Paper solution | 14-12-2022 | DWTU

• Subject Code: Design of water Treatment Units ( 3161306) 
• Date:14-12-2022
• Paper solved by Om sonawane

 Q.1 

(a) Explain the importance of alkalinity in coagulation along with chemical reaction.  

Alkalinity is important in coagulation because it helps to control the pH of the water during the coagulation process. Coagulation involves the addition of chemicals, such as aluminum or iron salts, to water in order to cause particles to stick together and form larger particles, or flocs. The pH of the water can greatly affect the efficiency of this process, as the chemicals used for coagulation are most effective within a specific pH range. Alkalinity helps to buffer the water and keep the pH within this optimal range, thus promoting efficient coagulation. Additionally, Alkalinity is important in chemical reactions involving acid-base reactions which are the basis of many coagulation reactions. 

(b) Explain the terms SOR and detention time along with their importance in design of sedimentation tank.

SOR, or "Sludge Occupancy Ratio," is a measure of how full a sedimentation tank is with sludge. It is calculated by dividing the volume of sludge in the tank by the total volume of the tank. The importance of SOR in the design of a sedimentation tank is that it helps to determine the size and capacity of the tank needed to effectively treat a given volume of water. A higher SOR means that the tank is more full of sludge, which can lead to reduced efficiency and increased maintenance requirements. Therefore, in designing a sedimentation tank, it's important to consider the SOR and make sure that the tank has sufficient capacity to handle the volume of water being treated. 

Detention time, also known as residence time, is the amount of time that water stays in the sedimentation tank. It is the ratio of the volume of the tank to the flow rate of water into the tank. The importance of detention time in the design of sedimentation tank is to ensure the solids in water settle properly. Longer detention time is needed for smaller particles to settle, and a tank with a larger volume will be required for this. Therefore, it's important to consider the detention time in the design of a sedimentation tank to ensure that the tank has sufficient volume to allow the particles to settle properly. 

(C) Draw a neat sketch of water treatment plant for ground water source and explain the different units.

Water treatment plant for ground water source. 

Screening Unit: It is the first unit of the treatment plant, where large debris and particles are removed from the water.

Coagulation and Flocculation Unit: Coagulants and flocculants are added to the water to cause particles to stick together, forming larger particles called flocs.

Sedimentation Unit: The water is then sent to a sedimentation tank, where the flocs settle to the bottom, allowing the clear water to be decanted off the top.

Filtration Unit: The water is then sent through filters, typically sand or gravel filters, to remove any remaining particles.

Disinfection Unit: The water is then disinfected with chlorine or UV light to kill any remaining microorganisms.

Storage Unit: The treated water is then stored in a tank or reservoir for distribution.

pH adjustment Unit : The pH of the water is adjusted to neutral if needed before distribution.

Testing Unit : The water is then tested for different parameters like turbidity, TDS, pH, and other contaminants before distribution.

Note: The exact treatment process may vary depending on the specific water source, treatment goals and regulations in place. 

Q.2 

(a) Explain Breakpoint Chlorination with graph. 

Breakpoint chlorination is a process used in water treatment to disinfect water by adding chlorine in increasing amounts until a "breakpoint" is reached, at which point the chlorine begins to rapidly oxidize any remaining organic matter in the water. This process is typically used to disinfect surface water sources, such as rivers and lakes, which may contain high levels of organic matter and other contaminants.

A Breakpoint chlorination graph is a plot of chlorine dosage (mg/L) versus the corresponding chlorine residual (mg/L) in water. The graph typically has two distinct regions.

In the first region, known as the "prebreakpoint" region, the chlorine residual increases linearly with the chlorine dosage. This is because in this region, the chlorine is reacting primarily with inorganic matter, such as ammonia, in the water.

In the second region, known as the "breakpoint" region, the chlorine residual increases rapidly with the chlorine dosage. This is because in this region, the chlorine is reacting with organic matter, such as dissolved organic carbon (DOC), in the water. At this point, the chlorine demand has been met, and a chlorine residual is maintained to ensure that the water is disinfected.

