chapter 2: conventional wastewater treatmentbaiyu/engi 9605 files/lecture-2-1_updated.pdf ·...

1

Chapter 2:

Conventional Wastewater

TreatmentWinter 2011

Faculty of Engineering & Applied Science

ENGI 9605 – Advanced Wastewater Treatment

Wastewater frequently contain

suspended and floating debris varying in size

from logs to small rags those solids can

blog and damage pumps or impede flow in

open channels and pipes

Screening the first step in treating

wastewater containing large solids

2.1 Physical treatment processes

(1) Why screening

2

1. Screening

(2) Screens in wastewater treatment

Coarse bar racks (Screens) have openings ≤ 2.5 in. protect wastewater lift pumps

set in a channel inclined 22º-45º to the horizontal to

facilitate cleaning

Mechanical cleaned medium screens

have bar openings 5/8 – 1.75 in.

the maximum velocity through the openings 2.5fps

Fine screens

have bar openings 1/32 in. remove suspended and

settleable solids in pretreating certain industrial waste streams

high head loss + high installation and operation cost rarely

employed in handling municipal wastewater3

Preliminary wastewater treatment(Viessman et al., Water Supply and Pollution Control, 2009 )

4

Primary and secondary wastewater treatment

(Al-Malack, Water Supply and Wastewater Engineering, 2007 )5

(Viessman et al., Water Supply and Pollution Control, 2009 )

Coarse bar racks

6

Mechanical cleaned medium screen

(3) Design equations

Head-loss across bar screens can be represented by:

Where

hL = head loss, ft (m)

Va = approach velocity, ft/sec (m/s)

Vb = velocity through bar openings, ft/sec (m/s)

g = acceleration of gravity, ft/s2 (m/s2) (g = 32.2 ft/s2 = 9.8 m/s2)

Aa = Area of channel, ft2 (m2)

Ab = Area of screen, ft2 (m2)

Va Aa = Vb Ab

7

Example 2-1: A mechanical bar screen is to be used in an

approach channel with a maximum velocity of 1 m/s. The

bars are 15 mm thick, and the openings are 25 mm wide.

Determine:

(1) The velocity between the bars

(2) The head loss in meters

(4) Shredding

Grinders/ barminutors/ comminutors installed before

screening during wastewater treatment in some cases

Cut solids to about ¼ to ¾ in. in size

8

Mixing can be classified as

2. Mixing and flocculation

(1) Why mixing and flocculation

Continuous-rapid mixing (less than 30s) or rapid

mixing

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blending of chemicals with wastewater (e.g., the addition

of alum or iron salts prior to flocculation + dispersion of

chlorine into wastewater for disinfection)

blending of miscible liquids

addition of chemicals to sludge and biosolids to improve

their dewatering characteristics

microflocs are produced

10

Continuous mixing (i.e., ongoing)

used where contents of a reactor or holding tank or basin

must kept in suspension (e.g., in equalization basins,

flocculation basins, suspended growth biological treatment

processes, aerated lagoons, and aerobic digesters)

Flocculation one application of continuous

mixing

form aggregates or flocs from finely divided particles and

from chemically destabilized particles

follow rapid mixing where chemicals have been added to

destabilize the particles agglomerate microflocs to

larger ones that can be removed readily by settling or

filtration

Tertiary wastewater treatment processes


13

Mixing devices

(2) System design for rapid mixing

Mechanical agitators most common

(e.g., using an impeller in a tank with stator

baffles) type of impellers

Pneumatic agitators through hydraulic

action (e.g., a jet injector mixer)

Static in-line blenders inserted into a

pipe for blending normally for gas-water

contact

Turbine Impellers

Paddle Impellers

Propeller Impellers


A vertical-shaft impeller-type mechanical rapid

mixer for dispersion of chemicals into water

14

15

Turbine Impellers

Paddle Impellers(Al-Malack, Water Supply and Wastewater Engineering, 2007 )

16

Propeller Impellers

(Al-Malack, Water Supply and Wastewater Engineering, 2007 )

