Water treatment and water treatment
technologies are an essential line of defense to remove contaminants and
bacteria before the delivery of clean, potable water supplies for consumption.
Water sources can be subject to contamination and therefore require appropriate
treatment to remove disease-causing agents. Public drinking water systems use a
variety of methods to provide safe drinking water for their communities.
Depending on the continent, country and region, different water treatment
systems may be in operation depending on regional regulations and raw water
input. The following article provides an overview of the basic principles of
water treatment and the processes and technologies involved.
There are over
145,000 active public water systems in the United States (including
territories). Of these, 97% are considered small systems under the Safe
Drinking Water Act, meaning they serve 10,000 or fewer people.
While many of these active small
systems consistently provide safe, reliable drinking water to their customers,
many face a number of challenges in their ability to achieve and maintain
system sustainability. Some of these small system challenges include lack of
expertise to choose, operate, and maintain systems; lack of financial
resources; aging infrastructure; limited options for residual disposal; and state
agencies with limited resources to support the large number of small systems.
Water treatment: mimicking earth’s hydrological cycle
What is
water treatment?
Maintaining water treatment to
ensure a clean supply to meet growing global populations has been an ongoing
challenge throughout human history.
Thanks to significant technological
developments in water treatment, including monitoring and assessment,
high-quality drinking water can be supplied and enjoyed around the world.
Replicating the earth’s hydrological cycle in which water is continuously
recycled, treatment enables the same water to be cleansed through several
natural processes.
To ensure they do not present a
health risk, nearly all water sources require treatment before they can be
consumed. Many treatment systems are designed to remove microbiological
contamination and physical constituents, including suspended solids
(turbidity). Following this, a final disinfection stage is nearly always
included at the end of the treatment process to help deactivate any remaining
microorganisms. If a persistent disinfectant, such as chlorine, is added this
can also act as a residual to help prevent biological regrowth during water
storage or distribution in larger systems.
Water treatment consists of several
stages. This can include the initial pre-treatment by settling or through using
coarse media, filtration followed by chlorination, called the multiple barrier
principle. The latter allows effective water treatment and allows each stage to
treat and prepare water to a suitable quality for the next downstream process.
For example, filtration can prepare water to ensure it is suitable of UV
(ultraviolet) disinfection.
Depending on the quality and type of
the water entering a water plant, treatment may vary. For example, groundwater
treatment works abstract water from below ground sources such as aquifers and
springs. These sources tend to be relatively clean in comparison to surface
water, with fewer water treatment steps required.
Surface water treatment works take
water from above ground sources, such as rivers, lakes and reservoirs. This raw
water is subject to direct environmental input. As a result, multiple treatment
steps are required and individual processes are required that will enable the
configuration of different comabinations to clean and finally disinfect the
abstracted water.
Some water supplies may contain
disinfection by-products, inorganic chemicals, organic chemicals and
radionuclides. As a result, specialised water treatment methods may also be
part of water treatment to help control formation and removal.
Furthermore, under renewed
regulations, tighter limits could be placed on endocrine disrupting
chemicals as well as lead limits being halved.
How
does the water treatment process work?
Coagulation, flocculation and
sedimentation are processes used to remove colour, turbidity, algae and other
microorganisms from surface waters.
Chemical coagulants can be added to
the water for the formation of a precipitate, or floc to entrap these
impurities. After sedimentation and/or filtration, the floc is separated from
the treated water
Aluminium sulphate and ferric
sulphate are two of the most commonly used coagulants , although others are
available. Raw water quality near to the inlet of a mixing tank or flocculator
determines the rate in which coagulants are dosed in solution.
By adding coagulant at a point of
high turbulence, it is rapidly and thoroughly dispersed on dosing. The next
stage is the sedimentation tank. Here aggregation of the flocs takes, which
settle out to form sludge that will need to be removed.
One of the advantages of coagulation
ais that it reduces the time required to settle out suspended solids.
Furthermore, it can be very effective in removing fine particles that are
otherwise very difficult to remove.
The cost and the requirement for
accurate dosing, thorough mixing and frequent monitoring, are often cited as
the principal disadvantages of using coagulants for treatment of small
supplies. Bench scale coagulation tests can be used to determine which
coagulant to use for a specific raw water.
As a result, to remove colour and
turbidity, coagulation and flocculation are considered the most effective
treatment techniques. However, for small water supplies they may not be
suitable. This is due to the level of control required and volumes of sludge
generated.
Six
essential Water treatment technologies
A variety of water treatment
technologies are needed to work together, in sequence, in order to purify raw
water before it can be distributed. Here is a list of basic technologies often
used in water treatment works.
- Screens
Screens
are used on many surface water intakes to remove particulate material and
debris from raw water. Weeds and debris can be removed using coarse screens,
whereas smaller particles including fish can be removed using band screens and
microstrainers. Ahead of coagulation or subsequent filtration, microstrainers
are used as a pre-treatment to reduce solids loading.
- Gravel filters
Turbidity
and algae can be removed using gravel filters, which consist of a rectangular
channel or a tank divided into several sections and filled with graded gravel
(size range 4 to 30mm). An inlet distribution chamber allows the raw water to
enter through and flow horizontally through the tank, encountering first the
coarse and then the finer gravel. An outlet chamber collects the filtered water
with solids being removed from the raw water accumulate on the floor of the
filter.
- Slow sand filters
Turbidity,
algae and microorganisms can also be removed using slow sand filters. A simple
and reliable process, slow sand filtration is often suitable for the treatment
of small supplies provided that sufficient land is available. Slow sand filters
usually consist of tanks containing sharp sand (size range 0.15-0.30mm) to a
depth of between 0.5 to 1.5m.
- Activated carbon
Using
physical adsorption, contaminants can be removed using activated carbon. This
will be affected by the amount and type of the carbon, the nature and
concentration of the contaminant, retention time of water in the unit and
general water quality (temperature, pH, etc.).
One of the
mocst common mediums is granular activated carbon (GAC), although powdered
activated carbon (PAC) and block carbon are also sometimes used. Filter media
is contained in replaceable cartridges and a particulate filter at the outlet
of the cartridge is used to remove carbon fines from the treated water.
- Aeration
Aeration
is designed to transfer oxygen into water and remove gases and volatile
compounds by air stripping. A common method is packed tower aerators as a
result of their compact design and high energy efficiency. To achieve air
stripping various techniques can be used including counter current cascade
aeration in packed towers, diffused aeration in basins and spray aeration.
- Membrane processes
Reverse
osmosis (RO), ultrafiltration (UF), microfiltration (MF) and nanofiltration
(NF) are the most commonly used membranes for water treatment processes.
Previously applied to the production of water for industrial or pharmaceutical
applications, membranes are being applied to the treatment of drinking water.
Membrane processes can provide adequate removals of pathogenic bacteria,
Cryptosporidium, Giardia, and potentially, human viruses and bacteriophages. In
a notable case study, companies from the Netherlands and Denmark are working on
integrating enzymes into membrane technology for the removal of pesticides
and pharmaceutical residues from drinking water.
UV
water treatment: shining a light on disinfection
Invisible to the human eye,
ultraviolet (UV) light can be used to disinfect microorganisms in water
treatment processes. The wavelengths of UV light range between 200 and 300
nanometers (billionths of a meter). Ultraviolet radiation is produced at 254 nm
from special low-pressure mercury vapor lamps. This is the optimal wavelength
for disinfection and ozone destruction. Categorised as germicidal, this means
they are capable of inactivating microorganisms, such as bacteria, viruses and
protozoa. It's important to note that UV lamps never have contact with the
water; they can be mounted external to the water which flows through UV
transparent Teflon tubes or housed in a quartz glass sleeve inside the water chamber.
How does it work? The wavelengtsh of
UV light render bacteria, viruses and protozoa incapable of reproducing and
infecting.
UV disinfection can be used for the
primary disinfection technology of potable drinking water. Futhermore, the
process can also be used as a secondary form of disinfection. For example,
against microorganisms, such as Cryptosporidium and Giardia, which can be
chlorine-resistant.
In addition, UV light (either alone
or in conjunction with hydrogen peroxide) can destroy chemical contaminants
such as pesticides, industrial solvents, and pharmaceuticals through a process
called UV-oxidation.
Under ideal conditions, a UV unit
can provide greater than 99% reduction of all bacteria. However, even with this
performance, ultraviolet disinfection has two potential limitations: “point”
disinfection and also cells not being removed.
"Point" Disinfection can
occur if the UV units only kill bacteria at one point in a watering system and
do not provide any residual germicidal effect downstream. If just one bacterium
passes through unharmed (100% destruction of bacteria cannot be guaranteed),
there is nothing to prevent it from attaching to downstream piping surfaces and
proliferating.