The point where the graph changes from linear to steep is known as the breakpoint. It indicates the point at which the chlorine has oxidized all of the inorganic matter in the water, and is now reacting with organic matter.

It's important to note that the exact shape of the graph may vary depending on the specific water source and the amount of organic matter present. Additionally, the exact dosage of chlorine required to reach the breakpoint will also vary based on the specific water source and the desired level of disinfection.

(b) Define effective size and uniformity coefficient and explain their use in design of RSF. 

Effective size and uniformity coefficient are two measures used to describe the size and distribution of the particles in a granular filter bed, such as a rapid sand filter (RSF).

Effective size (ES) is a measure of the average particle size of a granular filter bed. It is commonly defined as the size of the sieve opening through which 10% of the sample passes and 90% is retained. It is used as an indicator of filter performance, as a filter with a smaller effective size will be able to remove smaller particles than a filter with a larger effective size.

Uniformity coefficient (UC) is a measure of the distribution of particle sizes in a granular filter bed. It is commonly defined as the ratio of the size of the sieve opening through which 60% of the sample is retained to the size of the sieve opening through which 10% of the sample is retained. It is used as an indicator of filter performance, as a filter with a lower uniformity coefficient will be able to remove a more uniform range of particle sizes than a filter with a higher uniformity coefficient.

Both Effective size and uniformity coefficient are used in the design of RSF. Effective size is used to determine the size of the filter media to be used in the filter bed and uniformity coefficient is used to determine the appropriate mix of media sizes to be used in the filter bed, so that the filter can effectively remove a wide range of particle sizes.

It's important to note that the values for effective size and uniformity coefficient will vary depending on the specific application and the desired level of filtration. 

(c) Enlist and explain the parameters which should be kept in mind while selecting the water treatment units. 

When selecting water treatment units, several parameters should be considered to ensure that the units are appropriate for the specific water source and treatment goals. Some of the key parameters to consider include:

Water Quality: The quality of the water source will affect the type and size of treatment units needed. For example, water with high levels of dissolved solids may require a larger ion exchange unit, while water with high levels of bacteria may require a larger disinfection unit.

Flow Rate: The flow rate of water to be treated will affect the size and capacity of the treatment units needed. Larger flow rates will require larger treatment units to handle the volume of water.

Treatment Goals: The specific treatment goals for the water, such as removing specific contaminants or achieving a specific level of disinfection, will affect the type and size of treatment units needed.

Space availability: The space available for the treatment units will affect the size and type of units that can be used. For example, a small footprint unit may be necessary if space is limited.

Cost: The cost of the treatment units, including both the initial cost and the ongoing cost of operation and maintenance, should be considered.

Operator Skill Requirement: The skill level required to operate and maintain the treatment units should be considered. Some units may require more technical expertise than others.

Energy consumption : The energy consumption of the unit should be considered as it will affect the overall cost of the treatment process.

Durability and maintenance: The durability of the treatment units and the ease of maintenance should be considered to minimize downtime and maintenance costs.

Compliance: The treatment units should comply with the local and national regulations for water treatment.

It's important to note that the specific parameters considered will vary depending on the specific application and treatment goals. Additionally, trade-offs may need to be made between different parameters, such as cost and treatment efficiency.

OR

(c) A bar screen is inclined at 60˚ from the horizontal. The circular bars have a diameter of 20 mm. Determine the headloss when the bars are clean and the velocity approaching the screen is 1 m/s.

The headloss across a bar screen can be calculated using the following formula:

Headloss = (velocity²/2g) * (1/sin(angle)) * (1/open area ratio)

Where:

• velocity is the velocity of the water approaching the screen in m/s
• g is the acceleration due to gravity (approximately 9.81 m/s²)
• angle is the angle of inclination of the screen in radians
• open area ratio is the ratio of the open area of the screen to the total area of the screen.

