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Drinking Water Treatment > Disinfection and Oxidation > Flashcards

Disinfection and Oxidation Flashcards

1
Q

What is the purpose of Disinfection in drinking water production?

(2 points)

A
  1. Inactivation/ removal of pathogenic micro-organisms in/from water
  2. Prevention of (re)growth in basins and distribution networks
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2
Q

What are the six main purposes of Advanced Oxidation in drinking water production?

A
  1. Break down of organic micro-pollutants
  2. Break down pharmaceuticals
  3. Increasing biodegradability of organics
  4. Removal of colour, taste and order
  5. Iron and Manganese oxidation prior to filtration
  6. Chemical ammonium removal
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3
Q

What is the difference between pathogenic, non-pathogenic and opportunistic pathogenic organisms?

A

Pathogenic: will definitely get ill if you ingest or inhale them (Colera)

Non-Pathogenic: You won’t get sick (Probiotics).

Opportunistic Pathogenic: Only some people will be infected and get sick usually people with suppressed immune systems (Legionella)

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4
Q

Explain the mechanisms of disinfection on a micro-organism in 4 steps?

A

First if the oxidation or rupturing of the cell wall

Then diffusion of the disinfectant into the cell

This will damage the DNA causing deactivation

Lastly use Filtration, Adsorption and Sedimentation for the removal

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5
Q

Disinfection kinetics (reaction rates) are dependent on:

4 points

A
  1. Micro-organism (not all micro-organisms have the same sensitivity to disinfection)
  2. Disinfectant
  3. Temperature
  4. pH
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6
Q

Disinfection efficiency is influenced by:

2 points

A
  1. Reaction Rates

2. Water matrix (e.g NOM and NH4 concentration)

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7
Q

Disinfection Kinetics: Explain Chick’s law with a formula?

A

dN/dt = -k * N

after integration:

ln(N/No) = -kt => log(N/No) = -kt

If you increase the time then the log reduction increases: there is a linear relationship between the contact time between the disinfection and micro-organisms and the log removal.

Where N = number concentration micro-organisms (CFU/100ml)

No = initial concentration micro-organisms (CFU/100ml)

k = inactivation rate coefficient (1/s)

t = time (s)

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8
Q

Disinfection Kinetics: Explain Watson law with a formula?

A

The requested percentage of disinfection (Kr) is determined with the time * concentration of disinfection usually to the power of 1.

C^n * t = Kr

Where C = concentration of disnfectant (mg/l)

n = empirical constant (-)

t = contact time (s)

Kr = value for percentage of inactivation (e.g. 99%)

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9
Q

Disinfection Kinetics: Explain Chick-Watson concept with a formula?

A

DEC is proportional to C*t, when you increase the Ct the log(N/No) will increase which creates a linear relationship between Ct and the log removal.

ln(N/No) = -Kcw * C * t

which can be written as:

DEC = - log(N/No) = C * t

Where:

N = Number concentration of micro-organisms (CFU/100ml)

No = Initial number concentration of micro-organisms (CFU/100 ml)

Kcw = specific lethality (l/(mg.s))

C = concentration of disinfectant (mg/l)

t = time (s)

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10
Q

What is an Index Pathogen and what is the main drawback with disinfection in drinking water treatment?

A

Index Pathogen: A pathogen that is representative of waterborne pathogenic viruses, bacteria and parasitic protozoa.

Challenge: In relation to the infection risk, the concentrations in finished water is so low that it is not possible to measure.

Indicator Organisms have similar properties as the index pathogens i.e they can be removed in the same way but they can be measured in the finished water which helps to determine the required disinfection capacity of a system.

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11
Q

What is the dutch regulation for infection risk of Index Pathogens?

A

Infection risk for all pathogens is the same at less than 10^-4.

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12
Q

Why are index pathogens difficult to measure in finished water?

A

The Dutch Regulation requires there to be an infection risk of less than 10^-4 for an index pathogen which means less than 1 infection per 10,000 individuals - therefore each pathogen will have a different tolerable concentration (very small) which is accepted for every m3 of finished water - these concentrations are too small to be measured which is why indicator organisms are used.

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13
Q

What do you use a QMRA for?

A

A QMRA is a process of estimating the risk from exposure to microorganisms It is used when it is not possible to measure the concentration of an index pathogen in finished water, if you know the input concentration you can work out the log reduction for that pathogen and then measure the Cin and Cout for the indicator pathogen and see if the log reduction is the same too meet the standards: 10^-4/ yr.

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14
Q

Draw a pH curve to show how the different concentrations of Cl (Chlorine), HOCl (Hypochlorous Acid) and ClO- (Hypochlorite) vary with PH in water.

