Water purification
refers to the removal of undesirable chemicals, biological contaminants,
suspended solids, and gases from water. It is typically purified for drinking
water but may be purified to different standards for medical, pharmacological,
chemical, and industrial purposes. Water may be filtered, and stilled for sedimentation
to reduce suspended solids, flocculated, chlorinated, and/or run through ultraviolet
(UV) light to purify it. Confirmation of water quality is determined by chemical
and microbiological testing, which, unfortunately, is relatively costly.
Water treatment
plants typically pre-treat the water through pumping and
containment, screening out any large debris (not needed for most groundwater), storage
for biological purification, and running through slow sand filters.
Pre-chlorination to kill off any bacterial fouling of the incoming pipes has
largely been discontinued as a pretreatment method due to the harmful
byproducts of chlorination. It is still used sometimes to remove algae. Slow
sand filters capture some contaminants and allow for biological treatment as
the water runs through them very slowly. In alluvial groundwater aquifers that
occur in subsurface unconsolidated river sands (alluvium), the natural flow of
the water through the sand acts as a slow sand filter for the river water. Those
aquifers are considered to be younger groundwater since the water filtered down
through the alluvium from the river over a few years or decades. Slow sand
filters are also used to treat water in swimming pools where the water is
continuously filtered slowly through a large drum filled with sand. These must
be replaced every so often as the pressure changes due to solids buildup.
Aeration
is often used to treat water. Packed Tower Aeration utilizes tall towers and
can remove VOCs, disinfection byproducts, CO2, H2S, and other organic
compounds. Multi-stage bubble aeration can remove the same compounds but is
less efficient than packed tower aeration. However, it may be cheaper and more
applicable to smaller water systems.
pH
adjustment is an important water treatment technology. If the source
water is too acidic then lime, soda ash, or sodium hydroxide (caustic soda) may
be added to raise the pH to neutral (7). The addition of lime will increase the calcium
ion concentration and the water’s hardness. Removal of dissolved gases with
degasifiers is another method of raising water pH. Making the water slightly
alkaline aids the coagulation and flocculation process. It also limits the
leaching of lead from pipes and solders as occurs with more acidic water
sources.
Corrosion control
via phosphate feeds is another water treatment step in some systems
that are deemed in need of it. Phosphoric acid, zinc orthophosphate, or other
compounds may be added to treat the water. These compounds can remove orthophosphates
that are “believed to combine with lead and copper in plumbing materials to
form insoluble compounds, thus reducing lead and copper release at the tap.”
Coagulation
and flocculation are the next steps in water treatment. Coagulation
involves the addition of coagulants such as aluminum sulfate (alum) or iron III
salts such as iron III chloride. Through physical and chemical processes this causes
precipitation and then merging with suspended solids particles in the water
which can be flocculated out. Suspended solids in water may include inorganic
clay and silt particles, algae, bacteria, viruses, protozoa, and natural organic
matter. Coagulation and flocculation are considered to be two parts of the same
overall process. In the 1960s polymers were developed to aid flocculation and
in some cases to replace the inorganic metal salt coagulants. Polydiallyldimethylammonium
chloride (polyDADMAC) is a commonly used polymer to aid flocculation.
Sedimentation
in a sediment basin is the next step after flocculation. Here the water
clarifies as the floc settles to the bottom of the basin. The upper part of the
water is the clearest and is passed on over a weir. Overflow rates from
sedimentation tanks are typically 0.5 to 1.0 gallons per minute per square foot.
The tanks must be dep enough so that the water movement does not disturb the
floc, or sludge that falls to the bottom. This sludge, which typically makes up
3 to 5 % of the total water volume must be periodically removed and treated. Typical
detention times are 1.5 to 4 hours and typical sedimentation tank depths are 10
to 15 feet. Sludge treatment and disposal can get expensive for water treatment
plants. Other clarification methods are Lamella clarifiers which involve
inclined plates and tubes to increase the surface area so more particles can be
removed, and floc blanket clarifiers which remove sediment by trapping it in a
layer of suspended floc as the water is forced upward.
Dissolved
air flotation utilizes pressurized air that makes bubbles when it returns
to atmospheric pressure and those bubbles trap suspended solids. It is used
where normal sedimentation in tanks is considered to be inadequate. In this
method, the floc floats on the surface and is skimmed off while the clarified
water below is transferred to the next step in the treatment process which is filtration.
