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CHAPTER ONE

INTRODUCTION

DEFINITIONS
Term

Definition

Reaction
order

An empirical coefficient used to describe the relationship
between reaction rate and concentration for a compound in a
given reaction

Reactor

A vessel or tank in which physical, chemical, biological
reactions take place

Sludge

Any material produced during primary, secondary or
advanced wastewater treatment that has not undergone any
process to reduce pathogen or vector attraction

Steady state The condition that occurs when the concentration of a
constituent does not change over time
Temperatur A coefficient used to correct reaction rate constants for the
e coefficient effect of temperature

Term

Definition

Stoichiometry

The proportions in which chemical elements combine or are
produced and the weight relations in any chemical reaction

Storm water

Part of the precipitation that appears in surface streams
...
Various synonyms such as
municipal wastewater, sewage, etc
...


Wastewater
treatment

The removal of constituents so that the treated effluent can
be returned to the environment or reused safely
...


Unit process

A subset of complete treatment process in which constituent
conversion or removal is accomplished by physical forces such as
gravity sedimentation or by chemical or biological reactions
...

• Wastewater when stored goes septic due to decomposition of organic
matter and creates nuisance conditions
...

• Wastewater also contains nutrients which can stimulate aquatic
growth
...

• For these reasons immediate removal after generation followed by
treatment, reuse or disposal into the environment is necessary to
protect public health and the environment
...

• Confirm to national / regional standards and regulations
...

• The need to avoid discharge of nutrients (that promote aquatic growth)
and trace constituents
...

• Inefficient aeration system design
...

• Limitation is sludge reuse and ultimate disposal option
...

• Total maximum daily load: The total amount of pollutant load a given
water source (ex
...

• Air emissions: VOCs are of concern because they can be carcinogenic
...

• The HAPS regulation is specified in 40 CFR61
• OSHA Act: Safety provisions to be included in facilities design
...

• Air quality standards: for the regulation of air emissions from treatment
facilities
...


CLASSIFICATION OF WASTEWATER TREATMENT METHODS
• Constituents found in wastewater are removed by physical, chemical
and biological means
...

• Physical unit processes may involve screening, mixing, flocculation,
sedimentation, flotation, filtration and adsorption
...

• Chemical unit processes may include: precipitation, gas transfer,
adsorption, and disinfection
...
Substances are converted to gas (carbon dioxide that escapes to
atmosphere) and cell tissue that can be settled out be sedimentation
...

• Primary refers to the application of physical unit processes
...

• Tertiary refers to the combination of all the three processes
(physical, chemical and biological processes)
...

• Mass (Material)-balance analysis is used to describe that take place in
a reaction
...


PREPARATION OF MASS BALANCES
• Prepare a simplified schematic flow diagram of the system
or process
...

• List all pertinent data and assumptions to be used in
preparation of the material balance
...


DEFINITION, REPRESENTATION AND BOUNDARY LIMITS OF A
COMPLETE MIX REACTOR

• Simplified word statement:

Accumulation = Inflow - outflow + generation
• Symbolic representation:
𝒅𝑪
𝑽 = 𝑸𝑪 𝟎 − 𝑸𝑪 + 𝒓 𝒄 𝑽
𝒅𝒕
• Substituting –kC for rc yields:
𝒅𝑪
𝑽 = 𝑸𝑪 𝟎 − 𝑸𝑪 − (𝒌𝑪)𝑽
𝒅𝒕
𝒅𝑪
• Steady state simplification: ( = 𝟎)
𝒅𝒕

0 = 𝑸𝑪 𝟎 − 𝑸𝑪 − (𝒌𝑪)𝑽
• Solving for rc one gets
𝑸
𝒓𝒄 =
𝑪 − 𝑪𝟎
𝑽

REACTORS USED IN WASTEWATER TREATMENT
• Batch reactor: Flow enters is treated and then discharged and the
cycle repeats
...

They are often used to blend chemicals or dilute concentrated
chemicals
...

• Complete mixing can be accomplished in round square reactors
...


PLUG FLOW REACTOR
• Water particles pass through the reactor with little or no longitudinal
mixing and exit the reactor in the same sequence that they enter
...

• This type of flow is approximated in long open tanks with a high length
to width ratio in which longitudinal dispersion is minimal or absent
...

• The flow can be operated in either down flow or up flow mode
...
It can also
be arranged in multiple stages
...

• The expanded porosity of the fluidized bed packing medium can be
varied by controlling the flow rate of the fluid
...

• In homogenous reactions, the reactants are distributed uniformly
throughout the fluid so that the potential for reaction at any point
within the fluid is the same
...

• Examples of irreversible reactions are given below (1,2 and 3):

RATE OF REACTION

Specific reaction rate
………
...


𝒅 𝐥𝐧(𝒌)
=
𝒅𝑻

𝑬
𝑹𝟐

• After integration between T1 and T2 ;

𝑳𝒏

𝒌𝟐
𝒌𝟏

=

𝑬 𝑻𝟐− 𝑻𝟏
𝑹𝑻 𝟏 𝑻 𝟐

• K = Reaction rate constant at temperature T;
• T = kelvin temperature = 273
...
314 J/mole
...
99 cal/mole
...


𝒌𝟐
𝒌𝟏

𝑳𝒏

= 𝑪 𝑻𝟐 − 𝑻𝟏

𝒌𝟐
= 𝒆𝑪
𝒌𝟏
𝒌𝟐
𝒌𝟏

•

=

𝜽

𝑻𝟐− 𝑻𝟏

𝑻𝟐− 𝑻𝟏

where  = eC

Values of  vary between 1
...
1 for some biological systems

REACTION ORDER

HETEROGENEOUS REACTIONS
• Occur between one or more constituents that can be
identified with specific sites such as ion exchange resins
...

• Heterogeneous reactions are usually carried out in packed
and fluidized bed reactors
...

• Some reactions are the slowest and hence determine the
overall rate of reaction
...
Transport of reactants from the bulk fluid to fluid solid
interface (external surface of catalyst particle)
...
Inter-particle transport of reactants into the catalyst
particle (if it is porous)
...
Adsorption of reactants at interior sites of the catalyst
particle
...
Chemical reaction of adsorbed reactants to adsorbed
products (surface reaction)
...
Desorption of adsorbed products
...
Transport of products from the interior sites to the outer
surface of the catalyst particle
...

• The transfer is by molecular diffusion through concentration gradient

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