Rapid Sand Filter | PSM lectures | Gravity Filter | Environmental Engineering

Summary

The video script offers an in-depth look at rapid sand filters, a prevalent filtration method in water treatment, particularly in developed countries. It details the components, including the enclosure tank, filter media, and underdrainage system, and their assembly. The script explains the filtration process, emphasizing the importance of backwashing to remove trapped particles, and discusses the operation of the filter, including valve control and the significance of monitoring head loss. It concludes with the backwashing procedure, highlighting the reversal of water flow and the use of compressed air to agitate sand grains, ensuring effective cleaning without displacing the filter media.

Takeaways

• 🌟 Rapid sand filtration is a prevalent method for treating large volumes of drinking water in developed countries.
• 💧 The process requires power-operated pumps for backwashing and flow control, typically necessitating backwashing every two days with low turbidity water.
• 🏗️ The filter consists of an enclosure tank, filter media (sand layer), base material (gravel layer), and an underdrainage system.
• 🔍 The filter media's effectiveness is due to its high void space, achieved through the effective size and uniformity coefficient of the sand.
• 🚰 The filtration rate for rapid sand filters is substantial, ranging from 3000 to 6000 liters per hour per square meter of filter area.
• 🏭 The physical plant requirement for rapid sand filtration is about 20% of that for slow sand filters due to the higher filtration rates.
• 🔧 Additional components include wash water drops, an air compressor for backwashing, and a rate control device to maintain a constant filtration rate.
• 🛠️ Backwashing is crucial for cleaning the filter and is triggered by factors like filter run length, effluent turbidity, or head loss within the filter.
• ⏱️ The backwashing process involves reversing the flow, using compressed air and high-velocity water to dislodge trapped particles without disturbing the filter media.
• 📊 Head loss indicators are used to monitor when the filter needs backwashing, typically when the head loss reaches between 1.5 to 2.5 meters.

Q & A

What is a rapid sand filter?

A rapid sand filter is a common filtration system used in water treatment, especially in developed countries for treating large quantities of drinking water. It utilizes mechanisms like straining, settling, sedimentation, and adsorption to remove impurities from water.

What are the typical filtration rates for a rapid sand filter?

The filtration rates for a rapid sand filter are quite high, with a capacity of 3000 to 6000 liters per hour per square meter of filter area.

How often does a rapid sand filter require backwashing?

A continuously operating rapid sand filter usually requires backwashing about every two days when raw water of relatively low turbidity is used.

What are the main components of a rapid sand filter?

The main components of a rapid sand filter include an enclosure tank, filter media (sand layer), base material (gravel layer), and an underdrainage system.

What is the purpose of the gravel layer in a rapid sand filter?

The gravel layer in a rapid sand filter serves as support for the sand layer and ensures that the sand is uniformly distributed, which is crucial for maintaining the filter's efficiency.

What is the function of the underdrainage system in a rapid sand filter?

The underdrainage system in a rapid sand filter collects the filtered water uniformly over the area of the gravel bed and provides uniform distribution of backwash water without disturbing the gravel bed and filter media.

Why is backwashing necessary in a rapid sand filter?

Backwashing is necessary to clean out trapped particles and prevent the filter from clogging. It helps maintain the filter's efficiency and extends its operational lifespan.

How is the backwashing process initiated in a rapid sand filter?

Backwashing is initiated by closing the inlet and outlet valves, opening the air valve to agitate the sand grains with compressed air, and then opening the water valves to create an upward flow of water at a high velocity to dislodge and remove the deposits.

What are the factors that determine when a rapid sand filter needs backwashing?

The need for backwashing in a rapid sand filter can be determined by the length of the filter run, monitoring the turbidity of the effluent water, or by the head loss within the filter, which indicates when the filter is clogged enough to need cleaning.

What is the difference between a perforated pipe system and a pipe and strainer system in the underdrainage of a rapid sand filter?

In a perforated pipe system, water enters or exits through small holes along the pipe, while in a pipe and strainer system, water is distributed through lateral drains with strainers that have holes on their surface. The perforated pipe system is known for high velocity wash, whereas the pipe and strainer system is for low velocity wash.

How long does the entire backwashing process take in a rapid sand filter?

The entire process of backwashing a rapid sand filter, including rinsing and restarting the supplies, typically takes about 15 minutes, with a specified minimum backwash time of 5 minutes.