To calculate the headloss across the bar screen:

1. Convert the angle of inclination from degrees to radians (60 degrees = 1.047 radians)
2. Calculate the open area ratio: Open area ratio = (π/4)*(20 mm/2)²/(20 mm/2)² = 0.785
3. Substitute the values into the formula: headloss = (1²/2*9.81) * (1/sin(1.047)) * (1/0.785) = 0.096 m

Therefore, the headloss across the bar screen when the bars are clean and the velocity approaching the screen is 1 m/s is approximately 0.096 m. 

It's important to note that this is an approximate value and that the actual headloss may vary depending on the specific conditions of the system, such as the specific design of the bar screen and the properties of the water. 

Q.3 

(a) Differentiate between orthokinetic and perikinetic flocculation. 

Orthokinetic and perikinetic flocculation are two different types of flocculation that occur in water treatment processes.

Orthokinetic Flocculation is a type of flocculation that occurs when particles in water move relative to each other in the same direction and at the same velocity. The particles collide and stick together to form larger particles or flocs. Orthokinetic flocculation is typically characterized by relatively rapid floc formation and is typically used in applications where high water flows and short detention times are required.

Perikinetic Flocculation, on the other hand, is a type of flocculation that occurs when particles in water move relative to each other in different directions and at different velocities. The particles collide and stick together to form larger particles or flocs. Perikinetic flocculation is typically characterized by relatively slow floc formation and is typically used in applications where low water flows and long detention times are required.

In summary, orthokinetic flocculation occurs when particles move in the same direction and velocity and perikinetic flocculation occurs when particles move in different directions and velocities. Orthokinetic flocculation is used for high water flows and short detention times and Perikinetic flocculation is used for low water flows and long detention times.

(b) What is the volume required for a rapid mix basin to be used to treat 0.05m3/s of water if detention time is 1 minute? Also find out the power input if the water temperature is 20˚ C and velocity gradient is 700 s-1. Take µ =1.002 x10-3 N-s/m2.

The volume required for a rapid mix basin can be calculated using the following formula:

Volume = Flow rate x Detention time

Where:

Flow rate is the flow of water to be treated in m³/s

Detention time is the amount of time the water spends in the basin in seconds

In this case, the flow rate is 0.05 m³/s and the detention time is 1 minute (60 seconds).

So, the volume required for the rapid mix basin is:

Volume = Flow rate x Detention time = 0.05 x 60 = 3 m³

To find the power input in the rapid mix basin, we can use the formula:

Power input = (µ x Velocity gradient² x Volume) / 2 

Where:

µ is the dynamic viscosity of water in N-s/m²

Velocity gradient is the reciprocal of the mixing time in s-1

Volume is the volume of the basin in m³

In this case, the water temperature is 20 C, and the dynamic viscosity of water is 1.002 x10-3 N-s/m2, the velocity gradient is 700 s-1 and the volume of the basin is 3 m³.

So, the power input in the rapid mix basin is: 

Power input = (µ x Velocity gradient² x Volume) / 2 = (1.002 x10-3 x 700² x 3) / 2 = 2.862 KW

Therefore, the volume required for the rapid mix basin is 3 m³ and the power input required is 2.862 KW. 

(c) Design the flocculator and check for Gxt for design flow of 18 MLD. Assume following data :

Detention time = 30 minutes
Depth of flocculator = 3.5 m
Velocity gradient = 30 s-1 

To design a flocculator for a flow of 18 MLD, we need to determine the dimensions of the flocculator based on the detention time, depth, and velocity gradient.

First, convert the flow from MLD to m3/s:

18 MLD = 18 x 106 / (24 x 60 x 60) = 0.5 m3/s

Next, convert the detention time from minutes to seconds:

30 minutes = 30 x 60 = 1800 seconds 

To find the area of flocculator, use the formula:

Area = Flow rate x Detention time / Depth

Area = 0.5 x 1800 / 3.5 = 102.857 m²

To check for Gxt (the product of the velocity gradient and the detention time) we can use the formula:

Gxt = Velocity gradient x Detention time

Gxt = 30 x 1800 = 54,000

The value of Gxt for flocculator design should be between 20,000 and 200,000 s, a value of 54,000 s falls in this range, so the design is appropriate. 

