water supply and treatment – i

7/22/2019 Water Supply and Treatment I

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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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water supply and treatment – i

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