Transcript Water Purification and Treatment

Water Purification and
Treatment
Natural and Municipal
American History of Water
Purification
• Until the late 1800s,
Americans got their
water from ponds, lakes,
rivers, wells, and
rainwater holding tanks.
• Wastewater (including human
wastes) was dumped into holes,
dry wells, leaching cesspools (pits
lined with broken stone), into rivers,
lakes, or onto the ground.
• By the late 1800s, cities and towns
were constructing sewer systems,
and about one-quarter of all urban
households had a flush toilet
• However, some sewer wastes were
still dumped WITHOUT treatment
into lakes and streams
• Other cities and towns were
downstream and this polluted
their water
• Many community leaders believed that
natural waters would purify themselves.
• But, as population increased, diseases
carried by water also increased and the
concentration of intestinal bacteria in
drinking water rose.
E coli bacteria
Cholera bacteria
• As a direct result, water filtering and
chlorination of a community’s water
soon began.
• Nowadays, communities are also
responsible for treating their wastewater
BEFORE it is released into the
environment
Global Water-Purification System
• We can think of the hydrological cycle as
the Earth’s NATURAL water-purification
system, powered by the sun.
OCEANS – the primary water source
ATMOSPHERE – the principal carrier of
water
LAND – the location of the major water
users
• Overall, water is neither LOST nor GAINED
Purifying Water through the
Hydrological Cycle
 The hydrological
cycle-evaporation,
condensation,
precipitation,
infiltration and
runoff--removes
most dissolved
substances
Evaporation, Condensation
• Thermal energy from the Sun causes water
to evaporate from oceans and other water
sources
• Dissolved heavy metals, minerals and
molecular substances are left behind
Precipitation
Pure rainwater and snowflakes are the best
natural supply of clean water
 There is some contamination from
atmospheric gases (natural and humancaused)

Infiltration and Runoff


When rainwater and snowmelt are
absorbed by soil (infiltration) or travel over
the soil surface (runoff), then impurities
(such as organic substances from living
creatures) become suspended or
dissolved in this water.
A few cm below the surface, bacteria feed
on these substances and turn them into
carbon dioxide, water and other simple
compounds, repurifying the water.
Filtration at Deeper Levels
At deeper levels, water passes through
gravel, sand and even pores in rock
(filtration) where bacteria and
suspended material are filtered out
• Slightly acidic groundwater passing
through rocks with soluble compounds
(e.g., magnesium and calcium minerals)
ADDS minerals to water, making the
water ‘hard’

Summary of Nature’s WaterPurification System
These three processes make up the natural
water purification system:
• Evaporation, followed by condensation,
removes nearly all dissolved substances
• Bacterial action converts dissolved
organic contaminants into simple
compounds
• Filtration through sand and gravel
removes nearly all suspended matter
Can we depend on nature
to purify our water?
• If the conditions are right, we could depend
on this natural purification system.
• This means the water has (1) ENOUGH
TIME in the soil to have bacteria clean it
and (2) traveling through ENOUGH SAND
and GRAVEL to filter it.
• This system does NOT work if it is
OVERLOADED or water is drawn from it
before it can be cleansed.
Reservoir Model
• Another way to think of the Hydrological
Cycle is as a model made up of various
reservoirs with water simultaneously
entering and leaving these reservoirs.
Glaciers, ice caps
Soil moisture
Atmosphere
Seasonal snow cover
Shallow groundwater
Oceans
Deep groundwater
• The RESIDENCE TIME or storage
time in these reservoirs can be as
short as days or as long as
thousands of years.
• The reservoirs with the lowest water
capacity tend to have the shortest
residence time because they have
large inputs and outputs.
Glaciers, ice
caps
Soil moisture
Atmosphere
Seasonal snow cover
Shallow groundwater
Oceans
Deep groundwater
Atmosphere: replaced every 8 days
Glaciers and ice caps: 20 to 100 years
Soil moisture: 1-2 months
Seasonal snow cover: 2-6 months
Shallow groundwater: 100 to 200 years
Deep groundwater: to 10,000 years (e.g.,
Ogallala aquifer in central and western US)
Municipal Water Treatment
• Water taken from rivers and lakes for
community use is cleaned twice, both
before and after it is used.
• Pre-use purification is usually called water
treatment.
Water source: River,
lake, reservoir
Prechlorination
Screening
Flocculation
Sand filtration
Post-chlorination
Settling
Optional further treatment:
1) Aeration
2) pH adjustment
3) Fluoridation
Houses,
businesses,
schools, etc.
Municipal Water Treatment
1. Filtering through metal screens to remove
fish, sticks, beverage containers, etc.
2. Prechlorination—chlorine may be added
because it is a powerful disinfecting agent.
Municipal Water Treatment
3. Flocculation—Alum (aluminum sulfate)
and slaked lime (calcium hydroxide) are
added to remove suspended particles. They
react together to form aluminum hydroxide,
a sticky, jelly-like material that traps the
suspended particles. This is allowed to
settle out.
Municipal Water Treatment
4. The water is further filtered by passing
through sand to remove any remaining
suspended particles.
5. Post-chlorination—operators adjust the
chlorine concentration to make sure
there is a low but sufficient level to
protect the water from bacterial
infestation.
Municipal Water Treatment
6. There may be several more steps, such
as aeration (spraying the water into the
air to remove odors and improve taste),
pH adjustment, and fluoridation (to
reduce tooth decay).
Chlorination of Water
“The single most common cause of human
illness … is unhealthful water supplies.” (p.90, text)
Chlorination, the addition of chlorine to
water to kill harmful organisms, is
done in several ways:
1. Chlorine gas (Cl2) is bubbled into the
water. The gas is not very soluble in
water but does produce a watersoluble chlorine compound.
2. A water solution of sodium
hypochlorite, the active ingredient
in household bleach, is added.
3. Calcium hypochlorite, either as
small pellets or a powder, is
dissolved in the water. This
substance is commonly used in
swimming pools and is an
ingredient in bleaching powders
used in the home.
Pros and Cons of Chlorination
Under some conditions, chlorine in water
reacts with organic compounds
produced by decaying organic
material (plant and animal) to form
substances harmful to human health.
One group of such substances is
TRIHALOMETHANES (THMs).
Chloroform is a THM and a carcinogen.
The EPA has placed a limit of 80 ppb
on THM concentration in municipal
water-supply systems.
The benefit of using chlorination to
purify water supplies has to be
balanced against the risk from THMs.
Are there any alternatives?
Eliminating THM Health Risks
1. Pass treated water through
ACTIVATED CHARCOAL.
Disadvantage: Charcoal filters are
expensive to install and operate.
Also, they need to be replaced
often.
Eliminating THM Health Risks
2. Don’t use charcoal at all—instead,
use ozone (O3) or ultraviolet (UV)
light to disinfect the water.
Disadvantage: Doesn’t protect
the treated water from becoming
recontaminated. Also, ozone
itself can be hazardous if not
handled properly.
Eliminating THM Health Risks
3. Eliminate the pre-chlorination step.
Chlorine would be added only once,
after most of the organic material
has been removed.
Disadvantage: THMs can still form,
and the lower chlorine concentration
could lead to bacterial growth.