It's important to note that the design of flocculator may vary depending on the specific water source and treatment goals, and other factors such as the specific design of the flocculator and properties of the water. 

OR

Q.3 

(a) Write the design criteria for coarse screen. 

The design criteria for a coarse screen in a water treatment plant typically include the following:

Screening Size: The size of the openings in the screen should be large enough to prevent clogging, but small enough to remove the desired size of debris and particles from the water. The screening size is typically determined by the size of the largest particles that need to be removed. 

Flow Rate: The flow rate of water to be screened should be considered when designing the screen, as a larger flow rate will require a larger screen and a higher velocity to maintain efficient operation.

Headloss: The headloss across the screen should be kept to a minimum to reduce energy costs and ensure efficient operation. The headloss can be minimized by designing the screen with a high open area ratio and by optimizing the screen's angle of inclination.

Durability: The screen should be designed to withstand the forces of the water flow and the abrasion of debris, as well as the environment conditions it will be exposed to. The screen should be made of durable materials that are resistant to corrosion and wear.

Maintenance: The screen should be designed to be easy to maintain and clean, to minimize downtime and maintenance costs. The screen should be accessible for cleaning and inspection, and the cleaning process should be as easy as possible. 

Compliance: The screen should comply with the local and national regulations for water treatment and discharge.

Space availability: The screen should be designed based on the available space and the layout of the water treatment plant.

Cost: The cost of the screen, including both the initial cost and the ongoing cost of operation and maintenance, should be considered.

It's important to note that the specific criteria considered will vary depending on the specific application and treatment goals. Additionally, trade-offs may need to be made between different criteria, such as cost and screening efficiency. 

(b) If a 1.0 m3/s flow water treatment plant uses ten sedimentation basins with an overflow rate of 15 m3/m2.d , what should be the surface area of each tank ? 

To calculate the surface area of each sedimentation basin, you would divide the flow rate of the water treatment plant (1.0 m3/s) by the overflow rate of the basins (15 m3/m2.d).

1.0 m3/s / 15 m3/m2.d = 0.067 m2/s

Since there are ten basins, you would then divide the total surface area needed by the number of basins to find the surface area of each individual tank.

0.067 m2/s / 10 = 0.0067 m2/s or 6.7 m2 per basin. 

(c) Design a tube settler module of rectangular cross section with following data :

Design flow =1.7 MLD
Diameter of tube = 50 mm.
Length of tube = 1.0 m.
Angle of inclination = 60˚ to horizontal. 

A tube settler module with a rectangular cross section can be designed using the following steps:

Determine the surface area of the tube settler module: The surface area of the tube settler module can be calculated by multiplying the length of the tube by the width of the tube. In this case, the width of the tube is equal to the diameter of the tube (50 mm) and the length of the tube is 1.0 m, so the surface area is 0.05 m x 1.0 m = 0.05 m^2. 

Determine the total number of tubes needed: The total number of tubes needed is determined by dividing the design flow (1.7 MLD) by the flow per tube (which is determined by the surface area of the tube settler module). In this case, the flow per tube is 0.05 m^2, so the total number of tubes needed is 1.7 MLD / 0.05 m^2 = 34,000 tubes 

Determine the height of the tube settler module: The height of the tube settler module can be determined by multiplying the length of the tube (1.0 m) by the tangent of the angle of inclination (60˚). The height of the tube settler module is 1.0 m x tan(60) = 1.73 m

Determine the width of the tube settler module: The width of the tube settler module can be determined by multiplying the total number of tubes needed (34,000) by the diameter of the tube (50 mm). The width of the tube settler module is 34,000 x 0.05 m = 1.7 m 

So the dimensions of the tube settler module will be 1.73 m (height) x 1.7 m (width) x 1.0 m (length) and will require 34,000 tubes with a diameter of 50mm and inclination of 60 degrees to the horizontal. 

 Q.4 

(a) Write the sources and composition of sludge generation from drinking water Treatment plant.

