Activated Sludge Process - 3...

Disinfection of Wastewater...

Primary, secondary and even tertiary treatment cannot by expected to remove 100 percent of the incoming waste load and as a result, many organisms still remain in the waste stream. To prevent the spread of waterborne diseases and also to minimize public health problems, regulatory agencies may require the destruction of pathogenic organisms in wastewaters. While most of these microorganisms are not pathogens, pathogens must be assumed to be potentially present. Thus, whenever wastewater effluents are discharged to receiving waters which may be used for water supply, swimming or shellfishing, the reduction of bacterial numbers to minimize health hazards is a very desirable goal.

Disinfection is treatment of the effluent for the destruction of all pathogens. Another term that is sometimes also used in describing the destruction of microorganisms is sterilization. Sterilization is the destruction of all microorganisms. While disinfection indicates the destruction of all disease causing microorganisms, no attempt is made in wastewater treatment to obtain sterilization. However, disinfection procedures applied to wastewaters will result in a substantial reduction of all microbes so that bacterial numbers are reduced to a safe level.

In general, disinfection can be achieved by any method that destroys pathogens. A variety of physical or chemical methods are capable of destroying microorganisms under certain conditions. Physical methods might include, for example, heating to boiling or incineration or irradiation with X-rays or ultraviolet rays. Chemical methods might theoretically include the use of strong acids, alcohols, or a variety of oxidizing chemicals or surface active agents (such as special detergents). However, the treatment of wastewaters for the destruction of pathogens demands the use of practical measures that can be used economically and efficiently at all times on large quantities of wastewaters which have been treated to various degrees.

In the past, wastewater treatment practices have principally relied on the use of chlorine for disinfection. The prevalent use of chlorine has come about because chlorine is an excellent disinfecting chemical and, until recently, has been available at a reasonable cost. However, the rising cost of chlorine coupled with the fact that chlorine even at low concentrations is toxic to fish and other biota as well as the possibility that potentially harmful chlorinated hydrocarbons may be formed has made chlorination less favored as the disinfectant of choice in wastewater treatment. As a result, the increased use of ozone (ozonation) or ultraviolet light as a disinfectant in the future is a distinct possibility in wastewater disinfection. Both ozone and ultraviolet light, as well as being an effective disinfecting agent, leave no toxic residual. Ozone will additionally raise the dissolved oxygen level of the water. However, ozone must be generated and has only recently begun to compete favorably with chlorination in terms of economics. Ultraviolet light has recently undergone studies to determine its effectiveness and cost when used at large wastewater treatment plants. While the study is not yet complete, ultraviolet light now appears effective and economically competitive with chlorination as a disinfectant.

The use of both chlorine and ozone as chemical disinfectants and their disinfecting properties and actions will be considered individually. However, since chlorine continues to be used extensively as a disinfectant, we will mainly be concerned with the principles and practice of chlorination.

Chlorination...

Chlorination of wastewater is the application of chlorine to a wastewater to accomplish some definite purpose. The purpose of chlorination may not always be disinfection and may, in fact, involve odor control or some other objective which will be noted. Chlorine may be applied in two general ways, gaseous and liquid. In general, the effective chemical form of chlorine that either destroys the microbe or acts against odor, etc., is the same. Gaseous forms of chlorine are generally first dissolved in water prior to addition to the wastewater stream, while liquid forms of chlorine (called hypochlorites) are sold in the form of water soluble salts. Because chlorine gas generally costs less than hypochlorites, it is normally used in treatment plants except in rare instances where only a relatively small amount of chlorine is needed or where the possible danger from gaseous chlorine overrides economic considerations. The application of chlorine is usually controlled by special devices which are known as chlorinators, chlorinizers or by similar names.

Reactions of Chlorine in Wastewaters...

