This set of Environmental Engineering Multiple Choice Questions & Answers (MCQs) focuses on " Dual Media and Mixed Media Filter".
1. What is the specific gravity of Anthracite used in a dual media filter?
Explanation: The specific gravity of Anthracite is 1.4 and due to this, the beds of it can be expanded at a same backwash rate as sand bed when the size of anthracite grains is twice of sand grains.
2. In which of the following filter, two layers filtering media is provided for filtration?
Explanation: In dual media filter, filtration takes place from coarse to fine media by the provision of two layers filtering media called as dual media and that is why it is called as a dual media filter.
3. What is the size of sand used in a dual media filter?
Explanation: The size of sand used in a dual media filter is 0.5-1mm and it has a thickness of 20 to 30 cm.
Explanation: Anthracite is a coarser filter medium used in the dual media filter. It is costlier than sand and it is used as a substitute for sand.
Explanation: Anthracite is used as a top layer in dual media filter having a thickness of 30-40cm.
6. How the performance of rapid sand filter can be improved?
Explanation: The performance of rapid sand filter can be improved by capping sand filters with coal where the top 15 cm layer of sand is replaced with Anthracite.
Explanation: The size of Anthracite used in dual media filter is 1.25-2.5mm and it is placed over a sand of 0.5 to 1mm size.
Explanation: In mixed media filter, the filtration is from coarse to fine and impurities and suspended matter are stored and removed throughout the full depth of the bed.
9. What is the ratio in which coal, sand and garnet are present in the mixed media filter?
Explanation: In mixed media filter, the coal is placed on top, sand in the middle and garnet at the bottom in the proportion of 60%, 30%, 10% respectively.
10. Which of the following order is correct regarding the specific gravity of the materials used in the mixed media filter?
Explanation: Anthracite coal has a specific gravity of 1.42, sand has a specific gravity of 2.65 and Garnet sand has a specific gravity of 4.2. So, coal has lower, sand has medium and Garnet sand has the highest specific gravity between them.
11. Which of the following order is correct regarding the thickness of layer of the materials used in the mixed media filter?
Explanation: Anthracite coal has a thickness of 45 cm, sand has a thickness of 22.5cm and Garnet sand has a thickness of 7.5 cm. So, coal has highest, sand has medium and Garnet sand has a lower thickness of layer between them.
12. Which of the following is the best filter in terms of rate of filtration?
Explanation: Dual media filter has greater depth of filter media in comparison to rapid sand filter, whereas mixed media filter is an improved version of dual media filter having three materials for filter layers resulting in increased filter run times and water quality.