17


Jet injection mixer

Jet injection of chemical

into the water flow in a

pipe

Static mixing elements

that are inserted into a

pipe for in-line blending

of chemicals

18

The degree of mixing is based on the power provided

which is measured by the velocity gradient (the

difference in velocity between adjacent layers of the

fluid )2

1

V

PG

Where

G = root mean square velocity gradient, S-1

P = power input to water, ft-b/s (N-m/s)

µ = absolute viscosity of water , b-s/ft2 (N-s/m2 or

kg/m-s) μ=0.00131 N-s/m2

V = volume of tank/pipe, ft3 (m3)

19

RtQV Where

V = volume of tank/pipe, ft3 (m3)

Q = discharge rate, cfs (m3/s)

tR = the mean residence time, s

The volume of the tank or pipe containing the

water can be computed by the equation

G (Sec-1 )tR (Sec )

Typical tR and G for rapid mixing basins

Example 2-2: Chemical addition in a 25-mgd

wastewater treatment plant will be performed in two

parallel mixing basins with mechanical mixers. The

mixing intensity as measured by the root mean velocity

gradient should be a minimum of 750 s-1, and the mean

residence time should be 15 s at the design flow. Find the

volume of the tanks and the power required for the mixers.

The water temperature varies between 15 and 20 ºC

annually. Assume the mechanical mixer is 70% efficient

in transferring power to the water.

20

21


22

Mixing devices

(3) System design for flocculation

The common mechanical mixing devices paddle

(reel) flocculators and vertical-turbine mixers

The paddle flocculator consists of a shaft with

protruding steel arms that support wooden, plastic or

steel blades the paddle shaft could be located

transverse (more common) or parallel to the flow The paddle units slowly rotate (1-10 rpm) collision

among the floc particles that are held in suspension by

the agitation growth of the suspended particles from

colloids to settable floc

23


The paddle flocculator

The vertical-turbine mixer

24

The paddle flocculators in water treatment

(Al-Malack, Water Supply and Wastewater Engineering, 2007 )

25

The paddle flocculator with the paddle shaft located

transverse to the flow

The paddle flocculator with the paddle shaft located

parallel to the flow(Al-Malack, Water Supply and Wastewater Engineering, 2007 )

26

The power dissipated in water by a paddle flocculator

can be calculated as:

2

13

kACP

pd

Where

P = power dissipated, ft-b/s (N-m/s)

Cd = coefficient of drag

A = Paddle area, ft2 (m2)

ρ = density of the water, b-s2/ft4 (kg/m3)

vp = velocity of the paddle relative to water, ft/s (m/s)

K = ratio of the rotational velocity of water to the

velocity of the paddle, normally 0.25-0.50, with 0.3

being a common value

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For a paddle flocculator:

rNp 2

Where

vp = velocity of the paddle relative to water, ft/s (m/s)

r = distance from shaft to center of paddle, ft (m)

N = rotational speed, rev/s

For a flocculator tank with n symmetrical paddle arms

with blades at r of r1,r2…rj, rotating at N rev/s, the power

dissipated is:

2

211

333

j

iid rNkAnC

P

Where Ai = area of each paddle blade, ft2 (m2)

Example 2-3: A 25-mgd wastewater treatment plant has

a flocculation tank 64 ft long and 100 ft wide with a water

depth of 16 ft. The four horizontal shafts of paddle

flocculator are in compartments separated by baffles. All

the paddle units have four arms with two blades, with

radii of 3.0 and 6.0 ft measured from the shaft to the

center of the 0.80-ft-wide boards. The total length of the

paddle boards across the tank is 90 ft. Assume a ratio of

water velocity to paddle velocity of 0.3, rotational speed

of 3 rev/min, Cd equal to 1.8, and water temperature of

50F. Calculate the velocity gradient.

