GENERAL WATER TREATMENT
1.
Softening system
Water is referred to as “hard” when it contains more minerals (namely calcium and magnesium) than ordinary water; the degree of water hardness increases as more calcium and magnesium dissolve. Hard water can clog pipes, damage boilers, heat exchangers and many other devices. It also causes a higher risk of lime scale deposits in industrial, commercial and household water systems. This is where water softening proves to be an important process in reducing water hardness in different applications.
Level of hardness in water
| Hardness | odh | oF | CaCO3 mg/l |
|---|---|---|---|
| Very soft water | 0 - 4 | 0 - 7 | 0 - 70 |
| Soft water | 4 - 8 | 7- 14 | 70 - 140 |
| Moderately hard water | 8 - 12 | 14 - 22 | 140 - 220 |
| Fairly hard water | 12 - 18 | 22 - 32 | 220 - 320 |
| Hard water | 18 - 30 | 32 - 54 | 320 - 540 |
| Very hard water | Over 30 | Over 54 | Over 540 |
Softening process
Removal of these minerals is accomplished by softening the water through an ion exchange process. The softening system mainly removes calcium (Ca2+) and magnesium (Mg2+), which will be replaced by other ions as sodium (Na+). Iron ions may also be removed during softening. These ions exchangers such as sodium chloride (NaCl) will be placed in a brine tank. The softener contains a microporous exchange resin that will be saturated with sodium.
As water passes through the resin bed, calcium and magnesium ions attach to the resin beads and the sodium is released from the resin into the water. After softening a certain quantity of hard water, the beads of resin become saturated with calcium and magnesium ions; at this stage, the exchange resin must be regenerated.
Softening resin
Softening resins are one of the strong acid cation types, based on a polystyrene copolymer and operated in the sodium form. When in operation, the resin has a greater affinity for calcium and magnesium, and hence exchange with the sodium ions on the resin occurs. The process is reversible, so when all the active sites on the resin are taken up, it can be regenerated with salt solution.
However, several parameters affect the quality and performance of the resin:
1- The presence of oxidative chemicals, such as free chlorine, causes physical breakdown of the resin.
2- A high level of iron leads to the fouling of the resin beads.
3- A high level of suspended matter can also be a source of fouling of the resin beads.
Types of softeners
Softeners with timer valves
Water softeners with a timer valve are programmed to regenerate at a specific time of the day, usually when the demand for soft water is expected to be very low (for example at 3 am). The softener can be set to regenerate every day, two or three days, week, or on a specific day during the week.
Removal of these minerals is accomplished by softening the water through an ion exchange process. The softening system mainly removes calcium (Ca2+) and magnesium (Mg2+), which will be replaced by other ions as sodium (Na+). Iron ions may also be removed during softening. These ions exchangers such as sodium chloride (NaCl) will be placed in a brine tank. The softener contains a microporous exchange resin that will be saturated with sodium.
As water passes through the resin bed, calcium and magnesium ions attach to the resin beads and the sodium is released from the resin into the water. After softening a certain quantity of hard water, the beads of resin become saturated with calcium and magnesium ions; at this stage, the exchange resin must be regenerated.
Softening resin
Softening resins are one of the strong acid cation types, based on a polystyrene copolymer and operated in the sodium form. When in operation, the resin has a greater affinity for calcium and magnesium, and hence exchange with the sodium ions on the resin occurs. The process is reversible, so when all the active sites on the resin are taken up, it can be regenerated with salt solution.
However, several parameters affect the quality and performance of the resin:
1- The presence of oxidative chemicals, such as free chlorine, causes physical breakdown of the resin.
2- A high level of iron leads to the fouling of the resin beads.
3- A high level of suspended matter can also be a source of fouling of the resin beads.
Types of softeners
Softeners with timer valves
Water softeners with a timer valve are programmed to regenerate at a specific time of the day, usually when the demand for soft water is expected to be very low (for example at 3 am). The softener can be set to regenerate every day, two or three days, week, or on a specific day during the week.
Softeners with metered valves
In this type of softeners, water softener regeneration is based on the amount of water that has passed through the unit; the softener will only regenerate once the specified amount of water has passed through it. This is the main advantage of this type of softeners, as they save water by preventing unnecessary regenerations.
Duplex water softeners with metered valves
Twin tank water softeners have two resin tanks rather than one. Usually, one is in service and the other on standby. The flow through the softener is monitored so that, when the "on-line" tank is exhausted, the control system swaps flow to the other tank and regenerates the one taken out of service. That way, soft water is available even during regeneration, so the system continues to swap from one tank to the other, regenerating as necessary, at any time of the day.
Softening System Cycles
Five-cycle, full automatic water softener is generally used for water softening. This type of softener is automatically and regularly backwashed and regenerated, which is essential for an efficient operation.
The five cycles are the following:
1-Service: the water being softened
2-Backwash: the direction of the flow is reversed and the silt, sediment and iron particles are flushed from the resin.
3-Brine: the brine solution is slowly passed through the resin, until the latter retains as much sodium as possible.
4- Rinse: excess brine is rinsed from the resin.
5-Fast rinse: the resin bed is compacted for maximum operating efficiency.
How to size a water softener
In order to properly size a water softener, several parameters regarding the water to be treated are to be considered:
1-Hardness level (level of calcium carbonate): measured in grains per gallons (1 gpg = 17.14 ppm).
2-Manganese: measured in ppm (parts per million), and Iron: measured in ppm (parts per million), if available in the water. For every 1 ppm or mg/l of iron or manganese, 3 gpg of hardness needs to be added to the total hardness value.
3-Daily water consumption.
Let’s consider the below examples and calculations for more details:
a- Daily consumption
A population of 40 people (a building including 10 apartments). Each person consumes 80 gallons per day. The total daily consumption is thereby 40 x 80 = 3,200 gallons.
b- Calcium carbonate as CaCO3
A hardness level of 250 ppm. To convert it to grains per gallons, the value is divided by 17.14, which equals 14.58 gpg.
