2.5 times heavier than air. One volume of chlorine, when vapourized, will yield approsimately 460 volumes of gas. While not explosive nor iIlfkdlnmable, chlorine gas is capable of supporting combuvtion at high temperatures (250°C). Chlorine gas is supplied for use as a compressed liquid in tank cars made of special steel containers or in steel cylinders which must be handled very carefully. From containers or cylinders the compressed liquid is vaporized. As regards the mechanism of chlorination, there are several reactions $hat occur simultaneously when chlorine is added to waste waters. A first reaction is hydrolysis :
ur :
+ H,O tHOCl + I€+f C1C1, + OHHOCl + C1-
C1,
(11- 8 ) (11-9)
A second reaction is ionization :
H O C l Z H+
+ OC1-
(11-10)
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W a s t e W a t e r Disinfection. W a s t e W a t e r and Sludge Chlorination
Rypochlorites, calcium hypochlorite, for example, generate the same reactions. Regardless of the type of reaction, liquid-gaseous chlorine results in the formation of hypochlorous acid and hypochlorite ion:. in water. Hypochlorous acid (HOC1) and hypochlorite ion (OC1-) are referred to, in practice, as free available chlorine. Liquid-gaseous chlorine tends to lower the pH value. I n clean waler, a t pH = 6, the percentage of total free available chlorine is 96.87, as HOCl and 3.2 yoas OCl- ; at a pH value of 9, the percentages are reversed. Waste waters contain numerous substances (Fe++,NO;, 807,etc.) which react with ROC1 and reduce it to the stable chloride ion. Oxidation of reducing substances by chlorine produces the so-called immedinte chlorine demand. This reaction is proportional to the amount of reducing th.e present substances, with the result that free chlorine is not present the water for longer than a small fraction of a second. Waste waters also contain appreciable amounts of compounds of nitrogen which react with chlorine ; the most important reaction is t h a t of hypochlorous acid with ammonia :
+ COG1 -, NH,Cl + H,O (monochloramine) NH2C1 + HOCl -+ NHCl, + H,O (dichloramine) NHCl, + HOCl NCl, + H 2 0 (trichloramine) NH,
-
(11-11) (11-12)
(11- 1 3 )
Chloramines are combined available chlorine while HOCl and O(’1are ,free available chlorine. Chloramine formation depends on pH value, temperature, time of contact and concentration of each reacting substance. Monochloramine, for example, predominates at pH = 5 ; lower p H values favour the formation of NC1,. A substantially complete oxidation’ of ammonia and iveduction of chlorine is achieved at a ratio of 2 : 1; in a relatively short time the break point is reached, when all ammonia and oxidizing chlorine disappear completely from the water. Reaching the breakpoint is a function of the pH value at which the reaction occurs ; for pH values between 6.5 and 8.5, the break point is reached in about 30 min. The advantages of performing the chlorination in the immediate neighbourhood of the break point are: odours and tastes are destroyed ; a good disinfection is ensured. The amount of chlorine that remains free in water after chlorination is called residual cMorine. Experience with waste waters to be treated will generally establish a relationship between the residual chlorine content and the time of contact necessary to ensure the desired bacteriological results. The amount of chlorine is considered sufficient if, after a 30 min chlorination, the amount of residual chlorine ranges between 0.5 to 1.Omg/l ; it is assumed that the amount of residual chlorine after a 15 min chlorination must be at least 2 mg/l to alehievean optimum bacteriological effect.
444
Waste Water Disinfection
The amounts of chlorine necessary for waste water chlorination vary with the amount of polluting substances, the degree of disinfection to be accomplished, the temperature of waste waters, etc. Out of the total amount of chlorine used, about 30% is consumed by settleable solids, about 40 yo by remaining suspended solids and the finely divided matter, and about, 30 yo by dissolved solids. As these matters are removed, the amount of chlorine also required decreases. Large suspended solids must be separated through coarse or fine screens even if no other treatment is applied later on, because chlorine penetrates the inside of suspended solids only with difficulty. Waste waters under treatment, containing large amounts of hydrogen sulphide, require greater amounts of chlorine as compared t o those needed by fresh waste waters. 11.1.4.2 Hgpoehlorites. The hypochlorites commonly used for waste water disinfection are : sodium hypochlorite and calcium hppochlorite. Their effects as disinfectants are similar t o those obtained by liquid or gaseous chlorine. However, while hypochlprites are more safely handled than chlorine, the operation is much more difficult and they are more expensive. Sodium hypochlorite is supplied as a solution containing 10 to 15°/o of available chlorine. It must be stored in a cool, dark location as it is r&t ively unstable. Calcium hypochlorite is a dry white powder containing about 70 yo of available chlorine. It is relatively stable. When dissolved in water i t forms a precipitate, particularly with hard water. The sludge causes lii~ncilingdifficulties and feed equipment clogging. Ayueoub: hypochlorite solutions are extremely corrosive and should be prepared and stored in nun-reactive containers. Dry calcium hypochlorite should be stored in a cool dry area out of direct sunlight. Reaction with certain organic coi~ipoundsmay cause fire or explosion. For these reasons calcium hypochlorite is very seldom preferred t o xodium hypochlorite or liquid or ga wous chlorine. 11.7.4.3 Chlorine Dioxide. Chlorine dioxide, CIOz, is recommended for wiste xaterx whose required degree of disinfection is very high and for waters with a high ammonia content. Chlorine dioxide is an intense yellow-greenish gas that is quite unstable and which, under certain condiiions, is explosive. It cannot be shipped in containers asis chlorine gas, because of its explosive nature, and it must be generated a t the point of use and applied immediately. At ordinary temperatures, it is 2.4 times heavier than air and can be compressed to a liquid. The use of chlorine dioxide is limited because of its hazard potential and high cost. As regards its bactericidal efficiency, a t a neutral pH, it is similar t o that of liquid or gaseous chlorine; at pH = 8.5, the efficiency increases substantially.
446
Waste W a t e r Disinfection. W a s t e Water and Sludge Chlorination
11.1.5
Disinfection by Ozone
Ozone is an actt.ive oxidizing agent; it.s chemical formula is 0,. It is generated by pa,ssing cleaned dried a.ir through the corona formed between high tension elect,rodes. The silent elect.ric discharge taking place in t,he air convert,s slightly less than 1percent of the oxygen present, into ozone. Cooling the air increases the efficiency of generation. The generat ion p h n t is elaborate, costly and requires skilled operation. Ozone is a pale blue gas having a characteristic pungent, odour. I n t,he concentrations usually employed, the odour is detectable hut the colour is not. visible. Ozone is an unstable gas with each rriolecule breaking down 1.0 libemte one atom of nascent osggen. Immediately after generation, it is dispersed int.0 t,he wrat.er undw 1-rcatment..I n sufficient concentrations, it is a.n effective bactericide but. the gas must come int,o.direct. contact witjh the microorga,nisms (bacteria, and more recently, viruses) to dest.roy. them. I t is toxic ; fish may l t e killed if a high dosage is present in water. Ozone will react with ositliza1)lc~ maher in the waste waters wit.11 some reduction in HOD, and the dosage needed for effective disinfection will clepcmd on t,he quality of the waste waters. The optimum efficiency of ozone is attained at ;t pH value ranging between 6.0 snd 7.0. I)osa,ges of .5 to 8 mg/l in the secondary efflumt pr0mot.e an eff icientj disinfec;tion of wastx waters. Because of t,he nature of ozone, the residud is short-lived. I n matm', t.he half-life has been estimated to 'tie 20 rnin. I)ue t.o the form of ozone and its short; life, it must, be step-fetl into waste waters to provide the period of contact needed to a,cc.omplish disinfection. Intimate mixing of an ozone-enriched air stream with the wastc waters as well as maintenance of contact for an adequate period of tinit? is essent.ia1. The major problerris to be considered are : t h e satisfying of ozone demand ;the mpid rise of the gas to the liquid surface of the contact chamber, and escape of ozonizetl air bul)bles ; a,nd the relatively short half-life of ozom?. 11.1.6
Constructions
- Equipment
Chlorine is fed into waste waters either directly or as a solution. Directed feed is nccomplished with chlorine gas. I t is fed through simple devices that do not require water, air or energy consumption. However, the use of this method is limited only to small treatment plants. It is very important that, the point of application of chlorine in waste waters should ,be a t least a t 1.20 m below water level. Feeding as n solution requires the previoufi preparation of a solution of chlorine g a s in water. The solution is fed through pressure or vacuum chlorinators.
446
Waste Water Disinfection
The pressure f e e d type chlorinator utilizes a mechanical-diaphragm type of chlorine-control valve to reduce the cylinder pressure ahead of the metering device. The metered chlorine gas is fed into an injector where it is mixed with water and fed to the point of application. The vacuum-,feed type cldorinutor (Figure 11-1) has developed to a high degree of efficiency and it incorporates all the safety features required to prevent t,he accidental discharge of chlorine-gas. The chlorine pressure regulating valve, operating in conjunction with the vacuum, is designed to Rhut off the flow of chlorine-gas in the event of vacuum failure. The
LEGEND
ul/orrne gas
m l q fi/orrne so/utron Wafer
Figure 11 -1. Vacuum chlorinator, flow diagram.