Filtration
can involve the use of one or more of several types of water filters. Filtration
can capture many of the pathogens, providing some disinfection, but some still
get through the filters. The most common type of filter is a rapid sand filter.
These have vertical layers of sand and an activated carbon layer at the top to
filter out organic compounds that affect taste and odor. The filter is periodically
cleaned by reversing the flow or backflushing. Compressed air may be used as
well. Pressure filters are employed by some water companies where the filter
medium is enclosed in a steel casing under pressure.
Advantages {of pressure filters}
·
Filters out much smaller particles than paper
and sand filters can.
·
Filters out virtually all particles larger
than their specified pore sizes.
·
They are quite thin and so liquids flow
through them fairly rapidly.
·
They are reasonably strong and so can
withstand pressure differences across them of typically 2–5 atmospheres.
·
They can be cleaned (back flushed) and reused
Slow sand filters
require space and land as water moves slowly through them. They rely on
biological treatment more than physical filtration. They utilize graded layers
of sand with the finest at the top. Wikipedia via a 2014 article in Water
Supply describes more interesting things about slow sand filters:
“Filtration depends on the development of a thin
biological layer, called the zoogleal layer or Schmutzdecke, on the surface of
the filter. An effective slow sand filter may remain in service for many weeks
or even months, if the pretreatment is well designed, and produces water with a
very low available nutrient level which physical methods of treatment rarely
achieve. Very low nutrient levels allow water to be safely sent through
distribution systems with very low disinfectant levels, thereby reducing consumer
irritation over offensive levels of chlorine and chlorine by-products. Slow
sand filters are not backwashed; they are maintained by having the top layer of
sand scraped off when the flow is eventually obstructed by biological growth.
Slow Sand Filtration
Bank filtration
involves using the sediments in a riverbank as a kind of sand filter or alluvium as shown
below.
Microfiltration such as ultrafiltration
membrane filtration, or the chemical method of ion exchange can
remove dissolved ions. Activated carbon filters can remove organic chemicals and
chlorine. Nanofiltration (NF) uses semi-permeable membranes to
separate and purify water by removing particles and solutes with a molecular
weight between 200 and 1,000 daltons. Nanoscale graphene-based filters can
remove just about anything while letting the water molecules pass through. Water
softeners typically employ ion exchange to remove calcium and magnesium ions, replacing
them with sodium and potassium ions. This is cation exchange since the elements
are positively charged ions. Anion exchange utilizes negatively charged ions
(anions). It can remove arsenic, chromium-6, cyanide, nitrate, perchlorate,
per- and polyfluoroalkyl substances (PFAS), sulfate, and uranium. Deionization via
ion exchange is sometimes employed following reverse osmosis with the product
being high-purity deionized water. Ion exchange can also remove nitrates,
arsenic, and heavy metals. Granular activated carbon filters are made from
coal, peat, wood, or coconut shells. These filters can remove inorganic
contaminants including antimony, arsenic, beryllium, fluoride, selenium,
thallium, and uranium. They can also remove organic compounds, including those
that affect taste and odor as well as disinfection byproducts and VOCs. Contaminants
are trapped by adsorption
Disinfection
is the next step which kills off any remaining pathogens. Possible pathogens
include viruses, bacteria, including Salmonella, Cholera, Campylobacter, and
Shigella, and protozoa, including Giardia lamblia and other cryptosporidia. Disinfection
is often done in a disinfection tank where the water is held to achieve
sufficient ‘contact time’ with the disinfectant at the desired concentration.
Chlorine compounds are commonly used for disinfection. These may include chloramine,
chlorine dioxide, sodium hypochlorite (bleach), or calcium hypochlorite. Unfortunately,
chlorine compounds yield potentially harmful byproducts. These include trihalomethanes
(THMs) and haloacetic acids (HAAs). Both are carcinogenic in large quantities
and are regulated by the EPA. Chlorine dioxide can detonate. It is fast-acting
but yields chlorite as a byproduct that is regulated to low levels. Chloramines
are being used more often since they don’t make THMs and HAAs, but they are
less potent than sodium hypochlorite. Chloramines derive from ammonia and
chlorine. They can also experience nitrification with the end results being undesirable
nitrates. They can also leach metals, including lead. Chlorine compounds have
limited effectiveness against pathogenic protozoa that form cysts in water such
as Giardia lamblia and Cryptosporidium.
