fundamental concepts sedimentation and flow equalization n.
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Fundamental Concepts: Sedimentation and Flow Equalization. Sedimentation. Sedimentation is the gravitational accumulation of solids at the bottom of a fluid (air or water). Uses for Sedimentation. Removal of solids from drinking water Removal of solids from waste waters

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  • Sedimentation is the gravitational accumulation of solids at the bottom of a fluid (air or water)
uses for sedimentation
Uses for Sedimentation
  • Removal of solids from drinking water
  • Removal of solids from waste waters
  • Settling of solids from air emissions
  • Removal of solids from runoff water
types of settling
Types of Settling

Four types of sedimentation occur:

  • Discrete settling
  • Flocculant settling
  • Hindered settling
  • Compression
examples of settling types
Examples of Settling Types




types of sedimentation
Types of Sedimentation
  • In discrete settling individual particles settle independently
  • It occurs when there is a relatively low solids concentration
types of sedimentation1
Types of Sedimentation
  • In flocculant settling, individual particles stick together into clumps called flocs
  • This occurs when there is a greater solids concentration and chemical or biological reactions alter particle surfaces to enhance attachment
types of sedimentation2
Types of Sedimentation
  • In hindered settling, particle concentration is great enough to inhibit water movement
  • Water must move in spaces between particles
types of sedimentation3
Types of Sedimentation
  • Compression settling occurs when particles settle by compressing the mass below
factors influencing sedimentation
Factors Influencing Sedimentation
  • Particles
    • size
    • density
    • shape
    • charge
  • Fluid
    • velocity
    • density
    • viscosity
sedimentation rate
Sedimentation Rate
  • Stoke’s Law
    • Used for spherical particles
    • Assumes no fluid mixing, so usually will not work for gasses
sedimentation rate1
Sedimentation Rate

vp = particle settling velocity (m/s or ft/s)

p = particle density (kg/m3 or lbm/ft3)

w = fluid density (kg/m3 or lbm/ft3)

d = particle diameter (m or ft)

g = gravitational acceleration (9.81 m/s2 or 32.2 ft/s2)

μ = dynamic viscosity (kg/m·s or lbm/ft·s)

properties of water
Properties of Water
  • 1 lbm/ft·s = 1.48816 kg/m·s
  • Stoke’s Law can be used to determine the surface area of a settling tank
    • The critical velocity for settling is equal to the settling velocity of the smallest particle to be completely removed
    • The overflow rate is equal to the volumetric flow rate into the tank divided by the surface area


    • Setting the overflow rate equal to the critical settling velocity allows time to capture the smallest particles of interest

OFR = over flow rate (m/s or ft/s )

vc = critical settling velocity (m/s or ft/s)

Q = the flow rate into the basin (m3/s or ft3/s)

A = the surface area of the basin (m2 or ft2)

flow equalization baen 465
Flow Equalization (BAEN 465)
  • Flow rates to treatment plants vary throughout the day
  • Treatment processes are more stable if the flow rate is consistent
  • Equalization tanks store wastewater during peak flow periods and are drawn down during low flow to keep downstream flow constant
  • These also help smooth out fluctuations in BOD concentration to downstream processes
equalization tank design
Equalization Tank Design
  • Need information on daily flow cycle (input):
    • average flow rate for each time interval
    • BOD concentration for each time interval
  • Assume wastewater leaves the tank at a constant flow rate (average rate)
  • Tank must be large enough to store all input in excess of daily average flow (output) and may allow for additional storage
  • Mass balances on BOD determine changes in concentration in tank effluent going to downstream processes
equalization tank design1
Equalization Tank Design

Time when flow exceeds avg flow

equalization tank design2
Equalization Tank Design
  • Rearrange table so first row is time when flow first exceeds the average flow rate:
equalization tank design3
Equalization Tank Design
  • Now calculate the difference between flow into and out of tank (ΔV) and the cumulative volume in the tank (Σ(ΔV)) for each time period:
equalization tank design4
Equalization Tank Design
  • Minimum storage volume required
    • Maximum value of Σ(ΔV)
    • In preceding example this is 5897 m3
  • May design for additional storage, e.g., for downstream processes being shut down for a period of time.
equalization tank design6
Equalization Tank Design
  • Mass balance on BOD5 for example: