|
Comparing
Disinfectants:
Chlorination
(Gas)
At normal pressures, elemental chlorine is
a toxic, yellow-green gas, and is liquid at high pressures.
ADVANTAGES
Chlorine is very
effective for removing almost all microbial pathogens and is appropriate as
both a primary and secondary disinfectant.
LIMITATIONS
Chlorine is a
dangerous gas that is lethal at concentrations as low as 0.1 percent air by
volume. There is a resistance build up by some bacteria/virus.
PROCESS
Chlorine gas is
released from a liquid chlorine cylinder by a pressure-reducing and
flow-control valve operating at a pressure less than atmospheric. The gas is
led to an injector in the water supply pipe where highly pressurized water
is passed through a Venturi orifice creating a vacuum that draws the
chlorine into the water stream. Adequate mixing and contact time must be
provided after injection to ensure complete disinfection of pathogens. It
will be necessary to control the pH of the water.
EQUIPMENT
A basic system
consists of a chlorine cylinder, a cylinder-mounted chlorine gas vacuum
regulator, a chlorine gas injector, and a contact tank or pipe. Prudence
and/or state regulations would require that a second cylinder and gas
regulator be provided with a change-over valve to ensure continuity of
disinfection. Additional safety and control features may be required. A gas
chlorinator should be installed in a room or chamber with direct emergency
access to outside air and fitted with an exhaust fan ventilation system.
Federal and state safety regulations must be observed. If not onsite, a
self-contained breathing apparatus and a chlorine cylinder-repair kit should
be available within a reasonable time frame and/or distance.
CHEMICALS
Chlorine gas is
supplied as liquid in high-pressure cylinders.
Chlorination
(Sodium-hypochlorite solution)
Sodium-hypochlorite
is available as a solution in concentrations of 5 to 15 percent chlorine,
but is more expensive than chlorine gas (available as chlorine).
ADVANTAGES
Sodium hypochlorite
is easier to handle than gaseous chlorine or calcium hypochlorite.
LIMITATIONS
Sodium hypochlorite
is very corrosive and should be stored with care and kept away from
equipment that can be damaged by corrosion. Hypo-chlorite solutions
decompose and should not be stored for more than one month. It must be
stored in a cool, dark, dry area. There is a resistance build-up by some
bacteria/virus
PROCESS
Sodium-hypochlorite
solution is diluted with water in a mixing/holding tank. The diluted
solution is injected by a chemical pump into the water-supply pipe at a
controlled rate. Adequate mixing and contact time must be provided.
EQUIPMENT
A basic liquid
chlorination system, or hypochlorinator, includes two metering pumps (one
serving as a standby), a solution tank, a diffuser (to inject the solution
into the water), and tubing.
CHEMICALS
Sodium-hypochlorite
solution is readily available. Sodium hypochlorite can also be generated
onsite by electrolysis of sodium-chloride solution in specialized
proprietary equipment. The only supplies required are common salt and
electricity. Hydrogen is given off as a by-product and must be safely
dispersed.
Chlorination
(Solid-calcium hypochlorite)
Calcium hypochlorite
is a white solid that contains 65 percent available chlorine and dissolves
easily in water.
ADVANTAGES
When packaged,
calcium hypochlorite is very stable, allowing a year’s supply to be bought
at one time.
LIMITATIONS
Calcium hypochlorite
is a corrosive material with a strong odour that requires proper handling.
It must be kept away from organic materials such as wood, cloth, and
petroleum products. Reactions between calcium hypochlorite and organic
material can generate enough heat to cause a fire or explosion. Calcium
hypochlorite readily absorbs moisture, forming chlorine gas. Therefore,
shipping containers must be emptied completely or carefully resealed.. There
is a resistance build-up by some bacteria/virus
PROCESS
Calcium hypochlorite
may be dissolved in a mixing/holding tank and injected in the same manner as
sodium hypochlorite. Alternatively, where the pressure can be lowered to
atmospheric, such as at a storage tank, tablets of hypochlorite can be
directly dissolved in the free-flowing water by a proprietary device that
provides flow-proportional chlorination with gravity feed of the tablets.
EQUIPMENT
The equipment used to
mix the solution and inject it into the water is the same as that for sodium
hypochlorite. Solutions of 1 or 2 percent available chlorine can be
delivered by a diaphragm-type, chemical feed/metering pump or by tablet
chlorinator.
CHEMICALS
Calcium hypochlorite
can be purchased in granular, powdered, or tablet form.
Chloramine
Chloramines are
formed when water containing ammonia is chlorinated or when ammonia is added
to water containing chlorine (hypochlorite or hypochlorous acid).
ADVANTAGES
An effective
bactericide that produces fewer disinfection by-products, chloramine is
generated onsite.
Usually,
chloramine-forming reactions are 99 percent complete within a few minutes.
