particle settling

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ABE 482—Environmental Engineering in Biosystems September 10, 2008 Lecture 3

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Page 1: Particle Settling

ABE 482—Environmental Engineering in Biosystems

September 10, 2008

Lecture 3

Page 2: Particle Settling

Today

� Important terms/definitions

� Waste treatment processes (overview)

� Pre-treatment processes

� Gravity settlers

� Mechanical sorting

� Neutralization

� Sedimentation (particle settling velocity)

� Coagulation/flocculation

Page 3: Particle Settling

Terms/Definitions

� Organic

� Inorganic

� Organic matter

� Inorganic matter

� Organic loading

� Organic matter degradation

Page 4: Particle Settling

Terms/Definitions

� Total oxygen demand (TOD)

� Biochemical oxygen demand (BOD)

� Chemical oxygen demand (COD)

Page 5: Particle Settling

Terms/Definitions

� Total solids

� Volatile solids

� Fixed solids

� Suspended solids

� Total dissolved solids (TDS)

� Volatile suspended solids (VSS)

Page 6: Particle Settling

Terms/Definitions

� Total Kjeldahl nitrogen (TKN)

� Organic nitrogen

� Inorganic nitrogen

� Ammonia nitrogen (NH3-N)

� Ammonium nitrogen (NH4-N)

� Nitrate (NO3-)

� Total organic carbon (TOC)

� Total carbon (TC)

Page 7: Particle Settling

Terms/Definitions

� More complete list available on class website

� Will discuss/define later as they become important

Page 8: Particle Settling

Waste Treatment Processes

� Physical (organic and inorganic waste)

� Screening, sedimentation, filtration, reverse osmosis, carbon adsorption, ammonia stripping

� Chemical (inorganic waste)

� Neutralization, coagulation, ion exchange, chlorination

� Biological (organic waste)

� Activated sludge, aerobic and anaerobic digestion, composting

Page 9: Particle Settling

Pre-Treatment Processes

� Mechanical sorting

� Gravity sorting

� Neutralization

� Sedimentation

� Coagulation/flocculation

Large solids

Suspended solids

Page 10: Particle Settling

Mechanical Sorting

� Screening (solid waste)

� Bar screens separate material based on

size

� Usually mechanically vibrated or shaken

� Centrifugation (liquid waste)

� Centrifugal force used to separate large particles

� Cost efficient only for very small volumes

Page 11: Particle Settling

Gravity Sorting

� Gravity tables (solid waste)

� Perforated tables which allow certain sized particles to pass down through them, using the force of gravity

� Gravity settling (liquid waste)

� Most common physical process for removing suspended solids from wastewater

� Use for:

� Removing grit (sand particles >0.2 mm dia)

� Clarifying raw sewage and concentrating the settled solids

� Clarifying biological suspensions and settled floc

� Gravity thickening of primary or secondary sludges

Page 12: Particle Settling

Neutralization

� Used for wastewater that is either acidic (low pH) or basic (high pH)� Acid waste: add lime

� Acid mine drainage

� Basic waste: add acid solution� Concrete wash water

� Acid + base � Salt + H2O� Assists in precipitation of suspended solids

including metals� “Neutralizing odours” is the same process!

� Baking soda (basic) + smelly fatty acids = less stinky fridge!

Page 13: Particle Settling

Sedimentation

� Separation of larger, suspended solids from liquid waste

� Occurs when particles act as discrete particles

� Removes grit, sand, silt, iron and Mn

� Use theory of particle settling to calculate settling rate and/or settling times of particles in liquid

Page 14: Particle Settling

Particle Settling (Stokes’ Law)

� Settling velocity is when fluid drag force equals gravitational force (terminal velocity)

� For small particles and low terminal velocity, fluid flow around particle is laminar� Stokes’ Law applies

� v = terminal velocity (cm/s), g = accel due to gravity (981 cm/s2), d = particle diameter (cm), µ = viscosity of liquid

(g/cm-s), ρs, ρl = density of solid particle and liquid (g/cm3)

µ

ρρ

18

)(2

lsgdv

−=

Page 15: Particle Settling

Particle Settling (Reynold’s Number)

� To validate assumption of laminar flow,

calculate Reynold’s Number (Re)

� ρ is density of the liquid (g/cm3)

� Re < 0.2 � fluid flow is laminar (Stokes’ Law is valid)

µ

ρdvRe =

Page 16: Particle Settling

Particle Settling (Newton’s Law)

� When Re>1000, Newton’s Law is valid for

settling of spherical particles

l

lsgdv

ρ

ρρ )(32 −=

Page 17: Particle Settling

Particle Settling (Newton-Rittinger Equation)

� When 0.2<Re<1000, flow around particle

is neither laminar nor turbulent

� Newton-Rittinger equation applies

� Where Q is the coefficient of resistance (from graph)

dgQ

vl

ls

ρ

ρρ )(

3

42 −=

Page 18: Particle Settling
Page 19: Particle Settling

Particle Settling (Newton-Rittinger Equation)

� Newton-Rittinger Equation must be solved

iteratively

1. Calculate estimated v using either Newton’s or Stokes’ equation

2. Calculate Reynold’s number.

3. Find the Q value that corresponds with the Reynold’s number.

4. Use this value of Q to calculate v using the N-R equation.

5. Repeat steps 2, 3 and 4 until the value of Q does not change between iterations.

Page 20: Particle Settling

Example 1Calculate settling velocity

� Assume that a silica particle (density = 2.65 g/cm3) is spherical and has a diameter of 1 mm. The particle is settling in water (density = 1.0, viscosity = 0.01 poise). What is the particles settling velocity?

Page 21: Particle Settling

Sedimentation

Colloidal particles

www.filtration-and-separation.com/settling/settling.htm

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Page 23: Particle Settling

Coagulation & Flocculation

� A chemical-physical procedure where

particles too small for practical removal by sedimentation are destabilized and clustered together for faster settling

� Particles do NOT act as discrete particles

� Coagulation: chemical process used to

destabilize colloidal particles

� Flocculation: gentle mixing of the suspension to promote particle contact

Page 24: Particle Settling

Coagulation & Flocculation

� The colloid particles are agglomerated to

form larger solids called floc

� The most common coagulant is Aluminum Sulfate (Al2(SO4)3)

� Organic polymers may also be used alone or in combination with the Alum to improve flocculation

� Flocs settle via sedimentation

� Remember: we still haven’t dealt with dissolved solids yet!! � later in the term

Page 25: Particle Settling

Coagulation/Flocculation

Page 26: Particle Settling

Coagulation/Flocculation

Page 27: Particle Settling

Process for Removal of Suspended Solids by Coagulation/Flocculation

Page 28: Particle Settling

Next Day

� Waste treatment processes (overview)

� Solid treatment processes

� Composting

� Digestion

� Activated sludge

� Trickling filters

� Lagoons