C- Magnesium and iron
A level of 1 ppm of iron and 1.5 ppm of magnesium. This equals a level of 3 gpg of iron and 4.5 gpg of magnesium.
Thus, the total hardness value is: 14.58 + 3 + 4.5 ≈ 22
By multiplying the daily consumption by the calculated equivalent hardness value, the following number of grains per day is obtained: 22 x 3,200 = 70,400 grains. Therefore, for the softener to regenerate once every three days, its capacity should be 70, 400 x 3 = 211,200 grains.
Each one cubic foot of a typical high capacity resin can remove 30,000 grains. So the required quantity of resin is 211,200 / 30,000 =7 cu.ft.
Twin tank water softeners have two resin tanks rather than one. Usually, one is in service and the other on standby. The flow through the softener is monitored so that, when the "on-line" tank is exhausted, the control system swaps flow to the other tank and regenerates the one taken out of service. That way, soft water is available even during regeneration, so the system continues to swap from one tank to the other, regenerating as necessary, at any time of the day.
Softening System CyclesFive-cycle, full automatic water softener is generally used for water softening. This type of softener is automatically and regularly backwashed and regenerated, which is essential for an efficient operation.
The five cycles are the following:
1-Service: the water being softened
2-Backwash: the direction of the flow is reversed and the silt, sediment and iron particles are flushed from the resin.
3-Brine: the brine solution is slowly passed through the resin, until the latter retains as much sodium as possible.
4- Rinse: excess brine is rinsed from the resin.
5-Fast rinse: the resin bed is compacted for maximum operating efficiency.
How to size a water softener
In order to properly size a water softener, several parameters regarding the water to be treated are to be considered:
1-Hardness level (level of calcium carbonate): measured in grains per gallons (1 gpg = 17.14 ppm).
2-Manganese: measured in ppm (parts per million), and Iron: measured in ppm (parts per million), if available in the water. For every 1 ppm or mg/l of iron or manganese, 3 gpg of hardness needs to be added to the total hardness value.
3-Daily water consumption.
Let’s consider the below examples and calculations for more details:
a- Daily consumption
A population of 40 people (a building including 10 apartments). Each person consumes 80 gallons per day. The total daily consumption is thereby 40 x 80 = 3,200 gallons.
b- Calcium carbonate as CaCO3
A hardness level of 250 ppm. To convert it to grains per gallons, the value is divided by 17.14, which equals 14.58 gpg.
C- Magnesium and iron
A level of 1 ppm of iron and 1.5 ppm of magnesium. This equals a level of 3 gpg of iron and 4.5 gpg of magnesium.
Thus, the total hardness value is: 14.58 + 3 + 4.5 ≈ 22
By multiplying the daily consumption by the calculated equivalent hardness value, the following number of grains per day is obtained: 22 x 3,200 = 70,400 grains. Therefore, for the softener to regenerate once every three days, its capacity should be 70, 400 x 3 = 211,200 grains.
Each one cubic foot of a typical high capacity resin can remove 30,000 grains. So the required quantity of resin is 211,200 / 30,000 =7 cu.ft.
2.
Water filtration/Pressure filters
The removal of suspended and colloidal particles by media filtration is based on their deposition on the surface of filter grains while the water flows through a bed of these grains (filter media). The quality of the filtrate depends on the size, and geometry of both suspended solids and filter media, as well as on the water analysis and operational parameters.
The most common filter media in water treatment are sand and anthracite. The effective grain size for fine sand filter is in the range of 0.35 - 0.5 mm, and 0.7 - 0.8 mm for anthracite filter. In comparison to single sand filter media, dual filter media with anthracite over sand permit more penetration of the suspended matter into the filter bed, thus resulting in more efficient filtration and longer runs between cleaning. The design depth of the filter media is a minimum of 31 inches (0.8 m).
The design filtration flow rates are usually 4 - 8 gmp/ft2 (10-20 m/h), and the backwash rates are in the range of 16-20 gpm/ft2 (40-50 m/h). For feed waters with a high fouling potential, flow rates of less than 4 gpm/ft2 (10 m/h) and / or second pass media filtration are recommended. During operation, influent water to be filtered enters at the top of the filter, percolates through the filter bed, and is drawn off through the collector system at the bottom. Periodically, when the differential pressure increases between the inlet and outlet of the pressure filter is 4 - 9 psi (0.3 – 0.6 bar), the filter is backwashed and rinsed to carry away the deposited matter. Backwash time is normally of 10 minutes.
The filtration process is used to remove suspended organic and inorganic matter, and some pathogenic organisms. Pressure filters also reduce the level of bacterial particles, thus reducing the need for disinfection. Filtration system has also shown that iron with a low level could be removed. However, pressure filters do not completely remove all organic chemicals and dissolved inorganic substances, such as heavy metals, or trihalomethane (THM). Additional treatment processes are required for water that is high in dissolved solids (such as sodium, nitrite, sulphate, etc.) and unusually turbid water.
In order to size the correct filter, the first thing to do is to analyze the water in order to check if additional treatment is required. Afterwards, the diameter of the filter and the volume of necessary media can be determined.
For example, if the required flow rate is 50 m3/h and the selected filtration rate 20 m/h, the filter should have a diameter of 1.80 m. It is important to analyze the quality of water, because it will determine the filtration rate.
Typical Aquarius filter
The most common filter media in water treatment are sand and anthracite. The effective grain size for fine sand filter is in the range of 0.35 - 0.5 mm, and 0.7 - 0.8 mm for anthracite filter. In comparison to single sand filter media, dual filter media with anthracite over sand permit more penetration of the suspended matter into the filter bed, thus resulting in more efficient filtration and longer runs between cleaning. The design depth of the filter media is a minimum of 31 inches (0.8 m).