In order to determine at what points in the treatment process, and how much chlorine should be applied to accomplish the purpose desired, it is necessary to know what reactions occur when chlorine is mixed with a wastewater. When chlorine is mixed with pure water, it immediately dissolves, forming fist hypochlorous acid and then hypochlorites :

The above two forms of chlorine (hypochlorous acid and hypochlorite ion) are called "free" residual chlorine, as opposed to the reaction products of chlorine with other compounds that can also be detected using analytical techniques that are called "combined" residual chlorine. Free residual chlorine is a more effective disinfecting agent than combined residual chlorine, and generally hypochlorous acid is a much more effective disinfectant than hypochlorite ion. In wastewaters, free residual chlorine is seldom detected and chlorine is usually found in the "combined" residual form.

Chlorine is an extremely active oxidizing chemical that will react with many substances in wastewaters. If small amounts of chlorine are added to wastewaters it will react rapidly and is thus consumed. For example, chlorine first reacts readily with such substances as hydrogen sulfide, ferrous iron, manganese or thiosulfates, which may have their origin from industrial wastes. However, almost any "reducing" compound capable of reacting with chlorine (an oxidizing compound), will react. If all of the chlorine is consumed in these reactions, no disinfection will result.

Chlorine generally reacts in a prescribed order, first with inorganic reducing compounds. If enough chlorine is added to react with these substances, then the addition of more chlorine will result in reactions of chlorine with the organic matter that is present. This forms chloroorganic compounds, which have little or no disinfecting action. Again, if enough chlorine is added to react with all the reducing compounds and all the organic matter, then the addition of a little more chlorine will react with ammonia or other nitrogeneous compounds to produce chloramines or other combined forms of chlorine which also have disinfecting action but are not as effective as free chlorine.

The continued addition of chlorine will result in the destruction of the chloramines and the formation of free chlorine. While chlorine is seldom applied to this level in wastewater treatment, the addition of chlorine in sufficient dosages to where free chlorine is formed is called "breakpoint" chlorination.

For effective chlorine disinfection both sufficient chlorine dosages as well as contact time are necessary. Generally both of these factors must be worked out experimentally and other factors will affect the effectiveness of chlorination. Among the principal factors are bacterial numbers, pH, temperature and contacting. In "pure" systems bacterial kill at a particular chlorine dosage is directly related to the number of bacteria present when the chlorine is first added. pH will affect the form of chlorine present and, generally, at neutral pH's hypochlorous acid, the more effective form of chlorine, is favored. Temperature affects the speed with which chemical reactions take place and colder temperatures are less favorable for disinfection. Proper contacting or mixing or agitation, is necessary to make sure that the chlorine applied contacts or reaches the vital parts of the microbial cell.

The precise mechanism of the disinfecting action of chlorine is not fully known. However, chlorine is capable of undergoing a wide variety of reactions and probably reacts with the microbial cell at several levels. At high concentrations, massive oxidation takes place and membranes and all organic components are affected. At lower concentrations chlorine probably affects vital protein systems as well as membranes. From the point of view of wastewater treatment, the mechanism of action of chlorine is much less important than its effects as a disinfecting agent.

The quantity of reducing substances, both organic and inorganic, in wastewaters, varies, so that the amount of chlorine that has to be added to wastewater for different purposes will also vary. The chlorine used by these organic and inorganic reducing substances is defined as the chlorine demand. Chlorine demand is equal to the amount of chlorine added minus that remaining as combined chlorine after a period of time, which is generally 15 minutes. This relationship can be written as :

Chlorine Demand = (Applied Chlorine Dose) - (Chlorine Residual)

It is important to note that disinfection is carried out by that amount of chlorine remaining after the chlorine demand has been satisfied. This quantity of chlorine in excess of the chlorine demand is defined as residual chlorine and expressed as milligrams per liter. For example, if a chlorinator is set to feed 50 lbs. of chlorine per 24 hours and the wastewater flow is at a rate of 0.85 mgd and the chlorine as measured after 15 minutes contact is 0.5 mg/L, the chlorine feed or dose is :

