chapt7 primarytreatment(primarysedementation)[1]

cmutsvangwa:, Wastewater Engineering, Dept. of Civil and Water Eng., NUST 10/5/2006 7-1

Chapter 7

PRIMARY TREATMENT (PRIMARY SEDIMENTATION) Primary treatment Primary treatment marks the first stage in BOD removal and about 40% of BOD loading is removed. Primary treatment is accomplished in sedimentation tanks. The main objectives are:

to settle suspended matter under the action of gravity to form primary sludge. This relieves the load on subsequent treatment processes. Approximately 2/3 of suspended solids are removed.

to reduce the volumes of high water content in secondary sludges. Secondary sludges are difficult to dewater

to remove BOD and approximately (about 1/3 BOD is removed in primary sedimentation). tanks.

Sedimentation Downward movement of small suspended particles by gravity. Sedimentation is classified upon the characteristics and concentration of suspended materials:

discrete particles flocculent dilute suspension

Discrete particles (Type 1) Particle whose size, shape and specific gravity do not change with time i.e. non-interactive settling of particles from a dilute suspension. Examples are grit and sand, and their mass is constant.

Dep

th

Flocculent particle

Settling path of discrete particle

Time

Fig 1 Settling paths of discrete particles

Chapter 7: Primary treatment_ sedimentation

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Flocculent particles (Type 2) Particles which agglomerate (coalesce/flocculate) during settling i.e. no constant characteristics. Their mass varies during the process of settling and an increase in mass causes a faster rate of settlement. Dilute suspension Concentration of particles is not sufficient to cause significant displacement of water as they settle. Hindered particles Or called zonal settlement, the particles interact and the concentration of particles is high. Settlement is slow because as particles move down (large in numbers), water is displaced upwards hindering downward settlement. Settling velocities of particles The settling velocities for can be determined from Newton or Stokes laws. Newtons law is for all regimes of flows and Stokes law is for laminar flow. Newtons law is given as:

( )wd

wss C

gdv

3

4 = Or ( )134 = s

ds SC

gdv (For perfect spherical

spheres i.e. shape factor, =1). The shape factor accounts for the irregularity of the particle.

Specific gravity w

ssS

= Where: Cd =Newtons drag coefficient Ac =cross-sectional area of particle perpendicular to the direction of motion vs -settling velocity of particle The coefficient of drag varies with shape and the regime of flow which is defined by the Reynolds number, Re:

For laminar flow, Re1 Re24=dC

Transitional flow, 1103 Cd=0.4 and

dv ws=Re or dvs=Re

Where; =shape factor


2


d =diameter of particle

For laminar flow, Newtons law becomes ( )18

12 = ss Sgdv (which is Stokes Law)

Or ( )

18

2ws

sgdv = ( )18

2ws

sSSgdv =

Where; Ss =specific gravity of particle Sw =specific gravity of liquid To use the above equations for nonspherical particle, the diameter d must be the diameter of equivalent spherical particle. The volume of the equivalent spherical particle:

23

234

sphernonsphere ddV =

= spherenondd = 33.024.1 Estimate values for the shape factors (Sincero, 1996) are in Table 1: Table 1; Estimated values of shape factors

Material shape factor ( ) Angular sand 0.64 Sharp sand 0.77 Worn sand 0.86 Perfect sphere 1.0

In wastewater treatment flow is usually laminar and transitional, but the sphericity is not always 1, i.e. particles not always spherical. The effects of irregular shape are not pronounced in low settling velocities. This suite most sedimentation processes because they are designed to remove small particles which settle slowly.

==

,

,cos,cosdensitymass

ityvisabsoluteityvisKinematic

Therefore dynamic viscosity, = Example 1 Determine the terminal settling velocity for a sand particle with an average diameter of 0.5mm and a density of 2600kg/m3 settling in water at 20oC.


3


Solution: 1 Determine the terminal settling velocity using Stokes Law:

2 Check the Reynolds number and assuming =0.85 for sand.

NR =93.2 3 Because the Reynolds number is greater than 1, then it should be computed

from the equation: 34.0Re3

Re24 ++=dC

4




5

Example 2 Determine the terminal settling velocity of a discrete spherical particle having a diameter of 0.6mm and specific gravity of 2.65. T=22oC.


Example 3 Determine the terminal settling velocity of a discrete worn sand particle having a measured diameter of 0.6mm and specific gravity 2.65. T=22oC. Solution


6


7

ept of

THEORETICAL DESIGN OF SEDIMENTATION BASINS Theoretical design of sedimentation process is generally based on the concthe ideal settling basin. A particle entering the basin will have a horizontal velocityequal to the velocity of fluid. Rectangular basins

Fig. 2: Rectangular horizontal flow sedimentation tank



Fig. 3: Inlet and sludge removal mechanisms

Fig. 4 Typical inlets for sedimentation tanks Chapter 7: Primary treatment_ sedimentation

8


9

Fig. 5 Rectangular basin

Speed of horizontal flow, DB

Qarea

QvH ==

Time of horizontal flow, veocity

travelofceDisTHtan=

QDBL

DBQLTH

=

=

Settling zone

Outlet zone

Inlet zone

vs

D

vH

Area

B

D

Time for falling the depth, D



sD v

D

veloctysettling

DepthT ==

But time of fall = horizontal flow time:

DH TT =

Q

DBLVD

s

=

AQ

BLQVs == =surface loading rate (m

3/m2.day)

BLA = =surface area of the tank

Therefore the depth of a basin is not a factor in determining the size particle that can be removed completely e determining factor is the urface loading, which correspond to the terminal settling velocity of particles that

are 100% removed in the basin.

