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WATER SUPPLYAND TREATMENT I
Assist. Prof. Serdar DORUELITU Environmental Engineering Department
2013-2014 Autumn Semester
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WATER SUPPLY
AND TREATMENT I
The Hydrologic Cycle and Water AvailabilityGroundwater Supplies
Surface Water Supplies
Water Treatment
Softening
Ion Exchange and Reverse Osmosis
Lime Soda Softening
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Hydrologic cycle includes the precipitation of water
from clouds, infiltration into the ground or runoff into
surface watercourses, followed by evaporation and
transpiration of the water back into the atmosphere.
The Hydrologic Cycle and
Water Availability
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Figure 1. The hydrologic cycle in diagram form
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Precipitation
Precipitation is the term applied to all forms of
moisture originating in the atmosphere and falling to
the ground (e.g., rain, sleet, and snow).
Precipitation is measured with gauges that record in
inches of water.
The depth of precipitation over a given region is
often useful in estimating the availability of water.
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Evaporation and Transpiration
Evaporation and transpiration are the two ways
water reenters the atmosphere.
Evaporation is loss from free water surfaces while
transpiration is loss by plants.
Same meteorological factors that influence evaporation
(solar radiation, ambient air temperature, humidity,
wind speed, etc.) also impact the transpiration.
Because evaporation and transpiration are so
difficult to measure separately, they are often
combined into a single term, evapotranspiration.
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Water on the surface of earth that is exposed to the
atmosphere is called surface water.
Surface waters include rivers, lakes, oceans, etc.
Through the process of percolation, some surface
water (especially during a precipitation event) seeps
into the ground and becomes groundwater.
Both groundwater and surface water can be used as
sources of water for communities.
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Groundwater
Groundwater is both an important direct source of
water supply and a significant indirect source of
supply as a large portion of the flow to streams is
derived from subsurface water.
Water exists both near and far below the soil surface.
Groundwater Supplies
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Zone of Aeration (Vadose Zone)
Near the surface of earth, soil pore spaces containboth air and water. This zone is known as the zone
of aeration, or vadose zone.
It may have zero thickness in swamplands and be
several hundred feet thick in arid regions.
Moisture from the zone of aeration cannot be tappedas a water supply source because this water is held
to the soil particles by capillary forces and is not
readily released.
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Aquifer
Below the zone of aeration is the zone of saturation,
where the pores are filled with water. Water withinthe zone of saturation is referred to as groundwater.
A stratum that contains a substantial amount of
groundwater is called an aquifer.
Porosity
The amount of water that can be stored in the
aquifer is equal to the volume of the void spaces
between the soil grains. The fraction of voids volume
to total volume of the soil is termed porosity.
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Specific Yield
Not all of this water is available for extraction and
use because it is so tightly tied to the soil particles.
The amount of water that can be extracted is knownas specific yield.
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Table 1. Typical aquifer parameters
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Fudge Factor (K)
The fudge factor K is the coefficient of permeability,
an indirect measure of the ability of a soil sample totransmit water.
It varies dramatically for different soils, ranging from
about 0.05 m/d for clay to over 5000 m/d for gravel.
The coefficient of permeability is commonly measured
in the laboratory by using permeameters, whichconsist of a soil sample through which a fluid, such
as water, is forced. The flow rate is measured for a
given driving force through a known area of soil
sample, and the permeability is calculated.
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The water held by the reservoirs can be saved for the
dry periods. The objective is to build these reservoirs
sufficiently large to have dependable supplies.
One method of arriving at the proper reservoir size is
by constructing a mass curve. In this analysis thetotal flow in a stream at the point of a proposed
reservoir is summed and plotted against time. On
the same curve the water demand is plotted, and the
difference between the total water flowing in and the
water demanded is the quantity that the reservoir
must hold if the demand is to be met.
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Figure 2. Mass curve showing required storage volumes
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A mass curve is actually of little use if only limited
stream flow data are available. One yearsdata yieldvery little information about long-term variations.
For example, was the drought in the above example
the worst drought in 20 years, or was the year
shown actually a fairly wet year?
In such cases, it is necessary to predict statisticallythe recurrence of events such as droughts and then
to design the structures according to a known risk.
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Water Treatment
Many aquifers and isolated surface waters are ofhigh water quality and may be pumped from the
supply and transmission network directly to any
number of end uses, including human consumption,
irrigation, industrial processes, and fire control.
However, such clean water sources are the
exception to the rule, particularly in regions withdense populations or regions that are heavily
agricultural. Here, the water supply must receive
varying degrees of treatment prior to distribution.
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A typical water treatment plant is diagramed in
Figure 3.
Such plants are made up of a series of reactors or
unit operations.
Here, the water flows from one process to the next
to achieve a desired end product.
Each operation is designed to perform a specific
function, and the order of these operations is important.
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Figure 3. A typical water treatment plant
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Softening
Some waters (both surface waters andgroundwaters) need hardness removed to use them
as a potable water source.
Hardness is caused by multivalent cations (orminerals)such as calcium, magnesium, and ironthat dissolve from soil and rocks (particularly
limestone).
While hardness does not cause health problems, it
does reduce the effectiveness of soaps and cause
scale formation.