447
W a s t e W a t e r Disinfection. W a s t e W a t e r and Sludge Chlorination
injector and diaphragm check valve ensure the feeding of chlorine gas a,s a solution in adequate doses to the waste waters to be chlorinated (Figures 11-1, 11- 2 ) ,
Figuie 11 -2.
Chlorination installation with automatic control of chlorine flows.
Chlorine-gas dose can be controlled manually or automatically, function of waste water flow and quality. The location of the manual feed rate adjuster device of chlorine gas is shown in Figure 11-1. Water flow is controlled manually with the valve on the water supply pipe. The automatic control of chlorine-gas dose function of waste water characteristics and residual chlorine desired respectively, is of major importance due to daily variations in the chlorine demand of waste waters
448
W a s t e W a t e r Disinfection
and the necessity for close regulation of the disinfection process. The most effective method of chlorination control is the compound loop system. This arrangement uses two separate and independent signals to regulate the chlorination device :a flow-proportional signal to the chlorine metering orifice ; and a chlorine dosage signal to the vacuum regulating valve on the dosage control device. The chlorination mechanism combines these two signals to achieve a wide range of operation in excess of 100 to 1. The dosage signal is transmitted from R chlorine residual analyser T-ia a vacuum or electrical transmitting device. The controls on the a n a l y e r hhould be adjustable to accomodate loop times from 1to 7 min and correction times from 0 to 20 s. Accessories normally include a precision vacuum gtuge t o monitor the vacuum signal to the chlorination device. It is not good practice to interchange the signals, that is, to send the dosage signal t o the chlorine metering orifice positioner mid the flow proportional signal to the vacuum regulator. Chlorinators ore most commonly f e d with chlorine-gas .from c y l i d e r s or containers. In Roinania, a cylinder contains 50 kg of chlorine (gross weight 100 kg) and a container 650 or 1150 kg of chlorine, equivalent to 4,50 1 or 800 1 of chlorine respectively (gross weight 1000 kg or 1500 kg respectively). I f chlorine consumption per 24 h exceeds 460 to 600 kg, the use of evaporators is preferable in order to avoid an excessive number of cylinders or containers in service. The liquid chlorine is changed t o gas in an evaporator before being fed to the chlorinator. It is desirable to install one evaporator for each chlorinator with a minimum of two evaporators, each capable of supplying the maximum chlorine demand. Chlorine installations are usually provided with weighing scale so that the amount of chlorine remaining in the containers as well as the total amount fed can be determined at any time. Scales should be sized to accomodate the maximum number of cylinders required to serve the maximum chlorine feed rate. Scales may bc-mounted flush with the floor or on the floor surface within an enclosed box. With above floor mounting, it is desirable to have overhead hoist equipment available. Many instalhations have a loss-of-weight recorder to provide a continuous record of chlorine feed. Usually, chlorine ins tallations using cylinders are not equipped with hoisting devices. Manual handling of cylinders is generally used in plants where chlorine requirements are maximum, up to 100 kg per day. Handling of containers requires hoisting equipmetbt. It is desirable to use a poweroperated hoist and travel, particularly when it is necessary to change containers frequently. The hoist should have a minimum capacity of 2 tons and be equipped with an approved type of lif ting-beam container grab. Materids used t o manufacture pipes, valves, etc. should generally meet the requirements regarding the corrosiveness of chlorine and its compounds. Pipe lines for dry chlorine gas or liquid chlorine going from cylinder or container to the chlorinators or evaporators should be extra 29-742
449
Waste W a t e r Disinfection. W a s t e W a t e r and Sludge Chlorination
heayv, black and genuine wrought-iron pipes. Joints are made usiiy litharge and glycerine. Lines should be insulated when necessary i o prevent the gas from chilling and the liquid from overheating. F1exil)lv connections from the container to the header should be extra heaT-. Wet chlorine is very corrosive and must be handled with special materials such as silver, hard rubber, glass or certain plastics. Solution feed limb from the chlorinator to the point of application may be rubber how, rubber-lined pipe or plastic pipe. Rubber hose in sizes up to 50 mm i b the most widely used. Rubber-lined steel pipes are available in sizes from 40 mm diameter up. Plastic pipe, or plastic-lined pipe, or any of the highimpact polyvinyl chloride materials are suitable for solution feeding. Diffusers for insertion through corporation cocks a e generally made of silver or hard rubber, while large diffusers am usually rubber-lined a r i d covered steel pipes. Chlorinator overflow lines and gas vent lines a l e made of rubber or plastic. Chlorinators are supplied with water either from the public supply system or from the plant effluent. A separately pumped supply with n o direct connections to the potable water supply is used in some cases, snd in all cases where the plant is interconnected with the chlorinator. Commonly, about 330 1of water are required daily per kilogram of chlorinc. The amount of water must be enough t o maintain a chlorine solution strength which does not exceed 3500 mg/l. The pressure a t the point of chlorine application, known as injector back-pressure, is another parameter - besides water flow - which must be considered for the good operation of a chlorinator. Generally, for each chlorinator the manufacturer supplies the so-called operating curves specifying the amount of water and pressure required for a given amount of chlorine to be a'pplied against a given back-pressure. The minimum operating pressure is about 1.76kg/cni2 (1.76 atm) with 0 kg/cm2 back-pressure on the injector, and this must be increased to provide about three times the actual back-pregsure. I f necessary, the pressure must be attained by pumping. When the flow is larger, chlorine solution is injected into the waste waters through diffu,sers. Regardless of the fact that there are pipes or channels, diffusers are made of one or more perforated pipes placed horizontally or vertically and across the directtion of water flow. Each orifice of the diffusor must provide a flow of 0.06 to 0.12 l/s and a velocity of 3 mjs. Normally, the orifices are not smaller than 10 mm in diameter. A more rapid dispersion of chlorination solution in waste waters can be accomplished if adequate devices for initial mixing are provided. For this reasons, the solution is dispersed either in a hydraulic jump or in a conduit or channel designed to provide turbulent flow. Less recommended are mechanical mixers located in chambers, where the order of magnitude of the detention time is several seconds ; in this case, the diffusors will be used upstream of tbe miser. Initial mixing prior to the contact chamber must not be confused with mixing in the contact chamber, which is to be avoided because it generates short-circuiting in the chamber and leads t o unreliable disinfection results.
45 0
Waste Water Disinfection
The contact chamber is one of the main components of the chlorination installation. The purpose of the contact chamber is to ensure the detention tirrie necessary for the chlorine compounds to reduce the bacteria to ;Lcceptable levels. It is extremely important to minimize short-circuiting and dead spaces through basin configuration and flow pattern control. C'ontact chambers must not be designed as circular basins as in the common practice ; the best configuration to avoid short-circuiting through t h e chamber and to accomplish a plug-flow regime is that of a chamber (baffled basin) with long narrow sections, the length to width ratio of which is 10 : 1 or even more. Directional changes tend to encourage short-circuiting, so that such changes must be kept to a minimum. Guide vanes may be used for direct ional changes to prevent short-circuiting in basins where several directional changes are necessary because of space limitations. Chlorinc. improves the settling characteristics of solids. Since t h e waste waters to be treated still, sometimes, contain suspended solids, it is impeiatiw that adequate facilities for their removal out of t h e ch;iinbws should be piovided. Chlorme culinder
Figurc 11 - 3. Layout of a chlorination station, Iiomanian standard design.
-I
4 65
I
Residual chlorine, in the effluent of the chlorination chamber is determined by means of a residual chlorisje recorder monitor. Systems employing residual chlorine control recorders, usually have high and low residuul alarms. Installation using evaporators require alarms giving warning of low temperature and low water level in the water bath. The chlorination installation can sometimes fail when waste water chlorination ceases. The most common reasons leading to interruption of chlorination are : exhaustion of chlorine feed, power failure, water supply failure, equipment break-down, etc. A stand-by system, the use of portable chlorination, or of pre-chlorination system are to be highly recommended.
