3: Water Treatment Overview
After reading this section, you should be able to:
- Describe water quality characteristics
- Describe disinfection processes
- Describe the classes of water quality
- Explain water quality characteristics
Water Quality Characteristics
Water functions as a solvent due to its polarity and small molecular size. It has the ability to form hydrogen bonds which allows it to dissolve many types of molecules. It is often referred to as a “universal solvent”. Microorganisms rely on water in order to grow. As such, pathogens which cause human illness are found in untreated surface water and groundwater. Waterborne diseases include typhoid fever, cholera, giardia, dysentery, E coli., hepatitis A, cryptosporidium, and Salmonella.
Public water system treatment plants use a combination of different treatment methods to remove or inactivate all disease-causing organisms and viruses. The process is referred to as a multi-barrier treatment approach. The final step in this approach involves chemical disinfection using chlorine or chloramines. Complete sterilization like the process used in hospitals is not necessary in water treatment.
Factors Influencing Disinfection
Several physical characteristics of a microorganism and chemical factors can influence the disinfection of water and include:
pH
Worldwide, many agencies have historically limited the range of pH values of distributed water between 6.5 and 8.5. Although this range is not a regulatory limit, many jurisdictions have used it as one. The antimicrobial activity of chlorine is optimal at a neutral pH of 7.0. But an increase in pH will reduce its efficacy (pH over 8.0).
Temperature
The activity of most disinfectants will increase as the temperature increases. Chlorine is very effective at temperatures above 65 ℉. Temperature measurements for calculating treatment contact times is important.
Turbidity
Turbidity is the measure of the relative clarity of a liquid. Cloudy (turbid) water would be due
To the presence of clay, silt, tiny inorganic and organic matter and other microorganisms. This would definitely interfere with the disinfection process and is why a multi-barrier treatment approach with filtration precedes disinfection.
Organic Matter
A disinfectant like chlorine or bromine will react with natural organic residue found in water. This reduces the amount of disinfectant available. But more importantly the formation of chemical disinfection by-products called trihalomethanes (THMs) also occurs. These compounds are regulated by the EPA. Consequently, most water agencies in California now use chloramines instead of chlorine as a primary disinfectant.
Inorganic Matter
Inorganic compounds found in residual silts or a compound such as ammonia (NH3) in water will reduce the amount of disinfectant.
Reducing Agents
A reducing agent loses electrons and is oxidized in a chemical reaction. Examples of reducing agents include earth metals, formic acid, and sulfite compounds. Chlorine combines with reducing agents, thereby affecting the amount of chlorine available to disinfect. Examples of reducing agents in water include hydrogen sulfide (H2S), ferrous ion (Fe+2), manganous ion (Mn+2), ammonia (NH3), and nitrite (NO2-).
Microorganisms
The physical characteristics of a microorganism can influence the disinfection process for example, spores are resistant to disinfectants because the spore coat and cortex act as a barrier. But most microorganisms can be filtered or settled out prior to disinfection.
Removal Processes
A multi-barrier approach is used to remove pathogens before they are killed or inactivated (if a virus) by chemical disinfection.
- Coagulation. Coagulation is often the first step in water treatment. During coagulation, chemicals with a positive charge are added to the water (floc). The positive charge neutralizes the negative charge of dirt and other dissolved particles in the water. When this occurs, the particles bind with the chemicals to form slightly larger particles. Common chemicals used in this step include specific types of salts, aluminum, or iron. In an international report published in 1998, it was found that coagulation and sedimentation alone can remove between 27 and 84 percent of viruses and between 32 and 87 percent of bacteria. Usually, the pathogens that are removed from the water are removed because they are attached to the dissolved substances that are removed by coagulation.
- Sedimentation. This refers to one of the steps water treatment plants use to separate out solids from the water. During sedimentation, flocs settle to the bottom because they are heavier than water.
- Filtration. Once the flocs have settled to the bottom of the water, the clear water on top is filtered to separate additional solids from the water. During filtration, the clear water passes through filters that have different pore sizes and are made of different materials (such as sand, gravel, and charcoal). These filters remove dissolved particles and germs, such as dust, chemicals, parasites, bacteria, and viruses. Activated carbon filters also remove any bad odors.
