Separations in Environmental Engineering

Question 1

The size of particle is based on 4 properties:

Answer 1

1. the geometry
2. light scattering effect
3. flow characteristics
4. electrical mobility

Question 2

What is aerodynamic size?

Answer 2

The diameter of a unit density sphere which the same settling velocity VS as the particle in question.

Question 3

EPA Hi-Vol sampler:

Used to separate the aerosol from air stream. What are the 3 mechanisms which capture the particles?

Answer 3

1. Interception
2. Inertial Impaction
3. Diffusion

Question 4

Explain interception of a paricle?

Answer 4

Particles will follow the streamline and stick to the fibre by adsorption. Larger particles it tends to stick to the surface stronger, this is a property of adsorption as will have larger van der Waal forces.
Efficiency increases with increasing particle size

Question 5

Explain Interial impaction?

Answer 5

When the particle is too large the particle will not follow the streamline, but will drop out and hit the fibre directly
Efficiency increases with increasing particle size

Question 6

Explain diffusion?

Answer 6

When the particle is small it is driven by concentration difference, so will slowly diffuse due to Brownian diffusion and stick to the surface.

The smaller the particle side it will diffuse faster, so efficiency increases with decreasing particle size.

Question 7

PM10 particles are bigger and can be separated easier by two inertial effects:

Answer 7

Impaction and cyclone.

Impaction: The efficiency depends on the gas flow rate

Cyclone: The particle in the air that is sucked in are forced to undertake many circular motions inside the cyclone. Heavy particles fall and light particles rise.

Question 8

Difference between impaction and filtration?

Answer 8

Impaction: separation by inertia -> large particle cannot follow the streamline

Filtration: separation by size exclusion (Sieve)

Question 9

Explain a virtual impactor sampler?

Answer 9

: Inlet flow down into the impactor, while a clean air flow sweeps across the main airstream and sweeps particles smaller than 2.5 microns to another channel.

The major flow carrying the smaller particle sand there is a minor flow carrying larger particles with a small fraction of smaller particle which travel down the sampler.

Question 10

Explain a cascade impactor?

Answer 10

The principle depends on the inertial and the gravitational force.

The large particle drops out while the small particle carry on through to the next stage.
Particles impact on a flat plate to the next stage.

Normally the top stage has a large jet and low velocity to favour collection of large particles. The lower stage has a small jet and high velocity to catch the small particles.

The efficiency depends on the size of the particles and the flow rate of the incoming stream.

Question 11

There are three main problems with cascade impactor separators, what are they?

Answer 11

1. Particle Bounce
   Particle can bounce due to dropping on the collector at high speed, resuspending it.
2. Overloading:
   Platform can get saturation so you cannot collect more particles.
3. Velocity Requirement:
   A higher velocity is needed for collecting smaller particles, this can be achieved by operating the impactor at a lower pressure or by reducing the size of the orifice on top of the impaction plate.

Question 12

What is a bioaerosol?

Answer 12

Airborne particles that are living or derived from living organisms.

Question 13

Explain communicable, non-communicable and hospital acquired diseases?

Answer 13

Communicable – originated from human

Non-communicable – originated from ambient environment

Hospital acquired – endogenous in human, opportunistic infection

Question 14

What is the rule of thumb when sampling airborne pollutants?

Answer 14

Sampling times should be

long enough to collect a detectable and representative number of particles

short enough to avoid overloading and masking the colonies

Question 15

There are three principles of collection? What are they?

Answer 15

1. Inertial impaction
2. Liquid impingement
3. Filtration

Question 16

What is interial impaction?

Principles of collection

Answer 16

Inertial forces cause the large particles will drop off the trajectory and impact a surface of a pumping device.

Can have issues with particle bound and blow off.
When the flow rate is too high, you can crush them down too hard and kill the organism due to impact stress.

Question 17

What is Liquid Impingement?

Principles of collection

Answer 17

the inertial forces impact the particles onto a surface which is submerse in liquid.

If the flowrate of the air is too high it will push all the liquid to the side making everything airborne, you want everything bubbling through the liquid.

Need to control the flow rate to ensure everything is contacting the liquid.
Assist by particle diffusion within the air bubbles leaving the impaction region.
Living things will die when it is too dry, so this helps the microbes be healthy.

Question 18

What is filtration?

Principles of collection

Answer 18

Size exclusion with hole size of filter material.
Particles will need to be extracted from the filter.
Filters are normally made of polycarbonate, mixed cellulose ester or PVC.

Question 19

What are the 5 advantages of Liquid impingement over inertial impaction?

Answer 19

• Prevents dehydration
• measure the collected volume
• Avoids errors due to overgrowth or crowing
• Clusters of organisms can be separated
• Particles in liquid can be counted directly with cell counters

Question 20

What are the 4 disadvantages of Liquid impingement over inertial impaction?

Answer 20

• Lower biological activity due to physical stress of of impaction onto a impingers glass bottom
• Particles may bounce and be resuspended
• collected particles may also be re-aerosolized
• additional sampling process