451
Waste Water Disinfection. Waste Water and Sludge Chlorination
Chlorine installations require adequate room for the maintenance of equipment and handling of chlorine cylinders and containers. Figure 11-3). Usually, the room accomodating the chlorinator is located a t or above ground level. The chlorinator and its auxiliary equipment are placed in a chamber, while cylinders and containers are located in another chamber with access only through the outside door. The surface areas of the two chambers should be designed to allow the rapid access to the entire equipment for maintenance and repair. Informatively, for a plant with only one chlorinator supplying less than 100 kg chlorine per day, the area accomodating the chlorination installation should be about 6 m2; for a plant with two chlorinators, 16 m2; about 1 5 m2 are required for emch chlorine feed evaporator unit). In the chamber accomodating the chlorinator a consta m’ tem&mturp of about 21°C should be provided to ensure the good operarion of the chlorination installation. In the chamhers where the cylinders and containers are located, the temperature should be around 10°C. Chambers are preferably heated by hot water sybtems for safety reasons and due to the uniform temperature ensured. Stearn heating is not recommended because of the temperature extremes that might be experienced in t hc case of system failure. Electrical heating is suitable, and forced-air heJ,ting would be appropriate if an independent system is provided for the chlorination room or building. Ventilation is required for all chlorine equipment room3. The ventilation intake or exhaust fan must be located a t floor level. Ordinary practice consists of an exhaust fan with guard and shutters mounted a t floor level on an exterior wall, and operated imtermittently as required. An alternate and superior method consists of an exterior fan with the intake duct running to the chlorine room floor. Provisions must be made for fresh-air intake through either a louvered entrance door or a separate intakcl. . Good practice requires an air exchange every 3 minutes. The chamber accomodating the chlorinators must always have an i?ispectiota window - not too large - to enable the supervision of the chlorinator operation without coming into the room. This window is usually located in the entrance door. Any other possible sajety measures ensuring the good operation of chlorinators must be taken : fire control, automatic switches for energy supply, light signals in case of failure, etc.
11.2 Chlorination of Waste Waters and Sludges for Other Uses I n a treatment plant chlorine is used not only for disinfection but for other purposes as well. The chlorination of waste waters before being mechanically treated is called prechZorination ;after mechanical treatment, intermediate ch orination.
t
452
Chlorination of Waste Waters Waters and Sludges
Odoilr control. So long as aerobic conditions prevail in the treatment plant, odour is not offensive. The most odouriferous substances are generally products of anaerobic decomposition. Among them are hydrogen sulphide, indole, skatole, mercaptan, and cadaverin. Some of the foul odours released by these substances remain detectable even when they are dispersed in a million times their own volume of air. As a rule, hydrogen sulphide is the forerunner of all waste waters odours. It occurs whenever sulfates are broken down by bacteria, and it is particularly strong when the carrying water -is rich in sulfates or when industrial waste waters that contain sulfates are admitted to the sewerage system. The presence of hydrogen sulphide in confined spaces is dangerous because it is an extremely toxic gas, and lethal accidents might consequently follow. When sewers flow partly full, hydrogen sulphide released a t the waste water surface is oxidized into sulfurous and sulfuric acid, which attack concrete, mortar, iron, etc. Odour generation in sewerage systems, particularly in sludge collecting systems and long-outfalls may be checked by up-sewer chlorination. Odours that have been formed may be swept out by aeration or oxidized by chlorine. Hydrogen sulphide is destroyed by chlorine in the ratio of their combining weights (Cl,: H,S = 70. : 34.1, or 2.1 mg of C1, for each mg of H,S), but larger amounts of chlorine must be added to waste waters because chlorine also combines with other substances present. Chlorination of waste waters ahead of settling tanks does not measurably inteIfere with digestion of the sludge, because the small amounts of chlorine added ale quickly absorbed. Up to 50 mg/l of chlorine may be needed to control the odour of raw waste waters. Chlorination of supernatant from sludge digestors or sludge-dewatering processes may be useful. The chlorination of stack gases, particularly in the heat drying of sludge, is an emergency measure. If waste water is naturally fresh when it arrives at the treatment plant or is kept fresh by upstream sewer chlorination, the control of odours becomes largely a matter of safety. Reduction of biochemical oxygen demand. Chlorination leads to a reduction of BOD, to a delay in organic matter decomposition in the waste waters of the treatment plant and streams respectively. At the same time it prevents : the generation of anaerobic conditions in the sewerage system and controls odours developed; the damaging of concrete and mortar due to the corrosive action of waste waters ; the penetration of septic or waste waters under digestion into the treatment plant; the process of decomposition of organic matters in the waste waters a t the mechanical treatment units of the plant is inhibited, thus preventing odour development within the biological treatment units of the plan and especially in the biological filters: the worsening of the quality of returned sludge ( 5 9.3.2) ; continuation of the decomposition process during sludge thickening (0 10.4.1);it delays the immediate oxygen demand of water in streams (Subchapter 5.3) and of the supernatant coming from the sludge digester (Subchapter 10.2).
453
Waste Water Disinfection. Waste W a t e r and Sludge Chlopination
Chlorination may reduce BOD in one of the folloming ways : o\idation of BOD-exerting substances ; formation of disinfecting substances with nitrogen compounds (chloraminex, for example) ; formation of nondecomposable substances (with carbon compounds and unsaturated compounds, for example). The reduction of BOD depends upon the quality of the waste waters as well as the concentration of the clilorint.. Chlorination of fresh waste waters to a trace residual in 1 5 niiii may lower BOD by only 10 yo; doses above 100 ml/l may be needed to lower it by as much as 35 :(, [204]. The temporary lowering of the oxygen requirements of waste waters discharged into streams by chlorination may help maintain aerobic conditions, when the oxygen supplied by the water itself and by reaeration would otherwise be unable to keep pace, until the mother stream is reached or tributary waters establish a satibfactory oxygen balance. However, sludge deposits may intelfere with iiormally expected performances. Generally, the chlorine dosages used to control oclours will also control septicity. Destruction and Control of Damaging Orgccriisne Growth i,i the Sewerage System, Treatmettt Plant and Slreanzs. Filamentous organisms such as : 8phaerotiZus fiatans, Leptonsitus and Beggiatoa which clog the channels or narrow their flowing section, and also lead to the increase of the activated sludge flocs ( $ 9.3.2), are destroyed ; Ysychoda f l y growth is controlled as well as the formation of abnormally large aiuounts of sludge that might pond biological filters ( S 9.3.1) ; the growth of algae or other aquatic organisms in the surface waters is also controlled. Corrosiow. If the formation of hydrogen sulphide is prcvented as a result of waste water chlorination, then the hazard of corroding the metal and concrete constructions in the treatment plan1 and streams is avoided. Hydrogen sulphide in water is oxidized to form sulphuric acid which is corrosive and attacks metals ; from this reaction calcium sulpllsle also results which attacks constructions. Sludge Bu&l;ing ( 5 9.3.2) is caused especially by tJIJe filaineritous bacteria Spliaerotilus natans. Hulking can be prevented If the returned sludge is chlorinated long enough to give the chlorine a 2 t o 3 niin mixing (stirring) time. Pats are removed mash efficiently from skimming tanks if chlorine is blowning at the tank bottom together with compressed air (Subchapter 7.2).
11.3 Dcchlorination The chlorine doses used to disinfect waste waters are sometimes in excess. Thus, residual chlorine in anlourits larger than necessary still remain in the water, eventually finding its way into streams. Although the amounts of chlorine are generally small, when they are disposed in streams with low flows, they can he a nuisance due to the toxicity.Consequently, the
454
Dechlorination
effluent of the treatment plant must be dechlorirmted before reaching the \tream ; this means the removal of excess chlorine from the waste waters. Dechlorination of waste waters can be accomplished by adding redu(.11igchemicals, passing them through beds of granular atetivattedcarbon, ;I i d by aeration. The reducing chemicals are :sulphur dioxide, (SO,), sodium itietabisulpliite, (Na,S,O,), and sodium sulphite, (Na,SO,). SuZphur dioxide reacts with chlorine in waste water according to the ecluation : SO,
+ C1, + H,O
--f
H,SO,
+ 2HC1
(11-14)
Generally, 1mg/l of sulphur dioxide is required to dechlorinate 1mg/l residual chlorine. Sulphur dioxide is available in cylinders, like chlorine, under a pressure of approsimately 240 kN/m2. The equipment for metering sulphur dioxide delivery is, for all practical considerations, identical to that for chlorine. Sulphur dioxide is mixed in waste water by a mechanical mixer. The dechlorination reaction taking place ina contact chamber between sulphur dioxide :ind both free and combined chlorine is rapid, a few seconds at most. Quantitative checking of the reaction is performed downstream of the contact chamber where residual chlorine is to be determined. Xodium rnetnbisulphite is a white or cream-coloured powder, which rc+ldily disolves in water, changing into sodium hiosulphite ( NaHS03). The reaction taking place with sodium metabisulphite and waste waters to he dechlorinated is : 2HOC1 + H,O
+ Na2Sz05+ 2NaHS0, + 2 HC1
(11-15)
This is a stoichiometric ratio of 1.34 units of metabisulphite required to dechlorinate each unit of chlorine. In practice, it is well t o size the equipment on the basis of 1.5 units of metabisulphite to dechlorinate each unit of chlorine. The construction materials for a dechlorination system using a solution of sodium metabisulpliite are the same as those used for handling hypochlorite solutions. Actiwuted carbon. The dechlorination process involves a chemical reaction between chlorine and water with carbon acting as a catalyst. The stoichiometric reaction for dechlorination is : a
2 C1,
+ C + 2H,O-+
4HC1
+ CO,
(11-16)
The dechlorination process depends on chlorine concentration and flow rate, the physical characteristics of the carbon, waste water cwnditions alnd the chemical state of the free chlorine, the lastjfactor also being the most important. The dechlorination reaction is accompanied by adsorption of organic and inorganic contaminants. The most readily oxidizable products will be oxidized by chlorine, while others, adsorbed on the carbon surface, are oxidized by the nascent oxygen formed during the surface reaction.