The design filtration flow rates are usually 4 - 8 gmp/ft2 (10-20 m/h), and the backwash rates are in the range of 16-20 gpm/ft2 (40-50 m/h). For feed waters with a high fouling potential, flow rates of less than 4 gpm/ft2 (10 m/h) and / or second pass media filtration are recommended. During operation, influent water to be filtered enters at the top of the filter, percolates through the filter bed, and is drawn off through the collector system at the bottom. Periodically, when the differential pressure increases between the inlet and outlet of the pressure filter is 4 - 9 psi (0.3 – 0.6 bar), the filter is backwashed and rinsed to carry away the deposited matter. Backwash time is normally of 10 minutes.
The filtration process is used to remove suspended organic and inorganic matter, and some pathogenic organisms. Pressure filters also reduce the level of bacterial particles, thus reducing the need for disinfection. Filtration system has also shown that iron with a low level could be removed. However, pressure filters do not completely remove all organic chemicals and dissolved inorganic substances, such as heavy metals, or trihalomethane (THM). Additional treatment processes are required for water that is high in dissolved solids (such as sodium, nitrite, sulphate, etc.) and unusually turbid water.
In order to size the correct filter, the first thing to do is to analyze the water in order to check if additional treatment is required. Afterwards, the diameter of the filter and the volume of necessary media can be determined.
For example, if the required flow rate is 50 m3/h and the selected filtration rate 20 m/h, the filter should have a diameter of 1.80 m. It is important to analyze the quality of water, because it will determine the filtration rate.
Typical Aquarius filter
3.
Activated carbon filtration
Activated carbon is a form of carbon that has been processed to make it extremely porous, and thus it has a very large surface area available for adsorption or chemical reactions. Due to its high degree of microporosity, just 1 gram of activated carbon has a surface area in excess of 500 m2.
Applications
Activated carbon filtration is effective in removing organic contaminants from the water. Organic chemicals which are often responsible for taste, color and odor in the water will also be removed by the carbon. Moreover, activated carbon will adsorb chlorine and remove some organic chemicals that can be harmful, such as trihalomethanes (THM) and pesticides. However, activated carbon filtration does not remove microbes, sodium, nitrate and fluoride.
Activated carbon is widely used in the treatment of drinking water, domestic water, wastewater, medicines, etc. The two most common types of raw material used to produce activated carbons are coconut shell and bituminous coal.
One of the main differences between these two types of activated carbon is the size of the pores; those can be divided into three general sizes: micropores (diameter in the range of less than 2 nm), mesopores (diameter in the range of 2 – 50 nm), and macropores (diameter in the range of above 50 nm).
Bituminous coal:
Coal based activated carbon originates from coal that has undergone a steam activation process to create its activated carbon form. During activation, it creates millions of pores at the surface of the carbon, thus increasing the total surface area. Bituminous coal has fewer micropores, but more mesopores and macropores.
Coconut shell:
To create this activated carbon form, the coconut undergoes a steam activation process. During activation, it creates millions of pores at the surface of the carbon thus increasing the total surface area. Coconut shell has fewer mesopores and macropores, but more micropores, which means that it has much higher capacity to adsorb small molecules such as volatile organic chemicals (VOC). The coconut AC has almost twice the saturation capacity compared to bituminous coal.
Removal of organic matter and chlorine:
As already specified, activated carbon will remove organic matter and chlorine. If an activated carbon filtration is designed to remove these two parameters at the same time, design criteria for organic removal will override design criteria for dechlorination. Since organic adsorption onto activated carbon is a slower process than dechlorination, a system that has been designed for organic removal will be efficient for dechlorination. However, when the design is only for dechlorination, consideration must be given to any dissolved organics that may be present in water. These organics can reduce the capacity of carbon for dechlorination by occupying the available sites used for dechlorination.
Properties of activated carbon media
Pore size:
Particles size:
There is a wide range of mesh sizes: 20x50, 12x40 and 8x30, etc.
This parameter is important since it has a direct impact on the adsorption capacity, backwash rate requirements, and pressure drop. The smaller the activated carbon particles, the faster the dechlorination rate. However, the smaller particles will have a greater pressure drop with the media bed.
Hardness:
A measure of the activated carbon’s resistance to attrition and its ability to withstand frictional forces imposed by backwashing.
Ash content:
Reflects the purity of the carbon. It is the inorganic residue left after heating of the raw material. Ash content has to be as small as possible, especially when the water is hard.
Forms:
There are two main forms of activated carbon: Powder activated carbon and granular activated carbon.
Applications
Activated carbon filtration is effective in removing organic contaminants from the water. Organic chemicals which are often responsible for taste, color and odor in the water will also be removed by the carbon. Moreover, activated carbon will adsorb chlorine and remove some organic chemicals that can be harmful, such as trihalomethanes (THM) and pesticides. However, activated carbon filtration does not remove microbes, sodium, nitrate and fluoride.
Activated carbon is widely used in the treatment of drinking water, domestic water, wastewater, medicines, etc. The two most common types of raw material used to produce activated carbons are coconut shell and bituminous coal.
One of the main differences between these two types of activated carbon is the size of the pores; those can be divided into three general sizes: micropores (diameter in the range of less than 2 nm), mesopores (diameter in the range of 2 – 50 nm), and macropores (diameter in the range of above 50 nm).
Bituminous coal:
Coal based activated carbon originates from coal that has undergone a steam activation process to create its activated carbon form. During activation, it creates millions of pores at the surface of the carbon, thus increasing the total surface area. Bituminous coal has fewer micropores, but more mesopores and macropores.
Coconut shell:
To create this activated carbon form, the coconut undergoes a steam activation process. During activation, it creates millions of pores at the surface of the carbon thus increasing the total surface area. Coconut shell has fewer mesopores and macropores, but more micropores, which means that it has much higher capacity to adsorb small molecules such as volatile organic chemicals (VOC). The coconut AC has almost twice the saturation capacity compared to bituminous coal.
Removal of organic matter and chlorine:
As already specified, activated carbon will remove organic matter and chlorine. If an activated carbon filtration is designed to remove these two parameters at the same time, design criteria for organic removal will override design criteria for dechlorination. Since organic adsorption onto activated carbon is a slower process than dechlorination, a system that has been designed for organic removal will be efficient for dechlorination. However, when the design is only for dechlorination, consideration must be given to any dissolved organics that may be present in water. These organics can reduce the capacity of carbon for dechlorination by occupying the available sites used for dechlorination.