Disinfection Process
After the water has been filtered, water treatment plants may add one or more chemical disinfectants (such as chlorine, chloramine, or chlorine dioxide) to kill any remaining parasites, bacteria, or viruses. To help keep water safe as it travels to homes and businesses, water treatment plants will make sure the water has low levels of the chemical disinfectant when it leaves the treatment plant. This remaining disinfectant kills pathogens living in the pipes between the water treatment plant and your tap.
In addition to or instead of adding chlorine, chloramine, or chlorine dioxide, water treatment plants can also disinfect water using UV light and ozone. They work well to disinfect water in the treatment plant, but these disinfection methods do not continue killing germs as water travels through the pipes between the treatment plant and your tap (no disinfection residual).
Ultraviolet Rays
Advances in UV disinfection have been made in recent years. An Ultraviolet (UV) disinfection system transfers electromagnetic energy from a mercury arc lamp to an organism's genetic material (DNA and RNA). When UV radiation penetrates the cell wall of an organism, it destroys the cell's ability to reproduce. UV radiation, generated by an electrical discharge through mercury vapor, penetrates the genetic material of microorganisms and retards their ability to reproduce. Unfortunately, this method leaves no disinfectant residual and is expensive.
Heat
Primarily used in emergencies, heating water to a boil for 5 minutes will kill all microorganisms which may have breached the distribution system. From a practical perspective, it is an expensive and inefficient way to disinfect water for a public water system.
Chemical Disinfection
Iodine
Iodine is a good disinfectant and has been utilized for potable water treatment since the early 1900’s in concentrations between 2.5 – 7 ppm. The US Army continues to issue iodine-based tablets to field soldiers and other personnel to disinfect water in emergencies. It is expensive and is also not recommended for use as a primary disinfectant due to lack of knowledge on long-term toxic effects .
Bromine
Bromine is primarily used as an alternative disinfectant for swimming pools, spas and cooling tower water, but not for municipal drinking-water, partly due to cost and partly to concerns about the formation of brominated DBPs.
Ozone
Ozone is used to disinfect water along with reducing taste and odors. Drawbacks of ozone for disinfection include a high cost, big physical footprint, lack of residual, difficulty in storing, and maintenance requirements.
Ozone has its advantages and disadvantages. Ozone treatment has the ability to achieve higher levels of disinfection than chlorine or UV, however, the capital costs as well as maintenance expenditures are not competitive with available alternatives.
Chlorine
At room temperature, chlorine is a yellow-green gas that is heavier than air and has a strong irritating odor. It can be converted to a liquid under pressure or cold temperatures. Chlorine is mainly used as bleach in the manufacture of paper and cloth and to make a wide variety of products. Chlorine appears as a greenish yellow gas with a pungent suffocating odor. Chlorine does not burn but, like oxygen, supports combustion. Long-term inhalation of low concentrations or short-term inhalation of high concentrations has ill effects. Vapors are much heavier than air and tend to settle in low areas.
Disinfection Action
Chlorine destroys organisms by oxidizing their cellular material. It is extremely effective for inactivating pathogens with one exception, Cryptosporidium. Leaving chemistry out of this discussion, let’s consider what happens if 1 mg of chlorine gas is added to 1 liter of pure distilled water. Chemical reactions will occur to form two chlorine species which are referred to as free chlorine. That free chlorine “residual” is 1 mg/liter and has the ability to disinfect. Now consider 1 liter of water as it exists in nature. Let’s also say there are no pathogens in the water. It will still contain natural organic compounds, non-organic elements like iron and manganese, and other compounds found in nature like nitrites, ammonia, and hydrogen sulfide. Chlorine will also react with them to form other compounds but won’t have the ability to disinfect. Consequently, you won’t have 1mg/liter of free chlorine residual. It will be less. You would need to add more chlorine gas to get back to 1mg/liter.
Chlorine Reactions in Water
Chlorine and water will react to form hypochlorous acid HOCL and hydrochloric acid HCl. These are weak compounds and HOCL will dissociate further into the hypochlorite ion OCL⁻. Hypochlorous acid and the hypochlorite ion are what make up the free chlorine residual. As a disinfectant, the hypochlorite ion is only 1% as effective as hypochlorous acid.
These reactions are reversible and pH dependent.
- between pH 3.5 and 5.5, HOCl is the predominant species.
- between about pH 5.5 and 9.5, both HOCl and OCl⁻ species exist in various proportions.
- above pH 8, OCl⁻ predominates.