A

At pH 0 = 80% Cl2 and 20% HOCl and 0% ClO-

At pH 4 = 0% Cl2 and 100% HOCl and 0% ClO-

At around pH 7.5 = 0% Cl2 and 50% HOCl and 50% ClO-

At pH 14 0% Cl2 and 0% HOCl and 100% ClO-

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15
Q

Name four applications of using Chlorine?

A
  1. Transport Chlorination - to prevent (re)growth in pipes or distribution networks
  2. Pre-Chlorination (Process Chlorination) to avoid algal growth, reduce ammonia in sand filters and open reservoirs.
  3. Main Disinfection (Primary Disinfection) - to produce hygienically safe water
  4. Post Chlorination (Secondary Disinfection) - To avoid regrowth in drinking water pipes (recontamination).
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16
Q

One of the effects of using Chlorination is for the purpose of disinfection. Think of the Chick-Watson formula and state what influences the disinfection capacity in this case?

A

Influenced by:

pH: effects the equilibrium between HClO and OCl-

Ct: The contact time

Temperature: Equilibrium between HClO and OCl-.

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17
Q

What influences the formation of disinfection bio-products when using chlorination?

A

Depends on concentration of NOM, dosage, pH and time

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18
Q

How do you reduce the effect of disinfection by-products when using Chlorination?

A

Reduce concentration of NOM by:

Coagulation/Flocculation
Activated carbon filtration
Ion exchange

19
Q

What are the main drawbacks of using Chlorination for Disinfection to produce drinking water?

A

Can react with NOM to produce disinfection bioproducts which are carcinogenic & production of chlorinated compounds can also cause birth defects

20
Q

When Chlorine reacts with inorganic compounds one effect is the production of Monochloramine, Dichloramine and Trichloramine. Write the chemical reaction for each.

A

Monochloramine:

NH4+ + HClO => NH2Cl + H3O+

Dichloramine:

NH2Cl + HClO => NHCl2 + H2O

Trichloramine:

NH2CL + HClO => NCl3 + H2O

21
Q

What is the purpose of breakpoint chlorination? Write the chemical formula for the reaction.

A

To remove ammonium from the water

2NH2Cl + HClO + 2H2O => N2 + 3H3O+ + 3Cl-

22
Q

Explain breakpoint chlorination you may use a diagram to help with your discussion

A

When you dose free chlorine as (HClO - Hypocaloric Acid) if Ammonium (NH4+) is present in your water they will react and form NH2Cl (Monochlorime) 1 mol of cl2 will give you 1 mol of NH2Cl.

At a certain point everything is converted to NH2Cl.

If you continue to dose more free chlorine then you will start to reduce the NH2Cl by a factor of 2 until it reaches break point where no NHCl or NH4+ is present anymore.

If you still continue to dose more free chlorine you will only be left with HOCl

23
Q

Ozone disinfection is effective for the inactivation of Bacteria, Viruses and Protozoa.

True or False?

A

True - Bacteria and Viruses will die quickly but are only very effective for Protozoa at high temperatures and needs a much higher Ct value or a longer contact time.

24
Q

Ozone can react with NOM explain a benefit of this reaction and a drawback?

A

Ozone can react with big molecular structures causing a breakdown into smaller molecules that are easily biodegradable.

You may need to add a biofiltration step after ozonation as it is not a persistent disinfectant, to remove the biodegradable compounds to prevent regrowth in the distribution network - this could be costly.

25
Q

Explain why often Ozonation is considered to be a multi-barrier approach in drinking water production?

A

Ozonation reacts with NOM and can oxidise organic micro-pollutants producing biodegradable compounds or metabolites and some pollutants do not react at all with ozone therefore these all must be removed by an additional treatment step after ozonation such as GAC filters, slow sand filtration etc.

The Multi-barrier approach is just an integrated system of using different treatment technology and processes to reduce contamination of drinking water.

26
Q

What is the main drawback of using Ozoneation to produce drinking water?

A

Ozone reacts with Bromide to produce Bromate which is a carcinogenic compound

27
Q

When using ozonation what is the dutch standard for bromide in ug/l?

A

1 ug/l Bromide for ozonation and if oxidation then 5 ug/l

28
Q

What are the main advantages of dosing Ozone using a sidestream system?

A

Overall very effective as you can dose ozone in very fine bubbles and more dedicated flow.

  1. Less ozone is needed (-13%) this has a finical benefit
  2. Less Bromate is produced (-38%)
29
Q

Why when designing an Ozone contact chamber are plug-flow systems most favourable?

A

You get a high efficiency as the reaction rate is exponential & you have a more defined residence time the contact time with ozone is the same with every drop of water passing through the system.