Properties of activated carbon media
Pore size:
| Pore type | Pore size |
|---|---|
| Micropores | <2 nm |
| Mesopores | 2-50 nm |
| Macropores | > 50 nm |
Particles size:
There is a wide range of mesh sizes: 20x50, 12x40 and 8x30, etc.
This parameter is important since it has a direct impact on the adsorption capacity, backwash rate requirements, and pressure drop. The smaller the activated carbon particles, the faster the dechlorination rate. However, the smaller particles will have a greater pressure drop with the media bed.
Hardness:
A measure of the activated carbon’s resistance to attrition and its ability to withstand frictional forces imposed by backwashing.
Ash content:
Reflects the purity of the carbon. It is the inorganic residue left after heating of the raw material. Ash content has to be as small as possible, especially when the water is hard.
Forms:
There are two main forms of activated carbon: Powder activated carbon and granular activated carbon.
1-Powder activated carbon (PAC):
Is a pulverized carbon with a size predominantly between 10 and 50 nm, and is generally added directly to process units, such as raw water intakes, rapid mix basins, clarifiers, and gravity filters. PAC is not commonly used in a dedicated vessel, due to the high head loss that would occur.
2-Granular activated carbon
Is a kind of irregular shaped particles with sizes ranging from 0.2 to 5 nm. GAC is usually held in a fixed filter bed and operated until the carbon bed is exhausted. The carbon is then removed from the vessel and reactivated.
4.
Ultra-violet sterilization
The ultra violet sterilization is a process for removing harmless organisms from the water, by exposing it to high intensity ultra-violet light. UV light has the ability to affect the function of living cells by altering the structure of the cells nuclear material, or DNA. This method of treatment penetrates and permanently alters the DNA of the microorganisms through a process called thymine dimerization.
The microorganisms, which can range from bacteria and viruses to algae and protozoa, are inactivated and rendered unable to reproduce or infect. UV light is generated by applying a low voltage through a gas mixture, resulting in a discharge of photons.
UV lights
The term light refers to electromagnetic radiation of any wavelength, whether visible or invisible to the human eye. Wavelength of visible light to the human eye ranges between 400 and 700 nanometers; UV light ranges between 100 and 400 nanometers. It is divided into four distinct spectral areas: Vacuum UV, UV-C, UV-B and UV-A (see the light spectrum chart below). The UV-C spectrum (200 to 280 nanometers) is the most lethal range of wavelengths for microorganisms. Optimal UV germicidal action occurs at 254 nm. Outside these wavelengths, there is a drop off in the effectiveness, although wavelengths are still absorbed by the DNA.
Water quality and operating conditions
It is important to determine the water quality before installing a UV system. Several parameters should be considered since they can reduce the transmission of UV light through the water. The most important factors are the hardness, the level of iron and turbidity. Suspended solids need to be reduced to 5 microns, and water hardness should not exceed 120 ppm. Iron levels exceeding 0.3 ppm, will cause staining on the quartz sleeve.
Dosage and flow rate
UV dosage is a critical function of UV disinfection, because the extent of inactivation is proportional to the dose applied to the water. UV dosage is a function of two variables: UV intensity and contact time with the radiation source. A UV system with high intensity requires less contact time to achieve adequate disinfection, whereas a system with low intensity will require longer contact time. However, each microorganism requires a different dose to be inactivated.
UV dose (mJ/cm2) = intensity (mW/cm2) x time (sec).
The national sanitation foundation (NSF) requires a minimum of 40 mJ/cm2 dose for waters that are not microbially safe, and a minimum of 16 mJ/cm2 dose for water that are already treated or microbially safe.
Inactivation comparison (Cl2 vs UV)
The microorganisms, which can range from bacteria and viruses to algae and protozoa, are inactivated and rendered unable to reproduce or infect. UV light is generated by applying a low voltage through a gas mixture, resulting in a discharge of photons.
UV lights
The term light refers to electromagnetic radiation of any wavelength, whether visible or invisible to the human eye. Wavelength of visible light to the human eye ranges between 400 and 700 nanometers; UV light ranges between 100 and 400 nanometers. It is divided into four distinct spectral areas: Vacuum UV, UV-C, UV-B and UV-A (see the light spectrum chart below). The UV-C spectrum (200 to 280 nanometers) is the most lethal range of wavelengths for microorganisms. Optimal UV germicidal action occurs at 254 nm. Outside these wavelengths, there is a drop off in the effectiveness, although wavelengths are still absorbed by the DNA.
Water quality and operating conditions
It is important to determine the water quality before installing a UV system. Several parameters should be considered since they can reduce the transmission of UV light through the water. The most important factors are the hardness, the level of iron and turbidity. Suspended solids need to be reduced to 5 microns, and water hardness should not exceed 120 ppm. Iron levels exceeding 0.3 ppm, will cause staining on the quartz sleeve.
Dosage and flow rateUV dosage is a critical function of UV disinfection, because the extent of inactivation is proportional to the dose applied to the water. UV dosage is a function of two variables: UV intensity and contact time with the radiation source. A UV system with high intensity requires less contact time to achieve adequate disinfection, whereas a system with low intensity will require longer contact time. However, each microorganism requires a different dose to be inactivated.
UV dose (mJ/cm2) = intensity (mW/cm2) x time (sec).
The national sanitation foundation (NSF) requires a minimum of 40 mJ/cm2 dose for waters that are not microbially safe, and a minimum of 16 mJ/cm2 dose for water that are already treated or microbially safe.
Inactivation comparison (Cl2 vs UV)
5.
OZONATION
Ozone (O3) is created when diatomic oxygen (O2) is exposed to an electrical field or UV light. Exposure to these high levels of energy causes a portion of the diatomic oxygen molecules to split into individual oxygen atoms, which combine with diatomic oxygen molecules to form ozone.
Ozone delivers oxidizing power either directly or through the generation of hydroxyl-free radicals (OH-) in the decomposition of dissolved ozone into water.
