Physical water/wastewater

treatment processes

Tentative schedule (I)

• Week 1: Introduction

• Week 2: Overview of water/wastewater treatment processes

• Week 3: Major contaminants (Chemicals and pathogens)

• Week 4: Preliminary treatment (Screen)

• Week 5: Preliminary treatment (Grit Chamber)

• Week 6: Sedimentation 1

• Week 7: Sedimentation 2

• Week 8: Mid-term

Sedimentation

Classification of settling

• Based on the concentration of the particles and

the ability of the particles to interact

• Type 1: discrete, nonflocculent particles in a

dilute suspension

• Type 2: flocculent particles in a dilute

suspension

• Type 3: intermediate concentration of particles,

considerable interparticle forces

• Type 4: high concentation of particles, significant

interparticle forces (compression)

Type III settling

• Also called zone or hindered settling.

• The settling of an intermediate concentration of

particles in which the particles are so close

together that interparticle forces hinder the

settling of neighboring particles

• The mass of particles settle as a zone.

• Example: the settling that occurs in the

intermediate depths in a final clarifier for the

activated sludge process

Type IV settling

• Also called compression settling

• The settling of particles that are of such a high

concentration that the particles touch each other

and settling can occur only by compression of

the compacting mass.

• Example: the compression settling the occurs in

the lower depths of a final clarifier for the

activated sludge process

Wastewater treatment plant

(typical municipal wastewater)

Settling of a concentrated

suspension

Designing of final clarifiers for

the activated sludge processes

• Based on solids flux concept

– The rate of solids thickening per unit area in plan view (kg/h-m2)

• Gs = CtVt

– Where Gs = solid flux by gravity

– Ct = solid concentration

– Vt = hindered settling velocity

• Gb = CtVb

– Where Gb = bulk flux

– Vb = bulk velocity

• Total solid flux (Gt) for gravity setting and bulk movement

– Gt = Gs + Gb = CtVt + CtVb

Designing of final clarifiers for

the activated sludge processes

Designing of final clarifiers for

the activated sludge processes

• Vb = Qu/A

– Where Vb = bulk velocity

– Qu = flowrate of the underflow

– A = plan area of the tank

• Mt = Q0C0 = QuCu

– Where Mt = mass rate of solids settling

– Q0 = influent flowrate of the tank

– C0 = influent solids concentration

– Cu = underflow conc.

• AL = Mt/GL = Q0C0/GL

– AL = limiting cross-sectional area

– Where GL = limiting flux

Designing of final clarifiers for

the activated sludge processes

• Finally,

• Vb = Qu/A = Mt/CuA =

GL/Cu

– Qu = Mt/Cu

– Mt/A = GL

Designing of final clarifiers for

the activated sludge processes

Example 1: Final clarifier

• Batch settling tests have been performed using an

acclimated activated sludge to give the data in Table 9.6

• The design mixed liquor flow to the final clarifier is 160

L/s, the MLSS is 2500 mg/L, and the underflow

concentration is 12,000 mg/L. Determine the diameter of

the final clarifier.

Example 1. Analysis

Actual sedimentation basins

(circular)

• Inlets: center or on the periphery

– Center: < 9.14 m in diameter: downward flow

– Center: > 9.14 m in diameter: upward flow

• Outlet: weir

• The depth of a circular clarifier: the depth

at the side of the tank (swd)

Actual sedimentation basins

(circular)

Actual sedimentation basins

(circular)

Actual sedimentation basins

(Circular)

Actual sedimentation basins

(circular)

Theoretical vs. actual

retention time

• Actual retention time is affected by

– Dead spaces in the basins

– Eddy currents

– Wind currents

– Thermal currents

• If there are dead space, Mean

t/Theoretical t < 1

• If short circuiting is occuring, Mean

t/Median t < 1

Settling basin and tracer studies

Main design criteria

• Overflow rate (design settling velocity)

• Detention time

• Depth

Sedimentation

in water treatment plants (I)

• Two types: Plain sedimentation and sedimentation for

chemically coagulated water

• Plain sedimentation

– Water with high turbidity (due to silt)