Chlorine dose in mg/L	Chlorine residual in mg/L	Chlorine demand in mg/L
7.1	0.5	6.6

Theoretically, while microorganisms are killed as the chlorine demand is being satisfied, disinfection is generally the result of chlorine residual or the amount of chlorine remaining after the chlorine demand has been satisfied. Thus, measurement of chlorine residual is an important part of the operator's duties. Chlorine is seldom applied to wastewaters to reach "breakpoint" levels. This is because the amount of chlorine required prior to observing free available chlorine would be very high (approximately 150 mg/L). Generally chlorine is applied only to give a combined residual. It should be noted that in some of the more recent advanced wastewater treatment processes with high quality effluents where reduced inorganics and organic compounds are produced, it may be possible to chlorinate to sufficient dosages to have free available chlorine while at lower chlorine dosages, and at the same time affect the removal of ammonia from the wastewater.

Purposes of Chlorination...

Chlorine is added to wastewater for a number of different purposes and chlorine dosages and management will vary with the specific purpose. In general, chlorine applied before any treatment is given (pre-chlorination), during treatment (plant chlorination), or after normal treatment measures have been carried out (postchlorination). A few of the more important purposes of chlorination are listed below.

Disinfection...

Chlorine is a very effective disinfecting agent and has been the agent of choice in reducing bacterial numbers in wastewater effluents. As noted, neither primary nor secondary methods of wastewater treatment can completely eliminate pathogenic bacteria which are always potentially present. When wastewaters or treated effluents are discharged to bodies of water which are, or may be used as a source of public water supply, or for recreational purposes, treatment or disinfection for the destruction of pathogenic organisms is required to minimize the health hazards of pollution to these receiving waters.

Chlorination for disinfection requires that essentially all of the pathogens in the wastewater plant effluent be destroyed. At the same time it should be noted that many but not all of the nonpathogenic microorganisms are also destroyed. As noted, no attempt is made to sterilize wastewater and this is not only unnecessary but impractical. In some instances sterilization might be detrimental where other treatment dependent upon microbial activity may follow chlorination. Fortunately pathogenic microorganisms are less resistant to chlorine than most nonpathogens so that disinfection can be effected without sterilization. Chlorination as commonly practiced in wastewater treatment is insufficient to inactivate all of the enteric (intestinal) viruses which may be present in wastewater.

To accomplish disinfection, sufficient chlorine must be added to satisfy the chlorine demand and leave a residual chlorine that will destroy bacteria. Special laboratory equipment is necessary to measure the destruction of bacteria and the tests require several days to complete. Thus, bacteriological examinations are not practical for the day-to-day control of the application of chlorine. Laboratory experiments and actual plant experience have shown that if sufficient chlorine is added to wastewater so that 15 minutes after the chlorine has been added, a residual chlorine concentration of 0.5 mg/L is present in the wastewater, disinfection will usually be accomplished. This follows the general pattern of toxicity of most disinfectants and both concentration and contact time are important. Generally a small concentration acting over a long period of time would have the same effect as a large concentration acting over a short period of time. For the elimination of all entering viruses, for example, both longer contact times and higher chlorine dosages than now used must be employed. In actual operation the practical control of chlorination for disinfection is by measurement of the residual chlorine. By this means, test results can be obtained in a few minutes and the chlorinators adjusted to the proper feed rate.

Disinfection of wastewater is defined in terms of fecal coliforms. The requirements are that fecal coliform levels shall not exceed 400 organisms per 100 ml at any time or exceed a monthly geometric average of 200 organisms per 100 ml when disinfection is required to protect the best intended uses of the water in question.

It may be that a 0.5 mg/L residual after 15 minutes will not meet this bacteriological standard at all wastewater treatment plants. In this case experiments must be made to determine the residual chlorine value that must be obtained to comply with the standard if it is applicable. This reaction is then used to control the chlorine application.