This reaction yields three simultaneous processes: oxidation, disinfection and decomposition. During the oxidation process, ozone-directly and through the highly reactive hydroxyl- free radicals can break chemical bonds of organic compounds. For example, the components of the cell walls of microorganisms can be oxidized and broken down by ozone. This process facilitates disinfection by disrupting and lysing cell walls, exposing the contents of the cell to further oxidation and inactivation.
All common bacteria, viruses, molds, cysts and parasites can be destroyed by ozone in this manner. In the end, ozone decomposes to diatomic oxygen, leaving no unwanted residual taste or odor.
Mass Transfer
Mass transfer is the process in which ozone gas is dissolved into water. Ozone mass transfer may be achieved by bubbling ozone gas through a column of water via diffusion stones, or by mixing the ozone with the water via an integrated mass transfer system utilizing venturi injection.
Ozone Generators
There are two principal technologies used to generate ozone: UV and corona discharge. UV ozone generators utilize UV light at 185-nanometer wave-length that shines on fed gas (air or concentrated oxygen) flowing through a suitable tube chamber.
For most commercial-industrial processes, ozone is generated by corona discharge reactor cells. A corona discharge is a diffuse and continuous luminous electrical discharge that occurs when a high voltage electric field is produced between conductive and dielectric surfaces.
The ozone concentration and total output from an ozone reactor cell or generator is a function of three-controllable parameters: the power applied to the reactor cell, the oxygen concentration and the flow rate of the feed gas through the reactor cell.
Monitoring and Control
The substantial oxidizing and disinfecting power of ozone must be carefully monitored and controlled to ensure that the production of ozone matches the requirements of the application.
Inadequate ozone production may yield inadequate oxidation and disinfection, and too much ozone may damage the product or process equipment.
Ozone should be monitored to limit worker exposure and automatically shut down the system if necessary.
Ozone delivers oxidizing power either directly or through the generation of hydroxyl-free radicals (OH-) in the decomposition of dissolved ozone into water.
This reaction yields three simultaneous processes: oxidation, disinfection and decomposition. During the oxidation process, ozone-directly and through the highly reactive hydroxyl- free radicals can break chemical bonds of organic compounds. For example, the components of the cell walls of microorganisms can be oxidized and broken down by ozone. This process facilitates disinfection by disrupting and lysing cell walls, exposing the contents of the cell to further oxidation and inactivation.
All common bacteria, viruses, molds, cysts and parasites can be destroyed by ozone in this manner. In the end, ozone decomposes to diatomic oxygen, leaving no unwanted residual taste or odor.
Mass Transfer
Mass transfer is the process in which ozone gas is dissolved into water. Ozone mass transfer may be achieved by bubbling ozone gas through a column of water via diffusion stones, or by mixing the ozone with the water via an integrated mass transfer system utilizing venturi injection.
Ozone GeneratorsThere are two principal technologies used to generate ozone: UV and corona discharge. UV ozone generators utilize UV light at 185-nanometer wave-length that shines on fed gas (air or concentrated oxygen) flowing through a suitable tube chamber.
For most commercial-industrial processes, ozone is generated by corona discharge reactor cells. A corona discharge is a diffuse and continuous luminous electrical discharge that occurs when a high voltage electric field is produced between conductive and dielectric surfaces.
The ozone concentration and total output from an ozone reactor cell or generator is a function of three-controllable parameters: the power applied to the reactor cell, the oxygen concentration and the flow rate of the feed gas through the reactor cell.
Monitoring and Control
The substantial oxidizing and disinfecting power of ozone must be carefully monitored and controlled to ensure that the production of ozone matches the requirements of the application.
Inadequate ozone production may yield inadequate oxidation and disinfection, and too much ozone may damage the product or process equipment.
Ozone should be monitored to limit worker exposure and automatically shut down the system if necessary.
6.
Chlorination
Chlorination of potable water is essential to avoid contamination and protect people from waterborne infections and parasitic diseases.Chlorine is generally the disinfectant of choice as it is reasonably efficient, cheap and easy to handle. In all but the smallest water treatment plants, chlorine is added to water as either in aqueous solution or chlorine gas. Smaller supplies on the other hand, may use tablets of hypochlorite.
When added to water, the following reaction occurs instantaneously:
Cl2+H2O -> HOCl + H+ + Cl-
Hypochlorous acid is a weak acid that dissociates partially in water as follows:
HOCl -> H+ + OCl-
At 20 °C and pH 7.5, there is an equal distribution of HOCl and OCl-. At pH 8, about 30 % of the free chlorine is present as HOCl, and at pH 6.5, 90 % is present as HOCl.
Free chlorine
The term free chlorine refers to the sum of hypochlorous acid and hypochlorite ion. Since HOCl is a considerably more efficient disinfectant than OCl-, and free chlorine (even as hypochlorite is more effective than combined chlorine (reaction of chlorine with ammonia)), the international standards recommend that disinfection be carried out at pH less than 8 and at a free chlorine concentration >0.5 mg/l.
The following table summarizes the CT values for the chlorine disinfectant.
Where C = chlorine concentration in mg/l, and T = the contact time in minutes required to inactivate 99.9% of micro-organisms
Although the major source of exposure to chlorine is drinking water, free chlorine in water is not particularly toxic to humans. 100% of the TDI (total daily intake) was allocated to drinking water, giving a health-based GV (guide value) of 5 mg/liter for the sum of hypochlorous acid and hypochlorite ion. Based on the taste and odor threshold of free chlorine, it is doubtful however that the consumer would tolerate such a high level of chlorine. Most individuals are able to taste chlorine at concentrations below 5 mg/liter, and some at levels as low as 0.3 mg/liter. The health-based GV for chlorine should not be interpreted as a desirable level of chlorination.
Chlorine demand
The total amount of chlorine which reacts with compounds like iron and manganese, and with organics and ammonia is referred to as the chlorine demand, and can vary widely between the different types of water. It is the difference between the amount of chlorine added to the water (the chlorine dose) and the total chlorine detectable in the water. The chlorine demand for some waters, such as river water, can increase considerably, particularly after heavy rain.