Design principles

rectangular tanks should be long and narrow incoming flocs should be distributed uniformly over the width and depth of outer weir should be wide enough to reduce high velocities. rectangular

tanks are hydraulically more stable and control of large volumes of water is easier

more compact than radial flow tanks (circular tanks) limited length of outflow weir available complicated weir arrangements e.g. 1/3 of tank length

arge treatment plants with large flows

in the settling zone. Ths

Area

L

B

tank

they are commonly used for l Chapter 7: Primary treatment_ sedimentation

10


11

particle removal independent of tank depth they have uniform flow between inlet and outlet less susceptible to flow disturbances efficient use of the basin volume may be obtained by subdividing it vertically

increases the surface are of basin and tube setters or plate settlers).

esign parameters

Retention time It is equal to the volume of water in tank divided by the flow rate

by addition of horizontal trays. This reduces surface overflow rate (

provide at least two tanks D

( )flowofrateQ

kofvolumecapacityDBLT tan==

ing flow divided by the plan area (effective surface area)

2 to 4 hrs for discrete particles 4 to 6hrs for flocculent particles

Surface loading Incom

AQ

BLQ =

low rate maximum rate if flow pe

Weir overf

r unit length of outlet weir

( )( )mweirsoflengthtotaldaymflowimumWOR /max

3

= , (m3/m.day)

ave the disadvantage of having a small weir length. If the weir ry high, this may result in excessive velocities at the outlet.

The e, causing particles and flocs wh Recommended values are in Table 2, but generally range fro 6m3/h.m to 14m3/h.m. It m

Surface dimensions to 20 times the depth

ent

Rectangular tanks hoverflow rate is ve

se velocities extend back into the settling zonich would have otherwise be removed as sludge to be drawn into the outlet.

aybe necessary to install inboard weir designs (Fig.6).

WL L=2 to 4 times width or 10:Lmax =48m, Lmin =12m for ease use of equipmDepth for discrete particles =2.5 to 5m



12

/hr for Flocculant particles

Table

Potable water (rectangular

Wastewater grit removal

Wastewater primary settlement

Depth for flocculate particle =3 to 4m

Horizontal velocities vH


13

Cir flow is radial, and it enters at the centre and is baffled to the periphery

inually decreases towards the perimeter, resulting in the change of the absolu ett herefore the particles follow a parabolic path as opposed to the straight line in rectangular

ks short circuit result also from the uneven distribution of velocities and also as

a result of wind currents flow control is difficult com o rectan weir overflow rates not a the entire circum sed for

overflow to prevent extremely thin sheets of water from being drawn off, overflow

ks a ed. T sist of V of metal plates, which reduce the effective overflow area. The plates should be level

g sludge removal simpler and requires less maintenance

w disturb

cular tanks

horizontal velocity contte s ling velocity of a particle. T

tan

pared tproblem since

gular tanks ference is u

weirs on circular tan re install hey con -notches

to prevent short circuitin

occupy more land susceptible to flo ances

Fig. 5: Circular radial flow sedimentation tanks



ular sedimentation basin for city to treat 15000m3/day of water. An ove o Soluti

Design a rectangrfl w rate of 20m/day will produce satisfactory removal at a depth of 3.5m.

on

Com 3

1 pute surface area and provide two tanks at 7500m /day.

Chapter 7:

14

Primary treatment_ sedimentation


15

olution

Using the same data, design circular tanks. S

Fig.6 Inboard weir arrangement to increase the weir length



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s 1. Casey T. J, Unit treatment processes in water and wastewater engineering 2. Droste R., (1997), Theory and Practice of Water and Wastewater Treatment,

Reference

John Wiley, UK 3. Metcalf and Eddy, (1995), Wastewater engineering, treatment, disposal and

reuse, McGraw Hill, New York, USA 4. Schroeder E.D., (1971), Water and wastewater treatment, McGraw Hill, New

York, USA 5. Sincero A. G., (1996), Environmental Engineering: A design approach,

Prentice Hall, New Delhi 6. Peavy H. S., Rowe D. R., and Tchobanoglous G., (1985), Environmental

Engineering, McGraw Hill, New York, USA 7. Tchobanoglous and Schroeder Water Quality 8. Tebbutt T.H., (1977), Principles of water quality control, Pergamon Oxford,

UK


chapt7 primarytreatment(primarysedementation)[1]

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