28

To separate solids from liquid using the force of

gravity in sedimentation, only suspended solids

(SS) are removed

3. Sedimentation

(1) Why sedimentation

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Particles pollutants with adverse impacts to

aquatic life (e.g., damage fish gills, smother coral

reefs)

Particle settling clogs river, fills up reservoirs

Particles may carry adsorbed chemicals (e.g.,

PCBs in Hudson river)

30

(2) Type of sedimentation (settling)

Type I settling (free settling)

Type II settling (settling of flocculated particles)

Type III settling (zone or hindered settling)

Type IV settling (compression settling)

In wastewater treatment plants sedimentation is

used to reduce SS in the influent wastewater and to

remove settleable solids after biological treatment

33

(3) Type I settling (free settling or discrete settling)

The gravitational force on particle (FI, kg-m/s2)

Where

ρp = particle density and ρw = water density (kg/m3)

VP = particle volume (m3) and d = particle diameter (m)

g = acceleration due to gravity (9.81 m/s2)

3

6dggVF wppwpI

It means settling of discrete (nonflocculent)

particles (e.g., settling of sand particles in grit

chamber)

In type I settling a particle will accelerate

until the drag force (FD) equals the impelling

gravitational (due to weight) force on particle (FI)

settling occurs at a constant velocity, Vs (m/s)

(2.1)

34

The frictional drag force on spherical particles (FD)

depends on the particle velocity, fluid density, fluid

viscosity, particle diameter and the drag coefficient

2

2

swpD

D

VACF

Where

CD = drag coefficient and ρw = water density (kg/m3)

Ap = particle cross-sectional area in direction of flow = ¼

π d2 (m2)

Vs = particle settling velocity (m/s)

The drag coefficient CD , as a function of the Reynolds

number (NR) takes on different values depending on the

flow regime surrounding the particle laminar (NR <1),

transitional (NR = 1-2000) and turbulent (NR > 2000)

(2.2)

35

Where µ = viscosity of water (N-s/m2)

Type I settling assuming spherical particles in a

laminar flow regime

D

SwR

C

dVN

24

Then combine equations 2.1 to 2.3 the constant settling

velocity Vs can be calculated as the Stockes’ Law:

dVN Sw

R 34.0324

RR

DNN

C and

(2.3)

DI FF

18

)( 2gdV wP

s

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Overflow velocity (V0) in a sedimentation tank

Settling time = ts = H/V0

Detention time = tR = L/V

If ts ≤ tR SS removed

Q = average daily flow, gpd (m3/d)

A = surface plan area of the sedimentation tank, ft2 (m2)

LW

Q

A

QV 0

V0 an important design criterion in the sizing of

sedimentation tanks

To get desired settling with most efficient tank size

ts = tR

Where V0 = overflow rate, gpd/ft2 (m3/m-d) or the

surface settling rate ft/s (m/s)

37(Viessman et al., Water Supply and Pollution Control, 2009 )

An ideal rectangular clarifier settling discrete particles

with an overflow rate of V0

38

Sedimentation efficiency under different H?

(Shanahan, Water and Wastewater Treatment Engineering, 2006 )

39

Example 2-4: Four rectangular clarifiers in parallel,

each 200 ft long and 40 ft wide with a water depth of 12ft,

are used to settle a total flow of 20 mgd after a

flocculation process. Calculate the overflow rate,

detention time, and horizontal velocity. Also determine

the length of effluent weir required to meet a specification

of 20, 000 gpd/ft.


40

(4) Type II settling (settling of flocculated particles)

Type I settling assume uniform settling velocity

it relatively rare in water, especially wastewater

treatment

In treatment, many particles are present as a

particle falls, it collides with other particles and

they stick together to form larger particles

Chemicals are also added to enhance

coagulation and flocculation chemicals are

added to (quickly) cause coagulation

(destabilization and initial coalescing of colloidal

particles), which then (slowly) flocculate

(formation of large particles, flocs, from smaller

particles)

41

Particles flocculate during settling thus they increase

in size and settle at a faster velocity Type II settling

Examples of Type II settling

Design of clarifier for Type II settling requires

knowledge of settling velocity distribution

A batch settling tests are performed to evaluate the

settling characteristics of flocculent suspensions

Quiescent settling analysis

Primary settling of wastewater

Settling of chemically coagulated water and wastewater

42(Shanahan, Water and Wastewater Treatment Engineering, 2006 )

Apparatus for Quiescent settling analysis

Lab apparatus is a column

with diameter ≥ 5 inches to

reduce wall effects

Apparatus has a depth of 8

to 10 ft and sampling ports

every 2 ft.