Sludge is a byproduct generated during the treatment of drinking water in a water treatment plant. There are several sources and types of sludge that can be generated, including:

Coagulation And Flocculation: During the coagulation and flocculation process, chemicals such as alum or iron salts are added to the water to cause impurities to form into larger particles called flocs. These flocs settle to the bottom of the treatment tank and are removed as sludge.

Sedimentation: The settled flocs from coagulation and flocculation, as well as other suspended particles in the water, are removed during sedimentation. The settled particles are removed as sludge.

Filtration: During the filtration process, particles in the water are trapped in the filter media and removed. The trapped particles are removed as sludge.

Disinfection: In some cases, the addition of disinfectants such as chlorine can cause the formation of disinfection by-products, including trihalomethanes and haloacetic acids, which can form sludge.

The composition of sludge generated from drinking water treatment plant can vary depending on the source and type of sludge. However, it typically contains a mixture of inorganic and organic materials such as clay, silt, sand, algae, bacteria, and dissolved organic matter. Some of the sludge may also contain heavy metals, pathogens, and other contaminants. 

(b) Explain the necessity of recovery of water treatment chemicals from the sludge. 

Recovering water treatment chemicals from sludge is necessary for several reasons:

Economic: Water treatment chemicals are expensive, and recovering them from sludge can help reduce costs associated with purchasing and disposing of them.

Environmental: Disposing of sludge containing water treatment chemicals can be difficult and costly, and can also have negative impacts on the environment if not done properly. Recovering the chemicals can help reduce the amount of sludge that needs to be disposed of and can also help minimize the environmental impacts of disposal.

Resource Conservation: Recovery of water treatment chemicals from sludge can help conserve resources by reducing the need to extract and produce new chemicals. 

Chemical Efficiency: Many water treatment chemicals can be reused multiple times before they lose their effectiveness, so recovering them from sludge can help improve the overall efficiency of the treatment process.

Compliance: Some countries or regions have regulations on the disposal of sludge containing water treatment chemicals, so recovery can be necessary to comply with these regulations.

Overall, the recovery of water treatment chemicals from sludge can help to reduce costs, conserve resources, and minimize the environmental impacts of water treatment. 

(c) Design the under drainage system of RSF for filter length 6.25m and breadth 4.0 m. Assume necessary data. 

An underdrain system is an essential component of a rapid sand filter (RSF) as it helps to collect and remove the filtered water from the filter bed. The design of the underdrain system involves several steps, including determining the flow rate, selecting the underdrain pipe material, and determining the spacing of the pipes.

Determine The Flow Rate: The flow rate is the amount of water that needs to be removed from the filter bed. It can be calculated by multiplying the filter area (6.25 m x 4.0 m = 25 m^2) by the design flow rate (assume it is 1.0 m^3/m^2.d). So the flow rate is 25 m^2 x 1.0 m^3/m^2.d = 25 m^3/d or 25 m^3/h

Select The Underdrain Pipe Material: The underdrain pipe material should be able to withstand the conditions of the filter bed and be resistant to corrosion. PVC or HDPE pipe are commonly used materials for underdrain pipes.

Determine The Spacing Of The Pipes: The spacing of the pipes depends on the type of filter bed media used, the flow rate, and the size of the pipe. Typically, the pipes are spaced at a distance of 0.3m to 0.5m apart.

Design The System: The underdrain system can be designed by calculating the number of pipes required, and their layout. With the flow rate of 25 m^3/h and pipe spacing of 0.5m, the number of pipes required is 25 m^3/h / (0.5 m x 0.5 m x 0.3 m/pipe) = 250 pipes. The layout of the pipes can be in a parallel or a herringbone pattern under the filter bed.

Provide A Collection Sump: The underdrain system should have a collection sump to collect the filtered water and pump it to the next treatment process.

It is important to note that the design of underdrain system may vary depending on the specific conditions and requirements of the treatment plant, and it should be done by a professional engineer. 

Q.5 

(a) Write a short note on activated carbon filter for drinking water. 