Reactions with Substances in Water
Chlorine will react with natural organic and inorganic compounds. This will reduce the free chlorine residual for disinfection purposes and necessitate the addition of additional chlorine gas. Hydrogen sulfide is a common compound found in groundwater. It has an unpleasant smell and is responsible for many taste and odor complaints from retail customers who may be served with groundwater sources of supply. Chlorine is very effective at removing it.
When chlorine reacts with natural organic matter, a group of disinfection byproducts, known as THM’s or trihalomethanes may form. They may be harmful to certain segments of the population and are listed as a contaminant and regulated by the EPA. Consequently, many water agencies have switched from using the direct use of chlorine gas as a primary disinfectant.
Chloramination
Chloramines have been used as a disinfectant in water systems for many years, primarily to control taste and odor. They can maintain a detectable disinfectant residual in a distribution system. But chloramine is a weaker disinfectant than chlorine. Consequently, higher chloramine doses and longer contact times are required to achieve the same level of disinfectant effectiveness as chlorine. A big advantage of using chloramines is to significantly reduce trihalomethane (THM) levels in order to meet the regulatory requirements of the Surface Water Treatment Rule.
Chloramines are formed when chlorine reacts with ammonia. They will form when natural untreated water contains ammonia. To maintain very low THM levels but maintain effective disinfection residuals, treatment plant operators will apply chlorine gas with ammonia concurrently in the treatment plant influent, or sequentially at a different downstream location in order to create a high chorine residual in part of the treatment process.
Hypochlorite
Rather than use chlorine gas, hypochlorite is available as calcium hypochlorite or sodium hypochlorite. They are a good alternative to supplying chlorine. Calcium hypochlorite is available as a solid. Sodium hypochlorite is available in solution.
Chlorine Dioxide
Chlorine dioxide (ClO2) is an effective disinfectant. Chlorine dioxide does not form THM’s like chlorine gas. It is more costly, and more hazardous to use. Chlorine dioxide is highly reactive and flammable.
UV Disinfection and Treatment of Water
Ultraviolet (UV) rays are part of the light that comes from the sun. The UV spectrum is higher in frequency than visible light and lower in frequency compared to x-rays. The UV spectrum has a larger wavelength than x-rays and a smaller wavelength than visible light and the order of energy, from low to high, is visible light, UV, and x-rays.
UV is known to be an effective disinfectant due to its strong germicidal (inactivating) ability. UV disinfects water containing bacteria and viruses and can be effective against protozoans, such as Giardia lamblia cysts or Cryptosporidium oocysts. UV is used in the pharmaceutical, cosmetic, beverage, and electronics industries. In the United States, it is used for drinking water disinfection; however, high operating costs compared to disinfection by chlorination has limited its usage.
Because of safety issues associated with the reliance of chlorination and improvements in UV technology, UV has experienced increased acceptance in municipal water systems. Two classes of disinfection systems are certified and classified by the NSF under Standard 55, Class A and Class B Units.
- Class A These ultraviolet water treatment systems must have an intensity and saturation rating of at least 40,000 uw-sec/cm2 and possess designs that will allow them to disinfect and/or remove microorganisms from contaminated water. Affected contaminants should include bacteria and viruses. Class A point-of-entry and point-of-use systems covered by this standard are designed to inactivate and/or remove microorganisms, including bacteria, viruses, and Cryptosporidium oocyst and Giardia cysts from contaminated water. Systems covered by this standard are not intended for the treatment of water that has obvious contamination or intentional source contamination, such as raw sewage, nor are these systems intended to convert wastewater to drinking water. These systems are intended to be installed on visually clear water.
- Class B These ultraviolet water treatment systems must have an intensity and saturation rating of at least 16,000 uw-sec/cm2 and possess designs that will allow them to provide supplemental bactericidal treatment of water already deemed safe, such that no elevated levels of E. coli or a standard plate count of less than 500 colonies per 1 ml exists. NSF Standard 55 suggests Class B UV systems are designed to operate at a minimum dosage and are intended to reduce normally occurring non-pathogenic or nuisance microorganisms only. The Class B or similar non-rated UV systems are not intended for the disinfection of microbiologically unsafe water.