Disinfection when required must be a continuous process as it would be hazardous to discharge untreated effluent even for a short period of time. Proper contacting of the microbes with chlorine is important and the point of chlorine application must be at a place where the chlorine feed can be rapidly mixed with the entire flow of wastewater and where the mixture of chlorine and wastewater can be held for a minimum of 15 minutes before discharge into the receiving water.

Where the outfall pipe is long enough to provide at least 15 minutes for the effluent to flow from the plant to the stream, chlorination of the effluent as it leaves the plant can be used. Control, in this case, should be by measurement of the residual chlorine in the wastewater at the end of the outfall. Many times the end of the outfall is under water or at an inconvenient distance from the plant. It is advisable under such conditions to collect a sample of chlorinated wastewater making sure it is taken at a place where the chlorine is completely mixed with the wastewater, and hold the sample for 15 minutes before measuring the residual chlorine.

If the desired residual is 0.5 mg/L and actual chlorine residual is greater or less than 0.5 mg/L, the chlorine feed is decreased or increased until the proper residual is obtained. Since the chlorine demand of wastewater varies during the day, the chlorine feed required to maintain a 0.5 mg/L residual will vary. In a small wastewater plant (less than 1 mgd), the operator may not have time to check the residual chlorine repeatedly and adjust the rate of chlorine application. In this case, the chlorine feed is adjusted once daily to give the required residual at the time of maximum wastewater flow, which generally coincides with the time of maximum chlorine demand and in most plants occurs about 10:00 a.m. Then at all other times during the day the chlorine residual should be greater than 0.5 mg/L. This means that chlorine is being wasted, but the operator is sure that disinfection is being accomplished.

The amount of chlorine required to produce 0.5 mg/L residual in most secondary effluents will be between 40 and 50 lbs. per million gallons. By frequent adjustment of the chlorine feed it might be possible to save about five to ten lbs. of chlorine per million gallons. This can represent an economic factor in wastewater treatment plant operation and will vary with plant size. In a larger plant (10 mgd) the waste of chlorine might be significant and is worth trying to save. Therefore, residual chlorine values are measured possibly three or four times a day and the chlorinator adjusted each time. In still larger plants, it pays to make measurements frequently and it is often the practice to adjust the chlorine feed rate hourly.

Where the outlet sewer does not provide 15 minutes holding time at peak hourly flow, or 30 minutes holding at average rate of flow, a chlorine contact tank is built and so designed as to give the required 15 or 30 minutes contact time at maximum or average flows, respectively. In this case, chlorine is applied to the influent of the contact tank and the residual measured in the effluent. The object of disinfection is the destruction of pathogenic bacteria and the ultimate measure of effectiveness is in the bacteriological result. The measurement of residual chlorine does supply a tool for practical control. The 0.5 mg/L residual chlorine, while generally effective, is not a rigid standard but a guide that may be changed to meet local requirements. One special case would be the use of chlorine in the effluent from a plant serving a tuberculosis hospital. Studies have indicated that a residual of at least 2.0 mg/L should be maintained in the effluent from this type institution and that the detention period should be at least two hours at the average rate of flow instead of the 30 minutes which is normally used for basis of design.

Fish Toxicity...

Chlorine as well as chloramines are generally toxic to fish as well as harmful to aquatic biota even at low concentrations. The toxicity to aquatic life in a receiving water will depend upon the concentration of the residual chlorine, the relative amounts of chloramines if they are present, the amount of free chlorine, as well as the dilutions that take place in the receiving waters. Fifty percent of all rainbow trout and minnows, for example, have been reported killed by levels of about 0.2 mg/L of residual chlorine in 96 hours. Trout have been shown to "avoid" free chlorine levels of 0.001 mg/L. However, it should also be noted that this would represent chlorine levels measured after mixing and dilution in the receiving waters rather than in the effluent. It should also be noted that residual chlorine concentrations diminish with time and mixing as well as by temperature elevations.

