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choppedspleenMechanics

Feb 22, 2014 (3 years and 3 months ago)

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1

CE 548



Physical Unit Operations


2

3

Introduction


Physical unit operations
: operations used for the treatment of
wastewater in which change is brought about by means of the application
of physical forces.


Physical unit operations are a major part of treatment systems.

4

5

Screening


First unit operation encountered is wastewater treatment plants.


A screen is a device with generally uniform openings that is used to
retain coarse solids.


Screens may be divided into coarse (.25
-
6”) and fine (<.25”) screens.
Microscreens

(<50

m) are also available can be used to remove fine
solids from treated effluents.


The material that is retained by the screens is know as screenings.
Screenings are typically hauled to a landfill or may be buried on site or
incinerated depending upon the plant.

6

Traveling Screen
Screenings
7

Screening


Coarse Screens (Bar Racks):


Coarse Screens (Bar Racks): Composed or parallel bars or rods with
openings of 0.25
-
6” and are used to protect pumps, valves from
clogging by large objects and rags.


Bar racks may be hand cleaned or mechanically cleaned (T5
-
2 and
F5
-
3). The bars are mounted at some angle to the horizontal and the
allowable head loss is 6” or less.


Design of Bar Racks: Considerations should include: location (
upstream of grit chamber), approach velocity (1.25
-
3fps); clear
openings between bars or mesh size,
headloss

through the screens
(<6”), screenings and controls. Use 2 units or 1 unit and a by
-
pass.


Head
-
loss across bar screens can be represented by (equation 5
-
1):












g
v
V
C
h
L
2
1
2
2
velocity

approach

openings

through
velocity

0.6)
-
(0.7

t
coefficien

discharge




v
V
C
8

9

Screening


Fine Screens


Fine Screens. Composed of wires, grating, wire mesh, or perforated
plates with openings.


The application range from primary treatment to the removal of
residual SS solids from biological treatment processes. Fine screens
may static, rotary drum or step type. F5
-
4, p.322 and T5
-
4, p.323.


Design of Fine Screens: Fine screens would be preceded by coarse
screens. Use at least 2 sized to handle peak flow.


Head
-
loss across fine screens can be obtained from manufacturer’s
rating tables or calculated using Eq. (5
-
2):



2
2
1







CA
Q
g
h
L
area

open

effective

screens

through

discharge

0.6)

value

(typical

t
coefficien

discharge





A
Q
C
10

Screening


Microscreens


Composed of filter fabrics with openings of 10


35

m
and are
fitted on a drum periphery.


The principle applications for
microscreens

are to remove suspended
solids from secondary effluent and from stabilization
-
pond effluent.


Typical suspended solids removal range from 10 to 80 percent with
an average of 55 percent.


Problems encountered with
microscreens

include incomplete solids
removal and inability to handle solids fluctuations.


Table 5
-
6 gives typical design information for
microscreens
.



11

Screening


Screenings:


Screenings are the material retained on bar racks and screens.


The quantity of screenings collected depends on the screen openings.


Screenings retained on coarse screens
: consists of debris such as
rocks, branches, leaves, paper, plastics, rags, etc. Typical data on the
characteristics and quantity of coarse screenings are shown in Table
5
-
7
.


Screenings retained on fine screens
: include small rags, plastic
material of various types, razor blades, grit, food waste, feces, etc.
Fine screenings must be handled and disposed of properly because it
contains
putrescible

matter and substantial grease and scum. Typical
data on the characteristics and quantity of fine screenings are shown
in Table
5
-
8
.

12

Flow Equalization


Flow equalization


Flow equalization is the damping of flow variations to achieve a
consistent flowrate. It a method used to:


overcome the operational problems caused by flow rate variations


improve performance of downstream processes


reduce the size and cost of downstream treatment facilities


Advantages (benefits):


enhancement of biological treat


shock loading


dilution of
inhibition


improved effluent quality


better sedimentation due to constant
loading


improved filter performance


Attractive option for upgrading performance of overloaded plants.

