WTO Sedimentation and Filtration 4

Question 1

What is the main disadvantage of a high backwash rate in a rapid sand filter?

Accepted Answer

It can cause media loss by washing fine sand out of the filter

Answer

Excessively high backwash rates expand the bed too much and can wash fine media grains out through the underdrain, resulting in media loss and increased replacement costs.

Question 2

In an upflow solids-contact clarifier, what serves as a primary flocculation and settling zone?

Accepted Answer

The recirculating sludge blanket

Answer

Upflow solids-contact clarifiers rely on a suspended sludge blanket through which raw water passes upward, providing contact between incoming particles and recirculating floc.

Question 3

The term 'negative head' in filtration refers to:

Accepted Answer

Pressure below atmospheric occurring within the filter bed, which can cause air binding

Answer

Negative head occurs when head loss through a clogged filter section exceeds the available pressure head, creating sub-atmospheric pressure that can release dissolved gases and cause air binding.

Question 4

Which of the following conditions promotes formation of mud balls in a rapid sand filter?

Accepted Answer

Excessive coagulant leading to strong floc with insufficient backwash energy

Answer

Mud balls form when strongly coagulated solids bind filter grains together and backwash energy is insufficient to break them apart, leading to clumps that grow over successive filter runs.

Question 5

A sedimentation basin is experiencing density currents. What is the most likely cause?

Accepted Answer

Temperature or turbidity differences between inflow and basin water

Answer

Density currents occur when inflowing water has a different density (due to temperature or suspended solids) than basin water, causing it to flow along the floor or surface rather than settling uniformly.

(WTO) Water Treatment Operator Certification Practice Test

(WTO) Water Treatment Operator Certification Practice Test

WTO Sedimentation and Filtration 4