Secondly, a second limitation can be
if bacteria cells are not removed in a UV unit but are converted into pyrogens.
The killed microorganisms and any other contaminants in the water are a food
source for any bacteria that do survive downstream of the UV unit.
One notable development to UV
systems is the scaling up of light-emitting diode technology, known
as UV-LED, with 2018 witnessing a tipping point on power density and purchasing
price.
Ozone
water treatment: harnessing the power of lightning
Like a lightning storm, ozone is
created when oxygen is exposed to the discharge of a powerful electric current
through air. While widely used in Europe for many years to treat municipal
drinking water, it has not had a similar acceptance in the US.
Ozone can be used throughout water
treatment, for example during pre-oxidation, intermediate oxidation or final
disinfection as it has excellent disinfection and oxidation qualities. Usually,
it is recommended to use ozone for pre-oxidation, before a sand filter or an
active carbon filter (GAC). Following ozonization these filters can remove the
remaining organic matter (important for final disinfection).
Ozonation is carried out by an
electric discharge field as in the CD-type ozone generators, or by ultraviolet
radiation (UV-type ozone generators). Ozone can also be achieved through
electrolytic and chemical reactions, in addition to commerical methods.
In general, an ozonation system
includes passing dry, clean air through a high voltage electric discharge,
i.e., corona discharge, which creates and ozone concentration of approximately
1% or 10,000 mg/L. In treating small quantities of waste, the UV ozonation is
the most common while large-scale systems use either corona discharge or other
bulk ozone-producing methods.
Raw water is then passed through a
venturi throat which creates a vacuum and pulls the ozone gas into the water or
the air is then bubbled up through the water being treated. Since the ozone
will react with metals to create insoluble metal oxides, post filtration is
required.
Ozone is highly reactive and, as a
result, has a very short half-life once dissolved into water. The natural
reaction is for ozone to return to its oxygen form, with a reaction time
typically taking 10-20 minutes at 20 degrees Celsius.
Advantages to ozone water treatment
include the minimisation of inorganic, organic and microbiological problems and
taste and odour problems. Furthermore, no additional chemicals are added to the
water.
Meanwhile disadvantages include a
lack of germicidal or disinfection residual to inhibit or prevent growth.
Furthermore, the system may require pre-treatment for hardness reduction.
Types
of water treatment chemicals (and why they are used)
Chemical disinfection of
drinking-water includes any chlorine-based technology, such as chlorine
dioxide, as well as ozone, some other oxidants and some strong acids and bases.
Except for ozone, proper dosing of chemical disinfectants is intended to
maintain a residual concentration in the water to provide some protection from
post-treatment contamination during storage.
Disinfection of household
drinking-water in developing countries is done primarily with free chlorine,
either in liquid form as hypochlorous acid (commercial household bleach or more
dilute sodium hypochlorite solution between 0.5% and 1% hypochlorite marketed
for household water treatment use) or in dry form as calcium hypochlorite or
sodium dichloroisocyanurate. This is because these forms of free chlorine are convenient,
relatively safe to handle, inexpensive and easy to dose.
Chlorine is the most widely used
primary disinfectant and is also often used to provide residual disinfection in
the distribution system. Monitoring the level of chlorine in drinking water
entering a distribution system is normally considered to be a high priority (if
it is possible), because the monitoring is used as an indicator that
disinfection has taken place. Residual concentrations of chlorine of about 0.6
mg/l or more may cause problems of acceptability for some consumers on the
basis of taste.
Chlorine dioxide breaks down to
leave the inorganic chemicals chlorite and chlorate. These are best managed by
controlling the dose of chlorine dioxide applied to the water. Chlorite can also
be found in hypochlorite solution that has been allowed to age.
Proper dosing of chlorine for
household water treatment is critical in order to provide enough free chlorine
to maintain a residual during storage and use. Recommendations are to dose with
free chlorine at about 2 mg/l to clear water (< 10 nephelometric turbidity
units [NTU]) and twice that (4 mg/l) to turbid water (> 10 NTU).
Monochloramine, used as a residual
disinfectant for distribution, is usually formed from the reaction of chlorine
with ammonia. Careful control of monochloramine formation in water treatment is
important to avoid the formation of di- and trichloramines, because these can
cause unacceptable tastes and odours.
A number of other chemicals may be
added in treatment. These include substances such as sodium hydroxide for
adjusting pH and, in certain circumstances, chemicals for fluoridation of
drinking-water.