455
Waste Water Disinfection. Waste Water and Sludge Chlorination
Dechlorination is accomplished through rapid filters with up-to-down flow, running by gravity or pressure. For an influent with free residual chlorine of 3 to 4 mg/l, the surface loading of the filter can be of about 0.002 m3/m2.sat a 15 t o 20 min contact time with the activated carbon.
11.4 Design
- Examples
Chlorination plants for waste waters must be able to supply chlorine doses ranging between 4 and 50 nigjl. Chlorine doses vary as a function of the amounts of waste waters and sludge and of the purpose of the chlorination. Table 11-1 displays the chlorine doses required to obtain 0.5 t o 1.0 mg/l residual chlorine after 15 min contact time. TABLE 11- 1 Chlorine doses necessary to obtain 0.5 to 1.0 mg/l of residual chlorine after 15 min contact time
Waste waters or sludges
I
I
Raw waste waters Primary settling tank effluent Chemical treatment rffluent Biological filters effluent Activated sludge units effluent Sand filter effluent Raw waste waters for odour control Raw waste waters f o r corrosion prevention Ra w waste water for prevention of biological filter cloggidg (it is fed periodically, €or 8 h) Influent raw waste water for Inihoff tanks for prevention of foam forming Influent raw waste water to increase efficiency of skimming tanks Returned sludge l o prevent its bulking (2-3 min contact time) Sludge in thickening tanks upstream of sludge digesters t o obtain 1 mg/l of residual chlorine in t h e supernatant in t h e tanks
Chlorine dose, mg/l 6- 24 3- 18 3- 12 3- 9 3- 9 1- 6 max. 20 2- 10
.
20- 50
3- 15 1 .O- 1.5 1- 10 max. 50
In Romania, the design of chlorination plants must take into account the standard design “Chlorination Plant with Chlorine-gas for Waters and Waste Waters Treatment” (Figure 11 -3). Contact chambers are sized for a 30 min contact time. The amount of sludge deposited in these chambers (96% moisture content), is about 0.10 1 per capita daily using liquid or gaseous chlorine and of 0.17 1 per capita daily using calcium hypochlorite. The removed sludge is sent to the primary settling tanks. The store of cylinders or containers is sized to allow storing the amount of chlorine required for a 30 day period. The amount of water necessary t o prepare the solution of chlorine-gas is 4 to 5 1 per min for one standard chlorinator (or 350 1 of water each day per kg of chlorine) at a minimum pressure of 1.75 atm.
456
Chlorine, as chlorinated lime, was used t p deodorize and disinfect waste waters for the first time in London in 1854. In the United States of America, chlorine gas was used for the first time only in 1887 for the same purpose, while liquid chlorine only since 1914. In the U.S.A., about 30% of all municipal treatment plants have chlorination facilities, the percentage being much higher for large treatment plants. Today, chlorine is meci for waste water disinfection as well as for a aeries of technological purposes in the treatment process (odour control, reduction of BOD percentage, etc.).
11.1 Waste, Water Disinfection It must be ment,ioned that. disiizfection is not synonymous with sterilization. When all living organisms in water are destroyed, then the latter is sterile. Sterile water is required for drug manufacturing, researches, etc. However, not even potable water must be sterile. Waste waters are usually disinfected after their mechanical and biological treatment, the disinfection being applied immediately upstream of the discharge section of the waste waters in the stream. Sometimes, disinfection is used for raw waste waters as well. From the health control view-point, the disinfection,process is one of the most important taking place in the treatment plant. The pathogenic agents of many human diseases, present in domestic waste waters, contain bacterial and viral agents from the intestinal, urogenital and respiratory systems. In the past, disinfection efficiency was determined by measuring the removal of bacterial indicators using the most probable number analysis. In recent years, improved sampling and analytical methods have enabled the recovery of many species of enteric and respiratory viruses from waste waters. Since the effects of disinfection do not appear equally on viruses and bacteria, bacterial indicator tests do not accurately reflect the presence or absence of all pathogens in treated
438
Waste Water Disinfection
wast.e waters. Also, since infectious hepatitis is known to be water-borne, there is concern that other viral agents remaining in waste waters may transmit diseases through the water route [ Z ] . 11.1.1 Types of Disinfectants and Disinfection Methods
To be of practical service, disinfectants used for waste waters, such as drinking waters, must possess the following properties : they must destroy the kinds and numbers of pathogens that may be in municipal water or waste waters and must do so within a practicable period of time, over the expected range of water temperature, and p,ossible fluctuations in composition, concentration, and condition of the waters or wastewaters t o be treated ; they must be neither toxic to man and ,his domestic animals nor unpalatable or otherwise objectionable in the required concentrations ; they must be dispensable at reasonable cost and safe and easy to store, transport, handle and apply ;their strength or concentration in the treated water must be easily, quickly and preferably automatically determinable ; they must persist within disinfected water in sufficient concentration t o provide reasonable residual protection against its possible re-contamination before use. Today, the disinfection methods and disinfectants used are somewhat limited in number, chemical methods being the most widely-spread and chlorine as liyuid-gas the most used disinfectant. Ch~micaZdisinfection uses as disinfectants the following chemicals : halogens, chlorine, bromine and iodine as well as a number of chlorine compounds on the one hand, and ozone and potassium permanganate on the other. Chlorine, in its various forms i R easily handled and has a reasonable cost [140]. Bromine and iodine are used on a limited scale for the disinfection of swimming-pool waters. Oxonp and potassium permanganate are relatively expensive [248]. Metal, ions, for example, silver and copper ions can be employed as disinfectants but their action is only partial, i.e. copper ions are strongly algicidal but only weakly bactericidal ; strong acids and alkalis (pH < 3 and pH > 11)kill all pathogenic bacteria but, unfortunately, at the same time damage the constructions and equipment with which waste waters come into contact. Cationic detergents are strongly destructive, anionic detergents only weakly so. Disinfection by heat is a safe and commended practice where drinking water is conserned, but it is an inadequate method where waste waters are involved. Disinfection by light as a result of sun rays and artificially produced ultraviolet light can be employed only in exceptional circumstances and for small amounts of waste waters (i.e. waste waters from hospitals), Gamma radiation has been investigated recently as a method of waste water treatment and disinfection but it is used today oaly for sterilizing surgical instruments.
430
Waste W'ater Disinfectwn. Waste W a t e r and Sludge Chlorination
From review of the various disinfection methods mentioned above, the cost of the substances used, their handling and use, it follows that, generally, it is only chlorine and its compounds that can be economically used for waste water disinfection. 11.1.2
Theory of Chemical Disinfection
Green and Stumpf [ 7 8 ] have shown that in the disinfection process, chlorine and its compounds react with enzymes (Chapter 4) that are essential t o the metabolic process of living cells. Cells die when these key substances are inactivated. Enzyme destruction also remains the primary lethal mechanism of disinfectants when a radical process, such as heat, coagulates cell c o n t p t s . Because enzymes are generated within t8he cell plasma, chemical disinfection theoretically proceeds in two steps : the first, that of penetrating the cell wall, and the second, reaction with the cell enzymes. Factors governing chemical disinfection technology fall essentially into several categories, as follows : T h e nature of the organisms to be destroyed and their concentratioti, distribution and condition in the water to be disinfected : non-spore forming bacteria are less resistant to disinfectants than spore-forming bacteria ; among the enteric bacteria, Esch.coZi appears to be somewhat more resistant tthan the pathogenic bacteria; a number of enteric viruses, however, are measurably more resistant to chlorination than Esch. coli (i.e. Foliomyelitis virus Type 1).The virus of infectious hepatitis appears to be an especially hardy organism. The disinfectant reacts more actively if organisms are distributed uniformly through the water and in motion. Stirring and mixing of waste waters will do this. T h e nature, distribution and concentration of the disinfecting substance and its reaction products in the water to be disinfected. To provide maximum efficiency in the disinfection process, disinfectants must be uniformly distributed in water, and their concentration must correspond to the resistance of the organisms to be destroyed. Besides the already mentioned advantages displayed by chlorine and its compounds, it is worth mentioning that in water, another series of compounds with disinfecting qualities are generated. T h e nature and condition of the water to be disinfected. Suspended matter may shelter embedded organisms against chemical disinfection as well as against destructive light rays. Organic matters in large amounts lead to an increase of the required chlorine dose, because chlorine is consumed for organic matter oxidation. T h e higher the temperature of waste waters, the quicker are aquatic organisms destroyed. T h e time of contact. The longer the time, the greater is the opportunity for destruction. However, a minimal time of displacement is the governing factor for disinfection.