Chlorine can be effectively eliminated by the addition of dechlorinating chemicals. Because the chlorine concentrations of concern in the receiving waters are usually below the level of detection by the orthotolidine method, a more sensitive analytical method is now required. It has also been shown that small amounts of chlorine can greatly increase the toxicity of various industrial effluents.

Generally the National Academy of Sciences indicates that aquatic life will be protected as long as the concentration of residual chlorine does not exceed 0.003 mg/L at any time or place. They further not that aquatic organisms will tolerate short-term exposure to relatively high levels of chlorine and recommend that total residual chlorine should not exceed 0.05 mg/L for a period up to 30 minutes in any 24 hour period.

Dechlorination...

At times it is necessary to dechlorinate, or remove chlorine from a wastewater by the addition of dechlorinating agents. Generally this must be done to counteract the reactive effects of chlorine in effluent samples, for example, in determining coliforms or BOD's. At present only a small number of treatment plants in New York State dechlorinate, however, in the future it may become a more common practice to dechlorinate treated wastewater. Generally, the most common chemicals used for dechlorination are sulfur dioxide, sodium bisulfate, sodium sulfite, sodium thiosulfate and activated carbon. The chemical equivalents required for dechlorination can be calculated, however, laboratory experiments should be used to help to define the required dose. For laboratory samples Standard Methods give the recommended dosages for dechlorinating chemicals.

Chlorine Hazards...

Chlorine is a yellow green gas that is extremely toxic as well as corrosive in moist atmospheres. Chlorine is about two-and-a-half times as heavy as air. Chlorine is not flammable or explosive and will not freeze, even at the lowest temperatures. Chlorine will react readily with water, moisture or moist tissues. While dry chlorine gas will not attack iron, copper, lead and some other metals and alloys, moist chlorine readily attacks most metals. Thus, with moisture, chlorine must be handled in corrosion resisting materials such as silver, glass, rubber and certain plastics. Chlorine can be detected at very low levels and has a characteristic sharp odor. At moderately low levels chlorine can be penetrating and very irritating to mucous membranes. A very small percentage in air causes severe coughing. Heavy exposure can be fatal.

Physiological Effect of Breathing Air Chlorine Mixtures...

Effect of Exposure	Parts of Chlorine Gas Per Million of Air By Volume (ppm)
Slight symptoms after several hours exposure	1
Irritates throat	10 - 15
Causes coughing	30
Dangerous in 30 minutes	40 - 60
Fatal in a few breaths	1,000

Mild exposure to chlorine produces no cumulative effects and complete recovery usually occurs. Inhalation of chlorine gas will cause an initial restlessness, anxiety, a severe irritation of the throat, and the production of excessive saliva. These symptoms are followed by coughing, retching, vomiting and difficulty in breathing. Individuals suffering from asthma and certain types of chronic bronchitis are particularly affected. Exposure of the skin to liquid chlorine will result in severe irritation and blisters.

Wastewater treatment plant operators should be constantly alert for any chlorine leaks as well as thoroughly familiar with the properties of chlorine, the proper ways to handle it and protective as well as first aid measures associated with emergencies. Individuals working with chlorine should be trained in the use of self-contained breathing apparatus. While several types of gas masks should be available at the plant, it should be noted that the usual industrial canister type gas mask is not effective when chlorine in the air exceeds 1 %. Hence, they are not recommended in dealing with chlorine gas. The plant should be supplied with a chlorine gas mask of a design approved by the Bureau of Mines. When the oxygen content is limited (below 16 %) a self-contained "supplied air" or oxygen supply type breathing apparatus is recommended. The masks should be located in readily accessible points, away from any areas that are likely to be contaminated by chlorine gas. Masks should be checked regularly. However, it is to be emphasized that whenever a room must be entered that may contain chlorine gas, great care must be taken. When approaching this situation the door must be carefully opened and left ajar to check for the smell of chlorine gas. An individual should never enter a room containing harmful levels of chlorine without :

- A self contained air supply
- Protection to the eyes
- Protective clothing
- Help standing by
- Notifying proper authorities

First Aid Measures for Exposure to Chlorine...

