What is Left in a WWTP

1
What is Left in a WWTP?
2
Disinfection

• Disinfection Selective destruction of
  disease-causing organisms.
• In wastewater treatment treatment concerned with
  some bacteria, viruses and amoebic cysts
  (Entamoeba histolytica, which can occur in cysts-
  surrounded by protective wall)
• To prevent waterborne diseases such as
  typhoid,cholera, bacillary dysentery.

3
Methods for Disinfection

• Chemical agents Chlorine and its compounds,
  bromine, iodine, ozone, phenol, alcohols, soaps
  and detergents, hydrogen peroxide, acids, bases.
• The most common disinfectants are oxidizing
  chemicals and the most commonly used in North
  America is Chlorine

4
Methods for Disinfection

• Physical Agents Heat (boiling), UV. The use of
  UV systems has been increasing dramatically in
  the last few years. This technology will tend to
  substitute chlorination in wastewater (and water)
  treatment.
• Mechanical Means For example filtration,
  ultrafiltration, nanofiltration

5
Mechanisms of Disinfection

• Damage of cell wall
• Alteration of cell permeability (phenol)
• Alteration of colloidal nature of protoplasm
  (heat)
• Inhibition of enzyme activity (chlorine or any
  oxidant).

6
Selecting the Appropriate Type of Disinfection

• Germicidal and viricidal effect Needs to be
  sufficient to achieve treatment objectives
• Stable Residual Specially important in water
  treatment to prevent re-contamination during
  distribution
• Safety Some chemicals are highly toxic, such as
  chlorine.
• Formation of disinfection by-products.

7
How Effectiveness of Disinfection is Measured

• Pathogens is a very broad description Lots of
  them At a small concentration Difficult to
  isolate
• Use a surrogate or indicator organism. The
  majority of bacteria that cause disease are
  excreted by man. Each person discharges 100 to
  400 billion coliform bacteria per day. Coliform
  bacteria is the indicator organism measured in
  WWTP.
• If coliforms present, can assume water is
  contaminated, if not, water is most probably free
  from disease causing bacteria.

8
How Effectiveness of Disinfection is Measured

• Coliform bacteria include Escherichia Coli and
  Aerobacter.
• In wastewater, two measurements are typically
  done Total Coliform Bacteria and Fecal Coliform
  Bacteria. Slightly different tests are done for
  the two measurements (see Metcalf and Eddie,
  pages 95-99)
• Results expressed as most probable number (MPN)
  per 100 ml

9
Disinfection Using Chlorine or its Compounds

• Chlorine can be added as a
• Gas (Cl2(g)) - lots of plants use this method.
  Chlorine gas is highly toxic.
• Sodium hypochlorite (NaOCl) - Commonly know as
  bleach
• Calcium hypochlorite (Ca(OCl)2 - solid
• Fairly complex chemistry in water and wastewater

10
Dissolution of Chlorine in Water

• There are three basic reactions occurring when
  chlorine is added to water
• Dissolution
• Cl2(g)? Cl2 (l)
• Very fast reaction. Ratio of two concentrations
  given by Henrys Law constant (H).

11
Dissolution of Chlorine in Water

• Hydrolysis
• Cl2(l) H2O ? HOCl H Cl-
• Chlorine reacts with water to generate
  hypochlorous acid, hydrogen ions (acid) and
  chloride.
• Very fast reaction.
• Equilibrium constant
• Equilibrium constant is fairly large. The
  reaction proceeds to the hydrolysis of the
  chlorine to a large extent. Large quantities of
  chlorine can be dissolved in water

12
Dissolution of Chlorine in Water

• Hypochlorous acid is a weak acid, it can
  dissociate
• HOCl ? H OCl-
• Hypochlorous acid dissociates into hydrogen ions
  and hypochlorite.
• At a pH of 7.6, half of the hypochlorous acid is
  dissociated to hypochlorite.
• Equilibrium constant not as favorable to products
  as for hydrolysis of of hypochlorous acid.It
  increaseds with temperature

13
What is the effect of pH in Chlorine Speciation?

• Look at equations for chlorine in water
• Dissolution Cl2(g)? Cl2 (l)
• Hydrolysis Cl2(l) H2O ? HOCl H Cl-
• Dissociation HOCl ? H OCl-

14
Speciation of Chlorine Species as a Function of pH
15
Why Do We Care About Species Present?

• Hypochlorous acid is a much stronger oxidant than
  hypochlorite. For a given total chlorine
  concentration (CTCl2(l) HOClOCl- Free
  Residual Chlorine), more disinfection
  efficiencies are obtained at lower pH.
• It either takes more time, or at higher dosages
  if hypochlorite is the predominant species.