440
Waste Water Disinfection
From an examination of these categories, it follows that the three factors that can be adjusted to maximize disinfection efficiency are: the nature and concentration of the disinfectant ; the degree of stirring to which water is subjected ; and the time of contact between organisms and disinfectant. Certain changes in the quality of waste waters can be made to generate favourable conditions for disinfection, for instance, changing water pH or temperature. However, they require additional treatment and adequate equipment respectively which raise the cost of the disinfection process. 11.1.3
Kinetics of Chemical Disinfection
Under ideal conditions all cells of a single species of organism are discrete units equally susceptible t o a single sort of disinfectant ; the disinfectant and cells are uniformly dispersed in the water; the disinfectant stays substantially unchanged in chemical composition and substantially constant in concentration throughout the period of contact ; the water contajns no interfering substances. Under such conditions the rate of disinfection is a function of the following variables : the time of contact ; the concentration of the disinfectant; and the temperature of water. Tiwhe of contact - under the above-mentioned ideal conditions follows Chick’s law of disinfection [ S O ] . This states that y, the number of organisms destroyed in unit time, is proportional to N , the number of organisms remaining, or : d,
__ =
dt
k . ( N o- y ) ;
(11-11,
where : k
- coefficient of proportionality (rate of killing), or the rate constant with dimension ( t - 1 ) j N o - initial number of organisms. By integration between the limits y = 0 at t = 0, and y = y, at t = t : (11-2).
or : N- exp (-
NO
k.t).
(11-3)
However, even under the ideal conditions assumed, certain deviations from Chick’s law are possible: the rate of killing ( k ) rather than being constant may increase or decrease with time. Increase in rate of killing can be explained either as a result of the slow diffusion of chemical disinfectants through the cell wall and a rate of killing accelerating with the aecumulation of disinfectant within the cell, or by the consequence o f
441
Waste Water Disinfection. Waste Water and Sludge Chlorination
a time lag before the disinfectant can reach a lethal number of vital centers in the organism. Decrease in the rate of killing is generally explained by an increased resistance of cells to disinfection action. Concentration of Disinfectant. For changing concentrations of disinfectant, the observed disinfecting efficiency is generally approximated by the relationship : c'
where : c tp
.tp = constant ;
(11 -4)
- concentration of disinfectant ; - the time required to effect a
constant percentage of the organisms killed ; n - coefficient of dilution or, according to van't Hoff, a measure of the order of the reaction. Examples of time-concentration relationships for chlorine as HOCl =6.3 and 99 yokilling of Esch. coli and enteric viruses at 0 to 6°C are : co*8ti-tp for Coxsackie virus A2 ; c0."-tp = 1.2 for poliomyelitis virus 1 ; and co*86.t, = 0.098 for adenovirus 3 ; all in comparison with co.86.tp = 0.24 for Esch. coli [40].When n > 1,the efficiency of the disinfectant decreases rapidly as it is diluted ; when n < 1, time of contact is more important than dosage ; when n = 1, concentration and time are of equal weight and a first-order reaction may be in progress. Equation 11 -4 is empirical ;it is the result of laboratory observations. The concentration of organisms can also be expressed by an empirical equation : c q - N p= constant; (11-5) where : c - concentration of the disinfectant ; N p - concentration of organisms that is reduced by a given percentage in a given time ; - coefficient of disinfectant 8trength. q Temperature of Disin-feetion. I f the rate of disinfection is determined by the rate of diffusion of the disinfectant through the cell wall or by the rate of the reaction with an enzyme, temperature effects usually conformably to the ven't Hoff - Arrhenius relationship : (I1 -6)
where : !C2, TI- two absolute temperatures (in degrees Kelvin) for which the rates are to be compared ; t l , t , - the times required for equal percentages of destruction with fixed concentrations of disinfectant ; - activation energy (normally in calories) and a constant E characteristic of the reaction ; - the gas, constant (1.99 cal per deg C , for example). R 4 42
Waste Water Disinfection
For T,- T , = 10, tho useful ratio t&, called Q,,, is related to E approximately as follows at normal water temperatures : log Ql0 11.1.4
= log (t,/t,) =
E/39,000.
(11-7)
Disinfection by Chlorinc and Chlorine Compounds
4'hloriue is available as a liquid, A gas (chlorine) or as chlorine compounds {chlorinated lime, calcium hppochlorite, etc.). Liquid or gaseous chlorine is the preferred form for waste water treatment, because of its lower cost as compared to chlorine compounds. Elemental chlorine is a poisonous yellow-green gas at ordinary temperaturc and prc::+\me. Other properties of chlorine are : atomic weight (('1) 35.46 ; molecular weight (Cl,) 70.914 ; specific weight 3.12 g/l a t 15°C and 760 mm Hg ; liquefying point - 31.1"C' ; freezing point - 102°C ; the solubility of chlorine in water 1-aries with temperature : 1 litre of water will dissolve 6 g of chlorine a t 25°C ; 7.3 g of chlorine at 20°C ; 10 g of chlorine at lO"C, a n d 14.6 g at 0°C. The odour of chlorine in air is detected a t a concentration of 3.5 ppm in volume ; it becomes extremely irritating at concentrations of 40 to 60 ppni ; breathing of chlorine gas with concentrations of 40 to 60 ppm for 3 0 to 40 inin is dangerous ; at concentrations of 1000 ppm it becomes lethal. ZI.2.4.l Liquid or gaseous chlorine. Chlorine is generally purchased as liquid ch7orine or chlorine gnu which has been liquefied by compression. Liquid chlorine is not inflammable, it is clear, amber in color and, about 1.J t irnes heavier than water. C'hlorine gas has a highly irritating Bnd penetrating odour and is about 2.5 times heavier than air. One volume of chlorine, when vapourized, will yield approsimately 460 volumes of gas. While not explosive nor iIlfkdlnmable, chlorine gas is capable of supporting combuvtion at high temperatures (250°C). Chlorine gas is supplied for use as a compressed liquid in tank cars made of special steel containers or in steel cylinders which must be handled very carefully. From containers or cylinders the compressed liquid is vaporized. As regards the mechanism of chlorination, there are several reactions $hat occur simultaneously when chlorine is added to waste waters. A first reaction is hydrolysis :
ur :
+ H,O tHOCl + I€+f C1C1, + OHHOCl + C1-
C1,
(11- 8 ) (11-9)
A second reaction is ionization :
H O C l Z H+
+ OC1-
(11-10)
443
W a s t e W a t e r Disinfection. W a s t e W a t e r and Sludge Chlorination
Rypochlorites, calcium hypochlorite, for example, generate the same reactions. Regardless of the type of reaction, liquid-gaseous chlorine results in the formation of hypochlorous acid and hypochlorite ion:. in water. Hypochlorous acid (HOC1) and hypochlorite ion (OC1-) are referred to, in practice, as free available chlorine. Liquid-gaseous chlorine tends to lower the pH value. I n clean waler, a t pH = 6, the percentage of total free available chlorine is 96.87, as HOCl and 3.2 yoas OCl- ; at a pH value of 9, the percentages are reversed. Waste waters contain numerous substances (Fe++,NO;, 807,etc.) which react with ROC1 and reduce it to the stable chloride ion. Oxidation of reducing substances by chlorine produces the so-called immedinte chlorine demand. This reaction is proportional to the amount of reducing th.e present substances, with the result that free chlorine is not present the water for longer than a small fraction of a second. Waste waters also contain appreciable amounts of compounds of nitrogen which react with chlorine ; the most important reaction is t h a t of hypochlorous acid with ammonia :
+ COG1 -, NH,Cl + H,O (monochloramine) NH2C1 + HOCl -+ NHCl, + H,O (dichloramine) NHCl, + HOCl NCl, + H 2 0 (trichloramine) NH,
-
(11-11) (11-12)
(11- 1 3 )
Chloramines are combined available chlorine while HOCl and O(’1are ,free available chlorine. Chloramine formation depends on pH value, temperature, time of contact and concentration of each reacting substance. Monochloramine, for example, predominates at pH = 5 ; lower p H values favour the formation of NC1,. A substantially complete oxidation’ of ammonia and iveduction of chlorine is achieved at a ratio of 2 : 1; in a relatively short time the break point is reached, when all ammonia and oxidizing chlorine disappear completely from the water. Reaching the breakpoint is a function of the pH value at which the reaction occurs ; for pH values between 6.5 and 8.5, the break point is reached in about 30 min. The advantages of performing the chlorination in the immediate neighbourhood of the break point are: odours and tastes are destroyed ; a good disinfection is ensured. The amount of chlorine that remains free in water after chlorination is called residual cMorine. Experience with waste waters to be treated will generally establish a relationship between the residual chlorine content and the time of contact necessary to ensure the desired bacteriological results. The amount of chlorine is considered sufficient if, after a 30 min chlorination, the amount of residual chlorine ranges between 0.5 to 1.Omg/l ; it is assumed that the amount of residual chlorine after a 15 min chlorination must be at least 2 mg/l to alehievean optimum bacteriological effect.
