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Water & Wastewater Systems 4 > L7- Disinfection > Flashcards

L7- Disinfection Flashcards

1
Q

Disinfection

A

Inactivation of pathogens, m.o, bacteria, viruses by chemical, physical, mechanical or irradiation processes

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2
Q

Overall disinfection methods & main ones used in wastewater treatment

A

Chemical: via oxidative species - (Cl, Br, I, O3); heavy metals; acids/bases// alcohol

Physical: heat, filtration

Mechanical: sedimentation, desanding

Irradiation: UV, ultrasound

UV irradiation, ozonation, chloronation

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3
Q

Mechanism of chlorination disinfection and equipment

A

Direct oxidation of cell wall allowing cellular constituents to migrate out of cell
Modification of cell wall permeability
Inhibition of enzyme activity & damage to cell DNA/RNA

Equipment: Baffled contact chamber or long pipeline (act as ideal PFR)

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4
Q

Mechanism of ozone disinfection and equipment

A

Direct oxidation of cell wall allowing cellular constituents to migrate out of cell
Damage to cell DNA/RNA
Breakage of C-N bonds -> depolymerisation

Equipment: bubbling ozone gas through liquid contact chamber

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5
Q

Mechanism of UV disinfection and equipment

A

Photochemical damage to DNA/RNA
Inactivate cell

Equipment: Contact chamber- open or closed channels

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6
Q

Chlorination Advantages & Disadvantages

A

Adv: Residual reactions, low cost, simple to operate

Dis: Formation of carcinogenic DBP, toxicity of discharged Chlorine

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7
Q

What does chlorine react with in wastewater and list the makeup of Available chlorine:

A

Chlorine reacts with NH3 in wastewater:

Free: HOCl- and OCl-
Combined: NH2Cl, NHCl2 (Chloroamines) and NCl3 (nitrogen trichloride)

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8
Q

Breakpoint chlorination curve:

A

Flatline: immediate demand- readily oxidizable substance react with chlorine

Increase: formation of chloramines creates combined residual

Decrease: chloramines converted to NCl3

Trough: low chlorine residual breakpoint

Increase: further increase in chlorine adds free residuals

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9
Q

Disinfection factors

A

Contact time
Conc., type and activity of disinfection
Temperature
Population & species of m/o

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10
Q

Chlorination mechanism/equation

A

Cl2 + H2O -> HOCl- + H+ + Cl-
HOCl- -> H+ + OCl-

But, HOCL- is more reactive than OCl-
Therefore use NaOCl or Ca(OCl)2
NaOCl + H2O -> HOCl + NaOH
Ca(OCl)2 + 2H2O -> HOCl + Ca(OH)2

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11
Q

Dechlorination with SO2 and activated carbon mechanism

A

SO2 + HOCl + H2O-> Cl- + SO4^2- + 3H+
SO2 + NH2Cl + 2H2O -> Cl- + SO4^2- + NH4 + 2H+

C + 2Cl2 + 2H2O -> 4HCl + CO2
C + 2NH2Cl + 2H2O -> CO2 + 2NH4+ + 2Cl-

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12
Q

Contact chamber dimensions

A

Length/width >40
Length = 5a (assuming 5 channels)
Width = c

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13
Q

Contact time equation & define variables

A

N/N0 = exp(-kt)

N, N0 = concentration and initial concentration of m/o (amount/100 mL)
t = contact time
k = inactivation rate constant of m/o

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14
Q

Graphically finding inactivation rate constant, k

A

ln(N/N0) = -kt

Plot ln(N/N0) against t

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15
Q

Modelling disinfection process - define variables

A

k = /*C^n

/\ = lethality coefficient
C = residual disinfectant concentration (mg/L)
n = empirical constant related to dilution:

n = 1 -> both conc. and time important
n>1 conc more important than time
n<1 time more important than time

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16
Q

Germicidal dose -define variables

A

ln(Nt/N0) = -/\ * CT = -/*D

Nt, N0 = m/o concentration at time t and initially (mg/L)
/\ = lethality coefficient
C = residual disinfectant concentration (mg/L)
D = germicidal dose (mg min/L)

17
Q

Effect of temperature - define variables

A

ln(/\2 / /\1) = [E(T2-T1)]/[RT1*T2]

/\1, /\2 = lethality coefficients at T1 and T2
E = activation energy (J/mol)
R = universal gas constant (=8.3144 J/mol K)

18
Q

Effect of wastewater characteristics on Ozonation

A

Organic content/Oil & grease-> ozone demand

Natural organics -> ozone decomposition & demand

TSS -> increase ozone demand & shield bacteria

Nitrate/nitrites-> reduces effectiveness of ozone

Temperature -> rate of ozone decomposition

19
Q

Ozone dose - define variables

A

Ozone dose = Qg/Ql*[Cgin - Cgout]

Qg, Ql = gas and liquid flow (L/min)
Cgin, Cgout = concentration of ozone in feed and off gas (mg/L)

20
Q

Ozone advantages and disadvantages

A

Adv: Better disinfectant than chlorine, strong oxidation reaction (decreases odour, turbidity, colour, organics)

Dis: at high conc. production of hazardout DBPs; unstable, low duration of disinfection (inadequate residual reactions)

21
Q

Disinfection by-products (DBPs)

A

Chemicals formed when disinfectant/strong oxidant reacts with organics in wastewater

Chronic impact to health and environment
Harm organisms; carcinogenic to pets

22
Q

Formation of DBPs

A

Free chlorine and organic acids (humic acid) reaction -> forms CHX3 (where X is Cl or Br normally)

Rate depends on presence of organics, free chlorine/bromide conc., pH, temp, time

23
Q

UV Dose- define variables

A

D = Iavg*t

D = UV dose (mJ/cm2 or mWs/cm2)
Iavg = average UV intensity (mW/cm2)
t = exposure time (sec)

24
Q

UV intensity with distance - define variables

A

Beer-Lambert law for reduction in UV intensity with distance:
ln (I/I0) = -ECx = kx = [A/x]x

I, I0 = light intensity at distance x and at light source (mW/cm2)
E = molar absorptivity/extinction coefficient of light absorbing solute at wavelength (L/mole*cm)
C = concentration of light absorbing solute (mole/L)
x = light path length (cm)
k = absorptivity (1/cm)
A = absorbance

If x =1, k = A

25
Q

Transmittance - define variables

A

T = (I/I0)*100

T = transmittance (mW/cm2)
I, I0 = light intensity at distance x and at light source (mW/cm2)

26
Q

Effect of water characteristics on UV disinfection

A

Natural organics -> strong absorbers of UV
Oil and grease -> accumulates on quartz sleeves of UV lamps and absorbs UV
Organics -> absorb UV
TSS-> shields bacteria & absorbs UV
Iron -> strong absorber of UV, precipitates on quartz

27
Q

Advantages and disadvantages of UV irradiation

A

Adv: Successfully reacts when no suspended solids, reactivation effect

Dis: reduces proliferation of m/o, destructs outer cell of m/o and DNA/RNA, no residual reactions

Water & Wastewater Systems 4 (9 decks)

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