The type of unit depends on the situation for use, source of water, and water quality. Transmitted UV light dosage is affected by water clarity. Water treatment devices are dependent on the quality of the raw water. When turbidity is 5 NTU or greater and/or total suspended solids are greater than 10 ppm, pre-filtration of the water is highly recommended. Normally, it is advisable to install a 5 to 20 micron filter prior to a UV disinfection system.
UV disinfection is based on the principles associated with wave lengths of light that damage the nucleic acids of water borne pathogens. UV radiation has three wavelength zones, UV-A, UV-B, and UV-C, and it is the last region, the shortwave UV-C, that has germicidal properties for disinfection. A low-pressure mercury arc lamp resembling a fluorescent lamp produces the UV light in the range of 254 manometers (nm). These lamps contain elemental mercury and an inert gas, such as argon, in a UV-transmitting tube, usually quartz. Traditionally, most mercury arc UV lamps have been the low-pressure type because they operate at a relatively low partial pressure of mercury, low overall vapor pressure (about 2 mbar), low external temperature (50-100o C), and low power. These lamps emit nearly monochromatic UV radiation at a wavelength of 254 nm, which is in the optimum range for UV energy absorption by nucleic acids (about 240-280 nm).
In recent years, medium pressure UV lamps that operate at much higher pressures, temperatures, and power levels have been installed. They emit a broad spectrum of higher UV energy between 200 and 320 nm.
An essential requirement for UV disinfection with lamp systems is an available and reliable source of electricity. While the power requirements of low-pressure mercury UV lamp disinfection systems are modest, they are essential for lamp operation to disinfect water. Since most microorganisms are affected by radiation around 260 nm, UV radiation is in the appropriate range for germicidal activity. UV lamps are available that produce radiation in the range of 185 nm, and they are effective in reducing microorganisms as well. They will also reduce the total organic carbon (TOC) content of the water.
For typical UV systems, approximately 95-percent of the radiation passes through a quartz glass sleeve and into the untreated water. The water is flowing as a thin film over the lamp. The glass sleeve is designed to keep the lamp at an ideal temperature of approximately 104° F.
UV radiation affects microorganisms by altering the DNA in the cells and impeding reproduction. UV treatment does not remove organisms from the water. It inactivates them. The effectiveness of this process is related to exposure time and lamp intensity, as well as general water quality parameters.
The exposure time is reported as microwatt-seconds per square centimeter (uwatt-sec/cm2), and the U.S. Department of Health and Human Services has established a minimum exposure of 16,000 µwatt-sec/cm2 for UV disinfection systems. Most manufacturers provide a lamp intensity of 30,000-50,000µwatt-sec/cm2. In general, coliform bacteria are destroyed at 7,000 µwatt-sec/cm2.
Since lamp intensity decreases over time with use, lamp replacement and proper pretreatment are key to the success of UV disinfection. In addition, UV systems should be equipped with a warning device to alert operators when lamp intensity falls below the germicidal range.
Used alone, UV radiation does not improve the taste, odor, or clarity of water. UV light is a very effective disinfectant, although the disinfection can only occur inside the unit. No residual disinfection in the water exists to inactivate bacteria that may survive or may be introduced after the water passes by the light source. The percentage of microorganisms destroyed depends on the intensity of the UV light, the contact time, raw water quality, and proper maintenance of the equipment.
If material builds up on the glass sleeve or the particle load is high, the light intensity and the effectiveness of treatment are reduced. At sufficiently high doses, all waterborne enteric pathogens are inactivated by UV radiation. The general order of microbial resistance (from least to most) and corresponding UV doses for extensive (>99.9%) inactivation are vegetative bacteria and the protozoan parasites Cryptosporidium parvum and Giardia lamblia at low doses (1-10 mJ/cm2) and enteric viruses and bacterial spores at high doses (30-150 mJ/cm2).
Most low-pressure mercury lamp UV disinfection systems can readily achieve UV radiation doses of 50-150 mJ/cm2 in high quality water; and therefore, efficiently disinfect waterborne pathogens. However, dissolved organic matter, such as natural organic matter, certain inorganic solutes such as iron, sulfites and nitrites, and suspended matter (particulates or turbidity) will absorb UV radiation or shield microbes from UV radiation, resulting in lower delivered UV doses and reduced microbial disinfection. Another concern surrounding disinfecting microbes with lower doses of UV radiation is the ability of bacteria and other cellular microbes to repair UV-induced damage and restore pathogenicity, which is a phenomenon known as reactivation.