- Be sure you know the location of breathing apparatus, first aid kits, and other safety equipment at all times.
- Remove clothing contaminated with liquid chlorine at once. Carry patient away from gas area, if possible to a room with a temperature of 70 ^O F. Keep patient warm, with blankets if necessary. Keep him quiet.
- Place patient on his back with his head higher than the rest of his body.
- Call a doctor and fire department immediately. Immediately begin appropriate treatment.
- Eyes. If even small quantities of chlorine have entered the eyes, hold the eyelids apart and flush copiously with lukewarm running water. Continue flushing for about fifteen minutes. Do not attempt any medication except under specific instructions from a physician.

Chlorine Leaks...

In general, daily inspection of all chlorine cylinders will avoid major problems. Small leaks, detected in early stages can usually be corrected. Before any new system is put into service it should be cleaned, dried and tested for leaks by pressurizing with 150 psi dry air and testing with soapy water applications. Prompt measures are necessary since chlorine leaks become progressively greater. Small leaks around valve stems can usually be corrected by tightening the packing nut or closing the valve. A leak can also be reduced by removing the chlorine as rapidly as possible. If it cannot be added to the process there are several chemicals which can be used to absorb the chlorine gas. For example, chlorine can be absorbed by using 1 1/4 pounds of caustic soda or hydrated line, or 3 pounds of soda ash per pound of chlorine. Therefore, to absorb 100 pounds of chlorine use 125 pounds of solid or flake caustic soda dissolved in 40 gallons of water. A 55 gallon drum may be used. The chlorine line should be well below the surface and mixing improves removal of chlorine.

If the leaking container can be moved, it should be transported to an outdoors area where minimal harm will occur. Keep the leaking part the most elevated so that gaseous chlorine will leak rather than liquid chlorine.

If the leak is large, all persons in the adjacent area must be warned and evacuated. Only authorized persons equipped with the proper breathing apparatus, and protective measures to the eyes and body should investigate. As noted, help should be standing by and all other persons should be cleared from the affected area. The following generalizations can serve as guidelines.

- The leak may be located by using a rag or brush on a stick soaked in a strong ammonia solution (about 5 % ammonia). When the rag is held close to the leak a white gas will be formed.
- Never apply water to a leak, nor consider submerging a chlorine cylinder (for example, in a pond or tank), since it will probably float and water is not an efficient absorbent for chlorine.
- Remember to keep windward of the leak.
- Remember that chlorine gas is heavier than air and will accumulate in the lower parts of a room or building.
- Remember that the fusible plug melts at 158 ^O F.
- Keep chlorine cylinder or container emergency repair kits available. Be familiar with their use and location.
- Leaks around valve stems and discharge outlets can usually be stopped.
- Leaks at fusible plugs and cylinder valves requires special handling and emergency equipment. The chlorine supplier must be notified immediately.
- Pin hole leaks in cylinder walls or ton tanks can usually be stopped by mechanical pressure applications (clamps, turnbuckles, etc.). This only temporary and may require your ingenuity.
- Leaking containers cannot be shipped.

Ultraviolet Light for Disinfection...

Ultraviolet light (UV) is electromagnetic radiation with a wavelength of approximately 4 to 400 nm (nanometers). These wavelengths are outside the region normally detected by the human eye, and therefore are considered to be invisible. Ultraviolet light has been used since the early 1900's in Europe for the disinfection of municipal water supplies. UV is currently receiving renewed attention as a method of wastewater disinfection. A major reason for this renewed interest is that UV adds nothing to the wastewater during the disinfection process.