444
Waste Water Disinfection
The amounts of chlorine necessary for waste water chlorination vary with the amount of polluting substances, the degree of disinfection to be accomplished, the temperature of waste waters, etc. Out of the total amount of chlorine used, about 30% is consumed by settleable solids, about 40 yo by remaining suspended solids and the finely divided matter, and about, 30 yo by dissolved solids. As these matters are removed, the amount of chlorine also required decreases. Large suspended solids must be separated through coarse or fine screens even if no other treatment is applied later on, because chlorine penetrates the inside of suspended solids only with difficulty. Waste waters under treatment, containing large amounts of hydrogen sulphide, require greater amounts of chlorine as compared t o those needed by fresh waste waters. 11.1.4.2 Hgpoehlorites. The hypochlorites commonly used for waste water disinfection are : sodium hypochlorite and calcium hppochlorite. Their effects as disinfectants are similar t o those obtained by liquid or gaseous chlorine. However, while hypochlprites are more safely handled than chlorine, the operation is much more difficult and they are more expensive. Sodium hypochlorite is supplied as a solution containing 10 to 15°/o of available chlorine. It must be stored in a cool, dark location as it is r&t ively unstable. Calcium hypochlorite is a dry white powder containing about 70 yo of available chlorine. It is relatively stable. When dissolved in water i t forms a precipitate, particularly with hard water. The sludge causes lii~ncilingdifficulties and feed equipment clogging. Ayueoub: hypochlorite solutions are extremely corrosive and should be prepared and stored in nun-reactive containers. Dry calcium hypochlorite should be stored in a cool dry area out of direct sunlight. Reaction with certain organic coi~ipoundsmay cause fire or explosion. For these reasons calcium hypochlorite is very seldom preferred t o xodium hypochlorite or liquid or ga wous chlorine. 11.7.4.3 Chlorine Dioxide. Chlorine dioxide, CIOz, is recommended for wiste xaterx whose required degree of disinfection is very high and for waters with a high ammonia content. Chlorine dioxide is an intense yellow-greenish gas that is quite unstable and which, under certain condiiions, is explosive. It cannot be shipped in containers asis chlorine gas, because of its explosive nature, and it must be generated a t the point of use and applied immediately. At ordinary temperatures, it is 2.4 times heavier than air and can be compressed to a liquid. The use of chlorine dioxide is limited because of its hazard potential and high cost. As regards its bactericidal efficiency, a t a neutral pH, it is similar t o that of liquid or gaseous chlorine; at pH = 8.5, the efficiency increases substantially.
446
Waste W a t e r Disinfection. W a s t e Water and Sludge Chlorination
11.1.5
Disinfection by Ozone
Ozone is an actt.ive oxidizing agent; it.s chemical formula is 0,. It is generated by pa,ssing cleaned dried a.ir through the corona formed between high tension elect,rodes. The silent elect.ric discharge taking place in t,he air convert,s slightly less than 1percent of the oxygen present, into ozone. Cooling the air increases the efficiency of generation. The generat ion p h n t is elaborate, costly and requires skilled operation. Ozone is a pale blue gas having a characteristic pungent, odour. I n t,he concentrations usually employed, the odour is detectable hut the colour is not. visible. Ozone is an unstable gas with each rriolecule breaking down 1.0 libemte one atom of nascent osggen. Immediately after generation, it is dispersed int.0 t,he wrat.er undw 1-rcatment..I n sufficient concentrations, it is a.n effective bactericide but. the gas must come int,o.direct. contact witjh the microorga,nisms (bacteria, and more recently, viruses) to dest.roy. them. I t is toxic ; fish may l t e killed if a high dosage is present in water. Ozone will react with ositliza1)lc~ maher in the waste waters wit.11 some reduction in HOD, and the dosage needed for effective disinfection will clepcmd on t,he quality of the waste waters. The optimum efficiency of ozone is attained at ;t pH value ranging between 6.0 snd 7.0. I)osa,ges of .5 to 8 mg/l in the secondary efflumt pr0mot.e an eff icientj disinfec;tion of wastx waters. Because of t,he nature of ozone, the residud is short-lived. I n matm', t.he half-life has been estimated to 'tie 20 rnin. I)ue t.o the form of ozone and its short; life, it must, be step-fetl into waste waters to provide the period of contact needed to a,cc.omplish disinfection. Intimate mixing of an ozone-enriched air stream with the wastc waters as well as maintenance of contact for an adequate period of tinit? is essent.ia1. The major problerris to be considered are : t h e satisfying of ozone demand ;the mpid rise of the gas to the liquid surface of the contact chamber, and escape of ozonizetl air bul)bles ; a,nd the relatively short half-life of ozom?. 11.1.6
Constructions
- Equipment
Chlorine is fed into waste waters either directly or as a solution. Directed feed is nccomplished with chlorine gas. I t is fed through simple devices that do not require water, air or energy consumption. However, the use of this method is limited only to small treatment plants. It is very important that, the point of application of chlorine in waste waters should ,be a t least a t 1.20 m below water level. Feeding as n solution requires the previoufi preparation of a solution of chlorine g a s in water. The solution is fed through pressure or vacuum chlorinators.
446
Waste Water Disinfection
The pressure f e e d type chlorinator utilizes a mechanical-diaphragm type of chlorine-control valve to reduce the cylinder pressure ahead of the metering device. The metered chlorine gas is fed into an injector where it is mixed with water and fed to the point of application. The vacuum-,feed type cldorinutor (Figure 11-1) has developed to a high degree of efficiency and it incorporates all the safety features required to prevent t,he accidental discharge of chlorine-gas. The chlorine pressure regulating valve, operating in conjunction with the vacuum, is designed to Rhut off the flow of chlorine-gas in the event of vacuum failure. The
LEGEND
ul/orrne gas
m l q fi/orrne so/utron Wafer
Figure 11 -1. Vacuum chlorinator, flow diagram.
447
W a s t e W a t e r Disinfection. W a s t e W a t e r and Sludge Chlorination
injector and diaphragm check valve ensure the feeding of chlorine gas a,s a solution in adequate doses to the waste waters to be chlorinated (Figures 11-1, 11- 2 ) ,
Figuie 11 -2.
Chlorination installation with automatic control of chlorine flows.