Ultraviolet
light (UV) is most effective at inactivating cysts, in low turbidity
water. It is less effective in water with turbidity. It can work well in concert
with chloramines. UV light can be used in conjunction with hydrogen peroxide
and/or chlorine which create radicals such as hydroxyls that can oxidize
contaminants. It can remove dangerous microcontaminants such as 1,4-dioxane,
N-nitrosodimethylamine (NDMA), and methyl tert-butyl ether (MTBE).
Solar UV
radiation is also used in water disinfection. Ionizing radiation, bromine, and
iodine are minor methods of disinfection. Fluoride is often added to water at
very low concentrations to reduce tooth decay. Small amounts of phosphate ions
may be added to raise pH in order to prevent the leaching of lead from pipes, which
is known as plumbosolvency. In a few groundwater sources radium is removed via
ion exchange. Sometimes natural fluoride levels in water are too high and it is
removed via activated alumina and bone char filtering.
Boiling water removes
some pathogens. Adsorption with granular activated carbon removes many organic
compounds and their tastes and odors. Home water filters and fish tanks utilize
activated carbon. Metallic silver nanoparticles may also be used. They are
anti-bacterial and can remove pesticide residues. Filtered water from these
filters should be used quickly and the filters should be replaced regularly.
Biological
treatment of drinking water uses indigenous bacteria to remove
contaminants. According to the EPA:
“The process has a vessel or basin called a bioreactor
that contains the bacteria in a media bed. As contaminated water flows through
the bed, the bacteria, in combination with an electron donor and nutrients,
react with contaminants to produce biomass and other non-toxic by-products. In
this way, the biological treatment chemically “reduces” the contaminant in the
water.”
It can remove
nitrates and perchlorates. It destroys contaminants so there is no need for
contaminant removal.
Distillation
can remove 99.9% of contaminants by recondensing the vapor from boiling water
but contaminants with higher boiling points can get through.
Reverse
osmosis is a common way to get high-purity water. It is often used in
conjunction with nanofiltration. According to Wikipedia via Puretec Water:
“Reverse osmosis involves mechanical pressure applied
to force water through a semi-permeable membrane. Contaminants are left on the
other side of the membrane. Reverse osmosis is theoretically the most thorough
method of large scale water purification available, although perfect
semi-permeable membranes are difficult to create. Unless membranes are
well-maintained, algae and other life forms can colonise the membranes.”
Reverse osmosis
may be single pass, double pass, or a combo of both. It is a process that is both
water-intensive and energy intensive.
Other methods
are used to treat contaminated water such as crystallization into hydrates with
low molecular weight gases such as CO2. The water is then separated from the
hydrate crystals. In situ chemical oxidation (ISCO) involves injecting
oxidizers into contaminated soil or groundwater. Bioremediation utilizes microorganisms
to remove contaminants such as alkanes, perchlorates, and metals. It is also
used for contaminated soil and groundwater. Hydrogen peroxide (H2O2) is very
good at disinfecting water. It can be delivered from chemical plants but is
apparently much more potent if made onsite utilizing a gold-palladium catalyst to
make the H2O2 from ambient hydrogen and oxygen. It is very good at killing E.
coli bacteria.
Distilled, or demineralized
water may upset a person’s natural fluid balance. It increases the elimination
of electrolytes. There are recommendations for safe, ideal, and non-safe magnesium
concentrations, calcium concentrations, and water hardness. High water hardness
has been linked to increases in gallstones, kidney stones, urinary stones,
arthrosis, and arthropathies.
Emerging water
treatment technologies include improvements in nanotechnology, advanced
oxidation processes (AOPs), electrocoagulation, forward osmosis for salt
removal, membrane distillation, biological nutrient removal to remove nitrogen and
phosphorus, carbon nanotube filters, solar desalination, microbial fuel cells
which use particles in water to make electricity, and emerging
nanomaterials.
Portable and Home Water Treatment
Home water
filtration systems utilizing activated charcoal are now very common. For
instance, I have a Brita filter. One new trend in portable water treatment is
electric water pitchers. These work well but the filters need to be replaced every
200 gallons or every three months, and the filters cost from $20 to $60. Thus,
this is mainly a solution for those who can afford it.