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Total hardness (TH) is defined as the sum of the
multivalent cations in the water.
Calcium (Ca2+) and magnesium (Mg2+) tend to be
the largest components of hardness, so TH is
typically approximated as the sum of these two
components.
However, iron (Fe2+ and Fe3+), manganese (Mn2+),
strontium (Sr
2+
), and aluminum (Al
3+
) may also bepresent in water supplies.
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Typical units for hardness are mg/L as CaCO3 and
meq/L.
By using these units, the contributions of different
substances (e.g., calcium and magnesium) can beadded directly.
The units of mg/L of a particular substance, such as
10 mg/L of Ca2+, cannot be added directly to the mg/L
of a different substance, such as 5 mg/L of Mg2+.
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To convert a concentration in mg/L to meq/L, divide
the concentration by the substances equivalentweight (EW):
C = concentration in mg/L
Cq= concentration in meq/L
EW = equivalent weight in g/eq or mg/meq
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A substances equivalent weight is calculated bydividing its atomic weight (AW) or molecular weight
(MW) by its valence or ionic charge (n, which is
always positive).
Here, AW or MW has units of g/mole and n has units
of equivalents/ mole (eq/mol).
To convert to the standard unit mg/L as CaCO3, themeq/L concentration is multiplied by the equivalent
weight of CaCO3, which is 50.0 g/eq or 50.0 mg/meq.
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Water is classified as soft or hard depending on the
amount of hardness ions present. The water in the
above example would be classified as very hard.
Surface water is generally soft because fewer
minerals dissolve in it.
Water treatment plants typically distribute
moderately hard water, in the range of 80 to 90 mg/L
as CaCO3.
It is difficult to rinse off soap if the water is too soft,
and some hardness can protect the distribution
system from corrosion.
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Table 2. Water hardness classifications
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Reverse Osmosis
Point-of-use reverse osmosis units are used inhomes.
Large-scale units are used in locations with severelylimited freshwater supplies, but copious quantities of
saltwater.
Ion Exchange and Reverse Osmosis
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Figure 4. (A) A typical point-of-use reverse
osmosis unit; (B) schematic
(A) (B)
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Reverse osmosis uses high pressure to push water
molecules through a membrane, resulting in treated
water on one side and a concentrated wastewater
on the other.
The process produces water with low dissolved
minerals and removes some bacteria.
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However, reverse osmosis is slow and generates
large quantities of wastewater; it is also expensive tooperate.
Home units often come with an activated carbonfilter to remove chlorine and improve the taste of the
water.
For waters with high hardness, they will typically
follow a whole-house ion exchange softener.
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Ion Exchange
Ion exchange, or zeolite, softening is most
applicable to waters that are high in non-carbonate
hardness because it can be removed without
chemical addition.
On the other hand, total hardness should be less
than 350 mg/L as CaCO3.
Ion exchange softeners are often used in residences
that have wells.
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The hard water passes through a column containing
resin. The resin adsorbs the hardness ions, exchangingthem for sodium typically. This is why softened water
often tastes salty and why people on low-sodium
and sodium-free diets should avoid drinking it.
Once the resin no longer removes the amount of
hardness desired, a concentrated salt is used to
regenerate the resin (remove the hardness ions) sothat it can be reused.
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As long as the resin is relatively fresh (i.e., it has
plenty of sodium remaining), essentially 100% of thehardness will be removed.
Because not all the hardness needs to be removed,
part of the water can bypass the system so that,when the treated and untreated water mix, the
desired hardness is obtained.
This scenario, of course, is a classical material
balance.
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Figure 5. (A-B) Typical ion exchange
water softener; (C) schematic
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Lime Soda Softening
Although some municipal water treatment plants useion exchange, most use chemical precipitation.
The pH of the water is increased, often through theaddition of lime.
Either quicklime (CaO, unslaked lime) or hydrated
lime (Ca(OH)2, slaked lime) is used. Although lime isa calcium species, it is very effective at softening
water. Sodium hydroxide can be used, but it is more
expensive.
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As the pH increases to approximately 10.3,
carbonate becomes the dominant species of
alkalinity, and CaCO3(scale) precipitates.
As the pH increases to approximately 11, magnesium
precipitates as magnesium hydroxide (Mg(OH)2).
Non-carbonate hardness is more expensive to
precipitate because a carbonate (typically, soda ash,
Na2CO3) must be added.
Therefore, calcium carbonate hardness is targeted
for removal first, then magnesium carbonate hardness,
and finally calcium non-carbonate hardness.
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Carbon dioxide in water forms carbonic acid, which
must be neutralized (by lime or caustic addition) orremoved (through air stripping) before the pH will
rise.
Due to solubility constraints, precipitation can reducetotal hardness to as low as 40 mg/L as CaCO3(the
practical solubility limit).
Due to time constraints, excess lime (lime over thestoichiometric amount) is typically added.
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Figure 7 shows a general treatment train forsoftening.
The precipitates are removed through settling.Recarbonation (adding carbon dioxide to the water) is
used to lower the pH to ensure that any fine particles
not removed in the settling tank resolubilize and that
the distributed water has a pH near neutral.
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