Chlorine-gas dose can be controlled manually or automatically, function of waste water flow and quality. The location of the manual feed rate adjuster device of chlorine gas is shown in Figure 11-1. Water flow is controlled manually with the valve on the water supply pipe. The automatic control of chlorine-gas dose function of waste water characteristics and residual chlorine desired respectively, is of major importance due to daily variations in the chlorine demand of waste waters
448
W a s t e W a t e r Disinfection
and the necessity for close regulation of the disinfection process. The most effective method of chlorination control is the compound loop system. This arrangement uses two separate and independent signals to regulate the chlorination device :a flow-proportional signal to the chlorine metering orifice ; and a chlorine dosage signal to the vacuum regulating valve on the dosage control device. The chlorination mechanism combines these two signals to achieve a wide range of operation in excess of 100 to 1. The dosage signal is transmitted from R chlorine residual analyser T-ia a vacuum or electrical transmitting device. The controls on the a n a l y e r hhould be adjustable to accomodate loop times from 1to 7 min and correction times from 0 to 20 s. Accessories normally include a precision vacuum gtuge t o monitor the vacuum signal to the chlorination device. It is not good practice to interchange the signals, that is, to send the dosage signal t o the chlorine metering orifice positioner mid the flow proportional signal to the vacuum regulator. Chlorinators ore most commonly f e d with chlorine-gas .from c y l i d e r s or containers. In Roinania, a cylinder contains 50 kg of chlorine (gross weight 100 kg) and a container 650 or 1150 kg of chlorine, equivalent to 4,50 1 or 800 1 of chlorine respectively (gross weight 1000 kg or 1500 kg respectively). I f chlorine consumption per 24 h exceeds 460 to 600 kg, the use of evaporators is preferable in order to avoid an excessive number of cylinders or containers in service. The liquid chlorine is changed t o gas in an evaporator before being fed to the chlorinator. It is desirable to install one evaporator for each chlorinator with a minimum of two evaporators, each capable of supplying the maximum chlorine demand. Chlorine installations are usually provided with weighing scale so that the amount of chlorine remaining in the containers as well as the total amount fed can be determined at any time. Scales should be sized to accomodate the maximum number of cylinders required to serve the maximum chlorine feed rate. Scales may bc-mounted flush with the floor or on the floor surface within an enclosed box. With above floor mounting, it is desirable to have overhead hoist equipment available. Many instalhations have a loss-of-weight recorder to provide a continuous record of chlorine feed. Usually, chlorine ins tallations using cylinders are not equipped with hoisting devices. Manual handling of cylinders is generally used in plants where chlorine requirements are maximum, up to 100 kg per day. Handling of containers requires hoisting equipmetbt. It is desirable to use a poweroperated hoist and travel, particularly when it is necessary to change containers frequently. The hoist should have a minimum capacity of 2 tons and be equipped with an approved type of lif ting-beam container grab. Materids used t o manufacture pipes, valves, etc. should generally meet the requirements regarding the corrosiveness of chlorine and its compounds. Pipe lines for dry chlorine gas or liquid chlorine going from cylinder or container to the chlorinators or evaporators should be extra 29-742
449
Waste W a t e r Disinfection. W a s t e W a t e r and Sludge Chlorination
heayv, black and genuine wrought-iron pipes. Joints are made usiiy litharge and glycerine. Lines should be insulated when necessary i o prevent the gas from chilling and the liquid from overheating. F1exil)lv connections from the container to the header should be extra heaT-. Wet chlorine is very corrosive and must be handled with special materials such as silver, hard rubber, glass or certain plastics. Solution feed limb from the chlorinator to the point of application may be rubber how, rubber-lined pipe or plastic pipe. Rubber hose in sizes up to 50 mm i b the most widely used. Rubber-lined steel pipes are available in sizes from 40 mm diameter up. Plastic pipe, or plastic-lined pipe, or any of the highimpact polyvinyl chloride materials are suitable for solution feeding. Diffusers for insertion through corporation cocks a e generally made of silver or hard rubber, while large diffusers am usually rubber-lined a r i d covered steel pipes. Chlorinator overflow lines and gas vent lines a l e made of rubber or plastic. Chlorinators are supplied with water either from the public supply system or from the plant effluent. A separately pumped supply with n o direct connections to the potable water supply is used in some cases, snd in all cases where the plant is interconnected with the chlorinator. Commonly, about 330 1of water are required daily per kilogram of chlorinc. The amount of water must be enough t o maintain a chlorine solution strength which does not exceed 3500 mg/l. The pressure a t the point of chlorine application, known as injector back-pressure, is another parameter - besides water flow - which must be considered for the good operation of a chlorinator. Generally, for each chlorinator the manufacturer supplies the so-called operating curves specifying the amount of water and pressure required for a given amount of chlorine to be a'pplied against a given back-pressure. The minimum operating pressure is about 1.76kg/cni2 (1.76 atm) with 0 kg/cm2 back-pressure on the injector, and this must be increased to provide about three times the actual back-pregsure. I f necessary, the pressure must be attained by pumping. When the flow is larger, chlorine solution is injected into the waste waters through diffu,sers. Regardless of the fact that there are pipes or channels, diffusers are made of one or more perforated pipes placed horizontally or vertically and across the directtion of water flow. Each orifice of the diffusor must provide a flow of 0.06 to 0.12 l/s and a velocity of 3 mjs. Normally, the orifices are not smaller than 10 mm in diameter. A more rapid dispersion of chlorination solution in waste waters can be accomplished if adequate devices for initial mixing are provided. For this reasons, the solution is dispersed either in a hydraulic jump or in a conduit or channel designed to provide turbulent flow. Less recommended are mechanical mixers located in chambers, where the order of magnitude of the detention time is several seconds ; in this case, the diffusors will be used upstream of tbe miser. Initial mixing prior to the contact chamber must not be confused with mixing in the contact chamber, which is to be avoided because it generates short-circuiting in the chamber and leads t o unreliable disinfection results.
45 0
Waste Water Disinfection
The contact chamber is one of the main components of the chlorination installation. The purpose of the contact chamber is to ensure the detention tirrie necessary for the chlorine compounds to reduce the bacteria to ;Lcceptable levels. It is extremely important to minimize short-circuiting and dead spaces through basin configuration and flow pattern control. C'ontact chambers must not be designed as circular basins as in the common practice ; the best configuration to avoid short-circuiting through t h e chamber and to accomplish a plug-flow regime is that of a chamber (baffled basin) with long narrow sections, the length to width ratio of which is 10 : 1 or even more. Directional changes tend to encourage short-circuiting, so that such changes must be kept to a minimum. Guide vanes may be used for direct ional changes to prevent short-circuiting in basins where several directional changes are necessary because of space limitations. Chlorinc. improves the settling characteristics of solids. Since t h e waste waters to be treated still, sometimes, contain suspended solids, it is impeiatiw that adequate facilities for their removal out of t h e ch;iinbws should be piovided. Chlorme culinder
Figurc 11 - 3. Layout of a chlorination station, Iiomanian standard design.
-I
4 65
I
Residual chlorine, in the effluent of the chlorination chamber is determined by means of a residual chlorisje recorder monitor. Systems employing residual chlorine control recorders, usually have high and low residuul alarms. Installation using evaporators require alarms giving warning of low temperature and low water level in the water bath. The chlorination installation can sometimes fail when waste water chlorination ceases. The most common reasons leading to interruption of chlorination are : exhaustion of chlorine feed, power failure, water supply failure, equipment break-down, etc. A stand-by system, the use of portable chlorination, or of pre-chlorination system are to be highly recommended.
451
Waste Water Disinfection. Waste Water and Sludge Chlorination
Chlorine installations require adequate room for the maintenance of equipment and handling of chlorine cylinders and containers. Figure 11-3). Usually, the room accomodating the chlorinator is located a t or above ground level. The chlorinator and its auxiliary equipment are placed in a chamber, while cylinders and containers are located in another chamber with access only through the outside door. The surface areas of the two chambers should be designed to allow the rapid access to the entire equipment for maintenance and repair. Informatively, for a plant with only one chlorinator supplying less than 100 kg chlorine per day, the area accomodating the chlorination installation should be about 6 m2; for a plant with two chlorinators, 16 m2; about 1 5 m2 are required for emch chlorine feed evaporator unit). In the chamber accomodating the chlorinator a consta m’ tem&mturp of about 21°C should be provided to ensure the good operarion of the chlorination installation. In the chamhers where the cylinders and containers are located, the temperature should be around 10°C. Chambers are preferably heated by hot water sybtems for safety reasons and due to the uniform temperature ensured. Stearn heating is not recommended because of the temperature extremes that might be experienced in t hc case of system failure. Electrical heating is suitable, and forced-air heJ,ting would be appropriate if an independent system is provided for the chlorination room or building. Ventilation is required for all chlorine equipment room3. The ventilation intake or exhaust fan must be located a t floor level. Ordinary practice consists of an exhaust fan with guard and shutters mounted a t floor level on an exterior wall, and operated imtermittently as required. An alternate and superior method consists of an exterior fan with the intake duct running to the chlorine room floor. Provisions must be made for fresh-air intake through either a louvered entrance door or a separate intakcl. . Good practice requires an air exchange every 3 minutes. The chamber accomodating the chlorinators must always have an i?ispectiota window - not too large - to enable the supervision of the chlorinator operation without coming into the room. This window is usually located in the entrance door. Any other possible sajety measures ensuring the good operation of chlorinators must be taken : fire control, automatic switches for energy supply, light signals in case of failure, etc.
11.2 Chlorination of Waste Waters and Sludges for Other Uses I n a treatment plant chlorine is used not only for disinfection but for other purposes as well. The chlorination of waste waters before being mechanically treated is called prechZorination ;after mechanical treatment, intermediate ch orination.
t
452
Chlorination of Waste Waters Waters and Sludges
Odoilr control. So long as aerobic conditions prevail in the treatment plant, odour is not offensive. The most odouriferous substances are generally products of anaerobic decomposition. Among them are hydrogen sulphide, indole, skatole, mercaptan, and cadaverin. Some of the foul odours released by these substances remain detectable even when they are dispersed in a million times their own volume of air. As a rule, hydrogen sulphide is the forerunner of all waste waters odours. It occurs whenever sulfates are broken down by bacteria, and it is particularly strong when the carrying water -is rich in sulfates or when industrial waste waters that contain sulfates are admitted to the sewerage system. The presence of hydrogen sulphide in confined spaces is dangerous because it is an extremely toxic gas, and lethal accidents might consequently follow. When sewers flow partly full, hydrogen sulphide released a t the waste water surface is oxidized into sulfurous and sulfuric acid, which attack concrete, mortar, iron, etc. Odour generation in sewerage systems, particularly in sludge collecting systems and long-outfalls may be checked by up-sewer chlorination. Odours that have been formed may be swept out by aeration or oxidized by chlorine. Hydrogen sulphide is destroyed by chlorine in the ratio of their combining weights (Cl,: H,S = 70. : 34.1, or 2.1 mg of C1, for each mg of H,S), but larger amounts of chlorine must be added to waste waters because chlorine also combines with other substances present. Chlorination of waste waters ahead of settling tanks does not measurably inteIfere with digestion of the sludge, because the small amounts of chlorine added ale quickly absorbed. Up to 50 mg/l of chlorine may be needed to control the odour of raw waste waters. Chlorination of supernatant from sludge digestors or sludge-dewatering processes may be useful. The chlorination of stack gases, particularly in the heat drying of sludge, is an emergency measure. If waste water is naturally fresh when it arrives at the treatment plant or is kept fresh by upstream sewer chlorination, the control of odours becomes largely a matter of safety. Reduction of biochemical oxygen demand. Chlorination leads to a reduction of BOD, to a delay in organic matter decomposition in the waste waters of the treatment plant and streams respectively. At the same time it prevents : the generation of anaerobic conditions in the sewerage system and controls odours developed; the damaging of concrete and mortar due to the corrosive action of waste waters ; the penetration of septic or waste waters under digestion into the treatment plant; the process of decomposition of organic matters in the waste waters a t the mechanical treatment units of the plant is inhibited, thus preventing odour development within the biological treatment units of the plan and especially in the biological filters: the worsening of the quality of returned sludge ( 5 9.3.2) ; continuation of the decomposition process during sludge thickening (0 10.4.1);it delays the immediate oxygen demand of water in streams (Subchapter 5.3) and of the supernatant coming from the sludge digester (Subchapter 10.2).