UV disinfects by altering the DNA of the bacterial cells exposed to it. It has been found that UV radiation with a wavelength of approximately 254 nm is most efficient for disinfection purposes. In practical use at wastewater treatment plants, UV light is produced by low pressure mercury lamps. These lamps which provide radiation of 253.7 nm, are usually housed in specially fused quartz sleeves. Glass sleeves cannot be used as glass absorbs UV light with great efficiency. This would result with no or little UV light reaching the wastewater; therefore, very little disinfection would occur. The quartz sleeves serve as an electrical insulator by preventing the wastewater from contacting the electrical portion of the lamp and also as a temperature buffer so the bulb may remain at its optimum operating temperature for maximum efficiency.


The usual configuration is for the mercury lamps to be in a closed unit. This is done for safety and to promote complete mixing. The closed unit contains baffles which direct the wastewater flow so that the microorganisms will spend a maximum amount of time close to the ultraviolet source. For the ultraviolet disinfection process to be effective, the UV radiation must be directed on the bacteria. The UV unit therefore attempts to expose all bacteria to the radiation at a reasonably close range (i.e. 1.4 inch). To achieve disinfection, most bacteria require 6,000 to 13,000 microwatt seconds of exposure. Commercial available UV disinfection units can provide in excess of 30,000 microwatt seconds. This high amount serves as a safety factor as the mercury discharge lamp output deteriorates with use.

The three main disadvantages of UV are: (1) high cost of operation, (2) anything which will prevent the UV light from reaching the bacteria will prevent an effective kill, and (3) UV light tends to ionize compounds and break them apart (i.e. nitrate could become nitrite in UV light), causing toxic effects on the effluent. Suspended solids, slime growth, turbidity, and color are some of the factors which have an adverse effect on UV disinfection.


Better design of the disinfection units has attempted to solve some of these problems. For example, most units have automatic wipers for the quartz tubes to control slime build-up. Recently, experimenters have been able to achieve effluent limits of 200 fecal coliform/100 mL in wastewater with a suspended solids content of 50 mg/L at a flow of several million gallons per day. This efficiency is attributed to design improvements in the disinfection units.

Safety of Ultraviolet Disinfection...

Ultraviolet light poses a special problem because it is invisible. Intense UV exposure can result in first temporary and eventually permanent damage to the eye, possibly leading to blindness. Should it be necessary to perform work inside the UV unit, be sure it is off and remains off during the maintenance procedure. The symptoms of intense UV exposure are the feeling of sand in the eyes, although more is present. Exposure of skin will result in reddening of the skin, similar to sunburn, but possibly, depending on the exposure, much more severe.

Ozonation of Wastewaters...

Ozone is the triatomic form of oxygen, this is it is composed of three oxygen atoms. Ozone's chemical symbol is O₃. Under normal conditions ozone is unstable and quickly decomposed to the more stable gaseous oxygen, O₂. Because ozone is unstable and cannot be stored successfully, it must be generated at the point of application. Most simply, ozone can be generated by passing oxygen, or air containing oxygen, through an area having an electrical discharge or spark. You may have noticed a clean smell in the air after a thunder and lightning storm. The clean smell was most likely caused by ozone formed by lightning bolts passing through the atmosphere.

To generate a sufficient quantity of ozone for a wastewater treatment plant, ozonators developing a corona discharge are used. These ozonators have two large area metal electrodes separated by a dielectric and an air gap. An alternating electric current is applied to the electrodes creating an electrical discharge. At the same time air or oxygen is passed through the air gap. As the air or oxygen flows through the air gap, and the electrical discharge, a portion of the oxygen is converted to ozone. The dielectric is necessary to spread the electric discharge over the entire electrode area and avoid producing an intensive single arc.

A side product from the corona discharge is the generation of a large amount of heat. The air or oxygen flow in the air gap is not great enough to cool the electrodes. Since high temperatures cause ozone to very rapidly decompose to oxygen, it is necessary to provide a cooling system for the electrodes. At present, two types of cooling systems are used; they are (1) air cooled, and (2) water cooled.