– Very long detention time (30 days) and extremely large volume

• Sedimentation for chemically coagulated water

– Determining factors: the characteristics of the water, the

coagulant used, and the degree of flocculation

– The main design parameters (settling velocities, the required

overflow rates, and detention times) should be determined only

by experimental settling tests

Batch settling test (procedure)

• Samples are removed at

periodic time intervals and the

suspended solids

concentration are determined

• The percent removal is

calculated for each sample

and plotted on a graph as a

number versus time and depth

of the collection

• Interpolation are made

between the plotted points and

the curves of equal percent

removal are drawn

Sedimentation

in water treatment plants (II)

• Water coagulated with alum

– Produce light flocs

– Overflow rate: 20.4 to 32.6 m3/d-m2

– Weir or orifice channel loading: 149 to 224 m3/d-m2

– Detention time: 2 to 8 hours (4 to 6 hours common)

• Water coagulated with iron salts

– Produce dense flocs

– Overflow rate: 28.6 to 40.8 m3/d-m2

– Weir or orifice channel loading:199 to 273 m3/d-m2

– Detention time: 2 to 8 hours (4 to 6 hours common)

Sedimentation

in water treatment plants (III)

• Water after softening

– Overflow rate: 28.6 to 61.2 m3/d-m2

– Weir or orifice channel loading:273 to 323

m3/d-m2

– Detention time: 4 to 8 hours

Example 2: Clarifier for water

treatment

• A rectangular clarification basin is to be designed for a

rapid sand filtration plant. The flow is 30,000 m3/day, the

overflow rate or surface loading is 24.4 m3/d-m2, and the

detention time is 6 h. Two sludge scraper mechanisms

for square tanks are to be used in tandem to give a

rectangular tank with a length to width ratio of 2:1.

Determine the dimensions of the basin.

Sedimentation in wastewater

treatment plants

• Primary sedimentation:

– To remove settleable solids from raw wastewaters

• Secondary settling procedure

– To remove the MLSS (activated sludge)

– To remove any growths that may slough off the filter

(trickling filters)

• Sedimentation for chemically coagulated water

– To remove flocculated suspended solids (advanced

or tertiary wastewater treatment plants)

Wastewater treatment plant

(typical municipal wastewater)

Water treatment plant

(typical surface water)

Batch settling test (procedure)

• Samples are removed at

periodic time intervals and the

suspended solids

concentration are determined

• The percent removal is

calculated for each sample

and plotted on a graph as a

number versus time and depth

of the collection

• Interpolation are made

between the plotted points and

the curves of equal percent

removal are drawn

Designing of final clarifiers for

the activated sludge processes

• Finally,

• Vb = Qu/A = Mt/CuA =

GL/Cu

– Qu = Mt/Cu

– Mt/A = GL

Recommended criteria

for primary clarifier (I)

Recommended criteria

for primary clarifier (II)

• Detention time: 45 min to 2 hr

• Multiple tanks should be used when the

flow > 3.8MLD

• Peak weir loading

– < 248 m3/d-m (< flow of 3.8 MLD)

– < 373 m3/d-m (> flow of 3.8 MLD)

• BOD5 removal

– Correlated to detention time and overflow rate

BOD5 removal vs. overflow rate

BOD5 removal vs. retention time

Example 3: Primary clarifier

• A primary clarifier for a municipal wastewater treatment

plant is to be designed for an average flow of 7570 m3/d.

The regulatory agency criteria for primary clarifiers are

as follows: peak overflow rate = 89.6 m3/d-m2, average

overflow rate = 36.7 m3/d-m2, minimum side water depth

= 3.0 m, and peak weir loading = 389 m3/d-m. The ratio

of the peak hourly flow to the average hourly flow is 2.75.

Determine:

1. The diameter of the clarifier

2. The peak weir loading if peripheral weirs are used.

Is it acceptable?