Contact time
Disinfection with chlorine is not instantaneous; it takes some time for the pathogens present in the water to be inactivated. The time taken for different types of microbes to be killed varies widely. In general, amoebic cysts are very resistant and require a lot of exposure. Bacteria, including free-living vibrio cholerae are rapidly inactivated by free chlorine under normal conditions, while 2 mg/l after 30 minutes may be required to kill amoebic cysts. Thus, it is important to ensure that adequate contact time is available before water enters a distribution system or is collected for use.
Contact time in piped supplies is normally assured by passing the water, after addition of chlorine, into a tank from which it is then abstracted. In small community supplies, this is often the storage reservoir (storage tank).
Chlorine residual
Chlorine persists in water as “residual” chlorine after dosing, which helps minimize the effects of re-contamination by inactivating microbes that may enter the water supply after chlorination. It is important to take this into account when estimating requirements for chlorination in order to ensure that residual chlorine is always present.
The level of required residual chlorine varies with the type of water supply and local conditions. In chlorinated water supplies, there should always be a minimum of 0.5 mg/l residual chlorine after 30 minutes contact time in water.
Cl2+H2O -> HOCl + H+ + Cl-
Hypochlorous acid is a weak acid that dissociates partially in water as follows:
HOCl -> H+ + OCl-
At 20 °C and pH 7.5, there is an equal distribution of HOCl and OCl-. At pH 8, about 30 % of the free chlorine is present as HOCl, and at pH 6.5, 90 % is present as HOCl.
Free chlorine
The term free chlorine refers to the sum of hypochlorous acid and hypochlorite ion. Since HOCl is a considerably more efficient disinfectant than OCl-, and free chlorine (even as hypochlorite is more effective than combined chlorine (reaction of chlorine with ammonia)), the international standards recommend that disinfection be carried out at pH less than 8 and at a free chlorine concentration >0.5 mg/l.
The following table summarizes the CT values for the chlorine disinfectant.
Where C = chlorine concentration in mg/l, and T = the contact time in minutes required to inactivate 99.9% of micro-organisms
| Micro-organism | E.coli | Rotavirus | Giardia lamblia cysts |
| Free chlorine pH 6 to 7 | 0.034-0.05 | 0.01-0.05 | 47-150 |
Although the major source of exposure to chlorine is drinking water, free chlorine in water is not particularly toxic to humans. 100% of the TDI (total daily intake) was allocated to drinking water, giving a health-based GV (guide value) of 5 mg/liter for the sum of hypochlorous acid and hypochlorite ion. Based on the taste and odor threshold of free chlorine, it is doubtful however that the consumer would tolerate such a high level of chlorine. Most individuals are able to taste chlorine at concentrations below 5 mg/liter, and some at levels as low as 0.3 mg/liter. The health-based GV for chlorine should not be interpreted as a desirable level of chlorination.
Chlorine demand
The total amount of chlorine which reacts with compounds like iron and manganese, and with organics and ammonia is referred to as the chlorine demand, and can vary widely between the different types of water. It is the difference between the amount of chlorine added to the water (the chlorine dose) and the total chlorine detectable in the water. The chlorine demand for some waters, such as river water, can increase considerably, particularly after heavy rain.
Contact time
Disinfection with chlorine is not instantaneous; it takes some time for the pathogens present in the water to be inactivated. The time taken for different types of microbes to be killed varies widely. In general, amoebic cysts are very resistant and require a lot of exposure. Bacteria, including free-living vibrio cholerae are rapidly inactivated by free chlorine under normal conditions, while 2 mg/l after 30 minutes may be required to kill amoebic cysts. Thus, it is important to ensure that adequate contact time is available before water enters a distribution system or is collected for use.
Contact time in piped supplies is normally assured by passing the water, after addition of chlorine, into a tank from which it is then abstracted. In small community supplies, this is often the storage reservoir (storage tank).
Chlorine residual
Chlorine persists in water as “residual” chlorine after dosing, which helps minimize the effects of re-contamination by inactivating microbes that may enter the water supply after chlorination. It is important to take this into account when estimating requirements for chlorination in order to ensure that residual chlorine is always present.
The level of required residual chlorine varies with the type of water supply and local conditions. In chlorinated water supplies, there should always be a minimum of 0.5 mg/l residual chlorine after 30 minutes contact time in water.
7.
Iron removal
Iron is one of the most abundant metals in the earth’s crust. It is found in natural fresh waters at levels ranging from 0.5 to 50 mg/litre. Iron may also be present in drinking water as a result of the use of iron coagulants or the corrosion of steel and cast iron pipes during water distribution. Excessive iron and rust in tap water can stain fixtures and laundry, and give tap water a rusty tinge and metallic taste. The EPA (Environmental protection agency) limit of iron in the water is 0.3 ppm.
There are two main forms of iron in the water:
-Ferrous (Fe++) which is dissolved in the water at any pH level.
-Ferric (Fe+++) which is insoluble in water.
There are several methods for the removal of iron from the water; the most common types of which are described here below:
1-Birm filtration
Birm is an efficient media for the reduction of dissolved iron and manganese compounds from raw water. It may be used in either gravity fed or pressurized water treatment systems. Acting as a catalyst between the oxygen and the soluble iron compounds, birm enhances the oxidation reaction of Fe++ to Fe+++ and produces ferric hydroxide which precipitates and may be easily filtered.
Birm may also be used for manganese reduction. In these applications, the water to be treated should have a pH of 8.0 – 9.0 for best results. If the water also contains iron, the pH should be below 8.5. High pH conditions may cause the formation of colloidal iron, which is very difficult to filter out.
Conditions for operation
Pre-treatment
In some cases, an additional treatment is required in order to improve the efficiency of the birm filter.