Safe and Clean Drinking Water for People Who Need
It and New Ways to Help
Millions or
even up to 2 billion people around the world still lack access to safe and
clean drinking water. The problem is most prevalent in rural areas where people
get water from rivers, lakes, or hand-dug wells. One emerging solution utilizes
an aluminum foil coated with a special material called layered double hydroxide
(LDH). This material acts like a magnet, attracting and trapping microbes. It was
developed by an environmental engineer. It is being touted not as a standalone
treatment but as a complementary one that is inexpensive and fast-acting.
“In laboratory tests we found the LDH foil remarkably
efficient, removing over 99% of E. coli bacteria, a common indicator of water
contamination, from water samples within a few hours. We found that its
efficacy also extends beyond E. coli, targeting a wide range of waterborne
pathogens, including bacteria, viruses and parasites. This means that the LDH
foil offers comprehensive protection against various diseases.”
The process
relies on “electrostatic attraction, drawing the microbes towards the LDH
foil like iron filings to a magnet. Other chemical and physical forces
contribute to making pathogens bind to the LDH surface, ensuring their
effective removal from the water.”
This solution is
looking promising for use in Africa where there is a need for clean water
access. The LDH foil can be provided for less than $7 per person annually. More
research is needed, and field trials are underway for this emerging inexpensive
clean water solution.
Nanoconfined Single Atom Catalysts (SACs) – New Research
for Advanced Oxidation Processes Looks Promising but Still Needs Tweaked
New research from
a paper in Nature is improving the technique of advanced oxidation processes
(AOPs) via nanoconfined single-atom catalysts. According to Tech Xplore:
“This new method resulted in an astonishing 34.7-fold
increase in the rate of pollutant degradation compared to traditional methods.
The efficiency of oxidant use also improved significantly, from 61.8% to 96.6%.
The system proved to be highly effective in degrading various electron-rich
phenolic compounds, demonstrating robustness in different environmental
conditions and maintaining high performance in real lake water tests.”
According to the
paper’s abstract:
“The introduction of single-atom catalysts (SACs) into
Fenton-like oxidation promises ultrafast water pollutant elimination, but the
limited access to pollutants and oxidant by surface catalytic sites and the
intensive oxidant consumption still severely restrict the decontamination
performance. While nanoconfinement of SACs allows drastically enhanced
decontamination reaction kinetics, the detailed regulatory mechanisms remain
elusive.”
References:
Study
reveals new catalytic pathway for efficient water pollution control. Science X
staff. Phys.org. August 27, 2024. Study reveals new catalytic pathway
for efficient water pollution control (msn.com)
Nanoconfinement
steers nonradical pathway transition single atom fenton-like catalysis for
improving oxidant utilization. Yan Meng, Yu-Qin Liu, Chao Wang, Yang Si,
Yun-Jie Wang, Wen-Qi Xia, Tian Liu, Xu Cao, Zhi-Yan Guo, Jie-Jie Chen &
Wen-Wei Li. Nature Communications volume 15, Article number: 5314 (2024), Nanoconfinement steers nonradical
pathway transition in single atom fenton-like catalysis for improving oxidant
utilization | Nature Communications
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Emerging
Trends in Water Purifier Technology: 2023. Drink Prime. Latest Water Purification and
Filtration Technology (drinkprime.in)
Safe
Drinking Water Act: Overview of Drinking Water Treatment Technologies. U.S.
EPA. Overview of Drinking Water Treatment
Technologies | US EPA
Aluminum
foil that can clean water—researchers develop coating that attracts and traps
dangerous microbes. Taufiq Ihsan. TechXplore. September 2, 2024. Aluminum foil that can clean
water—researchers develop coating that attracts and traps dangerous microbes
(msn.com)
Tap
into technology: New innovations in water filtration systems. Yackulic
Khristopher. Android Headlines. August 26, 2024. Tap into technology: New innovations
in water filtration systems (msn.com)
Influence
of operating parameters on the performance of a household slow sand filter. Abhilash
T. Nair; M. Mansoor Ahammed; Komal Davra. Water Supply (2014) 14 (4): 643–649. Influence
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