Initially suspended

sediment is poured into

column, well mixed and

allowed to settle

Samples are taken at each

port (#1-#5) with selected

time intervals C/C0

determined

C/C0 vs time and depth is

plotted on a graph

43

(Tchobanoglous et al., Wastewater Engineering, 2003)

The percentage removal at certain time t2

2222

54

5

443

5

332

5

221

5

1 RR

h

hRR

h

hRR

h

hRR

h

h

44

(5) Type III settling (zone or hindered settling) and

Type IV settling (compression settling)

Type III settling the settling of an intermediate

concentration of particles

At high particle concentration, particles are close to each

other interparticle forces hinder settling of neighbouring

particles

Mass of particles settle as a zone

Type IV settling compression settling

Particles touch each other settling occurs by

compression of the compacting mass

It occurs in the lower depths of final clarifiers of activated

sludge

45(Shanahan, Water and Wastewater Treatment Engineering, 2006 )

46

(6) Type of sedimentation tanks

Rectangular and circular settling tank

Rectangular tank usually have chain-drive scrapers

to bring sludge to withdrawal trough in tank bottom

typically 3m deep for water treatment

Circular tank inflow at center and outflow along

perimeter weir or radial collection troughs circular

rake arm to rake sludge to center (in water treatment) or

with suction pipes (in wastewater treatment)

47

Rectangular tank better hydraulic characteristic in

long and narrow settling tank



Circular tank circular sludge sweep is relatively

trouble free

49


Sedimentation tanks for wastewater treatment

After the water has been settled, some fine

solids/flocs may still be in suspension removal of

these fine solids can be achieved by filtration

Filtration follows sedimentation to separate

nonsettleable solids from wastewater by passing

through a porous medium

Filtration help to produce high-quality

wastewater effluent

4. Filtration

(1) Why filtration

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52

(2) Description of a typical granular filter system


53

During filtration, water level is 0.91 to 1.2 m above sand

water passes downward through the media water

passes into the underdrain system

Flow of filtered water flow is controlled by the rate of

flow controller (RFC) standard filtration rate is 1.36

l/s-m2 of filter bed

Influent and effluent valves are open + washwater and

waste washwater valves are closed

Particles larger than 1µm captured by sedimentation

and interception

Particles smaller than 1µm captured by diffusion

Most diffcult particles to capture about 1µm in size

Dual media provide better capture than single media


usually

sand

crushed

anthracite

garnet

Cross section of the media in a dual-media fiter

Filter media

55

Specific gravity relative density = ρmedia/ρwater

Effective size size that 90% of the grains by weight

are larger than

Uniformity coefficient measurement of variation in

particle size of filter media

= size that 60% of the grains by weight are smaller and

40% of the grains by weight are larger than / effective size

56

Classifications of filters based on the

selection of media

Single-medium filters (used in water)

have one type of medium

usually sand or crushed anthracite coal

length to width ratio is 1 : 1.5 to 1 : 2 Dual-medium filters (used in water and wastewater)

have two types of media

usually crushed anthracite and sand

almost square with length to width ratio of 1 : 1

Multi-media filters (used in water and wastewater)

have three types of medium

usually crushed anthracite, sand, and garnet

57

Underdrain system to support the filter media, collect

the filtered water and distribute the water for backwashing

and air for scouring



Undertrain system for air scouring and water backwashing

(a) cross section of the filter (b) detail of the air-water nozzle

59

(3) Head lost and backwash

Head loss caused by accumulation of

particles on top and within the depth of the

filter

Head loss through a clean granular-media

filter generally less than 3ft (0.9m)

With accumulation of impurities head

loss gradually increases until the filter is

backwashed usually at 8-10ft (2.4-3.0 m)

60

Objective of backwash to remove

accumulated particles on the surface and within

the filter medium

Backwash performed using wash water or

air scouring

During backwash, the sand bed expands bed

expansion is between 20 to 50%

Backwash flow between 10.2 to 13.6 l/s-m2

Backwash continues till the waste washwater is

relatively clear

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