Water and Wastewater Disinfection

Water and Wastewater Disinfection

Disinfection

•         Disinfection is any process to destroy or prevent the growth of microbes

•         Many disinfection processes are intended to inactivate (destroy the infectivity of) the microbes by physical, chemical or biological processes

•         Inactivation is achieved by altering or destroying essential structures or functions within the microbe

•         Inactivation processes include denaturizing of:

•          proteins (structural proteins, enzymes, transport proteins)

•         nucleic acids

•         lipids (lipid bi-layer membranes, other lipids)

Properties of an Ideal Disinfectant

•         Broad spectrum:  active against all microbes

•         Fast acting:  produces rapid inactivation

•         Effective in the presence of organic matter, suspended solids and other matrix or sample constituents

•         Nontoxic; soluble; non-flammable; non-explosive

•         Compatible with various materials/surfaces

•         Stable or persistent for the intended exposure period

•         Provides a residual (sometimes this is undesirable)

•         Easy to generate and apply

•         Economical

Disinfectants in Water and Wastewater Treatment

•         Free Chlorine

•         Mono-chloramines

•         Ozone

•         Chlorine Dioxide

•         Mixed Oxidants

•         Electrochemically generated from NaCl

•         UV Light

•         Low pressure mercury lamp (monochromatic)

•         Medium pressure mercury lamp (polychromatic)

•         Pulsed broadband radiation

Summary of Disinfectants for Microbes in Water and Wastewater

•         Historically, the essential barrier to prevention and control of waterborne microbial transmission and waterborne disease.

•         Free chlorine: HOCl (hypochlorous) acid and OCl- (hypochlorite ion)

•         HOCl at lower pH and OCl- at higher pH; HOCl a more potent germicide than OCl-

•         strong oxidant; relatively stable in water (provides a disinfectant residual)

•         Chloramines:  mostly NH₂Cl:  weak oxidant; provides a stable residual

•         Ozone, O₃ , strong oxidant; provides no residual (too volatile and reactive).

•         Chlorine dioxide, ClO₂,, strong oxidant; unstable residual (dissolved gas)

•         Concerns due to health risks of chemical disinfectants and their by‑products (DBPs), especially free chlorine and its DBPs

•         UV radiation

•         low pressure mercury lamp: low intensity; monochromatic at 254 nm

•         medium pressure mercury lamp: higher intensity; polychromatic 220-280 nm)

•         reacts primarily with nucleic acids: pyrimidine dimmers and other alterations

Some Factors Influencing Disinfection Efficacy and Microbial Inactivation

•         Microbial strain differences and microbial selection:

•         Disinfectant exposure may select for resistant strains

•         Physical protection:

•         Aggregation

•         particle-association

•         protection within membranes and other solids

•         Chemical factors:

•         pH

•         Salts and ions

•         Soluble organic matter

•         Other chemical (depends on the disinfectant

Factors Influencing Disinfection Efficacy and Microbial Inactivation - Water Quality

•         Particulates: protect microbes from inactivation;

Dissolved organics:  protect microbes from inactivation; consumes or absorbs (for UV radiation) disinfectant; Coat microbe (deposit on surface)

•         pH:  influences microbe inactivation by some agents

•         free chlorine more effective at low pH where HOCl predominates

•         neutral HOCL species more easily reaches microbe surface and penetrates)

•         negative charged OCl^-  has a harder time reaching negatively charged microbe surface

•         chlorine dioxide is more effective at high pH

•         Inorganic compounds and ions:  influences microbe inactivation by some disinfectants; depends on disinfectant