453
Waste Water Disinfection. Waste W a t e r and Sludge Chlopination
Chlorination may reduce BOD in one of the folloming ways : o\idation of BOD-exerting substances ; formation of disinfecting substances with nitrogen compounds (chloraminex, for example) ; formation of nondecomposable substances (with carbon compounds and unsaturated compounds, for example). The reduction of BOD depends upon the quality of the waste waters as well as the concentration of the clilorint.. Chlorination of fresh waste waters to a trace residual in 1 5 niiii may lower BOD by only 10 yo; doses above 100 ml/l may be needed to lower it by as much as 35 :(, [204]. The temporary lowering of the oxygen requirements of waste waters discharged into streams by chlorination may help maintain aerobic conditions, when the oxygen supplied by the water itself and by reaeration would otherwise be unable to keep pace, until the mother stream is reached or tributary waters establish a satibfactory oxygen balance. However, sludge deposits may intelfere with iiormally expected performances. Generally, the chlorine dosages used to control oclours will also control septicity. Destruction and Control of Damaging Orgccriisne Growth i,i the Sewerage System, Treatmettt Plant and Slreanzs. Filamentous organisms such as : 8phaerotiZus fiatans, Leptonsitus and Beggiatoa which clog the channels or narrow their flowing section, and also lead to the increase of the activated sludge flocs ( $ 9.3.2), are destroyed ; Ysychoda f l y growth is controlled as well as the formation of abnormally large aiuounts of sludge that might pond biological filters ( S 9.3.1) ; the growth of algae or other aquatic organisms in the surface waters is also controlled. Corrosiow. If the formation of hydrogen sulphide is prcvented as a result of waste water chlorination, then the hazard of corroding the metal and concrete constructions in the treatment plan1 and streams is avoided. Hydrogen sulphide in water is oxidized to form sulphuric acid which is corrosive and attacks metals ; from this reaction calcium sulpllsle also results which attacks constructions. Sludge Bu&l;ing ( 5 9.3.2) is caused especially by tJIJe filaineritous bacteria Spliaerotilus natans. Hulking can be prevented If the returned sludge is chlorinated long enough to give the chlorine a 2 t o 3 niin mixing (stirring) time. Pats are removed mash efficiently from skimming tanks if chlorine is blowning at the tank bottom together with compressed air (Subchapter 7.2).
11.3 Dcchlorination The chlorine doses used to disinfect waste waters are sometimes in excess. Thus, residual chlorine in anlourits larger than necessary still remain in the water, eventually finding its way into streams. Although the amounts of chlorine are generally small, when they are disposed in streams with low flows, they can he a nuisance due to the toxicity.Consequently, the
454
Dechlorination
effluent of the treatment plant must be dechlorirmted before reaching the \tream ; this means the removal of excess chlorine from the waste waters. Dechlorination of waste waters can be accomplished by adding redu(.11igchemicals, passing them through beds of granular atetivattedcarbon, ;I i d by aeration. The reducing chemicals are :sulphur dioxide, (SO,), sodium itietabisulpliite, (Na,S,O,), and sodium sulphite, (Na,SO,). SuZphur dioxide reacts with chlorine in waste water according to the ecluation : SO,
+ C1, + H,O
--f
H,SO,
+ 2HC1
(11-14)
Generally, 1mg/l of sulphur dioxide is required to dechlorinate 1mg/l residual chlorine. Sulphur dioxide is available in cylinders, like chlorine, under a pressure of approsimately 240 kN/m2. The equipment for metering sulphur dioxide delivery is, for all practical considerations, identical to that for chlorine. Sulphur dioxide is mixed in waste water by a mechanical mixer. The dechlorination reaction taking place ina contact chamber between sulphur dioxide :ind both free and combined chlorine is rapid, a few seconds at most. Quantitative checking of the reaction is performed downstream of the contact chamber where residual chlorine is to be determined. Xodium rnetnbisulphite is a white or cream-coloured powder, which rc+ldily disolves in water, changing into sodium hiosulphite ( NaHS03). The reaction taking place with sodium metabisulphite and waste waters to he dechlorinated is : 2HOC1 + H,O
+ Na2Sz05+ 2NaHS0, + 2 HC1
(11-15)
This is a stoichiometric ratio of 1.34 units of metabisulphite required to dechlorinate each unit of chlorine. In practice, it is well t o size the equipment on the basis of 1.5 units of metabisulphite to dechlorinate each unit of chlorine. The construction materials for a dechlorination system using a solution of sodium metabisulpliite are the same as those used for handling hypochlorite solutions. Actiwuted carbon. The dechlorination process involves a chemical reaction between chlorine and water with carbon acting as a catalyst. The stoichiometric reaction for dechlorination is : a
2 C1,
+ C + 2H,O-+
4HC1
+ CO,
(11-16)
The dechlorination process depends on chlorine concentration and flow rate, the physical characteristics of the carbon, waste water cwnditions alnd the chemical state of the free chlorine, the lastjfactor also being the most important. The dechlorination reaction is accompanied by adsorption of organic and inorganic contaminants. The most readily oxidizable products will be oxidized by chlorine, while others, adsorbed on the carbon surface, are oxidized by the nascent oxygen formed during the surface reaction.
455
Waste Water Disinfection. Waste Water and Sludge Chlorination
Dechlorination is accomplished through rapid filters with up-to-down flow, running by gravity or pressure. For an influent with free residual chlorine of 3 to 4 mg/l, the surface loading of the filter can be of about 0.002 m3/m2.sat a 15 t o 20 min contact time with the activated carbon.
11.4 Design
- Examples
Chlorination plants for waste waters must be able to supply chlorine doses ranging between 4 and 50 nigjl. Chlorine doses vary as a function of the amounts of waste waters and sludge and of the purpose of the chlorination. Table 11-1 displays the chlorine doses required to obtain 0.5 t o 1.0 mg/l residual chlorine after 15 min contact time. TABLE 11- 1 Chlorine doses necessary to obtain 0.5 to 1.0 mg/l of residual chlorine after 15 min contact time
Waste waters or sludges
I
I
Raw waste waters Primary settling tank effluent Chemical treatment rffluent Biological filters effluent Activated sludge units effluent Sand filter effluent Raw waste waters for odour control Raw waste waters f o r corrosion prevention Ra w waste water for prevention of biological filter cloggidg (it is fed periodically, €or 8 h) Influent raw waste water for Inihoff tanks for prevention of foam forming Influent raw waste water to increase efficiency of skimming tanks Returned sludge l o prevent its bulking (2-3 min contact time) Sludge in thickening tanks upstream of sludge digesters t o obtain 1 mg/l of residual chlorine in t h e supernatant in t h e tanks
Chlorine dose, mg/l 6- 24 3- 18 3- 12 3- 9 3- 9 1- 6 max. 20 2- 10
.
20- 50
3- 15 1 .O- 1.5 1- 10 max. 50
In Romania, the design of chlorination plants must take into account the standard design “Chlorination Plant with Chlorine-gas for Waters and Waste Waters Treatment” (Figure 11 -3). Contact chambers are sized for a 30 min contact time. The amount of sludge deposited in these chambers (96% moisture content), is about 0.10 1 per capita daily using liquid or gaseous chlorine and of 0.17 1 per capita daily using calcium hypochlorite. The removed sludge is sent to the primary settling tanks. The store of cylinders or containers is sized to allow storing the amount of chlorine required for a 30 day period. The amount of water necessary t o prepare the solution of chlorine-gas is 4 to 5 1 per min for one standard chlorinator (or 350 1 of water each day per kg of chlorine) at a minimum pressure of 1.75 atm.
456