Water Treatments Methods
After separating most floc, the water is filtered as the final step to remove
remaining suspended particles and unsettled floc. The most common type of filter
is a rapid sand filter. Water moves vertically through sand which often has a
layer of activated carbon or anthracite coal above the sand. The top layer
removes organic compounds which could include dangerous disinfection by-products
as well as those with taste and odor. The space between sand particles is larger
than the smallest suspended particles, so simple filtration is not enough. Most
particles pass through surface layers but are trapped in pore spaces or adhere
to sand particles. So not just the top layer of the filter cleans the water, but
effective filtration extends into the depth of the filter. This property of the
filter is key to its operation: if the top layer of sand blocked all particles
the filter would quickly clog. To clean the filter, water is passed quickly
upward through the filter, opposite the normal direction (called backflushing or
backwashing) to remove embedded particles. Prior to this, compressed air may be
blown up through the bottom of the filter to break up the compacted filter media
to aid the backwashing process, this is known as air scouring. This contaminated
water can be disposed of, along with the sludge from the sedimentation basin, or
it can be recycled by mixing with the raw water entering the plant.
Some water treatment plants employ pressure filters. These work on the same
principle as rapid gravity filters differing in that the filter medium is
enclosed in a steel vessel and the water is forced through it under pressure.
Where sufficient land and space are available, water may be treated in slow sand
filters. These rely on biological treatment processes for their action rather
than physical filtration. Slow sand filters are carefully constructed using
graded layers of sand with the coarsest at the base and the finest at the top.
Drains at the base convey treated water away for disinfection. When bringing a
new slow sand filter bed into use, raw water is carefully decanted onto the
filter material to a water depth of one to three metres, depending on the size
of the filter bed. The water passing through the filter for the first few hours
is recirculated and not put into supply. Within a few hours, a film of bacteria,
protozoa, fungi, and algae builds on the surface of the sand. This is the
Schmutzdecke layer that removes all the impurities. An effective slow sand
filter may remain in service for many weeks or even months if the pre-treatment
is well designed and produces an excellent quality of water which physical
methods of treatment rarely achieve.
Disinfection with aggressive chemicals like chlorine or ozone is normally the
last step in purifying drinking water. Water is disinfected to destroy any
pathogens which passed through the filters. Possible pathogens include viruses,
bacteria including Escherichia coli, Campylobacter and Shigella and protozoans
including Giardia lamblia and other Cryptosporidium. Many water treatment
systems add sufficient disinfection agent to ensure that an effective
concentration remains in the water throughout the distribution system. In many
cities, this period can be many days.
The most common disinfection method is some form of chlorine such as chlorine
gas, sodium hypochlorite, chloramine or chlorine dioxide. The water and chemical
mix are allowed to sit in a large tank, called a clear well. The water must sit
in the clear well to ensure that the water is in contact with the disinfectant
for a minimum amount of time because it takes time to inactivate the harmful
microbes. Chlorine is a strong oxidant that kills many microorganisms and
remains in the water to provide continuing disinfection. Other disinfection
methods include using ozone, which acts very rapidly, or ultraviolet light,
which is almost instantaneous.
Chlorine gas and sodium hypochlorite are the most commonly used disinfectants,
because they are inexpensive and easy to manage. They are effective in killing
bacteria, but have limited effectiveness against protozoans that form cysts in
water (Giardia lamblia and Cryptosporidium, both of which are pathogenic).
Chlorine gas and sodium hypochlorite both have strong residuals in the water
once it enters the distribution system.
The main drawback in using chlorine gas or sodium hypochlorite is that these
react with organic compounds in the water to form potentially harmful levels of
the chemical by-products trihalomethanes (THMs) and haloacetic acids, both of
which are carcinogenic and regulated by the U.S. Environmental Protection Agency
(EPA). The formation of THMs and haloacetic acids is minimized by effective
removal of as many organics from the water as possible before disinfection
and/or by adding ammonia immediately after chemical disinfection is completed.
Formerly, it was common practice to chlorinate the water at the beginning of the
purification process, but this practice has mostly been abandoned to minimize
the production of THMs.
Chloramines are not as effective disinfectants compared to chlorine gas or
sodium hypochlorite, but do not form THMs or haloacetic acids. They are
typically used only in stored and distributed treated water. An example of this
sort is proceeses using ozone for primary disinfection which is very quickly
accomplished then using monochloramine to create a residual level of
disinfectant in the water. Chlorine dioxide is another rapid acting disinfectant
against bacteria but unlike ozone it leaves a long lasting residual in the
water. Despite these beneficial characteristics, it is rarely used because it
may creates excessive amounts of chlorate and chlorite, both of which are
regulated to low allowable levels.
Ozone is a very strong, broad spectrum disinfectant and is widely used in Europe
to disinfect water. It is a most effective method to inactivate harmful
protozoans that form cysts and works well against almost all other pathogens. To
use ozone as a disinfectant, it must be created on site and added to the water
by bubble contact. Other benefits of ozone are that it does not form any
dangerous by-products and does not add any taste or odor to the water. One of
the main disadvantages of ozone is that it leaves no disinfectant residual in
UV radiation can be used to disinfect water as well. UV radiation is very
effective at inactiavitng cysts, as long as the water has a low level of colour
so the UV can pass through without being absorbed. The main drawback to the use
of UV radiation is that it, like ozone treatment, leaves no residual
disinfectant in the water.
Many environmental and cost considerations affect the location and design of
water purification plants. Groundwater is cheaper to treat, but aquifers usually
have limited output and can take thousands of years to recharge. Surface water
sources should be carefully monitored for the presence of unusual types or
levels of microbial/disease causing contaminants. The treatment plant itself
must be kept secure from vandalism and terrorism. The large quantities of
dangerous chemicals suggests special training for workers and emergency
personnel. Facilities typically responsibly dispose of waste and prevent them
from contaminating the treatment components and the source water. All facilities
disinfect finished water, but the exact method of disinfection can be
controversial, and the costs and benefits of different methods weighed.
Water Filters -