The formation of oxides of nitrogen also takes place in the corona discharge. Oxides of nitrogen react with water to form nitric acid which would in time attack the materials inside the ozonator. To avoid this problem and extend the useful life of the ozonator, the air or oxygen flowing through the air gap in the ozonator must be moisture free. This is accomplished by cooling the compressed gas to remove the moisture before allowing it to enter the air gap.

The concentration of the ozone leaving the ozonator is approximately 1 to 2% by weight and is applied to the wastewater to be disinfected. As with chlorination, the effectiveness of disinfection is depended on the concentration of the disinfectant, thorough mixing and contact time. To satisfy the mixing and contact time requirements, three general types of contactors are usually used: (1) packed bed, (2) sparged column, and (3) sparged column with mixing. The most efficient contactor design will vary from treatment plant and may be different from the best for another wastewater with different conditions.

Advantages and Disadvantages of Ozonation...

The advantages of ozonation include :

- Eliminates odors
- Reduces oxygen demanding matter, turbidity and surfactants
- Removes most colors, phenolics and cyanides
- Increases dissolved oxygen
- Production of n o significant toxic side products
- Increases suspended solids reduction

The disadvantages of ozonation include :

- High capital cost
- High electric consumption
- Highly corrosive, especially with steel or iron and even oxidizes Neoprene

To minimize the disadvantages of ozonation in wastewater treatment plants, some innovations have been developed: use and recycle of oxygen feed to air gap and improved design of ozone contactors. These innovations, of course, are efforts to increase the effectiveness of ozonation systems while minimizing costs associated with ozone generation. Typically, once-through air feed/air cooled systems require about 6 to 9 kwh/lb while recycled oxygen feed/air cooled systems require about 2.5 to 3.5 kwh/lb. It can be seen from these power consumption figures that if pure oxygen is readily available, the cost of ozone generation can be cut dramatically.

Disinfection Using Ozone...

Ozone is thirteen times more soluble in water than oxygen. When first introduced into wastewater, very little disinfection occurs. The ozone is rapidly consumed, satisfying the ozone demand of inorganic salts and organic matter dissolved in the wastewater. The disinfecting properties of the ozone come into play only after the ozone demand is satisfied. When the demand is satisfied, research studies indicate, ozone brings about disinfection 3100 times faster than chlorine. It has also been found that disinfection occurs within contact times of 3 to 8 seconds. Typical ozone dosages needed to reach the disinfection stage vary with the quality of the effluent. Dosages between 5 to 15 mg/L are commonly cited for disinfection of secondary wastewater effluents. Ozone also exhibits excellent virocidal properties at these dosages but with longer contact time of about 5 minutes needed. It has also been found that any residual ozone in the effluent of the contactor disappears in a matter of seconds outside the contactor.

Other Uses of Ozone in Wastewater Treatment...

- Ozone has the ability to remove solids from wastewater by oxidation and physical floatation. A Foam develops when wastewater is ozonated. It has been found that this foam traps a significant amount of solids and nutrient material such as phosphates and nitrates.
- pH has been found to increase very slightly because of ozonation. This is probably the affect of carbon dioxide being driven out of the solution by the gas feed in the ozone contactor.
- Color and turbidity are reduced by addition of ozone. This is brought about by chemical oxidation of the substance causing the color or turbidity.
- Some minor nitrification occurs, but not at levels high enough to consider ozonation as an effective nitrification process.

Safety of Ozone...

The Maximum Allowable Concentration (MAC) of ozone in air, as established by the American Council of Governmental Industrial Hygienists is 0.1 ppm by volume for continuous human exposure. The threshold odor of ozone is 0.01 ppm. This means a person working near an ozone-handling area should be able to detect the presence of ozone at levels far below the MAC. The odor of ozone has been described as similar to that of cloves, new mown hay, nitric acid, etc., depending on the concentration. Concentrations greater than 1 ppm are extremely pungent and are considered unsafe for prolonged human exposure, and therefore should be avoided.