16
Comparing Killing Efficiency of Different
Chlorine Species
17
Reactions of Hypochlorous Acid with Ammonia

• Hypochlorous acid is a very strong oxidant.
• It reacts with ammonia present in effluent from
  plants. Very significant if plant is not
  nitrifying
• Different species, with different levels of
  disinfection (oxidation) capabilities are formed.
• Need to account for these to provide adequate
  disinfection.

18
Reactions of Hypochlorous Acid with Ammonia

• NH3 HOCl ? NH2Cl (monochloramine) H2O
• NH2Cl HOCl ? NHCl2 (dichloramine) H2O
• NHCl2 HOCl ? NCl3 (nitrogen trichloride) H2O

19
Reactions of Hypochlorous Acid with Ammonia - A
Few Definitions

• Free Residual ChlorineCl2(l) HOClOCl-
• Combined Residual Summation of all Chloramines
• Total Residual Chlorine Free Residual Chlorine
  Combined Residual Chlorine
• All concentrations are expressed as Cl2

20
Calculating Concentrations as Cl2

• Total reduction of Cl2 can be written as
• Need to calculate electrochemical equivalent
  amount of Cl2 to other compounds
• Hypochlorite is electrochemically equivalent to
  chlorine, so it contains 35.52 mg of available
  chlorine

21
Calculating Concentrations as Cl2

• Each chlorine atoms in all chloroamines molecules
  will undergo a two-electron reduction to
  chloride.
• Monochloroamine contains 71 grams of available
  chlorine.
• Dichloramine contains 142 grams of available
  chlorine.
• Trichloramine contains 213 grams of available
  chlorine.

22
Reactions of Hypochlorous Acid with Ammonia -
Breakpoint Chlorination

• Hypochlorous acid will first react with readily
  oxidizable substances, such as Fe2, Mn2 and
  H2S. At this point of process, hypochlorous acid
  concentration in liquid is zero.
• After these compounds are oxidized, hypochlorous
  acid will react with ammonia to form various
  chloramines, with mono being formed first. All
  chlorine is present in the form of combined
  chlorine.

23
Reactions of Hypochlorous Acid with Ammonia -
Breakpoint Chlorination

• For molar ratios of chlorine to ammonia less than
  one mono and dichloroamine are formed. The rate
  of formation of these two compounds dependent on
  pH and temperature.
• At chlorine dosages higher than this, nitrogen
  trichloride will be formed and the remaining
  chloramines will be oxidized to nitrous oxide gas
  and nitrogen gas. This is the breakpoint
• This results in ammonia removal from wastewater.
• See a decrease in the total residual chlorine
  concentration

24
Reactions of Hypochlorous Acid with Ammonia -
Breakpoint Chlorination

• Continous addition of chlorine past the break
  point, will result in a proportinal increase of
  the free residual chlorine.
• Need to account for these transformations in a
  total residual chlorine level is to be achieved.
• Chlorine demand is equal to the chlorine dose
  applied minus the total residual chlorine
  achieved.

25
Reactions of Hypochlorous Acid with Ammonia -
Breakpoint Chlorination
Dose
26
Breakpoint chlorination.

• If a target residual chlorine concentration is to
  be maintained, need to provide a dose such that
  demand is satisfied and residual is obtained.
• Theoretically, require 7.6 g of Cl2 to bring one
  g of N-NH4 to break point. In practice, a ratio
  of 8 is typically observed
• Breakpoint chlorination leads to the effective
  removal of nitrogen from the effluent wastewater.

27
Kinetics of Bacterial Death By Disinfection

• To design a chlorine contact chamber, need to
  know kinetics of disinfection.
• Several factors affect rate of bacterial kill
• Temperature (moderate effect) pH (strong effect
  since affect speciation) presence of organic
  matter (can be strong effect and disinfectant
  will oxidize organic material) concentration and
  types of organisms (strong effect).

