Particle Settling
Content
ABE 482—Environmental Engineering in Biosystems
September 10, 2008 Lecture 3
Today
Important terms/definitions Waste treatment processes (overview) Pre-treatment processes
Gravity settlers Mechanical sorting Neutralization Sedimentation (particle settling velocity) Coagulation/flocculation
Terms/Definitions
Organic Inorganic Organic matter Inorganic matter Organic loading Organic matter degradation
Terms/Definitions
Total oxygen demand (TOD) Biochemical oxygen demand (BOD) Chemical oxygen demand (COD)
Terms/Definitions
Total solids Volatile solids Fixed solids Suspended solids Total dissolved solids (TDS) Volatile suspended solids (VSS)
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)
Terms/Definitions
More complete list available on class website Will discuss/define later as they become important
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
Pre-Treatment Processes
Mechanical sorting Large solids Gravity sorting Neutralization Suspended solids Sedimentation Coagulation/flocculation
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
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
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!
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
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
gd ( ρ s − ρ l ) v= 18µ
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)
2
Particle Settling
(Reynold’s Number) To validate assumption of laminar flow, calculate Reynold’s Number (Re)
ρdv Re = µ
ρ is density of the liquid (g/cm3) Re < 0.2 fluid flow is laminar (Stokes’ Law is valid)
Particle Settling (Newton’s Law)
When Re>1000, Newton’s Law is valid for settling of spherical particles
2
v =
3 gd ( ρ s − ρ l )
ρl
Particle Settling
(Newton-Rittinger Equation) When 0.2<Re<1000, flow around particle is neither laminar nor turbulent Newton-Rittinger equation applies
4 ( ρ s − ρl ) v = dg ρl 3Q
2
Where Q is the coefficient of resistance (from graph)
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.
Example 1
Calculate 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?
Sedimentation
Colloidal particles
www.filtration-and-separation.com/settling/settling.htm
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
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
Coagulation/Flocculation
Coagulation/Flocculation
Process for Removal of Suspended Solids by Coagulation/Flocculation
Next Day
Waste treatment processes (overview) Solid treatment processes
Composting Digestion Activated sludge Trickling filters Lagoons
September 10, 2008 Lecture 3
Today
Important terms/definitions Waste treatment processes (overview) Pre-treatment processes
Gravity settlers Mechanical sorting Neutralization Sedimentation (particle settling velocity) Coagulation/flocculation
Terms/Definitions
Organic Inorganic Organic matter Inorganic matter Organic loading Organic matter degradation
Terms/Definitions
Total oxygen demand (TOD) Biochemical oxygen demand (BOD) Chemical oxygen demand (COD)
Terms/Definitions
Total solids Volatile solids Fixed solids Suspended solids Total dissolved solids (TDS) Volatile suspended solids (VSS)
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)
Terms/Definitions
More complete list available on class website Will discuss/define later as they become important
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
Pre-Treatment Processes
Mechanical sorting Large solids Gravity sorting Neutralization Suspended solids Sedimentation Coagulation/flocculation
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
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
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!
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
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
gd ( ρ s − ρ l ) v= 18µ
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)
2
Particle Settling
(Reynold’s Number) To validate assumption of laminar flow, calculate Reynold’s Number (Re)
ρdv Re = µ
ρ is density of the liquid (g/cm3) Re < 0.2 fluid flow is laminar (Stokes’ Law is valid)
Particle Settling (Newton’s Law)
When Re>1000, Newton’s Law is valid for settling of spherical particles
2
v =
3 gd ( ρ s − ρ l )
ρl
Particle Settling
(Newton-Rittinger Equation) When 0.2<Re<1000, flow around particle is neither laminar nor turbulent Newton-Rittinger equation applies
4 ( ρ s − ρl ) v = dg ρl 3Q
2
Where Q is the coefficient of resistance (from graph)
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.
Example 1
Calculate 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?
Sedimentation
Colloidal particles
www.filtration-and-separation.com/settling/settling.htm
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
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
Coagulation/Flocculation
Coagulation/Flocculation
Process for Removal of Suspended Solids by Coagulation/Flocculation
Next Day
Waste treatment processes (overview) Solid treatment processes
Composting Digestion Activated sludge Trickling filters Lagoons
