Particle Settling
Transcript of Particle Settling
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
� Gravity sorting
� Neutralization
� Sedimentation
� Coagulation/flocculation
Large solids
Suspended solids
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
� 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
−=
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 =
Particle Settling (Newton’s Law)
� When Re>1000, Newton’s Law is valid for
settling of spherical particles
l
lsgdv
ρ
ρρ )(32 −=
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 −=
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 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?
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