28
Kinetics of Bacterial Death By Disinfection

• Several factors affect rate of bacterial kill
• The type of disinfectant (we are considering
  chlorine)
• The concentration at which chlorine is present
• The contact time between the chlorine and the
  organisms to be killed

29
The Effect of Contact Time on Rate of Kill

• The most commonly used expression to reflect this
  effect is Chicks Law

30
Taking Concentration Into Account

• Concentration of disinfectant will also affect
  the rate at which bacteria are killed
• Rate of kill will be maximum if both contact time
  and disinfectant concentration are both high. Can
  have adequate kill rates if C is low but t is
  high and vice-versa.
• Commonly use the term Ct product when dealing
  with disinfection systems.

31
Correlation for Estimating Kill Efficiency as
function of Ct

• A correlation often used to estimate the amount
  of residual chlorine required to achieve a
  certain kill efficiency is
• Assume a chlorination tank with detention time of
  30 minutes. We want to destroy 99.9999 of the
  coliforms in the effluent. What residual chlorine
  concentration is required?

32
Calculating Required Residual Concentration.

• Required residual concentration
• MOE Guidelines for Chlorine Contact Chambers 30
  minutes for average annual flow and 15 minutes
  for peak hourly flow. Whichever gives the larger
  tank volume

33
What To Do If Residual Chlorine Concentration is
Too High?

• It may be possible that the required residual
  chlorine concentration for adequate disinfection
  is higher than what the MOE allows to discharge
  through the plant effluent.
• Can use the process of dechlorination. Addition
  of sulfur dioxide will lead to to conversion of
  hypochlorous acid to chloride. In practice, 1
  mg/L of SO2 is required to remove a residual of 1
  mg-Cl2/L

34
Desirable Characteristics for a Chlorination
Contact Tank

• Plug-flow or completely mixed?
• Plug-flow is preferable, since in theory, the
  hydraulic retention time of each slice of fluid
  is the same.
• Should prevent short-circuiting in the tanks.

35
Some Advantages of Using Chlorine as a
Disinfectant

• Reliable
• Cheap
• Simple
• Provides a Stable Residual

36
Some Disadvantages of Using Chlorine as a
Disinfectant

• Extremely toxic to people.
• Corrosive.
• Taste and odor
• Formation of trihalomethanes (chloroform,
  dichloromethane, trichloromethane, etc) by
  reacting with organic matter in water/wastewater.
  These compounds are known carcinogens.

37
UV Disinfection

• Physical method of inactivating pathogens.
• Mechanism of UV Disinfection
• Radiation with an wavelength of around 260 nm
  penetrates the cell wall and cell membrane of
  microorganisms and is absorbed by cell material
  such as DNA and RNA and promotes changes that
  prevents replication to occur

38
UV Disinfection - Mechanisms

• Emissions from Low Pressure Hg lamps as compared
  to spectral curve for cell inactivation. Maximum
  of absorption (265 nm) coincides to wavelength of
  radiation emitted (254).

39
UV Disinfection - Mechanism

• Schematic of the effect of UV on DNA

40
UV Disinfection - Mechanism

• The mechanism involves absorption of a UV photon
  photon by pyrimidine bases (principally thymine)
  where two pyrimidine bases are next to each other
  on the DNA chain.
• The photochemistry involves formation of a
  dimer that links the two bases together,
  disrupting the DNA, preventing replication of DNA
  and formation of RNA and therefore proteins and
  enzymes.

41
UV Disinfection - The Concept of Dosage

• For chlorine disinfection, presented the concept
  of Ct product as an indication of disinfecting
  potential.
• For UV disinfection, a similar quantity is UV
  dosage

42
Complicating Factors in Design of UV Systems

• There is an intensity field in an UV reactor.
  Each lamp radiates in all directions. This is
  called dissipation.
• The water, non-biological materials and even
  other lamps will absorb radiation emitted by
  other lamps.
• Bacteria are present in flocs in effluent to be
  disinfected. The higher the TSS concentration in
  effluent, the higher dosage required to reach all
  bacteria present.
• Pilot tests strongly recommended for UV systems

43
An Example of a Full-Scale UV System.
44
UV Disinfection

• Advantages of UV disinfection over chlorine
  disinfection
• No by-products of disinfection are known to be
  formed. With chlorine, there is strong potential
  for the formation of trihalomethanes (THM).
• Short detention times UV disinfection requires a
  six-to-10-second contact time, compared to a
  15-to-30-minute contact time for chlorine.
• UV disinfection presents no dangers in terms of
  handing chemicals . Chlorine highly toxic to
  humans.