Activated carbon filters (ACF) are commonly used in drinking water treatment plants to remove impurities such as chlorine, chloramines, pesticides, and volatile organic compounds (VOCs) that can give the water an unpleasant taste and odor. Activated carbon is a form of carbon that has been treated to increase its adsorption properties, making it highly effective in removing impurities from water. 

Activated carbon filters work by adsorption, which is the process of attracting and holding impurities to the surface of the carbon. The impurities are attracted to the carbon surface by chemical and physical interactions and are held there until they are removed from the filter. The carbon used in activated carbon filters can be made from a variety of materials, such as coal, coconut shells, and wood.

Activated carbon filters can be used as a standalone treatment or in combination with other treatment methods. They are available in several forms including granular activated carbon (GAC) and powdered activated carbon (PAC) filters. GAC filters are filled with small beads of activated carbon, while PAC filters contain a fine powder of activated carbon.

Activated carbon filters have a limited capacity to adsorb impurities and eventually will become saturated, so regular replacement is necessary for the optimal performance. They are also not effective in removing certain pollutants such as bacteria, viruses, and dissolved inorganic contaminants.

Overall, activated carbon filters are an effective method for removing impurities such as chlorine, chloramines, pesticides, and VOCs from drinking water, providing clean and safe water for consumption. 

(b) Explain the advantages of Nalgonda method over adsorption method for fluoride removal. 

The Nalgonda method and the adsorption method are both used for fluoride removal from drinking water. However, the Nalgonda method has several advantages over the adsorption method:

Cost-effective: The Nalgonda method is considered to be more cost-effective than the adsorption method as it does not require the use of expensive adsorbents or chemicals. 

High Removal Efficiency: The Nalgonda method can remove fluoride from water at a higher efficiency than the adsorption method. It can remove up to 98% of fluoride from water, while the adsorption method typically removes around 80-90% of fluoride.

Simple And Easy To Operate: The Nalgonda method is relatively simple and easy to operate as it does not require the use of specialized equipment or chemicals.

Low Maintenance: The Nalgonda method requires less maintenance than the adsorption method as it does not require replacement of adsorbents or chemicals.

Versatility: The Nalgonda method can be used to treat water sources with high fluoride content, and also can be used for the removal of other ions like arsenate, chromate, and nitrate.

The Nalgonda method involves treatment of water with alum and lime. Alum is added to form flocs which are then removed by sedimentation and filtration. Lime is added to raise the pH and precipitate fluoride as calcium fluoride. The resulting sludge is removed and the water is then passed through a sand filter.

However, the Nalgonda method has some limitations as well, such as, the water treated by this method may have high turbidity, presence of iron and manganese in water can cause problems and also it is not suitable for high TDS water. 

(c) Write down the Fe and Mn removal by oxidation method along with chemical reaction.

Iron (Fe) and Manganese (Mn) are naturally occurring elements that can be present in groundwater sources in the form of dissolved ions. High concentrations of Fe and Mn can cause staining, discoloration, and turbidity in water, making it unpleasant to drink and difficult to treat. One method to remove Fe and Mn from water is through oxidation.

The oxidation method for Fe and Mn removal involves the use of chemicals that oxidize the dissolved Fe(II) and Mn(II) ions into their insoluble forms, Fe(III) and Mn(IV) respectively, which can then be removed by sedimentation or filtration. The most common oxidizing agents used for this purpose are chlorine, hydrogen peroxide, potassium permanganate, and potassium dichromate.

The chemical reactions for Fe and Mn removal by oxidation are as follows:

Fe(II) + Cl2 → Fe(III) + Cl- (chlorine oxidation)

Fe(II) + H2O2 → Fe(III) + 2OH- (Hydrogen peroxide oxidation)

Mn(II) + KMnO4 → Mn(IV) + 2OH- (Potassium permanganate oxidation)

Fe(II) + K2Cr2O7 → Fe(III) + KCrO4 (Potassium dichromate oxidation)

It's important to note that the choice of oxidizing agent will depend on the specific water chemistry and the presence of other contaminants. Also, the excess of oxidizing agent can be harmful for the aquatic life and human consumption. Therefore, the dosage of oxidizing agent should be carefully controlled. 