13

Flow Equalization


Flow equalization


Types:


On
-
line: achieves both flow and organic damping


Off
-
line: achieves mainly flow damping


Volume requirements (example
5
-
2
)

14

Grit Removal


Grit is sand, gravel, cinders, eggshells, seeds, coffee grounds and
other heavy solid material that is much heavier than the organics in
wastewater. Specific gravity (
sg
) of approximately
2.7
but can be as
low as
1.3
versus an organic
sg

of
1
.


Grit is predominantly inert and relatively dry and highly variable in
composition ranging from
13
-
65
% moisture with a volatile content of
1
-
56
%. It has a bulk density of
100
lb/ft
3
.



Normally, grit chambers are designed to remove all grit that will be
retained on a
0.21
-
mm
-
diameter (
65
mesh) screen, and some
chambers are based on
0.15
mm
-
diameter (
100
mesh).


Grit chambers are typically located after the bar racks but before the
primary sedimentation tanks.

15

Grit Removal


The function of the grit chambers include:


Protect moving mechanical equipment from abrasion and wear.


reduce formation of heavy deposits in pipelines.


reduce the frequency of digester cleaning caused by excessive
accumulation of grit.


Types of grit chambers: horizontal flow, aerated or vortex.


Rectangular Horizontal
-
flow grit chamber
:


Designed to maintain a velocity of
1.0
ft/s and provide sufficient
time for grit particles to settle.


Organic particles will be carried out at this velocity.


Designed to remove grit particles that will be retained on a
65
-
mesh screen (
0.21
mm
dia
).

16

Grit Removal


Aerated Grit Chambers
:


Because some organic matter is settled with grit, grit washing is
usually done in order to clean the grit before disposal


Aerated grit is used in order to eliminate the need for grit
washing.


Typical design parameters are given in Table
5


17


The velocity governs the size of particle to be removed.

o
if velocity is too great, grit will be carried out

o
if velocity is too small, organic will be removed with grit

o
velocity is controlled by quantity of air.


17

Grit Removal


Vortex
-
type Grit Chambers,
Two Types:


PISTA unit:



Wastewater enters and exits tangentially


Rotating turbine maintains constant flow velocity and promotes
separation of organics from grit.


Grit settles by gravity into the hopper.

18

Grit Removal


Teacup unit:


Flow enters tangentially at the top of unit and a free vortex is generated.


Effluent exits the center of the top of the unit from a rotating cylinder.


Grit settles by gravity to the bottom


Organics including those separated by centrifugal forces exit with
effluent.


19

Primary Sedimentation


Objective:
to remove readily
settleable

solids and floating material.


Can be used as the principal treatment to provide for the removal of:


settleable

solids


free oil and grease and other floating material


a portion of the organic load


When used ahead of biological treatment, their function is to reduce the
load on biological units.


Primary sedimentation removes
50


70
%
ss

and
25


40
% BOD.

20

Primary Sedimentation


Design consideration:


Detention time
: Important since wastewater solids are
heterogeneous in nature and susceptible to flocculation.


Normally detention time vary from
1
-
1
/
2
to
2
-
1
/
2
hours.


Temperature effect is limited, however, at lower temp settling is retarded
(Figure
5


48
).


Surface
-
Loading rates:


Sedimentation tanks are normally designed on the basis of surface
-
loading
(overflow rate) expressed as gal/ft
2
.d. (Q/
A
surface
)


overflow rate must be set low enough to ensure satisfactory performance
at peak rates

21

Primary Sedimentation


Design consideration:


Scour velocity:


In sedimentation basins, horizontal velocities should be kept low so that
settling particles are not scoured from the bottom of the basin.


Critical velocity is given by:

0.03)
-
0.02

values

(typical

factor

friction
particles

of

diameter
gravity
particles

of
gravity
specific
0.06)
-
0.04

values

(typical

constant

cohesion
velocity

scour

critical
















f
d
g
s
k
V
where
f
gd
s
k
V
H
H
;
)
1
(
8
2
/
1
22

Primary Sedimentation


Sedimentation tank performance:


BOD and TSS removal
: Typical performance data for the removal
of BOD and TSS are presented on Fig.
5
-
46
. The figure is derived
using the following relationship:


constants

empirical
time

detention
efficiency

removal

expected
where;
45)
-
(5

Eq.







b
a
t
R
bt
a
t
R
,