45
Aeration - Introduction

• Objectives of aeration in biological systems
• To transfer oxygen to microorganisms (to MLSS) at
  a rate that is large enough so that oxygen never
  becomes limiting factor in process operation.
• Need to satisfy the requirements from a
  stoichiometric point of view, while still
  providing enough air to have a DO level that does
  not hinder microbial growth. Typically 2-3
  mg-O2/L

46
Aeration - Introduction

• Objectives of aeration in biological systems
• Addition of air to system also serves to keep
  solids in suspension. Air also serves for mixing.
• This function is one of the reasons why a very
  large bioreactor should not be built. This is
  because air required to keep solids in suspension
  is larger than air required to satisfy growth
  requirements

47
Aeration Devices - Types

• Two main types of aeration devices, depending on
  where they are located
• Submerged aerators Air introduced below liquid
  surface by blowing air to diffusers. Air is
  provided by blowers
• Surface (mechanical aerators)Mechanically
  agitate water to promote transfer of oxygen to
  the water from the atmosphere above the liquid.

48
Example of Submerged Aeration Fine Bubble
Diffusers
49
Example of Surface Aeration Rotor Brush Aerators
50
Different Types of Aeration Devices
51
What Do We Want to Learn Regarding Aeration?

• In previous classes, calculated oxygen
  requirement for biological growth (carbon removal
  and nitrification).
• This oxygen is going to be provided with air and
  through a device at a certain efficiency.
• Want to know how much air will need to provide to
  satisfy requirements and maintain a residual DO.

52
Basics of Oxygen Transfer

• For system in equilibrium consisting of a liquid
  in contact with a gas, the ratio of the
  concentrations of a solute is given by Henrys
  Law

Cg
Gas
CL
Liquid
53
Basics of Oxygen Transfer

• Henrys Law Constant can be presented in other
  forms
• For aeration, the solute we are considering is
  oxygen

54
Gas/Liquid Mass Transfer - The Two Film Model
(Lewis and Whitman - 1924)
55
Gas/Liquid Mass Transfer - The Two Film Model
(Lewis and Whitman - 1924)

• Two films exist at the gas-liquid interface One
  gas and one liquid.
• Films provide resistance to pass of gas molecules
  between bulk-liquid and bulk-gaseous phase.
• For slightly soluble gases, resistance in liquid
  film
• For very soluble gases, resistance in gaseous
  film
• Oxygen is slightly soluble, resistance in liquid
  film

56
Assumptions

• Resistance in liquid film.
• Concentration at interface in equilibrium
• PBPi and since then
  CICSConcentration at saturation

57
Based on Assumptions

• Assuming a linear concentration profile in the
  liquid film

58
Based on Assumptions
59
Factors Affecting Oxygen Transfer Rate

• Look at Equation
• Need to consider factors that affect CS
  (concentration in liquid phase when at
  equilibrium with gas phase)
• Need to consider factors that affect KLa

60
Factors Affecting CS

• Temperature As temperature increases saturation
  concentration of dissolved oxygen decreases with
  temperature See notes, page 131. For 0C, with
  atmosphere, Cs 14.6 mg-O2/L, while at 30C, CS
  is 7.6 mg-O2/L.
• Everything else constant, the oxygen transfer in
  South Carolina in the summer will be lower than
  in Hamilton in the winter.

61
Factors Affecting CS

• Partial pressure of oxygen

62
Factors Affecting CS

• Partial pressure of oxygen Obviously dependent
  if we are using air or oxygen to aerate the
  system. There are plants that use pure oxygen as
  source of oxygen.
• Presence of dissolved salts, particulates and
  surface active substances.

63
Putting Factors Affecting CS Into OTR Equation

• Initial Equation
• Now, need to consider factors that affect KLa

64
Factors Affecting KLa

• Temperature
• Wastewater characteristicsValues for KLa in
  wastewater less than in tap water
• Reason for this is presence of surface active
  agents, which have hydrophilic and hydrophobic
  regions
• Concentrate at interface, with hydrophilic end
  into water and hydrophobic end into gas phase,
  leading to retardation of molecular diffusion.

