Objective 01: Subsidence Flashcards Preview

3B1-11: Boiler Water Pretreatment > Objective 01: Subsidence > Flashcards

Flashcards in Objective 01: Subsidence Deck (16)
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How does an Intermittent, Batch-Type Settling Basin work?

1. A tank is filled with water containing suspended solids and allowed to stand.
2. After the sediment has settled, the clarified water is drawn off the top and the settled solids are removed from the bottom.
3. The tank is then ready for another cycle.


Which is more popular for subsidence, particularly for large volumes of water: Intermittent Methods or Continuous Methods?

Continuous Methods


How does a Continuous Method of Subsidence work?

1. Put water through an open vessel in which the velocity of water is reduced to a point that permits subsidence.
2. Solids settle to the bottom, while clarified water rises to the top and is drawn from the tank overflow.

3. Solids are removed from the bottom by either:

a) mechanically operated rakes or scrapers continuously

b) periodically flushing with high-pressure jets


What kinds of suspended solids cannot be removed by simple subsidence?

1. Low specific gravity
2. Very fine particle size


What are 2 physical methods of improving subsidence?

1. Increasing the basin size
2. Increasing the retention time


What are 2 ways that use chemicals of improving subsidence?

1. Coagulation
2. Flocculation


How does Coagulation improve subsidence?

It increases the size of the particles and promotes settling.



Chemicals called coagulants cause fine particles to gather together into a larger mass that will settle more readily.



Coagulant in the water causes a spongy substance known as floc to be produced. The floc has a very large surface area which traps the very fine suspended particles in the water, thus forming larger particles.


How does Flocculation improve Subsidence?

It increases the size and volume of the coagulated particles.


What are 2 ways of improving flocculation?

1. Gentle agitation of the water
2. Changing the direction of water flow, using baffles or other mechanical means


Are Coagulation and Flocculation chemical processes?

No, they are mechanical (physical) processes. The chemicals only aid in the settling of the small particles.


What are the 3 most common coagulants?

1. Aluminum Sulphate - Al 2 (SO 4 ) 3
2. Aluminum Hydroxide - Al(OH) 3
3. Sodium Aluminate - Na 2 Al 2 O 4


What conditions are most conducive to Coagulation?

1. Moderate temperature
2. pH between 5.5 and 8.0


How does it negatively affect subsidence if pH is not controlled?

The floc can dissolve.


How does a typical Water Clarifier work?

1. Raw water enters the top
2. Coagulant is also introduced at the top through the chemical feed pipe
3. Mechanically driven agitators mix the water and coagulant together as they travel down the central zone of the clarifier
4. By the time the water reaches the bottom of the mixing zone, particles have coagulated
5. The coagulated particles form a sludge blanket in the bottom portion of the upflow zone and this blanket acts as a filter for the upflowing water
6. The clarified water is drawn off, through a submerged collector pipe at the top of the upflow zone
7. The coagulated particles (sludge) collect in the conical section at the bottom and periodically removed through the desludging valve