OR

Q.5 

(a) Write a short note on Arsenic removal by adsorption. 

Arsenic is a naturally occurring element that can be present in groundwater sources in the form of dissolved ions. High concentrations of arsenic can be harmful to human health and can cause various health problems, such as cancer, cardiovascular disease, and skin lesions. Adsorption is a widely used method for removing arsenic from water.

Adsorption is a process in which contaminants are removed from water by attracting and holding them to the surface of an adsorbent material. The most commonly used adsorbent materials for arsenic removal are iron oxide-coated sand, activated alumina, and granular activated carbon.

The adsorption process works by attracting the arsenic ions to the surface of the adsorbent material, where they are held until they are removed from the water. The adsorbent material can be used in a variety of forms, including granular, powdered, or as a coating on a support material. 

The adsorption process can be carried out in a batch or continuous mode. The batch mode is simpler and less expensive but less efficient than the continuous mode. The continuous mode uses adsorbent beds in columns or packed beds and can achieve higher removal efficiency.

It's important to note that the adsorbent material may lose its adsorption capacity over time and will need to be replaced or regenerated. Also, the adsorbent material should be carefully selected as some adsorbents can release the adsorbed arsenic back into the water.

(b) Explain Cascade aeration with the help of neat sketch. 

Cascade aeration is a process used in water treatment to remove dissolved gases and other dissolved contaminants from water. It is a type of mechanical aeration where air is forced through the water in a series of steps, or "cascades."

The process begins with a pre-aeration step where water is passed through a diffuser or other type of nozzle that creates a fine mist or spray of water droplets. This causes a large surface area to come into contact with the air, allowing dissolved gases and other contaminants to be removed. The water then flows through a series of cascading steps, each with its own diffuser or nozzle that creates a fine mist or spray of water droplets. This increases the contact area between the water and air, allowing more dissolved gases and other contaminants to be removed.

The final step is the post-aeration step where water is passed through a final diffuser or nozzle that creates a fine mist or spray of water droplets. This step helps to ensure that any remaining dissolved gases and other contaminants are removed before the water is discharged.

Cascade aeration is often used in conjunction with other treatment methods such as coagulation, flocculation, and sedimentation.

The following is a simple sketch of cascade aeration process: 

[pre-aeration] [cascade 1] [cascade 2] [cascade 3] [post-aeration]

Water in Water out 

It's important to note that cascade aeration can be an energy-intensive process, and the design and operation of the system should be done by a professional engineer to ensure optimal performance and energy efficiency. 

(c) Enlist the methods for water softening and explain any one in detail. 

Water softening is a process used to remove dissolved minerals, such as calcium and magnesium, that can cause hardness in water. There are several methods that can be used to soften water, including:

Ion Exchange: This method involves passing water through a bed of resin beads that have been coated with a salt. The resin beads exchange the dissolved minerals in the water for sodium ions.

Reverse Osmosis: This method involves passing water through a semi-permeable membrane that only allows water molecules to pass through, while rejecting dissolved minerals.

Lime Softening: This method involves adding lime to the water, which causes the dissolved minerals to precipitate out of the water, forming a solid that can be removed by sedimentation or filtration.

Electrolytic Softening: This method involves passing an electrical current through the water to remove dissolved minerals by precipitation.

Complexation: This method involves adding a chelating agent to the water, which binds to dissolved minerals and makes them more easily removable by filtration or sedimentation.

I will explain the ion exchange method in detail:

The Ion Exchange method for water softening is based on the ion exchange principle. The ion exchange resin is a synthetic polymer containing positively charged functional groups. This resin is used in a water softener to remove the positively charged ions like calcium and magnesium from hard water, which are replaced by positively charged sodium ions. The ion exchange resin is initially loaded with sodium ions, which are then replaced by the calcium and magnesium ions found in hard water.