UV inactivates microbes primarily by chemically altering nucleic acids. However, the UV-induced chemical lesions can be repaired by cellular enzymatic mechanisms, some enzymes act independent of light (dark repair) and other enzymes require visible light (photo-repair or photo-reactivation). Therefore, achieving optimum UV disinfection of water requires delivering a sufficient UV dose to induce greater levels of nucleic acid damage; and thereby, overcome DNA repair mechanisms.
UV units have a maximum flowrate capacity and some equipment have minimum flowrates. If the flow is too high, water will pass through without enough UV exposure. If the flow is too low, heat may build up which can damage the UV lamp. A UV unit with minimum flow requirements should not be placed on the water line supplying pressure stations in a non-recirculating system. UV units are most often used in constant flow systems.
UV lamps do not burn out as normal florescent lamps do. Instead, the UV lamps will solarize, which reduce their intensity to about 60% of a new lamp after about one year of continuous use. When lamps are new, they will generate a dosage level near 60,000 µW-s/cm2. When the dosage drops to 30,000 µW-s/cm2, the minimum dosage needed to effectively kill bacteria, lamps should be replaced. Lamp life will be shortened significantly if the lamp is turned on and off more frequently than once every eight hours.
Water should be sampled and tested for bacteria counts regularly. Sample before and after the UV unit to test its performance. Water should also be sampled in the distribution since bacterial regrowth can occur downstream of the UV unit.
As water passes through the UV unit, minerals, debris, and other material in the water will deposit onto the quartz or Teflon sleeve. This activity will limit the penetration of UV rays through the sleeve and into the water. To maintain high clarity, the glass around the lamp must be cleaned regularly. Cleaning frequency depends on the water quality and will be minimal with RO treatment upstream.
UV light intensity meters are available which indicate the penetration of UV light through the glass sleeve and the water. Low intensity means the UV dose is too low to provide adequate disinfection. This meter will indicate when cleaning or lamp replacement is needed.
Ozone
Ozone is one of the most powerful water treatment compounds available to system managers today. It is a technology that has been in continual commercial use for over 100 years and has distinct properties that allow disinfection of even heavily compromised water streams. With the 1996 reauthorization of the Safe Drinking Water Act, ozone was named as among the best available technologies for water system compliance with National Primary Drinking Water Regulations as overseen by the US Environmental Protection Agency.
Ozone (O3) is formed when oxygen molecules are exposed to electron flow. Ozone molecules are unstable and will lose the third oxygen atom over time. Ozone is formation characterization:
- Ozone generators provide an electron flow between dielectric and SS tubes.
- Oxygen is passed through the gap between dielectrics resulting in ozone generation.
- Oxygen feed gas must be dry and free of particles.
- Ozone generators must be cooled, and cooling water removes .90 percent of the heat that is generated.
Ozone is a powerful oxidant with high disinfectant capacity. Ozone residuals between 0.3 to 2.0 mg/L inactivate viruses. Inactivation rates range from >3.9-log to >6-log, and occur within very short contact periods, 5 seconds. Microorganisms in natural waters are very sensitive to ozone. Giardia and enteric viruses are inactivated by ozone, as a primary disinfectant, with 5 minutes of contact time. Ozone residuals of 0.5 to 0.6 mg/L result in 3-log and 4-log removals, respectively. When ozone is used as a primary treatment, the criterion for its use is based on ozone residuals, competing ozone demands, and a minimum contact time to meet the required cyst and viral inactivation requirements.
Ozone is the strongest oxidant and strongest disinfectant available for potable water treatment. This unique material can be utilized for a number of specific water treatment applications, including disinfection, taste and odor control, color removal, iron and manganese oxidation, hydrogen sulfide removal, nitrite and cyanide destruction, oxidation of organics such as phenols, pesticides, and some detergents, algae destruction and removal, and as a coagulant aid. Even though ozone is the strongest chemical disinfectant available for water treatment, some refractory organics are not oxidized, or oxidize too slowly. In such cases, ozone can be combined with UV radiation and/or hydrogen peroxide to produce hydroxyl free radicals, HO-, which is a stronger oxidant than molecular ozone, O3. Deliberate production of hydroxyl free radicals starting with ozone has been termed ozone advanced oxidation. Groundwater that is contaminated with chlorinated organic solvents and some refractory hydrocarbons are being treated successfully with ozone advanced oxidation techniques.
