Question

In a filtration plant water is filtered using layers of
A) Sand
B) Oil
C) Sand and clay
D) Sand, fine gravel and medium gravel

Answer 1

Sewage contains a large amount of toxic organic matter. Microorganisms are used in the sewage treatment plant for removing this toxic matter. Sewage treatment is very necessary to decrease the toxicity of sewage and also to maintain a safe and healthy environment, as well as to promote human welfare.

Complete answer:
Sewage or the wastewater treatment plant comprises of two stages, namely,
a) Primary Treatment
Which involves the removal of large or even small-sized particles in the wastewater through physical processes.
b) Biological Treatment

Aerobic microorganisms are inoculated into the sewage treatment plant, which in turn utilize the organic components of the sewage and thereby reduce the toxicity. This can be measured by Biological oxygen demand. After the biological treatment, the sludge is pumped from the treatment plant into a larger tank. This large tank contains anaerobic bacteria that leads to the digestion of sludge. During this process, biogas is produced and is used as a source of energy.

Sand filters are being used as a step in the process of water treatment. This filter type will capture the particles to very small sizes, and does not have a cut off size below which these particles can always pass through. One of the easiest ways to remove hydrocarbons from water is by using a belt oil skimmer. It is a low maintenance and easy to use tool for removing oil from water or coolant. But it can filter only specific compounds.

Soils can filter and thereby clean water. The ability of soil to filter water is dependent upon particle size and how fast water flows through soil. It is considered that more time is required to filter water through sand and clay. In wastewater treatment, discharged or wastewater enters a grit and sand removal tank, where the speed of wastewater is slowed down to allow sand, grit, and pebbles to settle down.

Therefore the correct answer is option ‘D’.

Note: The process of filtration is an expensive process when it comes to water treatment and water purification. Maintenance and lack of regulation are disadvantages of filtration. Water treatment filters are not regulated by any health commission or department; therefore the effectiveness of filtration and purification can vary between manufacturers. But filters can be used as a mechanism of water treatment or purification.