Lecture 4A

Physical Wastewater Treatment

Agenda: Physical Wastewater Treatment

• overview of physical wastewater treatment

• screening

• flow equalization

• mixing

• flocculation

• sedimentation

• flotation

• aeration

4A-2

Physical Wastewater Treatment:

Overview

4A-3

Physical Unit Operations

Physical Unit Operations or Unit Operations are

operations in wastewater treatment by means of or through

the application of physical forces

Examples

• screening

• comminution

• grinding

• flow equalization

• mixing

• flocculation (agitation via mixing)

• sedimentation

• flotation

• aeration

• filtration

4A-4

Physical Unit Operations:

Applications and Devices

4A-5

4A-6

Screening and Flow Equalization

4A-7

Screening

Screening remove coarse materials from the incoming

flow stream. These materials, if not removed, could:

• damage subsequent process equipment

• reduce overall treatment reliability and effectiveness

• contaminate waterways

A Screen is a device with openings used for retaining

solids found in the influent wastewater to the treatment

plant

Materials removed are known as screenings.

4A-8

Coarse Screen, Fine Screen, & Microscreen

4A-9

Screening

• screening element may consist of parallel bars, rods or

wires, grating, wire mesh, or perforated plate

• screen openings geometry typically circular or

rectangular slots

• coarse screens with parallel bars / rods: “bar rack”

• fine screens often with perforated plates, wedgewire

elements, and wire cloth with small openings

4A-10

Types of Coarse Screens (bar racks)

hand-cleaned coarse screens

• used frequently ahead of pumps in small wastewater

pumping stations or at the headworks of small-to-

medium sized treatment plant

• often used for standby screening in bypass channels

– for service during high-flow periods

– to sub-in while mechanically cleaned screens are being repaired

– as backups in the event of power failure

mechanically cleaned bar screens

• mechanically cleaned reduce operating and

maintenance problems

• sub-divided into 4 designs/types

4A-11

4A-12

Pros & cons of

different bar screens

4A-13

Fine Screens

Fine Screens are used over a broad spectrum of

applications:

• preliminary treatment (following coarse screens)

• primary treatment (as a substitute of clarifiers)

• treatment of combined sewer overflows

• removal of solids causing clogging problems

Types of Fine Screens

• static (fixed), rotary drum, or step type

• openings vary from 0.2 to 6 mm

• headloss generally range from 0.8 – 1.4 m

4A-14

Types of Fine Screens

static wedgewire

drum

step

4A-15

Flow Equalization

Flow Equalization is the damping of flowrate variations to achieve a constant or nearly constant flowrate.

• to overcome operational problems caused by flowrate variations

• to improve performance of downstream processes

• to reduce size and cost of downstream treatment facilities

Main Applications of Flow Equalization

• dry-weather flows to reduce peak flows and loads

• wet-weather flows in collection systems experiencing inflow and infiltration

• combined stormwater and sanitary system flows

4A-16

Flow Equalization – Pros and Cons

Benefits

• biological treatment is enhanced by reducing shock loadings, inhibiting substances, and fluctuating pH

• effluent quality and thickening of secondary sedimentation tanks is improved

• effluent filtration performance is improved

• damping of mass loading improves chemical feed control and process reliability

Disadvanages

• large land areas may be needed

• equalization facilities need odor control (e.g., cover)

• additional operation and maintenance

• capital cost

4A-17

Flow Equalization – Configurations

inline arrangment

• all of the flow passes through the equalization basin

offline arrangement

• only the flow above some predetermined flow limit is

diverted into the equalization basin

• pumping requirement is minimized

• used to capture the “first flush” from combined collection

systems

4A-18

Two Approach of Flow Equalization

4A-19

Flow Equalization – Design Considerations

• where should the equalization facilities be

located?

• what type of equalization configuration should

be used?

• what is the required basin volume?

• what are the features that should be

incorporated into design?

• can the deposition of solids and potential odors

be controlled?