At ambient temperatures, ozone is an unstable gas, partially soluble in water; generally, more soluble than oxygen. Due to its instability, ozone quickly reverts to oxygen. Ozone cannot be produced at a central manufacturing site, bottled, shipped, and stored prior to use. It must be generated and applied on-site. The installation of an ozone production plant requires storage of pure oxygen on-site as the feed gas. Ozone is generated for commercial uses using corona discharge or ultraviolet radiation. The UV technique produces low concentrations of ozone, whereas corona discharge produces ozone concentrations in the range of 1 – 4.5 % when dry air is fed to the ozone generator. When concentrated oxygen is used as the feed gas, gas phase ozone concentrations of up to 14 to 18% can be produced. Since ozone is only partially soluble in water, once it has been generated it must be contacted with the water to be treated in such a manner as to maximize the transfer of ozone from the gas phase into water. For this purpose, many types of ozone contactors have been developed. However, as higher concentrations of ozone gas are employed, contacting system designs become more critical because of the lower gas to liquid ratios.
The use of oxygen as the feed gas can result in oxygen super saturation of the treated water causing operational problems and corrosion in the distribution system. Ozone contacting system options include atmospheric tall towers or pressurized gas to liquid mass transfer processes. Fine bubble diffusers, static mixers, or venturi injectors can be used to mix the gas with the water to be treated in full flow or side stream configurations. Once dissolved in water, ozone is available to act on water contaminants to accomplish its intended purposes of disinfection and/or oxidation. At pH levels of 3-6, ozone is present primarily in its molecular form (O3). However, as the pH rises, the decomposition of ozone to produce the hydroxyl free radical (HO-) becomes increasingly rapid. At pH 7 about 50% of the ozone transferred into water produces HO-. At pH >10, the conversion of molecular O3 to HO- is virtually instantaneous.
Because ozone is such a powerful oxidant/disinfectant, the trick to applying it to solve water treatment problems is to do so in a manner that is effective for water treatment, yet at the same safe for the people in the vicinity. Ozone safety issues are handled easily by using proper ambient ozone monitoring, tank venting, and ozone destruction. In the case of systems driven solely by a pumping/injector system, ozone may be produced under vacuum, which ensures no leakage of ozone into the operating environment.
The five basic components of an ozone system include:
- Gas preparation–either drying gas to a suitable dew point or using oxygen concentrators
- A suitable electrical power supply
- A properly sized ozone generator(s)
- An ozone contacting system
- Ozone off-gas destruction or suitable venting system
Moisture in the feed gas causes two operating problems:
- The amount of ozone produced by application of a given electrical energy level is lowered as relative humidity rises. Consequently, it is usually cost-effective to dry the air to a recommended dew point of minus 65’C (-65’C or -76’F) or lower.
- Ozone generated using air in the presence of moisture allows small amounts of nitrogen oxides to react with the moisture to produce nitric acid. In this instance, gas condensation at the cooling/heat transfer surfaces produces a corrosive compound which can cause corrosion problems in the ozone generation equipment with concomitant increases in equipment maintenance requirements.
Because of the high oxidative qualities of gas-phase ozone and the chance of moisture from a failing feed gas unit, system managers must take extra care to make certain that all components in the ozone generator, ozone supply line, ozone gas to liquid mass transfer equipment and the contact vessel are ozone-compatible.
For large scale ozone systems, the equipment for cleaning and drying feed gases can become quite complex. For example, effective air drying can involve multiple treatment steps including air filtration, compression, cooling, desiccation, and final filtration prior to passage into an operating corona discharge ozone generator.
A need exists for efficient ozone contacting and destruction of excess ozone in contactor off-gases. Absent an effective ozone off-gas destruct unit, excess ozone would be present for people in the vicinity to breathe, which is not recommended because of its strong oxidizing nature. Additionally, ozone is heavier than ambient air, and can settle in the vicinity, and attack oxidizable materials. Destruction of contactor off-gas ozone is readily accomplished thermally (370’C), catalytically, thermal-catalytically, and by passing the off-gas through granular activated carbon. Care should be exercised in selecting an ozone destruct method whenever very high concentrations of ozone are encountered.