2-Greensand filtration
Manganese Greensand is formulated from a glauconite greensand and is capable of reducing iron, manganese and hydrogen sulfide from water through oxidation and filtration. Soluble iron and manganese are oxidized and precipitated by contact with higher oxides of manganese on the greensand granules. Precipitates are then filtered and removed by backwashing. When the oxidizing capacity power of the manganese greensand bed is exhausted, the bed has to be regenerated with a weak potassium permanganate (KMnO4) solution.
Low pH, lack of chlorine oxidant or lack of permanganate oxidant are the most likely conditions which may lead to media destruction.
3-Aeration/filtration
Aeration provides the dissolved oxygen needed to convert the iron (and manganese) from their ferrous (and manganous) forms to their insoluble oxidized ferric and (manganic) forms. The aeration process applies to raw water with a maximum iron level of 5 mg/l. It takes 0.14 ppm of dissolved oxygen to oxidize 1 ppm of iron.
The reaction can be expressed as follows:
4Fe2+ + O2 + 8OH- + 2H2O -> 4Fe (OH)3
There are many ways to provide the aeration. Either the water being treated is dispersed into the air, or air is bubbled into the water. Other aeration methods include cascade trays, cone aerators, and porous air stones.
Operation of the aeration process requires careful control of the flow throughout the process. If the flow becomes too great, not enough air is applied to oxidize the iron. If the flow is too small and the aeration is not cut back, the water can become saturated with dissolved oxygen and, consequently, become corrosive to the distribution system.
The speed at which the bivalent iron is oxidized by the oxygen depends on several factors, particularly temperature, pH, level of iron, and oxygen contents.
After the oxidation of the iron is completed, the water must be filtered to remove the ferric hydroxide. The filtration rate should be between 5 to 15 m/h. The weight of iron retained per unit of filter surface varies from 200 to 1000 g Fe per m2 of sand, depending on the circumstances. In general, dual-media filters (anthracite & sand) are particularly suitable for iron removal applications.
4-Chlorination/filtration
Iron in water can be oxidized by a strong oxidant (Cl2, ClO2, KMnO4) converting it to ferric hydroxide. This method is generally used when the level of dissolved iron is high in the water (above 10 ppm - although this process can be used at any level of chlorine). Chlorine solution is injected with a chemical feed pump. The contact time needed to oxidize the ferrous iron depends on two factors: the level of iron in the water, and the quantity of injected chlorine. Once the precipitated iron begins to form, it is removed by the filtration. Backwashing the filter on a regular basis is important, in order to remove the precipitated iron. In some cases, it is recommended to install a carbon filter after the sand (or sand & anthracite) filter, in order to remove the excess of injected chlorine in the water. The oxidation process should occur at a pH of about 8, which is the optimum rate of oxidation. Soda ash injected with the chlorine will increase the pH to the optimum level. In order to improve the filtration process, it is recommended to inject coagulant.
-Ferrous (Fe++) which is dissolved in the water at any pH level.
-Ferric (Fe+++) which is insoluble in water.
There are several methods for the removal of iron from the water; the most common types of which are described here below:
1-Birm filtration
Birm is an efficient media for the reduction of dissolved iron and manganese compounds from raw water. It may be used in either gravity fed or pressurized water treatment systems. Acting as a catalyst between the oxygen and the soluble iron compounds, birm enhances the oxidation reaction of Fe++ to Fe+++ and produces ferric hydroxide which precipitates and may be easily filtered.
Birm may also be used for manganese reduction. In these applications, the water to be treated should have a pH of 8.0 – 9.0 for best results. If the water also contains iron, the pH should be below 8.5. High pH conditions may cause the formation of colloidal iron, which is very difficult to filter out.
Conditions for operation
- Dissolved oxygen content should be equal to at least 15% of the iron (or iron and manganese).
- Water pH range should be 6.8 – 9.0.
- Alkalinity should be greater than two times the combined sulfate and chloride concentration.
- Bed depth should be 30 – 36 in.
- Free board should be 50% of bed depth (min).
- Service flow rate should be 3.5 – 5 gpm/sq.ft.
- Backwash rate should be 10 – 12 gpm/sq.ft.
- Free chlorine concentration should be less than 0.5 ppm.
- Hydrogen sulfide should be removed prior to the Birm filter.
- Organic matter should not exceed 4-5 ppm.
Pre-treatment
In some cases, an additional treatment is required in order to improve the efficiency of the birm filter.
- If the level of dissolved oxygen in the water is under 15%, the addition of air will be required. Either oxygen will be introduced to the inlet water via an injector, or the water will be aerated.
- If the influent water has a pH of less than 6.8, neutralizing additives such as soda ash may be used prior to the Birm filter to raise the pH.
2-Greensand filtration
Manganese Greensand is formulated from a glauconite greensand and is capable of reducing iron, manganese and hydrogen sulfide from water through oxidation and filtration. Soluble iron and manganese are oxidized and precipitated by contact with higher oxides of manganese on the greensand granules. Precipitates are then filtered and removed by backwashing. When the oxidizing capacity power of the manganese greensand bed is exhausted, the bed has to be regenerated with a weak potassium permanganate (KMnO4) solution.
Low pH, lack of chlorine oxidant or lack of permanganate oxidant are the most likely conditions which may lead to media destruction.
 |
Conditions for operation
|
3-Aeration/filtration
Aeration provides the dissolved oxygen needed to convert the iron (and manganese) from their ferrous (and manganous) forms to their insoluble oxidized ferric and (manganic) forms. The aeration process applies to raw water with a maximum iron level of 5 mg/l. It takes 0.14 ppm of dissolved oxygen to oxidize 1 ppm of iron.
The reaction can be expressed as follows:
4Fe2+ + O2 + 8OH- + 2H2O -> 4Fe (OH)3
There are many ways to provide the aeration. Either the water being treated is dispersed into the air, or air is bubbled into the water. Other aeration methods include cascade trays, cone aerators, and porous air stones.Operation of the aeration process requires careful control of the flow throughout the process. If the flow becomes too great, not enough air is applied to oxidize the iron. If the flow is too small and the aeration is not cut back, the water can become saturated with dissolved oxygen and, consequently, become corrosive to the distribution system.
