Transcript Water Treatment - mcdowellscience

Water
Treatment
Water Sources and Water Treatment
Drinking water should be essentially free of disease-causing microbes, but
often this is not the case.
– A large proportion of the world’s population drinks microbially
contaminated water, especially in developing countries
Using the best possible source of water for potable water supply and
protecting it from microbial and chemical contamination is the goal
– In many places an adequate supply of pristine water or water that
can be protected from contamination is not available
The burden of providing microbially safe drinking water supplies from
contaminated natural waters rests upon water treatment processes
– The efficiency of removal or inactivation of enteric microbes and
other pathogenic microbes in specific water treatment processes has
been determined for some microbes but not others.
– The ability of water treatment processes and systems to reduce
waterborne disease has been determined in epidemiological
studies
The water treatment process generally
uses the steps below:
1. Pre-treatment: controls algae and arrests any
biological growth
2. Aeration: along with pre-chlorination, removes
dissolved iron and manganese
3. Coagulation: prepares for flocculation
4. Sedimentation: for solids separation, that is,
removal of suspended solids trapped in the floc
5. Filtration: removing particles from water
6. Disinfection: for killing bacteria
Pre-treatment
1. Pumping and containment: water must be pumped from its source or directed into pipes or
holding tanks.
2. Screening: remove large debris such as sticks, leaves, rubbish and other large particles
3. Storage: water from rivers may also be stored in bankside reservoirs for periods between a few
days and many months to allow natural biological purification to take place.
4. Pre-conditioning: water rich in hardness salts is treated with sodium carbonate to
precipitate calcium carbonate out
5. Pre-chlorination: water is chlorinated to minimize the growth of fouling organisms on the pipework and tanks
Aeration
In the process of aeration, water first goes to aeration tanks where large quantities of air are
injected into and bubble through the water. Aeration can reduce tastes and odors and can
oxidize soluble iron.
Aeration Tank
Chemical Coagulation-Flocculation
Removes suspended particulate and colloidal substances
from water, including microorganisms.
Coagulation: colloidal destabilization
• Typically, add alum (aluminum sulfate) or ferric chloride
or sulfate to the water with rapid mixing and controlled
pH conditions
• Insoluble aluminum or ferric hydroxide and aluminum
or iron hydroxo complexes form
• These complexes entrap and adsorb suspended
particulate and colloidal material.
Coagulation
Aluminum sulfate and other chemicals are added to the water, which cause small particles to
clump together (coagulate), forming floc. These floc attract dirt particles, making them eventually
heavy enough to sink to the bottom of the water storage tank. Some of the floc begins to settle
during this stage.
Sedimentation
Gravity and time are all that is needed to remove the floc from the water. The water and floc flow
into a sedimentation basin. These settling basins are typically twelve to eighteen feet deep and
hold the water for two to four hours at a forward velocity of 0.5 to 3 feet per minute.As the water sits
there, the heavy floc settle to the bottom, where they remain until removal.
Filtration
Water passes through layers of gravel, sand and perhaps charcoal, which serve to filter out any
remaining particles. Suspended materials (such as floc, microorganisms, algae, silt, iron, and
manganese) are filtered out when water passes through beds of granular material, usually
composed of layers of sand, gravel, coal, garnet, or related substances.
Filtration Processes Used
Rapid filtration
– used in United States
– fast filtration rates through media (sand or
anthracite)
– backwashing needed
Slow sand filtration
– common in United Kingdom and Europe
– slow filtration rates through media (sand
and gravel)
– removal of biological layer needed
– higher removal rates for all microorganisms
Granular Media Filtration
• Used to remove suspended particles (turbidity) incl. microbes.
• Historically, two types of granular media filters:
– Slow sand filters: uniform bed of sand;
– low flow rate <0.1 GPM/ft2
– biological process: 1-2 cm “slime” layer (schmutzdecke)
– Rapid sand filters: 1, 2 or 3 layers of sand/other media;
– >1 GPM/ft2
– physical-chemical process; depth filtration
• Diatomaceous earth filters
– fossilized skeletons of diatoms (crystalline silicate);
powdery deposit; few 10s of micrometers; porous
Slow Sand Filters
• Less widely used for large US municipal water supplies
• Effective; widely used in Europe; small water supplies;
developing countries
• Filter through a 3- to 5-foot deep bed of unstratified sand
• flow rate ~0.05 gallons per minute per square foot.
• Biological growth develops in the upper surface of the sand is
primarily responsible for particle and microbe removal.
• Effective without pretreatment of the water by
coagulation-flocculation
• Periodically clean by removing, cleaning and replacing the
upper few inches of biologically active sand
Microbial Reductions by Slow Sand Filtration
• Effective in removing enteric microbes from water.
• Virus removals >99% in lab models of slow sand filters.
– Up to 4 log10; no infectious viruses recovered from filter effluents
• Field studies:
– naturally occurring enteric viruses removals
• 97 to >99.8 percent; average 98% overall;
• Comparable removals of E. coli bacteria.
– Virus removals=99-99.9%;
– high bacteria removals (UK study)
• Parasite removals: Giardia lamblia cysts effectively removed
– Expected removals almost 99%
Roughing Filter
•Used in developing
countries
•inexpensive
•low
maintenance
•local materials
•Remove large solids
•Remove microbes
•1-2 log10
bacterial
reduction
•90% turbidity
reduction
Adsorbers and Filter-Adsorbers
Adsorbers:
• Granular activated carbon adsorption
–
–
–
–
remove dissolved organics
poor retention of pathogens, esp. viruses
biologically active; develops a biofilm
can shed microbes into water
Filter-adsorbers
• Sand plus granular activated carbon
– reduces particles and organics
– biologically active
Fluoridation & Disinfection
In most places, Fluoride (F-) is added to water to reduce tooth decay. During disinfection, diseasecausing organisms are destroyed or disabled. Chlorine (Cl2) is the most common disinfectant used
in the United States because it is practical, effective and economical. Ozone is used in some
places as well. A small amount of chlorine is added to kill any bacteria or microorganisms that may
be in the water.
Disinfection:
A Key Barrier Against Microbes in
Water
• Free chlorine still the most commonly used
disinfectant
• Maintaining disinfectant residual during treated water
storage and distribution is essential.
– A problem for O3 and ClO2, which do not remain in water for
very long.
– A secondary disinfectant must be used to provide a stable
residual
• UV radiation is a promising disinfectant because it
inactivates Cryptosporidium at low doses
– UV may have to be used with a chemical disinfectant
to protect the water with a residual through distribution
and storage
Storage
Finished water (the term water treatment professionals use) is stored in holding tanks, in order to
serve homes and businesses in the community.
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