4A-20

Two Approach of Flow Equalization

4A-21

Mixing, Flocculation,

Sedimentation, Flotation, Aeration

4A-22

Mixing and Flocculation

Mixing: Applications

• mixing of one substance completely with another

• blending of miscible liquids

• flocculation of wastewater particles

• continuous mixing of liquid suspensions

• heat transfer

most mixing operations can be classified as continuous-

rapid (less than 30s) or continuous (i.e., ongoing)

4A-23

Common Mixers in

Wastewater Treatment

4A-24

Typical Impellers for Mixing

4A-25

Gravity Separation Theory

• removal of suspended and colloidal materials

from wastewater stream by means of

gravitational settling

• sedimentation = separation of suspended

particles heavier than water by gravitational

settling

• sedimentation basin = sedimentation tank,

clarifier, settling basin, settling tank

• accelerated gravity settling refers to the removal

of particles in suspension by gravity settling in

an accelerated flow field

4A-26

4A-27

Gravity Separation Theory

• sedimentation is used for the removal of

– grit, TSS (primary settling basins)

– biological floc (in activated-sludge settling basin)

– chemical floc (after chemical coagulation)

– solids removals (in sludge thickners)

• primary purpose of sedimentation is to produce

a clarified effluent

4A-28

Grit Removal

Grit consists of sand, gravel, cinders, or other heavy solid

materials that have subsiding velocities or specific gravities

substantially greater than those of the organic matter/solids

in wastewater

• removal of grit is often done in grit chambers or

centrifugation units

• grit chambers often follow screens but go before primary

sedimentation tanks

• removal of grit is essential ahead of centrifuges, heat

exchangers, and high-pressure diaphragm pumps

4A-29

Grit Chambers

Grit Chambers are provided to:

• protect moving mechanical equipment from

abrasion and abnormal wear

• reduce formation of heavy deposits in pipelines,

channels, and conduits

• reduce frequency of digestion cleaning caused

by excessive accumulation of grit

4A-30

Types of Grit Chambers

horizontal flow type -

• horizontal flow; flow velocity controlled by dimension of

the unit, the influent distribution gate, and a weir at the

exit

aerated type -

• spiral-flow aeration tank

• spiral velocity is induced and controlled by tank

dimension and quantity of air supplied

vortex type –

• cylindrical tank with flow entering tangentially, creating a

vortex flow pattern

• grit separation by virtue of centrifugal and gravitational

forces

4A-31

Grit Removal

4A-32

Grit Chamber:

Vortex Type

Grit Chamber:

Aerated Type

4A-33

Primary Sedimentation

Primary Sedimentation is used as a preliminary processing

for incoming wastewater

• typical primary sedimentation tanks should remove 50 –

70% of the suspended solids and 25 – 40 % of the BOD

• sedimentation tanks have also been used as stormwater

retention tanks (with a detention period of 10 to 30 min)

for overflows from combined sewers or storm sewers;

this allow a substantial portion of the organic solids to be

removed

4A-34

Primary

Sedimentation

Tank:

Rectangular Tank

top-down view

side view

4A-35

Primary

Sedimentation

Tank:

Circular Tank

center feed peripheral feed

4A-36

Sedimentation Tank Performance

Efficiency of sedimentation basins with respect to the

removal of BOD and TSS is reduced by:

• eddy currents formed by the inertia of the incoming fluid

• wind induced circulation cells formed in uncovered tanks

• thermal convection currents

• cold or warm water causing the formation of density

currents along the bottom or across the top of the tank

• thermal stratification in hot arid climate

4A-37

Flow Patterns in Rect. Sedimentation Tanks

4A-38

Short Circuiting and Hydraulic Stability

• in an Ideal Sedimentation Basin, a given block of

entering water should remain in the basin for the full

detention time

• in practice, sedimentation basins are seldom ideal and

considerable short circuiting is often observed

• to determine if short circuiting exists and to what extent,

tracer studies should be performed and time-

concentration curves by developed and analyzed

• if in repeated tests, the time-concentration curves are

similar, then the basin is stable

• if the time-concentration curves are not repeatable,

the basin is unstable and the performance of the basin

will be erratic

4A-39

Temperature and Wind Effects

• temperature effects can be significant in sedimentation

basins

• a 1oC temperature differential between incoming

wastewater and wastewater in the sedimentation tank

will cause a density current to form

• wind blowing across the top of open basins can cause

circulation cells to form, reducing the effective volumetric

capacity of the sedimentation basins

• for both the temperature and the wind effects, the

reduced performance depends on the material being

removed and its characteristics

4A-40

Design Considerations

• if all solids in wastewater were discrete particles of

uniform size, density, and shape, then the removal

efficiency of these solids would be dependent on the

surface area of the tank and time of detention

• the depth of the tank would have little influence, provided

that horizontal velocities would be maintained below the

scouring velocity

• however, solids in most wastewater are heterogeneous

in nature, and the conditions under which they are

present can range from total dispersion to complete

flocculation

• in general, weir loading rates have little effect on the

efficiency of primary sedimentation tank

4A-41

Detention Time

• bulk of the finely divided solids reaching primary sedimentation tanks is incompletely flocculated but is susceptible to flocculation

• flocculation is aided by eddying motion of the fluid within the tanks and proceeds through the coalescence of fine particles, at a rate that is a function of their concentration and of the natural ability of the particles to coalesce upon collision

• coalescence of a suspension of solids becomes more complete as time elapses, thus, detention time is a consideration in the design of sedimentation tanks

• the mechanics of flocculation are such that as the time of sedimentation increases, less and less coalescence of remaining particles occurs

4A-42

Detention Time

• primary sedimentation tanks are often designed to

provide 1.5 to 2.5 h of detention based on the average

rate of wastewater flow

• (tanks that provide shorter detention periods (0.5 to 1 h)

are sometimes used for preliminary treatment ahead of

biological treatment units)

• in cold climates, water viscosity increases with lower

temperatures, thus slowing down the settling of particles

in clarifiers an increase in detention time is necessary

to maintain the same particles removal efficiency

• at 10oC, the detention period is 1.38 times that required

at 20oC in order to achieve the same efficiency

• clarifier should be designed to ensure adequate

performance4A-43

Temperature Dependence of

Required Detention Time

4A-44

Flotation

Flotation is a unit operation used to separate solid or liquid

particles from a liquid phase

• separation is brought about by introducing fine gas

(usually air) bubbles into the liquid phase

• the bubbles attach to the particulate matter, and the

buoyant force of the combined particle and gas bubbles

is great enough to cause the particle to rise to the

surface

• particles that have a higher density than the liquid can

thus be made to rise

• the rising particles with lower density than the liquid can

also be facilitated (e.g., oil suspension in water)

4A-45

Flotation

• in wastewater treatment, flotation is used mainly to

remove suspended matter and to concentrate

biosolids

• main advantages of flotation over sedimentation are that

very small or light particles that settle slowly can be

removed more completely and in a shorter time

• once the particles have been floated to a surface, they

can be collected by a skimming operation

• the degree of removal can be enhanced through the use

of various chemical additives, changing the nature of

air-liquid or liquid-solid interface to trap air bubbles

• dissolved-air flotation is frequently used for thickening of

waste biosolids

4A-46

Dissolved-air Flotation

• flow passed through pressurized tank and pick up air at

elevated pressure

• when stream enters the flotation tank, pressurized gas is

released via bubbling

4A-47

Dispersed-air Flotation

• air is induced and dispersed into the liquid by pumping

action of the inductors

4A-48

Oxygen Transfer

Oxygen Transfer is the process by which oxygen is

transferred from the gas phase to the liquid phase.

• oxygen transfer is commonly applied in the biological

treatment of wastewater

• functioning of aerobic processes such as activated

sludge, biological filtration, and aerobic digestion

depends on the availability of oxygen

• low solubility of oxygen in water

• to transfer large quantities oxygen, need to provide

additional interfaces for transfer

• air or oxygen can be introduced into the liquid, or the

liquid in the form of droplets can be exposed to the

atmosphere

4A-49

Oxygen Transfer in Clean Water

• for a given volume of water being aerated, aeration

devices are evaluated on the basis of the quantity of

oxygen transferred per unit of air introduced to the water

for equivalent conditions (T, chemical composition of the

water, depth at which the air is introduced, etc.)

• accepted test method involves the removal of dissolved

oxygen (DO) from a known volume of water by the

addition of sodium sulphite followed by reoxygenation to

near the saturation level. The DO of the water volume is

monitored during the reaeration period by measuring DO

concentration at several different points / sampling

locations selected to best represent the contents of the

tanks

4A-50

Aeration Systems

4A-51

End of Lecture