The speed at which the bivalent iron is oxidized by the oxygen depends on several factors, particularly temperature, pH, level of iron, and oxygen contents.
After the oxidation of the iron is completed, the water must be filtered to remove the ferric hydroxide. The filtration rate should be between 5 to 15 m/h. The weight of iron retained per unit of filter surface varies from 200 to 1000 g Fe per m2 of sand, depending on the circumstances. In general, dual-media filters (anthracite & sand) are particularly suitable for iron removal applications.
4-Chlorination/filtration
Iron in water can be oxidized by a strong oxidant (Cl2, ClO2, KMnO4) converting it to ferric hydroxide. This method is generally used when the level of dissolved iron is high in the water (above 10 ppm - although this process can be used at any level of chlorine). Chlorine solution is injected with a chemical feed pump. The contact time needed to oxidize the ferrous iron depends on two factors: the level of iron in the water, and the quantity of injected chlorine. Once the precipitated iron begins to form, it is removed by the filtration. Backwashing the filter on a regular basis is important, in order to remove the precipitated iron. In some cases, it is recommended to install a carbon filter after the sand (or sand & anthracite) filter, in order to remove the excess of injected chlorine in the water. The oxidation process should occur at a pH of about 8, which is the optimum rate of oxidation. Soda ash injected with the chlorine will increase the pH to the optimum level. In order to improve the filtration process, it is recommended to inject coagulant.
8.
DEIONIZATION
A deionizer is an ion exchange unit which removes any ionized substance (salt) from water; it treats water by means of cation and anion exchange columns. A great variety of substances can be found dissolved in water. However, a few inorganic salts can be found more frequently than others.
Most of the total dissolved minerals in surface water or non-brackish well water mainly consist of calcium and magnesium based salts. Those are called “hardness salts”, as they cause water hardness. There is a difference between hardness and total dissolved solids: hardness consists of only calcium and magnesium salts, while total dissolved solids means all the salts that may exist in water (hardness minerals included). Demineralization removes all the minerals that are in water.
Process
The salt molecule consists of two parts: a cation, which has a positive charge, and an anion which has a negative charge. When the salts dissolve in water, these two parts separate and are maintained separate due to the very high dielectric constant of water and to the solvation phenomena.
As the untreated water passes through the cation resin column, the positively charged ions of calcium, magnesium, sodium, potassium, etc., are exchanged for hydrogen ions that were placed on the cation resin during regeneration of the resin with hydrochloric acid. The hydrogen ions combine with the carbonates, chlorides, nitrates, sulfates, silicates, etc., that are in the water forming weak solutions of carbonic acid, hydrochloric acid, nitric acid, sulfuric acid, silicic acid, etc. The decationized water will have a low pH (about 3.0) as a result of the weak acid present in the water.
The decationized water then passes through the anion resin column where the carbonates, chlorides, nitrates, sulfates, etc., are “split” away from their respective acids and are replaced by hydroxide ions that were placed on the anion resin during regeneration of the resin with sodium hydroxide. The hydrogen ions that are available as a result of the “splitting” of the acids are free to combine with the hydroxide ions.
The result is as follows:
Hydrogen (H+) + Hydroxide (OH-) = HOH = H2O (water)
Water quality
Deionization must be considered as a highly specific means and not as a general water treatment, inclusive of filtration, oxidation, sterilization, etc. The water to be deionized must be clear, colorless, with no iron, oil, or organic matter in excess of 50 ppm. To remove such undesirable elements, a suitable pretreatment is necessary.
The use of deionized water is growing steadily with constant refinements in the electronics and chemical industries.
The desired quality of the treated water is dependent on the application and is expressed in terms of the conductance of the water in micromhos/cm. Using the most common anion resin (weakly basic), a water quality of 100 micromhos/cm to 10 micromhos/cm can be obtained. For silica and carbon dioxide removal, strongly basic anion must be used, and the resulting water quality can be 20 micromhos/cm to 1 micromhos/cm.
To obtain water quality better than 1 micromhos/cm, mixed bed deionizers must be used.
ProcessThe salt molecule consists of two parts: a cation, which has a positive charge, and an anion which has a negative charge. When the salts dissolve in water, these two parts separate and are maintained separate due to the very high dielectric constant of water and to the solvation phenomena.
As the untreated water passes through the cation resin column, the positively charged ions of calcium, magnesium, sodium, potassium, etc., are exchanged for hydrogen ions that were placed on the cation resin during regeneration of the resin with hydrochloric acid. The hydrogen ions combine with the carbonates, chlorides, nitrates, sulfates, silicates, etc., that are in the water forming weak solutions of carbonic acid, hydrochloric acid, nitric acid, sulfuric acid, silicic acid, etc. The decationized water will have a low pH (about 3.0) as a result of the weak acid present in the water.
The decationized water then passes through the anion resin column where the carbonates, chlorides, nitrates, sulfates, etc., are “split” away from their respective acids and are replaced by hydroxide ions that were placed on the anion resin during regeneration of the resin with sodium hydroxide. The hydrogen ions that are available as a result of the “splitting” of the acids are free to combine with the hydroxide ions.
The result is as follows:
Hydrogen (H+) + Hydroxide (OH-) = HOH = H2O (water)
Water quality
Deionization must be considered as a highly specific means and not as a general water treatment, inclusive of filtration, oxidation, sterilization, etc. The water to be deionized must be clear, colorless, with no iron, oil, or organic matter in excess of 50 ppm. To remove such undesirable elements, a suitable pretreatment is necessary.
The use of deionized water is growing steadily with constant refinements in the electronics and chemical industries.
The desired quality of the treated water is dependent on the application and is expressed in terms of the conductance of the water in micromhos/cm. Using the most common anion resin (weakly basic), a water quality of 100 micromhos/cm to 10 micromhos/cm can be obtained. For silica and carbon dioxide removal, strongly basic anion must be used, and the resulting water quality can be 20 micromhos/cm to 1 micromhos/cm.
To obtain water quality better than 1 micromhos/cm, mixed bed deionizers must be used.