65
Factors Affecting KLa

• Introduce factor to account for differences in
  KLa from tap water to wastewater
• Can determine a by evaluating KLa separately in
  both wastewater and in tap water.
• Normally, KLa determined under operational
  conditions and with MLSS, and determined value is
  referred to as a KLa

66
Typical Values for a
67
Putting Factors Affecting KLa Into OTR Equation

• We had the following equation
• Including the factors affecting KLa

68
Factors Included In General Equation

• The Equation obtained was

Oxygen saturation concentration at temperature T
and 14.7 psi pressure (atmosphere)
69
Typical Values for a
70
How Can We Get Parameters for Equation?

• Parameters used in equation for OTR are highly
  dependent on environmental conditions in the
  bioreactor. This affects a and b.
• For standardization purposes and to allow
  comparing apples with apples, manufacturers of
  aeration equipment, report oxygen transfer rate
  of equipment tested at standard conditions.

71
Standard Conditions for Testing Aeration Equipment

• Temperature 20 C
• Pressure 14 psi 1 atm
• Oxygen Concentration in Liquid Phase 0 mg-O2/L
• Standard Oxygen Transfer Rate SOTR

72
Test For Determining SOTR

• Need a reactor with clean water and aeration
  equipment in it with air being supplied at a
  given rate.
• Test started by adding sodium sulfite in the
  presence of cobalt catalyst to remove all oxygen
  from the water.
• Then turn air on and measure oxygen concentration
  with time.

73
Typical Dissolved Oxygen Concentration Profile
During Test

• Considering time zero as the point at which
  aeration system was turned back on

74
How to Get KLa From DO Profile

• Solve differential equation
• By integration

75
How to Get KLa From DO Profile

• Solving the integral
• Use non-linear regression to minimize sum of
  squares of differences between model and
  experimental data.

76
Relating SOTR with Actual OTR

• We can relate the Actual Oxygen Transfer Rate in
  the Field (AOTR) with the Standard Oxygen
  Transfer Rate (SOTR)

77
Other Means of Expressing OTR

• The most common method is to express OTR in terms
  of oxygen transfer efficiency (OTE). OTE is
  defined as the amount of oxygen transferred to
  the liquid per unit time divided by the amount of
  oxygen supplied by air per unit time

Density of Air
Fraction of Oxygen in Air 0.232 kg-O2/kg-Air
78
Other Means of Expressing OTR

• OTE can be calculated for Standard Conditions
  (SOTE) and for actual conditions (AOTE).
• Note that by definition
• Which means that

79
Typical Efficiencies For Aeration Systems
80
Other Means of Expression OTR

• On a power consumption basis kg-O2/kWh
• Divide OTR by power usage of aeration device.
• Power usage can be estimated based on actual
  power measurements, or by plate readings and
  assuming efficiency for system.
• Can size the aeration system in terms of HP.
• Used commonly for surface aerators - Small Plants

81
Back To the Main Question

• In previous classes, calculated oxygen
  requirement for biological growth (carbon removal
  and nitrification).
• This oxygen is going to be provided with air and
  through a device at a certain efficiency.
• Want to know how much air will need to provide to
  satisfy requirements and maintain a residual DO.

82
Work an Example

• Nitrifying activated sludge system with actual
  oxygen requirements (AOR) of 1,000 kg-O2/day.
  Temperature is 20C and pressure is 1 atm.
• Want to maintain a residual DO of 2 mg-O2/L using
  ceramic discs for fine bubble aeration.
• How much air need to provide?

83
How much Air Need to Supply?

• Step 1 Check with manufacturers, they typically
  give us SOTE values for aeration equipment.

Assume SOTE 0.30
84
How much Air Need to Supply?

• Then use the previously derived equation
• Assume that b is 0.95. This is very reasonable
  for typical municipal wastewater.
• Need a value for a

85
How much Air Need to Supply? - Typical Values
for a
86
How much Air Need to Supply?

• Can substitute all the values in equation
• This means that 11 of the oxygen supplied by the
  air will be transferred to liquid phase.
• We know how much oxygen needs to be supplied to
  liquid phase 1,000 kg-O2/day

87
How much Air Need to Supply?

• To calculate amount of air to be supplied to
  satisfy microbial oxygen demand and to provide a
  residual DO

88
Critical Aspects on Previous Analysis

• When doing real calculations, need to get actual
  SOTE from manufacturer.
• Value assumed for a critical on calculations. If
  existing plant, then should try to measure it
• Neglected effects of depth in tank on the oxygen
  saturation concentration, leading to conservative
  value. Can do detailed calculation using EPA
  Manual EPA Design Manual for Fine Pore Aeration
  Systems (EPA/625/1-89/023)