LIMITATIONS
Chloramine is a weak
disinfectant. It is much-less effective against viruses or protozoa than
free chlorine.
Chloramine is
appropriate for use as a secondary disinfectant to prevent bacterial
regrowth in a distribution system. Nitrogen trichloride appears to be the
only detrimental reaction. It may be harmful to humans and imparts a
disagreeable taste and odour to the water. The use of the proper amounts of
each chemical reactant will avoid its production. There is a resistance
build-up by some bacteria/virus
PROCESS
Chlorine (gaseous
solution or sodium hypochlorite) is injected into the supply main followed
immediately by injection of ammonia (gaseous solution or as ammonium
hydroxide). As before, adequate mixing and contact time must be provided.
The mix of products produced when water, chlorine, and ammonia are combined
depends on the ratio of chlorine to ammonia and the pH of the water.
Chlorine-to-ammonia ratios of 5:1 should not be exceeded. If the pH drops
below 5, some nitrogen trichloride may be formed.
EQUIPMENT
The generation of
chloramines requires the same equipment as chlorination (gaseous or aqueous
hypochlorination), plus equipment for adding ammonia (gaseous or aqueous).
CHEMICALS
Chemicals used to
generate chloramine from ammonia and chlorine gas depend on the
ammonia-based chemical used. Anhydrous ammonia is the least expensive, while
ammonium sulphate is the most expensive.
Ozonation
Ozone, an allotrope
of oxygen having three atoms to each molecule, it is the most powerful
oxidizing and disinfecting agent practically available to man. It is formed
by passing dry air through a system of high-voltage electrodes. There is no
resistance build-up possible by any bacteria or virus as in the case of
other mediums.
ADVANTAGES
Requiring the
shortest contact time and lowest dosage of all, ozone is widely used, as a
primary disinfectant in many parts of the world—but is relatively new to the
U.S. Ozone leaves no residual toxins, flavours or chemicals and there is no
resistance possible by any bacteria/virus. It also does not produce
halogenated organic materials unless a bromide ion is present when used in
combination, as a secondary disinfectant.
LIMITATIONS
Ozone gas is unstable
and must be generated onsite. A secondary disinfectant, usually bromine or
chlorine may
be required when a long-term residual disinfectant in water needs to be
maintained and where, for practical reasons, ozone cannot be reintroduced or
recirculated.
PROCESS
The five major
elements of an ozonation system are:
• Air preparation or
oxygen feed;
• Electrical power
supply;
• Ozone
generation—usually using a corona discharge cell/s
• Ozone contact
chamber; (if required)
• Ozone exhaust-gas
destruction. (If Required)
EQUIPMENT
Ozonation equipment
can include air-preparation equipment; an ozone generator, contactor,
destruction unit; and instrumentation and controls. The capital costs of
ozonation systems are relatively low. Operation and maintenance are
relatively simple and the cheapest of all. Electricity represents 26 to 43
percent of total operating and maintenance costs for small systems and that
consumption is very low being a high voltage, low amperage operation.
CHEMICALS
For some
applications, pure oxygen is a more attractive ozone feed gas than air
because it:
• has a higher
production density;
• requires lower
energy consumption;
• more than doubles
the amount of ozone that can be generated per unit, and
• it requires smaller
gas volumes for the same ozone output, thus lowering costs for ancillary
equipment.
Ultraviolet Light
(UV)
Ultraviolet (UV)
radiation is generated by a special lamp. When it penetrates the cell wall
of an organism, the cell’s genetic material is disrupted and the cell is
unable to reproduce.
ADVANTAGES
UV radiation
effectively destroys bacteria and viruses. A secondary disinfectant must be
used to prevent regrowth of micro-organisms. UV radiation can be attractive
as a primary disinfectant for small systems because:
• it is readily
available;
• it produces no
known toxic residuals;
• it requires short
contact times, and
• the equipment is
easy to operate and maintain.
LIMITATIONS
UV radiation may not
inactivate Giardia lamblia or Cryptosporidium cysts, and
should be used only by ground water systems not directly influenced by
surface water—where there is virtually no risk of protozoan cyst
contamination. UV radiation is unsuitable for water with high levels of
suspended solids, turbidity, colour, or soluble organic matter. These
materials can react with or absorb the UV radiation, reducing the
disinfection performance.
PROCESS
The effectiveness of
UV-radiation disinfection depends on the energy dose absorbed by the
organism, measured as the product of the lamp’s intensity (the rate at which
photons are delivered to the target) and the time of exposure. If the energy
dosage is not high enough, the organism’s genetic material might only be
damaged instead of destroyed. To provide a safety factor, the dosage should
be higher than needed to meet disinfection requirements.
EQUIPMENT
UV lamps and a
reactor.
CHEMICALS
No chemical oxidant
required; therefore, micro-organisms can be killed without generating
by-products of chemical oxidation or halogenation.
Source: National Drinking Water Clearinghouse.
|
Useful Links
