Lecture Notes CHEM4012 Lecture 4 2013 2014

CHEM4012-Ahmed Al-Dallal/2013-2014

CHAPTER FOUR Preliminary and Primary Treatment processes

Part-3

Flotation, Aeration & Chemical Treatment

PART- A

Flotation


Flotation General

Driving force is difference in specific gravity

In settling, particle SG>1, so particles sink

In flotation, particle SG


Typical Flowsheet of DAF Clarification

Dispersed Air Flotation


Dissolved Air Flotation Process

without Recycle

Dissolved Air Flotation Process with

Recycle


Flotation Design Analysis

Mass Balance

Determine the goal usually a design equation

A/S - Ratio of mass air release rate to net solids application rate

as basis for design/operation

Define system boundary

Look for inputs, outputs and reactions

State Assumptions

Steady state

Solids volume


Mass Balance

Mass rate of air in, dMain/dt


Mass rate of air out, dMaout/dt

Mass Balance


The Goal: A/S

Dissolved Air Flotation Process

without Recycle

An analysis similar to that conducted for

the recycle case gives:


Rules of Thumb

A/S = 0.005 to 0.06 mL air/mg solids

P = 275 to 350 kPa (40 to 50 psi)

R/Q = 0.15 to 1.2

Surface loading = 0.2 to 1.5 L/m2-min (0.5 to 4 gpm/ft2)

Solids loading = 0.1 to 1 kg/m2-hr (0.5 to 5 lb/ft2-hr)

Detention time = 10 to 60 min.

Pilot Testing 1/2

Refer to Fig. 5-55 (pp. 422)

Pressure tank water volume equivalent to R

Graduated cylinder wastewater volume equivalent to Q

Pressurize and shake

Discharge into graduated cylinder containing wastewater

Underflow concentration after desired detention time gives clarification

performance

Float consistency after desired detention time give thickening performance

Solids/water interface velocity give surface loading rate

Repeat for several R/Q (i.e. several A/S)

Different series at various P, detention times


Pilot Testing 2/2

Flotation versus Settling


Example 5-12 1/2

(mg/mg)

Gauge

Example 5-12 2/2

.d

(mg/mg)


Example 5-12 3/2

(mg/mg)

(461.9+400) 74.8 m2

PART-B

Aeration


Aeration provides oxygen to the aerobic microorganisms

degrading organic materials in the wastewater treatment

system.

Solubility of Gases -

equilibrium:


Two Film Theory

Oxygen diffuses faster through the gas film than

the water film so the water film is limiting and:


Diffusion and Mass Transfer 1/4

Diffusion and Mass Transfer 2/4 From 2 film theory:


Determination of KLa

Experimental

System specific so must simulate field conditions

Reduce initial D.O.

Chemical addition, bubble w/N2, etc.

Record temp., atm. Pressure

Start aeration

Record C, t

Determination of KLa Linear

Regression


Determination of KLa Nonlinear

Regression (ASCE Standard)

Factors Affecting K La 1/2

1. Bubble size

2. Mixing or Turbulence

increased mixing shears the liquid film so thickness, DL, decreases.

Since KL = DL/ L, increasing mixing increases KL, and therefore KLa, if other factors

remain constant.

3. Waste Composition

Dissolved solids that affect surface tension can affect L. Usually effect is reduction in

L so KLa increases.

Large molecules can impede diffusion, reducing DL, and, therefore, reducing KLa.

La by reducing L, or reduce it by reducing DL.

Empirical correction factor for 1-3:


Factors Affecting K La 2/2

4. Temperature

As temperature increase, DL, increases.

Arrhenius style adjustment

= temperature correction coefficient

= 1.01 to 1.03

M&E uses the typical value of 1.024

Oxygen Transfer Capacity

Mass O2/unit energy input

lb O2/hp-hr or kg O2/kWh

Directly related to Kla

Used in design

If know required rate of O2 supply, RO2V (e.g., lb O2/hr)

Then RO2V/N = required aeration power

-hr ) = hp

Manufacturer usually supplies capacity under standard test

conditions, No

Must correct to field conditions


Designers must correct this stated capacity

to that expected under field conditions.

The Corrections


Different D.O. Conditions Cwalt

Oxygen solubility correction factor

versus elevation


Example

PART-C

Chemical Treatment


Chemical Treatment

Neutralization

Coagulation/flocculation

Precipitation

Chemical oxidation

Chemical reduction

Neutralization

PROBLEMS

(highly) variable pH

usually little or no buffering

pH for biological treatment = 6.5 to 8.5

OPTIONS

Mix acidic and alkaline waste streams

Lime or MgOH slurries or NaOH for acidic wastewater

H2SO4, HCl solutions or CO2 for alkaline streams

Multistage neutralisation for strong wastes


Titration Curve

Coagulation & Flocculation


Coagulation & Flocculation

Removal of suspended solids and colloids

Colloids: particles of 0.1 - 1 nm size

do not settle in sedimentation

often electrostatically charged surface

most WW colloids have negative charge

coagulation with high-valence cations

hydrophobic colloids (clays etc) have limited

stability in aqueous media

easily coagulated with electrolytes

hydrophilic colloids (eg proteins) have high affinity to water

(solvation envelope)

coagulation more difficult and slow

Suitable chemicals and conditions difficult to predict

ww

Parameters to optimize:

pH (usually around 7)

coagulant dose

poly-electrolyte dose

time and stirring speed

for rapid mixing and slow stirring (flocculation)


Typical coagulants & poly-electrolytes

Rapid Mixing and Flocculation Rapid mixing is used to:

particles

Microflocs are produced

Flocculation is used to:

microflocs to larger ones


Devices

Agitation in rapid mixing and flocculation is

performed by:

Design1/8


Design2/8

Design3/8

Velocity Gradient

The rate of particle collision G

Shear force G

Total number of particle collisions GT

Rapid Mixing

Mixing devices

Detention time

Types of impellers


Design4/8

Mixing Devices

Design5/8


Design6/8 Rotary Mixing

Rotary mixing devices can be

Turbine Impellers

Design7/8


Design8/8

Example Rapid Mixing

A square rapid-mixing basin, with a depth of water equal to 1.25

times the width, is to be designed for a flow of 7570 m3/d. The

velocity gradient is to be 790 mps/m, the detention time is 40

seconds, the operating temperature is 10 C, and the turbine shaft speed is 100 rpm. Determine:

The basin dimensions

The power required


Example Rapid Mixing

Precipitation

Main focus on heavy metals due to:

toxicity

accumulation in biosphere

main concern in urban/industrial areas

Precipitate as metal hydroxide or sulfide

Theoretical solubility of most soluble species

solubility constant

pH

concentration of precipitating agent


Solubility

Diagrams

Achievable Heavy Metal Removal

Documents

Lecture Notes CHEM4012 Lecture 4 2013 2014