3 Physical Disinfectants
Chemical disinfectants protect humans against waterborne microbial diseases. However, it has several disadvantages, including the formation of toxic by-products,
taste, and odor problems. Moreover, in the longer term, there is pressure on the
industries to reduce the production of chlorine-based disinfectants for environmental
issues. In a commercial scale, membrane-based processes are available to remove
microbes as well as chemical contaminants. It operates without chemical disinfection, or at least to reduce the number of chemicals used for disinfection. Further,
pressure-driven membrane processes (microfiltration, ultrafiltration, nanofiltration,
and reverse osmosis) can remove chemical contaminants and microbes based on
their pore size. From the point of toxicological issues, this membrane process would
prevent the formation of disinfection by-products and undesirable chemicals. The
main issues in the membrane-based processes are the removal efficiency of microbes
and biofouling. As an alternative to chemical disinfection, UV radiation is also
capable of removing bacteria and viruses. This part describes the principles involved
in the physical processes.
3.1 Ultraviolet Radiation
UV radiation energy waves ranges from 100 to 400 nm. The optimum UV range is
between 245 and 285 nm for germicidal effects. This process does not produce any
toxic by-products or taste and odor problems. It has several demerits such as high
cost as compared to the chemical disinfectant, UV lamps maintenance, and photoreactivation of enteric bacteria. The effectiveness is decreased in the effluents by
substances like phenolic compounds, humic substances, lignin sulfonates, and ferric
iron. The presence of suspended matter may protect microbes from UV light, leading
to the implementation of pretreatment techniques (filtration) in water treatment.
UV light damages microbial RNA or DNA at a wavelength of 260 nm. This
causes thymine dimerization, which blocks the replication of nucleic acid and
effectively inactivates microbes. In the case of viruses, UV light attacks the genome,
followed by a virus protein coat. Figure 6.3 shows the UV treatment of drinking
water. A minimum dose (16,000 μW s/cm
2 ) is recommended for drinking water.
This leads to a 99.9% reduction of bacterial coliforms (DeMers and Renner 1992).
The factors that affect the performance are biological films, reactor geometry, short
circuiting, microorganism clumping, and turbidity (Sawyer 1992). The continuous
exposure to UV radiation does not change the water chemistry. As a result, there is
no formation of THM or other disinfection by-products.
`
124
N. R. Srinivasan et al.
Chemical disinfectants protect humans against waterborne microbial diseases. However, it has several disadvantages, including the formation of toxic by-products,
taste, and odor problems. Moreover, in the longer term, there is pressure on the
industries to reduce the production of chlorine-based disinfectants for environmental
issues. In a commercial scale, membrane-based processes are available to remove
microbes as well as chemical contaminants. It operates without chemical disinfection, or at least to reduce the number of chemicals used for disinfection. Further,
pressure-driven membrane processes (microfiltration, ultrafiltration, nanofiltration,
and reverse osmosis) can remove chemical contaminants and microbes based on
their pore size. From the point of toxicological issues, this membrane process would
prevent the formation of disinfection by-products and undesirable chemicals. The
main issues in the membrane-based processes are the removal efficiency of microbes
and biofouling. As an alternative to chemical disinfection, UV radiation is also
capable of removing bacteria and viruses. This part describes the principles involved
in the physical processes.
3.1 Ultraviolet Radiation
UV radiation energy waves ranges from 100 to 400 nm. The optimum UV range is
between 245 and 285 nm for germicidal effects. This process does not produce any
toxic by-products or taste and odor problems. It has several demerits such as high
cost as compared to the chemical disinfectant, UV lamps maintenance, and photoreactivation of enteric bacteria. The effectiveness is decreased in the effluents by
substances like phenolic compounds, humic substances, lignin sulfonates, and ferric
iron. The presence of suspended matter may protect microbes from UV light, leading
to the implementation of pretreatment techniques (filtration) in water treatment.
UV light damages microbial RNA or DNA at a wavelength of 260 nm. This
causes thymine dimerization, which blocks the replication of nucleic acid and
effectively inactivates microbes. In the case of viruses, UV light attacks the genome,
followed by a virus protein coat. Figure 6.3 shows the UV treatment of drinking
water. A minimum dose (16,000 μW s/cm
2 ) is recommended for drinking water.
This leads to a 99.9% reduction of bacterial coliforms (DeMers and Renner 1992).
The factors that affect the performance are biological films, reactor geometry, short
circuiting, microorganism clumping, and turbidity (Sawyer 1992). The continuous
exposure to UV radiation does not change the water chemistry. As a result, there is
no formation of THM or other disinfection by-products.
`
124
N. R. Srinivasan et al.