Recommended criteria

for secondary clarifier

Suggested depth

for final clarifiers

Recommended criteria

for final clarifier

• Detention time: 1.0 to 2.5 hr

• Multiple tanks should be used when the

flow > 3.8MLD

• Peak weir loading

– < 248 m3/d-m2 (< flow of 3.8 MLD)

– < 373 m3/d-m2 (> flow of 3.8 MLD)

Designing of final clarifiers for

the activated sludge processes

Example 4: Final clarifier

• A final clarifier is to be designed for an activated sludge treatment

plant serving a municipality. The state’s regulatory agency criteria

for final clarifiers used for activated sludge are as follows: peak

overflow rate = 57.0 m3/d-m2, average overflow rate = 24.4 m3/d-m2,

peak solids loading = 244 kg/d-m2, peak weir loading = 373 m3/d-m,

and depth = 3.35 to 4.57 m. The flow to the reactor basin prior to

junction with the recycle line = 11,360 m3/day. The maximum

recycled sludge flow is 100% of the influent flow and is constant

throughout the day. The MLSS = 3,000 mg/L, and the ratio of the

peak hourly influent flow to the average hourly flow is 2.50.

Determine

1. The diameter of the clarifier

2. The depth of the clarifier

3. The peak weir loading if peripheral weirs are used. Is it

acceptable?

Inclined-settling devices

• To increase overflow rates (3-6 times)

• To increase the capacity of existing

clarifier

• Can be used both circular and rectangular

clarifier

Inclined-tube settler

Inclined-tube settler on a

circular clarifier

Inclined-tube settler on a

rectangular clarifier

Inlet and outlet hydraulics

(a rectangular tank) (I)

• Inlet (an orifice flume)

– The discharge from the most distant orifice

from the influent pipe be at least 95 % of the

that from the most closest orifice

– Q = 0.6 A 2𝑔ℎ

• Where Q = discharge from an orifice (m3/s)

• A = orifice area (m2)

• h = head loss (m)

• g = gravitational acceleration

Inlet and outlet

for a rectangular tank

Inlet details

Outlet details

Inlet and outlet hydraulics

(a rectangular tank) (II)

• Outlet: suppresed weir or V-notched weir

• Suppressed weir

– Q = 1.84LH3/2

• Where Q = discharge (m3/s)

• L = weir length (m)

• H = head (m)

• V-notched weir

– Q = 1.40H5/2

Inlet and outlet hydraulics

(a rectangular tank) (III)

• H0 =√(𝑑2 +2𝑄2

𝑔𝑏2𝑑+

2

3𝑛2𝐿𝑄2

𝑏2𝑟4/3𝑑𝑚

)

– Where H0 = upstream water depth (m)

– d = downstream water depth (m)

– Q = total discharge (m3/s)

– b = channel width (m)

– n = Manning’s friction factor

– L = channel length (m)

– r = mean hydaulic radius (m)

– 𝑑𝑚 = mean depth (m)

Inlet and outlet hydraulics

(a rectangular tank) (IV)

• 𝑑𝑚 = H0 – 1/3(H0 –d)

• r = 𝑑𝑚b/2 𝑑𝑚+b

• The minimum value of d = critical depth

(yc) = (q2/g)1/3 where q = the discharge per

unit width of channel

Example 5

• A rectangular setting tank basin is 21.33 m wide by 42.67 m long

and has a flow of 0.2146 m3/s. The inlet flume is an orifice channel

with eight orifices that are 216 mm in diameter, each with an area of

0.0366 m2. The difference in the elevations of the water surface at

the influent pipe (that is, the center of the flume) and at the last

orifice in the flume is 0.0061 m/ This head loss is due to the friction

and form loss in the flume. The effluent weir plate consists of 90 C

V-notch weirs spaced at 203 mm centers. The effuent channel is

48.77 m in length and extends across the downstream end of the

basin, which is 21.33 , and 13.72 m upstream along each side. The

effluent channel is rectangular in cross section and is 0.533 in width.

There is a 102 mm freefall between the crests of the V-notch weirs

and the maximum water depth. The Manning friction coefficient is

0.032, and there is a freefall at the effluent box. Determine,

• 1. The ratio of the flow from the last influent orifice to the flow from

the influent orifice nearest to the influent pipe

• 2. the head loss on the V-notch weirs