L7- Disinfection Flashcards
Disinfection
Inactivation of pathogens, m.o, bacteria, viruses by chemical, physical, mechanical or irradiation processes
Overall disinfection methods & main ones used in wastewater treatment
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
Mechanism of chlorination disinfection and equipment
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)
Mechanism of ozone disinfection and equipment
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
Mechanism of UV disinfection and equipment
Photochemical damage to DNA/RNA
Inactivate cell
Equipment: Contact chamber- open or closed channels
Chlorination Advantages & Disadvantages
Adv: Residual reactions, low cost, simple to operate
Dis: Formation of carcinogenic DBP, toxicity of discharged Chlorine
What does chlorine react with in wastewater and list the makeup of Available chlorine:
Chlorine reacts with NH3 in wastewater:
Free: HOCl- and OCl-
Combined: NH2Cl, NHCl2 (Chloroamines) and NCl3 (nitrogen trichloride)
Breakpoint chlorination curve:
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
Disinfection factors
Contact time
Conc., type and activity of disinfection
Temperature
Population & species of m/o
Chlorination mechanism/equation
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
Dechlorination with SO2 and activated carbon mechanism
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-
Contact chamber dimensions
Length/width >40
Length = 5a (assuming 5 channels)
Width = c
Contact time equation & define variables
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
Graphically finding inactivation rate constant, k
ln(N/N0) = -kt
Plot ln(N/N0) against t
Modelling disinfection process - define variables
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
Germicidal dose -define variables
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)
Effect of temperature - define variables
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)
Effect of wastewater characteristics on Ozonation
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
Ozone dose - define variables
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)
Ozone advantages and disadvantages
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)
Disinfection by-products (DBPs)
Chemicals formed when disinfectant/strong oxidant reacts with organics in wastewater
Chronic impact to health and environment
Harm organisms; carcinogenic to pets
Formation of DBPs
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
UV Dose- define variables
D = Iavg*t
D = UV dose (mJ/cm2 or mWs/cm2)
Iavg = average UV intensity (mW/cm2)
t = exposure time (sec)
UV intensity with distance - define variables
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
Transmittance - define variables
T = (I/I0)*100
T = transmittance (mW/cm2)
I, I0 = light intensity at distance x and at light source (mW/cm2)
Effect of water characteristics on UV disinfection
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
Advantages and disadvantages of UV irradiation
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