A plug flow has a narrower spread of resistance time in the reactor compared to fully mixed which has a larger spread of resistance time in the reactor.

30
Q

CT concept: In an ozone plug flow reactor what is meant by CT10 and how can it improve the system?

A

CT10 is the HRT when 10% of the water has passed the reactor - a safety net if your system has short-circuiting or dead zones.

31
Q

In an ozone disinfection treatment system, an engineer detects reactor is short-circuiting?

What is short-circuiting and how can it be prevented?

A

Short-circuiting occurs when the flow path of water in the reactor is following a direct path in and out and does not enter the mixing zone and therefore is not in contact with the ozone. Most likely a fully mixed system.

You can use a plug flow but ensure that is properly designed to prevent “dead zones” add extra baffles to make better use of the volume in the reactor.

32
Q

What is the difference between disinfection and oxidation?

A

Disinfection you rupture the cell wall of a micro-organism and interfere with the DNA and RNA

Oxidation you breakdown molecules

33
Q

Why is Ozone prefered over Chlorination?

A

The Trihalomethane produced in Chlorination are much more difficult to control, Bromate concentrations are easier to control.

Ozone is not that effective for cryptosporidium and gardia and you need a very high concentration of Chlorine whereas Ozone more effective for protozoa

34
Q

For the purpose of disinfection using UV, are low-pressure UV or high-pressure UV lamps better?

A

Low pressure because they operate at a similar wavelength (254 nm) compared to the optimal wavelength for disinfection and have a high radiative intensity of 100% effective whereas medium-pressure lamps have a much broader wavelength but are not strong enough (0-80% effective)

35
Q

UV can inactivate micro-organisms but they are not dead explain this using two micro-organisms repair mechanisms?

A

Micro-organisms have repair mechanisms to repair DNA:

Photoreactivation in the light

Dark Repair in the dark

36
Q

Which type of UV lamp can lead to more micro-organism repair mechanisms and why is this the case?

A

Low Pressure lamps because they have less energy which can lead to dark repair compared to medium pressure lamps which destroy the enzymes

37
Q

What are the advantages of UV disinfection over chemical disinfection/ Ozone

A
  1. No formation of byproducts
  2. No storage and transport of chemicals is required
  3. No overdoes possible
  4. Short contact times (3-5 seconds)
38
Q

What are the disadvantages of UV disinfection over chemical disinfection/ Ozone

A
  1. Bacterial DNA may repair
  2. No residual disinfection capacity during distribution
  3. UV light can cause skin irritation and eye damage
39
Q

In UV Photolysis explain what is UV Absorbance and what will influence the effectiveness?

A

Every micropollutant will have its own unique absorbance wavelength.

The amount of UV adsorbed will depend on the composition of the water:

  • NOM can absorb UV and then there may not be enough particles left for the specific micro-pollutant you want to break down
  • Nitrate (NO3) can also adsorb UV
  • Particles that cause turbidity can scatter UV particles diminishing the UV transmittance of the water
40
Q

In UV Photolysis explain what is UV Absorbance and what will influence the effectiveness?

A

Every micropollutant will have its own unique absorbance wavelength.

The amount of UV absorbed will depend on the composition of the water:

  • NOM can absorb UV and then there may not be enough particles left for the specific micro-pollutant you want to break down
  • Nitrate (NO3) can also adsorb UV
  • Particles that cause turbidity can scatter UV particles diminishing the UV transmittance of the water
41
Q

Explain UV advanced oxidation? and how it differs from other disinfection methods?

A

You dose Hydrogen Peroxide (H2O2) and then radiate the water with UV and produce OH radicals that are highly reactive and will attack and destroy a wide range of contaminants.

UV advanced oxidation is selective which means it will attack all bonds compared to ozone which is much more selective i.e is not effective for some micropollutants

42
Q

An engineer wants to test if a UV lamp is working effectively conducting a biodosimetry. He administers a dose of UV and calibrates a Dose-Response Curve for the target organism. In the full-scale system, he administers the same dose and measures the organism inactivation in the reactor and compares the results to see if they match the curve. He sees that the results do not match what could be a reason for this?

A
  1. Lamp age - decrease of 20%
  2. Lamp Fouling
  3. Watermatrix - UV adsorption
  4. Poor hydraulic flow design of reactor: stagnant zones not mixing in the reactor
43
Q

When designing a UV disinfection and AO system what can affect the UV efficiency?

A
  1. Lamp age - decrease of 20%
  2. Lamp Fouling
  3. Watermatrix - UV adsorption
  4. Poor hydraulic flow design stagnant zones not mixing in the reactor

Drinking Water Treatment (6 decks)

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