Ozone is a critical process for non-reverse osmosis purification. It is usually coupled with biologic activated carbon filtration. The process reduces TOC and trace chemical pollutants, removes protozoans, kills viruses, and is a flocculation aid. Ozone treatment is an oxidation process used as a disinfection and oxidant prior to biologic activated carbon filtration.
Instrumentation and controls for ensuring effective and safe operation of ozone systems are concerned with applying ozone effectively and affordably. System processes control ozone generation, oxygen usage, drying, ozone injection and diffusing, and ozone destruction.
The instrumentation monitors each step, and each step has an alarm associated with the process.
Key Terms
- Coagulation – a process that is followed by sedimentation and filtration will remove 90 to 95-percent of the pathogenic organisms depending on which chemicals are used; alum usage in coagulation can increase virus removals up to 99-percent.
- Disinfection – a process that destroys harmful organisms physically or chemically
- Filtration – a process through granular filters is an effective means of removing pathogenic and other organisms form water; removal rates vary from 20 to 99-percent, depending on the coarseness of the filter media and the type of effectiveness of pretreatment.
- Sedimentation – a process through which pathogenic organisms settle out by gravity, assisted by chemical floc; proper design of sedimentation processes can effectively remove 20 to 70-percent of the pathogenic microorganisms
Review Questions
- List the factors that affect chlorination of drinking water?
- Describe disinfection as a process in drinking water treatment.
- What is the purpose of chloramination in drinking water?
- What is ozonation of drinking water?
- How does UV light effect microorganisms?
- Describe the primary reason that chlorine may not be used for drinking water disinfection at a particular drinking water facility.
Chapter Quiz
-
________ is the process designed to kill or inactivate most microorganisms in water, including essentially all pathogenic (disease-causing) bacteria.
- Sterilization
- UV treatment
- Application of chlorine
- Disinfection
-
Which of the diseases listed is not caused by a waterborne pathogenic organism?
- Gastrointestinal anthrax
- Shigellosis
- Polio
- All of these are water borne diseases
-
Hypochlorous Acid and hypochlorite when considered together are also known as the ________.
- chlorine dose
- chlorine demand
- free chlorine
- chlorine residual
-
_________ is a gas that does not form carcinogenic compounds like those compounds formed by chlorine disinfection. It also is not affected by ammonia, and it is a very effective disinfectant at higher pH levels. It is also used to diminish taste and odor problems in the distribution system. It is especially useful in inhibiting
Legionella
.
- Chloramination
- Chlorine dioxide
- Hydrogen peroxide
- Potassium permanganate
-
Because of safety issues associated with the reliance of chlorination and improvements in _____ technology, it has experienced increased acceptance in municipal water systems. Two systems are certified and classified by the NSF under Standard 55. The type of unit depends on the situation for use, source of water, and water quality. The dosage is affected by water clarity. When turbidity is 5 NTU or greater and/or total suspended solids are greater than 10 ppm, pre-filtration of the water is highly recommended. It is advisable to install a 5 to 20 micron filter prior to the disinfection system.
- ozone
- chloramination
- ultraviolet light
- chlorine dioxide
-
______ is the strongest oxidant and strongest disinfectant available for potable water treatment. This unique material can be utilized for specific water treatment applications, including disinfection, taste and odor control, color removal, iron and manganese oxidation, hydrogen sulfide removal, nitrite and cyanide destruction, oxidation of organics such as phenols, pesticides, and some detergents, algae destruction and removal, and as a coagulant aid.
- Ozone
- Chloramination
- Ultraviolet light
- Chlorine dioxide
-
A major problem with disinfection using _____ is that bacterial regrowth can occur downstream of the application site in the distribution.
- Ozone
- Chloramination
- Ultraviolet light
- b and c are correct
-
An alternative used by many water agencies to manage the formation of DBP’s would be ______ disinfection.
- Ozone
- Chloramination
- Ultraviolet light
- Chlorine dioxide
-
_______ is a critical process for non-reverse osmosis purification. It is usually coupled with biologic activated carbon filtration. The process reduces TOC and trace chemical pollutants, removes protozoans, kills viruses, and is a flocculation aid. It is an oxidation process used as a disinfection and oxidant prior to biologic activated carbon filtration.
- Ozone
- Chloramination
- Ultraviolet light
- Chlorine dioxide
-
Which of the listed factors does not have an influence on the disinfection of water with chlorine?
- pH,
- Reducing agents
- Hardness
- Temperature