Water
disinfection technologies must offer effective microorganism
inactivation, safe and cost-effective operations, ease of
use, a chlorine residual, and disinfection by-product formation
below the maximum contaminant levels. The most common method
of disinfection, chlorination, is available in gaseous, liquid,
and solid forms. Chlorine can also be
generated on-site in the form of hypochlorite or mixed oxidants.
Chlorine combined with ammonia is called chloramination. Other
disinfection alternatives include chlorine dioxide, ozone, and
ultraviolet.
Each
of these technologies has benefits and disadvantages. In
general, chlorine technologies provide the chlorine residual
required by the EPA; however, none of the chlorine alternatives
are very safe and all maximize production of total trihalomethanes
(TTHMs). Liquid and solid chlorine also are more labor intensive
due to corrosivity and degrading concentration that requires
mixing and dilution ratios to be constantly adjusted. Chloramination
forms much lower TTHM levels, but the disinfectant residual
is very weak and can create taste problems, especially when
mixed with free chlorine downstream. Chlorine dioxide, ozone,
and UV are all very effective disinfectants, much more so
than chlorine, but none leave the required chlorine residual.
Most non-chlorine alternatives also have higher operating
and capital costs.
Another
alternative, on-site generation of chlorine-based oxidants,
is gaining popularity with several thousand installations
from a variety of manufacturers. The process utilizes salt,
water, and electricity to generate a disinfectant solution,
which is collected in a day tank and injected into the water
at a dosage suitable for treatment objectives. On-site generation
eliminates the storage and transport of hazardous chemicals,
while still leaving an acceptable chlorine residual. On-site
generation of mixed oxidants, in contrast to hypochlorite,
offers additional advantages of superior microorganism inactivation
microflocculation, a more durable chlorine residual, reduced
TTHM formation, biofilm removal, ammonia oxidation at sub-breakpoint
doses, and oxidation of iron, manganese, and sulfides. |
