UNIT OPERATION

EXPERT LECTUREON

Delivered BY:

DR. N. J. MISTRY, PROFESSORCivil Engineering Department

Sardar Vallabhbhai National Institute of Technology, Surat - 7

UNIT OPERATIONSScreeningMixingFlocculationSedimentationFloatationElutriationVacuum FiltrationHeat Transfer and Drying

PRELIMINARY TREATMENT SYSTEM

Approach ChannelScreen ChamberGrit ChamberSkimming Tank )Oil and Grease Traps)

PRIMARY TREATMENT SYSTEM

Above +PSTReduces 60 – 70 % of Settleable Suspended Solids and30 – 32 % of Organic Suspended Solids

BAR SCREEN

Clarifier

Plot for Computation of Volume of an Equalization Basin

EQUALIZATION TANK

SCREEN CHAMBER

Screen Type Size of Clear Opening

Coarse Screen More than 50 mm

Medium Screen 25 – 50 mm

Fine Screen 10 – 25 mm

DESIGN CONSIDERATIONS

Where

hL = head loss (m)

β = bar shape factor

W = width of bar facing the flow

b = clear spacing between the bars

hV = velocity head of flow approaching the rack (m)

= inclination of the bars

Head loss is also computed by the expression

hL = 0.0729 (v2 – v2h)

Where hL = head loss (m)

v = velocity of flow through screens, m/s

vh = velocity of flow before screens, m/s (approach velocity)

sinθv

h4/3

bW

βL

h

GRIT CHAMBER

Design Criteria

Flow maximum

Peaking factor 2.5 – 3.0

Detention time, t 30 – 90 s (usually 60 s)

Flow through velocity, vh 0.2 – 0.4 m/s (usually 0.3 m/s)

Settling velocity, vs 0.016 – n0.021 m/s for particles of 0.2 mm diameter

0.01 – 0.015 m/s for particles of 0.15 mm diameter

and specific gravity of 2.65

Liquid depth, D 1 – 1.5 m

Length, L 3 – 25 m

Quantity of grits 0.022 - 0.075 m3/1000 m3 of flow

Overflow rate 1200 – 1700 m3/m2-d (at Qpeak) (1200 – 1700)

GRIT CHAMBER

DETERMINING SETTLING VELOCITY

Where

g = acceleration due to gravity (usually assumed as 9.81 m/s2

Pw = density of water (usually assumed as 1000 kg/m3

Ps = density of solid particles

(normally of specific gravity 2.65 = 2.65 x 1000 = 2650 kg/m3

d = diameter of particles (usually assumed as 0.2 mm = 0.2 x 10-3 m

CD = coefficient of drag force depends on flow conditions

(or Reynolds number, Re)

g3C

)d4(V

wD

wss

ρ

ρρ

SKIMMING TANK (OIL AND GREASE TRAP)

Detention time 3 – 30 min.

Surface area 250 m2/m3 flow

PST as a Load Reducing Unit for Secondary Treatment System

SKIMMING TANK

PRIMARY SEDIMENTATION TANK

Dimensions

Rectangular Tank: Range Typical

Length (m) 15 – 100 30

Width (m) 3 – 30 10

Depth (m) 2.5 – 5 4

Circular Tank:

Diameter (m) 3 – 60 30

Depth (m) 3 – 5 4

Bottom slope, mm/mm 0.02 – 0.05 0.03

DETENTION TIME (Hydraulic Retention Time, HRT)

V = Q x t

(days)Q

Vt

24(hours)Q

Vt

Where

t = detention time in days

V = volume of basin in m3

Q = flow of wastewater in m3/d

Schematic Diagram for Concept of Short-circuit of Flow

FLOW THROUGH VELOCITY OR HORIZONTAL VELOCITY, Vh

t

l

taken time

travelled distance Vvelocity, throughFlow

h

The volume of the tank, V = L x B x D

The cross-sectional area of the tank, AX= B x D

time(liquid) tank of volume

tV

Q Rate,Flow The

th

VL

tL

hV

hVQ

xorA

xAQ

hV

hV

XAQ

D)BLV( tL

DBQ

SURFACE LOADING RATE (SLR) OR OVERFLOW RATE (OFR)

)

)/2

3

(m area surface

(mflow SLR

d

= settling velocity (m/d)

Therefore, numerically SLR = Vs

WEIR LOADING RATE (WLR)

d/mm(m) length weirtotal

/d)(m rateflow RateOverflow Weir 3

3

Depth of Tank D = Vs x t (Vs = SLR)

Total depth D = net depth + free board + sludge zone depth

= D + 0.3 + D1

= D+0.3+10% of D

75

THANK YOU FOR YOUR

ATTENTION